Physiology of Movements in the Stems of Seedling Pisum sativum L. cv Alaska

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Abstract
Phototropic response in etiolated pea (P. sativum L. cv. Alaska) seedlings is poor. The curvature induced by unilateral blue light can be hastened and increased in magnitude by a previously administered red light pulse followed by several hours of darkness. Phytochrome is involved in the red light effect. Phototropic response was almost completely inhibited by removal of the apical bud and hook, but it was restored if exogenous IAA was applied apically to the cut stump. The stem contains the phototropic photoreceptor and response mechanism. Perception of gravity and gravitropic response were also localized in the stem, but gravitropism was scarcely inhibited by decapitation. The kinetics and curvature pattern of gravitropism differed greatly from those of phototropism. Like phototropism, stem nutation required auxin and was promoted by red light. Unlike phototropism, photoenhanced nutational curvature required the apical hook and was propagated as a wave down the stem. Naphthylphthalamic acid inhibited, in order of decreasing effect, nutation, phototropism/gravitropism and growth. Phototropism, gravitropism and nutation appear to represent distinct forms of stem movement with fundamental differences in the mechanisms of curvature development.