OPTIMIZATION OF GROWTH STAGE SPECIFIC NITROGEN FERTILIZATION

IMPROVES STRAWBERRY GROWTH AND YIELD

By

BHAGATVEER SINGH SANGHA

A THESIS PRESENTED TO THE GRADUATE SCHOOL

OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF

MASTER OF SCIENCE

UNIVERSITY OF FLORIDA

2017

© 2017 Bhagatveer Singh Sangha

To my honorable and loving parents

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ACKNOWLEDGMENTS

First and foremost, I would like to thank God for blessing me with this wonderful

opportunity of gaining knowledge. I am also grateful to my parents for their continued support in

every way all these years. I want to express my sincere gratitude towards my advisor, Dr.

Shinsuke Agehara for his continued support in every way for two years and a half. I feel truly

blessed and fortunate to be mentored by him. Working under him was really a great learning

experience, both professionally and personally. I would also like to thank all my lab members for

their continued help and support during field and greenhouse trials.

5

TABLE OF CONTENTS

page

ACKNOWLEDGMENTS ...............................................................................................................4

LIST OF TABLES ...........................................................................................................................7

LIST OF FIGURES .........................................................................................................................8

LIST OF ABBREVIATIONS ..........................................................................................................9

ABSTRACT ...................................................................................................................................10

CHAPTER

1 LITERATURE REVIEW .......................................................................................................12

Strawberry ...............................................................................................................................12 Plant Structure ........................................................................................................................14

Cultivation in Florida ..............................................................................................................16 Florida127 ........................................................................................................................17

Florida Radiance ..............................................................................................................18 Strawberry Festival ..........................................................................................................19 FL 05–107 .......................................................................................................................20

Nutrient Requirements ............................................................................................................21 Physical Observation of Plants ........................................................................................21

Soil and Water Analysis ..................................................................................................22 Leaf Analysis ...................................................................................................................22

Nitrogen ..................................................................................................................................22 General Information ........................................................................................................22

Nitrogen Fertilization In Strawberry ...............................................................................23 Previous Strawberry Nitrogen Fertilization Research ............................................................24 Current N Fertilization in Florida ...........................................................................................29 Objective .................................................................................................................................30

2 OPTIMIZATION OF GROWTH-STAGE SPECIFIC NITROGEN FERTILIZATION

IMPROVES STRAWBERRY GROWTH AND YIELD.......................................................34

Introduction .............................................................................................................................34

Materials and Methods ...........................................................................................................36 Results.....................................................................................................................................39 Discussion ...............................................................................................................................41 Conclusion ..............................................................................................................................45

3 USING A SCANNER BASED RHIZOTRON SYSTEM TO CHARACTERIZE ROOT

MORPHOLOGICAL RESPONSES OF BARE-ROOT STRAWBERRY

TRANSPLANTS TO NITROGEN FERTILIZATION RATES ............................................56

6

Introduction .............................................................................................................................56

Materials and Methods ...........................................................................................................58 Rhizotron Construction and Planting ..............................................................................58

Sprinkler Irrigation and Nitrogen Treatments .................................................................58 Use of Scanner for Digital Root Images ..........................................................................59 Other Measurements ........................................................................................................59 Data Analysis ...................................................................................................................59

Results and Discussion ...........................................................................................................60

Conclusion ..............................................................................................................................61

4 PRACTICAL IMPLICATIONS OF GROWTH STAGE SPECIFIC NITROGEN

FERTILIZATION IN STRAWBERRIES ..............................................................................68

Current N Recommendations vs Growers’ Practice ...............................................................68

Background .............................................................................................................................68 Suggested New Recommendations .........................................................................................69

LIST OF REFERENCES ...............................................................................................................72

BIOGRAPHICAL SKETCH .........................................................................................................78

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LIST OF TABLES

Table page

1-1 Strawberry nutrition facts for 100 grams of fresh strawberries. ........................................31

1-2 Top 10 strawberry producing nations in 2011. ..................................................................31

1-3 Planting dates and bed configuration for Florida strawberry production ..........................32

1-4 N fertilization recommendations for strawberries grown in central Florida on sandy

soils. ...................................................................................................................................32

1-5 Main functions of macro and micro nutrients. ...................................................................33

1-6 Deficient, optimum and excess concentrations of nutrients in foliar analysis of

strawberries. .......................................................................................................................33

2-1 N treatments for early, mid and late seasons during the 2013-14 season. .........................46

2-2 N treatments for early, mid and late seasons during the 2014-15 season. .........................46

2-3 Significance of main and interaction effects of N rate and cultivar for monthly and

total marketable yield for 2013–14 season according to ANOVA. ...................................46

2-4 Significance of main and interaction effects of N rate and cultivar for monthly and

total marketable fruit number for 2013–14 season according to ANOVA. .......................47

2-5 Effects of cultivar and N rates on strawberry shoot biomass, canopy width and

soluble solid content (SSC). ...............................................................................................47

2-6 Main effects and interactions of N rate and cultivar for monthly and total marketable

yield for 2014–15 season according to ANOVA. ..............................................................48

3-1 Significance of effects of N rate on average root length and increase in crown

diameter recorded at the end of the experiment. ................................................................62

4-1 Current N fertilization recommendations for strawberries grown in central Florida on

sandy soils. .........................................................................................................................71

4-2 New N fertilization recommendations for different strawberry cultivars grown in

central Florida on sandy soils. ...........................................................................................71

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LIST OF FIGURES

Figure page

2-1 Early or December marketable yields for ‘Florida Radiance’, ‘FL 127’, ‘Strawberry

Festival’ and ‘FL 05 – 107’ as a function of early season N application during the

2013-14 season...................................................................................................................49

2-2 Total marketable yields for ‘Florida Radiance’ and ‘FL 127’ as a function of early

season N application during the 2013-14 season. The N treatments were varied from

Oct. 15 – Dec. 15. ..............................................................................................................50

2-3 Early and total marketable yields for Florida Radiance and Florida 127 as a function

of early season N application during the 2014-15 season. The N treatments were

varied from Oct 22 – Dec. 14.. ...........................................................................................51

2-4 Total unmarketable yield for Florida Radiance and Florida 127 during the 2014-15

season. ................................................................................................................................52

2-5 Canopy width and shoot biomass for Florida Radiance and Florida127 recorded 14

weeks after transplanting (WAT) as a function of early season N application during

the 2014–15 season.. ..........................................................................................................53

2-6 Correlation between leaf area values recorded 14 weeks after transplanting (WAT)

and total marketable yield for Florida Radiance and Florida 127 during the 2014-15

season. ................................................................................................................................54

2-7 Correlation between shoot biomass values recorded 14 weeks after transplanting and

total marketable yield for Florida Radiance and Florida 127 during 2014-15 season. ......54

2-8 Correlation between end of season brix values and early N rate for Florida Radiance

and Florida 127 during the 2014-15 season. ......................................................................55

3-1 End of season crown diameter, number of new crown roots, total root length and

canopy area for ‘Florida Radiance’ as a function of early season N application.. ............63

3-2 Correlation of leaf area and number of new crown roots with crown diameter

recorded at the end of experiment. .....................................................................................64

3-3 End of season canopy area for ‘Florida Radiance’ as a function of early season N

application. Solid lines show fit to the following models: linear (for canopy area at

21, 28, 35 and 43 DAP). ....................................................................................................65

3-4 End of season root surface area for ‘Florida Radiance’ as a function of early season

N application. Solid lines show fit to the following models: linear (for root surface

area at 43 DAP). .................................................................................................................66

3-5 Correlation between leaf area and root length recorded at the end of experiment. ...........67

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LIST OF ABBREVIATIONS

ABA Abscisic acid

ANOVA Analysis of variance

ATP Adenosine tri–phosphate

B Boron

Ca Calcium

Cu Copper

Fe Iron

GCREC Gulf Coast Research and Education Center

IAA Indole-3-acetic acid

iPA Isopentenyl adenosine

K Potassium

Mg Magnesium

Mn Manganese

Mo Molybdenum

N Nitrogen

NH4 Ammonium

NO3 Nitrate

P Phosphorus

PAR Photosynthetically active radiation

S Sulfur

UAN Urea ammonium nitrate

WAT Weeks after transplanting

Zn Zinc

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Abstract of Thesis Presented to the Graduate School

of the University of Florida in Partial Fulfillment of the

Requirements for the Degree of Master of Science

OPTIMIZATION OF GROWTH STAGE SPECIFIC NITROGEN FERTILIZATION

IMPROVES STRAWBERRY GROWTH AND YIELD

By

Bhagatveer Singh Sangha

August 2017

Chair: Shinsuke Agehara

Major: Horticultural Sciences

Different N rates are generally recommended based on crop growth stage to ensure

optimal N fertilization for strawberry production. The objective of this study was to determine

optimal growth-stage specific N fertilization rates for strawberry cultivars grown in Florida.

Field experiments were conducted to test four (0.56, 0.84, 1.12 and 1.40 kg/ha/d) and five N

rates (0.22, 0.67, 1.12, 1.57 and 2.02 kg/ha/d) during the establishment to early harvest season

(Oct- Dec) for the 2013–14 and 2014–15 growing seasons respectively. Except the treatment

period, all plants were fertilized at 0.84 and 1.12 kg/ha/d in the 2013–14 and 2014–15 seasons

respectively. The increased N rates resulted in increased canopy width and shoot biomass. Linear

and exponential (rise to maximum) responses to N rates were observed not only for early

marketable yield, but also total marketable yield. Minimal effects of increased N rates were

observed on fruit quality. It was anticipated that increased N rates changed root morphological

responses of bare-root strawberry transplants that led to increased uptake efficiency and thus

increased early yields.

A greenhouse experiment was conducted during the 2016–17 season with the objective to

characterize root morphological responses of bare-root strawberry transplants to N fertilization

rates using a scanner based rhizotron system. Bare-root transplants (‘Florida Radiance’) were

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planted into the rhizotrons. Thereafter, six N rates of 0.56, 1.12, 1.68, 2.24, 2.80 and 3.36 kg/ha/d

were evaluated for 33 days. At the end of the experiment, total root length increased with N rates

up to 2.24 kg/ha/d and decreased with further increase in N rates. The number of new crown

roots increased linearly with N rates up to 3.36 kg/ha/d. This enhancement in root growth

resulted in linear increases in crown diameter and canopy area with increasing N rates. These

results suggest that during the establishment period, using higher N rates (2-3 kg/ha/d) than

typical in-season rates (0.84-1.68 kg/ha/d) is beneficial for young bare-root strawberry

transplants to promote new root development and shoot establishment.

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CHAPTER 1

LITERATURE REVIEW

Strawberry

Strawberry is amongst the most popular and economically important small fruit crop in

the world (Santos and Chandler, 2009). It belongs to family Rosaceae and genus Fragaria. There

are over 20 different cultivated species of the strawberry plant. There are numerous strawberry

species native to temperate regions all around the world (Trinklein, 2012). Fragaria ×ananassa

is the most commonly cultivated species for commercial cultivation. The symbol ‘×’indicates

that it is of hybrid origin and union of two species native to the Americas, Fragaria virginiana

and Fragaria chiloensis. Fragaria virginiana is a species native to North America. It has small

but highly aromatic berries. Fragaria chiloensis is a wild species native to Chile. Both of these

species were taken to France in the 17th century where they were widely grown in gardens.

