高中统考 Chapter 18 Growth and Development

Chapter 18 Growth and
development

Learning objectives:
1. Know the growth curve of living organisms
2. Understand the growth and development of plant meristematic
shoot tip and root tip
3. Understand the secondary growth of shoot.
4. Understand the growth curve of human being
5. Know the plant hormones and understand their functions
6. Understand the relationship between phytochrome and plants’
growth and development.
7. Know the photoperiod of plants.

Growth vs development
• Growth means the increase of number of cells of organism, the
increase of body size and weight of the organisms due to the
accumulation of protein.
• Development means various cells are produced through the
differentiation processes of cells to form tissues, organs and systems.

Growth curve
• Growth changes of organisms can be determined by periodically
measuring of certain life characteristics such as length, height,
diameter, volume, fresh mass, and dry mass etc.
• Plant growth are usually measured with dry mass.
• Dry mass means the mass of a biological sample after the water
content has been removed; usually by placing the sample in an oven.
• These data are plotted against the time in a graph, producing a
growth curve.

Using fresh mass to measure growth
Advantages Disadvantages
Easier and convenient to measure Inaccurate and inconsistent
No need to destroy the specimen The mass is affected by the fluctuation
of the amount of water in the organismThe same organism can be used for
repeated measurement

Using dry mass to measure growth

Sigmoid curve S函数
• Generally, individuals and populations
with limited growth exhibits a sigmoid
shape growth curve.
• A: Lag phase (slow beginning growth
phase)
• B: Log phase (exponential phase / rapid
• growth phase)
• C: S-phase (slow mature growth phase)
• D: Stable phase (stationary growth
phase)
• E: Decline phase
• F: Death phase

A: Lag phase
• Growth is slow at the beginning,
called lag phase.
• This is because individual cell
number is not that much, and the
cell division is just started.

B: log phase
• The growth rate is the highest at
log phase.

C: S-phase
• The growth rate of the organism
starts to decrease.

D: stable phase
• Organisms stops to grow when
body size of the adult reaches its
limit.
• After this, the cells getting older
and loss its activeness, this is
known as decline phase. Finally,
the cell bursts and die, called
death phase.

E: decline phase
• After this, the cells getting older
and loss its activeness. Finally, the
cell bursts and die, called death
phase.

F: death phase
• The cell bursts and die, called
death phase.

Growth curve of human
• First stage is infancy.
• During infancy, the rate of
growth is high whereas there is
less differentiation happen.
• Second stage is called
childhood, growth slows down
during this stage.

• Growth rate will speed up
during the third stage, it is
called adolescence.
• After this stage, the rate of
growth and differentiation are
basically balanced, and growth
rate starts to slow down.

• Once enter the fourth stage,
adulthood, most of the growth
of tissues and organs focus on
the renewal and repairing of
the injured or worn out tissues.
• Finally, during the aging stage,
various function of mechanisms
declines.

Differential growth in human
• The ratios of various parts of the
human body will change according
to their age, this is due to the
growth of various parts of human
body are not the same

• The ratio of size of the head
decreases with the increasing of the
age.
• The head grow rapidly during the
beginning stage of growth.
• After that, growth rate of the head
slow down.

• The ratio of the length of the legs
increase with the increasing of age
• The growth rate of legs is faster
than the other parts of the body.

• During growth, the ratio of the
length of the trunk is almost remain
unchanged.
• The speed of growth of the trunk is
almost same as the speed of growth
of whole body.

The growth curve of an insect
• A stair case shape curve for
insects with exoskeleton.
• The horizontal / almost flat
section correspond to the
time when the insect stops
growing.
• Its size is restricted by its
exoskeleton.
• These stages are called an
instar.

The growth curve of an insect
• The vertical lines refers to
the insect’s moulting or
ecdysis.
• The insect cast away their
exoskeleton and use air to
expand their body quickly.

Growth curve of annual plant
• In the growth curve of an annual plant, it
can be seen at B there is a decrease in mass.
• This is as a result of endosperm being used
up in theseed during germination.
• However, as green leaves develop,
photosynthesis begins and the plant gains
mass.
• Towards the end of the
• growing season the curve flattens out and
declines as can be seen at point C, as seeds
are dispersed.

