RECOVERY OF PHOTOSYNTHESIS AFTER ENVIRONMENTAL-STRESS IN SOYBEAN GROWN UNDER ELEVATED CO2

Episodes of high temperature and water deficit may be more frequent un der predicted future climates of warmer mean temperatures and elevated CO2. This study investigated whether the effects of an episode of hig h sir temperature (HT, 43 degrees C as a daily maximum), water deficit (WD), or both, had the same effect on the recovery of photosynthesis and on leaf water relations of soybean [Glycine max (L.) Merr., cv. Fi skeby V] grown at ambient CO2 (aCO(2)) or elevated CO2 (eCO(2)). An 8- d period of HT, WD, or both (HTWD) were imposed during early seed fill ing of soybean grown in glasshouses at either 362 or 685 mu mol mol(-1 ) CO2. Photosynthesis (Amax), stomatal conductance (g(s)), and water r elations were measured in fully expanded upper-canopy leaves. Immediat ely after the 8-d treatments at 60 d after sowing (DAS), Amax was redu ced by 31, 48, and 64% in aCO(2) and by 28, 39, and 49% in eCO(2) unde r HT, WD, and HTWD, respectively, but no significant interactions were detected. At 60 DAS, g(s) was reduced by WD and HTWD in aCO(2) but no t by HT while there was little change in g(s) by WD, HT, and HTWD unde r eCO(2). Amar fully recovered under WD in eCO(2) by 66 DAS, while Ama x remained reduced under WD in aCO(2). Under each CO2 concentration, a lmost full recovery of Amar occurred under HT by 75 DAS but under HTWD Amar never attained control values. At 60 DAS, early morning leaf wat er potential (Psi) was lower after HT, WD, and HTWD and Amax was a neg ative function of Psi, at each CO2 concentration. The results suggest that full recovery of Amax from WD was only possible under eCO(2), bec ause at aCO(2), immediately after the stress episode, Psi was below th e threshold for chloroplast damage.