| The Sensitive Plant |
The Sense of Touch
Climbing plants with entwining tendrils, like bryony (Bryonia) have touch sensors on their tendrils. The model above, and
below, shows a group of epidermal cells from the tendril of Bryonia dioica (red bryony). Each epidermal cell has one to
three hemispherical domes protruding from its outer surface. These domes contain cytoplasmic projections of the cell
and the cell membrane is especially thin, giving the dome flexibility to deform when pressed. The sensors respond most
strongly, however, to being pushed from the side, as the tendrils respond to rubbing against rough surfaces (as would
happen when the wind blows once they contact a surface). The sensors on one side of the tendril (the ventral or lower
surface) inhibit tendril coiling, those on the other side trigger coiling. The tendrils will therefore coil in one direction once
they contact a suitable support, wrapping around the object to support the plant.
Climbing plants with entwining tendrils, like bryony (Bryonia) have touch sensors on their tendrils. The model above, and
below, shows a group of epidermal cells from the tendril of Bryonia dioica (red bryony). Each epidermal cell has one to
three hemispherical domes protruding from its outer surface. These domes contain cytoplasmic projections of the cell
and the cell membrane is especially thin, giving the dome flexibility to deform when pressed. The sensors respond most
strongly, however, to being pushed from the side, as the tendrils respond to rubbing against rough surfaces (as would
happen when the wind blows once they contact a surface). The sensors on one side of the tendril (the ventral or lower
surface) inhibit tendril coiling, those on the other side trigger coiling. The tendrils will therefore coil in one direction once
they contact a suitable support, wrapping around the object to support the plant.
Functional anatomy of the mechanoreceptor cells in tendrils of Bryonia dioica Jacq. Engelberth et al.
1995. Planta 196: 539-550.
The tendrils of Passiflora caerulea, D.T. Mac Dougal, Botanical Gazette, 1893, 18: 123-130.
Plant signalling: the inexorable rise of auxin. A.J. Fleming, 2006. Trends in Cell Biology 16: 397-402.
Mechanisms of control of leaf movements, Satter & Galston, 1981. Ann. Rev. Plant Physiol 32: 83-110.
Reaction wood and the regulation of tree form. Sinnott, 1952. Am. J. Bot. 39: 69-78.
Plant Physiology. Salisbury and Ross.
1995. Planta 196: 539-550.
The tendrils of Passiflora caerulea, D.T. Mac Dougal, Botanical Gazette, 1893, 18: 123-130.
Plant signalling: the inexorable rise of auxin. A.J. Fleming, 2006. Trends in Cell Biology 16: 397-402.
Mechanisms of control of leaf movements, Satter & Galston, 1981. Ann. Rev. Plant Physiol 32: 83-110.
Reaction wood and the regulation of tree form. Sinnott, 1952. Am. J. Bot. 39: 69-78.
Plant Physiology. Salisbury and Ross.
The diagram below is a 3D representation of the model of these tactile bleps as elucidated by Engelberth et al. Inside
the bleb or dome is a pocket of protoplasm, continuous with the protoplast of the cell through a cytoplasmic canal - a
knob-like projection of the cell, lined by the continuous cell-surface membrane. Inside the cytoplasm of this pocket is a
ring of membrane-bound vesicles or sacs that store calcium ions (calcium store) and beneath this is a cytoskeletal ring.
Surrounding the neck of the blp, where it passes into the cell wall, is a ring of cellulose microfibrils and around the
circumference of the bleb is a ring of callose.
the bleb or dome is a pocket of protoplasm, continuous with the protoplast of the cell through a cytoplasmic canal - a
knob-like projection of the cell, lined by the continuous cell-surface membrane. Inside the cytoplasm of this pocket is a
ring of membrane-bound vesicles or sacs that store calcium ions (calcium store) and beneath this is a cytoskeletal ring.
Surrounding the neck of the blp, where it passes into the cell wall, is a ring of cellulose microfibrils and around the
circumference of the bleb is a ring of callose.
Mechanism
The following account gives a likely model of the mode of action of these sensors, as suported by experimental evidence.
Forces acting on the blep deform the thin cell wall of the protrusion. In particular, the sensor is much more sensitive to
side-on (lateral) forces (large orange arrow) than to those acting directly from above (smaller yellow arrow). These
lateral forces will deform the callose ring (which will spring-back into shape when the force is removed) which in turn
pushes against the cell-surface membrane lining the blep cavity, stretching and deforming it. Note that the callose ring
may act as an amplifier - focusing the force onto a small region of the membrane. When the membrane is deformed,
electrical signals due to the flow of positively charged calcium ions initiates electrical waves within the cell, beginning in
the blep. Most of this calcium comes from a ring of membranous organelles that store calcium inside the cytoplasm of
the blep. There is a ring of cytoskeletal proteins beneath this calcium-storage region.
There are several possibilities: stretching the membrane may open ion channels (e.g. calcium channels) inside the cell
membrane, allowing some calcium to enter the cell (from the cell wall spaces) making the inside of the cell less negative
(remember cell membranes act as electrical capacitors and store charge, with the inside of the cell negative relative to
the outside in the untriggered state). This calcium signal may then trigger the release of more calcium from the internal
stores, triggering a wave of calcium (these steps also act to amplify the signal). Alternatively, the membrane may pass
its signal to the calcium stores by another mechanism. One such possibility involves the cytoskeletal ring - the
membrane may push on this when it is deformed, triggering some change in this ring, active or passive, which pulls open
calcium channels in the calcium store. Alternatively, the cytoskeletal ring may be resistant to deformation and elastic and
so help to restore the shape of the blep to its resting state.
Once the electrical signal occurs throughout the cell, it must be passed onto others, especially if the signal exceeds a
certain threshold. This could occur via the flow of calcium through the plasmodesmata that connect the sensory cell to
neighbouring cells. Alternatively, or in addition, there may be a flow of electric current through the mesh-like cell wall. A
signal could also be passed from cell-to-cell by activating the cytoskeleton network in the cell. There are connections
between the cytoskeletal ring of each blep and the mesh of cytoskeletal filaments beneath the cell-surface membrane
and some signal may be passed along these (possibly a physical signal, according to tensegrity theory).
Slower and longer-lasting signals may also pass between the cells as chemical messengers or hormones - an
octadecanoid molecule is known to act as such a signal.