Chance seedlings of crosses between two species appeared that were the probable ancestors of

modern day strawberry, Fragaria ×ananassa. The strawberry production was well established in

United States by the end of 18th century and early 19th century. The strawberry species differ

mainly on the basis of number of chromosomes. Strawberries can be diploid, tetraploid,

hexaploid and octoploid (Staudt, 2008). Most current commercial cultivars in the state of Florida

are ‘Strawberry Festival’, ‘Florida Radiance’, ‘Florida 127’, ‘FL 05-107’ and ‘Winter Dawn’

(UF|IFAS, 2016).

The historical accounts show that strawberry was mentioned as old as 234 BCE when a

roman senator is known to have promoted the medicinal use of strawberries (Hummer and

Hancock, 2009). Fragaria vesca is believed to be the first domesticated strawberry in the world,

grown widely across Europe in 14th and 15th centuries. F. virginiana and F. chiloensis became

more prevalent in 16th and 17th centuries respectively. F. chiloensis is believed to have grown in

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Chile for over 1000 years. Presently cultivated Fragaria×ananassa was discovered to be a

hybrid of F. chiloensis and F. virginiana by Duchesne in 1766. It is also called as ‘pineapple

strawberry’ because its aroma resembles that of a pineapple. It remains the dominant species for

commercial strawberry cultivation till date.

Strawberries are delicious low calorie fruits (Adda Bjarnadottir, 2012). They are a good

source of vitamins and minerals. The health benefits include maintaining levels of cholesterol

and blood pressure, reduced inflammation and decreased oxidative stress. The strawberry

nutrition facts are briefly summarized in table 1-1. Strawberry is widely known for its

characteristic aroma, flavor and taste (Giampieri et al., 2012). It is widely consumed as fresh

fruit or in processed forms such as jams, jellies and juices. In general, a fully ripe strawberry

contains 10% total soluble solids and 90% water. It is an extremely rich source of Vitamin C

(58.8 mg/100g). Strawberries are a rich source of dietary fiber and fructose that aids in

controlling blood sugar levels in the body by slowing down the pace of digestion. Recent

research evidence shows that nutritional properties of strawberry have a great potential in

preventing cardiovascular and cancer related health problems (Basu et al., 2010) (Seeram, 2008).

Strawberry is rich in anthocyanins and anti-carcinogenic materials such as ellagic acid,

containing more than 25 types of anthocyanin pigments (da Silva et al., 2007).

Commercial strawberry production is practiced throughout the world with major planting

regions being North America, Europe, Southwest Asia and Australia (Wu et al., 2012). The

United States is the largest strawberry producer in the world followed by Turkey and Spain.

Table 1-2 summarizes the strawberry production and acreage of top ten world nations.

About 81% of the US strawberry production is for fresh market. California is the top

strawberry producing state in Unites States followed by Florida and Oregon (Boriss, 2006). The

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majority of US strawberry exports are to Canada. In 2014, US recorded export of 273.6 million

pounds of fresh strawberries, with Canada receiving 83 percent of the total strawberry exports.

On the other hand, a large portion of US strawberry imports (around 82% in 2014) are supplied

by Mexico.

California, Florida and Oregon are major contributors towards total US strawberry

production (Wu et al., 2012). The majority of Florida’s production of strawberries if for the fresh

market. The winter production of strawberries in Florida results in strawberries being produced

and marketed year round in United States (Boriss, 2006). Florida is the largest producer of winter

grown strawberries in the US. Due to overlapping production seasons, Florida strawberry

growers face a tough competition from Mexican strawberry imports in the winter. Florida

growers get higher prices for strawberries early in the season (November and December), until

Mexican imports start coming in January. Thus, development of early yielding strawberry

cultivars and improved nutrient management strategies can prove helpful to improve early season

yields in Florida strawberries.

Plant Structure

The vegetative structure of a strawberry plant can be divided into five basic parts- leaves,

crown, roots, runner and daughter plants (Plants, 2010).

The crown is basically a short and thick stem that gives rise to both root and shoot

portions of the plant. Leaves and inflorescence arise from the upper growing point of the crown

whereas roots grow from the lower portion. Side crowns develop in late fall or early winter from

axillary buds at the base of each leaf. An adequate number of side crowns ensures a productive

canopy. More than 4 crowns per plant potentially hinders the growth and yield by negatively

affecting fruit size (David T. Handley, 2003). The initial crown diameter during planting

influences marketable yield of strawberry (Le Mière et al., 1998). During strawberry planting,

15

the transplants should be planted with crown at the level of the soil such that upper and lower

growing portions remain above and below the soil respectively (Peres et al., 2006). Too deep and

shallow planting results in reduced growth of respective shoot and root portions of the plant.

The strawberry leaf is trifoliate, each containing 3 leaflets. The leaves are arranged in a

spiral fashion (Poling, 2012) in such a way that every sixth leaf remains above the first leaf

(David T. Handley, 2003). The leaves capture light for the process of photosynthesis. Adequate

development of plant canopy is important for improved yield and fruit quality. After planting of

bare-root strawberry plants, overhead sprinkler irrigation is used to sustain the leaf growth of

transplants until the root system becomes efficient enough for the uptake of water and nutrients

(Poling, 2012).

Strawberry roots are positively geotropic and can grow rapidly (Neri and Savini, 2004).

The adventitious root system develops from the crown, primary roots go several inches below

the soil and then give rise to lateral roots. There are about 20-30 primary roots per plant, growing

4-6 inches long. A greater portion of the roots are concentrated in the upper three inches of soil

(David T. Handley, 2003). Nutrient composition and physical soil properties greatly affect root

growth and morphology (Sas et al., 2003).

The inflorescence develops at the growing tip of the crown (Poling, 2012). There are 5

sepals in a strawberry flower that cover the flower at bud stage. Stamens and Pistils are

respective male and female parts of the flower. The receptacle is a cone shaped structure that

contains a large number of pistils. It is this receptacle that develops into a fruit at maturity. The

edible berry is actually an enlarged and matured receptacle with imbedded seeds or the actual

fruit’ that are normally called achenes.

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Runner plants are used commercially for strawberry propagation (Andriolo et al., 2014).

Runner growth is stimulated by warm temperatures (David T. Handley, 2003). As a result, the

majority of runners develop in summer. The first runner plant develops from the first internode,

and subsequent runner growth occurs at the second node where runner plants develop. As the

runner plants grow and develop roots during the summer, they are dug up in the coming fall and

stored as bare-root transplants at 0ºC until planting. Any additional runners growing after

planting are clipped to encourage growth of branch crowns for a productive canopy. The

strawberry plants are also propagated by plug transplants (Hochmuth et al., 2006).

Cultivation in Florida

Strawberries in Florida are grown as annuals rather than perennials. The commercial

strawberry planting time in Florida is late fall, from late September to mid-October. Flowering

and fruiting begin in November and generally continue until April (Chandler et al., 1994). In

Florida, strawberries are cultivated in an annual hill production system (Santos and Chandler,

2009). There are two types of transplants used for commercial cultivation in Florida: bare-root

and containerized (plug) transplants (Peres et al., 2006). The former are most commonly used for

commercial cultivation. However, the field establishment of bare-root transplants is difficult

because of a desiccated root system. The transplants are shipped from nurseries in California,

North Carolina and Canada. The roots become desiccated during shipping. Thus, they require

overhead sprinkler irrigation for the first seven to twelve days after planting for successful

establishment. This prevents wilting and aids in development of a root system efficient for water

and nutrient uptake. The bare-root strawberry transplants are planted in double rows on raised

beds. At the time of bed formation, the beds are fumigated with methyl bromide and chloropicrin

for control of weeds, nematodes and soil borne diseases. Just after the fumigation, beds are

covered with black high density polyethylene mulch (Santos and Chandler, 2009). 2-3 inches

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deep planting holes are recommended to cover all roots while planting (Peres et al., 2006). The

strawberry transplants should be placed with crown at soil level. Crown placement below soil

level reduces establishment whereas exposed crowns above soil surface result in dehydrated and

wilted plants. The planting information and bed configuration for state of Florida is summarized

in table 1-3 (Peres et al., 2006). Drip irrigation is the most commonly used method for irrigation

of strawberries in Florida (Hochmuth and Albregts, 1994). Strawberries are irrigated with a

single drip tape at the center of each bed with a 12 inch emitter spacing. The drip irrigation

requires roughly half the amount of water that is required by sprinkler irrigation, thus resulting in

improved irrigation efficiency.

Application of fertilizers through drip irrigation (fertigation) increases efficiency of

application of leachable nutrients like N and K (Locascio and Martin, 1985). The strawberry

recommendations for P and K are based on the calibrated Mehlich-1 soil test results and vary

according to the soil-test levels of these nutrients. The current university recommendations for N

fertilization include application of 168 kg N per acre for the entire season of 200 days. The

recommendations are to apply 0.34 kg of N/acre/d for first two weeks after sprinkler irrigation

and then gradually increase to 0.84 kg/ha/d during the months of February and March (Table 1-

4) (Hochmuth and Albregts, 1994).

Below are some of the major cultivars used for strawberry production in Florida:

Florida127

It is a new strawberry cultivar released from University of Florida in 2013 (Vance M.

Whitaker, 2015b). The fruit of this cultivar is marketed under the Sensation® brand. It is a result

of a cross between ‘FL 05–107’ (WinterstarTM) (female parent) and unreleased breeding

selection FL 02 – 58 (male parent). The fruit has excellent flavor and aroma with higher soluble

solids content than ‘Florida Radiance’. It is a short-day plant with a robust growing habit. The

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fruits are uniform and conic to broad-conic in shape. The fruit is relatively greater in size as

compared to the fruit of ‘Florida Radiance’. However, it is more susceptible to water damage

that results in cracking and development of water soaked spots in ripe fruit. The bright red color

of the fruit develops at a comparatively lower pace as compared to Florida Radiance. Thus it is

recommended to have picking interval of ‘Florida 127’ with one day longer as compared to

Florida Radiance to allow optimum external color development.

A plant spacing of 15–16 inches is recommended since it has a more vigorous growing

habit as compared to ‘Florida Radiance’. However, the early and total season yields of this

cultivar are not significantly different from ‘Florida Radiance’. The bare-root strawberry

transplants of Florida 127 require ten to twelve days of sprinkler irrigation for 8 to 12 hours per

day depending on the temperature. The nutrient requirements of this cultivar are less in the early

season as compared to ‘Florida Radiance’. Because of its vigorous growing habit, it responds

more strongly to N fertilization with respect to vegetative growth. For deep, sandy soils of west-

central Florida, drip irrigation exceeding 1 hour per day may result in nutrient leaching.

Florida 127 is highly resistant to anthracnose fruit rot. However, it is more susceptible to

botrytis fruit rot and Podosphaera aphanis (causal agent of powdery mildew) than ‘Florida

Radiance’. It is also highly susceptible to crown and root rots caused by Phythophthora

cactorum.

Florida Radiance

This cultivar was released in 2008 and remains the dominant variety among strawberry

growers of central Florida (Vance M. Whitaker, 2013). It is a result of a 2001 cross between

‘Winter Dawn’ (female parent), a cultivar released in 2005 from UF/IFAS breeding program,

and FL 99–35 (male parent). It is popularly cultivated not only in Florida but also in other winter

production regions of the world including Australia and southwest Spain. It is marketed as

19

‘Florida Fortuna’ in markets outside of United States and Canada. The long fruit stems and

medium plant vigor allows efficient fruit picking. Fruit size is larger than ‘Strawberry Festival’

and almost same on average as that of FL 05–107 (WinterstarTM). The fruits are less firm than

‘Strawberry Festival’ and ‘FL 05–107’.

