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.

Above: Bryonia dioica, Red-berried Bryony also called simply Red Bryony and also White Bryony (presumably because of its greenish-white flowers) but this should not be confused with Bryonia alba, also called White Bryony which has berries that ripen straight from green to black and do not turn red. Indeed, the two have been confused taxonomically in the past. As the name suggests, Bryonia dioica is dioecious: it has separate male and female plants, whilst Bryonia alba is often (if not always) monecious: with seperate male and female flowers on the same plant. Bryonia belongs to the Cucurbitaceae family (squashes, pumpkins, melons, gourds and their relatives). The fruit of Bryonia dioica is only about as large as a pea and contains 3-6 seeds. Although the plant has traditional medicinal uses, it is also very poisonous, including the berries, only the young spring shoots have ever been safely eaten, after boiling.

The tendrils respond to rough stroking textures as they brush against a support, but do not respond to raindrops. The leaves are five-lobed, with the middle lobe the longest. The leaves are rough on both surfaces, being covered with short white prickles. Note that once the tendrils grapple a support, they pull the shoot towards it by coiling and contracting. Since both ends are fixed, they can not spiral in a way that would generate twisting tension and so one part of the tendril coils clockwise, another part anticlockwise, with the two coiled segments separated by a short uncoiled hinge. A tendril will coil within minutes of rubbing against a rough surface, but if it loses its grip early on then it will often slowly uncoil and try again. Relying on nearby supports, the stem of Bryonia is not very strong and is quite brittle.

Above: the greenish-white flower of Bryonia dioica, with black angular and ovoid cremocarps of Alexanders (Symrnium olusatrum). The tendency for some Bryonia types to be monoecious and others to be dioecious, made this plant a model for studying the biology of sex determination. Bryonia has X and Y sex chromosomes, Y being the male chromosome, and this was apparently the first experimental system for which genetic control of sex was elucidated. Note, however, this is not the only system of sex determination used in flowering plants.

The diagram below is a 3D representation of the model of the tactile blebs on Bryonia tendrils 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 bleb, 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 neighboring 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. Prior 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.

Adhesive Discs and Suckers

Not all climbing plants rely on tendrils which grapple supportive structures, some, such as the Boston Ivy use tendrils armed with adhesive discs to gain purchase on the trees they climb up. The strength of these adhesions is immense and this has attracted the attention of engineers wishing to mimic their properties. In the Boston Ivy, Parthenocissus tricuspidata, a member of the grape family Vitaceae, each tendril consists of a main axis and 5 to 9 alternate branchlets. Each branchlet has a small swelling at its tip. This swelling fills with mucilage as the tendril develops. Repeated contact with a nearby support is sensed by the tendril and this acts as a stimulus for the final development of the swelling which differentiates into an adhesive disc.

The surface cells of the disc develops into elongated epidermal cells, resembling clusters of fingerlike protrusions (each epithelial cell is one fingerlike process) which grow and mould to the shape of the surface and then secrete adhesive mucilage. This mucilage contains carbohydrates and some 21compounds rich in N, S and O: elements which tend to carry a negative charge and so are good at forming electrostatic bonds with the surface of the support (opposite charges attract, van der Waals forces). In addition, the center of the disc raises up, forming a cup-shaped structure and generating suction (acting as a true sucker) to help cement the seal. In the mature disc the epidermis degenerates and the disc shrinks.

Tendrils of the Passion Flower Passiflora discophora end in an adhesive pad and also usually also bear two side-branches, near the tendril apex, which branch again with each branch ending in an adhesive pad, making 5 adhesive pads in all. Each pad begins growth as a callus which penetrates gaps and spaces in a rough surface and produces extracellular glue. The tendril axis contains a tough lignified core and after about 2 to 6 weeks the tendrils die and desiccate with the fixed woody tendril remaining fixed in place.

Tendrils of climbing plants may also coil, once an attachment has been made, pulling the climber closer to its support. Virginia Creeper (Parthenocissus quinquefolia, grape family, Vitaceae) has branched tendrils, with each branch ending in an adhesive disc that secrete an adhesive which penetrates tiny gaps, then the tendrils coil.

