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Drought is the most important stress for reducing wheat (Triticum aestivum L.) yield and wateruse efficiency (WUE) in the U.S. Southern High Plains (SHP). Adoption of cultivars with higher yield and WUE under drought conditions is critical in the area. The objective of this study was to investigate the physiological basis of yield determination and WUE of wheat in the SHP. A 2-yr field experiment was conducted in 10 genotypes under dryland and irrigated conditions. The newer cultivars or more drought tolerant genotypes had higher yield, biomass, WUE, and water-use efficiency for biomass (WUEbm) under drought. Genotypes with higher yield had more seeds per spike and higher 1000-kernel weight (TKW). The WUE or WUEbm was determined by yield or biomass as genotypic differences in evapotranspiration were not significant. Biomass at anthesis significantly contributed to higher yield under drought. Yield, spikes per square meter, TKW, and harvest index were correlated to spike, stem, and total dry weights per unit area at anthesis. Single stem dry weight was linearly related to seeds per spike. For dryland wheat, remobilization of stem C reserves contributed to yield in 1 yr and to seeds per spike and seeds per square meter in both years. The amount of remobilization was linearly related to single stem dry weight at anthesis. The results of this study indicated that stem dry weight at anthesis may be an important trait for high yield in the SHP environment. [ABSTRACT FROM AUTHOR]