Question

How do the tropism and nastic movement of plants can help photosynthesis

Answer 1

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.




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 movement