Carbon Isotope Discrimination in Photosynthesis of CAM Plants

Abstract

SUCCULENT plants capable of Crassulacean acid metabolism (CAM) show extremely variable carbon isotope discrimination ratios1,2. This ratio, usually expressed as a δ13C value referred to a standard3, has emerged as a useful diagnostic criterion to determine photosynthetic pathways in higher plants3–5. Species with the C4 photosynthetic pathway6 show less negative δ13C values than do species with the C3 photosynthetic pathway. The difference in isotope discrimination is believed to be a result of the different fractionation characteristics of the primary phosphoenolpyruvate (PEP) carboxylase of C4 plants and primary ribulosediphosphate (RuDP) carboxylase of C3 plants7. Within a species, the δ13C value for total carbon does not usually vary by more than ±0.5‰ and it is largely insensitive to environmental conditions during growth8, although the soluble components within a particular leaf may show some variation9. Examples of the constancy of δ13C values are shown in the genus Atriplex, where C4 species have less negative values (for total carbon) of −8 to −12‰ than C3 species which range between −25 and −27‰ (ref. 8).