The responses of predawn leaf water potential (φwp), leaf conductance to water vapour diffusion (g), CO2 assimilation rate (A) and carbon isotope competition (δ13C) to a soil drying cycle were assessed in Pinus pinaster, a drought-avoiding species with high stomatal sensitivity to drought, and Quercus petraea, a drought-tolerant species with lower stomatal sensitivity to drought, under present (350 μmol−1) and elevated (700 μmol−1) atmospheric CO2 concentrations ([CO2]). In P. pinaster, decreasing A in response to drought was associated with increasing plant intrinsic water use efficiency (A/g) and with decreasing calculated intercellular [CO2] (C1), suggesting a stomatal limitation of A. In contrast, in Q. petraea, A/g declined and C1 increased during the drying cycle, which suggests a non-stomatal origin for the decrease in A. In P. pinaster, a negative relationship was observed between the gas exchange-derived values of Ci/Ca and δ13C, which conforms to the classical two-step carbon isotope discrimination model. In Q. petraea, the relationship between C1/Ca and δ13C was positive. Possible causes of this discrepancy are discussed. Lower g values were observed under elevated [CO2] than under present [CO2] in Q. petraea, whereas g was unaffected in P. pinaster. A stimulation of A by elevated [CO2] was found in P. pinaster but not in Q. petraea. In both species, A/g was markedly higher under elevated than under present [CO2]. Whether the differences in the g response to elevated [CO2] found here can be generalized to other drought-avoiding and non-avoiding species remains to be assessed.
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