The English Ivy, Hedera helix, also uses an adhesive, but this is secreted by root hairs as the adhesive appendages are adventitious roots (roots growing from the climbing stem). Only these aerial roots secrete an adhesive consisting of a nanocomposite material which contains uniform nanoparticles which are organic and 60-80 nm (60 to 80 billionths of a meter) in diameter. It is thought that the matrix of the adhesive not only forms electrostatic bonds with the surface, but also with the nanoparticles, giving the adhesive itself tensile strength - it is not always enough to simply have the adhesive stick to the surface, but it must also be able to bond with itself so that the adhesive does not fracture.

Circumnutation

Circumnutation is the circular motion often shown by growing shoots, roots and leaves. It is due to differential growth: in order to elongate, plant  cells must transiently weaken their cell walls, so this occurs on only one side of the shoot at any one time. However, this is likely more than a simple growth phenomenon and probably helps growing plant parts avoid obstacles and locate gaps to grow through. The period of a rotation of a growing tip varies between tens of minutes to a few hours. Twining vines that climb by coiling their whole stem around a support, do so by wide circumnutations, like a cowboy circling his rope before he throws it, and when they encounter a support they continue to rotate around it in a passive response that requires no special sensory reaction. The sensitive tendrils of climbers like Bryonia also circumnutate, which helps them locate a support by random movement.

Neuroid and Nervous 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 localized 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 specialized 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 specialized in signal transduction, or nervous system, and a more general system in which electrical signals are passed from one cell to its neighbors. 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. As more research is carried out in this area it is becoming apparent that more plants than previously supposed make use of a nervous system, from algae to apple trees, and it seems likely that all plants make use of a nervous system. 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. Action potentials in plants also travel at only one-hundredth to a thousandth of the speed they do in animals (2 cm/s compared to 1-20 m/s), so the plant nervous system is much slower.

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.).

Plants exhibit two primary types of long-range electrochemical signal. An electrochemical signal is an electrical signal generated by the flow of ions (charged chemical species). The action potential (AP) of plants and animals is one such signal. Action potentials are all-or-none in character, that is the size of the AP is (usually) of no importance in encoding information, but the number of action potentials and the rate and rhythmicity of action potential firing are all important. In this sense an AP is a binary signal: it is either ON or OFF.  Action potentials travel mainly in the phloem in plants. In plants action potentials can encode a variety of signals, such as responses to wounding, touch, change in illumination, cold and cell expansion. For example, applying a flame to a leaf of Aloe vera will generate action potentials. Thus we can say that plants do indeed have a nervous system.

However, plants also pass a type of long-range prolonged graded potential, called a variation potential (VP). A graded potential is an electrical response in which the height of the signal generally encodes stimulus strength and the duration of the signal may also encode stimulus duration. Thus, a VP is not all-or-none like an AP and is 'analogue' rather than 'digital'. Graded potentials are used in neuroid systems in many organisms and are usually local in nature, since the height of the signal diminishes with distance from the signal source. (If an AP loses signal height this does not matter as long as the AP is discernible above background noise, so an AP is generally used for longer-range signalling). Plants are unusual in using the VP for long-range signaling too. This is almost unknown in the animal kingdom, though some jellyfish neurons do use long-range graded potentials in addition to action potentials.

Variation potentials encode a variety of signals, often connected to wounding, local burning, organ damage or removal and also in response to hydraulic signals. Changes in xylem pressure can travel as pressure waves along the xylem at the speed of sound (about 1500 m/s). These might be produced by an increase in turgor pressure following rainfall, an embolism (blockage) in a xylem vessel, bending of the plant, and also wounding. These hydraulic waves travel through the xylem, generating variation potentials in parenchyma cells which are then carried to the phloem. Such a signal can travel through a region of dead tissue (which is able to conduct water) as an hydraulic wave before being regenerated as a VP in living tissue on the other side. The speed of variation potentials varies and they may be slower than action potentials (in darkness) or faster (in light). Such signals may, for example, signal stomata closure after wounding to conserve water.

Differences in electric potential can be measured between cells in different parts of woody plants, such as between the stem and the leaf. Electric potential is the potential energy per unit charge and as gravitational potential energy drives the acceleration of falling objects, so electric potential energy drives the flow of electric charge or electric current. These differences follow a daily rhythm, changing with the light-dark cycle. It is possible that electrical signals in plants have some function in regulating and coordinating daytime and nighttime activities in plants and may be part of their biological clocks.

