Blue light-dependent proton extrusion by guard-cell protoplasts of Vicia faba

Abstract

In plant leaves, light regulation of stomatal movement, and hence of gas exchange¹, involves a non-photosynthetic photosensory system sensitive to blue light^2–6, which transduces many other plant movement responses. The exact nature of the primary photo-reactions of this system is still not known⁷. Stomatal opening is, in general, based on swelling of the guard cells driven by ion uptake¹, and it has been suggested that energy for such uptake may be provided by a chemi-osmotic extrusion of protons across the plasmalemma^1,8,9. We report here that guard-cell protoplasts from Vicia faba, irradiated continuously with photosynthetically saturating, high-fluence-rate red light, extrude H⁺ ions in response to a short pulse (30s) of blue light. The response is relatively long-lived, such that a high rate of H⁺ extrusion continues for several minutes after the pulse stimulation, indicating that continuous input of blue light energy is not required for a sustained response. The kinetic relation between pulse input and response output matched closely that found for the stomatal response in intact leaves⁶. In agreement with the chemi-osmotic hypothesis, activation of an electrogenic plasmalemma H⁺-ATPase is postulated as the cause of blue light-induced H⁺ extrusion.