Plant Reproductive Physiology

Photoperiodism Temperature Nutrition

Pollination Development after fertilization Maturity, ripening and senescens

Juvenile stage Transitional stage Maturity stage Senescence stage

Vegetative growth and unable to flower even if plant grows an environment for flowering

◦ a physiological state of plant before flower differentiation◦ Flowering cannot be induced◦ plants often differ in appearance from the adult.

Phase length varies:◦ annual – shorter eg. Weed will be at juvenile stage 4-5 d after germinated perennial – longer at juvenile stage

eg. in certain shrubs up to 40years Morphologies:◦ Simple primary leaf to trifoliate leaves◦ beans: adult – compound leaf; juvenile – simple leaf◦ leaves lobe◦ rapid growth

Usually, the basal part of tree is juvenility and the top is mature or adult in physiology.

1. Long-day treatment - shorten the juvenility form 5 ～ 10 year to 1 year of birch

2. Grafting- speed up flowering of fruit crops in 2-3 year.

3. GAs treatments- can induce flowering in juvenility of ivy, cypress and fir.

Have both juvenile and mature tissue May revert back to juvenile if environmental

conditions are right. Involves the transition of a vegetative

meristem, producing leaves and stems, into a floral meristem, producing flowers.

Flower Initiation and Developmenta. Irreversible change in which to bud (meristem) changes from growing vegetative tissue to reproductive tissue

b. Improper conditions can cause flower buds to abort1. High temp2. Moisture stress

c. Flowers can be induced naturally or through PGR (plant growth regulators)

Stage where plants are ready to flower. Flowering - ultimate expression of mature state

◦ Changes influence by environment◦ Environment serve as expression changes regulator

Changes in physiology and morphology◦ Transformation of primodium of stem, leaf or

vegetative part to primodium reproductive organ◦ One way transformation

Many plants produce flowers independent of environmental conditions

Factors influence transformation of the juvenile into the mature:

1.Temperature – Vernalization2.Photoperiodism3.Light intensity4.Drought stress5.Low fertility levels (especially N)

The final stage in a plant’s life cyclea. May occur naturally or accelerated by environmental

conditions including pathogenic attackb. Cell and tissues deterioratec. Partial senescence is when plant organs age and

eventually died. Complete senescence is when the whole plant dies.

Monocarpic plant – flowering and fruiting oncePolycarpic plant – many times/repeat

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Four genetically regulated pathways to flowering have been identified1. The light-dependent pathway2. The temperature-dependent pathway3. The gibberellin-dependent pathway4. The autonomous pathway

Plants can rely primarily on one pathway, but all four pathways can be present

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The autonomous pathway does not depend on external cues except for basic nutrition

It allows day-neutral plants to “count” nodes and “remember” node location

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Upper Axillary Bud Released from Apical Dominance Lower Axillary Bud Released from Apical Dominance

Intact plant Shoot removed Replacement shoot

Shootremovedhere

5 nodes*removed

5 nodes*replaced

Intact plant Shoot removed Replacement shoot

Shootremovedhere

13 nodes*removed

13 nodes*replaced

*nodes = leaf bearing node

Autonomous Pathway--Plants Can Count

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Shootremovedhere

Shootremovedhere

Shoot Florally Determined Shoot Not Florally Determined

a. b.

Intact plant

Shootremoved

Rooted shoot Floweringrooted shoot

Intact plant

Shootremoved

Rooted shoot Floweringrooted shoot

Autonomous Pathway--Plants Can Remember

Not-Florally Determined Plants are said not to remember...Florally Determined plants are said to remember

1. Julien Tournois (1910) – 1st found about LD in flowering

2. George Klebs (1918) – observe the function of LD in flowering

3. Garner & Allard (1920) –found photoperiod ⇨relative duration of light and dark towards control flowering of certain crops. eg. tobacco – flowering once expose to short light period (SDP)

4. Hamner & Bonner (1938) – phenomena night break where dark disturbance during day time are not/a bit effect but light disturbance during night time inhibit flowering (SDP) or initiate flowering (LDP)

