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Effects of Elevated CO2 on the Isoflavone Content of Soybean Seeds
Caldwell, C.R., Britz, S.J. and Mirecki, R.M.  2005.  Effect of temperature, elevated carbon dioxide, and drought during seed development on the isoflavone content of dwarf soybean [Glycine max (L.) Merrill] grown in controlled environments.  Journal of Agricultural and Food Chemistry 53: 1125-1129.

The authors write that "the beneficial effects of isoflavone-rich foods have been the subject of numerous studies (Birt et al., 2001; Messina, 1999)," and that "foods derived from soybeans are generally considered to provide both specific and general health benefits," presumably via these substances.  Hence, it is only natural they should wonder how the isoflavone content of soybean seeds might be affected by the ongoing rise in the air's CO2 content, and that they would conduct a set of experiments to find the answer.

What was done
Well-watered and fertilized soybean plants were grown from seed to maturity in pots within two controlled-environment chambers, one maintained at an atmospheric CO2 concentration of 400 ppm and one at 700 ppm.  The chambers were initially kept at a constant air temperature of 25C.  At the onset of seed fill, however, air temperature was reduced to 18C until seed development was complete, in order to simulate average outdoor temperatures at this stage of plant development.  In a second experiment, this protocol was repeated, except that the temperature during seed fill was maintained at 23C, with and without drought (a third treatment), while in a third experiment, seed-fill temperature was maintained at 28C, with or without drought.

What was learned
In the first experiment, where air temperature during seed fill was 18C, the elevated CO2 treatment increased the total isoflavone content of the soybean seeds by 8%.  In the second experiment, where air temperature during seed fill was 23C, the extra CO2 increased total seed isoflavone content by 104%, while in the third experiment, where air temperature during seed fill was 28C, the CO2-induced isoflavone increase was 101%.  Finally, when drought-stress was added as a third environmental variable, the extra CO2 boosted total seed isoflavone content by 186% when seed-fill air temperature was 23C, while at a seed-fill temperature of 28C, it increased isoflavone content by 38%.

What it means
Under all environmental circumstances studied, enriching the air with an extra 300 ppm of CO2 increased the total isoflavone content of soybean seeds.  In addition, the percent increases measured under the stress situations investigated were always greater than the percent increase measured under optimal growing conditions.  Consequently, the direct effects of atmospheric CO2 enrichment on the health-promoting properties of soybean seeds are likely universally beneficial and a boon to the entire human race, especially in light of the fact that Bernacchi et al. (2005) characterize the soybean as "the world's most important seed legume."

What it means
Bernacchi, C.J., Morgan, P.B., Ort, D.R. and Long, S.P.  2005.  The growth of soybean under free air [CO2] enrichment (FACE) stimulates photosynthesis while decreasing in vivo Rubisco capacity.  Planta 220: 434-446.

Birt, D.F., Hendrich, W. and Wang, W.  2001.  Dietary agents in cancer prevention: flavonoids and isoflavonoids.  Pharmacology & Therapeutics 90: 157-177.

Messina, M.J.  1999.  Legumes and soybeans: overview of their nutritional profiles and health effects.  American Journal of Clinical Nutrition 70(S): 439s-450s.

Reviewed 25 May 2005