Paper Reviewed
Li, Y., Yu, Z., Jin, J., Zhang, Q., Wang, G., Liu, C., Wu, J., Wang, C. and Liu, X. 2018. Impact of elevated CO2 on seed quality of soybean at the fresh edible and mature stages. Frontiers in Plant Science 9: Article 1413, doi: 10.3389/fpls.2018.01413.

There has been growing interest to understand the potential impacts of rising atmospheric CO2 on the nutritional quality of plants. And in this regard, scientists have reported both enhancements and declines in various nutritional measures in studies examining this topic (see the review of Dong et al., 2018 as an example).

Working with four soybean cultivars (Zhongke-maodou 1, Zhongke-maodou 2, Zhongke-maodou 3 and Hei-maodou), Li et al. (2018) were the latest research group to investigate this phenomenon, assessing the effects of elevated CO2 on the nutritional quality of soybean seeds at the fresh edible (R6) and mature (R8) stages of growth. In particular, they examined changes in crude protein, oil, isoflavones, free amino acids, fatty acids and mineral elements. Plants were grown in pots in open-top chambers at the Northeast Institute of Geography and Agroecology at the Chinese Academy of Sciences, Harbin, China, at either 390 or 550 ppm atmospheric CO2 concentrations during the growing season under well-watered and fertilized conditions.

Results from the study were mixed, with cultivar-specific responses that showed increases, decreases or no change in the nutritional parameters examined. Thus, in generalizing their findings, Li et al. say that elevated CO2 has "both positive and negative effects on the nutritional quality of soybean seeds."

Concerning the negative impacts, they note that elevated CO2 reduced the seed protein content during the R8 stage (but not the R6 stage), as well as some of the micronutrients examined (i.e., Zn and Fe). Focusing on the positive findings, however, the nine Chinese scientists report that soybean "is likely to benefit from the accumulation of seed fat and isoflavone" under elevated CO2. With respect to the accumulation of seed fat, Li et al. write that elevated CO2 "consistently increased oleic acid (18:1) concentrations by decreased linoleic acid (18:2) concentrations at R6 and R8," which finding "indicates that elevated CO2 improves soybean oil quality, with potential benefits for human health," while also noting that high levels of oleic acid "enhance the oxidative stability of soybean oil, giving it a longer shelf life." With respect to the accumulation of isoflavone, the scientists state that "the increase in isoflavone concentration of soybean observed in response to elevated CO2 suggests improved nutritional value of soybean under the scenario of rising CO2 levels," explaining that "foods with high levels of isoflavone have been recommended by the U.S. Food and Drug Administration" due to the "health-promoting effects of isoflavones on human vasomotor symptoms, the cardiovascular system, the breast, uterus, bone, and cognition," citing the works of Morrison et al. (2008) and Clarkson et al. (2011).

Such positive findings and the favorable health implications they infer suggest that it is important to evaluate all plant nutritional parameters when assessing the overall impact of rising atmospheric CO2 on the nutritional quality of plants. Unfortunately, too many people are quick to judge and base their assessment on changes solely on plant protein or micronutrient concentration, thus missing or overlooking important health benefits that elevated CO2 promotes elsewhere, benefits that can outweigh perceived health reductions.

References
Clarkson, T.B., Utian, W.H., Barnes, S., Gold, E.B., Basaria, S.S., Aso, T., et al. 2011. The role of soy isoflavones in menopausal health: report of The North American Menopause Society/Wulf H. Utian Translational Science Symposium in Chicago, IL (October 2010). Menopause: The Journal of the North American Menopause Society 18: 732-753.

Dong, J., Gruda, N., Lam, S.K., Li, X. and Duan, Z. 2018. Effects of elevated CO2 on nutritional quality of vegetables: A review. Frontiers in Plant Science 9: Article 924, doi: 10.3389/fpls.2018.00924.

Morrison, M.J., Cober, E.K., Saleem, M.F., McLaughlin, N.B., Fregeau-Reid, J., Ma, B.L., Yan, W., and Woodrow, L. 2008. Changes in isoflavone concentration with 58 years of genetic improvement of short-season soybean cultivars in Canada. Crop Science 48: 2201-2208.