CONCENTRATION AND DELTA-C-13 OF LEAF CARBOHYDRATES IN RELATION TO GAS-EXCHANGE IN QUERCUS-ROBUR UNDER ELEVATED CO2 AND DROUGHT

The variations of leaf carbohydrate concentration, carbon isotope disc rimination (Delta) of leaf soluble carbohydrate, gas-exchange and grow th during a soil drying cycle under 350 and 700 mu mol mol(-1) CO2 con centrations ([CO2]) in Quercus robur seedlings were analysed. In well- watered conditions, a doubling of [CO2] caused an increase of CO2 assi milation rate (A) (+47%) and a decrease of stomatal conductance for wa ter vapour (g) (-25%), and doubled the intrinsic water-use efficiency (A/g). The values of Delta were not affected by elevated [CO2] which w as consistent with the 2-fold increase of A/g. Elevated [CO2] also sig nificantly increased sucrose and starch leaf concentrations as well as aerial growth and plant dry weight. The stimulating effect of CO2 enr ichment on A and A/g was maintained in moderate drought conditions, bu t disappeared in the most severe drought conditions, Drought induced a n increase of hexose concentrations in both [CO2], but this effect was more pronounced under elevated [CO2], which may contribute to increas e osmoregulation, From the onset of drought, starch was depleted in bo th [CO2]. Carbon isotope discrimination decreased in response to droug ht, which corresponded to an increase in A/g according to the two-step model of isotopic discrimination, In contrast, the A/g values derived from instantaneous leaf gas-exchange measurements decreased along the drying cycle, The discrepancy observed between the two independent es timates of water-use efficiency is discussed in terms of time-scale in tegration. The results obtained with the isotopic approach using solub le carbohydrate suggest a predominant stomatal limitation of CO2 assim ilation in response to drought.