Growth curve of perennial plant
• Perennial plants grow year after
year. The growth curve takes the
form
• of lots of s-shaped curves. Each year,
spring and summer provide the best
• growing conditions i.e. plenty of light
and warmth for photosynthesis.
• During winter, growth slows down
and stops. Under these conditions,
annual rings can be formed in trees

Recap: plant meristem and plant
growth

Meristematic tissue 分生组织
• Tissue that contains undifferentiated meristematic
cells分生细胞.
• Meristematic cells can divide rapidly through mitosis
and they are indeterminate cells that are not
specialized yet.
• Meristem cells are small, thin-walled, closely
arranged, have relatively large nucleus, dense
cytoplasm and generally no vacuoles.
• Meristem are found in zones of the plant where
growth can take place.
Root meristem of onion.

Types of meristem
Based on the
location of
meristematic
tissues in plants, it
can be divided
into:
• Primary (apical)
meristem 初生分生
组织/顶端分生组织
• Secondary
(lateral)
meristem 次生分生组
织

Primary (apical) meristem
• It is located at the tip of the roots and the shoots of plants.
• The root apical meristems give rise to future roots.
• The shoot apical meristems give rise to flower, fruit, leaf and stem.

Three zones in the
root tips
• The zone of cell division is closest to
the root tip and is made up of the
actively-dividing cells of the root
meristem, which contains the
undifferentiated cells of the
germinating plant.
• The zone of elongation is where the
newly-formed cells increase in
length, thereby lengthening the
root.
• Beginning at the first root hair is
the zone of cell maturation where
the root cells differentiate into
specialized cell types. All three
zones are in approximately the first
centimeter of the root tip.

高中统考 Chapter 18 Growth and Development

Secondary (lateral) meristem
• It is located at the lateral side of roots and
stems and occurs as cylinders in older parts of
stem.
• It causes lateral growth (increase in diameter)
• It includes the vascular cambium 维管束形成层 and
cork cambium 木栓形成层.
• Vascular cambium produces secondary xylem 次
生木质部 and secondary phloem 次生韧皮部.
• Cork cambium forms new layers of protective
tissue (periderm) on top of the thicken surfaces
of roots and shoots.
• In a mature stem, all of the tissues found
outside the vascular cambium make up the
bark.
• Not all plants undergoes secondary growth.

Wood
• “Wood” is actually layers of
secondary xylem produced
by the vascular cambium.
• Heartwood, near the center
of the stem, contains old
xylem that no longer
conducts liquids.
• Sapwood surrounds
heartwood and is active in
fluid transport.

Annual rings or growth rings
• In most of the temperate zone, tree
growth is seasonal.
• Activities of cambium are affected by the
changes of weather.
• During spring when weather is warm, and
the rainfall is sufficient, the cells are more
active grow larger.
• At the end of growing season, the
weather is dry and cold, so the cells
produced are small and lesser.
• Annual rings can be used to estimate a
tree’s age and provide information about
past climate and weather conditions.

Annual growth rings of a tree
trunk(A) A Douglas fir
(Pseudotsuga menziesii) is
born. (B) Growth is rapid,
forming relatively broad,
even rings. (C) “Reaction
wood” is formed to help
support the tree after
something fell against it. (D)
Growth is straight but
crowded by other trees. (E)
Competing trees are
removed, and growth is
again rapid. (F) Fire scars the
tree. (G) Narrow rings are
caused, probably by a
prolonged dry spell. (H)
Narrow rings may have been
caused by an insect.

Difference in plant and animal growth
Feature Plants Animals
Pattern of growth Often can grow
continuously
Tend to grow to a
maximum size
How growth happens Mainly by cell
enlargement (increase in
cell size)
Increasing the number of
cells
Where cell division
happens
Mainly at meristems –
found at the tips of
shoots and roots
In most tissues
Cell differentiation Many cells can
differentiate
Most cells lose the ability
to differentiate at an early
stage

18.4 Hormones and development
of plants

Plant hormones
• Plant hormones are signal molecules
produced within plants, that occur in
extremely low concentrations.
• They exert strong control over plant
development and can either act locally
or in more distant part of the plant.
• Plants utilize simple chemicals, which
move more easily through their tissues
through xylem and phloem.
• Amount of plant hormones secreted is
affected by following factors: light, day
span, temperature, gravity force and is
regulated by certain internal
environment factors.

