Stomatal responses to changing irradiance in Phaseolus vulgaris L.

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The effects of air temperature (T a ), leaf-air vapour pressure differences (VPD) and water deficit on stomatal responses to changing irradiance 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 closing were faster than in well-watered conditions. Stomata were more sensitive to VPD and water stress than to Ta. The higher the VPD or Ta the more rapid was the stomatal response, except in stressed plants where there was no significant effect of Ta