EFFECTS OF LIGHT ON ELONGATION AND BRANCHING IN PEA ROOTS

Abstract

The characteristic morphology of the branched tap-root system of an adult plant is determined by the number and location of lateral root meristems initiated by the primary root during the course of its development. Much interest has centered around the identification of the specific factors responsible for the initiation of the cellular divisions leading to the precise formation of lateral root primordia. It was shown by Thimann (16) and others that auxin plays a role in the initiation of lateral roots in the primary root of such plants as the garden pea, Pisum sativum. Evidence from recent studies of isolated pea roots grown in sterile nutrient medium (17) indicated that some other factor or factors act together with auxin to control lateral root formation in this plant. The continued study of these unknown factors revealed some remarkable effects of light on root growth? effects which may contribute to our understanding of the processes controlling root growth. Relatively few investigators have described direct effects of illumination on the growth of this usually subterranean organ of the plant body. Scattered reports in the literature including such early work as that by Darwin (2) and others (7), demonstrated an inhibitory effect of white light on root elongation. Segelitz (15) reported that isolated corn roots grown in culture in the dark extend more rapidly than roots exposed to white light, an effect attributed to the production of auxin by roots in the light. Robbins and Maneval (14) reported that the growth of excised corn roots was favorably influenced by light, an effect also reported by White (20) for the excised roots of wheat grown in sterile culture. Naundorf (8) showed that illumination causes increased auxin production in the roots of intact or seedless Helianthus plants, which results in a slight acceleration of root elongation. According to Naundorf, auxin stimulation of root elongation may occur when suboptimal concentrations of auxin are normally present in the root. Phototropic responses by roots, either positive or negative, are recorded for a number of different species of plants and attempts have been made (8) to explain these responses in terms of differential auxin production in the light and dark. White (21) has stated that light has no observable effect on the growth of tomato roots in a sterile nutrient medium. Robbins (13) reported a peculiar periodic formation of root hairs by isolated roots of Datura grown in