Dark time more function in determine reaction in photoperiod

Flowering responseof Japanese morningglory (left) and blackhenbane (right) todaylength of 24-hperiod.Note the prominentflowers (arrows) inJapanese morningglory under shortdays and in blackhenbane under longdays . Plants of eachspecies under bothphotoperiod regimesare of the same age

Lo

ng

day

s

S

ho

rt d

ays

1. Short day plant (SDP) The plant can only flower under day length shorter than its critical day length of 24 h cycle. eg. chrysanthemumsie. the critical day length to induce flowering must be less than some maximum.

2. Long day plant (LDP)The plant can only flower under daylength longer than its critical day length of 24 h cycle. eg. Chinese cabbage,

beet etc. ie the critical day length must be longer than a minimum

3. Day neutral plant (DNP)Without critical daylength, they can flower in any day length of 24h cycle, if other conditions are satisfied. eg. tomato, cucumber, egg plant and bean. After bred for long time , most of crops are not sensitive to day length, eg. early rice, spring soybean, spring maize and cotton

Critical Day Period: It is the duration of the photoperiod or the dark

period that ultimately determines whether the plant has to go through vegetative growth or to produce flowers. referred as the day length of 24h cycle - the shortest day length for LDP flower and the longest day length for SDP flower.

The chemical nature of the receptor is a the molecule PHYTOCHROME. - biological compound that absorbs light

Two types : -Phytochrome far red (PFR) -Phytochrome red (PR)

- interconvertible

* Plants measure the ratio of Pfr/Pr. LDP would flower when the ratio is high SDP would flower when the ratio is low

Since Pfr is labile and is broken down at night or reverts back to Pr - the longer the night, the lower the phytochrome (Pfr) content.

Thus, phytochrome is like the sand in an egg timer; the relative amount of Pfr remaining at the end of the night would be an indication of the day length.

Flowering in SDP:◦ Short day plants flower when the night period is long. ◦ In day light or red light, phytochrome red (Pr) is

converted to phytochrome far red (Pfr). The conversion actually only requires a brief exposure to white or red light.

◦ In the dark, Pfr is slowly converted back to Pr. A long night means that there is a long time for the conversion.

◦ Under short day conditions (long night) at the end of the night period the concentration of Pfr is low.

◦ In SDP, low Pfr concentration is the trigger for flowering.

Flowering in LDP:◦ Long day plants flower when the night period is short.◦ In day light (white or red) the Pr is converted to Pfr.◦ During periods when the day light period is long but

critically the dark period is short, Pfr does not have long to breakdown in the dark. Consequently there remains a higher concentration of Pfr.

◦ In LDP, high Pfr concentration is the trigger to flowering.

Dark period more important in photoperiodism reaction because interruption during night will inhibit flowering in SDP but promote flowering in LDP

In short day plants, Pr promotes flowering when Pfr suppresses it, when it is vice versa in long day plants.

Flowering hormone? Leaves detected photoperiod (at least

one leaf must be present for the plantto fllower) The receptor of photoperiod is located within the

leaf. Photoperiodism sensor - leaf bud

Buds produce flowers Bud meristem changes from vegetative to

flower growth

Stimulus transferred◦ Cut off all leaves after expose to photoperiod inhibit

flowering◦ Cut off all leaves 20 -36hrs later promote flowering

In 2005 a substance, mRNA (FL mRNA) was finally isolated that was found to be moving from leaf to flower meristem.

This mRNA provides a link between the phytochrome system (the receptor), its activation of genes in the leaf (mRNA synthesis) and the differentiation of the meristem into the flower structure.

◦ Florigen - name given to the proposed flowering "hormone“.

1. Flower industry eg. Chrysanthemum – sdp2. Selection of variety/cultivar for vegetable

and field cropsManipulating Flowering Response Use artificial light or dark to simulate

daylength. Light at night to lengthen day.

◦ 10 pm to 2 pm. Cover with black cloth to shorten the day.

