Interactive effects of CO 2 enrichment and drought stress on gas exchange and water-use efficiency in Alnus firma

Abstract

Independent and interactive effects of atmospheric CO 2 enrichment and drought stress on leaf conductance, photosynthetic performance, transpiration and water-use efficiency in 2-year-old Alnus firma, a common pioneer tree species, were assessed. Measurements were conducted in a controlled environment laboratory at three CO 2 concentrations [350 (ambient), 600 and 900 (enrichment) μmol mol −1] and combined with five water regimes [leaf water potential of higher than −0.3 (well-watered), −0.5 and −0.8 (moderate drought), −1.0 and lower than −1.2 (serious drought stress) MPa]. Under well-watered conditions, rates of net photosynthesis significantly ( P<0.01) increased with increasing CO 2 concentrations; leaf conductance significantly decreased. With drought stress established, leaf conductance, photosynthesis and transpiration decreased. However, leaf water-use efficiency increased with drought stress, with potential transpiration affected sooner than potential photosynthesis. The combined effects of CO 2 enrichment and drought stress on water-use efficiency were significant in that the result of net photosynthesis was stimulated while transpiration in CO 2 enriched plants resembled that of unenriched plants under conditions of drought stress. The results presented here suggest that if a doubling of atmospheric CO 2 concentration occurs by the mid-21st Century, then the photosynthetic rate of A. firma in drought-affected regions may be expected to increase. A reduction in total water use, however, is not indicated.