Carbon isotope discrimination and the integration of carbon assimilation pathways in terrestrial CAM plants

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Abstract
Measurement of carbon isotope discrimination (Δ) of organic plant material integrates the combination of C4 and C3 carboxylation processes during the phases of CAM through dark and light periods. These processes are tempered by environmental conditions which regulate CAM activity at the molecular, biochemical and ecological level. The factors contributing to short‐term changes in Δ are discussed in terms of the day‐night changes in metabolite pools and integration via on‐line, instantaneous discrimination techniques. Thus, the isotope signature of newly fixed carbon in malic acid reflects the balance between diffusion and carboxylation limitation together with direct and indirect effects of respiratory metabolism. Leakage of CO2 during decarboxylation leads to greater discrimination being expressed than is predicted from existing models. Over the timescales of seasonal growth and productivity, most constitutive CAM and C3‐CAM intermediate plants show little variation in Δ (2–4‰). The changes induced by developmental and environmental signals and genetic regulation of CAM are compared for stem and leaf succulents. The role of CAM as a potentially highly productive photosynthetic pathway is contrasted with the induction of CAM as a maintenance mechanism in response to environmental stresses. Analyses of Δ have already contributed much to our understanding of the distribution and regulation of CAM, and in turn can also be used to analyse phylogenetic relationships and the origins of CAM as determined from palaoecological evidence.