Vernalization ⇨ the process by which flowering is promoted by prolonged exposure to the cold of a typical winter ◦ plant expose to low T to initiate and promote

flowering common in biennials and some perennial

plants reactions are varies among spesies/variety

eg. apple, cherry, pear, peaches carrots, cabbage, bulbs like tulip, onion

a. cold storage is used to preserve (sets) bulbs during winter this causes the sets to flower and produce seed in spring

b. to devernalize- sets are exposed to 27oC for 2-3 weeks before planting

Temperature and time of vernalization ◦ -4℃—12℃. Most efficient 1 ～ 2 ℃◦ Reaction between temperature and time:

- in the range of vernalization temperature, the lower T, the shorter time.- the lower temperature for vernalization the plant needs, the longer time lasts.

Table 8-1 Temperature and time for vernalization of wheatTypes temperature range(℃ ) days Spring wheat 5 ～ 15 5 ～ 8Semi winter wheat 3 ～ 6 10 ～

15Winter wheat 0 ～ 3 40 ～

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Kinetics of the vernalisation

• effective on actively growing plant.ie. growing plant – under gone vegetative stage, seed

stage (annual) or bulb

- Seeds must imbibed water (50% of seed dry weight) and the germination process has been initiated and exposed to low temperature ( vernalized). The vernalized seeds are dried and stored.

- other plants in particular the biennial eg. cabbage, must

reach a certain minimum size or age before they can be vernalized ie. whole plant

• In general, the plant flowering need long day period and higher temperature after finishing vernalization, which induces flower differentiation

The site/part of vernalization is the growing point (shoot apex)

eg. celery - shoot apex exposed to low temperature, other part of plant to normal (higher) temperature, the plant can flower.

-shoot apex to normal (higher) temperature, other part of plant low temperature, the plant remain vegetative

• The main part sensitive to low temperature is shoot apical meristem - bud, apex meristems

1. Facultative (quantitative)◦ Flowering will appear earlier (faster) once

expose to low T eg. winter annual cereal

2. Absolute (obligate)◦ MUST expose to low T then flowering appear

eg. biennial plants (cabbage)◦ Exposure to desired period

Lang (1957) found that GA function = vernalin, where GA promote flowering without vernalisation

Vernalin hypothesis: After passing vernalization, plant can form vernalin,which can transfer from one part to others and promote flower.

Vernalization only act the meristem of shoot apex. The effect can transfer form the cell to cell, not from organ to organ.

Control GA3 Low T

In relation to flowering and fruiting or duration of plant life, plants are group into:

Perennial plants Able to flower and produce seeds and fruit for an indefinite number of

growing seasons- may be herbaceous or woody-in deciduous plants all the leaves fall, and the tree is bare, at a particular time of year-in evergreen plants, the leaves drop throughout the year, and so the plant is never completely bare

Annual plants Grow, flower, and form fruits and seeds, and typically die within one growing season- usually herbaceous

Biennial plantsHave two-year life cycles-they store energy the first year and flower the second year

 

1. Treatment with vernalization and devernalizationeg. onion for seed production

2. Induced crops to flower: forcing plants for earlier or late bloom blooms indoor

3. Selecting sowing datedifferent types and characters

SDP initiate flowering at low T. eg. Chrysanthemum

As supplementary or complimentary to each other

Vernalisation cancellation Before finishing vernalization, the effect will

lost under high temperature Devernalization - 25 ～ 40℃ 。 eg. vernalized onion bulbs expose to high

temperature after vernalisation◦ Direct planting flowering◦ Keep in warm temperature (2-3wks) not

flowering

Klebs (1918) – ratio of carbohydrate with inorganic nutrient esp N (C:N) high – will promote flowering

Kraus & Kraybill (US) – flowering on tomato plants was controlled by CHO:N level◦ CHO:N low – delay flowering & less flower (N high)◦ CHO low, N low – less vegetative part, less flower◦ CHO:N high – faster and no of flower increase

◦ there is no C:N critical for flowering