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 (Ψ wp ), leaf conductance to water vapour diffusion (g), CO 2 assimilation rate (A) and carbon isotope competition (δ 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 μmol mol -1 ) and elevated (700 μmol mol -1 ) atmospheric CO 2 concentrations ([CO 2 ]). 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 [CO 2 ] (C i ), suggesting a stomatal limitation of A. In contrast, in Q. petraea, A/g declined and C i 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 δ 13 C, which conforms to the classical two-step carbon isotope discrimination model. In Q. petraea, the relationship between C i /C a and δ 13 C was positive. Possible causes of this discrepancy are discussed. Lower g values were observed under elevated [CO 2 ] than under present [CO 2 ] in Q. petraea, whereas g was unaffected in P. pinaster. A stimulation of A by elevated [CO 2 ] was found in P. pinaster but not in Q. petraea. In both species, A/g was markedly higher under elevated than under present [CO 2 ]. Whether the differences in the g response to elevated [CO 2 ] found here can be generalized to other drought-avoiding and non-avoiding species remains to be assessed.