Carbon isotope ratios of central Mexican Crassulaceae in natural and greenhouse environments.

Measurements of carbon isotope ratios of central Mexican Crassulaceae collected over a broad habitat range show consistent patterns of CAM activity with no indications of substantial flexible photosynthetic pathways between C3 and CAM. The three genera studied - Echeveria, Pachyphytum, and Graptopetalum - are all closely related to Dudleya in which considerable flexible metabolic response has been demonstrated. Comparative measurements of carbon isotope ratios for field collected and greenhouse reared samples of the same taxa showed a uniform occurrence of slightly more negative δ 13C values, but no indication of substantial flexible metabolic response.