Temperature effects on tocopherol composition in soybeans with genetically improved oil quality

Tocopherol, a natural antioxidant, typically accounts for a smalt percentage of soybean (Glycine max L. Merr.) oil. Alleles that govern the expression of polyunsaturated fatty acids in soybean germplasm are influenced by temperature. However, little is known about the environmental influences on tocopherol expression. The objective of this study was to assess the influence of temperature on tocopherol composition in soybean germplasm that exhibit homozygous recessive and dominant alleles that govern the predominant omega-6 and omega-3 desaturases. The control cv. Dare and three low-18:3 genotypes (N78-2245, PI-123440, N85-2176) were grown under controlled-temperature environments during reproductive growth. Analysis of crude oil composition at various stages of seed development revealed a strong negative correlation between total tocopherol content and growth temperature. The relative strength of this correlation was greater in the germplasm that exhibited homozygous alleles governing the omega-6 desaturase than those governing the omega-3 desaturase. The decline: in total tocopherol with reduced temperature was attributed predominantly to loss of gamma-tocopherol However, gamma-tocopherol concentration also was directly related to 18:3 concentration in all genotypes. Thus, low-18:3 oils contained both a lower content and a lower concentration of gamma-tocopherol. Although the biochemical basis for this observation is unknown, the antioxidant capacity df gamma-tocopherol appeared to be directly associated with changes in oil quality that were mediated more by genetic than by environmental influences on 18:3 concentration. Another aspect of this work showed that low-18:3 soybean varieties should be expected to contain more a-tocopherol, especially when grown under normal commercial production environments. This condition should be regarded as another beneficial aspect of plant breeding-approaches to the improvement of soybean oil quality.