Carbon isotope ratio and the extent of daily CAM use by Bromeliaceae

Use of carbon isotope ratio (δ13C) to resolve photosynthetic pathways (C3, C4 or CAM) has limitations imposed by the use of intermediate photosynthetic modes by certain plant taxa. Diel gas-exchange patterns, leaf δ13C values and nocturnal tissue acidification were determined for 50 Bromeliaceae. δ13C values for well watered plants reflected the proportion of daily CO2 uptake occurring at night. Thirteen per cent of species with δ13C values typical of C3 plants (i.e. from -22.6 to -31.5‰) showed nocturnal acidification and either a small proportion (< 10%) of daily CO2 uptake occurring nocturnally or internal CO2 recycling during part of the night. None altered CAM expression in response to short-term drought, but the contribution of CAM to daily carbon gain became proportionally more important as C3 CO2 uptake failed. Surveys of plant communities using solely the carbon isotope technique underestimate the number of CAM-equipped plants.