INTERACTIONS OF ELEVATED CO2 AND DROUGHT STRESS IN GAS-EXCHANGE AND WATER-USE EFFICIENCY IN 3 TEMPERATE DECIDUOUS TREE SPECIES

The effect of CO2 increase on gas exchange and water-use efficiency (W UE) in three temperate deciduous species (Fagus crenata, Ginkgo biloba and Alnus firma) under gradually-developing drought-stress was assess ed. Seedlings were grown within transparent open-top cabinets and main tained for 4 months at mean CO2 concentrations of either 350 (ambient; C-350) Or 700 mu mol mol(-1) (elevated; C-700) and combined with five water regimes [leaf water potential, Psi(w), higher than -0.3 (well-w atered), -0.5 and -0.8 (moderate drought), -1.0 and fewer than -1.2 MP a (serious drought-stress)]. Increase in CO2 concentration induced a 6 0 % average increase in net photosynthetic rate (P-N) under well-water ed conditions. The effect of C-700 became more pronounced with drought stress established, with an 80 % average increase in P-N at Psi(w), a s low as -0.8 MPa; leaf conductance to water vapour transfer (g(s)) an d transpiration rate (E), however, were significantly decreased. Conse quently, WUE increased under drought, through drought stress affected potential E sooner than potential P-N. The interaction of CO2 x drough t stress on WUE was significant in that P-N was stimulated while E in C-700 enriched plants resembled that of C-350 plants under drought. He nce if a doubling of atmospheric CO2 concentration occurs by the mid 2 1(st) century, then greater P-N in F. crenata, G. biloba and A. firma may be expected and the drought susceptibility of these species will b e substantially enhanced.