Analysis of Stomatal and Nonstomatal Components in the Environmental Control of CO(2) Exchange in Leaves of Welwitschia mirabilis.

In well-watered plants of Welwitschia mirabilis, grown in the glass-house under high irradiance conditions, net CO(2) assimilation was almost exclusively observed during the daytime. The plants exhibited a very low potential for Crassulacean acid metabolism, which usually resulted in reduced rates of net CO(2) loss for several hours during the night. In leaves exposed to the diurnal changes in temperature and humidity typical of the natural habitats, CO(2) assimilation rates in the light were markedly depressed under conditions resembling those occurring during midday, when leaf temperatures and the leaf-air vapor pressure differences were high (36 degrees C and 50 millibars bar(-1), respectively). Studies on the relationship between CO(2) assimilation rate and intercellular CO(2) partial pressure at various temperatures and humidities showed that this decrease in CO(2) assimilation was largely due to stomatal closure. The increase in the limitation of photosynthesis by CO(2) diffusion, which is associated with the strong decline in stomatal conductance in Welwitschia exposed to midday conditions, may significantly contribute to the higher (13)C content of Welwitschia compared to the majority of C(3) species.