Technical Abstract: Plant ash content has been highly correlated with plant water use efficiency (WUE), and delta 13 in field crops and grassland species, and proposed as selection criteria for WUE. Delta 18 has also been correlated with transpiration in herbaceous plants. The objectives of the study were to 1) evaluate the relationship of plant ash with delta 13, delta 18, and water use efficiency in 'Empire' apple over a three-year-period, 2) determine if yearly variation significantly affected the relationship of plant ash with delta 13 and delta 18, and 3) evaluate the value of the relationship between plant ash content with delta 13 and delta 18 for a population of Malus syverseii. 'Empire' leaf area index was negatively correlated with WUE and ash content, and positively correlated with delta 13 discrimination. Delta 13 discrimination was negatively correlated with 'Empire' ash content and WUE. The ash content of the shoots was significantly related to the WUE, and there were no yearly effects at a site. Within the Malus syverseii accessions, there was a grouping based on ash content that could be identified, and this outlier group also had the lowest delta 18 composition levels of the accessions. There were no correlations of leaf area, number, length, width or stomatal number with delta 18 composition, delta 13 composition, ash content or WUE for the Malus syverseii accessions. The seasonal evapotranspiration (ETo) was highly negatively correlated with delta 18 composition in 'Empire' from 2005 to 2007. Rather than a substitute for delta 13 discrimination measurement in assessing WUE, ash content analysis adds an additional dimension to understanding the dynamics of WUE in apple. This work has identified a unique population of Malus for further study.