Journal of Experimental Botany Effect of irradiance and vapour pressure deficit on stomatal response to CO2 enrichment of four tree species

The stomatal response of seedlings grown in 360 or 720 fitno \ mo r 1 to irradiance and leaf-to-air vapour pressure deficit (VPD) at both 360 and 720 //mol mol 1 CO2 was measured to determine how environmental factors interact with CO2 enrichment to affect stomatal conductance. Seedlings of four species with different conductances and life histories, Cercis canadensis (L.), Quercus rvbra (L.), Populus deltoides (Bartr. ex Marsh.) x P. nigra (L.), and Pinus taeda (L.), were meas-ured in hopes of identifying general responses. Conductance of seedlings grown at 360 and 720 //mol m o l 1 CO2 were similar and responded in the same manner to measurement CO2 concentration, irradiance and VPD. Conductance was lower for all species when measured at 720 than when measured at 360 //mol m o l 1 CO2 at both VPDs (-1.5 and-2.5 kPa) and all measured irradiances greater than zero (100, 300,600,>1600 //mol m~2 s~1). The average decrease in con-ductance due to measurement in elevated CO2 concen-tration was 32 % for Cercis, 29 % for Quercus, 26 % for Populus, and 11 % for Pinus. For all species, the abso-lute decrease in conductance due to measurement in CO2 enrichment decreased as irradiance decreased or VPD increased. The proportional decrease due to measurement in CO2 enrichment decreased in three of eight cases: from 0.46 to 0.10 in Populus and from 0.18 to 0.07 in Pinus as irradiance decreased from>1600 to 100 //mol m"2 s " 1 and from 0.35 to 0.24 in Cercis as VPD increased from 1.3 to 2.6 kPa