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Carbon isotope discrimination (Delta) and ash content (m(a)) have been proposed as selection criteria for grain yield of wheat grown under drought stress. The association between wheat grain yield and genotypic variation in Delta or m(a) has been found, however, to depend highly on the water regime imposed. In this work the relationships between A, ma and yield were studied in 20 bread wheat (Triticum aestivum L.) genotypes under the arid conditions of North-west Mexico, using four different water treatments (full irrigation, postanthesis water stress, preanthesis water stress and residual soil moisture stress) in two seasons (2000-2001, 2001-2002). These treatments simulated four primary wheat growing mega-environments representing approximately 70% of the wheat producing area worldwide. Carbon isotope discrimination was analyzed on grain at maturity in both seasons and on leaves at booting stage and anthesis in Season 2 as was ash content of the flag leaf and grain at maturity. Grain Delta correlated positively with yield under postanthesis and residual moisture stress. Leaf Delta was positively related with yield only under postanthesis water stress. Significant correlation was found between leaf ma and grain yield under full irrigation and postanthesis water stress. Leaf ma was positively correlated with grain Delta under postanthesis water stress. Grain ma was not associated with yield. These results suggest that under postanthesis water stress, grain Delta and leaf ma may be used as indirect selection criteria for yield, and leaf ma could be useful for predicting yield under well-watered conditions. (C) 2004 Elsevier B.V. All rights reserved.