Effects of moisture stress on the oil and protein components of soybean seeds

Abstract

Over a five year period, early-maturing indeterminate soybean lines were evaluated in parallel dryland and fully irrigated field experiments. Moisture stress effects on yield, seed weight, and oil and protein content of seeds were evaluated by comparing genotype x moisture regime means. Both the amount and distribution of rainfall varied among seasons, resulting in dryland yields from 6% to 84% of the fully irrigated experiments. Seed weight also responded to seasonal rainfall patterns with reductions owing to moisture stress contributing from 29% to 54% of the yield response. Between 57% and 68% of the seed weight loss was attributed to changes in weight per seed of oil and protein. An index relating response to stress to irrigated performance was calculated for each trait. Regression equations showed significant (P > 0.05) relationships between the effect of stress on seed weight and the resultant oil and protein responses to stress. When compared to the irrigated treatments, oil and protein percentages of dryland seed varied, depending on the balance between reduction in seed weight and response of these constituents to the pattern of stress. In one season when severe stress occurred very early in podfill, the result was reduced protein and increased oil percentages. In three other years the protein percentage of dryland seed increased, and was accompanied by a decrease in oil percentage. In a high rainfall season, non-significant (P > 0.05) increases in both oil and protein percentage were obtained. Sowing early-maturing genotypes into soils with more than 150 mm available soil moisture will usually avoid moisture stress until after podfill commences. Stress will usually increase during podfill, which will most often result in higher protein and lower oil content of dryland produced soybean seed.