The objectives of this project are to (1) study the physiology of genotypes with contrasting Carbon Isotope Discrimination (CID) to dissect the mechanisms underlying the observed isotope signature; (2 characterize the water use efficiency of genotypes contrasting for Oxygen Isotope Discrimination (0101), (3) apply bulk-breeding and mass selection techniques to develop unique germplasm by using genotypes with proven isotope signatures and elite varieties; and (4) develop mapping populations for CID and OID.