Relative humidity is a key factor in the acclimation of the stomatal response to CO2

Previous work has shown that stomata of growth chamber-grown Vicia faba leaves have an enhanced CO 2 response when compared with stomata of greenhouse-grown plants. This guard cell response to CO 2 acclimatizes to the environmental conditions on the transfer of plants between the two environments. In the present study, air relative humidity is identified as a key environmental factor mediating the changes in stomatal sensitivity to CO 2 . In the greenhouse environment, elevation of relative humidity to growth chamber levels resulted in an enhanced CO 2 response, whereas a reduction in the light level to that comparable to growth chamber conditions had no effect on stomatal CO 2 sensitivity. The transfer of plants between humidified and normal greenhouse conditions resulted In an acclimation response with a time-course matching that previously obtained in transfers of plants between greenhouse and growth chamber environments. The high stomatal sensitivity to CO 2 of growth chamber-grown plants could be reduced by lowering growth chamber relative humidity and then restored with its characteristic acclimation time-course by an elevation of relative humidity. Leaf temperature was unchanged during this restoration, eliminating it as a primary factor in the acclimation response. Humidity regulation of stomatal CO 2 sensitivity could function as a signal for leaves inside dense foliage canopies, promoting stomatal opening under low light, low CO 2 conditions.