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Chapter 36
GROWTH RESPONSES AND REGULATION
The ultimate control of plant growth and development is genetic.
Location of a cell in the plant body and environment influence gene expression in plants.
Chemical signals from adjacent cells may help the cell perceive its location in the plant body.
Environmental cues like changes in light and temperature influence gene expression.
Plant hormones are chemicals that are produce in one part of the plant and transported to
another part where they cause a physiological response.
SEED GERMINATION
A mature seed is one with a fully developed embryo.
Mature seeds are often dormant and may not germinate even if conditions are ideal.
Seed germination is influenced by external and internal factors.
External environmental factors: oxygen, light, water and temperature.
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Imbibition of water by the seed.
Oxygen for cell respiration from food in the endosperm.
Optimal temperature for most plants is 25o to 30oC (77o-86o F).
Some plants require exposure to freezing temperatures.
Some require light and some do not.
Internal factors are the maturity of the embryo and the presence or absence of chemical
inhibitors.
Germination pattern is different in monocots and dicots.
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Dicots produce a "hook" that pushes out of the ground.
Monocots grow produce a plant that grows straight out of the ground and is protected by
the coleoptile.
Stems and roots have the ability to grow throughout the lifetime of the plant: indeterminate
growth.
Leaves and flowers stop growing at some point: determinate growth.
PHOTOPERIODISM
Photoperiod is the length of daylight in a 24-hour day.
Short-day plants (long-night plants) flower when the night length is equal to or greater than
some critical period.
 Plant detects the shortening of the day or lengthening of the night.
 Minimum critical night length varies with the species.
 Fall flowers like poinsettias, chrysanthemums.
Long-day plants (short-night plants) flower when the night length is equal to or less than
some critical period.
 Plant detects the lengthening of the day and shortening of the night.
 Maximum critical night length varies with the species.
 Spring flowers.
Day-neutral plants do not respond to photoperiod.
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Many originated in the tropics where there is little difference in day length throughout
the year.
Tomato, beans, corn, cucumber, etc.
The photoreceptor for photoperiodism is a group of five blue-green pigments
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Each coded by different gene.
Collectively called phytochrome.
Found in the cells of all vascular plants.
Phytochrome detects the varying periods of day length.
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Blue-green pigment that absorbs light.
Open tetrapyrrole attached to a protein.
Phytochrome occurs in two forms: one form, Pr, absorbs red light at 660 nm and the other form,
Pfr, absorbs far-red light at 730 nm.
The shape of the molecule changes when light is absorbed.
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During the day the amount of Pfr increases due to red light.
At night Pfr, which is less stable, reverts slowly to Pr.
Pfr inhibits flowering in short-day plants (read long-night plants).
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These plants need long days and short nights in order to flower.
Long period of darkness allows all of the Pfr to revert to Pr and the inhibition
disappears: the plant blooms.
In long-day plants (read short-night plants) Pfr induces flowering.
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Long days produce large amount of Pfr
During the short night only part of the Pfr changes to Pf .
The plant has sufficient Pfr left to flower.
Experiments show that an internal clock is also at play here.
VERNALIZATION
Low temperature is needed by some plants in order to bloom.
It usually requires temperature between 0oC and 10oC.
The part of the plant needed to be exposed to low temperature varies: seed or apical meristem.
Other plants bloom early if exposed to low temperature, if not exposed then they bloom later.
Hormone level interacts with external factors and modifies their effect.
CIRCADIAN RHYTHMS
These internal timers or biological clocks of organisms.
They are alternating patterns of activity that occur at regular intervals.
 Approximate 24-hour period (20-30 hour periods).
 Independent of temperature and light cycles.
 Reset by the sun every day.
 Opening and closing of stomata, sleep movements, opening of flowers.
NASTIC MOVEMENTS
Changes in turgor cause plant part to move.
The movements are temporary and reversible.
The direction of the movement is predetermined and independent of the direction of the
stimulus.
In the case of Mimosa, an electrical impulse created by the stimulus induces a chemical signal
that increases the membrane permeability to certain ions thus changing the turgor of the cells.
e. g. sleep movement, solar tracking, sensitive leaves.
TROPISMS
Tropism is growth response to an external stimulus from a specific direction.
Changes are permanent and irreversible.
Phototropism is a response to the direction of light.
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A flavoprotein acts a photoreceptor of blue light (500 nm).
Gravitropism (syn. geotropism) is a response to gravity.
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Statoliths made of starch accumulate at the bottom of cells in the root cap in
response to gravity.
The side of the cell opposite to the statoliths elongates.
Gravitropism may be positive (toward) or negative (away from).
Thigmotropism is a response to contact with a solid object.
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Water, temperature, oxygen and chemicals may also cause tropism in plants.
Heliotropism is the ability to follow the sun's movement across the sky.
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It is triggered by blue light.
Many heliotropic leaves have pulvini at the base of the petiole.
THIGMOMORPHOGENESIS
Mechanical stress due to the action of wind, rain, etc. in exposed places causes plants to grow
shorter and stockier.
HORMONES
Plant hormones are chemical messengers.
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Produced in one part of the plant.
Transported to another part of the plant.
Causes a physiological response: regulate growth and development.
Each hormone type causes several responses.
The responses of different hormones overlap.
There are five classes of plant hormones.
1. AUXIN
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It is produce in the apical meristem of shoots, in young leaves and in seeds.
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It is transported downward in parenchyma cells.
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It causes cell elongation, inhibits lateral bud development, stimulates fruit development
and inhibits abscission.
2. GIBBERILLIN
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It is produced in young leaves, roots, shoot apical meristem and in the seed embryo.
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Method of transport in the plant is unknown.
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It promotes seed germination, cell division and elongation, fruit development, flowering
in some plants and breaks seed dormancy.
3. CYTOKININS
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They are produced in the roots.
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Travel upward in the xylem.
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Promote cell division and differentiation in which unspecialized cells become specialized,
stimulates lateral bud development, inhibits abscission and delays senescence.
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Zeatin was the first isolated naturally occurring cytokinin.
4. ETHYLENE
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It is a gaseous hormone produced in stem nodes, aging tissues and ripening fruits.
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It probably diffuses out of the tissue that produces it.
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It promotes ripening of the fruits and abscission, inhibits cell elongation, stimulates
germination of seeds and it is involved in responses to wounds and infections by
microorganisms.
5. ABSCISIC ACID
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It is produced in older leaves, the root cap and stems. Stressed plants produce abscisic
acid
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It travels in the vascular tissue.
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It inhibits seed germination and promotes winter and seed dormancy, formation of bud
scales. It causes the closing of stomata in plants under water stress
Grafting experiments suggest the existence of a substance (florigen) that causes flowering in
plants, and others that inhibit flowering.
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Neither has been isolated yet.
OTHER CHEMICALS INVOLVED IN PLANT GROWTH AND DEVELOPMENT.
Polyamines are organic molecules with two or more amine groups (- NH2).
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They may be involved in gene expression and increase the transcription of DNA and
translation of mRNA.
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They exist in high concentration in plant tissues and are not transported extensively
through the plant.
Systemin, a plant polypeptide, stimulates plant defenses that produce chemicals that disrupt
insect digestion.
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Present in very small quantities.
Oligosaccharins are cell-wall fragments consisting of short, branched chains of sugar residues.
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Present in quantities lower than hormones.
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Bind to membrane receptors and affect gene expression and have many effects on
plants.
Salicylic acid help to defend the plant against insect attack and promotes wound healing.