Paper Reviewed
Xi, W., Zhang, Q., Lu, X., Wei, C., Yu, S. and Zhou, Z. 2014. Improvement of flavor quality and consumer acceptance during postharvest ripening in greenhouse peaches by carbon dioxide enrichment. Food Chemistry 164: 219-227.

In our all-too-politically-correct world, carbon dioxide (CO2) frequently gets a bad rap, demonized for its potential and unproven effects on climate. However, if the truth be told, carbon dioxide is a magnificent molecule, essential to nearly all life on Earth. It is the primary raw material from which plants construct their tissues and grow during the process of photosynthesis. Perhaps it should come as no surprise, therefore, that plants perform this essential function ever better as atmospheric CO2 levels climb ever higher, a fact demonstrated in literally thousands of laboratory and field studies (see, for example, our Plant Growth Database). And because plants are the ultimate food source for animals and humans, we are all indebted to CO2 for its role in sustaining and promoting the growth of plants everywhere.

But there are other benefits to atmospheric CO2 enrichment beyond enhancing plant growth, as illustrated in the recent study of Xi et al. (2014). Publishing in the professional journal Food Chemistry, the six-member team of Chinese horticultural and food scientists "investigated the effectiveness of CO2 enrichment for improving fruit flavor and customer acceptance of greenhouse-grown peaches."

The rationale for their study stems from the fact that peaches are widely cultivated in greenhouses throughout northern China. Under such controlled conditions, the trees are afforded protection from the natural environment, including damaging low temperatures and high winds. But this protection does not come without a price. Plant photosynthesis can cause CO2 levels inside closed greenhouses to decrease during daylight hours to values below 200 parts per million, which values are half or less than half the CO2 concentration of normal outside air. As a result, Xi et al. state these "low CO2 levels may be a limiting factor for the productivity of fruit trees cultivated in greenhouses," and they may negatively impact the "development of fruit flavor quality" and aroma, which is not good for those in the peach growing business! Thus, the six scientists set out to explore how enriching greenhouse air with CO2 might mitigate these potential problems.

For their experimental design, Xi et al. (2014) divided a greenhouse into two parts using a hermetic barrier wall, supplying one side with CO2-enriched air and the other with ambient air to be used as the control. The enriched side of the greenhouse was maintained at an atmospheric CO2 value of 360 ppm (approximately twice that of the control) from 12:00 to 16:00 each day during the main CO2 shortage period, while "fruit sugar, organic acids, volatile contents and consumer acceptability were investigated, focusing on the period of postharvest ripening."

With respect to their findings, the Chinese researchers report that net photosynthesis was significantly increased in the trees growing in the CO2-enchanced portion of the greenhouse despite their receiving only a mere 4 hours of CO2 enrichment per day above those growing in the ambient or control portion of the structure. Elevated CO2 also improved fruit flavor and aroma, significantly increasing dominant sugar levels (sucrose and fructose), fruity aroma compounds (lactones), and floral scent compounds (norisoprenoids), while decreasing compounds that contribute to fruit sourness and undesirable aroma volatiles (Table 1).

Table1. Percent difference of various peach fruit compounds from trees grown in CO2 enriched air, relative to trees grown in ambient air, as measured in fruit picked on the day of harvest and five days after harvest. Data derived from Table 1 of Xi et al. (2014).

As a result of their findings, the authors conclude that "CO2 enrichment can significantly improve the flavor quality of 'Zaolupantao' peach fruits grown in greenhouse and their consumer acceptance." And if it can do that from a mere 4 hours of CO2 enrichment per day in a greenhouse, imagine what 24 hours of enrichment might promise for other fruiting plants growing out-of-doors, in natural environments, under present-day global atmospheric CO2 concentrations of 400 ppm and above? Hinting at the possibilities, Xi et al. cite the work of researchers studying other fruits, where similar CO2 benefits have been reported for tomato (Shahidul Islam et al., 1996; Zhang et al., 2014), strawberry (Wang and Bunce, 2004; Sun et al., 2012), and grapes (Bindi et al., 2001).

Yes, truth be told, atmospheric CO2 is a magnificent molecule, and those who continue to demonize it based on potential and unproven climatic effects, should wake up and smell the peaches. Or, they should at least eat one and taste how sweet its biological benefits can be!

References
Bindi, M., Fibbi, L. and Miglietta, F. 2001. Free air CO2 enrichment (FACE) of grapevine (Vitis vinifera L.): II. Growth and quality of grape and wine in response to elevated CO2 concentrations. European Journal of Agronomy 14: 145-155.

Shahidul Islam, M., Matsui, T. and Yoshida, Y. 1996. Effect of carbon dioxide enrichment on physico-chemical and enzymatic changes in tomato fruits at various stages of maturity. Scientia Horticulturae 65: 137-149.

Sun, P., Mantri, N., Lou, H., Hu, Y., Sun, D., Zhu, Y., Dong, T. and Lu, H. 2012. Effects of elevated CO2 and temperature on yield and fruit quality of strawberry (Fragaria x ananassa Duch.) at two levels of nitrogen application. PLoS ONE e41000.

Wang, S. Y. and Bunce, J. A. 2004. Elevated carbon dioxide affects fruit flavor in field-grown strawberries (Fragaria x ananassa Duch). Journal of the Science of Food and Agriculture 84: 1464-1468.

Zhang, Z.M., Liu, L.H., Zhang, M., Zhang, Y.S. and Wang, Q.M. 2014. Effect of carbon dioxide enrichment on health-promoting compounds and organoleptic properties of tomato fruits grown in greenhouse. Food Chemistry 153: 157-163.