Vanadate Inhibits Blue Light-Stimulated Swelling of Vicia Guard Cell Protoplasts

When supplied under low chloride concentrations, vanadate inhibits the blue light-stimulated swelling of Vicia faba L. guard cell protoplasts in a dose-dependent fashion. The volume of guard cell protoplasts incubated in 10 mm K-imino-diacetic acid, 0.4 m mannitol, and 1 mm CaCl(2) remained essentially constant under 1000 mumol m(-2) s(-1) red light, but increased an average of 27% after 8 min of the addition of 50 mumol m(-2) s(-1) blue light to the background red light. At 500 mum, vanadate completely inhibits the response to blue light. Vanadate also inhibits the swelling of guard cell protoplasts stimulated by the H(+)-ATPase agonist fusicoccin. The vanadate sensitivity of the blue light-stimulated swelling implicates a proton-pumping ATPase as a component of the sensory transduction of blue light in guard cells.