Field Studies of Leaf Conductance Response to Environmental Variables in Citrus

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Abstract
The relationship between leaf conductance and environmental variables was examined during 2 summer periods in 22-yr-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 [megapascals] by withholding water for 45 days. 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 vapor pressure difference. Response of leaf conductance to irradiance was evident only at low photon flux densities. A model which has been used successfully in other species to relate leaf conductance to atmospheric variables, i.e., irradiance, leaf-to-air vapor pressure difference and leaf temperature, was applied. Only 11% of the variations in the measurements were accounted for by the model. 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 2 variables showed evidence of feedback control of conductance. The hourly rate of change in leaf water potential showed several distinct oscillations during the day and was highly correlated with leaf conductance.