Concentration and δ13C of leaf carbohydrates in relation to gas exchange in Quercus robur under elevated CO2 and drought

Abstract

The variations of leaf carbohydrate concentration, carbon isotope discrimination (Δ) of leaf soluble carbohydrate, gas-exchange and growth during a soil drying cycle under 350 and 700 μmol 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 Δ 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.