Background
The authors write that "in China, tomatoes are often grown in greenhouses, in order to provide early-ripening fruit that meet the demands of consumers." But they say that "the quality of greenhouse tomato, as indicated by characteristics such as color and flavor, as well as content of ascorbic acid and carotenoids, is usually found to be poor." And because of those facts, they report that "many complaints about poor quality of tomato fruit have been made in the past few years (Baldwin et al., 2000), and consumers demand products with better flavor (Causse et al., 2003)."

What was done
In response to these widespread consumer complaints, Zhang et al. conducted a two-greenhouse study, where one was subject to prior standard operating procedures and the other differed by merely having a composting unit of the type described by Jin et al. (2009) placed in its center. And this single alteration led to greenhouse CO2 concentrations in the range of 800-900 ppm, as compared to the 100-250 ppm daytime concentrations characteristic of the control greenhouses due to their "hermetic conditions, which impair plant growth," as described by Klaring et al. (2007).

What was learned
In the words of the five Chinese researchers who analyzed the consequences of this one simple addition to prior standard greenhouse culture conditions, "the contents of health-promoting compounds, including lycopene, β-carotene, and ascorbic acid, as well as the flavor, indicated by sugars, titrable acidity, and sugar/acid ratio, were markedly increased in CO2 enrichment fruits." And they additionally state that "CO2 enrichment significantly enhanced other organoleptic characteristics, including color, firmness, aroma, and sensory attributes in tomato fruits."

Bolstering their observations, Zhang et al. add that "the finding of increased content of ascorbic acid by CO2 enrichment is consistent with the effect of CO2 enrichment on orange (Idso et al., 2002), and leaf lettuce (Jin et al., 2009)," while noting that the positive effects of elevated CO2 on flavor "have also been found in grape," citing Bindi et al. (2001). And they end their paper by adding that their conclusion that high concentrations of carotenoids and ascorbic acid "highlight the nutrient value of tomato," is additionally supported by the findings of Crozier et al. (1997), who also found the same to be true for tomato, as well as for onions, lettuce and celery.

What it means
Simply put, the tomato fruit plucked from the plants growing in the greenhouse with the composting unit in the midst of it looked better, felt better, smelled better, tasted better and were better for one's health. What more could one possibly ask? ... especially of what the U.S. Environmental Protection Agency wrong-headedly calls an air pollutant?

References
Baldwin, E.A., Scott, J.W., Shewmaker, C.K. and Schuch, W. 2000. Flavor trivia and tomato aroma: biochemistry and possible mechanisms for control of important aroma components. HortScience 35: 1013-1022.

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.

Causse, M., Buret, M., Robini, K. and Verschave, P. 2003. Inheritance of nutritional and sensory quality traits in fresh market tomato and relation to consumer preferences. Journal of Food Science 68: 2342-2350.

Crozier, A., Michael, E.J.L., McDonald, M.S. and Black, C. 1997. Quantitative analysis of the flavonoid content of commercial tomatoes, onions, lettuce, and celery. Journal of Agricultural and Food Chemistry 45: 590-595.

Idso, S.B., Kimball, B.A., Shaw, P.E., Widmer, W., Vanderslice, J.T., Higgs, D.J., Montanari, A. and Clark, W.D. 2002. The effect of elevated atmospheric CO2 on the vitamin C concentration of (sour) orange juice. Agriculture, Ecosystems and Environment 90: 1-7.

Jin, C.W., Du, S.T., Wang, Y., Condon, J., Lin, X.Y. and Zhang, Y.S. 2009. Carbon dioxide enrichment by composting in greenhouses and its effect on vegetable production. Journal of Plant Nutrition and Soil Science 172: 418-424.

Klaring, H.P., Hauschild, C., Heissner, A. and Bar-Yosef, B. 2007. Model-based control of CO2 concentration in greenhouses at ambient levels increases cucumber yield. Agricultural and Forest Meteorology 143: 208-216.