Analysis of Stomatal and Nonstomatal Components in the Environmental Control of CO2 Exchange in Leaves of Welwitschia mirabilis

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Abstract
In well-watered plants of Welwitschia mirabilis, grown in the glasshouse under high irradiance conditions, net CO2 assimilations 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 CO2 loss for several hours during the night. In leaves exposed to the diurnal changes in temperature and humidity typical of the natural habitats. CO2 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.degree. C and 50 millibars bar-1, respectively). Studies on the relationship between CO2 assimilation rate and intercellular CO2 partial pressure at various temperatures and humidities showed that this decrease in CO2 assimilation was largely due to stomatal closure. The increase in the limitation of photosynthesis by CO2 diffusion, which is associated with the strong decline in stomatal conductance in Welwitschia exposed to midday conditions, may significantly contribute to the higher 13C content of Welwitschia compared to the majority of C3 species.

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