Modelling Diurnal Trends of Leaf Water Potential in Transpiring Wheat

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Abstract
Measurements were made of the diurnal changes in the leaf water potential (.psi.L) of flag leaves of winter-sown and spring-sown wheat in 3 years, and on crops grown with different water stresses. Parallel measurements were also made of leaf conductance to water vapor and of the environmental variables necessary to calculate transpiration rate from the flag leaf. A range of models were fitted to the diurnal trends in evaporation, using cubic spline approximations, to interpret the changes in water potential and to develop a simple model for predicting leaf water potential. An Ohm''s Law analogue, with a constant resistance to flow through the soil-plant pathway, explained more than 50% of the variation in leaf water potential. This model was always inferior to models which incorporated a variable resistance (decreasing asymptotically with increasing flux) and/or a capacitance to simulate the hysteresis in the relationship between water potential and evaporation rate. The more sophisticated models usually accounted for more than 90% of the variation in hourly means of leaf water potential. The validity of this type of model, which considers only flow through the flag leaf, and the interpretation of the estimated parameters is discussed.