After several seconds of suitable stimulation, the tendril begins to coil. Initially, the coiling is probably driven by osmotic
changes - water moves from one side of the tendril to the other, leaving cells on one side plasmolysed and flaccid, whilst
cells on the other side become stretched and turgid. This creates a force with the side that loses water being pushed
inwards, on the inside of the curve. If the stimulus persists, then the tendril undergoes a second phase of irreversible
coiling. This phase is slower, taking 24-36 hours and probably involves differential growth with cells on the outside of the
curve elongating.
In the passion flower, Passiflora caerulea, the tendrils are sensitive even to cotton thread, but the strength of the
response increases with the force of the stimulus and the roughness of the surface. The tendrils do not respond to
smooth polished surfaces. The tendrils can coil around an object of any size that the tendril is able to reach around and
tendrils may also coil around one another. Prioir to receiving a stimulus, the tendrils make wide sweeping circular
movements, searching for a support. Coiling starts with about 30 seconds of stimulation and the tendril is capable of
seeking out crevices into which it goes, and then coils to form a tight plug. Between the support and the plant, the rest of
the plant which has not managed to wrap around the support, becomes coiled, drawing the shoot in nearer to its
support, and giving the tendril the elasticity of a spring, so that it can resist buffeting. Each tendril can coil with weight up
to about 20 g attached to it, though the usual operational load rarely exceeds 1 g. After coiling, the tendril matures,
thickening and strengthening. It takes about 350 to 750 g of load to break a single mature tendril, and with many tendrils
grappling a support, these vines are well able to resist wind forces.
The following account gives a likely model of the mode of action of these sensors, as suported by experimental evidence.
Forces acting on the blep deform the thin cell wall of the protrusion. In particular, the sensor is much more sensitive to
side-on (lateral) forces (large orange arrow) than to those acting directly from above (smaller yellow arrow). These
lateral forces will deform the callose ring (which will spring-back into shape when the force is removed) which in turn
pushes against the cell-surface membrane lining the blep cavity, stretching and deforming it. Note that the callose ring
may act as an amplifier - focusing the force onto a small region of the membrane. When the membrane is deformed,
electrical signals due to the flow of positively charged calcium ions initiates electrical waves within the cell, beginning in
the blep. Most of this calcium comes from a ring of membranous organelles that store calcium inside the cytoplasm of
the blep. There is a ring of cytoskeletal proteins beneath this calcium-storage region.
There are several possibilities: stretching the membrane may open ion channels (e.g. calcium channels) inside the cell
membrane, allowing some calcium to enter the cell (from the cell wall spaces) making the inside of the cell less negative
(remember cell membranes act as electrical capacitors and store charge, with the inside of the cell negative relative to
the outside in the untriggered state). This calcium signal may then trigger the release of more calcium from the internal
stores, triggering a wave of calcium (these steps also act to amplify the signal). Alternatively, the membrane may pass
its signal to the calcium stores by another mechanism. One such possibility involves the cytoskeletal ring - the
membrane may push on this when it is deformed, triggering some change in this ring, active or passive, which pulls open
calcium channels in the calcium store. Alternatively, the cytoskeletal ring may be resistant to deformation and elastic and
so help to restore the shape of the blep to its resting state.
Once the electrical signal occurs throughout the cell, it must be passed onto others, especially if the signal exceeds a
certain threshold. This could occur via the flow of calcium through the plasmodesmata that connect the sensory cell to
neighbouring cells. Alternatively, or in addition, there may be a flow of electric current through the mesh-like cell wall. A
signal could also be passed from cell-to-cell by activating the cytoskeleton network in the cell. There are connections
between the cytoskeletal ring of each blep and the mesh of cytoskeletal filaments beneath the cell-surface membrane
and some signal may be passed along these (possibly a physical signal, according to tensegrity theory).
Slower and longer-lasting signals may also pass between the cells as chemical messengers or hormones - an
octadecanoid molecule is known to act as such a signal.
After several seconds of suitable stimulation, the tendril begins to coil. Initially, the coiling is probably driven by osmotic
changes - water moves from one side of the tendril to the other, leaving cells on one side plasmolysed and flaccid, whilst
cells on the other side become stretched and turgid. This creates a force with the side that loses water being pushed
inwards, on the inside of the curve. If the stimulus persists, then the tendril undergoes a second phase of irreversible
coiling. This phase is slower, taking 24-36 hours and probably involves differential growth with cells on the outside of the
curve elongating.
In the passion flower, Passiflora caerulea, the tendrils are sensitive even to cotton thread, but the strength of the
response increases with the force of the stimulus and the roughness of the surface. The tendrils do not respond to
smooth polished surfaces. The tendrils can coil around an object of any size that the tendril is able to reach around and
tendrils may also coil around one another. Prioir to receiving a stimulus, the tendrils make wide sweeping circular
movements, searching for a support. Coiling starts with about 30 seconds of stimulation and the tendril is capable of
seeking out crevices into which it goes, and then coils to form a tight plug. Between the support and the plant, the rest of
the plant which has not managed to wrap around the support, becomes coiled, drawing the shoot in nearer to its
support, and giving the tendril the elasticity of a spring, so that it can resist buffeting. Each tendril can coil with weight up
to about 20 g attached to it, though the usual operational load rarely exceeds 1 g. After coiling, the tendril matures,
thickening and strengthening. It takes about 350 to 750 g of load to break a single mature tendril, and with many tendrils
grappling a support, these vines are well able to resist wind forces.
Gravitropism
Among the many signals plants can detect is gravity. Shoots and roots can sense the direction of the gravitational field,
with roots growing down towards it (positive gravitropism) and shoots growing upwards away from it (negative
gravitropism). The photograph below shows gravitropism in conifers growing on a slope - the roots have grown
donwards, growing down around obstacles like boulders along the way.
Among the many signals plants can detect is gravity. Shoots and roots can sense the direction of the gravitational field,
with roots growing down towards it (positive gravitropism) and shoots growing upwards away from it (negative
gravitropism). The photograph below shows gravitropism in conifers growing on a slope - the roots have grown
donwards, growing down around obstacles like boulders along the way.
How do roots sense gravity? The sensors appear to be cells located in the root cap of growing root-tips. These cells
contain heavy starch grains which move under gravity, for example if a root is dislodged to become horizontal, these
sensors will detect this change in orientation. A chemical signal, or plant hormone (phytohormone) called auxin plays a
key role here. Auxin is produced in the growing tips of shoots, in the tip meristems or shoot apical meristems (SAMs) (a
meristem is a region of cell division in a plant, where new cells are made). This auxin is carried down the shoot and into
the roots, to the root tip, through the phloem.
