14.1 Plant Tropisms and Hormonal Control

Plants depend on their immediate environment
for the materials and energy they require for
Plants are relatively tolerant of environmental
changes from which they cannot escape.
Plant growth and reproduction are
synchronised with seasonal changes, and with
local physical and climate conditions.
The environment provides the cues for many
stages in plant growth (flowering, ripening of
fruit and seed germination) to occur when
conditions are ideal.
Plants also respond to gravity, light and
Communication between cells in the plant is
required so plant responses are coordinated
and controlled.
Plants control and coordinate their response
using hormones.
A hormone is a chemical produced by a
specific cell(s) in an organism and move
throughout to stimulate other cells capable of
producing the desired response.
Hormones act as messengers between cells to
regulate function.
They control various metabolic functions
including rates of rxn’s, transport of substances
across membranes, secretion, and growth.
They transmit their signal to specific cells by
altering specific biochemical reactions in the
cell (ie. Production of an enzyme, turning on or
off a gene).
The target cell has a specific receptor for that
hormone which causes it to be affected.
Hormones work by interacting with a receptor
on the cell membrane or by passing through
the membrane directly into the cell to trigger a
biochemical event.
Hormones are effective even in low
Although hormones may pass through the entire
system, only those cells with the specific receptor are
able to respond to the hormone.
Few hormones affect the entire system, most target a
specific organ(s) and even only a specific type of cell
within that organ.
Plants have fewer hormones than animals, and often
their hormones are simpler and affect the entire
This is because plants have no system for transport like
the endocrine system. Plants have no glands and their
hormones are often produced as a direct result of
environmental stimuli (light, temperature etc.).
Hormonal effects are slower than nervous system
responses but often last longer.
Plant hormones move much slower as they have to
be transported a fair distance to where they take
effect in stimulating things such as flowering or
causing stems to bend.
Hormones are often transported using xylem and
phloem, from cell to cell and occasionally by air.
Movement of a plant hormone requires energy
(ATP molecules) for active transport which is ~10x
faster than by diffusion but still very slow.
In flowering plants hormones orientate the growth
of roots, stems and leaves and the timing of
reproduction (flowering), fruit ripening and seed
A growth response is triggered by environmental
factors. When the direction of growth response is
related to the direction of the stimuli’s origin the
response is called a Tropism.
If a plant grows towards a stimulus it’s called
positive tropism, growth away from the stimulus
is a negative tropism.
Pg 265 Hormone responsibilities in plants.
Plants often grow and bend towards a source
of light (positive phototropism).
This growth is stimulated by a chemical called
Auxin diffusing downward from the tip of a
growing grass shoot.
Auxin is produced continually in a growing tip
(meristem) and diffuses through layers of cells
as the tip has no vascular tissue.
Light interacts with the receptors that control
membrane permeability to auxin, thus the
auxin moves laterally away from the light.
This causes higher concentrations of auxin on
the side opposite the light source, which then
grows faster and causes the plant tip to literally
bend towards the light.
Auxins promote growth by working with
receptors in the specific growth region causing
a softening of cell walls allowing the cells to
elongate more rapidly under turgor pressure
(high internal fluid pressure), cells outside the
growing region appear to lack these receptors.
Auxins are also involved in growth in response
to the forces of gravity.
Geotropism is the response to gravity. Auxins
actually cause negative geotropism (cause the
plant to grow the opposite direction of gravity).
Auxin concentrations are always lower at the
base causing the plant to grow upwards.
Auxins also affect lateral growth in plants.
Auxin is produced in the apical tip moves
down the stem and inhibits the growth of
lateral buds. This is a phenomenon called
apical dominance.
This leads to a taller plant with fewer side
Promote cell elongation, like auxins, except
they promote growth in the entire plant.
Gibberellins are also an important factor in
promoting cell division, flowering in some
plants, fruit enlargement and seed germination.
Gibberellins are synthesized in flowers,
developing fruits, seeds and actively growing
buds, and elongating stems.
In the presence of auxin, cytokinins stimunlate
cell division and cell differentiation in plants.
The ration between auxin and cytokinins
determines the path of differentiation of new
Stems and leaves develop when there is more
cytokinins, and roots develop when there is
more auxin.
Cytokinins are involved in many other
functions in plants, but cell division and
differentiation is the primary function.
These are the best known growth-inhibiting
hormones. They have the opposite effect of
auxins, gibberellins and cytokinins and are
particularly important in regulation of plant
They help plants to tolerate adverse conditions
(drought, salinity, and low temps) by
promoting leaf drop, bud and seed dormancy
and increasing frost resistance.
Synthesized mainly in chloroplasts.
The dropping of ripe fruit, and unfertilized
flowers, and leaves in deciduous plants is known
as abscission and occurs as a result of the
disintegration of a special layers of cells at the base
of the organ being dropped due to ABA.
Development of seed dormancy and vernalisation
requires the presence of ABA to cause gene
expression in nuclei.
ABA also carries a message when too much water
is being lost to close guard cells. ABA is increased
particularly in times of drought, high temperature
and waterlogging.
The gas ethylene or ethene (C2H4) is a small
molecule, which is released by ripening fruit.
One of the effects is the stimulation of fruit
ripening. Ripening of fruit is accompanied by
colour change and softening of the flesh.
These changes help attract animals which disperse
the fruit and make it easy for seeds to be released
from the fruit.
Ethylene increases cellular respiration, breaking
down of starches and oils into sugars.
Ethylene production is stimulated by auxins and

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