‘Florida Radiance’ is also recommended for protected culture besides open field

cultivation. For regions of west central Florida, its optimal planting period is between October 5

and 15. Early planting can result in formation of elongated and unmarketable fruits. This cultivar

has a weak plant vigor. Although it has very low chilling requirement for initiation of flower

buds, extended chilling hours in the nursery can help for increased crown size and root mass for

this cultivar. Early season yields are higher than ‘Strawberry Festival’ and similar to ‘FL 05–

107’.

Bare-root transplants require 10–12 days of sprinkler irrigation for 8–12 hours per day. It

also performs exceptionally well as a plug transplant. The fertilization program is based on crop

requirement and soil fertility. Higher N application during mid and late seasons reduce fruit

quality. Application of 168–196 kg/ha of N and K from December to mid–March results in

excellent fruit yields. Drip irrigation for more than 1 hour per day may result in nutrient leaching.

It is resistant to anthracnose fruit rot. However, the resistance to botrytis fruit rot is less

than ‘Strawberry Festival’. It is highly susceptible to crown and root rots caused by

Phytophthora cactorum.

Strawberry Festival

The cultivar was named such to recognize the famous ‘Strawberry Festival’ that is

celebrated in Plant City, Florida to celebrate the industry in Hillsborough County (UF|IFAS,

2016). Released in 2000, it is known to produce a large number of robust runners in the nursery.

It is a result of a cross between ‘Rosa Linda’ (female parent) and ‘Oso Grande’ (Vance M.

20

Whitaker, 2015a). The cultivar grows moderately vigorous with a medium fruit size and long

fruit stems that allow efficient fruit picking. The fruits are uniform and medium in size with deep

red external color. It has a moderately acidic flavor and firm texture with excellent shipping

quality. It is highly resistant to rain damage.

The recommended planting period for this cultivar is October 10–20 with 15–16 inches

of in–row spacing. The cultivar responds greatly to higher N application rates. N rates of 168–

196 kg/ha are applied during growing season. For ‘Strawberry Festival’, it is suggested to start

with N applications of 0.56 to 0.75 kg/ha/d during establishment season. Irrigation interval

should not be longer than one hour per day. The cultivar has moderate to steady yields from

mid–December to March. Early planting can result in smaller fruit size later in the season.

It is resistant to Phytophthora root rot and moderately resistant to botrytis and

anthracnose fruit rot. The cultivar is however susceptible to Colletotrichum crown rot and

angular leaf spot.

FL 05–107

This cultivar was released in 2011 (UF|IFAS, 2016) . The fruits are medium-large sized.

Plants have a compact growth that encourages high planting densities with this cultivar. It is

result of a cross between ‘Florida Radiance’ (female parent) and ‘Earlibrite’ (male parent)

(Vance M. Whitaker, 2015c). It can be marketed under WinterstarTM brand. The resistance of this

cultivar to water damage is less than that of ‘Florida Radiance’. Similar to that of Florida 127,

the external bright red fruit color develops gradually and thus picking intervals for this cultivar

should be scheduled accordingly. The fruits have a lower acid content than ‘Strawberry Festival’

and ‘Florida Radiance’, resulting in a sweet flavor that improves gradually during ripening.

Early planting in this cultivar does not result in elongated fruits as observed in ‘Florida

Radiance’. It performs equally well under protected culture environments as well as field

21

conditions. Bare-root transplants of ‘FL 05–107’ require 10–12 days of sprinkler irrigation for 8–

12 hours per day, depending on temperature. The fertilization program is based on crop

requirement and soil fertility. Higher N applications during mid and late seasons reduce fruit

quality as in ‘Florida Radiance’. Irrigation intervals greater than one hour per day may result in

nutrient leaching.

The cultivar is resistant to anthracnose and susceptible to botrytis fruit rot. It is

susceptible to crown and root rots. Early and total season yields are intermediate to those of

‘Strawberry Festival’ and ‘Florida Radiance’ or can be same as that of ‘Florida Radiance’.

Nutrient Requirements

Strawberry is a fast growing crop. The rapid growth habit of strawberry demands

sufficient supply of nutrients synchronized with the growing stages of the crop (Medeiros et al.,

2015). The growth of strawberry is influenced largely by fertilization and other environmental

factors such as temperature, light and salinity. Optimal production of strawberry depends on the

availability of mineral nutrients supplied from the soil and through fertilization (Li et al., 2010).

The macronutrients required are N, Potassium (K), Calcium (Ca), Magnesium (Mg), Phosphorus

(P) and Sulfur (S). Of the above mentioned nutrients, N, P and K are classified as primary

elements and others as secondary elements. Iron (Fe), manganese (Mn), zinc (Zn), copper (Cu),

molybdenum (Mo) and boron (B) are classified as micronutrients and are required in scarce

quantities (Haifa, 2014). Table 1-5 briefly describes the functions of these nutrients.

In general, there are three basic tools for determining nutrient requirement of any crop

plant.

Physical Observation of Plants

Physical observation of visual symptoms can be used to detect deficiencies of N, P, K,

Ca, Mg, S, Zn and other nutrients. Abnormal symptoms in foliage, growth or significant

22

variations in yield as a result of application of one or other nutrients can be noticed to manage

nutrient requirement in strawberry.

Soil and Water Analysis

Soil analysis before planting can help to determine nutrient requirements for strawberries

based on the nutrient content of the soil. Soil pH strongly influences nutrient availability. There

will be reduced availability of N, P, K, Mg and Mo in acidic soils. Alkaline soils however will

have limited quantities of Zn, B, Fe, Mn and Cu. Soil pH of of 5.3 – 6.5 is the optimal range for

strawberry crop. Soil pH of around 6.5 is best suited for sandy soils whereas pH of 5.3 is

preferable for fine-textured soils.

Leaf Analysis

Leaf tissue analysis during the growing season is the best way to monitor nutritional

status in strawberry. It is recommended to collect leaf samples at the initiation of flowering and

continue it every two weeks throughout flowering and fruiting. This not only helps to determine

any nutrient deficiencies but also protects against applying excess nutrients. For strawberry, the

most recently matured trifoliate leaf is selected. The leaves sampled at fruiting are the best

indicator for N–P–K concentrations in the plant. Standard concentrations of nutrients from leaf

analysis of strawberries are described in table 1-6.

Nitrogen

General Information

N is the most important and essential nutrient for all plant species. N is present in soil in

both organic and inorganic (NO3 and NH4) forms, with more than 90% of soil N in organic form.

The N form taken up varies by plant species. Many crops including strawberry have greater

growth and yield responses and nutrient uptake rates when fertilized with nitrate (NO3) N

(Darnell and Stutte, 2001). The N supplied from natural sources is not sufficient to meet the plant

23

needs and thus N needs are met by application of commercial fertilizers (Mosaic) . The

increasing expectations of higher agricultural output to meet the growing demands of ever

increasing population puts tremendous pressure on available land resources. Such situations

demand sustainable use of agricultural inputs such as fertilizers to avoid losses in any form and

keep cost of production to a minimum. The sustainable use implies the ability to increase the

crop production while keeping the environmental impacts in an acceptable range. The increasing

population trend expects the global food production to double by 2050. As per the current

scenario, the prevailing crop production practices in the world are adding detrimental quantities

of N and phosphorous to the soil systems. Application of fertilizers in excess amounts not only

adds toxic amounts to soil and groundwater resources, but also disturbs the aquatic ecosystem

(Tilman et al., 2002) (Cassman, 1999). Eutrophication in water bodies is mainly a result of

leaching of nitrate residues in groundwater (Finkl and Charlier, 2003). The fertilizers used for

agricultural purposes are considered one of the major sources of nitrate pollution in groundwater

(Santos, 2010). Thus, there is a need to develop nutrient management strategies for sustainable

productivity of strawberries.

Nitrogen Fertilization in Strawberry

N is required by crops in optimum quantities. It is a vital component of chlorophyll and

aids in the development of proteins. It is an essential component of ATP and nucleic acids such

as DNA. N promoted enhanced interception and use efficiency of photosynthetically active

radiations (PAR) by increasing the leaf area of the plant. Adequate N amounts promote increased

vegetative growth and photosynthesis (Fredeen et al., 1991) . Excess and deficient applications

of N have their own diverse effects. Use of nutrient supply phasing technique to supply N as per

the growth stages of the crop improved growth and yield in broccoli (Nkoa et al., 2001).

Excessive supply of N in strawberry leads to vigorous vegetative growth and increased disease

24

incidence (Kirschbaum et al., 2010a). Insufficient supply of N results in reduced biomass and

yield attributes (Deng and Woodward, 1998). It has been noted that response to N fertilization in

strawberry is dependent on cultivar and the interaction of N rate and cultivar (Simonne et al.,

2001). N supply also affects fruit quality in strawberry (Nestby et al., 2005). As the strawberry

season advances, the N demands of root and crown portions of the plant decrease while the

accumulation of N, P and K increase in the growing fruit (Albregts and Howard, 1980). Nutrient

analysis of strawberry plants was performed on sampled plants through the season to determine

nutrient accumulation trends over the entire season. While there was negligible change in N

content of the plant from mid–season (27 kg/ha) to end of season (27.4 kg/ha), N accumulation

in the fruit increased from 10.4 to 31.4 kg/ha during this period. The fruit N content is also

dependent on the type of soil, fertilization, cultural factors and variety (Hochmuth and Cordasco,

1999).

Reduced N supply results in decreased anthocyanin synthesis in the fruit (Yoshida et al.,

2000). Application of excess N can result in higher proportion of fruit malformation (Hochmuth

and Cordasco, 1999). Application of higher N rates can result in a greater proportion of

malformed fruit by the end of season in strawberry (Albregts and Howard, 1982). Application of

N rates of 73, 146 and 218 lb N/acre during the season resulted in 3, 4.2 and 7.5 % of malformed

fruit respectively in the month of March while there was no significant increase in percent of

malformed fruit in the months of January and February. Other possible causes of malformed fruit

can be application of fungicides at flowering stage, poor quality of pollen and insects feeding on

flowers (Albregts and Howard, 1982).

Previous Strawberry Nitrogen Fertilization Research

In more than 40 years of fertilization research with strawberries in Florida, there has been

a huge change in the strawberry production practices, cultivars grown and use of new irrigation

25

and nutrient application strategies. Nutrient management is closely linked with the type of

irrigation system used (Hochmuth and Cordasco, 1999). The use of drip irrigation for

strawberries in Florida became more prevalent after 1990. Before that, sprinkler irrigation was

the most commonly used method of irrigation (Hochmuth et al., 1996). With overhead irrigation,

the fertilization trials involved evaluating strawberry yield responses to different fertilizer

sources, testing different fertilizer placement methods and response to different organic manures

(Hochmuth and Cordasco, 1999). The strawberry trials in 1970’s used blended N-P-K fertilizers

such as 6-8-8, 4-8-8, Osmocete (16-5-16) for testing cultivar responses to N rates(Hochmuth and

Cordasco, 1999). The yield responses to blended fertilizers were considered as response to N

alone as it was considered as limiting in Florida sandy soils. The fertilizer applications were

tested as shallow and deep placement methods where band application of fertilizers was done at

respective depths of 1 and 5 inches. The measurement of soluble salt concentrations at different

soil depths depicted that there was a 5 fold increase in soluble salt concentrations at shallow

placement as compared to deep placement method. This concluded that nutrients present at

greater depths were unavailable to strawberry plants for uptake (Albregts and Howard, 1973).

