Environmental stresses constitute one of the major limitations of crop yield worldwide. Effects of these stresses have been closely studied throughout the modern era of plant breeding. The vast majority of breeding projects designed to improve plant germplasm with regard to stress tolerance have involved phenotypic recurrent selection experiments, wherein controlled levels of a specific stress are applied to an array of plants. Relatively little plant breeding effort has gone into direct selection for improved plant consumptive use of water under stress environments. The photosynthetic enzyme, ribulose biosphosphate carboxylase/oxygenase has been shown to discriminate against the naturally occurring carbon isotope 13(C) in C3 plants. Considerable variation has been found for this trait, and significant correlations between water use efficiency and the ratio of carbon isotopes accumulated in plant tissues as a consequence of photosynthesis have been reported. The field experiments reported here confirm the relation of plant water use and carbon isotope discrimination, as reported for other plants, and serve to illustrate the importance of plant genetic background on the expression of these traits and related physiological and morphological plant-stress responses. Progeny analyses from interspecific crosses have provided an excellent means to assess the interplanning of genetic and environmental influences on expression of carbon isotope discrimination.