LEAF GAS-EXCHANGE AND CARBON-ISOTOPE COMPOSITION RESPONSES TO DROUGHTIN 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 conducta nce to water vapour diffusion (g), CO2 assimilation rate (A) and carbo n isotope competition (delta(13)C) to a soil drying cycle were assesse d in Pinus pinaster, a drought-avoiding species with high stomatal sen sitivity to drought, and Quercus petraea, a drought-tolerant species w ith lower stomatal sensitivity to drought, under present (350 mu mol m ol(-1)) and elevated (700 mu mol mol(-1)) atmospheric CO2 concentratio ns ([CO2]). In P, pinaster, decreasing A in response to drought was as sociated with increasing plant intrinsic water use efficiency (A/g) an d with decreasing calculated intercellular [CO2] (C-i), suggesting a s tomatal limitation of A, In contrast, in Q, pefraea, A/g declined and Ci increased during the drying cycle, which suggests a non-stomatal or igin for the decrease in A, In P, pinaster, a negative relationship wa s observed between the gas exchange-derived values of C-i/C-a, and del ta(13)C, which conforms to the classical two-step carbon isotope discr imination model, In Q, petraea, the relationship between C-i/C-a and d elta(13)C was positive, Possible causes of this discrepancy are discus sed, Lower g values were observed under elevated [CO2] than under pres ent [CO2] in Q. pefraea, whereas g was unaffected in P, pinaster, A st imulation 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 tha n under present [CO2], Whether the differences in the g response to el evated [CO2] found here can be generalized to other drought-avoiding a nd non-avoiding species remains to be assessed.