All living plant cells respond directly to environmental changes (external stimuli) and indirectly to morphogenetic responses (internal stimuli). There are four principal plant responses: tropisms, which are directional growth movements in response to such stimuli as light, gravity, touch, and water; turgor movements, which are curving motions in plant organs, such as leaves or flowers, caused by changes in turgor pressure initiated by the sun’s movement, day length, and touch; morphogenetic responses to day length, or photoperiodism; and the action of hormones and enzymes in growth regulation.
Experiments have shown that the responses of plants involve the production and movement within the plant of certain chemicals, the phytohormones. These compounds include auxins, gibberellins, and kinins, and are widespread in the plant kingdom. Their precise mode of operation, however, is not yet fully understood.
The autotrophic plant’s behavior is geared to the maximum exploitation of light, which is its only source of energy. It is therefore one of the most important environmental influences on plant orientation. Motile algae, such as Eu-glena and Volvox, show a positive phototactic response—they move bodily toward a light source. Sessile plants, however, grow toward a light source in a movement known as pho-totropism. The growth curvature common to all forms of tropism occurs when there is a differential growth of cells on the opposite side of the responding organ. This is caused by the growth-regulating hormone, auxin, stimulating cell elongation on one side of the organ. In plant shoots, positive phototropic growth has been found to result from the lateral transport of auxin, which is induced by the stimulus of light. The light is perceived by a pigment called blue light receptor. Phototropism is induced most effectively by blue light but how it causes auxin transport is unknown.
Gravity also has a decisive influence on plant orientation. The main roots of plants show a positive geotropism by growing downward, whereas shoots demonstrate negative geotropism by growing away from the force. Leaves show transverse geotropism. The geotropism in roots may be controlled by abscisic acid rather than auxin (which inhibits the geotropism of root cells). Geotropic curvature in roots is not very well understood, but it is thought that gravity is perceived by metaboli-cally inert starch grains (statoliths) in the root tips. These are displaced in the root tip tissue, for example, when the plant grows around an obstruction in the soil thus causing the statoliths to shift. They fall under gravity to the lowest side of the root where they inhibit growth and promote it on the upper side. This theory, however, does not explain why some plants which apparently lack statoliths, such as the onion (AHium sp.), still respond to gravity.
Chemicals also influence plant movements. Motile bacteria and algae demonstrate chemo-taxis, moving bodily toward chemicals, whereas parts of higher plants show chemot-ropism. For example, the male gametes of some species are attracted by chemicals released by the female gametes.
Plants also respond to contact. In a movement known as thigmotropism, tendrils and twining stems of climbing plants, such as the passionflower (Passiflora sp.), are induced to curl around supports that touch them. Water, or the lack of it, particularly in arid soils, also directs the orientation of root development— this is called hydrotropism.
In what may appear to be a phototropic movement the leaves and flowers of some plants open and close according to the light available or follow the sun’s movement through the sky. In these movements, however, light is perceived by a receptor pigment called phytochrome, which is sensitive to red light only, and the changes are due to the expansion and contraction of motor cells that gain or lose turgor pressure in response to the stimulus of the light. These motor cells are contained in an articulating structure called a pulvinus.
Plants such as the poppy (Papaver radica-tum) track the sun’s course through the day, the flower moving in a 180° curve. The closure of plants’ leaves at night, or sleep movements, is called nyctinasty and is due to the action of a pulvinus in the leaf petiole. The closure of flowers at night, as in tulips (Tulipa spp.), is not truly nyctinastic although it is a response to decreasing light; the motor cells are not organized into obvious pulvini.
Changes in development that are not oriented toward the direction of a stimulus are morphogenetic, and those induced by light are termed photomorphogenetic. The response to day and night length is known as photoperiod-ism and involves the detection of light by phytochrome. Such responses include plant and seed dormancy and leaf abscission. Leaf abscission is initiated by decreasing day length in autumn. The short days alter the balance of the hormones auxin (which inhibits abscission) and ethylene (which stimulates it) and promote the development of a weakened zone at the base of the leaf petiole. Winter dormancy in plants is thought to be controlled by a balance between abscisic acid and gibberellin hormones.
The effects of phytochrome mediation—the influence of light on plant growth—also include flowering during particular day or night lengths; this ensures that all individual plants belonging to the same species flower together at an appropriate season. For example, dill (An-ethum graveolensJ needs short nights to flower, whereas cocklebur (Xanthium penn-sylvanicumj needs long nights; in other words, dill flowers in summer but cocklebur flowers in winter.