Studies of Effects of Light on Growth Pattern and of Gibberellin Sensitivity in Relation to Age, Growth Rate, and Illumination in Intact Wheat Coleoptiles

Abstract

As dark-grown wheat coleoptiles approach maturity, elongation ceases first in basal and last in apical portions. Illumination of dark-grown coleoptiles with red, blue, or white light increases elongation of apical and decreases elongation of basal portions. Reciprocal photoreversibility with far-red radiation was established for both the promotion of elongation of apical and the inhibition of basal portions by red light and also for both the promotion of elongation of apical and inhibition of basal portions by blue light. Since all the light treatments given to dark-grown plants were given after cell division had ceased in the coleoptiles, the results demonstrate the nonessentiality of cell division for these photomorphogenic effects. With reference to controls that received no light, the light which light-grown plants received decreased the ratio of "apical" to "basal" elongation, in contrast to the light given to dark-grown plants, which increased this ratio. Gibberellin sensitivity was no greater in young, rapidly growing tissues than in older, more slowly growing tissues. Gibberellic acid promoted elongation of all portions of the coleoptile. Thus the effects of the light treatments on elongation of the dark-grown plants can not be attributed to the regulation of endogenous gibberellin levels. In dark-grown coleoptiles the action of red light, whether increasing elongation or decreasing elongation, and the action of gibberellic acid appear to be independent of each other.