Increases in grain yield of wheat by breeding for an osmoregulation gene: relationship to water supply and evaporative demand

The effect of water stress on yield increases produced by breeding for an o smoregulation gene (or) was examined using both backcross-bred lines (produ ced using allele identification in pollen grains) and inbred lines (produce d using leaf tests). Yields were measured in 39 field experiments spanning 8 seasons. These included experiments where water was supplied through drip irrigators and rain excluded with a mobile shelter. Several approaches to the measurement of stress environment were examined. The commonly used site mean yield, although most accessible and so utilising the most data, was l east effective in explaining variation in yield response. Comparatively hig h efficiency (84%) could be achieved using measurements of rainfall or evap orative demand for specific periods of crop ontogeny, but this suffered the limitation of being season or treatment specific. The results did, however , demonstrate the value of the pollen selection method in increasing yield under conditions of reduced water supply. In keeping with past climatic analyses, and with the theory underlying vari ations in leaf water potential to which osmoregulation responds, an index i ncorporating water supply and evaporative demand accounted well for the yie ld increase (85%) over the wide range of seasons and treatments examined. I t requires inputs of available soil water at sowing, rainfall, and free-wat er evaporation during the growing period, together with sowing and anthesis dates. The index was used to assess potential yield responses in the vario us climates covered by the Australian wheatbelt. Greatest potential lay at the drier, inland, margins especially where soils are lighter and water-hol ding capacities lower.