Albregts et.al tested the response of ‘Florida Belle’ strawberry cultivar to chicken manure that

was consecutively applied for three years annually. The soluble salt concentrations measured at

varied soil depths indicated that N and K had a downward movement in soil after manure

application. It was suggested to avoid large and frequent applications of poultry manure as it had

the tendency to accumulate large amounts of calcium in the soil (Albregts and Howard, 1981a).

Other trials involved testing strawberry yield responses to fertilizer application with an injection

wheel. However, it was observed that fertilizer application at mulching resulted in higher yields

as compared to fertilizer injection treatments at monthly intervals. The injection earlier in the

26

season was more beneficial as late season injections had minimal effect on strawberry yield

attributes (Albregts et al., 1989).

Using drip irrigation saves around 70 percent of water as compared with sprinkler

irrigation systems (Locascio and Myers, 1976). There was no advantage of drip over sprinkler

with respect to increased strawberry yields but the former helped in achieving similar yield

targets by using just one-third of the amount of water that was used for sprinkler irrigation

Applying 50 percent of N and K as drip injections increased strawberry yields as compared to

applying 100% in pre – plant applications (Hochmuth et al., 1996). The use of drip irrigation

increased the scope of nutrient management by allowing more flexibility in varying proportions

of pre-plant to injected fertilizer applications and also timing of fertilizer application. It can be

used for split applications on a daily or weekly basis (Hochmuth and Cordasco, 1999). In 1980’s,

another strawberry trial evaluated both controlled release and soluble fertilizers being injected all

pre-plant or 40 and 60 percent as pre-plant and injected respectively. The strawberry yields were

not affected by either way of application of controlled release fertilizers but there was a

significant interaction between soluble fertilizers and split application method indicating higher

importance of continued nutrient supply irrespective of the N source (Locascio and Martin,

1985). In drip irrigation, irrigation rate is also an important factor beside the fertilizer rates used

since the fertilizers are applied along with the irrigation water. Evaluation of drip irrigation rates

between 0.4 and 0.8 inches of water per week resulted in production of higher quality strawberry

fruits (Hochmuth and Cordasco, 1999). For sprinkler irrigation, Albregts et.al observed that

there was a significant interaction between N source and cultivar and also N rate and source.

However, there was no such interaction observed for drip irrigation (Hochmuth and Cordasco,

1999). In the year 2000, there was a significant interaction observed between N rates and cultivar

27

for drip irrigated strawberries. The response of ‘Camarosa’ and ‘Sweet Charlie’ strawberry

cultivars was linear and quadratic respectively to N rates (0.56, 0.84 and 1.12 kg/ha/d) for total

marketable yield (Simonne et al., 2001). In another study testing the effect of N fertilizer rates on

performance of strawberry cultivars (Santos and Chandler, 2009), it was noted that different

cultivars responded differently to range of N rates used for fertilization. The marketable yield in

‘Strawberry Festival’ showed a linear response when N rates ranged between 0.5 and 0.9

kg/ha/day while there was no such significant observation for ‘Winter Dawn’ strawberry

cultivar. The effect of pre – plant application of N fertilizers has also been evaluated. It was

concluded that pre–plant application of N had no significant effect on strawberry yields,

regardless of the source of N (Santos, 2010). It was found that increased strawberry yields can be

attributed solely to the application of pre–plant fertilizer containing elemental sulfur (S) since

there was no yield difference observed when the fertilizer formulation used a combination of N

(N) and sulfur (S). The effects of N fertilization on strawberry growth and yield have been tested

by several studies with conflicting results. While some studies report no effect of increased N

rates on growth and yield parameters, others report otherwise. A study evaluated response of

growth and yield variables in strawberry when fertilized with N and P in the presence and

absence of K (Medeiros et al., 2015). The study tested the effect of five different N and P

fertilization rates in the presence and absence of potassium. It was concluded that the tested

growth and yield variables increased at the greatest rate when highest doses of N and fertilization

were applied in the presence of potassium.

Another study evaluated the growth and yield responses in strawberry to weekly

injections of N for two seasons (Iatrou and Papadopoulos, 2016). The N rates of 0, 0.5, 1, 3 and 6

kg N/ha were used for the first year and 0, 1, 2 and 3 kg N/ha for the second year. While there

28

was no significant effect of N rates observed for total yield in the first season, the second season

witnessed 18, 20 and 18% more fruit yield compared with control treatments.

The effects of N application are not only limited to growth and yield but also fruit

quality. Numerous studies have reported significant effect of N fertilization on strawberry fruit

quality. The N application affects fruit firmness, fruit size, incidence of pests and fruit disorders

and fruit chemical components (Nestby et al., 2005).

It has been noted that late season foliar applications of N in the nursery increase N

mobilization to crown and root portions (Kirschbaum et al., 2010a). Foliar application of about

80 kg N/ha to the nursery in late summer helped with increased early yield and fruit number. The

foliar application of 80 kg N/ha was applied in three split applications in the months of August

and September. In strawberry production systems that utilize bare-root transplants, successful

plant establishment depends on the formation of new roots and leaves from the crown or root

reserves of the transplants (Kirschbaum et al., 2010b). Low N supply promotes root growth more

than shoot growth whereas higher supply of N enhances growth of shoots more than roots

(Vamerali et al., 2003) (Zeng et al., 1999). Application of N fertilizers in strawberry changes the

root structure of the plants by changes in levels of endogenous hormones such as Indole acetic

acid (IAA), abscisic acid (ABA) and isopentenyl adenosine (iPA) (Bo et al., 2009).

Certain parameters of the initial planting material might also have an impact on the

overall performance of strawberry cultivars. Pre-planting measurements of crown number and

root length correlated positively with strawberry yield (Bartczak et al., 2010). Crown diameter

recorded at planting had a strong linear relationship with total marketable yield for ‘Pajaro’ and

‘Camarosa’ strawberry cultivars (Bussell et al.). ‘Chandler’ and ‘Camarosa’ strawberries

29

recorded a positive linear relationship initial crown diameter at planting and end of season plant

dry weight (Johnson et al., 2005).

In some production systems, the strawberries are planted in the same field as a single

crop or in short rotation cycles. A study was conducted to evaluate the growth response of

strawberry roots to increasing strawberry residues (Neri et al., 2005). The experiment used glass

rhizotrons to evaluate root growth. Commercial compost was used a basic substrate in the

rhizotrons. To simulate the soil accumulation of strawberry plant residues, a 3 cm layer in the

rhizotrons was placed consisting of 0, 3, 10, 30 and 100% residue concentration. The residues

were prepared from 15 cm of soil surface. The observation concluded that root growth was

decreased by the presence of residues. The application of 30 and 100% concentrations of organic

residues not only decreased root growth but also above ground growth.

Current N Fertilization in Florida

By the end of the 1990’s, Florida’s strawberry fertilization research evaluated many

aspects for improved nutrient management such as different sources of N fertilizers, testing of

fertilizer placement methods, timing of application of fertilizers, effect of different irrigation

strategies on fertilizer application and cultivar responses to varied N sources and rates. Based on

all the fertilization work until this period, researchers have developed N fertilization

recommendations for drip irrigated strawberries in Florida. These recommendations were

developed in 1997. The UF recommendations suggest application of 168 kg of N/ha during the

entire season with 0-45 kg of N application pre-plant. It has been established that applying pre-

plant N fertilizers has a negligible effect on marketable yield. It is beneficial to apply Sulfur

coated N fertilizers in sulfur deficient soils in Florida. Based on transplant requirements, it is

suggested to apply 0.34 kg of N/ha/d during the initial 2-3 weeks of transplant establishment and

then gradually increase to 0.84 kg of N/ha/d by the end of the season (February-March).

30

However, the current growers’ practice is different, starting with an initial high

application of 1.68 to 3.36 kg/ha/d during the establishment period, gradually lowering to 0.84

kg/ha/d for the rest of the season.

Objective

Thus, the main objective of the study was to optimize growth stage specific N

fertilization rates for improved growth and yield of selected major strawberry cultivars, ‘Florida

Radiance’, ‘Florida 127’ (Sensation®), ‘Strawberry Festival’ and ‘FL 05–107’ (WinterstarTM).

31

Table 1-1. Strawberry nutrition facts for 100 grams of fresh strawberries.

General

information

Amount Vitamins and

mineralsz

Amount % Daily

values (DV)

Calories 32 Vitamin A 1 µg 0

Water 91% Vitamin C 58.8 mg 65

Protein 0.7g Vitamin D 0 µg ~

Carbs 7.7g Vitamin E 0.29 mg 2

Fat 0.3g Vitamin K 2.2 µg 2

Folate 24 µg 6

Calcium 16 mg 2

Iron 0.41 mg 5

Magnesium 13 mg 3

Phosphorus 24 mg 3

Potassium 153 mg 3

Manganese 0.39 mg 17

Source: (USDA, 2017)

Table 1-2. Top 10 strawberry producing nations in 2011.

Countries Production (1000 tons) Area (ha)

USA 1313 23260

Turkey 302 11967

Spain 263 6896

Egypt 240 5628

Mexico 229 6968

Russian federation 184 27000

Japan 177 6020

Republic of Korea 172 5816

Poland 166 50522

Germany 154 13848

Source: (FAOSTAT, 2017)

32

Table 1-3. Planting dates and bed configuration for Florida strawberry production

Planting regions Planting dates

North Florida Sept. 15 – Oct. 15

Central Florida Sept. 15 – Oct. 25

South Florida Oct. 1 – Dec. 1

Bed configuration (double rowed beds)

Bed width (inches) 48 – 60

Plant spacing (inches) 12 – 16

Row width (within a bed) 12 – 14

Days to first ripe fruit (from transplanting) 40 – 100

Planting density (number per acre) 16,000 – 22,000

Source: (Peres et al., 2006)

Table 1-4. N fertilization recommendations for strawberries grown in central Florida on sandy

soils.

N fertilization rate (kg/ha/d)

Total

(kg/ha)

Pre-planty

(kg/ha)

First 2

weeks

Nov. to

Jan.

Feb. and

Mar.

April

N 168 0 – 45 0.34 0.67 0.84 0.67

Source: (Hochmuth and Cordasco, 1999)

33

Table 1-5. Main functions of macro and micro nutrients.

Nutrient Main function

N (N) Formation of proteins (growth and yield)

Phosphorus (P) Cell division and ATP synthesis

Potassium (K) Sugar transport, stomata control and cofactor of many enzymes

Calcium (Ca) Major component of cell wall

Sulfur (S) Synthesis of cystin and methionine amino acids

Magnesium (Mg) Essential part of chlorophyll molecule

Iron (Fe) Chlorophyll synthesis

Manganese (Mn) Process of photosynthesis

Boron (B) Cell wall formation, pollen tube germination and elongation

Zinc (Zn) Auxin synthesis, enzyme activation

Copper (Cu) Metabolism of N and carbohydrates

Molybdenum (Mo) Component of nitrate–reductase and Nase enzymes

Source: (Haifa, 2014)

Table 1-6. Deficient, optimum and excess concentrations of nutrients in foliar analysis of

strawberries.

