Growth dynamics, transpiration and water-use efficiency in Quercus robur plants submitted to elevated CO 2 and drought

Abstract

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