HELIOTROPIC LEAF MOVEMENT RESPONSE TO H+/ATPase activation,H+/ATPase inhibition,AND K+ CHANNEL INHIBITION IN VIVO

The pulvinus, located at the base of soybean leaflets, is both the light perception and motor organ for heliotropic leaf movements. Our objective was to investigate the role of plasma membrane H⁺/ATPase and TEA-sensitive K⁺ channels in mediating pulvinar response to light. The plasma membrane H+/ATPase activator, fusicoccin, plasma membrane H⁺/ATPase inhibitors, vanadate and erythrosin-B, and the K⁺ channel blocker TEA were introduced to the intact pulvinus through the transpiration stream. The pulvinus was illuminated by a vertical light beam of 1,400 μmol m^-2 s^-1 to stimulate leaf movement. Leaf orientation was measured every 5 min for 60 min of illumination. All compounds tested inhibited pulvinar bending, but concentration and uptake time required for inhibition varied: 12.5 μM fusicoccin reduced leaf movement after 3 hr uptake, 2 mM vanadate reduced leaf movement after 6 hr uptake, 100 μM erythrosin-B reduced leaf movement after 3 hr uptake, and 15 mM TEA reduced leaf movement after 6 hr uptake. In all cases final leaf angle was reduced by higher concentrations and/or increased time for uptake of the chemical into the pulvinus. Results support the hypothesis that the proximal mechanism of heliotropic movement is similar to that of nyctinastic movements.