Macro–nutrients

Deficient

(%)

Optimal

(%)

Excess

(%)

N <1.5 1.9 – 2.8 >4.0

Phosphorus <0.20 0.25 – 0.4 >0.5

Potassium <1.2 1.6 – 2.5 >3.5

Calcium <0.6 0.7 – 1.7 >2.0

Magnesium <0.25 0.3 – 0.49 >0.8

Sulfur <0.20 0.4 – 0.6 >0.8

Micro–nutrients

(ppm)

(ppm)

(ppm)

Manganese <40 50 – 200 >350

Iron <30 60 – 250 >350

Zinc <15 20 – 49 >80

Copper <5 7 – 19 >20

Boron <19 30 – 64 >90

Molybdenum <0.5 >0.5

Source: (Ullio, 2010)

34

CHAPTER 2

OPTIMIZATION OF GROWTH-STAGE SPECIFIC NITROGEN FERTILIZATION

IMPROVES STRAWBERRY GROWTH AND YIELD

Introduction

Strawberry is a high value crop and requires optimum N fertilization for improved

growth and yield (Medeiros et al., 2015; Simonne et al., 2001). It has been reported that higher

strawberry yields can be achieved by a continuous supply of N irrespective of the N source

(Hochmuth and Cordasco, 1999). Both insufficient and excessive N fertilization can have

adverse effects on growth, yield and fruit quality of strawberries (Medeiros et al., 2015).

Insufficient N fertilization not only reduces the total biomass, but will also limit the yield. Excess

N fertilization hinders fruit development by stimulating prolonged vegetative growth

(Kirschbaum et al., 2010a). The exaggerated vegetative growth also increases the plant

susceptibility to pathogens. Furthermore, N fertilization influences fruit shelf life, firmness, size

and concentration of chemical components (Nestby et al., 2005). Lince et al. reported that

application of 225 kg N/ha effectively maintained fruit quality of strawberries for 21 days when

five N rates of 150, 225, 300, 450 and 600 kg N/ha were evaluated (Mukkun et al., 2000).

Despite these results, other studies show conflicting results with respect to fruit quality. Some

studies reported that fruits receiving N fertilization were less firm than the unfertilized ones

(Miner et al., 1997; Neuweiler, 1996) whereas another study reported the response otherwise

(Darrow, 1931). Some of the studies also reported no effect of N fertilization on fruit firmness

(Bell and Downes, 1961). Lacertosa et al. reported that fruit acid and sugar concentration

correlated inversely with fruit N content (Lacertosa et al., 1999).

Commercial strawberry production in Florida uses an annual hill production system

(Hochmuth et al., 1996). Strawberries are grown on raised beds covered with black high density

polyethylene mulch. Fertilizers are applied as injection treatments incorporated into irrigation

35

application using surface drip. Drip irrigation is reported to save about one third the amount of

water as compared to sprinkler irrigation while resulting in similar strawberry yields (Locascio

and Myers, 1976). The majority of strawberry production occurs in west central part of Florida,

which contains sandy soil types, high water tables and rapid infiltration (Santos, 2010). Thus,

there is a need for the development of appropriate fertilization programs that encourage the

sustainable use of fertilizer resources. The majority of fertilization research focused on fertility

involves studies with N because of its high leaching potential and high impact on plant growth

and yield. Previous studies on strawberry fertilization in Florida evaluated different N sources,

fertilizer placement methods, the effect of pre-plant fertilizers on yield and effects of N

fertilization rates and cultivars (Albregts and Howard, 1973; Hochmuth and Cordasco, 1999;

Simonne et al., 2001). It was reported that the application of N in the form of pre-plant fertilizers

had no significant effect on total marketable yield in strawberry (Santos and Whidden, 2008).

Simmone et al. (2001) reported significant interactions between N rates and strawberry cultivars

for total and monthly marketable yield. The results suggested that response of strawberry to

varied N rates was dependent on the type of cultivar. Other studies evaluated the response of

strawberry cultivars to different N rates (Hochmuth et al., 1996).

However, there is limited research on optimizing N fertilization rates based on strawberry

growth stages and cultivar. Different strawberry cultivars show varied responses to N

fertilization rates. Optimization of N fertilization based on growth stages of broccoli resulted in

increased growth and yield (Nkoa et al., 2001). Similarly for bare-root strawberry transplants, N

requirements may differ during the establishment period and later parts of the season.

Strawberry growers in Florida generally apply 168-224 kg of N/ha during the growing

season using drip irrigation. They start with an initial high application of 1.96-3.36 kg/ha/d

36

during establishment or early season (Nov. and Dec.) and then gradually lower to 0.84-1.40

kg/ha/d for the rest of the season (Jan. and Feb.). However, this contradicts with the current

university recommendations that suggest to start with a relatively lower rate of 0.34 kg/ha/d

during early season and then gradually increase to 0.84 kg/ha/d for the rest of the season. Thus,

the two practices mainly differ in the amounts of N applied during establishment or early season.

The main objective of this study was to optimize the N fertilization rate based on growth

stages for major strawberry cultivars grown in state of Florida. It also aimed to see if there were

any cultivar and N rate interactions for both the cultivars that would aid in developing growth-

stage specific N recommendations for increased yield and fruit quality.

Materials and Methods

The field experiments were conducted at Gulf Coast Research and Education Center

(GCREC) in Balm, Florida during 2013-14 and 2014-15 growing seasons.

For 2013–14 season, the respective pH and organic matter content of the soil were 7.3

and 1.5% respectively. The soil type of the region is classified as a Myakka fine sand siliceous

Hyper-thermic Oxyaquic Alorthod. Planting beds were made about 70 cm wide at the base, 61

cm wide at the top and 25.4 cm high with bed centers spaced 122 cm apart. Fumigation was

carried out in mid-September. Beds were fumigated with PicClor 60 (1, 3 -dichloropropene +

chloropicrin; 303 kg/ha) to eliminate the risk of nematodes, weeds and soil borne diseases. The

fumigant was delivered 20 cm deep using a standard fumigation rig with three knives per bed.

Just after fumigation, the beds were covered with black high density polyethylene mulch, a

single drip tape (0.25 gal/100 ft per minute) was installed in each bed at 2.5 cm depth with 30 cm

emitter spacing.

The bare-root transplants of ‘Florida Radiance’, ‘Florida127’ (SensationTM), Strawberry

Festival’ and ‘FL 05 – 107’ (WinterstarTM) were planted on October 3, 2013 placed 38 cm apart

37

in double rows. There was a spacing of about 45 cm between the plots with 20 plants per plot.

Sprinkler irrigation was carried out from October 3 to October 14. An initial rate of 0.84 kg/ha/d

of N was distributed over all treatment plots until October 14. From October 15 to December 15,

four N rates of 0.56, 0.84, 1.12 and 1.40 were evaluated (Table 2-1). Following December 15, all

treatment plots were fertilized with same N rates of 0.84 kg/ha/d. Fertilization was performed 3

times per week with a 6–2–4 formula at a reference rate of 0.56 kg/ha/d of N. Calcium nitrate

(15%N) was used as the supplemental N source. After establishment, plants were irrigated twice

a day with irrigation intervals in the morning and afternoon. Depending upon the

evapotranspiration for the area, irrigation intervals were 15 minutes/cycle during October to mid

– November, 30 min/cycle from mid – November to early December and 45 minutes per cycle

from early December to the end of the season. Fertilization (other than N) and pest control were

done according to university crop recommendations.

Plant canopy width was recorded at 8 and 15 weeks after transplanting (WAT). The

canopy width measurements were made perpendicular to the direction of the rows, using five

randomly selected plants per plot. The same plants from each plot were sampled for other

measurements during the season. Early and total marketable fruit weights were recorded starting

at 8 WAT. Early marketable fruit yield was defined as the yield until the end of December

whereas total marketable fruit yield consisted of all the 23 harvests during the entire season.

Total soluble solids content (SSC) and marketable quality after 7 days of storage at 4º C (on a

scale of 0 – 5, where 0 = non–commercial and 5 = optimum quality) was measured at 15, 18 and

22 WAT.

During the 2014-15 season, planting beds were 80 cm wide at the base, 70 cm wide at the

top and 25 cm high with a row spacing of 120 cm. Fumigation was performed with the same

38

method as the previous season and beds were covered with black high density polyethylene

mulch with a single drip tape placed at 2.5 cm depth at the center of each bed with 30 cm emitter

spacing.

The bare-root transplants of ‘Florida Radiance’ and ‘Florida127’ (Sensation®) (G.W

Allen Nursery, NS, Canada) were planted in double rows on October 9, 2014 spaced 37.5 cm

apart with 24 plants per plot. Following planting, the sprinkler irrigation (4.5 gal/min) was

carried out for two weeks (Oct 9- Oct 22) to ensure plant establishment. There was no fertilizer

application during this time. After sprinkler irrigation, the beds were irrigated with drip irrigation

twice a day. Depending upon the evapotranspiration, two irrigation cycles of 30 (Nov to Dec.)

and 45 (Jan- Feb.) minutes/cycle long were used for irrigation. Five N rates of 0.2, 0.6, 1.0, 1.4

and 1.8 were evaluated during the establishment or early season (Oct 22- Dec 14). (Table 2-2).

After this, all the treatment plots were fertilized with the same fertilizer rate of 1.12 kg/ha/d for

the rest of the season. Fertilization was performed with a Dosatron fertilizer injector. Since only

the N amounts were to be varied, Urea Ammonium Nitrate (UAN-32) was used as the source of

N whereas phosphorous and potassium were supplied by 0-2-8 formulation. Fertilization other

than N and pest control were performed as per crop recommendations.

Plant canopy width measurements were carried out at 14 WAT to evaluate the effect of

applied N rates. The measurements were made perpendicular to the direction of the rows by

random selection of 2 plants per plot. Leaf area, crown diameter and shoot and root biomass

measurements were also recorded for the same set of plants at 14 WAT. Leaf area measurements

were made with a leaf area meter and crown diameter was measured at the point of maximum

width of the crown. Brix measurements were recorded at 17 weeks after planting with a

refractometer. Strawberries were harvested 27 times between Nov 24, 2014 and Feb 26, 2015.

39

Fruit grading was performed in accordance with USDA grading standards. Fruits were graded

into marketable and unmarketable categories and fruit number and weight was recorded for each

category.

The experimental design for both the seasons was a split plot replicated four times with N

as the main plot factor and cultivar as the sub-plot factor. Data analysis was performed with R

software. The main effects of cultivar and N rates and their interaction were assessed using

ANOVA. Regression analysis was performed to evaluate cultivar yield responses to nitrogen

application rates. Regression was performed for linear, quadratic and exponential rise to

maximum responses. The model fit was based on lowest value of Akaike Information Criterion

(AIC). Correlation was also performed to determine the type of relation between certain growth

parameters and crop yield.

Results

For the 2013–14 season, the effects of cultivar, N rate and their interactions were

evaluated for monthly and total marketable yield (Table 2-3) and fruit number (Table 2-4) using

ANOVA. There were significant effects of cultivar and N rate for both monthly and total

marketable yield and fruit number, but no significant interactions between these two factors in

either trait. The main effect of N rate was not significant for marketable fruit yield and number in

January and February.

