Carbon: terrestrial C₄ plants

The carbon isotope composition of terrestrial C 4 plants depends on the primary carboxylation of phosphoenolpyruvate (PEP) and on the diffusion of CO 2 to the carboxylation sites, but is also influenced by the final carboxylation of ribulose‐1,5‐bisphosphate (RuBP). Several models have been used for reproducing this complex situation. In the present review, a particular model is applied as a means to interpret the effects of environmental and genetically determined factors on carbon isotope discrimination during C 4 photosynthesis. As a new feature, the model considers four types of limitation of the overall CO 2 assimilation rate. Both carboxylation reactions are assumed to be limited by either maximum enzyme activity or maximum substrate regeneration rate. The model is applied to experimental data on the effects of CO 2 , irradiance and water stress on short‐term discrimination by leaves of several C 4 species measured simultaneously with CO 2 gas exchange characteristics. In particular, different patterns of the influence of low irradiances on carbon isotope discrimination are interpreted as due to variations in that irradiance at which a transition from limitation by PEP regeneration rate and RuBP carboxylase activity to limitation by the regeneration rates of both substrates occurs. After discussing literature data on the effects of environmental conditions on carbon isotope discrimination by C 4 plants seasonal and developmental changes in carbon isotope composition, studies on the systematic and geographic distribution of C 4 plants, evolutionary and genetical aspects, and some ecological implications are reviewed.