Adapting Irrigation Management to Water Scarcity: Constraints of Plant Growth, Hydraulics and Carbon Assimilation.

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 optimize crop yields for available soil water.  The relationship between crop yield and evapotranspiration (ET) of many crops is linear.  In crops such as maize, this relationship is also steeply sloped, providing especially limited capacity to save ET without yield losses.  Results from seasonally-varied strategic application of deficit irrigation suggest potential for optimizing crop yields under limited soil water.  Maize given 40% of full ET during the late vegetative stage has reduced plant height and increased root growth with little effect on leaf area index (LAI).  Maize has extremely high water use efficiency (WUE) and conservative hydraulic control (e.g. stomatal closure under drought stress).  Maximum hydraulic conductance per unit leaf area was similar in full and deficit irrigation treatments, suggesting that reductions in biomass resulted from xylem dysfunction, stomatal closure, or photochemistry.   With little indication of permanent decline in carbon assimilation down to 40% of full ET, maize appears able to buffer grain yield reductions if soil water is available during the reproductive and maturation stages.  However, economic constraints on producers may make limited irrigation more feasible in crops with less steep slopes between crop yield and ET.