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

Summary An investigation was carried out into the photosynthetic pathways of the complete bromeliad flora of Trinidad (West Indies). Carbon-isotope ratios (¹³C 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 ¹³C 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 ¹³C 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.