A growing root consists of several zones. Behind the root cap, which shields the root and produces mucus to assist its
movement through the soil, is the root apical meristem (RAM) where mitosis gives rise to new cells. Behind this is the
zone is elongation, where the new cells expand and elongate, and then behind this is the zone of differentiation, where
the cells develop or differentiate into the different cell types that make up the mature root. The phloem develops in this
zone of differentiation, however auxin can move outside the phloem, across the tissues from cell-to-cell in specifically
controlled directions, by a process called polar auxin transport. This allows the auxin to reach the RAM or root-tip
meristem. From here, polar transport carries the auxin back along the rows of new cells, up the root. High concentrations
of auxin inhibit root elongation. In a vertical root, all sides have an equally low auxin concentration and all elongate
equally.
In a vertical root the auxin descending from the shoots will be uniformly distributed on all side of the root and the root will
grow straight down. This is illustrated below:
contain heavy starch grains which move under gravity, for example if a root is dislodged to become horizontal, these
sensors will detect this change in orientation. A chemical signal, or plant hormone (phytohormone) called auxin plays a
key role here. Auxin is produced in the growing tips of shoots, in the tip meristems or shoot apical meristems (SAMs) (a
meristem is a region of cell division in a plant, where new cells are made). This auxin is carried down the shoot and into
the roots, to the root tip, through the phloem.
A growing root consists of several zones. Behind the root cap, which shields the root and produces mucus to assist its
movement through the soil, is the root apical meristem (RAM) where mitosis gives rise to new cells. Behind this is the
zone is elongation, where the new cells expand and elongate, and then behind this is the zone of differentiation, where
the cells develop or differentiate into the different cell types that make up the mature root. The phloem develops in this
zone of differentiation, however auxin can move outside the phloem, across the tissues from cell-to-cell in specifically
controlled directions, by a process called polar auxin transport. This allows the auxin to reach the RAM or root-tip
meristem. From here, polar transport carries the auxin back along the rows of new cells, up the root. High concentrations
of auxin inhibit root elongation. In a vertical root, all sides have an equally low auxin concentration and all elongate
equally.
In a vertical root the auxin descending from the shoots will be uniformly distributed on all side of the root and the root will
grow straight down. This is illustrated below:
Neuroid System
Venus's fly-trap, Dionaea muscipula, is an obvious example of a rapid movement in plants. The trap consists
of two lobes or valves edged with spines and is a modified leaf. On the inside of each lobe are three sensory
trigger hairs, arranged in a triangle. Should an insect wonder into the trap and touch one of these trigger hairs
once - nothing apparently happens, but if it touches the same hair repeatedly, or several different hairs in short
succession, then the trap springs shut in about 0.3 seconds. The hairs interlock, trapping all but the largest
insects inside. The trap then secretes mucus to entangle the insect and seals tightly shut. Enzymes are
released, the insect is digested, and then the trap reopens, ready to operate again. Touching one of the
sensory hairs generates electrical currents within the trap. These currents are summed over space and time
(which requires the plant tissues to have an electrical memory), and repeated stimulation of the hairs causes
greater currents to flow, until, if a threshold is reached a rapid electrical pulse (action potential) travels across
the trap, causing it to close suddenly. This summing of the stimuli reduces the likelihood of a false alarm - it
would do no good for the trap to close every time a rain drop lands on it, but a living, moving insect is another
matter!
Electrical (electrochemical) signals, very similar to the electrical signals in animal nervous systems, are
actually quite common in plants and may indeed occur in all plants. As in animals these signals or pulses,
called action potentials, are used by cells that are some distance apart in the body to communicate rapidly.
These signals are created when localised parts of the cell membrane release stored electrical charge,
resulting in current flow. The cell membrane acts as a capacitor in a resting cell, storing electric charge. A
suitable stimulus may cause ion channels to open in the membrane, freeing the stored charge to flow across
the membrane. This flowing current may then trigger more ion channels in adjacent areas of the membrane to
open, causing a cascade or domino-effect as the signal pulse travels across the cell membrane. This signal
must then be relayed to adjacent cells and the plasmodesmata may serve this function - ions carrying the
electric current can move from cell to cell through the plasmodesmata channels that join neighbouring cells
together.
This kind of electrical junction is known as an electrical synapse in animal nervous systems and allows
electrical pulses to travel in both directions across it. Similar gap junctions also carry electrical signals
between non-nervous cells in animals, such as epidermal cells and cardiac cells. Animal nervous systems,
however, also have chemical synapses, which allow signals to travel in one direction only across the synapse,
which is vital for complex processing. Animal nervous systems also have elongated specialised cells called
neurones, acting like electrical wires, to carry signals from one place to another at great speed. Thus, we have
two distinct system-types here - a system with elongated cells specialised in signal transduction, or nervous
system, and a more general system in which electrical signals are passed from one cell to its neighbours. The
latter is called a neuroid system, and typically only transmits slowly and over short distances.
Some scientists refer to the plant system as a neuroid system, others as a nervous system. By and large the
plant system functions as a neuroid system, as signals are passed from one cell to its immediate neighbours
and the signals in the fly trap, for example, are local. Sponges are animals that lack a nervous system, but
these too have a neuroid system. However, in some plants action potentials can travel far and do so by
traveling in the phloem vessels. Phloem vessels, which transport sugary sap around the plant, have living
protoplasts (unlike xylem) and so can carry these signals. In vascular bundles, which contain phloem vessels,
the bundle sheath may provide electrical insulation - confining the signals to the vessels, much as animal
nerves are insulated. In this case, the plant system is a little more than a neuroid system, and is much more
like a nervous system proper, even though the phloem performs other functions and is not dedicated to signal
transduction in the way that nerve cells in animals are.
Many plants produce action potentials. More-or-less as dramatic as the fly trap is the sensitive plant, Mimosa.
Mimosa will fold its leaflets rapidly when touched (to deter predators, possibly causing them to fall off the leaf if
they are insects) and if the signal is strong enough it will spread further, causing one or more leaves to droop.
However, many less obvious cases are often overlooked. Even the humble tomato plant will send action
potentials around its body when it is touched! Many plants (if not all) will also generate action potentials when
damaged, for example by a grazer or by fire or cold. These signals can trigger other parts of the plant to
prepare its defenses (e.g. by producing toxins or taking measures to prevent water loss by closing stomata,
etc.).
