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Chapter 9 Plant Flowering Physiology
Chapter outline
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Floral induction
Vernalization
Photoperiodism
Molecular genetic control of flower
initiation and development
§1 Floral Induction (花诱导）
• Floral induction is the essential step in the
transition of vegetative to reproductive
stages of development.
• Floral induction needs several key
environmental factors for plants to undergo
important interior changes.
– Low temperature
– Photoperiod
FLORAL INITIATION
(花诱导）
• THE EVENTS OCCURRING IN THE
SHOOT APEX THAT SPECIFICALLY
COMMIT THE APICAL MERISTEM TO
PRODUCE FLOWERS
VEGETATIVE VS. FLOWERING
SHOOT APEX IN ARABIDOPSIS
VEGETATIVE
APEX
FLOWERING
APEX
THE SHOOT APEX AND PHASE CHANGES
THE SHOOT APICAL MERISTEM PASES THROUGH THREE
DEVELOPMENTAL PHASES:
1.
THE JUVENILE PHASE（幼年期）：grass plants: shorter (days
or weeks); wood plants: longer( years)
No response to low temperature or photoperiod
2.
THE ADULT VEGETATIVE PHASE(成年期）：
The plants reach the ripeness to flower state（花熟状态） that
can response to low temperature or photoperiod
3.
THE ADULT REPRODUCTIVE PHASE (生殖期）
flowering and producing seeds and fruits
JUVENILE AND ADULT FORMS OF ENGLISH
IVY (Hedera helix: 英国常春藤)
JUVENILE
ADULT
FLORAL INDUCTION : COMPETENCE
AND DETERMINATION
Able to
respond
when the
development
signal
(LT and
PP are
present
Or LT
Able to
continue
development even
after
removal
from plant
Apical meristem
undergoes
morphogenensis
§ 2 Vernalization（春化作用）
• The requirement for a low-temperature
treatment specifically to hasten flowering is
called vernalization（Lysenko).
• Vernalization is most commonly linked to a
long-day photoperiodic requirement.
• Gibberellins will normally substitute for a
cold treatment.
Winter-annual Arabidopsis
without vernalization
Winter-annual Arabidopsis
with vernalization
Without cold treatment plants show delayed flowering or remain vegetative
(left) and low temperature stimulates flowering (right)
1: Factors influencing vernalization
(1) Low temperature
Wheat types
Vernalization
Temp.
Days
Distribution
Winter wheat
0-3
40-45
North China
Semi-winter
wheat
3-6
10-15
Yellow River Area
Spring wheat
8-15
5-8
South & Northeast
China
Vernalization
• The range of effective
vernalization
temperature varies on
the species and duration
of exposure.
• The effect of
vernalization is
proportional to the
duration of treatment.
• Lower temperature
requires longer time,
probably due to slower
metabolic rate.
2): Factors influencing vernalization
(2) O2
(3) Water (dry seed do not respond to cold treatment)
(4) Sugar
(5) Light (accumulation of carbonhytrates and LD)
3). Devernalization (去春化作用）: break
vernalization by warm temperature（25-40C),
anoxia and so on.
Revernalization: continue to vernalize after break, so
vernilation is reversible!
.
2. Vernalization result in competence in
shoot apical meristem
1). The stages of perception: almost all the
development stages varying from seed to
different stages of seedlings depending on plant
species. Arabidopsis, winter wheat: imbibition;
cabbage: seedlings is at stage of stem(>0.5cm)
and leaf width(5cm).
2) The site of perception: the shoot apical meristem or
young leaves near apical tip.
3) Treatment temp. From 0 to 10 ℃,mostly 1-7 ℃ for
several weeks.
3. Vernalization causes many
variations in metabolism
1) Increased membrane permeability
2) Increased HMP pathway and high
NADPH
3) New RNA and protein synthesis
4) Increased respiration, photosynthesis rate
5) Reduced tolerance to cold
6) Increased GB in some plants.
4. Mechanism of vernalization on
floral evocation
1) “Vernalin (春化素）” of Melchers
(grafting experiment)
2) GB replaces vernalization of some plants
3) Vernalization causes change in gene
expression
FLC (flowering locus C) is temporarily switched
off by vernalization depending on coordinate
expression of VRN1 and VRN2.
6 weeks, 5ºC
GA
GA
• Gibberellins can
substitute for
vernalization for some
long-day plants. It is
ineffective for shortday plant and
caulescent long-day
plants.
