Carbon isotope ratios in crassulacean Acid metabolism plants: seasonal patterns from plants in natural stands

A year round study of photosynthesis and carbon isotope fractionation was conducted with plants of Opuntia phaeacantha Engelm. and Yucca baccata Torr. occurring in natural stands at elevations of 525, 970, 1450 and 1900 m. Plant water potentials and the daytime pattern of ¹⁴CO₂ photosynthesis were similar for all cacti along the elevational gradient, despite significant differences in temperature regime and soil water status. Carbon isotope ratios of total tissue and soluble extract fractions were relatively constant throughtout the entire year. Additionally, the σ¹³C values were similar in all plants of the same species along the elevational gradient, i.e. −12.5 ± 0.86 ‰ for O. phaeacantha and −15.7 ± 0.95 ‰ for Y. baccata. The results of this study indicate Crassulacean acid metabolism predominates as the major carbon pathway of these plants, which do not facultatively utilize the reductive pentose phosphate cycle of photosynthesis as the primary carboxylation reaction.