Evidence Against the Involvement of Gibberellins in the Differential Growth which Causes Geotropic Curvature

Abstract

The geotropic curvature of coleoptiles of three varieties of rice with widely differing endogenous gibberellin levels was studied. The kinetics of the geotropic curvature of coleoptiles from low gibberellin plants were very similar to those from high gibberellin plants. Treatment of the rice seedlings with a growth retardant reduced the rate of geotropic curvature to a similar extent in all three varieties; likewise the application of exogenous gibberellic acid increased the rate of geotropic curvature of all varieties. Thus in the rice coleoptile, geotropic curvature proceeded in a similar manner in the presence of high gibberellin levels or very low gibberellin levels. Horizontal sub-apical sections from light or dark grown Helianthus hypocotyls showed a gravity-induced growth differential despite the fact that such tissues were not sensitive to exogenous gibberellic acid treatment. It is argued that there is no convincing evidence that the establishment of gibberellin gradients in horizontal shoots are the cause of the differential growth which brings about geotropic curvature.