Recovery of Photosynthesis after Environmental Stress in Soybean Grown under Elevated CO 2

Episodes of high temperature and water deficit may be more frequent under predicted future climates of warmer mean temperatures and elevated CO 2 . This study investigated whether the effects of an episode of high air temperature (HT, 43°C as a daily maximum), water deficit (WD), or both, had the same effect on the recovery photosynthesis and on leaf water relations of soybean [ Glycine max (L.) Merr., cv. Fiskeby V] grown at ambient CO 2 (aCO 2 ) or elevated CO 2 (eCO 2 ). An 8‐d period of HT, WD, or both (HTWD) were imposed during early seed filling of soybean grown in glasshouses at either 362 or 685 ;μmol mo1 −1 CO 2 . Photosynthesis ( Amax ), stomatal conductance (g 5 ), and water relations were measured in fully expanded upper‐canopy leaves. Immediately after the 8‐d treatments at 60 d after sowing (DAS), Amax was reduced by 31, 48, and 64% in aCO 2 and by 28, 39, and 49% in eCO 2 under HT, WD, and HTWD, respectively, but no significant interactions were detected. At 60 DAS, g 5 was reduced by WD and HTWD in aCO 2 but not by HT while there was little change in g 5 by WD, HT, and HTWD under eCO 2 . Amax fully recovered under WD in eCO 2 by 66 DAS, while Amax remained reduced under WD in aCO 2 . Under each CO 2 concentration, almost full recovery of Amax occurred under HT by 75 DAS but under HTWD Amax never attained control values. At 60 DAS, early morning leaf water potential (ψ) was lower after HT, WD, and HTWD and Amax was a negative function of ψ, at each CO 2 concentration. The results suggest that full recovery of Amax from WD was only possible under eCO 2 , because at aCO 2 , immediately after the stress episode, ψ was below the threshold for chloroplast damage.