Crop‐Water Relations of Sunflower and Soybean under Irrigated and Dryland Conditions 1

A field study was established on a Woodburn silt loam (fine‐silty, mixed mesic Aquultic Argixeroll) to study the physiological responses of sunflower ( Helianthus annuus L.) and soybean [ Glycine max (L.) Merr.] under irrigated and dryland conditions. Diurnal measurements of leaf water potential (ψ L ), stomatal resistance ( R s ), leaf temperature ( T L ), and carbon exchange rates (CER) were made on individual leaves of both crops on four dates during a soil drying cycle that corresponded to bud formation through flowering in sunflower and flowering through pod formation in soybean. Soil water and leaf area index (LAI) measurements were also taken on the four dates. Dryland sunflower depleted soil water to a depth of 1.8 m, and maintained a minimum ψ L (−1.6 MPa) of 0.4 to 0.6 MPa lower than that of irrigated sunflower on all dates. Although increased Rs of dryland sunflower did not occur until 50 days into the soil drying cycle, 33 and 15% reductions in LAI and CER, respectively, were observed 22 days into the cycle. The data suggested that nonstomatal effects were most responsible for the reduced CER in dryland sunflower. Dryland soybean depleted 45% less soil water than that of dryland sunflower; nevertheless, both dryland crops maintained the same minimum ψ L on all dates. Stomatal closure, which occurred early in the soil drying cycle of dryland soybean, was accompanied by T L 4°C above ambient on occasion, a 50% reduction in CER, and a 68% reduction in LAI. A significant negative correlation ( r = −0.80) was observed between CER and R s of soybean, which suggests that stomatal closure was responsible for the reduced CER in dryland soybean.