Dynamic aspects of the response of the pulvinus in the leaf of bean plants(Phaseolus vulgaris L.) to photoexcitation

The pulvinus of bean (Phaseolus vulgaris L) responds to unilateral photo-ex citation by phototropic curvature. Osmotically active solutes and water are transported from its exposed to the opposite sector of ils motor tissue, r esulting in differential changes in turgor pressure in these sectors and ge neration of a trans-pulvinar torque. A null-point approach was used to non- invasively study these dynamic changes in the terminal leaflet of bean. A v ariable torque was applied perpendicular to the midrib, to restrain laminar movement by precisely and continuously counteracting the generated torque. This equilibrium prevented curvature of the pulvinus and the associated op posite axial changes in volume in the opposite sectors of its motor tissue. The laminar torques measured were used to estimate stresses (changes in tu rgor pressure) generated within the motor tissue. These stress values were used to derive the corresponding changes in osmotic pressures and in solute concentration. Skotonastically downfolded leaflets were excited with white light to study their combined dynamic response to photonastic and phototro pic excitation. Photonastically unfolded (horizontal) leaflets were excited with blue and red light, alone and in combination, to determine the spectr al dependence of the dynamic pulvinar responses to phototropic excitation b y itself.