A varied response among all the cultivars was recorded for both early (Figure 2-1) and

total marketable yield (Figure 2-2). Both ‘Strawberry Festival’ and ‘FL 05 – 107’ had a quadratic

response for early marketable yield to N application rates. ‘Florida Radiance’ was more

responsive to N rates than other cultivars, with early marketable yield increasing linearly with N

rates. The early marketable yield for ‘Florida127’ recorded an exponential rise to maximum

response to N rates. However, the responses to total marketable yield were not consistent for all

40

the cultivars. There was no significant response observed for total marketable yield of

‘Florida127’ and ‘FL 05-107’. Both ‘Florida Radiance’ and ‘Strawberry Festival’ recorded

linear increases in total marketable yield.

There were no significant cultivar and N rate interactions for any of the growth or fruit

quality variables. There were significant cultivar effects for shoot biomass and canopy width.

The highest shoot biomass was recorded for ‘Strawberry Festival’ and ‘FL 127’ while ‘Florida

Radiance’ registered the lowest value (Table 2-5). The canopy width values did not vary among

cultivars until 8 weeks after transplanting. However, at 15 weeks after transplanting, ‘Strawberry

Festival’ and ‘FL 127’ recorded the highest values for canopy width. There was an effect of N

rates on shoot biomass at 8 weeks after transplanting. The highest value of shoot biomass was

recorded at 1.12 kg/ha/d with no difference at higher N rates. There were effects of N rate on

canopy width both for 8 and 15 weeks after transplanting, with the highest values recorded at

1.40 and 1.12 kg/ha/d respectively. Both cultivar and N rates had negligible effects on fruit

quality. The values of soluble solid content (SSC) did not differ significantly among cultivars or

N rates at 15, 18 and 22 weeks after transplanting except for highest value of SSC recorded at

lowest N rate of 0.56 kg/ha/d at 22 WAT.

For the 2014-15 season, the effects of cultivar, N rate and their interactions were

evaluated for monthly and total marketable yield using ANOVA (Table 2-6). The cultivar effects

and interactions were significant only for December marketable yield. The effects of N

application rate were significant for both total and monthly marketable yield.

Both the cultivars recorded varied responses to N fertilization rates. The early marketable

yield for ‘Florida Radiance’ and ‘Florida127’ showed respective exponential and quadratic

responses towards early season N application rates (Figure 2-3).

41

Though the N rates were varied only during the early season (Oct 22 – Dec 14), the total

marketable yield showed linear and quadratic responses for ‘Florida Radiance’ and ‘Florida127’

respectively (Figure 2-3). However, both monthly and total unmarketable yields were unaffected

by early season N treatments for both the cultivars. In general, ‘Florida 127’ had a significantly

higher total unmarketable yield than Florida Radiance (Figure 2-4).

Similar responses were recorded for various growth parameters evaluated in response to

early season N fertilization rates. Florida Radiance was more responsive as compared to Florida

127. Canopy width and shoot biomass recorded 14 weeks after transplanting (WAT) showed a

linear response to early season N rates in ‘Florida Radiance’. The response was quadratic for

canopy width and shoot biomass recorded for ‘Florida127’ (Figure 2-5).

Thus, higher early season N rates not only improved vegetative growth and marketable

yield during early season but also total marketable yield and a productive canopy even after the

end of early season treatments. Leaf area (Figure 2-6) and shoot biomass (Figure 2-7) values

recorded at the end of early season treatments (14 WAT) also had a high correlation with the

total marketable yield for both cultivars.

The increase in N rate resulted in increased yields for both the cultivars. However, the

effects on fruit quality were minimal for both the cultivars. The increase in N rate from 0.22 to

2.02 kg/ha/d resulted in one degree decrease in brix value for ‘Florida Radiance’ whereas there

was no effect of increased rates on brix values for ‘FL 127’ (Figure 2-8).

Discussion

The cultivar and N rate interactions were not significant for monthly or total marketable

yield or fruit number (Table 2-3 and 2-4) during the 2013 – 14 season. There were no observed

significant interactions when strawberry yield response to three different fertilizer sources and

42

rates was evaluated for both sprinkler and drip irrigations (Hochmuth and Cordasco, 1999). The

interactions were significant for sprinkler irrigation but not for drip irrigation.

However, the effect of N rate impacted both total marketable yield and fruit number

(Table 2-3 and 2-4) during most of the year except for January and February. This can be

explained by the varied nutrient demands of plant organs during the season. As the strawberry

season advances in Florida sandy soils, the N demands of crown and root portions of the plant

decrease, with the fruit accumulating more nutrients than all the other plant parts combined

(Albregts and Howard, 1981b). It has been observed that only small quantities of the nitrogen

nutrient are needed in the late season to replace the nutrients removed by the fruit. Applications

of higher quantities during this period result in building up or leaching nutrients from the soil.

This implies that applications of higher N rates during early season had significant effect not

only on monthly yield during December (early season yield) but also total marketable yield, even

if the effects were not significant for January and February. The strawberries in Florida are

typically priced higher in November and December than rest of the season. Thus, higher early

season yields can prove more profitable for Florida strawberry growers. The application of

higher N rates during the early season promoted increased vegetative growth, with shoot biomass

and canopy width maximized at 1.0 and 1.25 lb N/acre/d (Table 2-5). The results for canopy

width are in agreement with a study where canopy width was not affected by nitrogen rates at 6

weeks after planting (WAT) but increased linearly with nitrogen rates at 12 WAT (Santos and

Chandler, 2009).

The effects of cultivar were significant for both monthly and total marketable yield and

fruit number (Table 2-3 and 2-4). All cultivars had varied responses to N application for early

and total marketable yield (Figure 2-1 and 2-2). Significant cultivar effects were also observed

43

for various growth parameters such as shoot biomass and canopy width (Table 2-5). This can be

attributed to the cultivar differences. Both ‘Strawberry Festival’ and ‘Florida127’ are known to

have robust growth and greater plant vigor as compared to ‘Florida Radiance’, which has a less

dense growing habit with more delicate leaved and petioles (Vance M. Whitaker, 2015a; Vance

M. Whitaker, 2013). Thus, the highest shoot biomass was recorded for ‘Strawberry Festival’ and

‘Florida127’ while ‘Florida Radiance’ registered the lowest value (Table 2-5). Canopy width was

also maximized for ‘Strawberry Festival’ and ‘Florida127’ while ‘Florida Radiance’ and ‘FL 05

– 107’ recorded the lowest values.

Similarly, for the 2014 – 15 season, the significance of effects of cultivar and N rates on

monthly and total marketable yield can be explained as discussed for the 2013 – 14 season.

‘Florida Radiance’ remained the most responsive to nitrogen rates even for the second season

and showed exponential and linear responses for early and total marketable yield respectively.

‘Florida 127’ however, showed quadratic response to both early and total marketable yield. The

response of ‘Florida 127’ during 2013 – 14 season was exponential. This difference can most

likely be attributed to the range of N rates used during the two seasons (0.5 to 1.25 lb N/acre/d

for 2013 – 14 and 0.2 to 1.8 lb N/acre/d for 2014–15 season). However, the cultivar and N rate

interactions for early marketable yield were also significant for the monthly marketable yield of

December during the 2014 – 15 season. The evaluation of N rates only during early season (Oct.

to mid Dec.) probably resulted in significant N and cultivar interactions during this period.

The early season N rates had no effect on total non-marketable yield which can be

influenced by many factors other than N fertilization such as environmental conditions, time of

the season and cultivar. Increased temperatures by the end of the season result in increased

disease incidence and over ripe and decayed fruit. Thus, variation in non – marketable yield

44

cannot be solely explained as a result of treatment differences. ‘Florida 127’ had higher yield

than ‘Florida Radiance’. This can be explained by vigorous vegetative growth of ‘Florida127’ as

compared to ‘Florida Radiance’ which has lower plant vigor. The results for non-marketable

yield are in agreement with a study where the cull fruit yield was not affected by nitrogen rates

but by cultivars where ‘Sweet Charlie’ cultivar had higher non-marketable yield than ‘Oso

Grande’ (Hochmuth et al., 1996).

The response of growth parameters such as shoot biomass and canopy width was similar

for both the cultivars as observed for early and total marketable yield. ‘Florida Radiance’ and

‘Florida 127’ had respective linear and quadratic responses for shoot biomass and canopy width

to early season N rates.

In terms of fruit quality, the soluble solids content or brix values did not vary

significantly for cultivars in response to early season N fertilization rates during both the

seasons. The results might be due to unusually high temperatures during both seasons.

Previously conducted studies have reported conflicting results with effect of N on fruit quality

parameters. In one study, it has been reported that acid and sugar concentrations in the fruit are

inversely related with N rates (Lacertosa et al., 1999) whereas another study reported significant

differences with higher nitrogen rates (Hennion et al., 1999).

In strawberry production from bare-root transplants, the majority of the root system is

desiccated at the time of transplanting, with limited water and nutrient uptake capacity. Thus,

successful production is dependent on formation of new roots and leaves using the reserves

stored in crowns (Kirschbaum et al., 2010b). The crown in strawberry is considered as an

important reservoir of starch. Once the autotrophic function of the strawberry plant is

reestablished, the role of crown reserves becomes less important. Thus, for both the seasons,

45

higher N rates during early season (Oct. to mid Dec.) enhanced nutrient reserves in the crown

that promoted increased vegetative growth. Increased shoot biomass and canopy width

developed a productive canopy that resulted in increased yields not only during early season, but

also total marketable yield during both the seasons. Both canopy width and shoot biomass

recorded at the end of early season treatments had a strong linear relationship with total

marketable yield.

This explains that even though the nutrient requirements of the transplants are low at the

time of planting, the majority of the desiccated root system is inefficient and cannot access the

greater portion of soil nutrients. This emphasizes the need to apply more nutrients at the initial

stages. As the plants continue to grow and develop a productive canopy and efficient root

system, smaller nutrient rates are sufficient to meet plant needs during later parts of the season.

Conclusion

The results suggest that application of higher N rates during the early season (Oct. to mid

Dec.) improves strawberry growth and yield with minimal effects on fruit quality. These effects

of increased early season N application are seen in improved early and total marketable yield.

Though the higher N rates were responsible for increased yields for all cultivars, ‘Florida

Radiance’ was overall more responsive than ‘Florida 127’, ‘Strawberry Festival’ and ‘FL 05–

107’.

46

Table 2-1. N treatments for early, mid and late seasons during the 2013-14 season.

Treatment

(#)

Sprinkler irrigation Early season Mid – late season

Total (Oct. 3 – Oct. 14) ( Oct. 15 – Dec. 15) (Dec. 15 – Feb. 20)

(kg/ha/d) (kg/ha) (kg/ha/d) (kg/ha) (kg/ha/d) (kg/ha) (kg/ha)

1 0.84 10 0.56 34.20 0.84 56.28 100.48

2 0.84 10 0.84 51.24 0.84 56.28 117.52

3 0.84 10 1.12 68.32 0.84 56.28 134.60

4 0.84 10 1.40 85.40 0.84 56.28 151.68 Planting date: 3 Oct. 2014

Table 2-2. N treatments for early, mid and late seasons during the 2014-15 season.

Treatment

(#)

Sprinkler irrigation Early season Mid – late season

Total (Oct. 9 – Oct. 21) ( Oct. 22 – Dec. 14) (Dec. 15 – Feb. 26)

(kg/ha/d) (kg/ha) (kg/ha/d) (kg/ha) (kg/ha/d) (kg/ha) (kg/ha)

1 0 0 0.22 11.88 1.12 82.88 94.76

2 0 0 0.67 36.18 1.12 82.88 119.06

3 0 0 1.12 60.48 1.12 82.88 143.36

4 0 0 1.57 84.78 1.12 82.88 167.66

5 0 0 2.02 109.10 1.12 82.88 192.00 Planting date: 9 Oct. 2014

Table 2-3. Significance of main and interaction effects of N rate and cultivar for monthly and

total marketable yield for 2013–14 season according to ANOVA.

