Auxin Factor in Branch Epinasty

Abstract

Until specific growth regulators were known to control the enlargement phase of the growth process in plant organs, the angular position of the plagiotropic branch could be analyzed only as the resultant of negative geotropism and an opposing epinasty. Epinasty of branches was first reported in 1872 by De Vries (3) and confirmed by Baranetzsky (1) in 1901. Rawitscher reviewed (13) subsequent studies by a few workers before 1932 but they had done little more than interpret what seemed to be an inherent property of the plagiotropic branch. The dependence of negative geotropism on a lateral transport of auxin to the lower side of a horizontal organ was demonstrated in 1930 by Dolk (4). More recently his work has been verified by Gillespie and Briggs (5), with supplementary proof supplied by Gillespie and Thimann (6) with IAA-C14. Horizontal coleoptiles of Avena develop a gradient of diffusible auxin from 35 to 40 % in the upper half to 65 to 60 % in the lower half of the organ. The mechanism for this lateral transport of auxin is still unknown but the associated geoelectric effect of Brauner (2) has recently been verified by Grahm and Hertz (8). It is logical to suspect a corresponding but reversed imbalance of auxin in lateral branches when the effect of gravity on its lateraltransport is removed by turning the plant on a horizontal clinostat, as in most demonstrations of epinastic curvatures. S6ding (17) seems to favor this basis for the resulting greater growth of the upper side of the branch, as suggested by the inconclusive evidence of Uyldert (19), and von Witsch (20) with shoots of Tradescantia and by the work of Munch (12) and of Snow (15,16) with auxin pastes. A recent note by Leike and von Guttenberg (11) describes an epinastic response of a defoliated Coleus branch to exogenous auxin applied to the disbudded tip or to the lower side of the youngest internode. This strongly suggests some lateral transport of the growth regulator to the upper side but in no work thus far has it been possible to demonstrate an excess of auxin in the upper (convex) tissues of an epinastic curvature produced in any way. This paper includes an independent confirmation and extension of the points reported by Leike and von Guttenberg, based on work done before their brief note was published. It has also been possible to measure by two methods the distribution of indoleacetic acid (IAA) within the growth zone of a lateral branch when the transport effects of gravity are eliminated.