Responses of gas exchange to humidity in populations of three herbs from environments differing in atmospheric water.

Responses to humidity of net photosynthesis and leaf conductance of single attached leaves were examined in populations of herbs from wet soil sites in Beltsville, Maryland and Davis, California, USA. Plants were grown in controlled environments under three conditions which differed in the magnitude of the day-night temperature difference and in daytime air saturation deficit. No population differences in response were found in Abutilon theophrasti. In Amaranthus hybridus stomatal conductance and net photosynthesis were more reduced by increasing leaf to air water vapor pressure difference (VPD) in the population from Beltsville, but only for the growth condition with a constant 25°C temperature. In Chenopodium album, stomatal conductance was more sensitive to VPD in the population from Davis, but only for the growth condition with 28/22°C day/night temperatures. Population differences in the sensitivity to VPD of leaf conductance were associated with differences in leaf area to root weight ratio. The relative reduction of net photosynthesis as VPD increased was greater than, equal to, or less than the relative decrease in substomatal carbon dioxide partial pressure. The pattern depended on species, and on growth condition. From these results one can not conclude that environmental humidity has been a strong selective force in determining sensitivity to humidity of stomatal conductance.