Control Systems In Plants
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Research on how plants grow toward light led to the discovery of plant
are chemical signals that coordinate the parts of the organism.
minute concentrations are required to induce substantial changes in the
phototropism is the growth of a shoot toward light.
and Francis Darwin observed that a grass seedling could bend toward light only
if the tip of the coleoptile was present.
If the tip
were removed, the coleoptile would not curve.
growth response occurred some distance below the tip.
proposed the hypothesis that some signal was transmitted downward from the tip
to the elongating region of the coleoptile.
Boysen-Jenson demonstrated that the signal was a mobile substance by separating
the tip from the remainder of the coleoptile by a block of gelatin.
contact was block but chemicals could pass.
seedlings behaved normally.
extracted the chemical messenger by removing the coleoptile tip and placing it
on a block of agar.
concluded the block contained a chemical produced in the coleoptile tip and it
stimulated growth as it passed down the coleoptile.
Plant hormones help coordinate growth, development, and responses to
classes of plant hormones.
plant growth and development by affecting the division, elongation, and
differentiation of cells.
mediate shorter-term physiological responses.
Each has a
multiplicity of effects depending on the site of action, the developmental stage
of the plant, and the concentration of the hormone compared to others.
from cell to cell involves the passage across cell walls.
very small concentrations.
May act by
altering the expression of genes, by affecting the activity of existing enzymes,
or by changing properties of membranes.
usually depends not on the absolute amount of the hormones but its relative
concentration compared to others.
balance may control the growth and development of the plant.
any chemical substance that promotes elongation of the coleoptiles.
natural auxin is called indoleacetic acid of IAA.
meristem is a major site of auxin synthesis.
moves down the shoot apex to the region of cell elongation and stimulates growth
concentrations, it may inhibit cell elongation.
probably due to a high level of auxin inducing the synthesis of another hormone,
ethylene, which generally acts as an inhibitor of plant growth.
directly through parenchyma tissue from one cell to the next.
from shoot tip to base and is called polar transport although it has nothing to
do with gravity.
auxin transport requires energy using proton pumps driven by ATP.
exits each cell by a specific carrier protein that is restricted to the basal
end of the cell.
pumps located at the plasma membrane also play a response by lowering the pH of
the wall which makes it more plastic and is free to take up additional water by
osmosis and this allows for elongation.
stimulates longer-term growth responses.
secondary growth by inducing cell division in the vascular cambium and by
influencing the differentiation of secondary xylem.
the formation of adventitious roots at the cut base of the stem.
seeds also synthesize auxin, which promotes the growth of fruit in many plants.
Used as a
by accident by Johannes van Overbeek using coconut milk on developing embryos.
because they stimulate cytokinesis or cell division.
in actively growth tissues in roots embryos and fruits.
target tissues by moving up the plant by the xylem sap.
cell division and influence the pathway of differentiation.
of Cytokinin to auxin controls the differentiation of cells.
Cytokinin than auxin, shoot buds appear.
auxin that Cytokinin, roots form.
transported from the shoot restrains axillary buds from growing causing the
shoot to lengthen.
entering the shoot system from the roots counter the action by signaling the
axillary buds to grow.
become more extensive, the increased level of cytokinins would signal the shoot
system to form more branches.
retard aging of some plant organs by inhibiting protein breakdowns, by
stimulating RNA and protein synthesis and by mobilizing nutrients from
Kurosawa discovered that the disease of rice was caused by a fungus and then
later determined that the hyperelongation of rice stems came from a chemical
young leaves are major sites of production.
growth in both the leaves and stem but have little effect on root growth.
cell elongation and cell division.
and auxin must be acting simultaneously.
in fruit growth.
plants both gibberellins and auxins must be present.
have a high concentration of gibberellins from the embryo.
water is imbibed, the release of gibberellins from the embryo signals the seeds
to break dormancy and germinate.
function to break dormancy in the resumption of growth by apical buds in spring.
in the terminal bud.
growth and directs leaf primordial to develop into the scales that will protect
the dormant buds during winter.
cell division of the vascular cambium.
both primary and secondary growth.
as a stress hormone to help the plant cope with adverse conditions.
discovered by ripening fruit in the presence of kerosene heaters.
produces ethylene which is a gaseous by-product.
It is a
gas that moves through the plants in the air spaces between cells.
move in the cytosol traveling from cell to cell through the symplast and in the
with a variety of aging processes in plants.
senescence is a progression of irreversible change that eventually leads to
at cell level, organ level or whole plant levels.
the degradation of cell walls which softens the fruit and decreases the
chlorophyll content causing the fruiting to ripen.
triggers the aging process, the aging cells then release more ethylene.
to ripen even spreads from fruit to fruit.
air prevents ethylene from accumulating and carbon dioxide inhibits the action
of whatever has not been flushed away.
of leaves is controlled by the ethylene and auxin levels.
is the loss of leaves at the base of the petiole.
cells of this layer are very thin. And enzymes hydrolyze the polysaccharides
which causes the leaves to fall.
leaf produces less and less auxin.
Tropisms orient the growth of plant organs toward or away from stimuli
are growth responses that result in curvatures of whole plant organs toward or
away from stimuli.
mechanism is differential rate of elongation of cells opposite sides of the
stimuli are used to induce and are gravity, light, and touch.
from auxin stimulating cell elongation on the darker side of the stem or some
other chemical messenger inhibiting elongation on the lighter side.
is the site of photoreception that triggers the growth response and are made of
pigment molecules that are most sensitive to blue light.
These are probably yell and related to riboflavin.
curve downward in response to gravity.
display positive gravitropism and shoots display negative gravitropism.
statoliths which are specialized plastids containing dense starch grains and
moving them to the low points of the cells.
they are located in certain cells of the root cap.
organs usually grow straight until they touch something and the contact
stimulates a coiling response caused by differential growth of cells on opposite
sides of the tendril.
developmental response to mechanical perturbation is called thigmomorphogenesis
and usually results from an increased production of ethylene in response to
chronic mechanical stimulation like wind.
Turgor movements are relatively rapid, reversible plant responses
from a rapid loss of turgor by cells within pulvini which are specialized motor
organs located at the joints of the leaf.
cells suddenly become flaccid after stimulation because the lose potassium which
causes water to leave the cells by osmosis.
plant conserve water.
point of stimulation, the message that produces this response travels wavelike
through the plant at a speed of about a centimeter per second.
messengers probably have a role in transmission but an electrical impulse can
also be detected and is called action potentials.
plants lower their leaves in the evening and raise them to a horizontal position
in the morning.
daily changes in the turgor pressure of motor cells in pulvini.
leaves are horizontal the cells on side of the pulvinus are turgid and those on
the other side are flaccid.
the opposing changes in volume is massive migration of potassium from one side
Photoperiodism synchronizes many plant responses to changes of season.
response to day length such as flowering is photoperiodism
control of flowering
Garner and Allard
if the plants were kept in light-tight boxes so that lamps could manipulate the
duration of light and dark, flowering would occur at other times.
plants require a light period shorter than a critical length to flower.
include mums, poinsettias, and soybeans.
plants tend to flower in late spring or early summer when the light period is
plants flower when they reach the correct stage of maturity regardless of day
length at the time.
discovered in the 1940ís that is was the length of night that controlled these
plants and not the length of day.
a pretreatment to cold required before they will flower.
This is called vernalization.
produce flowers but leaves detect the photoperiod.