Abstract
(1) The relationship between leaf conductance and environmental variables was examined during two summer periods in 22-year-old Shamouti orange trees. Soil water potential in the upper 0.6 m of the root zone was reduced from -0-01 to -0.87 MPa by withholding water for a period of 45 days. (2) Leaf conductance was dominantly controlled by soil water potentials lower than -0-02 MPa. At high soil water potentials, hourly midday conductance was negatively correlated with leaf-to-air vapour pressure difference. Response of leaf conductance to irradiance was evident only at low photon flux densities. (3) A model which has been used successfully in other species to relate leaf conductance to atmospheric variables, i.e. irradiance, leaf-to-air vapour pressure difference, and leaf temperature, was applied. In contrast to work in other species, only 1 1% of the variations in the measurements were accounted for by the model. (4) Average midday leaf conductance was linearly related to leaf water potential. Although hourly leaf water potential was not linearly related to leaf conductance, daily courses of the two variables showed evidence of feedback control of conductance. (5) The hourly rate of change in leaf water potential showed several distinct oscillations during the day and was highly correlated with leaf conductance.
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