Venus's fly-trap, Dionaea muscipula, is an obvious example of a rapid movement in plants. The trap consists
of two lobes or valves edged with spines and is a modified leaf. On the inside of each lobe are three sensory
trigger hairs, arranged in a triangle. Should an insect wonder into the trap and touch one of these trigger hairs
once - nothing apparently happens, but if it touches the same hair repeatedly, or several different hairs in short
succession, then the trap springs shut in about 0.3 seconds. The hairs interlock, trapping all but the largest
insects inside. The trap then secretes mucus to entangle the insect and seals tightly shut. Enzymes are
released, the insect is digested, and then the trap reopens, ready to operate again. Touching one of the
sensory hairs generates electrical currents within the trap. These currents are summed over space and time
(which requires the plant tissues to have an electrical memory), and repeated stimulation of the hairs causes
greater currents to flow, until, if a threshold is reached a rapid electrical pulse (action potential) travels across
the trap, causing it to close suddenly. This summing of the stimuli reduces the likelihood of a false alarm - it
would do no good for the trap to close every time a rain drop lands on it, but a living, moving insect is another
matter!
Electrical (electrochemical) signals, very similar to the electrical signals in animal nervous systems, are
actually quite common in plants and may indeed occur in all plants. As in animals these signals or pulses,
called action potentials, are used by cells that are some distance apart in the body to communicate rapidly.
These signals are created when localised parts of the cell membrane release stored electrical charge,
resulting in current flow. The cell membrane acts as a capacitor in a resting cell, storing electric charge. A
suitable stimulus may cause ion channels to open in the membrane, freeing the stored charge to flow across
the membrane. This flowing current may then trigger more ion channels in adjacent areas of the membrane to
open, causing a cascade or domino-effect as the signal pulse travels across the cell membrane. This signal
must then be relayed to adjacent cells and the plasmodesmata may serve this function - ions carrying the
electric current can move from cell to cell through the plasmodesmata channels that join neighbouring cells
together.
This kind of electrical junction is known as an electrical synapse in animal nervous systems and allows
electrical pulses to travel in both directions across it. Similar gap junctions also carry electrical signals
between non-nervous cells in animals, such as epidermal cells and cardiac cells. Animal nervous systems,
however, also have chemical synapses, which allow signals to travel in one direction only across the synapse,
which is vital for complex processing. Animal nervous systems also have elongated specialised cells called
neurones, acting like electrical wires, to carry signals from one place to another at great speed. Thus, we have
two distinct system-types here - a system with elongated cells specialised in signal transduction, or nervous
system, and a more general system in which electrical signals are passed from one cell to its neighbours. The
latter is called a neuroid system, and typically only transmits slowly and over short distances.
Some scientists refer to the plant system as a neuroid system, others as a nervous system. By and large the
plant system functions as a neuroid system, as signals are passed from one cell to its immediate neighbours
and the signals in the fly trap, for example, are local. Sponges are animals that lack a nervous system, but
these too have a neuroid system. However, in some plants action potentials can travel far and do so by
traveling in the phloem vessels. Phloem vessels, which transport sugary sap around the plant, have living
protoplasts (unlike xylem) and so can carry these signals. In vascular bundles, which contain phloem vessels,
the bundle sheath may provide electrical insulation - confining the signals to the vessels, much as animal
nerves are insulated. In this case, the plant system is a little more than a neuroid system, and is much more
like a nervous system proper, even though the phloem performs other functions and is not dedicated to signal
transduction in the way that nerve cells in animals are.
Many plants produce action potentials. More-or-less as dramatic as the fly trap is the sensitive plant, Mimosa.
Mimosa will fold its leaflets rapidly when touched (to deter predators, possibly causing them to fall off the leaf if
they are insects) and if the signal is strong enough it will spread further, causing one or more leaves to droop.
However, many less obvious cases are often overlooked. Even the humble tomato plant will send action
potentials around its body when it is touched! Many plants (if not all) will also generate action potentials when
damaged, for example by a grazer or by fire or cold. These signals can trigger other parts of the plant to
prepare its defenses (e.g. by producing toxins or taking measures to prevent water loss by closing stomata,
etc.).
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Nyctinastic Movements
These are literally 'sleep movements' and are movements related to rhythmic changes in day and night conditions.
Thermonasties are responses to diurnal temperature changes, whereas photonasties are responses to diurnal light
changes.
Many flowers will only open at certain times of day and then close again. This ensures that they expose their parts when
pollinating insects are at their busiest, but close to protect their parts (and to conserve aromatics and nectar) when
pollinating insects are inactive. Flowers that rely on moths for pollination will open at night-time. In the crocus and tulip,
the sepals are like petals and form part of the flower, in such a case the term 'tepals' is used to refer to both the petals
and sepals. In warm temperatures and in high light intensities, the upper surface at the base of the tepal expands faster
than the lower surface and the flower opens. A reversal in these growth rates close the flowers at night. These
movements are actual growth movements - the tepals are growing a little bit each day. The optimum growth temperature
of the upper tepal tissues is 10-17 degrees C, that of the lower tepal tissues, 3-7 degrees. Surprisingly, this creates a
very sensitive system and a change in temperature of just 0.2 to 1 C can be enough to trigger flower opening/closing.
Leaf Movements
Leaves are positively phototropic and will position themselves so as to intercept the maximum amount of light. If one leaf
become shaded by other leaves, then it will grow out from the shade toward the light. It is the leaf-stalk or petiole that
responds and grows in this way. This enables leaves to avoid overlapping one another and leaves tend to form instead a
leaf mosaic - a canopy of 'interlocking' leaves.
The leaves of many plants also move so as to track the sun a sit moves across the sky, keeping themselves optimally
illuminated. This movement, unlike phototropism in shoots and petioles, is not a growth movement and is truly reversible.
It is caused by water moving in and out of cells in the pulvinus (the swelling at the base of the petiole which forms a joint).
If the cells on one side of the pulvinus swell with water, then these cells will exert a pushing force, whilst at the same time
cells on the opposite side lose water and become soft and flaccid and so stop pushing. In this way the pulvinus can push
the leaf one way or another. This movement of water is apparently driven by the movement of potassium ions. If
potassium exits a cell, then this will lower the water potential outside the cell, causing water to leave the cell, following the
potassium by osmosis (as water will move from a region of higher water potential to a region of lower water potential by
osmosis - see transport in plants).
This solar tracking undertaken by leaves is also called heliotropism.
Many leaves undergo nictinastic movements - opening out during the day and closing at night. Extensor cells in the
cortex on one side of the pulvinus open the leaves by swelling with water and becoming turgid and stiff to create a
pushing force, whilst flexor cells in the cortex on the opposite side of the cortex lose water and become soft and flaccid.