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 downwards, 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:

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.

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:

Nature of the Gravity Sensor

Growing root tips have gravity-sensitive cells (gravireceptors) in the columella of the root cap. The root cap is a protective cap of tissue which protects the growing region of the root tip and secretes mucilage to help the root tip move through the soil. The columella (lit. 'little column') is the central column of cells in the root tip. Gravity-sensing cells also occur in the endodermis (innermost layer or layers of cortical parenchyma cells surrounding the vascular tissues) of green shoots and the pulvini of leaves responsible for correct positioning of the growing leaf.

These gravirecptor cells have one feature in common: the presence of amyloplasts (modified chloroplasts which lack chlorophyll but store starch, i.e. starch grains). The starch makes the amyloplasts weighty and if a cell is turned upside- down the starch grains will sediment at the bottom of the cell in about 5 minutes. Complete sedimentation is thought not to be necessary for triggering a response which is thought to occur after only a few seconds post-reorientation. This is the starch-statolith hypothesis. A statolith is a weighty 'stone-like' structure in an organism which is involved in sensing gravity.

Experiments have confirmed that the starch grains are required for gravity sensing. A working hypothesis is that the amyloplasts are attached to part of the cell's cytoskeleton. Movement of the amyloplasts can then send a signal throughout the cell via the cytoskeleton. The movement of the amyloplasts triggers calcium ion release from internal stores (via the PIP2 - IP3 signalling system which serves many diverse functions in eukaryotes). This rise in the cytosolic concentration of calcium within the cells occurs in two phases in Arabidopsis seedlings: a rapid spike in calcium concentration, followed by a slower more phasic response, in which calcium concentration is proportional to stimulus strength and gradually fades over 25 minutes or so. (See: Plieth, C and AJ Trewavas, 2002. Reorientation of Seedlings in the Earth's Gravitational Field Induces Cytosolic Calcium Transients. Plant Physiology 129: 786-796). Interestingly, the seedlings also responded to puffs of air by mobilising intracellular calcium ions, but in this case only the rapid spike was seen. The gravitropic response is accompanied by polar transport of calcium ions in the apoplast (plant cell wall network) which is essential for polar auxin transport.

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.

Plant Muscle and Reaction Wood

It is not just roots that respond to gravity. Shoots tend to grow away from the center 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 angiosperms 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 specialized 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 weight 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 center-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.

In many cases, at least, the contraction generated by reaction wood in angiosperms is generated by G-fibers, specialized sclerenchyma fibers with gelatinous cell-walls. These fibers are not only found in wood, but in secondary xylem more generally and may occur in primary and secondary phloem. The mechanism is not fully understood, but the gelatinous material swells when hydrated, but shrinks when it dries out. This shrinkage would generate tension or a contractile force. G-fibers then function as the muscles of dicotyledonous angiosperms, though many plants can also achieve movement without them. Nevertheless, G-fibers also occur in tendrils, like those of Bryonia dioica, generating the contractile force that causes them to coil, and also in the stems of many twining plants. They also occur in the curious sinusoidal stems of the Monkey-ladder Vine, Entada gigas, a member of the Pea family, Fabaceae.

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.

The blue arrows indicate the movements of auxin, synthesised in the shoot tip. The Cholodny-Went theory (1937) 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.

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:

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 kept in 24 hours light or total darkness.

Mechanism of Pulvinus Action

The flexor actively closes leaves, whilst the extensor acts to open the leaves.

When the leaves are open (in the light) the potassium ion concentration in the protoplasm of cells in the extensor is high. When the leaves begin to close much of this potassium is transferred into the walls of the extensor cells, where it enters the apoplast pathway. Chloride follows, driven by the electrochemical gradient (the negatively charged chloride ions are pulled along by the positively charged potassium ions). Water will also follow by osmosis, so that the extensor cells become flaccid and pliable, reversibly buckling under the weight of the leaf. Recall that there are two main pathways of water flow across plant tissues: the symplast, consisting of the protoplasts of cells connected by plasmodesmata, and the apoplast, consisting of the connected walls of plant cells which resemble porous fibrous meshes. Once in the apoplast pathway, the potassium and chloride ions can move through the collenchyma cells in the leaf. Collenchyma cells are supporting cells which have thickened cellulose cell walls.

