Growth and Yield of Sunflower and Soybean under Soil Water Deficits1

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Abstract
Quantitative data are needed to support the thesis that sunflower (Helianthus annuusL.) is more drought resistant than soybean [Glycine max(L.) Merr.]. In 1980 and 1981 a comparative study of crop growth, seed yield, and soil water extraction patterns was conducted on a Woodburn silt loam soil (fine‐silty, mixed mesic Aquultic Argixeroll) to characterize the crop responses under moderate and severe soil water deficits. Line source sprinkler designs were used on both crops to generate well‐irrigated, deficit‐irrigated, and dryland treatments. Crop growth and soil water measurements were taken at 7‐ to 10‐day intervals on both crops in 1980 and 1981, respectively. In the well‐irrigated treatments, sunflower and soybean produced comparable total dry matter (1400 and 1350 g m−2, respectively) with seed yields of 4.10 and 3.29 Mg ha−1, respectively. Soil water deficits reduced dry matter production in the deficit and dryland treatments by 22 and 50% in sunflower and by 18 and 70% in soybean; and reduced seed yield by 20 and 51% in sunflower and by 27 and 87% in soybean, respectively. Leaf area index (LAI) was the vegetative growth characteristic of sunflower most sensitive to soil water deficits, and seeds per plant was the yield component most affected. Soil water deficits did not affect the harvest index (HI) of sunflower. In soybean LAI and net assimilation rate (NAR) were reduced under severe soil water deficits. Pod number was the yield component most sensitive to soil water deficits in soybean. Dryland sunflower extracted soil water to a 1.8‐m depth and thus avoided severe water stress during its reproductive period. In contrast, dryland soybean depleted considerably less soil water from the 0.9‐ to 1.8‐m depth. As a result, dryland soybean suffered severe water stress during its reproductive period, which dramatically lowered HI and seed yield. This study confirms that sunflower may avoid prolonged drought better than soybean by extracting water from deeper in the soil profile.