Leaf closing is brought about when the flexor cells become swollen and turgid and the extensor cells falccid. These
movements are not growth movements (unlike the opening and closing of Venus' fly-trap) and are perfectly reversible. In
some plants, e.g. Mimosa, Abizzia, the leaves fold upwards when closing at night and the extensor cells are in the
uppermost part of the pulvinus. In plants like Phaseolus and Samanea, the leaves fold downwards when closing and the
extensor cells are in the lower part of the pulvinus. The structure of a typical pulvinus (as seen in transverse section) is
shown below:
These are literally 'sleep movements' and are movements related to rhythmic changes in day and night conditions.
Thermonasties are responses to diurnal temperature changes, whereas photonasties are responses to diurnal light
changes.
Many flowers will only open at certain times of day and then close again. This ensures that they expose their parts when
pollinating insects are at their busiest, but close to protect their parts (and to conserve aromatics and nectar) when
pollinating insects are inactive. Flowers that rely on moths for pollination will open at night-time. In the crocus and tulip,
the sepals are like petals and form part of the flower, in such a case the term 'tepals' is used to refer to both the petals
and sepals. In warm temperatures and in high light intensities, the upper surface at the base of the tepal expands faster
than the lower surface and the flower opens. A reversal in these growth rates close the flowers at night. These
movements are actual growth movements - the tepals are growing a little bit each day. The optimum growth temperature
of the upper tepal tissues is 10-17 degrees C, that of the lower tepal tissues, 3-7 degrees. Surprisingly, this creates a
very sensitive system and a change in temperature of just 0.2 to 1 C can be enough to trigger flower opening/closing.
Leaf Movements
Leaves are positively phototropic and will position themselves so as to intercept the maximum amount of light. If one leaf
become shaded by other leaves, then it will grow out from the shade toward the light. It is the leaf-stalk or petiole that
responds and grows in this way. This enables leaves to avoid overlapping one another and leaves tend to form instead a
leaf mosaic - a canopy of 'interlocking' leaves.
The leaves of many plants also move so as to track the sun a sit moves across the sky, keeping themselves optimally
illuminated. This movement, unlike phototropism in shoots and petioles, is not a growth movement and is truly reversible.
It is caused by water moving in and out of cells in the pulvinus (the swelling at the base of the petiole which forms a joint).
If the cells on one side of the pulvinus swell with water, then these cells will exert a pushing force, whilst at the same time
cells on the opposite side lose water and become soft and flaccid and so stop pushing. In this way the pulvinus can push
the leaf one way or another. This movement of water is apparently driven by the movement of potassium ions. If
potassium exits a cell, then this will lower the water potential outside the cell, causing water to leave the cell, following the
potassium by osmosis (as water will move from a region of higher water potential to a region of lower water potential by
osmosis - see transport in plants).
This solar tracking undertaken by leaves is also called heliotropism.
Many leaves undergo nictinastic movements - opening out during the day and closing at night. Extensor cells in the
cortex on one side of the pulvinus open the leaves by swelling with water and becoming turgid and stiff to create a
pushing force, whilst flexor cells in the cortex on the opposite side of the cortex lose water and become soft and flaccid.
Leaf closing is brought about when the flexor cells become swollen and turgid and the extensor cells falccid. These
movements are not growth movements (unlike the opening and closing of Venus' fly-trap) and are perfectly reversible. In
some plants, e.g. Mimosa, Abizzia, the leaves fold upwards when closing at night and the extensor cells are in the
uppermost part of the pulvinus. In plants like Phaseolus and Samanea, the leaves fold downwards when closing and the
extensor cells are in the lower part of the pulvinus. The structure of a typical pulvinus (as seen in transverse section) is
shown below:
Above: auxin movement in a vertical root (blue arrows) leads to a uniform distribution of auxin
and the root grows equally on all sides and remains straight - growing straight down.
and the root grows equally on all sides and remains straight - growing straight down.
However, in a root which is horizontal, the gravity sensors in the root cap send signals to the root to alter the transport
of auxin back along the root from the RAM by polar transport - it transports more auxin back along the lower side of the
root than along the upper side. This will cause high concentrations of auxin to accumulate in the lower half of the root,
and causing the underside to elongate less than the upper side, since the high auxin concentration here inhibits its
elongation. The result is that the root curves downwards as the upper surface elongates more. This is illustrated below:
of auxin back along the root from the RAM by polar transport - it transports more auxin back along the lower side of the
root than along the upper side. This will cause high concentrations of auxin to accumulate in the lower half of the root,
and causing the underside to elongate less than the upper side, since the high auxin concentration here inhibits its
elongation. The result is that the root curves downwards as the upper surface elongates more. This is illustrated below:
The diagram below illustrates the current model of polar auxin transport. Two neighbouring cells are illustrated.
Essentially protein pumps, possibly a protein called PIN, which span the cell-surface membrane, export the auxin from
one end of each cell and another type of protein pump (called AUX1) imports it across the cell-surface membrane at
the other end or pole of the cell. This ensures that the net flow of auxin is from the top to the bottom in the diagram.
Additionally, when outside of the cell in the cell-wall matrix, which tends to be acidic, auxin possibly exists largely as a
neutral molecule IAAH (indole acetic acid), but when inside the cell the higher pH causes the auxin to lose a proton (H+
ion) forming an anion, indole acetate or IAA-, which being a large and charged molecule cannot easily cross the
cell-surface membrane. Thus the auxin entering the cell at one end is changed into a form that cannot passively diffuse
back across the membrane and out the way it came. This would help ensure that the flux of auxin continues in one
direction.
Essentially protein pumps, possibly a protein called PIN, which span the cell-surface membrane, export the auxin from
one end of each cell and another type of protein pump (called AUX1) imports it across the cell-surface membrane at
the other end or pole of the cell. This ensures that the net flow of auxin is from the top to the bottom in the diagram.
Additionally, when outside of the cell in the cell-wall matrix, which tends to be acidic, auxin possibly exists largely as a
neutral molecule IAAH (indole acetic acid), but when inside the cell the higher pH causes the auxin to lose a proton (H+
ion) forming an anion, indole acetate or IAA-, which being a large and charged molecule cannot easily cross the
cell-surface membrane. Thus the auxin entering the cell at one end is changed into a form that cannot passively diffuse
back across the membrane and out the way it came. This would help ensure that the flux of auxin continues in one
direction.
Reaction Wood
It is not just roots that respond to gravity. Shoots tend to grow away from the centre of a gravitational field, upwards and
hopefully toward the light. The branches and trunks of trees also respond to gravity. A branch has to maintain its
position against its own weight and also reorient itself should the tree become dislodged and slanted from the vertical.
