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 (δ13C values) were used to distinguish obligate C3 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 δ13C values was found. The obligate CAM species had values of -10 to -20‰ and the obligate C3 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 C3. One species, Guzmania monostachia var. monostachia, was identified as a C3-CAM intermediate, and others may well exist in the Trinidad flora. The influence of factors such as source CO2, photosynthetic photon flux density and ambient humidity in determining the δ13C values is discussed.The taxonomic distribution of C3 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 C3 plants and less light-demanding CAM plants. The shade-tolerant bromeliads, which show a requirement for high ambient humidity, are all C3 plants. We discuss the phylogenetic origins of CAM and the epiphytic habit in the Bromeliaceae.