Tropisms and Nastic Movements

Tropisms are growth responses of plants that result in curvatures of plant organs toward or away from certain stimuli. Tropisms can be positive, in which case the plant will bend toward a stimulus, or negative, in which case the plant will bend away from a stimulus. Important tropisms in plants include phototropism, gravitropism, and thigmotropism.

Phototropism is the tendency for plant organs to bend in response to a directional light source. For example, light streaming in a window from one direction will often cause the stems of plants placed nearby to bend toward the window, a positive phototropism. Gravitropism is the tendency for plant organs to bend in response to gravity. In most plants, roots grow downward with gravity while shoots grow upward against gravity. Within hours, the shoot of a plant placed on its side will usually bend upward and the roots will bend downward as the plant reorients its direction of growth in response to gravity. Thigmotropism is the tendency for a plant organ to bend in response to touch. For example, the specialized touch-sensitive tendrils of many vining plants, such as pea, will bend toward the side receiving a touch stimulus. Continual stimulation can lead to the coiling of the tendril around an object, which enables vining plants to grasp objects on which they can climb.

For a plant organ to bend in response to a stimulus, differential growth of cells on either side of the organ is required. For example, for the stem of a plant to bend toward a light source, cells on the shaded side of the stem near the shoot tip must elongate faster than cells on the lighted side. Differential cell growth results from either the accumulation of growth-promoting substances on the shaded side, accumulation of growth inhibitors on the lighted side, or both. One substance that appears to mediate many tropisms is auxin, a plant hormone that promotes cell elongation. When the tip of a plant is lighted from one side only, auxin appears to accumulate on the shaded side of the tip, where it promotes more rapid cell elongation than occurs on the lighted side, resulting in the bending of the stem toward the light source.

Nastic movements are rapid movements of plant organs in response to a stimulus that results from alterations in cell volume in a specialized motor organ called a pulvinus. For example, handling of the touch-sensitive leaves of Mimosa pudica results in the folding of its leaflets within a few seconds and is an example of a thigmonastic movement. Leaf folding is due to the rapid uptake of water and increase in volume of some cells in the pulvinus located at the base of each leaflet, coupled with the rapid water loss and collapse of adjacent cells. Because nastic movements occur so rapidly, the movement of plant hormones (which can be slow) does not appear to be involved. Instead, rapidly propagated bioelectrical signals appear to mediate many nastic movements.

see also Hormones, Plant; Rhythms of Plant Life

Donald F. Cipollini

Bibliography

Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. Biology, 5th ed. Menlo Park, CA: Benjamin Cummings, 1999.

Hopkins, William J. Introduction to Plant Physiology, 2nd ed. New York: John Wiley & Sons, 1999.

tropism , involuntary response of an organism, or part of an organism, involving orientation toward (positive tropism) or away from (negative tropism) one or more external stimuli. The term tropism is usually applied to growth and turgor movements in plants; an involuntary orientation of a microorganism toward or away from an external stimulus is commonly called a taxic movement, or taxis —e.g., the negative phototaxis of certain protozoans that move away from light. Tropistic stimuli include light, heat, moisture, gravity, electricity, and chemical agents. Plant stems are positively phototropic and negatively geotropic, i.e., they grow toward light and against gravity; roots are the reverse, as well as positively hydrotropic (moisture-seeking). Tropistic growth in plants is believed to be triggered by the presence of plant hormones (see auxin ) that promote cell growth. Auxin action is apparently inhibited by light; hence, if a plant is placed in a position of unequal lighting, the cells on the shadier side elongate faster than those on the illuminated side, and the plant bends toward the light. There is also evidence that auxins are affected by gravity, i.e., they accumulate in the lower portions of the plant organs. Since an overconcentration of these hormones inhibits growth, the cells on the underside of a root elongate more slowly than those on the upper side, resulting in the root's downward growth. Generalized plant responses to a stimulus are called nastic movements, or nasties. These include the opening of bud scales and of flower petals, growth movements that occur in response to stimuli such as light and heat without regard for the direction of the stimulus. Some spring flowers exhibit thermonasties, i.e., their flowers open in response to warmth rather than the amount of light. Turgor movements are effected by changes in the water content of cells and are often quite rapid. Examples are the "sleep movements" of clover, the sudden drooping of the leaves of the sensitive plant (mimosa) when touched (thigmotropism), and the reactions of insectivorous plants to the presence of their prey. The exact mechanism controlling the sudden loss of water pressure in certain cells, producing turgor movements, is not clearly understood.

tropism(tropic movement) A directional response by a plant to a stimulus. It may be positive or negative (i.e. towards or away from the source of the stimulus). The plant may respond by growth or turgor changes so that parts of it bend towards, away from, or at right angles to the direction of the stimulus. The suffix -tropism is used in relation to responses to particular stimuli (e.g. phototropism is a response to light, geotropism to gravity, and chemotropism to a chemical substance).

tropism (tropic movement) A directional response by a plant to a stimulus. It may be positive or negative (i.e. towards or away from the source of the stimulus). The plant may respond by growth or turgor changes so that parts of it bend towards, away from, or at right angles to the direction of the stimulus. The suffix-tropism is used in relation to responses to particular stimuli; e.g., phototropism is a response to light, geotropism to gravity, and chemotropism to a chemical substance.

tropism The directional growth of a plant organ in response to an external stimulus, such as light, touch, or gravity. Growth towards the stimulus is a positive tropism; growth away from the stimulus is a negative tropism. See also geotropism; hydrotropism; orthotropism; phototropism; plagiotropism; thigmotropism. Compare nastic movements; taxis.

tro·pism / ˈtrōˌpizəm/ • n. Biol. the turning of all or part of an organism in a particular direction in response to an external stimulus.

tropism (tropic response) Response in growth and orientation of a plant or a part of it in relation to a directional, external stimulus, such as light or water.