Photosynthetic pathways in the Bromeliaceae of Trinidad: relations between life-forms, habitat preference and the occurrence of CAM.

An investigation was carried out into the photosynthetic pathways of the complete bromeliad flora of Trinidad (West Indies). Carbon-isotope ratios (δC values) were used to distinguish obligate C and crassulacean acid metabolism (CAM) species. Measurements were also carried out on some species in the field to test for day-night changes in leaf titratable acidity.A wide range of δC values was found. The obligate CAM species had values of -10 to -20‰ and the obligate C species of -23 to -35‰ CAM was found (a) in the majority of Tillandsia spp. (Tillandsioideae) and (b) in all species of Bromelioideae. The other genera of the Tillandsioideae appeared to be at least predominantly C. One species, Guzmania monostachia var. monostachia, was identified as a C-CAM intermediate, and others may well exist in the Trinidad flora. The influence of factors such as source CO, photosynthetic photon flux density and ambient humidity in determining the δC values is discussed.The taxonomic distribution of C and CAM species within the Bromeliaceae is analyzed in terms of the life-forms and ecological types recognized by Pittendrigh (1948). The most xerophytic species (the light-demanding "atmospherics") all show CAM and are restricted to the drier parts of the island. Most of the species with waterstoring "tanks" have a wide geographic distribution: these include light-demanding C plants and less light-demanding CAM plants. The shade-tolerant bromeliads, which show a requirement for high ambient humidity, are all C plants. We discuss the phylogenetic origins of CAM and the epiphytic habit in the Bromeliaceae.