Carbon Isotope Discrimination in Photosynthesis of CAM Plants

Abstract

SUCCULENT plants capable of Crassulacean acid metabolism (CAM) show extremely variable carbon isotope discrimination ratios^1,2. 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 plants^3–5. 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).