Expansion of apple growing to dry areas requires reliable approaches to select genotypes tolerant to water stress. We have simulated apple tree transpiration (Ec) from different commercial orchards and compared it with actual transpiration (Ea) derived from sap flow measurements. The modelling approach lies in rearranging a parameterized P-M Equation and incorporating in it a stomatal conductance model derived from meteorological variations. The model uses diurnal courses of variables instead

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... commonly used daily means. We then wrote a complex P-M Equation which already includes the canopy conductance as the function of radiation and vapour pressure deficit. For model calibration, an extensive experimental dataset of tree transpiration was collected from 2011 to 2013 in various sites of Albania and Kosovo, ranging from very warm to temperate. The daily pattern of canopy conductance for water vapour at canopy level provided by the model offers an unprecedented insight into rapid response to humidity of different cultivars as well as their light saturation levels. The satisfactory performance of this functional model should improve the precision of current empirical approaches followed to compute fruit tree water requirements and can be a promising tool for early and quick evaluation of water stress tolerance in apple genotypes.