The Role of the Peripheral Cell Layers in the Geotropic Curvature of Sunflower Hypocotyls: a New Model of Shoot Geotropism

Abstract

The rate of elongation of sunflower (Helianthus annuus L.) hypocotyl sections was found to be dependent on the rate of growth of the outermost cell layers (peripheral cell layers) of that tissue. Hypocotyl sections from which those layers had teen peeled grew but did not show typical geotropic curvature. A model of geotropic curvature is proposed where the differential growth causing curvature is due to a differential rate of elongation between the upper and lower peripheral cell layers of a horizontal shoot. In the model it is speculated that the peripheral cell layers are the site of both geoperception and georesponse. The model does not involve a lateral movement of a growth regulator and experiments with longitudinally bisected hypocotyl sections provided evidence consistent with this model but inconsistent with the Cholodny-Went theory of geotropism.