Technical Abstract: This study examined the interaction between a reflective particle film and water use efficiency (WUE) response of irrigated and non-irrigated apple trees over a wide range of environmental conditions. The objectives were to measure the specific gas exchange and WUE response of 'Empire' apple treated with a reflective particle film (PF), with and without supplemental irrigation, compared to an untreated control, with and without supplemental irrigation, over a range of leaf area indices (LAI), seasonal evapo-transpiration (ETo) and vapor pressure deficits (VPD) to determine the mechanisms of action affecting WUE in apple. Short-term whole canopy gas exchange studies and isotope discrimination analysis were used to test the hypothesis that WUE was modified by the use of a PF (3% Surround). In whole, canopy gas exchange studies, carbon assimilation (A) and transpiration (E) tended to increase, and WUE and canopy conductance (Gc) tended to decrease with VPD within each LAI class from 2 to 6. For VPD’s > 1 kPa, the 3% Surround irrigated treatment consistently had the greatest WUE, and other treatments were intermediate for LAI’s of 2 to 4. The 3% Surround irrigated and non-irrigated treatments had greater WUE than the control irrigated and non-irrigated treatments for VPD’s < 2 kPa, and there were no treatment effects for VPD’s > 2 kPa in the LAI range of 4 to 6. The 3% Surround non-irrigated was equivalent to the control non-irrigated treatment at VPD’s of 1 to 3 kPa but significantly lower at VPD’s of 3 to 4 kPa. Both 3% Surround irrigated and non-irrigated treatments had the greatest carbon isotope discrimination (delta 13 C), the control non-irrigated treatment had the lowest delta 13 C, and the control irrigated treatment was intermediate. Oxygen isotope enrichment (delta 18 O) was positively correlated with the mean growing season vapor pressure deficit and mean growing season evapotranspiration. Delta 13 C was significantly and positively correlated with delta 18 O. Seasonal water use efficiency (SWUE) was negatively correlated with delta 13 C, and there was an interaction with LAI. The seasonal water use of apple is better evaluated with stable isotope discrimination integrating seasonal variation, rather that the use of whole canopy gas exchange measurements that measure WUE for brief periods of time. Delta 13 C was an accurate measurement of apple WUE and indicated that the 3% Surround irrigated treatment had the greatest delta 13 C, and so the lowest WUE compared to the Control non-irrigated treatment at LAI’s from 4 to 6. The reduced WUE of the 3% Surround irrigated treatment compared to the control non-irrigated treatment is likely due to increased stomatal conductance from lower canopy temperature and increased canopy PAR diffusion that drove increased A. Delta 18 O was an indicator of seasonal water use over 6 growing seasons due to its high correlation with ETo. In 'Empire' apple, A can be increased with PF and irrigation treatments but at the cost of decreased WUE.