Auxins (IAA)
• Auxin normally accumulates
in the apical meristems of
shoots and roots, young
leaves, embryos of seeds
and cambium etc.
• Auxin is polar transported
by the parenchyma cells
from shoot tips to the base
of the plant.

How auxin trigger plant growth
• acid-growth hypothesis:
• IAA stimulates H+ pumps in
the cell membrane.
• H+ pumps secrete H+ into the
cell wall, decreasing its pH.
• This acidifies the cell wall
which activates pH-
dependent enzymes
expansins that breaks bonds
between cellulose
microfibrils.
• The wall "loosens" because
of the broken bonds and the
turgor pressure expands the
cell.

Effect on different concentration of auxin
• Low concentration of
auxins can promote the
growth of plants while high
concentration of auxin
inhibits the growth of
plants.
• Different organs of the
plant have different
responses to the
concentration of growth
hormones.

Effect of auxin
• Apical dominance
• High concentrations of auxin inhibit the growth of lateral buds in shoot apex
• Stimulate the formation of adventitious root
• adventitious roots are roots arise from an organ other than the root—usually
a stem
• Parthenocarpy单性结实
• the ovary子房 develop into fruit without fertilization
• the ovules 胚珠 would not develop into seeds
• produces seedless fruits - strawberry, cucumber, tomatoes etc.

Effect of auxin
• Stimulate flowering
• Herbicide
• high dose of auxin stimulate production of ethylene
• excess ethylene inhibit elongation growth, cause leaf abscission, and even
death
• Storage
• prevent germination of tubers – potato and bulbs – onion
• Preventing the abscission of flowers and fruits
• prevent the formation of abscission layer离层

What can you tell about the following
experiment?
Auxin promotes fruit development that is produced by achenes

Gibberellin
• Gibberellin, commonly abbreviated to GA,
is a member of a group of naturally
occurring acids.
• Gibberellins, abundant in seeds, are also
formed in young leaves and in roots
• Gibberellins move upward from the roots
in the xylem and thus do not show the
movement characteristic of auxins.

Function of gibberellin
• Gibberellins have little effect upon
pieces of coleoptile in tissue culture.
• Gibberellins promotes stem growth,
by increasing the length between
internodes.
• Gibberellins can be used to replace
vernalization春化作用, promoting
flowering and fruiting.
• Gibberellins also stimulate
germination of seeds.
1. Shows a plant lacking gibberellins and
has a internode length of "0" as well as it
is a dwarf plant. 2. Shows your average
plant with a moderate amount of
gibberellins and an average internode
length. 3.Shows a plant with a large
amount of gibberellins and so has a much
longer internode length because
gibberellins promotes cell division in the
stem.

How gibberellin works
• Stem elongation in plants as a
result of gibberellin treatment
involves both cell division and
cell elongation.
• In deep-water rice, stimulation
of internodes elongation is
partly due to increased cell di-
visions in the intercalary
meristems and partly due to
elongation of cells of the latter
who have divided with cell
elongation preceding the cell
divisions.

How gibberellin works
• GAs cause an increase in both mechanical
extensibility of cell walls and stress relaxation
(loosening) of the walls of living cells of stem.
• Unlike auxin, gibberellins does not cause
acidification.
• GA promotes the activity of the enzyme xyloglucan
transglycosylase (XET).
• XET hydrolyses xyloglucans of the cell walls internally
and causes molecular rearrangement in the cell wall
matrix which could promote extension of cell wall.
• XET may also facilitate penetration of proteins called
expansions into the cell wall causing cell wall
loosening and thus increasing mechanical extensibility
of cell walls.

抽苔(随着花芽的分化，茎开始迅速伸长，植株变高)

Parthenocarpy in pear fruit
UP, Unpollination; P, Pollination; GA4+7, GA4+7 75 mg L−1; fruits were collected at 3, 9, 14 and 153 DAA

Comparing the effects of IAA and GA
Effects IAA GA
Stem Growth + +
Parthenocarpy + +
Root initiation + -
Callus formation + -
Induction of amylase - +
Bolting and flowering - +

Cytokinin
• Cytokinins, which were named for
their ability to promote
cytokinesis, are a class of
phytohormone derived from
adenine.
• Common cytokinins found in plants
include the isoprenoid cytokinins
trans-zeatin (tz) and isopentenyl-
adenine (iP). Benzyladenine (BA) is
a commonly used synthetic
aromatic cytokinin.

