Distribution of crassulacean acid metabolism in orchids of Panama: evidence of selection for weak and strong modes.

Crassulacean acid metabolism (CAM) is one of three metabolic pathways found in vascular plants for the assimilation of carbon dioxide. In this study, we investigate the occurrence of CAM photosynthesis in 200 native orchid species from Panama and 14 non-native species by carbon isotopic composition (δC) and compare these values with nocturnal acid accumulation measured by titration in 173 species. Foliar δC showed a bimodal distribution with the majority of species exhibiting values of approximately -28‰ (typically associated with the C pathway), or -15‰ (strong CAM). Although thick leaves were related to δC values in the CAM range, some thin-leaved orchids were capable of CAM photosynthesis, as demonstrated by acid titration. We also found species with C isotopic values and significant acid accumulation at night. Of 128 species with δC more negative than -22‰, 42 species showed nocturnal acid accumulation per unit fresh mass characteristic of weakly expressed CAM. These data suggest that among CAM orchids, there may be preferential selection for species to exhibit strong CAM or weak CAM, rather than intermediate metabolism.