Concentration and σ¹³C of leaf carbohydrates in relation to gas exchange in<i>Quercus robur</i>under elevated CO₂and drought

Abstract

The variations of leaf carbohydrate concentration, carbon isotope discrimination (▵) of leaf soluble carbohydrate, gas-exchange and growth during a soil drying cycle under 350 and 700 μmol mol-1 CO2 concentrations ([CO2]) inQuercus 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 A 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. Soil drying cycle, elevated CO2, leaf gas-exchange, leaf carbohydrate concentrations, carbon isotope discrimination, growth, Quercus robur.