Role of Leaves in Phototropism

Abstract

Experiments with green seedlings of sunflower (Hellanthus annuus) indicate the existence of a phototropic mechanism which involves the leaves or cotyledons, and which can produce an asymmetry of auxin content without the involvement of lateral auxin transport, the classic explanationof phototropism in etiolated seedlings. The basic lines of evidence for the leaf-mediatedtropismare: (1) darkening of 1 cotyledon will cause curvature of the stem toward the lighted cotyledon; (2) the darkened cotyledon sustains an enhancedgrowth rate in the stem below it; (3) conversely, light suppressesthe growth-stimulating effects of a single cotyledon; and (4) more diffusible auxin is obtained from the stem be low darkened cotyledons than be low lighted ones.