Crop Water-Use Efficiency Under Strategic Deficit Irrigation.

Water shortages are responsible for the greatest crop losses around the world and are expected to worsen. In arid areas where agriculture is dependent on irrigation, various forms of deficit irrigation management have been suggested to achieve high yields with less water used by the crop (i.e. evapotranspiration, ET). The observed relationship between crop yield and ET can be curvilinear or linear with the majority of ET being crop transpiration. In arid regions, crops may have limited capacity to reduce transpiration without significant yield losses. However, strategic application of deficit irrigation may improve the yield-ET response. In particular, application of greater deficit during the late vegetative state with full or nearly full ET during the rest of the season consistently maintained yield similar to full ET treatments while saving approximately 17% of ET. Maize given 40% of full ET during the late vegetative stage had reduced plant height but little reduction in final leaf area index. Stress treatments showed conservative hydraulic control (e.g. midday stomatal closure). The efficiency of photosystem II (quantum yield) in maize fluctuated with water stress. The ability of photosystem II and stomatal conductance to recover when well-watered after stress suggests that reductions in biomass and yield resulted from stomatal closures, reduced photochemistry, or xylem dysfunctions that were temporary. With little indication of permanent decline in carbon assimilation after reducing ET by 45% of full ET in vegetative stages, maize appears able to achieve high grain yield if soil water is available during the reproductive and maturation stages. However, if plants were given full or nearly full irrigation during the entire vegetative period, deficits during the maturation period resulted in a greater proportion of yield losses than there were ET savings. Strategic deficit irrigation shows promise for increasing crop productivity and buffering crops again yield losses in water limited environments, although economic modeling is ultimately needed to determine its benefits.