Closing takes about 20 minutes and during closing the portassium and chloride ions flow to the flexor where they enter both the cell walls and protoplasts of the flexor cells. Water follows by osmosis, so that these cells become swollen and turgid and exert positive pressure to move the leaf. In darkness, when closed, potassium ions remain at high concentration in the collenchyma apoplast which acts as a potassium ion reservoir. concentration. Closing takes about 20 minutes.

Some potassium and chloride are also probably shunted through the symplast pathway. It is thought that specialised transfer cells pump potassium and chloride in and out of the symplast of the phloem. The reverse process happens when the leaves open - potassium and chloride leave the flexor cells, which become flaccid, and enter the extensor cells which become inflated, such that the action of the forces is reversed.

Mimosa pudica, the Sensitive Plant, is a good demonstration of pulvini power. Repeated touching of the leaves will cause the leaflets of each compound leaf to close, which again involves action of the pulvini.

Thigmomorphogenesis

Thigmomorphogenesis is a change in plant growth in response to touch. The most obvious example is the way climbing plants spiral around a supporting object, such as the stem of another plant. Many plants respond to repeated rubbing or bending by growing shorter with thicker stems. This is simulating movements caused by wind. Plants growing in windy places need to more resistant to the wind and so grow thicker stems. They are also generally shorter, either because this reduces loading on the stem, or because they have limited nutrients to invest and have invested more in growing wider, or because wind-exposed plants are likely to be isolated and so need not compete so much for light.

Trees growing in isolation are exposed more to high winds and grow shorter but thicker. Trees growing grouped together compete for the light and are sheltered by one-another and so invest in producing a narrower but taller trunk. Plants that are repeatedly shaken but not directly handled also show a similar response (also called seismomorphogenesis). This raises the possibility that this response may also occur in response to repeated browsing by herbivores. Browsing, pruning or damaging a tree also tends to cause a proliferation of shoots, especially in hardwoods.

Pollarding a tree, in which the canopy is cut back about two metres above the ground, results in the activation of dormant buds or production of new buds on the stem, called epicormic buds, which results in more shuts growing back. Clearly if a tree has lost a major limb, then replacing this with several limbs is a good insurance policy - if the tree is exposed to winds or herbivores, then producing more branches is a good insurance policy. Pollarding was done to encourage the growth of numerous new shoots, which had many uses, for example as fence posts, whilst keeping the new shoots out of the range of pigs, sheep and deer that may graze the woodland. Where such grazers are not kept Coppicing is often carried out - in which the tree is cut done near the base, leaving a tree stump to grow new shoots. Most conifers do not survive coppicing, but hardwoods have many epicormic buds to produce new shoots. A regenerating tree stump will draw upon food reserves in the roots, the parenchyma of the remaining wood, and also on nutrients supplied by other trees. Tree roots often naturally graft together beneath the ground, connecting their xylem together, which allows some nutrients to pass from nearby trees to the stump, or indeed any tree that is struggling. Sometimes such root grafts even occur between trees of different species.

These responses pose several problems for woodland management. First of all, pollarded trees are often pollarded at intervals, failure to do so can result in too many branches maturing, making the crown top-heavy, despite thickening of the trunk. This may cause the trunk to fail and split as the tree starts to fall apart. This is not as disastrous for the tree as it sounds, however, since pollarded trees extend their life-span and such failing pollards are often immensely old in any case. Also, the fragments may remain rooted and grow as separate trees. Felling trees that were close together, leaving some remaining trees rather isolated can increase windfall or wind-damage since a tree that previously grew taller and thinner, relying on its companions for support may now suddenly be more exposed to high winds.

Mechanical stimulation may produce shorter and stockier plants, but it may also reduce fruit yield. Spraying tomato plants once a day can severely reduce the yield, since the plant has invested more resources in strengthening its body and so has less to invest in fruit production. Many plants only require shaking for a few seconds each day in order to induce a response. These responses also pose problems for plant scientists, since when comparing treatments on plant growth they must ensure that each group of plants are handled in a similar way.

