Yield and Water Use Response of Cuphea to Irrigation in the Northern Corn Belt

Cuphea (Cuphea viscosissima Jacq. x C. lanceolata W.T. Aiton) may be prone to drought stress, yet little is known about the yield response of this new oilseed crop to irrigation. A field study was conducted in western Minnesota on a Barnes loam soil (fine-loamy, mixed, superactive, frigid Calcic Hapludolls) in 2002 and 2003 to compare yield and water use of irrigated and nonirrigated cuphea. Nonirrigated cuphea received only precipitation, while irrigated plants received supplemental watering to maintain soil water content near field capacity during the study. Crop drought stress was assessed by measuring photosynthesis, stomatal conductance, leaf water potential, and delta C-13 (C-13/C-12 stable isotope composition). In 2002, soil water content was similar for irrigated and nonirrigated plants throughout the growing season owing to timely rains, and seed yield was similar between treatments (873 kg ha(-1)). During 2003, irrigation resulted in a 2.7-fold increase in seed yield and greater harvest index than nonirrigated plants, while water use efficiency (WUE) of seed production for both treatments was similar (2.4 kg ha(-1) mm(-1)). Drought conditions occurred in 2003; by mid-August, leaf photosynthesis and leaf water potential of nonirrigated plants were 83 and 56% less, respectively, than for irrigated cuphea. Moreover, delta C-13 measurements of seed further confirmed that nonirrigated cuphea suffered significant drought stress. Our results indicate that cuphea is drought sensitive and that regions or soils prone to water deficits will likely require supplemental irrigation to increase seed yield.