Large variation in whole-plant water-use efficiency among tropical tree species

It is well known that whole-plant water-use efficiency (transpiration efficiency of carbon gain, TEC) varies among plant species with different photosynthetic pathways. However, less is known of such variation among tree species within the C-3 group. Here we measured the TEC of seven C-3 tropical tree species. Isotopic analyses (delta C-13, delta O-18, and delta N-15) and elemental analyses (carbon and nitrogen) were undertaken to provide insight into sources of variation in TEC. Plants were grown over several months in approx. 80% full sunlight in individual 38-l containers in the Republic of Panama. Soil moisture content was nonlimiting. Significant variation was observed in TEC among the C-3 tree species. Values ranged from 1.6 mmol C mol(-1) H2O for teak (Tectona grandis) to 4.0 mmol C mol(-1) H2O for a legume, Platymiscium pinnatum. Variation in TEC was correlated with both leaf N concentration, a proxy for photosynthetic capacity, and oxygen-isotope enrichment, a proxy for stomatal conductance. The TEC varied with C-isotope discrimination within species, but the relationship broke down among species, reflecting the existence of species-specific offsets.