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. Thus, phytochrome alternates between a form that is sensitive to red light at about 660 nm (peak absorption occurs at 666 nm) and a far-red sensitive form sensitive to far-red light at about 730 nm.

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

Plants clearly utilise light-energy for photosynthesis, but they also sense light using phytochrome. 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 affects 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.

Light is also used as a signal to control flowering. 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 of 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. For example in lettuce seeds: red light promotes germination; far-red light inhibits germination.

Q. Which form of phytochrome (Pr or Pfr) promotes germination, and which form inhibits it?

A. Pr senses red-light more efficiently than Pfr absorbs far-red light in direct sunlight. This Pr converts into Pfr, so in direct sunlight there is more Pfr present and so it is Pfr that stimulate germination. In darkness, Pfr converts back to Pr and so Pr inhibits germination! 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).

As a further example, 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).

Photoperiod

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.

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.

Other light-sensitive pigments in plants

Chlorophyll: responsible for absorbing the light energy used in photosynthesis. Chlorophyll is green since it absorbs blue light (400-500 nm) and red light (600-700 nm). Chlorophyll, however, does not appear to play any major role as a light sensor.

Cryptochrome: absorbs blue and violet light. This pigment triggers phototropism: the growth of some parts of a plant towards light (and other parts away from light) as described above. Cryptochromes occur in animals and plants and are important components in the biological clock that regulates circadian rhythms. they are also implicated in responses to the detection of magnetic fields, for example in birds. In some plants and animals it has been shown that responses to magnetic fields depend upon the response of cryptochrome to blue light. (More information will be provided on this topic as it becomes available).

Other responses of plants

Many parts of the plant are sensitive to a variety of stimuli. For example, the nodes of many grass stems and/or the bases of leaf sheaths are sensitive to gravity, exhibiting gravitropism (moving the plant as it grows to grow away from gravity). They contain statoliths and responses to a change in gravity start to occur after about 30 seconds if the plant is re-oriented. These movements occur at the same regions that sense the gravity, called pseudo-pulvini or false pulvini (singular: pseudo-pulvinus) to distinguish them from the pulvini of leaves that bring about reversible movements rather than growth responses. Thus the false pulvinus is both a gravity sensor and responds to gravity by re-orineting parts of the plant. Stamens, flower stalks, fruit, leaves and other parts of the plant may also respond to gravity. This area is still poorly researched.

One of the emerging themes of this article is that plants respond to a very wide variety of stimuli, albeit in slow motion compared to many reactions of animals. These responses are often poorly researched. The role of the plant nervous system, with its action potentials, remains poorly studied. Additionally, like the cells of other multicellular organisms, plant cells can communicate with each other by chemicals as well as electrical signals. Chemical messengers, such as phytohormones, cause signals within the responding plant cell, an intracellular cell-signalling mechanism. In contrast to studies on animals and the cellular slime mould Dictyostelium, and indeed bacteria, cell-signalling is very poorly understood in plants. Clearly much more research is urgently required in these fields of botany / plant science.

Download a pdf table of plant hormones.

References and Bibliography

• 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.
• Brenner, E.D., R. Stahlberg, S. Mancuso, J. Vivanco, F. Baluska and E. V. Volkenburgh, 2006. Plant neurobiology: an integrated view of plant signaling. Trends in Plant Science 111: 413-419.
• Gurovich, L. A.,  2012. Electrophysiology of Woody Plants, Electrophysiology - From Plants to Heart, Dr.Saeed Oraii (Ed.), ISBN: 978-953-51-0006-5, InTech, Available from:
• http://www.intechopen.com/books/electrophysiology-from-plants-to-heart/plant-electrophysiology
• Plant Physiology. Salisbury and Ross.
• Biology and physiology of vines. Vaughn, K.C. and Bowling, A.J. Horticultural Reviews, 38: 1-21.
  

See also: Plant Trichomes


Article updated:

22/2/2014 - damaged parts of the file replaced along with some additional material;
13 Jan 2016 - information added on adhesives in climbing plants.
21 May 2019 - Page maintenance (file was damaged again!)

11/Feb/2024