Background
"Phytoextraction," in the words of the authors, "has been defined as the direct use of living green plants in order to extract pollutants from contaminated soils and concentrate them into roots and easily harvestable shoots (Baker and Brooks, 1989; Raskin et al., 1994; Salt et al., 1995; Cunningham and David, 1996)."  They say this technique "offers a cost-effective and environmentally sound pollution-remediation option," but that "one of the key problems is how to enhance the uptake of metals by plants in order to increase absolute phytoremediation efficiency."  Hence, they decided to see what elevated CO2 could do in this regard.

What was done
Tang et al. grew individual sunflower (Helianthus annuus L.) and Indian mustard (Brassica juncea L. Czern.) plants from seed in pots with 1 kg natural topsoil laced with different concentrations of copper (Cu) for 24 days in ambient air of 350 ppm CO2, after which one-third of the plants were exposed to air of 800 ppm CO2 and another third to air of 1200 ppm CO2, which elevated concentrations were only supplied between the hours of 8 and 11 a.m. for 12 additional days.  At the end of this period, the plants were harvested and the concentrations of copper in their leaves, stems and roots were measured, after which bioaccumulation factors (BFs) were calculated as the ratios of average copper concentrations in leaves to the copper concentration in the soil.

What was learned
In the words of the authors, "Indian mustard and sunflower grew higher and larger, and had more and thicker leaves, and produced larger leaf areas, compared to the plants growing under ambient CO2 levels."  In addition, for sunflower plants, the ratio of the observed BF at 800 ppm CO2 to that observed at 350 ppm CO2 was 3.4 in natural soil, 10.9 in soil containing 100 mg Cu per kg soil, and 4.2 in soil containing 200 mg Cu per kg soil, while the similar ratios of observed BFs at 1200 ppm CO2 to that observed at 350 ppm CO2 were 1.2, 3.8 and 2.6, respectively.  For Indian mustard plants the results were even more spectacular.  The ratio of the observed BF at 800 ppm CO2 to that observed at 350 ppm CO2 was 14.5 in natural soil, 39.5 in soil containing 100 mg Cu per kg soil, and 2.5 in soil containing 200 mg Cu per kg soil, while the similar ratios of observed BFs at 1200 ppm CO2 to that observed at 350 ppm CO2 were 5.4, 17.8 and 1.2, respectively.

What it means
Tang et al. say these findings are significant "since the increase of plant biomass resulting from CO2 application could suggest that more metal be taken up from the contaminated growth media, and that the tolerance to metal toxicity be improved," adding that "obviously, this could help metal accumulators survive on the metal stress conditions, shorten the time needed for clean-up of contaminated sites, and, therefore, increase relative phytoremediation efficiency."  They also note that "the large increase in uptake of copper by Indian mustard and sunflower, and the alleviation of chlorosis in their leaves with elevated CO2, suggest that both species may be able to increase the internal recycling of deficient nutrients resulting from copper stress."  Hence, they conclude that "the use of CO2 fertilizer for triggering hyperaccumulation in plants, and increasing biomass production, could open up the way for enhanced phytoremediation and for phytomining."

References
Baker, A.J.M. and Brooks, R.R.  1989.  Terrestrial higher plants which hyperaccumulate metallic elements - a review of their distribution, ecology, and phytochemistry.  Biorecovery 1: 81-126.

Cunningham, S.D. and David, W.O.  1996.  Promises and prospects of phytoremediation.  Plant Physiology 110: 715-719.

Raskin, I., Kumar, P.B.A.N., Dushendov, S. and Salt, D.E.  1994.  Bioconcentration of heavy metals by plants.  Current Opinions in Biotechnology 5: 285-290.

Salt, D.E., Blaylock, M., Kumar, N.P.B.A., Dushenkov, V., Ensley, B.D., Chet, I. and Raskin, I.  1995.  Phytoremediation: A novel strategy for the removal of toxic metals from the environment using plants.  Biotechnology 13: 468-474.