Factor

Marketable yield (t/ha)

Dec. Jan. Feb. Total

ANOVA

(P value)

Cultivar <0.001 <0.001 0.019 <0.001

N rate 0.047 0.480 0.183 0.039

N rate * cultivar 0.804 0.588 0.234 0.676

47

Table 2-4. Significance of main and interaction effects of N rate and cultivar for monthly and

total marketable fruit number for 2013–14 season according to ANOVA.

Factor

Marketable fruit number

Dec. Jan. Feb. Total

ANOVA

(P value)

Cultivar <0.001 0.002 0.096 0.025

N rate 0.034 0.124 0.386 0.021

N rate * cultivar 0.601 0.511 0.255 0.849

Table 2-5. Effects of cultivar and N rates on strawberry shoot biomass, canopy width and soluble

solid content (SSC).

Cultivars

Shoot

biomass

(g)

Canopy width

(cm) SSC (%)

8 WAT 8

WAT

15

WAT

15

WAT

18

WAT

22

WAT

Florida Radiance 14 c 17 30 b 8.06 10.26 7.76

Strawberry Festival 18 ab 18 33 a 8.24 10.9 10.33

FL 05–107 16 bc 16 29 b 8.46 9.99 10.41

FL 127 19 a 18 33 a 8.75 13.36 9.86

Significance (≤0.05) * NS * NS NS NS

N rates

0.5 14 b 16 c 29 b 8.12 10.37 12.54 a

0.75 13 b 18 ab 31 ab 8.54 10.25 8.73 b

1.0 19 a 17 bc 32 a 8.51 13.36 8.48 b

1.25 19 a 19 a 33 a 8.35 10.53 8.60 b

significance (≤0.05) * * * NS NS * NS and * = non–significant and significant effects at the 5% level, according to analysis of variance. Values

followed with the same letter do not differ statistically based on Tukey’s HSD test

48

Table 2-6. Main effects and interactions of N rate and cultivar for monthly and total marketable

yield for 2014–15 season according to ANOVA.

Factor

Marketable yield (t/ha)

Dec. Jan. Feb. Total

ANOVA

(P value)

Cultivar <0.001 0.910 0.150 0.988

N rate <0.001 <0.001 0.020 <0.001

N rate * cultivar 0.009 0.834 0.047 0.301

49

Figure 2-1. Early or December marketable yields for ‘Florida Radiance’, ‘FL 127’, ‘Strawberry

Festival’ and ‘FL 05 – 107’ as a function of early season N application during the

2013-14 season. The N treatments were varied from Oct. 15 – Dec. 15. Solid lines

show fit to the following models: linear (early or December marketable yield for

‘Florida Radiance’), exponential (for ‘FL 127’), quadratic (for ‘Strawberry Festival’

and ‘FL 05 – 107’).

50

Figure 2-2. Total marketable yields for ‘Florida Radiance’ and ‘FL 127’ as a function of early

season N application during the 2013-14 season. The N treatments were varied from

Oct. 15 – Dec. 15. Solid lines show fit to the following models: linear (total

marketable yield for ‘Florida Radiance’ and ‘Strawberry Festival’). Dotted lines show

no significant linear relationship for ‘FL 127’ and ‘FL 05-107’.

51

Figure 2-3. Early and total marketable yields for Florida Radiance and Florida 127 as a function

of early season N application during the 2014-15 season. The N treatments were

varied from Oct 22 – Dec. 14. Solid lines show fit to the following models: quadratic

(early or December and total marketable yield for Florida 127), linear (total

marketable yield for Florida Radiance) and exponential (early or December

marketable yield for Florida Radiance).

52

Figure 2-4. Total unmarketable yield for Florida Radiance and Florida 127 during the 2014-15

season.

0

1

2

3

4

5

6

Florida Radiance Florida 127

b

a U

nm

arket

able

yie

ld

(t/h

a)

53

Figure 2-5. Canopy width and shoot biomass for Florida Radiance and Florida127 recorded 14

weeks after transplanting (WAT) as a function of early season N application during

the 2014–15 season. The N treatments were varied from Oct 22 – Dec. 14. Solid lines

show fit to the following models: quadratic (canopy width and shoot biomass for

Florida 127) and linear (canopy width and shoot biomass for Florida Radiance).

54

Figure 2-6. Correlation between leaf area values recorded 14 weeks after transplanting (WAT)

and total marketable yield for Florida Radiance and Florida 127 during the 2014-15

season.

Figure 2-7. Correlation between shoot biomass values recorded 14 weeks after transplanting and

total marketable yield for Florida Radiance and Florida 127 during 2014-15 season.

Leaf area (sq.cm)

Mar

ket

able

yie

ld (

t/h

a)

Mar

ket

able

yie

ld (

t/h

a)

Shoot biomass (g)

55

Figure 2-8 Correlation between end of season brix values and early N rate for Florida Radiance

and Florida 127 during the 2014-15 season.

Early season nitrogen rate

(kg/ha/d)

Solu

ble

soli

ds

conce

ntr

atio

n

(º B

rix

)

y = -0.84x + 10.26

r² = 0.184

0

2

4

6

8

10

12

14

0 0.5 1 1.5 2 2.5

Florida Radiance

y = -0.021x + 11.07

R² = 0.0001

0 0.5 1 1.5 2 2.5

Florida 127

56

CHAPTER 3

USING A SCANNER BASED RHIZOTRON SYSTEM TO CHARACTERIZE ROOT

MORPHOLOGICAL RESPONSES OF BARE-ROOT STRAWBERRY TRANSPLANTS TO

NITROGEN FERTILIZATION RATES

Introduction

Most commercial strawberry production in Florida uses bare-root strawberry transplants

(Santos and Chandler, 2009) shipped from nurseries in California, North Carolina and Canada.

Prior to shipping, transplants are dug up in the field, cleaned to remove the soil from roots, and

then packed tightly in a shipping box. As a result, the majority of roots are often desiccated at the

time of transplanting, limiting initial water and nutrient uptake capacity. Thus, successful

establishment of these transplants is dependent on growth and development of new roots

(Kirschbaum et al., 2010b). Although nutrient demand of strawberry transplants during

establishment is small, the common N fertilization practice used by Florida strawberry growers is

to apply relatively high rates of N during establishment and early harvest period (October to mid-

December) and then lower the amounts during the remaining season. The advantage of this

practice is supported by the results of the field trials in Chapter 2, indicating the importance of N

fertilization to promote initial growth that aids in the development of a productive canopy for the

entire season. Application of N fertilizers has a significant effect on root growth and structure in

strawberry plants (Bo et al., 2009). The Bo et al. (2009) study evaluated the root growth in

strawberry in response to organic, organic-inorganic compound fertilizer and no fertilizer control

treatments. The study also evaluated the concentration of indole-3-acetic acid (IAA), abscisic

acid (ABA) and isopentenyl adenosine (iPA) enzymes in the roots and leaves using enzyme-

linked immunosorbent assay. It was observed that concentrations of these hormones and

enzymes changed during the season in response to all fertilizer treatments. There were the same

concentrations of IAA and ABA in both root and leaf portions at the initial growth stage (20

57

days) and lower levels at later stage (60 days) as compared to those of control treatments. It was

concluded that root morphological changes in response to nitrogen fertilization could probably

be due to changes in concentrations of endogenous hormones.

In some production systems, strawberries are planted in the same field as a single crop or

in short rotation cycles. A study was conducted to evaluate the growth response of strawberry

roots to increasing strawberry residues using glass rhizotrons to study root growth responses to

strawberry plant residues in strawberry (Neri et al., 2005). The plant residues were collected,

oven dried, grounded and sieved to less than 1 mm. Commercial compost was used as a basic

substrate and the plant residues were mixed at different concentrations with commercial compost

on a dry weight basis. To simulate the soil accumulation of strawberry plant residues, a 3cm

layer of residues was placed on the rhizotrons consisting of 0, 3, 10, 30 and 100% residue

concentration. The observation concluded that root growth was decreased by the presence of

residues. The application of 30 and 100% concentrations of organic residues not only decreased

root growth but also above ground growth. However, the impact of N fertilization on initial root

development is unknown, although it is the period critical for new root development.

The evaluation of root system architecture under field conditions is challenging. It often

involves destructive measurements of roots that are labor intensive and can lead to loss of fine

roots while sampling. The use of a scanner based rhizotron system enables quantification of root

morphological responses through a non-destructive approach. It enables the scanning of the

entire root system that helps to evaluate different aspects of root growth such as root surface

area, root number and root elongation throughout the growing season without the need for any

destructive measurements. Thus, the main objective of the study was to characterize root

morphological responses of bare-root strawberry transplants to N fertilization rates during

58

establishment or early season (Oct- Dec) using a scanner based rhizotron system. Evaluating the

response of other parameters of planting material such as crown diameter, number of crown

roots, length of roots and canopy area to varied N fertilization rates could also be helpful.

Materials and Methods

Rhizotron Construction and Planting

The experiment was conducted in a greenhouse at UF/IFAS GCREC, Balm, Florida from

Oct. 20 – Dec. 3, 2016. Bare-root transplants of ‘Florida Radiance’ were planted in 30 open-top

rhizotrons and evaluated for root morphological responses to six different N rates during the

establishment period. Each rhizotron was constructed from 7×3.8 cm thick pressure treated

woods and two transparent, 4-mm thick acrylic sheets. Both sides (acrylic sheets) of the

rhizotron were shielded from light with opaque polystyrene sheets. Each rhizotron had a

22.9×30.5 cm window which was scanned using a flat-bed scanner (EPSON, Indonesia) to obtain

root images.

The same weight of soil with a known field bulk density was packed in all the open-top

rhizotrons. Rhizotrons were inclined at 30˚ on a bench in the greenhouse to encourage root

growth on the transparent sides facing downwards. Thirty uniform bare-root transplants of

‘Florida Radiance’ were selected and planted on Oct 20, 2016, with one plant in the center of

each rhizotron. Prior to planting, leaves of all transplants were removed to have a uniform set of

transplants from the very first day of planting and to record a more accurate trend in canopy

growth in response to N treatments. Initial crown diameter was recorded for each of the

transplants.

Sprinkler Irrigation and Nitrogen Treatments

After planting, plants were irrigated with micro-sprinklers for 10 days to simulate field

establishment. There was no fertilizer applied during sprinkler irrigation. Urea ammonium nitrate

59

(UAN-32) was used as a source of N. Six different N treatments of 0.56, 1.12, 1.68, 2.24, 2.80

and 3.36 kg/ha/d were evaluated in a completely randomized design, replicated five times. The

amount of UAN for each treatment was calculated such that each of the six N concentrations

were applied as 50 ml nutrient solution per day. Fertilization was done twice a day, using 25 ml

vials.

Use of Scanner for Digital Root Images

A tablet computer (Acer) with a USB port was used to operate the scanner. Before

scanning, the polystyrene sheet from the tilted side of the transparent wall was removed and the

rhizotron was placed on the scanner for the root images. The scanned images were saved to the

tablet and later analyzed for root surface area and root length. The canopy images of strawberry

transplants were also taken every other day and analyzed for canopy area. ImageJ was used for

analyzing the images for canopy area, root surface area and root length.

