STOMATAL RESPONSES TO CHANGING IRRADIANCE IN PHASEOLUS-VULGARIS L

The effects of air temperature (T-a), leaf-air vapour differences (VPD) and water deficit on stomatal responses to changing were studied in Phaseolus vulgaris L. Responses were approximately sigmoidal, with rates of closure being faster than the rates of opening. The mean half-time for closure was 5.4 min and for the opening 9.2 min. Under water deficit, both stomatal opening and dosing were faster than in well-watered conditions. Stomata were more sensitive to VPD and water stress than to T-a. The higher the VPD or T-a the more rapid was the stomatal response, except in stressed plants where there was no significant effect of T-a. Under water stress, stomata were more sensitive to water potential (psi) in the opening phase than in the closing phase, and the lower the psi the more rapid was the stomatal response to light change. Responses to changing irradiance in the field were much slower with opening and closing half-times averaging 28.5 and 17.6 min, respectively.