For example, if a young sapling is dislodged so that it stands at an angle, the branches will sense the change in gravity
pulling on them and alter their position as they continue to grow, as indeed will the stem which will be pulled back into a
vertical position as new wood forms. This is achieved by the formation of compression wood in conifers and tension
wood in angiosperms or broad-leaved hardwood trees (though there is some evidence that angiospserms may also form
a type of compression wood).
A branch which is more-or-less horizontal will tend to grow vertically elongated, giving it an oval cross-section. This
strengthens it against its own weight acting to bend it downwards. A conifer branch will also produce specialised wood
that forms where the weight of the branch would tend to compress the wood on the underside. This is the compression
wood (CW) which is distinctly reddish in a cut log and has shorter tracheids. This wood is designed to offer greater
resistance to the compressive forces and tends to push against the weight of the branch to keep it in position, or to
correct its position as needed. Angiosperms form tension wood (TW) toward the upper surface of the branch, which pulls
against the weight of the branch to achieve the same result.
It is not just roots that respond to gravity. Shoots tend to grow away from the centre of a gravitational field, upwards and
hopefully toward the light. The branches and trunks of trees also respond to gravity. A branch has to maintain its
position against its own weight and also reorient itself should the tree become dislodged and slanted from the vertical.
For example, if a young sapling is dislodged so that it stands at an angle, the branches will sense the change in gravity
pulling on them and alter their position as they continue to grow, as indeed will the stem which will be pulled back into a
vertical position as new wood forms. This is achieved by the formation of compression wood in conifers and tension
wood in angiosperms or broad-leaved hardwood trees (though there is some evidence that angiospserms may also form
a type of compression wood).
A branch which is more-or-less horizontal will tend to grow vertically elongated, giving it an oval cross-section. This
strengthens it against its own weight acting to bend it downwards. A conifer branch will also produce specialised wood
that forms where the weight of the branch would tend to compress the wood on the underside. This is the compression
wood (CW) which is distinctly reddish in a cut log and has shorter tracheids. This wood is designed to offer greater
resistance to the compressive forces and tends to push against the weight of the branch to keep it in position, or to
correct its position as needed. Angiosperms form tension wood (TW) toward the upper surface of the branch, which pulls
against the weight of the branch to achieve the same result.
More mature trees may have too much right to right themselves if dislodged into a large angle. However, in a thick trunk
reaction wood is still important in maintaining shape. It forms wherever a kink or burr in the trunk displaces the stem from
its centre-line of gravity and helps to keeps the trunk from curving too far as it grows. If a tree falls, but remains rooted,
then one of its side-branches that now points upwards will become the new leader shoot, or perhaps the tip of the trunk
will curve upwards as it grows toward the light, perhaps assisted by reaction wood. In this way trees with apparently
horizontal trunk sections may form. It is the combination of a plant's responses to gravity and light that keep it growing as
intended.
reaction wood is still important in maintaining shape. It forms wherever a kink or burr in the trunk displaces the stem from
its centre-line of gravity and helps to keeps the trunk from curving too far as it grows. If a tree falls, but remains rooted,
then one of its side-branches that now points upwards will become the new leader shoot, or perhaps the tip of the trunk
will curve upwards as it grows toward the light, perhaps assisted by reaction wood. In this way trees with apparently
horizontal trunk sections may form. It is the combination of a plant's responses to gravity and light that keep it growing as
intended.
Phototropism in Shoots
A tropism is a growth away or toward a stimulus. Photropism is growth toward or away from a source of light.
Roots tend to grow away from the light - they exhibit negative photropism, whereas shoots are positively
phototactic and grow towards the light.
If a growing shoot is evenly illuminated on all sides, say from a light source directly overhead, then it will grow
straight up toward the light. If, however, the light is brighter on one side than the other then the shoot tip (or the
region of the shoot just behind the tip) will bend toward the light.
A tropism is a growth away or toward a stimulus. Photropism is growth toward or away from a source of light.
Roots tend to grow away from the light - they exhibit negative photropism, whereas shoots are positively
phototactic and grow towards the light.
If a growing shoot is evenly illuminated on all sides, say from a light source directly overhead, then it will grow
straight up toward the light. If, however, the light is brighter on one side than the other then the shoot tip (or the
region of the shoot just behind the tip) will bend toward the light.
The blue arrows indicate the movements of auxin, synthesised in the shoot tip. The Cholodny-Went theory (1937)
proposes that auxin is transported away from the illuminated side of the shoot to the darker side (by lateral transport
across the width of the shoot). Thus, as this auxin moves back along the shoot the darker side receives more auxin.
Similar to roots, shoots grow from a meristem in the shoot-tip, which produces new cells by cell division (mitosis). Behind
the tip is a region of cell elongation, where the new cells elongate, before they mature further down the shoot. The auxin
concentrations found in shoots stimulate cell elongation (in contrast to roots in which a high auxin concentration inhibits
cell elongation) and so cells in the darker side of the shoot elongate more, causing the shoot to bend toward the light.
This theory has been hard to test directly, however, it is known that transport of auxin is necessary for phototropism in
shoots. Phototropism occurs specifically in response to blue light (which is a component of white light and natural yellow
sunlight). The theory is that a pigmented light receptor (called cryptochrome) respond to the blue light signal and then
trigger a cell-signaling cascade which results in a change in the level of phosphorylation of the auxin export pumps
required for polar transport of auxin.
proposes that auxin is transported away from the illuminated side of the shoot to the darker side (by lateral transport
across the width of the shoot). Thus, as this auxin moves back along the shoot the darker side receives more auxin.
Similar to roots, shoots grow from a meristem in the shoot-tip, which produces new cells by cell division (mitosis). Behind
the tip is a region of cell elongation, where the new cells elongate, before they mature further down the shoot. The auxin
concentrations found in shoots stimulate cell elongation (in contrast to roots in which a high auxin concentration inhibits
cell elongation) and so cells in the darker side of the shoot elongate more, causing the shoot to bend toward the light.
This theory has been hard to test directly, however, it is known that transport of auxin is necessary for phototropism in
shoots. Phototropism occurs specifically in response to blue light (which is a component of white light and natural yellow
sunlight). The theory is that a pigmented light receptor (called cryptochrome) respond to the blue light signal and then
trigger a cell-signaling cascade which results in a change in the level of phosphorylation of the auxin export pumps
required for polar transport of auxin.
