Carbon isotope composition and water-use efficiency in plants with crassulacean acid metabolism.

The relationship between water-use efficiency, measured as the transpiration ratio (g HO transpired g above- plus below-ground dry mass accumulated), and C / C ratio (expressed as δC value) of bulk biomass carbon was compared in 15 plant species growing under tropical conditions at two field sites in the Republic of Panama. The species included five constitutive crassulacean acid metabolism (CAM) species [Aloe vera (L.) Webb & Berth., Ananas comosus (L.) Merr., Euphorbia tirucalli L., Kalanchoë daigremontiana Hamet et Perr., Kalanchoë pinnata (Lam.) Pers.], two species of tropical C trees (Tectona grandis Linn. f. and Swietenia macrophylla King), one C species (Zea mays L.), and seven arborescent species of the neotropical genus Clusia, of which two exhibited pronounced CAM. The transpiration ratios of the C and CAM species, which ranged between 496 g HO g dry mass in the C-CAM species Clusia pratensis Seeman to 54 g HO g dry mass in the constitutive CAM species Aloe vera, correlated strongly with δC values and nocturnal CO gain suggesting that δC value can be used to estimate both water-use efficiency and the proportion of CO gained by CAM species during the light and the dark integrated over the lifetime of the tissues.