Calcium Effects on Stomatal Movement in Commelina communis L.

Stomatal movements depend on both ion influx and efflux; attainment of steady state apertures reflects modulation of either or both processes. The role of Ca(2+) in those two processes was investigated in isolated epidermal strips of Commelina communis, using the Ca(2+) chelator EGTA to reduce apoplastic Ca(2+). The results suggest that a certain concentration of Ca(2+) is an absolute requirement for salt efflux and stomatal closure. EGTA (2 millimolar) increased KCl-dependent stomatal opening in darkness and completely inhibited the dark-induced closure of initially open stomata. Closure was inhibited even in a KCl-free medium. Thus, maintenance of stomata in the open state does not necessarily depend on continued K(+) influx but on the inhibition of salt efflux. Opening in the dark was stimulated by IAA in a concentration-dependent manner, up to 15.4 micrometer without reaching saturation, while the response to EGTA leveled off at 9.2 micrometer. IAA did not inhibit stomatal closure to the extent it stimulated opening. The response to IAA is thus consistent with a primary stimulation of opening, while EGTA can be considered a specific inhibitor of stomatal closing since it inhibits closure to a much larger degree than it stimulates opening. CO(2) causes concentration-dependent reduction in the steady state stomatal aperture. EGTA completely reversed CO(2)-induced closing of open stomata but only partially prevented the inhibition of opening.