LEAF STABLE CARBON-ISOTOPE COMPOSITION RECORDS INCREASED WATER-USE EFFICIENCY OF C-3 PLANTS IN RESPONSE TO ATMOSPHERIC CO2 ENRICHMENT

1. A total of 17 temperate C-3 grass and herb species were grown for 5 weeks at three mole fraction treatments of atmospheric CO2 (350, 525 and 700 mu mol mol(-1)). Leaf stable carbon isotope compositions (delta(13)C) were determined to record long-term exchange responses together with instantaneous gas exchange measurements. The isotopic composition of the atmospheric CO2 (delta(13)C(a)) integrated over the course of the CO2 treatments was recorded biologically using the C-4 species Zea mays. 2. We found that increases in the mole fraction of atmospheric CO2 above current levels resulted in a sustained increase in instantaneous (photosynthesis, A/conductance, g(s)) leaf water-use efficiency (IWUE), as calculated from carbon isotope-derived p(i)/p(a) ratios. Grass species showed a marked decline in the magnitude of WUE increase as the CO2 mole fraction was increased from 525 to 700 mu mol mol(-1), a response which was absent in herb species. 3. Isotopic derivation of the ratio of intercellular CO2 mole fraction (p(i)) to that in the surrounding atmosphere (p(a)), considered as a set point of leaf metabolism, showed no significant (P = 0.06) changes in response to increases in the mole fraction of CO2, for herb and grass species. Measurements of p(i)/p(a) determined from measurements of leaf gas exchange differed significantly (P<0.01) from those derived from stable isotope ratios. These differences are attributed to contrasting stomatal behaviour between herb and grass species. 4. Leaf intercellular CO2 mole fraction and previously reported above-ground biomass responses to CO2 increases for the same species were positively correlated (P < 0.05). This suggests that as atmospheric CO2 levels continue to rise species showing sustained higher rates of leaf photosynthesis, may be translated into increased productivity depending on soil water and nutrient status.