ABSTRACT

During photosynthetic gas exchange, fractionation against CO2 occurs during both diffusion from the ambient air surrounding the leaf to the sites of carboxylation and subsequently during fixation by ribulose-1, 5-bisphosphate carboxylase/oxygenase. The overriding control of the extent of discrimination expressed by the plant will depend on the rate of supply of CO2 to the sites of fixation. When CO2 supply is not limiting, discrimination will reflect the fractionation of the respective carboxylation enzyme. Respiration is the release of CO2 from plants via decarboxylase enzymes. Growth and/or maintenance respiration (also termed dark respiration) produce CO2 via two decarboxylase enzymes resident in the tricarboxylic acid cycle, namely isocitrate dehydrogenase and 2-oxoglutarate dehydrogenase.