Thigmomorphogenesis
The response of plants to mechanical stress is called thigmomorphogenesis, which refers to touch affecting the shape
of plants. If a stem is frequently rubbing against another object then the stem responds by elongating more slowly and
thickening more, resulting in a shorter and stockier plant. Trees respond to wind stresses in a similar way - when grown
crowded together in a forest the trees shelter one another from winds and grow taller and narrower as they compete for
light. In contrast, a tree growing alone in an exposed field will grow to be much shorter and thicker. This is designed to
strengthen the stem.
Stomata
Stomata and their responses are considered under transport in plants.
Circadian Rhythms
Plant hormones (plant growth regulators or phytohormones)
Both plants and animals use hormones to coordinate their growth and development and to respond to certain stimuli.
Hormones are chemical signals and are generally slow-acting, and when a fast response is required then electrical
signals must be used (nervous system, Venus flytrap). Hormones are also generally longer-acting and so are good for
signaling slow and longer-term changes to a stimulus. Hormones are chemicals released by cells in one part of the body
and then travel through the body until they reach their target cells, for whom the message is intended. The target cells
then detect and respond to the hormone signal.
Phytohormones are small organic molecules synthesised by the plant and active in very low concentrations and they
promote or inhibit growth and developmental responses to environmental conditions.
Plant hormones and animal hormones compared:
Phytohormone table (pdf).
The response of plants to mechanical stress is called thigmomorphogenesis, which refers to touch affecting the shape
of plants. If a stem is frequently rubbing against another object then the stem responds by elongating more slowly and
thickening more, resulting in a shorter and stockier plant. Trees respond to wind stresses in a similar way - when grown
crowded together in a forest the trees shelter one another from winds and grow taller and narrower as they compete for
light. In contrast, a tree growing alone in an exposed field will grow to be much shorter and thicker. This is designed to
strengthen the stem.
Stomata
Stomata and their responses are considered under transport in plants.
Circadian Rhythms
Plant hormones (plant growth regulators or phytohormones)
Both plants and animals use hormones to coordinate their growth and development and to respond to certain stimuli.
Hormones are chemical signals and are generally slow-acting, and when a fast response is required then electrical
signals must be used (nervous system, Venus flytrap). Hormones are also generally longer-acting and so are good for
signaling slow and longer-term changes to a stimulus. Hormones are chemicals released by cells in one part of the body
and then travel through the body until they reach their target cells, for whom the message is intended. The target cells
then detect and respond to the hormone signal.
Phytohormones are small organic molecules synthesised by the plant and active in very low concentrations and they
promote or inhibit growth and developmental responses to environmental conditions.
Plant hormones and animal hormones compared:
- Plants have far fewer hormones than animals (as far as we know).
- In animals hormones are often produced far from their site of action within the body, whereas in plants they are
more often produced where they are required. (Why this difference?) - Animal hormones are very specific in what target cells they effect and in what effects they cause on these cells;
plant hormones are less specific. - Plant hormones work together more than animal hormones, and plant hormones show many cases of synergism
and antagonism.
Phytohormone table (pdf).
The leaves may respond to light and dark, opening in the light and closing in the dark, or there may be a circadian
rhythm (which is set by the light-dark cycle) in which the leaves continue opening and closing at the correct times of day
even when in continuous light or darkness. (Circadian rhythms usually free run for several days, but gradually the timing
is lost and may become chaotic, but is reset by exposure to a light-dark cycle).
The following account describes a model of pulvinus function (which is supported by experimental evidence).
Movements of ions into and out of the motor cells causes the cells to swell with water or to lose water and become
flaccid as appropriate.
The apoplast also acts as a store of potassium ions - the cellulose in plant cell walls has negative charges which can
bind positive ions. The flux of potassium ions through the apoplast may also involve the pumping of hydrogen ions which
occupy sites left vacant as potassium moves on, and thus helping the apoplastic transport of potassium.
The activation of the motor cells is thought to be triggered by a small influx of calcium ions into the motor cells. Calcium
ions act as messengers, triggering a wide range of responses, in a wide range of eukaryotic cells, so its is likely to be
involved here.
rhythm (which is set by the light-dark cycle) in which the leaves continue opening and closing at the correct times of day
even when in continuous light or darkness. (Circadian rhythms usually free run for several days, but gradually the timing
is lost and may become chaotic, but is reset by exposure to a light-dark cycle).
The following account describes a model of pulvinus function (which is supported by experimental evidence).
Movements of ions into and out of the motor cells causes the cells to swell with water or to lose water and become
flaccid as appropriate.
- When the leaves are open as in the light, potassium and chloride ions are high in concentration inside the
extensor motor cells and low in concentration inside the flexor cells. The extensor cells are turgid, holding the
leaves open. - Within about 20 minutes of moving a Samanea plant from white light into darkness, the leaves are closed.
- During closure, the turgid extensor cells pump out potassium ions (accompanied mostly by chloride ions, but also
other ions, which balance the electric charges) into the apoplast (cell walls) and water follows by osmosis. The
extensor lose water and become flaccid, their cell walls buckle and they can no longer exert a pushing force. - This water is thought to flow to the flexor cells, largely through the apoplast, especially of the collenchyma cells
that form the sheath around the vascular bundle. Collenchyma cells have thickened cell walls and so possess a
large apoplast for water movement. Some water also appears to be shunted through the phloem symplast in the
vascular bundles by transfer cells which pump the potassium and chloride across their membranes and into the
symplast, with the water following by osmosis. This water may flow from cell to cell in the phloem, through the
symplast, to be pumped out again, into the apoplast of the cortex, by more transfer cells. A suberised layer
prevents apoplastic loading of the vascular tissue, much as it does in plant roots. (See transport in plants). - This water reaches the flexor cells which pump in the potassium and chloride ions, across their cell membranes,
with the water following by osmosis causing the cells to swell and become turgid and so exert a pushing force
which closes the leaf. - Leaf opening would involve a reversal of the process, transporting water from the flexor to the extensor cells.
The apoplast also acts as a store of potassium ions - the cellulose in plant cell walls has negative charges which can
bind positive ions. The flux of potassium ions through the apoplast may also involve the pumping of hydrogen ions which
occupy sites left vacant as potassium moves on, and thus helping the apoplastic transport of potassium.
The activation of the motor cells is thought to be triggered by a small influx of calcium ions into the motor cells. Calcium
ions act as messengers, triggering a wide range of responses, in a wide range of eukaryotic cells, so its is likely to be
involved here.
