NORMAL AND LOW LINOLENIC ACID SOYBEAN STRAINS - RESPONSE TO PLANTING DATE

Enzymes controlling biosynthesis of fatty acids in the oil of soybean [Glycine mar (L.) Merr.] seeds are sensitive to temperature during the time of oil deposition. Soybean strains have been developed with low levels of linolenic acid (18:3) in an effort to improve oil flavor and stability. This study was conducted to determine the response of fatty acid composition in normal and low linolenic acid soybean strains to planting date, a production variable that may result in different temperatures during oil deposition. 'Century', C1640, and 9509 (three genotypes differing in linolenic acid due to alleles at the fan locus) were planted at four or five dates in each of 5 yr near West Lafayette, IN. Dates when 95% of the pods were mature and the fatty acid composition of the oil were determined for each plot. Fatty acid composition was not affected by planting date in either 1986 or 1990. In 1987 and 1988, levels of palmitic acid tended to decrease slightly and levels of stearic acid tended to increase slightly with successively later planting dates. From 1987 through 1989, linolenic acid was affected more than the other fatty acids by planting date. Regression of linolenic acid content on average maximum daily air temperature from 0 to 20 d prior to maturity demonstrated that 18:3 decreased 4.9 and 3.0 g kg-1 for each 1-degrees-C increase in temperature for Century and C1640, respectively. The data demonstrate that late planting in Indiana should not result in substantially increased levels of linolenic acid in strains containing the fan allele.