• Genetic evidences
suggested GA and
vernalization took
different pathways.
VERNALIZATION BLOCKS THE EXPRESSION OF THE GENE FLC
Winter annual
without cold
Winter annual
after 40 cold
days
FLC mRNA
Winter annual
without cold,
but with FLC
mutation
§ 3 Photoperiodism
or photoperiodicity
(光周期现象）:
1. Dicovery
In 1912, French Scientist J. Tournois found the reason
why Humulus (hops,山苦瓜) and Cannabis (hemp,大麻)
plants flower precociously during the winter in the
greenhouse was due to the shortening of daylength or the
lenghtening of night. However, World War I interrupted
his research and he did not live to continue it.
W.W. Garner and
H.A. Allard
found that
Maryland
mammoth
tobacco flowering
differently
responded in
Winter and
Summer in 1920
Left: wild type or
Maryland
mammoth tobacco
grown in green
house of winter
Right:
mammoth
tobacco
grown in
summer or
in winter of
green house
supplement
with light
Figure 19.1
September soybeans cultivar Biloxi (Garner and
Allard)
All soybeans sown over between May to August flowered within a
three-week period in September.
2. Photoperiod, photoperiodism and
photoperiodic response types
1). Photoperiod (光周期） refers to the
relative length of daylength and
nightlength in a 24 hours cycle.
2). Photoperiodism or photoperiodicity
refers to the responses of plant
development such as tuber development,
leaf fall, dormancy, in particular,
flowering to daylength and nightlength.
3). Photoperiodic response types:
Critical daylength (临界日长）: the threshhold daylength
which causes plants to flower, for the long-day plants, the
daylength must be longer than the critical daylength, but for the
short- day plants, the daylength must be shorter than the critical
daylength.
Critical dark period （临界暗期）: the threshhold dark period
which causes plants to flower, for the short-day plants, the dark
period must be longer than the critical dark period, but for the
long- day plants, the the dark period must be shorter than the
critical dark period.
The experiments showed that the critical dark period is more
important than critical daylength for plant flower induction.
Photoperiodic response types are as followings:
(1). Short-Day Plants (SDP:短日照植物) are plants
that flower when the daylength is shorter than the
critical daylength
（红叶藜）
（硫华菊）
（一品红）
（紫苏）
（日本牵牛花）
（苍耳）
（2）Long-Day Plants (LDP：长日照植物) are plants
flower in response to daylength longer than the critical
daylength
(茴香）
（天仙子）
（黑麦草）
（春黑麦）
（白芥）
(3). Day-Neutral Plants (DNP：日中性植物) are not sensitive
to daylength
(千日红）
Eggplant, tomato, paprika
The critical daylength of some plants
植物名称
SDP
菊花
苍耳
美洲烟草
晚稻
牵牛
一品红
大豆 曼德临（早
熟种）
临界日长（h） 植物名称
LDP
15
天仙子
15.5
木槿
14
拟南芥
12
大麦
15
小麦
12.5
17
15
大豆 曼德临（早
熟种）
比洛克西（晚熟种） 13～14
临界日长（h）
11.5
12
13
10～14
12以上
13
菠菜
红叶三叶 12
草
约14
白芥
燕麦
9
(4). Other types of photoperiodism
• LSDP plants (long-short-day plants：长短日照植
物) flower only if a certain number of short days
(flower development) are preceded by a certain
number of long days (floral induction) such as
Kalankoe daigremontiana).
• SLDP (short-long-day plants：短长日照植物) is
the reverse of LSD plants such as canterbury bell.
• IDP(Intermediate-daylength plants:中日性植物）
flower only in response to daylengths of
intermediate length such as sugarcane (critical
daylength:11.5-12.5).
• Amphophotoperiodism(两极光周期现象）:
plants do not flower unless daylength is longer
than 14 hours or shorter than 12 hours.
Qualitative and Quantitative
• Plants have a qualitative (obligate) requirement for
daylength meaning that they will not flower unless
it receives an appropriate photoperiod.
• Plants have a quantitative (facultative)
requirement for daylength meaning that they will
flower under inappropriate photoperiod but the
exposure of appropriate photoperiod will facilitate
flowering.
Appropriate photoperiod
• Giving plants the appropriate photoperiod can
induce flowering. Therefore, the appropriate
photoperiod is called the inductive treatment.
