Field Performance of Soybean Cultivars Varying for Water Use Efficiency.

In Ontario, Canada crop water use generally exceeds precipitation during the late reproductive stages of soybean.  This leads to soil water deficits that cause premature canopy senescence, thereby shortening the seed-filling period and reducing yield. Previous research shows that there is variation for water use efficiency (WUE) among commercial soybean varieties currently grown in Ontario. We hypothesized that soybean varieties with higher WUE would be less susceptible to yield losses under naturally-occurring water deficits.  Twenty-three Ontario-adapted varieties of similar maturity were compared for vegetative stage WUE in a greenhouse experiment.  Additionally, 20 of these were grown in a replicated field experiment in 2011 under both irrigated (water-replete) and unirrigated (rain-fed) conditions, to evaluate the effects of naturally-occurring water stress on yield and its components.  Significant variation for WUE was found among varieties in the greenhouse.  In the field, reduction in yield due to water stress was correlated with reductions in both crop biomass and pods per unit area, but not with reductions in seeds per pod or seed size.  Naturally-occurring water stress hastened maturity compared to water-replete conditions, and there was a significant water treatment x variety interaction for days to maturity. Varieties with higher WUE in the greenhouse were less susceptible to accelerated senescence under water deficit conditions in the field. Under water-replete conditions, WUE was positively correlated with yield.  These results suggest that increased WUE may be a desirable trait in soybean varieties developed for this production region.