Other Measurements

As the experiment involved evaluating response to N rates during establishment, the

experiment was terminated on December 3, 2016. At the end of the experiment, all the

transplants were taken out of the rhizotrons and crown diameter, leaf area, number of new crown

roots and longest root length were recorded. The average root length for each treatment was

measured by dividing the total length of all the roots by total number of roots at the end of the

experiment.

Data Analysis

The data was analyzed with R software. The main effect of N rates was assessed using

ANOVA. Regression analysis was performed to evaluate yield responses to nitrogen application

rates. Regression was performed for linear, quadratic and exponential rise to maximum

responses. The model fit was based on lowest value of Akaike Information Criterion (AIC).

60

Correlation was also performed to determine the relationship between certain root and shoot

growth parameters.

Results and Discussion

During the experiment, both initial and end of season crown diameter were recorded. The

change in crown diameter was recorded as increase in crown diameter from its initial value in

response to early season N treatments. The highest and lowest values of the increase in crown

diameter were recorded at 3.0 and 0.5 lb N/acre/d respectively (Table 3-1). Higher N rates

resulted in significant root elongation, with average root length maximized at 2.5 lb N/acre/d

with no significant effect of subsequent N rates (Table 3-1). The effect of increased N rates was

also evident for total root length, with an exponential response to N treatments (Figure 3-1). The

crown diameter, number of new crown roots and canopy area recorded at the end of the

experiment increased linearly with increased N applications during the early season (Figure 3-1).

The initial crown diameter of strawberry transplants influences total marketable yield in

strawberry (Bussell et al.; Johnson et al., 2005). The initial crown diameter, fresh and dry weight

and root length of strawberry transplants at planting have a strong correlation with total

strawberry yield (Bartczak et al., 2010; Johnson et al., 2005). Optimum crown size (8 – 11 mm)

of strawberry transplants promote earliness and improved fruit quality in strawberry (Moncada et

al., 2011). The crown is an important part of strawberry plant structure, that supports both above

and below ground growth. It is an important reservoir of starch (Macías-Rodríguez et al., 2002)

and is critical for establishment and growth of new roots in bare-root strawberry transplants with

a mostly desiccated root system at the time of planting (Kirschbaum et al., 2010b). Thus, higher

N rates lead to increased nutrient reserves in the crown that promoted increased crown size and

growth of new crown roots. A strong linear relationship was also recorded between crown

diameter and number of new crown roots (Figure 3-2). The response of canopy and root surface

61

areas to N rates was evaluated at 7, 14, 21, 28, 35 and 43 days after planting (DAP). The above

ground growth was more responsive to N treatments relative to the growth below ground. A

significant linear response was observed for canopy area at 21 DAP (Figure 3-3) and continued

until the end of the experiment. A strong linear relationship was also recorded between crown

diameter and leaf area (Figure 3-2) recorded at the end of experiment. However, there was no

significant effect of N treatments on root surface area until 43 DAP (Figure 3-4). Thus, increased

crown diameter first promoted increased canopy growth. The increased canopy growth promoted

the development of an efficient photosynthetic system that resulted in increased root growth.

There was also a strong linear relationship between leaf area and root length recorded at the end

of the experiment (Figure 3-5).

Conclusion

The results indicate increased above and below ground growth as a result of increased N

rates. The increased N rates promoted increased crown size and growth of new crown roots. The

increased canopy area as a result of increased crown diameter registered a more rapid response to

N rates than root growth. The root growth was promoted as a consequence of nutrients derived

from the photosynthetic system of the developed canopy. The increased canopy area promoted

root elongation whereas primary root formation was enhanced as a result of increased crown

diameter. Thus, increased N rates during early season (Oct. to mid Dec.) promote increased

canopy and root growth of strawberry transplants that result in increased early and total

marketable yields.

62

Table 3-1. Significance of effects of N rate on average root length and increase in crown

diameter recorded at the end of the experiment.

N rate (kg/ha/d)

Average root length

(cm)

Increase in crown diameter

(mm)

0.56 8.15 b 0.96 c

1.12 9.34 ab 1.09 c

1.68 8.48 b 1.57 bc

2.24 10.81 ab 1.60 bc

2.80 12.30 a 2.34 ab

3.36 9.46 ab 3.04 a

ANOVA

(P value)

0.014 <0.001

Means in a column followed by the same letter are not significantly different at P≤0.05 according to Tukey’s test.

ANOVA= Analysis of variance

63

Figure 3-1. End of season crown diameter, number of new crown roots, total root length and

canopy area for ‘Florida Radiance’ as a function of early season N application. Solid

lines show fit to the following models: linear (for crown diameter, number of new

crown roots and canopy area) and exponential (for root length).

64

Figure 3-2. Correlation of leaf area and number of new crown roots with crown diameter

recorded at the end of experiment.

Crown diameter (mm)

No. of

new

cro

wn r

oots

L

eaf

area

(cm

2)

65

Figure 3-3. End of season canopy area for ‘Florida Radiance’ as a function of early season N

application. Solid lines show fit to the following models: linear (for canopy area at

21, 28, 35 and 43 DAP).

66

Figure 3-4. End of season root surface area for ‘Florida Radiance’ as a function of early season

N application. Solid lines show fit to the following models: linear (for root surface

area at 43 DAP).

67

Figure 3-5. Correlation between leaf area and root length recorded at the end of experiment.

Leaf area (cm2)

Root

length

(cm

)

68

CHAPTER 4

PRACTICAL IMPLICATIONS OF GROWTH STAGE SPECIFIC NITROGEN

FERTILIZATION IN STRAWBERRIES

Current N Recommendations vs Growers’ Practice

The application of fertilizers for strawberry production in Florida is done with drip

irrigation (fertigation). The current recommendation for N fertilization by University of Florida

is 168 kg of N/ha for a season of 200 days. This recommendation is based on crop requirements

and initial soil N levels. It suggests to apply up to 45 kg of N at pre–plant and start with a rate of

0.34 kg/ha/d N for first two weeks after sprinkler irrigation, 0.67 kg/ha/d N during the months of

November, December and January and 0.84 kg/ha/d N for the months of February and March.

On the contrary, the current practice among Florida strawberry growers is to apply 168-224

kg/ha of N during the growing season using drip irrigation. They start with an initial high

application of 1.96 – 2.24 kg/ha/d of N during establishment or early season (Nov. and Dec.) and

then gradually lower to 0.84-1.40 kg/ha/d of N for the rest of the season (Jan. and Feb.). No pre –

plant N is applied.

Background

The university recommendation was developed in 1997. This N recommendation was

based on findings of numerous strawberry fertilization studies conducted by University of

Florida during the 1990’s. The fertilization studies evaluated the yield response to different N

rates over the entire season (Hochmuth et al., 1996), studied interaction between different

strawberry cultivars and N rates (Simonne et al., 2001) or evaluated the performance of different

cultivars in terms of growth and yield over a set of N rates tested over the entire season (Santos

and Chandler, 2009).

The studies so far have evaluated yield responses to N rates based on applications for the

entire season. However, there was limited research on evaluating varied N rates based on growth

69

stages of bare-root strawberry transplants. Unlike production of other crops from transplants, the

majority of strawberry production in Florida is achieved from bare-root strawberry transplants

shipped from nurseries in California, North Carolina and Canada. The majority of root system

becomes desiccated during shipping, resulting in limiting water and nutrient uptake capacity at

the time of transplanting. The live root system is thus confined to a very small portion of soil at

the time of transplanting. Transplant establishment depends on growth and development of new

roots with improved water and nutrient uptake capacity. The successful establishment of these

transplants is thus critical for improved strawberry growth and yield for the entire season.

This necessitates the development of a well – planned and sustainable fertilization

program throughout the growing season that aids in new root growth, increased uptake efficiency

and improved growth and yield. Thus, the fertilization program for strawberry production from

bare-root transplants should not only be based on crop requirements. At the time of planting,

even though the nutrient requirement of transplants is low, the small and desiccated root system

has a very low uptake efficiency and has limited access to a greater portion of the bed for

nutrients. Thus, there is a need to apply more nutrients in the initial stages. As the plants continue

to grow, the efficient root system has greater access to relatively larger areas of the bed. Thus,

applications of small N rates during later parts of the season is sufficient to meet the plant needs.

Suggested New Recommendations

Based on the findings of two growing seasons (2013–14 and 2014–15) and the

greenhouse trial, it can be concluded that:

1. Different strawberry cultivars have varied responses to the range of N rates used for

fertilization.

2. With respect to early and total marketable yield, 1.40 – 2.0 kg/ha/d of N is the optimal

rate during the early season (Oct. – mid Dec.) depending on cultivars.

3. The impact of early season N fertilization rates on unmarketable yield is minimal.

70

4. Increasing the early season N fertilization rate up to 2.0 kg/ha/d not only increases early

marketable yield but also marketable yield for the entire season.

5. The increased early season N rates promote the development of a productive canopy with

increased canopy area, canopy width and shoot biomass.

6. The impact of early season N fertilization rates on fruit quality is minimal.

7. The aboveground growth responds to increased early season N rates more rapidly than

root growth during establishment and early harvest period.

8. High N fertilization rates at 1.68 – 2.80 kg/ha/d during the establishment promote crown

root formation and root elongation.

Thus, new recommendations must take into account the advantages of increased N rates

during early season (Oct. – mid Dec.).

Table 4-1 and 4-2 describe the respective current and new recommendations respectively

for bare-root strawberry transplants grown in central Florida on sandy soils based on a

strawberry season from Oct. 15 to April 30.

71

Table 4-1. Current N fertilization recommendations for strawberries grown in central Florida on

sandy soils.

Nutrient

N fertilization rate (kg/ha/d)

Total

(kg/ha)

Pre-plant

(kg/ha)

First 2

weeks

Nov. to

Jan.

Feb. and

Mar.

April

N 168 0 – 45 0.34 0.67 0.84 0.67 Based on planting date: Oct. 15

Table 4-2. New N fertilization recommendations for different strawberry cultivars grown in

central Florida on sandy soils.

Cultivar

Sprinkler irrigation Early season Mid – late season

Total (Oct. 15 – Oct. 25) ( Oct. 26 – Dec. 15) (Dec. 16 – April 30)

(kg/ha/d) (kg/ha) (kg/ha/d) (kg/ha) (kg/ha/d) (kg/ha) (kg/ha)

Florida

Radiance

0 0 2.02 – 2.24 100–112 0.84 – 1.12 113 – 151 213 –

263

Florida 127

0 0 1.57 78.5 0.84 – 1.12 113 – 151 192 –

230

Strawberry

Festival

0 0 1.12 56 0.84 – 1.12 113 – 151 169 –

207

FL 05 –

107

0 0 1.40 70 0.84 – 1.12 113 – 151 183 –

221 Based on planting date: Oct. 15

Sprinkler irrigation: Oct. 15 – Oct. 25

No pre – plant fertilizer to be applied

72

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BIOGRAPHICAL SKETCH

Bhagatveer Singh Sangha is originally from Punjab, a small state in north–western India.

He belongs to a farming family that has been involved in the farming business from last four

generations. He completed his Bachelor of Science in agriculture from Punjab Agricultural

University, India in July 2014. Realizing increased scope of cultivation of horticultural crops in

India, he decided to pursue his Master of Science in Horticultural Sciences from University of

Florida. He joined as a graduate student at University of Florida in January 2015 and graduated

with a master’s degree in summer of 2017.