The effect of leaf temperature on stomatal conductance and net CO2 uptake was studied on French bean (Phaseolus vulgaris L.) using either dehydrated attached leaves (25-40% water deficit) or cut leaves supplied with 10(-4) M abscisic acid (ABA) solution to the transpiration stream. Decreasing leaf temperature caused stomatal opening and increased net CO2 uptake (which was close to zero at around 25° C) to a level identical to that of control leaves (without water deficit) at around 15° C. (i) The ABA effect on stomatal closure was modulated by temperature and, presumably, ABA is at least partly responsible for stomatal closure of french bean submitted to a drought stress. (ii) For leaf temperatures lower than 15° C, net CO2 uptake was no longer limited by water deficit even on very dehydrated leaves. This shows that dehydrated leaves retain a substantial part of their photosynthetic capacity which can be revealed at normal CO2 concentrations when stomata open at low temperature. In contrast to leaves fed with ABA, decreasing the O2 concentration from 21% to 1% O2 did not increase either the rate of net CO2 uptake or the thermal optimum for photosynthesis of dehydrated leaves. The quantum yield of PSII electron flow (measured by ΔF/Fm) was lower in 1% O2 than in 21% O2 for each leaf pretreatment given (non-dehydrated leaves, dehydrated leaves, and leaves fed with ABA) even within a temperature range in which leaf photosynthesis at normal CO2 concentration was the same in these two O2 concentrations. It is concluded that this probably indicates an heterogeneity of photosynthesis, since this difference in quantum yield disappears when using high CO2 concentrations during measurements.