FA composition determines the quality of vegetable oil. Soybean breeders have generated and used mutations in FA genes to develop altered FA profiles in the seed. However, the expression of the alleles and the relative activity of the gene products are often dependent on the environment, and these facts have hampered the breeding efforts. To investigate the environmental effect on FA composition of soybean seed oil in specific mutant material developed at the University of Guelph, a recombinant inbred line (RIL) population was developed from a cross between a low palmitate (16:0) line and a high-stearate (18:0) parent. The RIL population was field-tested across three environments over 2 yr. A combined ANOVA for FA composition was conducted to determine the year and location effects on the expression of FA alleles in this material. The results indicated that linolenic (18:3) level was most vulnerable to the environmental changes. Year effects accounted for a greater amount of variance than location effects for 16:0, 18:0, and 18:1, whereas location effects were more important than year effects for the relative amounts of 18:2 and 18:3. Genotype × environment (year, location) interaction effects were significant for the relative amounts of all five FA according to the combined ANOVA. Our results indicated that the extreme minimum daily temperatures during September seed fill period, rather than the means or the maximum temperature, may be responsible for the ratio of saturated vs. unsaturated FA in soybean oil.An improved understanding of environmental effects on FA composition of soybean seed oil could assist breeders and agronomists to interpret data, predict performance, and select proper cultivation strategies for altered FA cultivars. The effects of many environmental factors on the FA composition of soybean seed oil have been investigated, including temperature, year, planting date, planting location, seed-fill duration, photoperiod, light intensity and quality, drought stress, irrigation, total rainfall, mean solar radiation, soil type, and nutrition (1-7). Temperature has been consistently shown to be a predominant environmental factor affecting the FA composition, especially for the unsaturated FA. Elevated levels of oleate (18:1) and reduced linoleate (18:2) and linolenate (18:3) have been reported as a result of increased temperatures during seed development in many studies (3,5-9). However, a more precise estimate of the effects that various temperature profiles have on the final FA composition, during different periods in the growing season is still warranted, especially regarding saturated FA.In soybean breeding, the segregating progeny is often advanced to homozygous recombinant inbred lines (RIL) for evaluation and selection under multiple environments. For soybean oil quality improvement, lines with differently altered FA levels are often chosen as parents in order to combine the genes for desirable FA composition (10). In most of the previous studies regarding the environmental effect on the FA co...

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