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 C 4 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). © 1973 Nature Publishing Group.