Transcript Lecture 11

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.
2.
3.
Long-day treatment - shorten the
juvenility form 5～10 year to 1 year of
birch
Grafting- speed up flowering of fruit
crops in 2-3 year.
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 Development
a. Irreversible change in which to bud
(meristem) changes from growing vegetative
tissue to reproductive tissue
b. Improper conditions can cause flower buds
to abort
1. High temp
2. 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 – Vernalization
2. Photoperiodism
3. Light intensity
4. Drought stress
5. Low fertility levels (especially N)
The final stage in a plant’s life cycle
a. May occur naturally or accelerated by environmental
conditions including pathogenic attack
b. Cell and tissues deteriorate
c. Partial senescence is when plant organs age and
eventually die
d. Complete senescence is when the whole plant dies.
Monocarpic plant – flowering and fruiting once
Polycarpic plant – many times/repeat
Four genetically regulated pathways to
flowering have been identified
1. The light-dependent pathway
2. The temperature-dependent pathway
3. The gibberellin-dependent pathway
4. The autonomous pathway
Plants can rely primarily on one pathway, but
all four pathways can be present
13
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
15
Autonomous Pathway--Plants Can Count
Upper Axillary Bud Released from Apical Dominance Lower Axillary Bud Released from Apical Dominance
5 nodes*
removed
13 nodes*
removed
5 nodes*
replaced
13 nodes*
replaced
Shoot
removed
here
Shoot
removed
here
Intact plant
Shoot removed Replacement shoot
*nodes = leaf bearing node
Intact plant
Shoot removed Replacement shoot
16
Autonomous Pathway--Plants Can Remember
Shoot Florally Determined
Shoot
removed
here
Shoot Not Florally Determined
Shoot
removed
here
Shoot
removed
Shoot
removed
Intact plant
a.
Rooted shoot
Flowering
rooted shoot
Intact plant
Rooted shoot
Flowering
rooted shoot
b.
Not-Florally Determined Plants are said not to
remember...Florally Determined plants are said to
remember
17
1.
2.
3.
Julien Tournois (1910) – 1st found about LD
in flowering
George Klebs (1918) – observe the function
of LD in flowering
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
Short days
Long days
Flowering response
of Japanese morning
glory (left) and black
henbane (right) to
daylength of 24-h
period.
Note the prominent
flowers (arrows) in
Japanese morning
glory under short
days and in black
henbane under long
days . Plants of each
species under both
photoperiod regimes
are of the same age
1. Short day plant (SDP)
The plant can only flower under day length shorter than
its critical day length of 24 h cycle. eg. chrysanthemums
ie. 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 plant
to 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.
2.
Flower industry eg. Chrysanthemum – sdp
Selection of variety/cultivar for vegetable
and field crops
Manipulating 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 wheat
Types
temperature range(℃ )
days
Spring wheat
5～15
5～8
Semi winter wheat
3～6
10～15
Winter wheat
0 ～3
40 ～45
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
Facultative (quantitative)
1.
◦
Flowering will appear earlier (faster) once expose
to low T eg. winter annual cereal
Absolute (obligate)
2.
◦
◦
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 plants
Have two-year life cycles
-they store energy the first year and flower the second year
1. Treatment with vernalization and devernalization
eg. onion for seed production
2. Induced crops to flower:
forcing plants for earlier or late bloom
blooms indoor
3. Selecting sowing date
different 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