How cytokinin works
• Cytokinins are more abundant
in developing tissues and
organs, such as root tip, shoot
apex, cambium, and immature
organs.
• Cytokinins is transported by the
xylem.
• Cytokinin causes cell division,
but generally inhibit cell
elongation, which counters the
effect of auxin.

Function of cytokinin
• Tissue culture
• Nutrient storage
• Increase shelf life
• Overcome senescence
• Remove apical dominance
• Reduce dropping of premature
fruit

• Tissue culture
• Cytokinins induce
formation of new leaves,
chloroplasts in leaves,
lateral shoot formation
and adventitious shoot
formation.
BA = cytokinin
NAA = auxin

• Nutrient mobilization
• Cytokinin-treated leaves
become "sinks" for
nutrients such as amino
acids.
Cytokinin-treated leaves become "sinks" for
nutrients such as amino acids. Here you can see
in seedling B that the cytokinin-treated leaf on
the left attracted the radio-tagged amino acid
from the untreated leaf on the right. In seedling
C, when the tagged leaf is also treated with
cytokinin, there is not even the small amount of
leakage to the other leaf observed in the control
(seedling A).

• Overcoming Senescence:
• Cytokinins delay the senescence
of leaves and other organs by
mobilisation of nutrients.
• Cytokinins prevent the
production of hydrolytic enzymes
such as nucleases and proteases,
so that nucleic acids, proteins,
and chlorophyll are not destroyed.

• Increase shelf life
• Application of cytokinins to
marketed vegetables can keep
them fresh for several days.
• Shelf life of cut shoots and flowers
is prolonged by employing the
hormones.

• Remove apical dominance
• Presence of cytokinin in an area
causes preferential movement of
nutrients towards it.
• When applied to lateral buds,
they help in their growth despite
the presence of apical bud.
• They thus act antagonistically to
auxin which promotes apical
dominance.

• Reduce dropping of
premature fruit
• Presence of cytokinin in
an area causes
preferential movement
of nutrients towards it.
• Avoid stunted growth in
fruits
• Avoid dropping of fruits

Abscisic acid (ABA)
• Abscisic acid is often referred to as a
inhibitory rather than stimulatory
hormone.
• It is involved in the closure of stomata,
bud and seed dormancy and is known to
inhibit other hormonal actions.
• ABA can be transported by xylem from
the roots or phloem from the aerial
organs.
sto1 mutant plants are sensitive to soil desiccation. Mutant sto1 plants were unable to accumulate ABA following a hyperosmotic stress,
although their basal ABA level was only moderately altered. A, Representative photograph of wild-type and sto1 mutant plants exposed to
desiccation. Plants were grown in soil under a standard irrigation regime until four to five fully expanded leaves were formed, at which stage
irrigation was stopped. After 15 d, in coincidence with the appearance of clear symptoms of leaf desiccation, plants were rewatered and left to
recover for 48 h, at which time pictures were taken. B, Shoot fresh weights of desiccation-stressed wild-type and sto1 mutant plants after
rewatering. Values are means of 20 plants ± se.

Function of Abscisic acid (ABA)
• Closure of Stomata
• Bud Dormancy
• Seed Dormancy
• Abscission
• Flowering

Closure of
Stomata
• Large amounts of
abscisic acid in the
leaves causes the
stomata to close
which helps the plant
conserve water
during droughts.
• ABA binds to
receptors at the
surface of the plasma
membrane of the
guard cells.

Closure of
Stomata
• The receptors
activate several
interconnecting
pathways which
converge to
produce
• a rise in pH in the
cytosol;
• transfer of Ca2+ from
the vacuole to the
cytosol.

Closure of
Stomata
• These changes
stimulate the loss of
negatively-charged
ions (anions),
especially NO3− and
Cl−, from the cell
and also the loss of
K+ from the cell.

Closure of
Stomata
• The loss of these
solutes in the
cytosol reduces the
osmotic pressure of
the cell and thus
turgor.
• The stomata close.

Bud Dormancy
• ABA mediates the conversion of the
apical meristem into a dormant bud.
• The newly developing leaves growing
above the meristem become
converted into stiff bud scales that
wrap the meristem closely and will
protect it from mechanical damage
and drying out during the winter.

