We investigated responses of growth, leaf gas exchange, carbon-isotope discrimination, and whole-plant water-use efficiency to elevated [CO2] in seedlings of five leguminous and five non-leguminous tropical tree species. Plants were grown at CO2 partial pressures of 40 and 70 Pa. As a group, legumes did not differ from non-legumes in growth response to elevated [CO2]. The mean ratio of final plant dry mass at elevated to ambient [CO2] (ME/MA) was 1.32 and 1.24 for legumes and non-legumes, respectively. However, there was large variation in ME/MA among legume species (0.92 to 2.35), whereas non-legumes varied much less (1.21 to 1.29). Variation among legume species in ME/MA was closely correlated with their capacity for nodule formation, as expressed by nodule mass ratio, the dry mass of nodules for a given plant dry mass. Whole-plant water-use efficiency (WP) increased markedly in response to elevated [CO2] in all species. The ratio of intercellular to ambient CO2 partial pressures during photosynthesis remained approximately constant at ambient and elevated [CO2], as did carbon isotope discrimination, suggesting WP should increase proportionally for a given increase in atmospheric [CO2]. Results suggest that tree legumes with a strong capacity for nodule formation could have a competitive advantage in tropical forests as atmospheric [CO2] rises, and that water-use efficiency of tropical tree species will increase under elevated [CO2].
