Drought Stress in Ontario Soybean - What's Really Going On?.

Improving drought tolerance is a major goal of many soybean breeding efforts, but soil water deficits vary in timing and severity, leading to a wide array of physiological responses that limit yields in different ways.  Therefore, a mechanistic approach to genetic modification for increased drought tolerance requires knowledge about the actual nature of water deficit events typically occur in the target environment.  To investigate soybean yield losses to soil water deficits, we compared rainfed to water-replete (irrigated) soybean in replicated field trials at a high-yielding site in southern Ontario, Canada over three years.  Even though in-season precipitation was equal to or exceeded the long-term average and there was never any outward sign of water stress such as midday leaf wilting, irrigation always significantly increased yields (from 8% to 25%).  In all three years the number of pods per unit ground area was the yield component most strongly affected by water stress; number of seeds per pod and seed size were only mildly affected.  Only in one year did water stress strongly reduce canopy interceptance of solar radiation or total crop biomass accumulation.  However, in all three years water stress significantly hastened leaf senescence and advanced crop maturity.  Overall, these results indicate that soil water deficits can significantly reduce yields of otherwise high-yielding soybean crops in this region, even in years when there are no obvious outward signs of water stress.  Traits that increase the efficiency with which soil water is used to produce crop biomass may be beneficial in this environment where soil water deficits tend to be mild and transient.