Light-driven movements of the primary leaves of bean (Phaseolus vulgaris L.): A kinetic analysis

The skotonastic and phototropic responses of primary leaves of bean (Phaseo lus vulgaris L.) were studied, using one primary leaf as an untreated contr ol for its treated twin. A sudden, drastic step-down in photon fluence rate , Phi FR, resulted in the typical skotonastic downfolding. The magnitude an d kinetics of this response were independent of the preceding Phi FR as wel l as of the initial laminar elevation (at transition). The skotonastic resp onse was eventually followed by increase in laminar elevation, as part of a progressive adaptation to the new (very low) Phi FR. The pulvinus exhibite d positive and reversible phototropic curvature in response to adaxial and lateral exposure to high Phi FR. Adaxial exposure at different stages of it s adaptation to low Phi FR resulted in very similar positive phototropic re sponses (rate and magnitude). At saturation, abaxial exposure of the pulvin us to equivalent Phi FR resulted in full reversal, to the same level as exp osure of both sectors to low Phi FR, coinciding with the level of the non-e xposed controls. Pulvinar responses involve processes of solute transport, tightly coupled to transport of water. The results suggest that phototropic pulvinar responses and those that are part of the skotonastic and adaptati on responses are not only different, but are also independent of each other and can take place simultaneously.