Bud Dormancy
• ABA in the bud also acts to enforce
dormancy so if an unseasonably
warm spell occurs before winter is
over, the buds will not sprout
prematurely.
• Only after a prolonged period of cold
or the lengthening days of spring
(photoperiodism) will bud dormancy
be lifted.

Seed Dormancy
• It is important the seeds not germinate prematurely during
unseasonably mild conditions prior to the onset of winter or a dry
season.
• Abscisic acid induces seeds to synthesize storage proteins.
• Abscisic acid also inhibits the affect of gibberellins on stimulating de
novo synthesis of a-amylase.
• Not until the seed has been exposed to a prolonged cold spell and/or
sufficient water to support germination is dormancy lifted.

Abscission
• ABA also promotes abscission of leaves
and fruits (in contrast to auxin, which
inhibits abscission) by inducing the
formation of abscission zone.
• The abscission zone is composed of a top
layer that thin-walled parenchyma cells,
and a bottom layer that expands in the
autumn, breaking the weak walls of the
cells in the top layer. This allows the leaf
to be shed.

Abscission
• The dropping of leaves in the autumn is a
vital response to the onset of winter
when ground water is frozen — and thus
cannot support transpiration — and
snow load would threaten to break any
branches still in leaf.

Flowering
• ABA promotes short-day plant to flower in
long days.

Ethylene
• Gaseous hormone
• Promote plant organ maturation
• Ethylene is widely found in many
plants, especially ripening fruit
tissue, other organs such as flowers,
leaves, stems, roots, tubers and
seeds.
• It can be dissolved in water for
transport or diffuse in the cell gap in
a gaseous state.

Function of ethylene
• Promote fruit ripening
• Promote old leaf detachment and aging
• Promote the discharge of secondary secretion
• Increase female flowers of cucumber

Promote fruit ripening
• Very little ethylene in young fruits
• As the fruit grows, the synthesis of
ethylene accelerates
• Ethylene can increase the
permeability of the cell
membrane, accelerate the
respiration, so that the fruit
mature faster.
• Ethylene has been used to ripen
fruits, such as ripening citrus,
persimmons, bananas and cotton.

Promote old leaf detachment and aging
• Treatment of cotton leaves with ethylene can
accelerate the senescence and defoliation of
cotton leaves.
• If the concentration of ethylene is appropriate,
it does not affect the photosynthesis function
of the leaves and the growth of the young
leaves, only accelerates the aging of the old
leaves.
• Spraying of ethylene to promote the
detachment of the old cotton leaves, and
improve the ventilation and light transmission
conditions of the cotton stem base.
• Spraying ethylene on the grapes quickly causes
the leaves to fall but no the fruits, which can
improve the efficiency of harvest.
• In fruit cultivation, ethylene also has the effect
of thinning fruit.

Promote the discharge of secondary secretion
• The excretion of rubber tree
latex is affected by ethylene.
• After the rubber tree was
treated with ethylene, the
latex production rose the
next day.
• It also promotes the
production of plant
secondary biomass such as
lacquer漆树, pine, tolu吐鲁香
and Indian rosewood印度紫檀.

Increase the ratio of
female cucumber flowers
• The cucumber seedlings were
treated with ethylene to produce
female flowers in the early stages
and increases the total amount
of female flowers
• Auxin can also produce more
female flowers, because auxin
can induce ethylene synthesis, so
it is actually caused by ethylene.

Blossoming of plants
• Flowering is the beginning of
the reproductive stage of the
plant.
• The flowering process begins
with the differentiation of
flower buds, and flower buds
must bloom sooner or later.
• Therefore, whether the plant
can bloom depends on
whether there is differentiation
of flower buds.

Blossoming of plants
• The flowering of some plants is controlled
by inheritance. For example, peas can
grow as long as they grow and mature to
the stage of maturity without the need of
specific environmental factors.
• Other plants, although mature, still need
to wait for certain environmental factors
such as light or temperature to stimulate
the flowering, otherwise they will remain
in the nutrition stage.
• Temperature
• Light

Vernalization春化作用
• Latin vernus "of the spring“
• Annual or biannual plants require
vernalization (low temperature
treatment, about 3 °C) to proceed from
vegetative growth (i.e. the development
of roots, stems and leaves) to to the
reproductive stage (i.e. the development
of flowers, fruits and seeds)
• The embryo or the meristem tissue of
the plant senses the temperature.

