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 H2O transpired g(-1) above-plus below-ground dry mass accumulated), and C-13 /C-12 ratio ( expressed as delta C-13 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., Kalanchoe daigremontiana Hamet et Perr., Kalanchoe pinnata ( Lam.) Pers.], two species of tropical C-3 trees ( Tectona grandis Linn. f. and Swietenia macrophylla King), one C-4 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-3 and CAM species, which ranged between 496 g H2O g(-1) dry mass in the C-3 - CAM species Clusia pratensis Seeman to 54 g H2O g(-1) dry mass in the constitutive CAM species Aloe vera, correlated strongly with delta C-13 values and nocturnal CO2 gain suggesting that delta C-13 value can be used to estimate both water-use efficiency and the proportion of CO2 gained by CAM species during the light and the dark integrated over the lifetime of the tissues.