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 discrimination (Delta) of leaf soluble carbohydrate, gas-exchange and growth during a soil drying cycle under 350 and 700 mu mol mol(-1) CO2 concentrations ([CO2]) in Quercus robur seedlings were analysed. In well-watered conditions, a doubling of [CO2] caused an increase of CO2 assimilation rate (A) (+47%) and a decrease of stomatal conductance for water vapour (g) (-25%), and doubled the intrinsic water-use efficiency (A/g). The values of Delta were not affected by elevated [CO2] which was consistent with the 2-fold increase of A/g. Elevated [CO2] also significantly increased sucrose and starch leaf concentrations as well as aerial growth and plant dry weight. The stimulating effect of CO2 enrichment on A and A/g was maintained in moderate drought conditions, but disappeared in the most severe drought conditions, Drought induced an increase of hexose concentrations in both [CO2], but this effect was more pronounced under elevated [CO2], which may contribute to increase osmoregulation, From the onset of drought, starch was depleted in both [CO2]. Carbon isotope discrimination decreased in response to drought, 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 estimates of water-use efficiency is discussed in terms of time-scale integration. The results obtained with the isotopic approach using soluble carbohydrate suggest a predominant stomatal limitation of CO2 assimilation in response to drought.