Growth dynamics, transpiration and water-use efficiency in Quercus robur plants submitted to elevated CO2 and drought

Seedlings of pedunculate oak (Quercus robur L) were grown for one growing season under ambient (350 mu mol mol(-1)) and elevated (700 mu mol mol(-1)) atmospheric CO2 concentration ([CO2]) either in well-watered or in droughted (the water supply was 40% of the well-watered plants transpiration in both [CO2]) conditions. In the droughted conditions, gravimetric soil water content (SWC) was on average 4 10(-2) g g(-1) lower under elevated [CO2]. In well-watered conditions, biomass growth was 39% higher in the elevated [CO2] treatment than under ambient [CO2]. However relative growth rate (RGR) was stimulated by the elevated [CO2] only for 17 days, in July, at the end of the stem elongation phase (third growing flush), which corresponded also to the phase of maximum leaf expansion rate. Both the number of leaves per plant and the plant leaf area were 30% higher in the elevated [CO2] treatment than under ambient [CO2]. In the droughted conditions, no significant enhancement in biomass growth and in plant leaf area was brought about by the elevated [CO2]. Transpiration rate was lower in the elevated [CO2] conditions, but whole plant water use was similar in the two [CO2] treatments, reflecting a compensation between leaf area and stomatal control of transpiration. Transpiration efficiency (W = biomass accumulation/plant water use) was improved by 47% by the elevated [CO2] in well-watered conditions but only by 18% in the droughted conditions. Carbon isotope discrimination (Delta) was decreased by drought and was increased by the elevated [CO2]. A negative linear relationship was found between transpiration efficiency divided by the atmospheric [CO2] and Delta, as predicted by theory.