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.

References

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.