Concentration and {sigma}13C of leaf carbohydrates in relation to gas exchange in Quercus robur under elevated CO2 and drought

The variations of leaf carbohydrate concentration, carbon isotopediscrimination () of leaf soluble carbohydrate, gas-exchangeand growth during a soil drying cycle under 350 and 700 µmolmol^-1 CO₂ concentrations (CO₂]) inQuercus robur seedlings wereanalysed. In well-watered conditions, a doubling of CO₂] causedan increase of CO₂ assimilation rate (A) ( +47%) and a decreaseof stomatal conductance for water vapour (g) (–25%),anddoubled the intrinsic water-use efficiency (A/g). The valuesof A were not affected by elevated CO₂] which was consistentwith the 2-fold increase of A/g. Elevated CO₂ also significantlyincreased sucrose and starch leaf concentrations as well asaerial growth and plant dry weight. The stimulating effect ofCO₂ enrichment on A and A/g was maintained in moderate droughtconditions, but disappeared in the most severe drought conditions.Drought induced an increase of hexose concentrations in bothCO₂], but this effect was more pronounced under elevated CO₂],which may contribute to increase osmoregulation. From the onsetof drought, starch was depleted in both CO₂]. Carbon isotopediscrimination decreased in response to drought, which correspondedto an increase in A/g according to the two-step model of isotopicdiscrimination. In contrast, the A/g values derived from instantaneousleaf gas-exchange measurements decreased along the drying cycle.The discrepancy observed between the two independent estimatesof water-use efficiency is discussed in terms of time-scaleintegration. The results obtained with the isotopic approachusing soluble carbohydrate suggest a predominant stomatal limitationof CO2 assimilation in response to drought. Soil drying cycle,elevated CO₂, leaf gas-exchange, leaf carbohydrate concentrations,carbon isotope discrimination, growth, Quercus robur. Key words: soil drying cycle, elevated CO₂, leaf gas-exchange, leaf carbohydrate concentrations, carbon isotope discrimination, growth, Quercus robur