Breeding for Tolerance to Drought in Winter Wheat.

Although winter wheat is well adapted to many lower rainfall areas of the temperate world, it is also subject to periods of drought stress that vary in timing, intensity, and duration. In the Great Plains, where the majority of U.S. winter wheat is produced, the frequency and severity of these drought episodes are expected to increase according to most climate models. Breeding for these variable climates requires a sound understanding of the target environments and of drought-adaptive traits that provide benefits in drought years without penalties in favorable years. Analysis of historical winter wheat yield and weather data in Colorado has revealed that precipitation during crop establishment in fall and during the pre-flowering stage in spring were critical in determining yield. Attempts to identify secondary traits for selection of drought tolerance have generally not been successful, probably due to the multiple mechanisms wheat plants use to adapt to low moisture and to confounding effects from other stresses. Recently the USDA-funded Triticeae Coordinated Agricultural Project (TCAP) has provided opportunities to associate dense marker arrays with yield-related traits across a range of soil moisture availability, and to evaluate the usefulness of canopy spectral reflectance and root traits as indicators of drought tolerance. An important feature of the TCAP is the T3 database, a national repository for wheat and barley phenotypic and marker data, which will facilitate data analysis across multiple genotypes and environments and application of genomic selection strategies. New sources of germplasm that may provide useful diversity for drought tolerance include synthetic hexaploid wheats and improved cultivars and lines from Eastern Europe and Central Asia. Selection for drought tolerance in winter wheat will remain challenging, but new tools and germplasm should advance progress in the coming decades.