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Drought reduces photosynthesis in walnut (Juglans regia L.), but it is not known whether this is due mainly to the closure of stomata, or to possible effects on leaf biochemistry. In an attempt to answer this question we studied diurnal changes in the water status and gas exchange in droughted [50% crop evapotranspiration (ETc)] and fully irrigated (100% ETc) walnut trees, over 2 d. Stem water potential (Psi(s)) ranged from -0.5 MPa in the morning to -1.2 MPa in the afternoon under drought, and from -0.1 MPa to -0.4 MPa under full watering. Net CO2 assimilation (A(max)) ranged from 15 mu mol CO2 m(-2) s(-1) in the morning to 3 mu mol CO2 m(-2) s(-1) in the afternoon under drought, and from 25 mu mol CO2 m(-2) s(-1) in the morning to 10 mu mol CO2 mm(-2) s(-1) in the afternoon under full watering. At these times, stomatal conductance (g,) varied from 0.2 to 0.02 mol H2O m(-2) s(-1) and from 0.7 to 0.2 mol H2O m(-2) s(-1), respectvely. Drought reduced the internal CO, concentration (Q by about 55 mu mol mol(-1) on day-1, and by about 100 mu mol mol(-1) on day-2 and increased leaf temperature (T-l) by about 2 degrees-5 degrees C. The reductions in g(s) and C-i with drought suggest that lower photosynthesis was associated with stomatal closure. However, in each treatment, A(max) decreased during the day, while C-i was stable, suggesting that photosynthesis was also reduced by a direct effect of heat on leaf biochemistry. Both A(max) and g(s) correlated with T-l and with the leaf-to-air vapour pressure deficit (VPDl), but with different relationships for droughted and control trees. However, when stomatal limitations to photosynthesis were accounted for (i.e., based on the assumption that, under stomatal limitation, photosynthesis is proportional to C-i, a single relationship between A(max) and T-l described all the data (R-2 = 0.81). Thus, photosynthesis was limited by the closing of stomata under drought, and by a direct effect of heat on leaf biochemistry. These results suggest that hot and dry weather reduces photosynthesis and potential productivity in walnut in the absence of soil water deficit.