EFFECT OF YEARS AND PLANTING DATES ON FATTY-ACID COMPOSITION OF SOYBEAN GENOTYPES

Soybean [Glycine max (L.) Merr.] genotypes with modified fatty acid composition in the seed oil are being considered for commercial production. The stability of fatty acid composition across a range of environmental conditions will be an important factor for end users of the new soybean oils. The objective of this study was to evaluate the influence of years and planting dates on the fatty acid composition of soybean genotypes with elevated palmitic acid, elevated stearic acid, or reduced linolenic acid. Ten soybean lines with modified fatty acid composition and two common cultivars were grown in 1988, 1989, and 1990 at the Iowa State University Agricultural Engineering and Agronomy Research Center near Ames. Planting dates were 2 May, 16 May, 30 May, and 13 June. For the genotype with elevated palmitic acid, A21, there were significant differences among years, but the maximum difference was only 10 g kg-1. Effects of planting dates were not significantly different for the palmitic acid content of A21. For the genotype with elevated stearic acid, A6, differences among years, planting dates within individual years, and years x planting dates interaction were significant, but the effect of planting dates averaged across years was not significant. Differences among years were significant for the linolenic acid content of most genotypes; however, the maximum difference among years was only 3 g kg-1 for genotypes with an average of < 25 g kg-1 linolenic acid. Early planting dates favored lower linolenic acid content, but the maximum difference between 2 May and 13 June for any genotype with < 25 g kg - 1 linolenic acid was only 4 g kg-1. Average daily high temperature during the seed-filling period did not consistently explain the differences in fatty acid content among planting dates or years. Our data indicated that different planting dates could not be used as a substitute for years in determining environmental stability of fatty acid composition.