Environmental Constraints

Text Chapter 3

Subtropical Regions

• Most commercial citrus production

–23.5 - 40o NS Latitudes

• Minimum temperatures > -7oC (19.6oF

• Examples

–Rio Grande Valley ~ 26o NL

–Orlando, Florida ~ 28o NL

– Indio, California ~ 33o NL

Tropical - Subtropical Comparison

• Tropical between 23.5o NL SL

• Subtropical 100 tonnes ha-1

• Tropical 15 tonnes ha-1

–Minimum 0oC (32oF)

• Small fluctuations in DL 10o NL SL

• Small fluctuations in temp at low to mid elevations (-3oF/1000 Ft rule)

Lowland Tropical Regions

• Altitude 0 - 500 m

• Highest average temperatures

• Highest heat units

• Distinct wet-dry cycles

–Wet season onset of flowering

High Elevation Tropical Regions

• Fog reduces light intensity (Day)

–Lowers temp & CO2 assimilation

• Fog (Night) decreases radiation

– Increases temp

• Ultraviolet radiation increased

–Due to reduced particulate matter

–Leaf distortion & reduced growth

Limiting Factors

• Flower initiation av. < 24oC

• Vegetative growth > 12.5oC

• Heat units > 5000

– increased respiration

– < fruit solids & acids

Heat Unit Calculation

12.5C + 24.5C = 18.5

2

18.5 - 12.5 = 6

6 x 30 days =180 heat units / month

180 x 12 = 2160 heat units / year

Examples

Site Latitude Altitude HU• Weslaco 26o05’ N 40 m 3900 • Orlando 28o40’ N 30 m 3700• Indio, CA33o40’N -10 m 3900• Degania Is. 32o40’N -200 m 3600• Lmeira, Bz 22o30’S 700 m 3000• Colombia 4o20’N 400 m 5700

Diurnal Fluctuations

• Greater in subtropical regions

• Mean annual temps from 15 to 18oC

• But many exposed to -0oC

–Minimum of -10oC (14oF) Florida

• Freeze damage in US, Europe, Mexico, Australia, central China.

• Rarely in Brazil

Far East

• Minimal damage

–Large bodies of water

–Satsuma mandarin / Trifoliate orange

–Extended periods of cool temps

•Maximum freeze hardiness

•Not true in Texas and Florida

The Juvenile Plant

• Thorny (modified leaves)

• Upright unbranched growth habit

• Apical meristem abscises

– lateral buds break

–new shoots from lateral buds

• Process repeated Zig-zag growth (sympodial) Determinate habit

Duration of Juvenility

• Inversely related to

– tree vigor

–heat unit accumulation

• Example

–Lemons and limes < 2 years

–Others - 5 to 13 years

• Air layers ‘Tahiti’ lime - fruit in 1 year

Hardiness

• Seedling citrus > hardy than same cultivar budded to a root stock.

Influence of Climate

• Lowland tropical areas with high RF

–Shorter juvenile period

• Arid subtropical area

–Longer juvenile period

• Calamondin & ‘Key’ lime - 18 months

–30oC days, 25oC nights, 16 h DL

•No flushes (continuous growth)

The Budded Plant (Budling)

• Adult - so earlier production.

• Budling -

– Riverside, CA 30 mo. Market sz

–Mannar, Sri Lanka 15 mo.

• Difference?

–Riverside 1700 heat units

–Mannar 5700 heat units

Net CO2 Assimilation

• Linear photosynthetic photon flux (PPF) up to 700 mol m-2 s-1 (Fig. 3.3)

• PPF of 2000 m-2 s-1 represents full sunlight

• Thus max CO2 assimilation at 30% full sunlight

• Max growth when DL 12 h

Influence of High Radiation

• High light intensity may decrease CO2 assimilation

–High light increase leaf temp 10oC

–Optimum range 28 - 30oC

• > 35oC limits RuBisCo inc VPD Mid day stomatal closure poor

growth

Shoot and Leaf Growth

• Shoot growth - 2 to 5 distinct flushes

• Tropical - may be continuous

• Growth begins at >12.5oC (55oF)

• Spring flush

–March - April NL

–September - October SL

• Mean temps 12 - 20oC in this period

Other Flushes

• Summer flushes

–June - July NL

–January - February SL

• Fewer growing points

–Longer internodes

• Tropical - continuous

• Same annual growth - distribution?

Leaf Area

• 3 year old tree - 16,000 leaves (34 m2)

• 29 yr tree - 173,000 (203 m2)

• Interior PPF down to 50 mol m-2 s-1

–What should be done?

