Control of Ion Fluxes in Stomatal Guard Cells

Opening and closing of the stomatal pore is associated with very large changes in K‐salt accumulation in stomatal guard cells. This review discusses the ionic relations of guard cells in relation to the general pattern of transport processes in plant cells, in plasmalemma and tonoplast, involving primary active transport of protons, proton‐linked secondary active transport, and a number of gated ion channels. The evidence available suggests that the initiation of stomatal opening is regulated through the uptake mechanisms, whereas initiation of stomatal closing is regulated by control of ion efflux at the plasmalemma, and of fluxes to and from the vacuole. In response to a closing signal there are large transient increases in efflux of both Cl− (or Br−) and Rb+ (K+) at the plasmalemma, with also a probable increase in anion flux from vacuole to cytoplasm and decrease in anion flux from cytoplasm to vacuole. A speculative hypothetical sequence of events is discussed, by which the primary response to a closing signal is an increase in Ca2+ influx at the plasmalemma, producing depolarisation and increase in cytoplasmic Ca2+. The consequent opening of Ca2+‐sensitive Cl− channels, and voltage‐sensitive K+ channels (also Ca2+‐sensitive?) in the plasmalemma, and of a Ca2+‐sensitive nonspecific channel in the tonoplast, could produce the flux effects identified by the tracer work; this speculation is also consistent with the Ca2+‐sensitivity of the response to closing signals and with evidence from patch clamping that such channels exist in at least some plant cells, though not yet all shown in guard cells.