| Abstract: | It was found for two species that net carbon dioxide uptakerates were reduced at constant intercellular carbon dioxidepartial pressure when single attached leaves were exposed tolarge leaf to air water vapour pressure differences. Leaf temperature,irradiance, and ambient carbon dioxide and oxygen partial pressureswere kept constant. Net carbon dioxide uptake rates decreasedlinearly with increasing vapour pressure difference, even incases where transpiration rates were highest at intermediatevalues of vapour pressure difference. Decreases in net carbondioxide uptake rates were quickly reversible. Different windspeeds across the measured leaf, different vapour pressure deficitsaround the rest of the shoot, and transient responses of netcarbon dioxide uptake rate to abrupt changes in vapour pressuredifference all gave the same response of net carbon dioxideuptake rate to vapour pressure difference. The data show thatthe inhibition of net carbon dioxide uptake rate at a givenvapour pressure difference was not simply related to whole leaftranspiration rate or stomatal conductance. Key words: Vapour pressure difference, CO2 uptake rate, Leaf temperature |
