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 grow ing 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) l ower 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 phas e (third growing flush), which corresponded also to the phase of maxim um leaf expansion rate. Both the number of leaves per plant and the pl ant leaf area were 30% higher in the elevated [CO2] treatment than und er ambient [CO2]. In the droughted conditions, no significant enhancem ent in biomass growth and in plant leaf area was brought about by the elevated [CO2]. Transpiration rate was lower in the elevated [CO2] con ditions, but whole plant water use was similar in the two [CO2] treatm ents, 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-wat ered conditions but only by 18% in the droughted conditions. Carbon is otope discrimination (Delta) was decreased by drought and was increase d by the elevated [CO2]. A negative linear relationship was found betw een transpiration efficiency divided by the atmospheric [CO2] and Delt a, as predicted by theory.