Phytochrome
Phytochrome is a blue pigment present in plants (in low concentrations, so it does not make plants look blue!) that
absorbs red light and, in so doing, is converted to a green form that absorbs far-red light. When the far-red sensitive
form absorbs far-red light it is converted back to the red sensitive form. Pr is the form that absorbs red light (at 666 nm
wavelength) and Pfr is the form that absorbs far-red light (at 730 nm wavelength). Far-red light is light between
700-800 nm (infra-red light is light above 760 nm wavelength). Pfr also reverts back to Pr in the dark.
Phytochrome is a blue pigment present in plants (in low concentrations, so it does not make plants look blue!) that
absorbs red light and, in so doing, is converted to a green form that absorbs far-red light. When the far-red sensitive
form absorbs far-red light it is converted back to the red sensitive form. Pr is the form that absorbs red light (at 666 nm
wavelength) and Pfr is the form that absorbs far-red light (at 730 nm wavelength). Far-red light is light between
700-800 nm (infra-red light is light above 760 nm wavelength). Pfr also reverts back to Pr in the dark.
Phytochrome is used by plants to sense light.
Light induces or promotes the following reactions in germinating plant seedlings:
1. Chlorophyll synthesis (seedlings grown in the dark are pale and NOT green).
2. Leaf expansion (if leaves unfold under the ground it causes problems!).
3. Stem elongation inhibition (dark-grown seedlings have long stems).
4. Root development.
The process by which light effects development in plants is called photomorphogenesis.
Large seeds have abundant food reserves and once they germinate they do not need to photosynthesise for several
days. Dark-grown seedlings are said to be etiolated (French: etioler: to grow pale or weak) because of their long, thin
pale stems. It is a strategy to enable the seedling to place all its resources in growing a long shoot to break through the
soil. Once in the light it will then turn green, thicken and unfold its leaves.
Other reactions of plants to light:
1. Photosynthesis – production of organic molecules from CO2 and water!
2. Flowering control.
Often both light and temperature are involved, enabling the plant to determine the right time of day to open its flowers,
or the right time of year to shed its leaves. In many cases these responses (and the role of phytochrome) are
incompletely understood.
The role of phytochrome:
In sunlight there are slightly fewer far-red than red photons (the Sun is a yellow star!) although the levels at these two
wavelengths are roughly the same. To amplify this effect, Pr absorbs red light more efficiently than Pfr absorbs far-red
light, so that sunlight acts as a red-light stimulus. At dawn and dusk, the ration of FR:R light in sunlight increases, so
phytochrome can also detect these times of day.
E.g. 1. Lettuce seeds: red light promotes germination; far-red light inhibits germination.
Q. Which form of phytochrome (Pr or Pfr) promotes germination in lettuce, and which form inhibits it?
A. Pfr (produced in red light) promotes germination; Pr (produced in far-red light) inhibits it.
E.g. 2. In French beans red-light (Pfr) promotes unbending of the hypocotyl hook and triggers leaf expansion. The
hypocotyl is the stem of the seedling, between the radicle (first seedling root) and the cotyledons (first pair of ‘leaves’).
The hypocotyl is arched over at its tip, called the hypocotyl hook, protecting the cotyledons as it pushes through the
soil. Once through the soil it straightens and the cotyledons unfold). Seeds that require light for germination are said to
be photodormant seeds. (Imbibition: is the first uptake of water by a seed and is required for germination to begin).
E.g. 3. The photoperiod is the day length.
E.g. 4. Leaf canopy shading: more far-red light than red light penetrates tree canopies. Thus, shaded plants have more
Pr. In order to compensate for this, germination in shade-tolerant plants is less inhibited by far-red light, than it is in
shade-intolerant plants.
Light induces or promotes the following reactions in germinating plant seedlings:
1. Chlorophyll synthesis (seedlings grown in the dark are pale and NOT green).
2. Leaf expansion (if leaves unfold under the ground it causes problems!).
3. Stem elongation inhibition (dark-grown seedlings have long stems).
4. Root development.
The process by which light effects development in plants is called photomorphogenesis.
Large seeds have abundant food reserves and once they germinate they do not need to photosynthesise for several
days. Dark-grown seedlings are said to be etiolated (French: etioler: to grow pale or weak) because of their long, thin
pale stems. It is a strategy to enable the seedling to place all its resources in growing a long shoot to break through the
soil. Once in the light it will then turn green, thicken and unfold its leaves.
Other reactions of plants to light:
1. Photosynthesis – production of organic molecules from CO2 and water!
2. Flowering control.
Often both light and temperature are involved, enabling the plant to determine the right time of day to open its flowers,
or the right time of year to shed its leaves. In many cases these responses (and the role of phytochrome) are
incompletely understood.
The role of phytochrome:
In sunlight there are slightly fewer far-red than red photons (the Sun is a yellow star!) although the levels at these two
wavelengths are roughly the same. To amplify this effect, Pr absorbs red light more efficiently than Pfr absorbs far-red
light, so that sunlight acts as a red-light stimulus. At dawn and dusk, the ration of FR:R light in sunlight increases, so
phytochrome can also detect these times of day.
E.g. 1. Lettuce seeds: red light promotes germination; far-red light inhibits germination.
Q. Which form of phytochrome (Pr or Pfr) promotes germination in lettuce, and which form inhibits it?
A. Pfr (produced in red light) promotes germination; Pr (produced in far-red light) inhibits it.
E.g. 2. In French beans red-light (Pfr) promotes unbending of the hypocotyl hook and triggers leaf expansion. The
hypocotyl is the stem of the seedling, between the radicle (first seedling root) and the cotyledons (first pair of ‘leaves’).
The hypocotyl is arched over at its tip, called the hypocotyl hook, protecting the cotyledons as it pushes through the
soil. Once through the soil it straightens and the cotyledons unfold). Seeds that require light for germination are said to
be photodormant seeds. (Imbibition: is the first uptake of water by a seed and is required for germination to begin).
E.g. 3. The photoperiod is the day length.
- Short-day plants are plants that flower when the daylength (photoperiod) is short, e.g. spring flowering plants like
tulips and daffodils and autumn/winter flowering plants like snowdrops. Flowering in short-day plants is inhibited
by Pfr and stimulated by Pr, since Pr accumulates in darkness and Pfr accumulates in sunlight. - Long-day plants flower in summer (when photoperiod is long) and are stimulated to flower by Pfr, which is the
predominant phytochrome form in sunlight.
E.g. 4. Leaf canopy shading: more far-red light than red light penetrates tree canopies. Thus, shaded plants have more
Pr. In order to compensate for this, germination in shade-tolerant plants is less inhibited by far-red light, than it is in
shade-intolerant plants.