• Induction is not an all-or-none process. Although
some plants respond to single inductive cycle, the
initiation of floral primordia is more rapid and
more prolific if multiple cycles are given. Other
plants may exhibit fractional induction – a
summation of inductive photoperiods despite
interruption with noninductive cycles.
3. Perception and transduction of photoperiodic signal
Leaf; signal in leaves to stem apex
SDP Perilla（紫苏)
Although the actual change from the
vegetative to reproductive growth occurs
in apical meristems, the photoperiodic
signal is perceived not by the stem apex
but by the leaf.
Youngest fully expanded leaves are most
sensitive to photoperiod.
4. Dark period plays a critical role in plant flowering
Plants actually measure the length of
the dark period.
The light/dark ratio is not important.
SD plants only flower if the dark
period is longer than the critical dark
period
The central role of dark period in the SDP, Xanthium strumarium,
its critical dark period is 9 h. The number in brackets is dark length.
LD plants require a dark period
shorter than some critical maximum.
A light break in the middle of the
dark period will shorten the dark
period to less than maximum and
permit flowering to occur.
Light break
Red light (660nm) was the most
effective as a light-break and its
effect can be reversed by far-red,
suggesting the involvement of
phytochromes.
However, many LD plants are not
sensitive to light-breaks.
5. Photoreceptors and
photoperiodism
• Blue light is also effective in controlling
photoperoidism.
• The role of cry1 in flowering is not clear but
cry2 promotes flowering.
• PhyA seems to promote flowering but phyB
is inhibitory.
6. Flower Induction needs both appreciate photoperiod
and a certain number of appreciate photoperiod
Different plant
species require
various number of
appreciate
photoperiod.
Ussulay, 1-15days ,
e.g. Japanese
morning glory
needs only 1,
soybean 2-3days.
Soybean
7. Floral induction, floral organ
formation and gene expression
Three sequential stages to the flowering
process (and three different sets of genes)
• Flowering-time genes determine when the
plant initiates flowering
• Floral-identity genes commit
undifferentiated primordia to the production
of floral rather than vegetative structures.
• Organ-identity genes control the
development of floral parts.
CO (CONSTANS) encoding
a transcript factor inducing
FT expression!
FT(FLOWERING LOCUS T)
encoding a small protein that
is known to promote
flowering.
8. ABC model of flowering
• Put forward by Meyerowitz and Coeng in
1991,1994 that flowers are initiated in 4
successive whorls.
Three Types of Genes Control
Flower Development
A: Sepal(Se) + Petal(Pe)
B: Petal(Pe) + Stamen(St)
C: Stamen(St) + Carpel(Ca)
A ←→ C
In Arabidopsis, the genes are A: AP1/AP2; B:
AP3/PI and C: AG respectively
THE ABC MODEL FOR FLORAL ORGAN IDENTITY
(AP3/PI)
(AP1/AP2)
(AG)
Once A, B or C mutated or is lost, there will be
homeotic floral mutants(同源异型突变体）.
MUTATIONS IN FLORAL ORGAN IDENTITY GENES
(A)
(B)
(C)
QUADRUPLE MUTANT (ap1, ap2, ap3/pi, ag)
RESULTS IN THE PRODUCTION OF LEAF-LIKE
STRUCTURES IN PLACE OF FLORAL ORGANS
9、 Photperiod, latitude and application
1）、在北半球，任何纬度都是夏至时白天最长，冬至时白
天最短。
2）、在北半球，自春分到秋分，纬度越高，白天越长；纬
度越低，白天越短。自秋分到春分，纬度越高，白天
越短，纬度越低，白天越长。
3）、因此对于我国南方(广州一带，北纬23°)白天与黑夜
的长度变化不大，没有很长的白天，原产于此的多为
短日植物。
4）、北方(东北一带，北纬45°)在其春分—秋分的生长季
节中，日长很长，原产于此的多为长日植物而在秋
分—春分季节中温度太低，植物不宜生长。
5）、山东(北纬36°)自春分—夏至，日渐变长，适于长日
植物如小麦；自夏至—秋分，日渐变短，适于短日植
物如玉米、大豆。
6)、不可以把冬小麦到南方种植等！东北大豆(SDP)引种到
南方日照变短而提早开花，产量降低！
§5. Pollenation and fertilization
Male gametophyte is generated
from microspore mother cell
Female gametophyte generation
is generated from one megaspore
mother cell
（反足细胞）
(卵器）
Pollen will germinate after pollination occurs
Figure 14.17
Development of male
gametophytes
(from generative nucleus)
受精
Seed
Fruit