Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/30/2008
Publication Date: 10/1/2008
Citation: Britz, S.J., Kremer, D.F., Kenworthy, W. 2008. Tocopherols in soybean seeds: genetic variation and environmental effects in field-grown crops. Journal of the American Oil Chemists' Society. 85(10):931-936.
Interpretive Summary: Recent recommendations to classify alpha-tocopherol as the sole form of vitamin E to meet human nutritional requiements to the exclusion of other similar forms of tocopherol, and to increase the recommended daily intake of vitamin E, have resulted in over 90% of the U.S. population having inadequate intakes of vitamin E from food. Soybeans are a major source of dietary tocopherols, but contain relatively low amounts of alpha-tocopherol. Since ARS scientists have shown that warm temperatures and drought during seed development can increase the alpha-tocopherol content of soybean seeds several fold, this study was undertaken to compare the impact of different growing locations and planting dates on tocopherols in a wide range of soybean varieties grown in Maryland over a 4 year period. During 3 relatively “normal” years, 1999-2001, large 2-fold differences in the proportion of alpha-tocopherol were observed among 18 different soybean lines grown at 3 locations as part of the Maryland Uniform Soybean Trials. On average, the proportion of alpha-tocopherol was also greater in soybeans from warmer locations in Maryland or when beans developed during warmer parts of the growing season, but the differences were small compared to those as a result of genetic variation among the different soybean varieties. In 2002, there were large differences in the proportion of alpha-tocopherol compared to average values for the preceding 3 years, probably because of warmer temperatures and extreme drought across the State of Maryland. The effects were especially pronounced for earlier maturing varieties of soybean in which seed development coincided with high temperatures during August. In several varieties, the proportion of alpha-tocopherol approached or exceeded 30% of total tocopherols as compared to typical values of 5-10%. Soybean seeds are apparently capable of boosting the production of the vitamin E-form of tocopherol when induced by environmental stress. This suggests breeding programs could be designed to foster this response and generate soybean varieties with higher levels of both alpha-tocopherol and related tocopherols that may also have health benefits. This information should be useful to plant scientists and breeders, soybean growers, nutrition scientists, and climatologists assessing the impact of global climate change on the nutritional properties of crops.
Technical Abstract: The fraction of tocopherol (T) in soybean seeds present as alpha-tocopherol (aT) has been shown to increase several fold as a result of relatively mild increases in temperature or extreme drought during seed maturation, whereas total tocopherols (Ttot) remain approximately constant. These observations suggest that natural variation in weather or climate will affect T under field conditions. To test this hypothesis, we measured T in seeds from 18 soybean lines undergoing variety trials at several locations in Maryland between 1999 and 2002. Comparing lines, there were small but significant differences in Ttot as well as large 2-fold differences in aT (expressed either as absolute concentration or relative to Ttot). aT was weakly and inversely correlated with delta-T, another form of T, while aT/Ttot was not correlated with Ttot. As a result, it should be possible to breed for soybeans with elevated Ttot as well as aT. Seeds from locations on the Eastern Shore of Maryland (full season crops) had higher absolute and relative levels of aT compared to seeds from a (cooler) central Maryland location or seeds from a later planting (double crop) on the Eastern Shore. However, the effects of location or planting date are small compared to those of genetic line when considering the years 1999-2001, which are close to normal with respect to weather. In 2002, a year characterized by drought and warm temperatures at all locations, large 2 to 3.5-fold increases in the proportion of aT were observed in Maturity Group III and IV seeds, especially from full season crops grown at two locations on the Eastern Shore of Maryland.