Background
The authors write that "although post-combustion emissions from power plants are a major source of air pollution, they contain excess CO2 that could be used to fertilize commercial greenhouses and stimulate plant growth," but they worry about "the acidic pollutants in the flue gas" -- such as SO2 and NO2 -- and their possible negative effects on growth.

What was done
Xu et al. grew specimens of the C3 plant Alternanthera philoxeroides from stem cuttings in custom-built microcosm growth chambers supplied with different concentrations of CO2 with and without SO2 and NO2 at concentrations of 3.1 and 0.35 ppm, respectively, for approximately three weeks, after which they harvested the plants and determined their production of aboveground biomass at the various CO2 concentrations employed. Their choice of this particular plant derives from the fact that "A. philoxeroides was introduced in many countries as [a] forage crop and has proved nutrient value for food supplement of stock or humans (Dewanji, 1993; Bhatta and Das, 1996; Dewanji and Matai, 1996)," as they describe it.

What was learned
Compared with the ambient control treatment of 350 ppm CO2, and in the absence of the flue gases, the observed increase in aboveground biomass accumulation was 80% at 1000 ppm CO2, 120% at 3000 ppm, and 110% at 5000 ppm. What is more, the drop-off rate of the CO2-induced growth stimulation evident between 3000 and 5000 ppm CO2 was so small that the benefits of the added CO2 did not entirely disappear until the atmosphere's CO2 concentration had exceeded 16,000 ppm. When the flue gases were added, however, the CO2-induced increase in aboveground biomass accumulation dropped from 80% to 55% at 1000 ppm CO2 and declined all the way to zero somewhere between 2000 and 3000 ppm CO2.

What it means
The nine U.S. researchers conclude that "using excess CO2 from power plant emissions to optimize growth in commercial greenhouses could be viable." Nevertheless, they state that flue gases used for this purpose should be "diluted to optimize the CO2 enhancement effect and minimize the deleterious effects of the acidic components." We also note their results demonstrate that earth's plants are not harmed by super-elevated atmospheric CO2 concentrations, even when they are an order of magnitude or more greater than that of the globe's current mean (see Growth Response to Very High CO2 Concentrations in our Subject Index for many similar items).

References
Bhatta, R. and Das, T.K. 1996. Chemical and amino acid composition of alligator weed (Alternanthera philoxeroides). Indian Veterinary Journal 73: 799-800.

Dewanji, A. 1993. Amino-acid-composition of leaf proteins exracted from some aquatic weeds. Journal of Agricultural and Food Chemistry 41: 1232-1236.

Dewanji, A. and Matai, S. 1996. Nutritional evaluation of leaf protein extracted from three aquatic plants. Journal of Agricultural and Food Chemistry 44: 2162-2166.