Vernalization
• Example of the range of vernalization responses
among Brachypodium accessions. Plants were
exposed to cold for the indicated number of
weeks and then shifted to growth at 16 ̊ C ( plants
shown were grown for 60 d after a shift to 16 ̊ C).
Note there are accessions with sharp transitions
from quite delayed flowering to rapid flowering
after 2, 4, 6, or 8 wk of cold exposure.

Vernalization
• A genetic strain
of Arabidopsis (winter-
annual Arabidopsis)
requires vernalization for
flowering. Without it, the
plant is large and
vegetative (left), but when
exposed to a cold period,
it is smaller and flowers
(right).

Photoperiodism
• Photoperiodism is the physiological reaction of organisms such as
flowering to the length of day or night.
• The number of photoperiod cycles required for each plant varies
• Some need as much as 12 cycles of photoperiod to form flower buds.
• Plants can be classified under three groups according to the
photoperiods:
• Short-day plants
• Long-day plants
• Day-neutral plants

Critical day length临界日照
• The period of daylight,
specific in length for any
given species, that
appears to initiate
flowering in long-day
plants or inhibit flowering
in short-day plants.
• It is the value which 50 %
of the plants will flower.

Short-day plants (SDPs)
• Short-day plants (SDPs) flower only when the day
is shorter than a critical maximum.
• They include poinsettias and chrysanthemums as
well as Maryland Mammoth tobacco.
• Thus, for example, we see chrysanthemums in
nurseries in fall and poinsettias in winter.

Long-day plants (LDPs)
• Long-day plants (LDPs) flower only when the day
is longer than a critical minimum.
• Spinach and clover are examples of LDPs.
• Spinach tends to flower and become bitter in the
summer and is therefore normally planted in
early spring.

Importance of photoperiodic control
• Photoperiodic control synchronizes the flowering of plants of the
same species in a local population.
• This synchronization promotes cross-pollination and successful
reproduction.
• It also means that floriculturists can vary light exposures in
greenhouses to produce flowers at any time of year.

Night length is the
environmental cue
• In a greenhouse, the overall length of a
day or night can be varied irrespective
of the 24-hour natural cycle.
• For example, if cocklebur, an SDP, is
exposed to several long periods of light
(16 hours each), it will still flower as
long as the dark period between them
is 9 hours or longer.
• This 9-hour inductive dark period also
induces flowering even if the light
period varies from 8 hours to 12 hours.
• Hence, night length is the key
photoperiodic cue that determines
flowering

Interruption of dark period
• Biologists noticed that when the
inductive dark period was
interrupted by a brief period of
light, the flowering signal
generated by the long night
disappeared.
• It took several days of long nights
for the plant to recover and
initiate flowering.
• Interrupting the day with a dark
period had no effect on
flowering.

Does the color of light in a night interruption
matter?
• It was observed that red
light (660 nm) was most
effective in interrupting
the long night while far-red
light (730 nm) was
ineffective.
• Sequences of flashes given
in the middle of the night
ending in R acted as if an
interruption had been
effective while sequences
ending in FR light acted as
if no interruption had
occurred at all.

The leaves measure the photoperiod.
1.What measures the
length of darkness?
2.What carries the
messages to the
buds?
(SDP)

Phytochrome光敏素
• Phytochromes are a class of photoreceptor in
plants, bacteria and fungi use to detect light.
• Phytochromes are proteins.
• Phytochromes control many aspects of plant
development.
• the germination of seeds (photoblasty), the
synthesis of chlorophyll, the elongation of
seedlings, the size, shape and number and
movement of leaves and the timing of flowering in
adult plants. Phytochromes are widely expressed
across many tissues and developmental stages

Phytochromes exist in two forms – an active
form and an inactive form.
• The inactive form of phytochrome (Pr) is converted into the active
form when it absorbs red light (~660 nm)

• The active form of phytochrome (Pfr) is broken down into the inactive
form when it absorbs far red light (~725 nm)
• Additionally, the active form will gradually revert to the inactive form
in the absence of light (darkness reversion)

• Because sunlight contains more red light than moonlight, the active
form is predominant during the day
• Similarly, as the active form is reverted in darkness, the inactive form
is predominant during the night

Think about it
• During the day, is the phytochrome going to be in the Pfr stage or Pr
stage?

