It is a characteristics of plants, except for few unicellular plants such as Chlaymydomonas, that they do not exhibit movement of entire organism (locomotion). However, individual plant organs (stems, roots and leaves) exhibit movements. These movements are triggered in response to external environmental stimuli or by an internal timing mechanism, the biological clock. Therefore, these movements are also called plant responses. For example, when terminal leaflet of sensitive plant (Mimosa pudica) is touched, all leaflet fold rapidly, and the entire leaf droops. The plant responses are generally quite slow growth or turgor movements. Plants tend to adapt to new situation by modifying their growth.
Stimulus, Responses and Irritability
A stimulus is the change in the environment which produces an appreciable effect upon the living protoplast and bring about a visible reaction or response. The stimuli may be:
Mechanical that are caused by impact, friction and pressure, chemical caused by oxygen, or ethereal brought by light, heat or electricity wave. The response of a plant to the stimulus is the result of a fundamental property of protoplasm, the irritability or sensitivity, i.e., the ability to respond to a change in the environment. This property enables the protoplasm to adjust itself to environmental conditions. Most responses are either change in the turgor pressure of the cells, or in the form of differential growth, i.e., greater growth on one side and lesser on the other side of the plant organ.
Broadly plant movements are classified as:
Paratonic Movements: The movements induced by external stimuli, for example the movements of flower heads of sunflower in response to stimulus of sunlight.
Autonomic Movements: The movements induced by some internal timing mechamism, for example circumnutation, ciliary movements as a result of which whole organism moves from place to place, or cyroplasmic streaming that results in movement of cytoplasmic organelle.
The movement induced by external stimuli can be classified into three categories: tropic movements or tropism, nastic movement or nasties and tactic movement or taxes.
Tropic Movement or Tropisms
A tropism ia growth movement of a plant organ in response to an external stimulus in which the direction of the stimulus determines the directions of the response. The most common stimuli responsible for tropic movements are light, gravity, solid surface or touch and chemicals. Tropic responses may be positive or negative depending upon whether the growth is towards or away from stimulus respectively.
On the basis of nature of stimulus the tropic movements may be:
Phototropism: Movement of shoots and coleoptiles in response to stimulus of light.
Geotropism: Movements of roots in response to stimulus of gravity.
Chemotropism: The movements exhibited by hyphae of some fungi and pollen tubes in response to the stimulus of chemicals.
Hydrotropism: The movements of roots in response to stimulus of water.
Thigmostrxism: The movements exhibited by tendrils in response to the stimulus Df touch.
Aerotropism: The movements in response to oxygen in air.
The hydrotropism and aerotropism are considered as special kind of chemotropism.
Plant Response to Stimulus
The plant response to external stimuli takes places in three phases:
Perception: A unilateral stimulus, such as light, is perceived (at the tip of the coleoptile).
Transduction: Conduction of the influence, in the form of a growth hormone, to the site of response.
Response: Usually due to differential extension growth.
The Coleoptile – Experimental Organ
The coleoptile is a sheath enclosing the shoot in the embryo of membranes of the grass family. In the dark the coleoptile grows up like a stem. In the light it split open at the soil surface and the leaves grows out. The coleoptiles is very useful for investigating tropic responses.
Experiments on the response of the grass (Avena) coleoptile to unilateral light initiated the study of tropism and led to the subsequent discovery of auxin. Further developmental of these experimental methods gave evidence for the hypothesis that auxin distribution was responsible for tropic responses of stems and roots.
Phototropism – Plants & Light
The response of green plants to light is complex. In total darkness plant stems grow thin and week and with tiny underdeveloped leaves. Chlorophyll is not formed and stem and leaves appear pale yellow. The growing point with terminal bud is curved over and hooked. The condition of plants grown in the dark is described as etiolated. By contrast, the shoots in full light are short, with straight, thick, sturdy stems and large, dark green expanded leaves. Thus, light is essential for normal growth of plants.
Coleoptiles Curvature – A Classical Example of Photoperiodism
When germination seedlings (Avena coleoptile for example) receive light from one direction only (unilateral illumination) their stems grow towards the light source. The side adjacent tot he light source grows more slowly and that away from it more rapidly. Such a growth movement induced by light is called phototropism. The stem is said to be positively phototropic because the tip of the stem grows towards the light.
Charles Darwin and his son Francis (1880) were the first to investigate the response of reed canary grass and oat (Avena) coleoptiles to unilateral light. They found that tip of the coleoptile perceived the stimulus of unilateral light but that the growth response occurred lower down the organ. They concluded that some influence is transmitted from the tip to the grown region.
Fritz Went continued with this line of enquiry in 1928 and carried out a series of experiments. He removed tips of dark grown oat coleoptiles, placed these on agar gel blocks, and kept these blocks in dark for several hours. When the agar blocks were placed on one side of the coleoptile stumps, the result was a curvature of the coleoptile. Went explained the result that the growth hormone has passed from the coleoptile tip into the agar block and then to the stump.
Auxin – The Growth Hormone
Went named this hormone auxin (Gr. auxien = to increase). The auxin was isolated and found to be indole acetic acid (IAA) chemically. Auxin is synthesized in apical meristems and young leaves and is transported to the regions of growth in plant. It is found in highest concentrations at the tips of stems and roots, in young growing leaves and in flowers and fruits and in decreasing concentrations at a distance from these meristems. Transport of IAA is polar and is due to active transport.
Cholodny-Went Theory – Mechanism of Phototropism
Auxins promote cell enlargement and elongation of coleoptiles and stems occurs because of cell esnlargement. Cholodny in 1924 and Went in 1926 independently suggeated that the phototropic response is due to differential growth resulting froman unequal distribution of auxin (IAA). Went removet oat coleoptile root tip and placed it upon two agar blocks arranged to receive the auxin from illuminated and shaded sides separately. He allowed unilateral light to fall upon the excised tip and found that more auxin collected on the illuminated side. Went believe auxin is inactivated on illuminated side and that light also induced the lateral transport of auxin to the darkened side. However, current evidence does not favour inactivation of auxin by light.
Later in experiments with maize coleoptiles lateral transport of auxin and its unequal distribution was demonstrated. In an experiment the tips of coleoptiles grown in dark and light were excised and placed on agar blocks and it was found that in each case 40 units of IAA diffused into the blocks. In other experiment the coleoptiles were split and the two halves were separated by a thin glass cover slip, the same amounts of IAA are collected from the illuminated and the darken halves (20 units of IAA from each side). But when the coleoptile was partially separated at the base by a thin piece of glass, more auxin diffused from the darkened half than the illuminated, about 30 IAA units from darkened half and 10 IAA units from the illuminated half. The results suggest that asymmetric distribution of auxin produces an symmetric elongation of cells, followed by bending.
Photoreceptor for Phototropism
Research to identify the photoreceptor for phototropism began by attempts to match the action spectrum for phototropism with the absorption spectrum of pigments present in the light-sensitive tissues. Blue light is most effective in causing phototropism and red is inactive. This suggest that a yellow pigment is the photoreceptor. In fact two pigments may be involved. It is found that the absorption spectrum of carotene matches the action spectrum in the visible range, whereas the absorption spectrum of riboflavin matches the action spectrum of ultraviolet. Whilst it is probably that both substances are involved, it is not evident how, when activated by light, these pigments bring about the asymmetric distribution of IAA in stem or coleoptile tips.