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.