Vol. 81 No: 3

Title:
Tree water status and gas exchange in walnut under drought, high temperature and vapour pressure deficit

Authors:
A. ROSATI, S. METCALF, R. BUCHNER, A.N FULTON and B. LAMPINEN

pp: 415-420

Abstract:
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 (ET_c)] and fully irrigated (100% ET_c) walnut trees, over 2 d. Stem water potential (ψ_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 μmol CO₂ m^-2 s^-1 in the morning to 3 μmol CO₂ m^-2 s^-1 in the afternoon under drought, and from 25 μmol CO₂ m^-2 s^-1 in the morning to 10 μmol CO₂ mm^-2 s^-1 in the afternoon under full watering. At these times, stomatal conductance (g_s) 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 CO₂ concentration (C_i) by about 55 μmol mol^-1 on day-1, and by about 100 μmol mol^-1 on day-2 and increased leaf temperature (T_l) by about 2°-5°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 (VPD_l), 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 (R² = 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.

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