Absorption spectra of phytochromes
• Absorption spectra of the
two forms (Pr and Pfr) of
phytochromes.
• The Pr form absorbs
maximally at 660 nm
• The Pfr form absorbs
maximally at 730 nm.

Dual roles (sensory and regulatory) of the
phytochrome molecules.
• Phytochromes sense the
light environment
• They undergo a
photoconversion from the
inactive Pr form to the
active Pfr form
• They transduce the signals
through distinct signaling
pathways, which ultimately
leads to regulated gene
expression and appropriate
morphogenesis.
(suppresses stem growth)

Photo-regulation in seed germination
• Because seed storage has very limited nutrients, seeds must be
selected at an appropriate time to sprout to improve survival
opportunities.
• Seeds can be dormant for years, but when it is illuminated, such as
dying or falling of surrounding tree, Pfr initiates the germination
mechanism.

Photo-regulation in plant height
• In the forest, tall trees obscure the sunlight.
• The infrared light will still penetrate to the forest
floor, but most of the red light will be absorbed
by the chloroplast of the taller tree crown for
photosynthesis.
• The infrared light converts the phytochrome into
the Pr configuration, which in turn stimulates the
trees to allocate more nutrients for vertical
growth.
• When the trees themselves are higher, there are
more red light; and the Pfr configuration in turns
suppresses vertical growth of the tree.

Quiz
• In an experiment, leaves were removed from plants before the plants
were exposed to the inductive dark period. There were six plants in
each condition.
• Formulate a conclusion based on the result above.
• How would you modify the experiments shown in the table to find
out how many days of the inductive dark period are required before
the signal is produced? Would you expect the results to be different
for the intact plants and plants with only one leaf? Why or why not?

How flowering signal travels?
• Because the receptor of the photoperiodic stimulus (phytochrome in
the leaf) is physically separated from the tissue on which the stimulus
acts (the shoot apical meristem), the inference can be drawn that a
signal travels from the leaf through the plant’s tissues to the shoot
apical meristem.
• Unlike animals, plants do not have a nervous system, so the signal
must be a diffusible chemical.

Evidence of diffusible signals
• If a photoperiodically induced leaf is immediately removed from a plant
after the inductive dark period, the plant does not flower. If the induced
leaf remains attached to the plant for several hours, however, the plant
flowers. This result suggests that something is synthesized in the leaf in
response to the inductive dark period, and then moves out of the leaf to
induce flowering.
• If two or more cocklebur plants are grafted together, and if one plant is
exposed to inductive long nights and its graft partners are exposed to
noninductive short nights, all of the plants flower.
• In several species, if an induced leaf from one species is grafted onto
another, noninduced plant of a different species, the recipient plant
flowers. This indicates that the same diffusible chemical signal is used by
both species.

Florigen
• Although the diffusible signal was given a name, florigen (FT),
meaning “flower inducing,” decades ago, the nature of the signal was
only recently explained.
• Florigen is a small protein.
• Florigen is made in the phloem companion cells of a leaf.
• Florigen travels in the sieve tube elements from the leaf to the shoot
apical meristem.
• There FT combines with another protein to stimulate transcription of
genes that initiate flowering.

Some plants do not require an environmental
cue to flower
• Some plants flower in response to cues from an “internal clock.”
• For example, flowering in some strains of tobacco is initiated in the
terminal bud when the stem has grown four phytomers in length (stems
are composed of repeating units called phytomers).
• If a terminal bud and a single adjacent phytomer are removed from a plant
this size and planted, the cutting will flower because the bud has already
received the cue for flowering.
• But the rest of the shoot below the bud that has been removed will not
flower because it is only three phytomers long.
• After it grows an additional phytomer, it will flower.
• These results suggest that there is something about the position of the bud
(atop four phytomers of stem) that determines its transition to flowering.

The difference between photoperiodism and
vernalization
Photoperiodism vernalization
Stimuli Light
(red light)
Temperatire
(low temperature)
Reception organ Leaf Meristemic tissue
Receptor Phytochrome Ca2+
Effector Seeds, stems, leaves,
flowers
Seed, flower

• http://www.macmillanhighered.com/BrainHoney/Resource/6716/digi
tal_first_content/trunk/test/hillis2e/hillis2e_ch27_3.html

高中统考 Chapter 18 Growth and Development

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