Leaf gas exchange and carbon isotope composition responses to drought in a drought-avoiding (Pinus pinaster) and a drought-tolerant (Quercus petraea) species under present and elevated atmospheric CO2 concentrations

The responses of predawn leaf water potential (Psi(wp)), leaf conductance to water vapour diffusion (g), CO2 assimilation rate (A) and carbon isotope competition (delta(13)C) 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 mu mol mol(-1)) and elevated (700 mu mol mol(-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] (C-i), suggesting a stomatal limitation of A, In contrast, in Q, pefraea, A/g declined and Ci 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 C-i/C-a, and delta(13)C, which conforms to the classical two-step carbon isotope discrimination model, In Q, petraea, the relationship between C-i/C-a and delta(13)C was positive, Possible causes of this discrepancy are discussed, Lower g values were observed under elevated [CO2] than under present [CO2] in Q. pefraea, 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.