BLUE LIGHT-PROMOTED STOMATAL OPENING IN ABAXIAL EPIDERMIS OF COMMELINA-BENGHALENSIS IS MAXIMAL AT LOW-CALCIUM

The requirement for calcium in blue light-promoted stomatal opening, i n comparison with that in red light, was studied in epidermal strips o f Commelina benghalensis L. Blue light promoted stomatal opening in th e presence of a low level of calcium, whereas in red light opening was relatively tolerant to calcium. Stomatal opening under blue light was restricted by external calcium (above 5 mu M) or abscisic acid. When present in the incubation medium, EGTA increased the extent of stomata l opening under blue light. Verapamil (a calcium-channel blocker) and trifluoperazine (TFP, a calmodulin antagonist) reduced the stimulation of stomatal opening by blue light. Lanthanum, an external calcium-cha nnel antagonist, had no significant effect on stomatal opening under e ither blue or red light. These observations indicate that blue light-p romoted stomatal opening preferentially occurs at low levels of calciu m, and modulation by calmodulin is strongly suggested. We conclude tha t a fine-tuning of the calcium level within guard cells is essential d uring the transduction of the blue light signal.