Leaf Function

• Leaves C importers ~ 6 wks post bloom

• Stomata on abaxial surface take time to develop - poor transpiration contr.

• CO2 assimilation stabilizes in ~ 7 mo

• Healthy citrus leaves may for 3 yrs.

Root Growth

• Temp regulates root growth, water, and nutrient uptake

• Root growth begins at temp > 7oC

–May - June and August - Sept.

• Increases from 17 to 30oC

• Water & nutrient uptake inc 10 - 30oC

• Winter chlorosis

Water and Root Growth

• Root growth decreases significantly

–When soil moisture < 45% of field capacity.

• Excessive water for only a few days due to H2S produced by soil-bore bacteria caused by lack of oxygen.

Flower Induction and Differentiation

• Flower bud differentiation occurs during winter rest in subtropics

• Or during dry season in tropics

• Temperatures < 25oC rest

• Drought > 30 days induce flower bud

• Italy - water withheld from lemons - irrigated - blooms - control harvest

Anthesis

• Events leading to anthesis (Fig. 3.5)

• Min. threshold 9.4oC (48oF)

• Low to moderate temps during bloom (<20oC) protracted bloom

• Higher temps (25-30oC) shorter bloom

Factors Associated with Flowering

• Carbohydrates

–Girdling

–Alternate bearing mandarins

• Hormones - GA3 influence not clear

• Nutrition -

Carbohydrate / Nitrogen Ratio

• I II III IV

• C/NNNN CC/NNN CCC/NN CCCC/N

• No fruit No Yes No

• Age • Pruning

• Irrigation • Nitrogen fertilizer

Pollination and Fruit Set

• Bee activity minimal temp < 12.5oC

• Pollen germination best at 25 - 30oC

• Pollen tube growth through stylar canals - 2 days to 4 wks

–Depending on cultivar & temp.

Initial Fruit Set

• 100,000 - 200,000 flowers / tree

–Only 1 - 2 % produce fruit

• 1st drop - 3 - 4 weeks post bloom

–Defective flowers or no pollination

• High temps (>40oC) (104oF) may cause fruit drop in Navel Oranges.

Physiological Drop

• 2nd drop - May - June NL

– ( Nov-Dec SL)

–Fruitlets 0.5 - 2.0 cm

• Caused - competition for metabolites

–High temps. H2O stress stomatal closure assim CO2

–Neg. C balance fruit abscission

Longevity

• Freezes primary cause of short life.

• Aside from freezes:

–Humid subtrop - fruit in 15-20 yrs

–Med. Climates - peak 20-25 yrs

• Humid subtrop - life 100 years

• Mediterranean - life 200 years

Fruit Yields

• FL av grapefruit 41 max 120 Mg ha-1

–Oranges 31 Mg max 100 Mg ha-1

• Brazil and China 1/2 - lack irrigation

• Semiarid or arid - < hu < tree size < yield.

• % marketable fruit > in semiarid area

• < pests, < blemishes & > peel color

Fruit Growth

• Sigmoid pattern (Fig. 3.7)

• Phase I - Cell div. (all cells formed)

–1 - 1.5 mo - determines fruit size

• Phase II - Cell differentiation

• Phase III - Cell enlargement (6 mo)

Maturation

• Phase IV - Peel color from green to yellow or orange

–Slight increase in TSS

–Rapid decrease in TA

• Time 6 - 16 months for ‘Valencia’

– Influenced by water and temp.

External Quality

• Peel color (air & soil temps < 15oC)

–Chloroplasts Chromoplasts

•Carotenoids (grapefruit)

• Lycopenes (red grapefruit)

•Anthocyanin (blood oranges)

• Low vigor more color

• Excessive N poor color

Fruit Shape

• High temperatures sheepnosed fruit

–Excessive cell division in albedo near stem end.

Fruit Size

• Citrus 85 - 90% water by weight

–Fruit size function of water

–Leaves on detached fruitless twig wilt in few hrs

–Leaves on detached fruited twig remain turgid for many hours

• WHY?

• Water translocated to leaves is stored in fruit peel

• Irrigation scheduling

Internal Quality

• Carbohydrates 75 - 80% of TSS

–Regulates internal quality

• TSS decrease > rapid - low tropical than in sub-tropical areas (Fig. 3.8)

• TA - same relation (Fig. 3.9)

–Function of heat units

•Organic acids in Citric acid cycle

TSS:TA Ratio

• Determines edible quality

• ‘Valencia’ reach 9:1 in 7 mo in tropics

–Coastal subtropics - up to 16 mo

– (Fig. 3.10)

THE END