Enhanced Diaphototropic Response to Vectorial Excitation in Solar-Tracking Leaves of Lavatera cretica by an Immediately Preceding Opposite Vectorial Excitation

Abstract

The rate of the diaphototropic response (angular velocity of laminar reorientation) is constant under a constant level of vectorial excitation by an oblique beam (angle of incidence and fluence rate of light intercepted by the lamina). Laminar elevation decreases when the oblique beam is oriented towards the base of the lamina (BO) and increases when the beam is oriented towards the tip (TO). The responses to TO and BO excitations were enhanced considerably when such excitations were immediately preceded by exposure to equivalent excitations in the opposite directions (BO and TO, respectively). Plants exposed to several cycles of «solar» tracking under simulated conditions were more responsive to vectorial excitation, whether or not it was preceded by an opposite excitation, but the enhancement by a preceding opposite excitation remained the same. Exposure to vectorial excitation enhanced the response to a subsequent opposite excitation even when the lamina was mechanically restrained from responding throughout the first excitation, although the effect was somewhat reduced. Exposure of the leaf to non-vectorial light (normal to the lamina) for the same time and at an equivalent fluence rate as vectorial excitation did not enhance the response to a subsequent vectorial excitation. The results suggest that in addition to providing on-line directional information, vectorial excitation may produce longer-lasting effects on membrane transport.