Isoflavones in soybean seeds: Genetic variation and environmental effects in field-grown crops

Authors: Britz, Steven; Schomburg, Charlotte; KENWORTHY, WILLIAM - University Of Maryland

Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/2009
Publication Date: 12/7/2009
Citation: Britz, S.J., Schomburg, C.J., Kenworthy, W.J. 2009. Isoflavones in soybean seeds: Genetic variation and environmental effects in field-grown crops. Journal of the American Oil Chemists' Society. 88(6):827-832.

Interpretive Summary: Climate change is expected to bring about increased average temperatures and more variable precipitation patterns. In addition to questions about how these changes may affect yields of agricultural crops, there are concerns about possible changes in the content of important compounds (phytochemicals) present, especially in regard to compounds with known or suspected roles in human or animal nutrition, either beneficial or detrimental. Soybean seeds are an example of a major commodity that is the prime source of important phytochemicals such as isoflavones. Isoflavones are a type of flavonoid with both antioxidant activity and a demonstrated ability to act as a weak form of human hormone estrogen. Hence, isoflavones are often called “phytoestrogens”. Soy products are the major source of three different isoflavones (daidzein, genistein, and glycitein) in the American diet. Although a number of health benefits have been ascribed to various isoflavones, the literature is unclear as to the significance of these effects and also the possibility of negative effects, such the stimulation of breast cancer cells. Therefore, it is important to know the types and amounts of isoflavones in soy seeds and soy products and how they might be affected in the future. Unfortunately, isoflavone concentration and composition are highly variable with large differences between varieties (genetic lines) and also significant environmental effects often superimposed on the genetic differences. Numerous controlled environment and field studies indicate that isoflavones increase several fold when seeds mature under cooler conditions or when plants have adequate soil moisture. The problem is complex, since genetic differences, e.g., early season vs. late season seed maturation, can also affect the environment in which seeds mature. To date there is little information regarding how seed isoflavones in soybean lines differ in maturation timing between locations and years with different weather conditions. Therefore, soybean seeds from 15 lines representing a range maturity groups grown in 2001 and 2002 at three locations in Maryland (normal planting date at all three locations and a delayed planting at one location) were analyzed for isoflavone content. The locations represent typically warmer as well as cooler locations, while the years represent relatively normal (2001) and warm and dry years (2002). Comparing the 15 lines for both years, total isoflavone content varied significantly by a factor of two, but most of the lines were not significantly different from one another. Isoflavone composition as a percent of total isoflavones was similar, averaging 40% for daidzein (33-46% depending on line), 50% for genistein (46-56%) and 10% for glycitein (7-13%). Note that glycitein, although a minor component of whole seed, is a major isoflavone in soy germ and has been reported to provide some unique health benefits (e.g., neuroprotection). Isoflavone content decreased by 30-60% in 2002 compared to 2001 in four early maturing soybean lines grown in warmer locations but not when grown in a cooler location. Isoflavones were not affected consistently or to a large extent in later maturing lines at any location or planting date combination. Relative changes in genistein and daidzein, the major seed isoflavones, were similar whereas relative changes in glycitein were often much larger, much smaller, or even opposite in direction to changes in daidzein and genistein. Thus, the major isoflavones in early maturing soybean lines are more likely to be affected by changes in temperature and precipitation associated with climate change, but it is difficult to conclude from these data how glycitein content will be affected.

Technical Abstract: Both controlled environment and field studies indicate that isoflavones, a dietary source of a class of bioactive phytochemicals present primarily in soybean seeds, increase greatly when seeds mature under cooler conditions or when plants are well-watered. Environmental effects can be superimposed on genetic differences in isoflavone concentration and composition, but genetic differences, e.g., early vs. late seed maturation, can also affect the environment in which seeds mature. To date there is little information regarding how seed isoflavones in soybean lines differing in maturity group change between locations and years with different weather conditions. Therefore, soybean seeds from 15 lines representing four maturity groups grown in 2001 and 2002 at three locations in Maryland (full season at all three and double crop at one location). 2001 and 2002 represent relatively normal and warm and dry years, respectively. Total isoflavones averaged for both years and all locations/planting dates ranged between 4.7 µmol [g seed dry matter (SDM)]-1 in cv. MD95-5358 and 8.7 µmol [g SDM]-1 in cv. Stressland. Isoflavone composition averaged 40% daidzein (33-46% depending on line), 50% genistein (46-56%) and 10% glycitein (7-13%). Isoflavone content decreased 30-60% in 2002 compared to 2001 in early maturing soybean lines grown in warmer conditions but not in a cooler location. Isoflavones were not affected consistently or to a large extent in later maturity lines at any location or planting date combination. Relative changes in genistein, daidzein, and total isoflavons were similar whereas glycitein was much more variable. Early maturing soybean lines are more likely to be affected by changes in temperature and precipitation associated with climate change.