Background
The authors write that cadmium (Cd) "is a non-essential element that negatively affects plant growth and development processes, such as respiration and photosynthesis, water and mineral uptakes, cell division, and cellular redox homeostasis." On the other hand, they indicate that "elevated levels of CO2 improve photosynthesis of C3 plants, reduce stomatal conductance, and as a result, increase water-use efficiency, while aiding in the decrease of photorespiration and oxidative stress" and increasing "the ability of plants to combat abiotic stress, such as ozone, drought and salt."

What was done
Seedlings of two important forage crops -- Perennial ryegrass (Lolium perenne) and Italian ryegrass (Lolium multiflorum) -- were grown hydroponically within controlled environment chambers for three weeks in individual pots filled with one liter of half-strength Hoagland nutrient solution containing one of three different Cd concentrations (0, 4 and 16 mg/L) at atmospheric CO2 concentrations of either 360 or 1000 ppm, during and after which period a number of plant properties and processes were carefully measured.

What was learned
Jia et al. write that "root morphological parameters, including root length, surface area, volume, tip number, and fine roots, all decreased under Cd exposure," while "by contrast, elevated levels of CO2 significantly increased all those parameters in the presence of Cd, compared to the CO2 control, suggesting that elevated levels of CO2 had an ameliorating effect on Cd-induced stress."

In addition, the extra 640 ppm of CO2 increased the shoot dry weight of L. multiflorum by 68%, 92% and 90% and that of L. perenne by 65%, 61% and 67% at low, medium and high (0, 4, and 16 mg/L) cadmium concentrations, while it increased the root dry weight of L. multiflorum by 65%, 54% and 50% and that of L. perenne by 47%, 67% and 10%, under the same respective set of conditions.

Last of all, but by no means least of all, the seven scientists report that "total Cd uptake per pot, calculated on the basis of biomass, was significantly greater under elevated levels of CO2 than under ambient CO2," increasing by 42-73% in plant shoots. Yet at the same time, they report there was a reduction of Cd concentration within the plants' tissues at elevated CO2.

What it means
The seven scientists note that due to high Cd uptake under CO2-enriched conditions, the two Lolium species show great potential for use in the phytoremediation of Cd contaminated soils in a CO2-enriched world of the future. And at the same time, because of much greater biomass production, the Cd concentration reduction in their tissues suggests that the ongoing rise in the air's CO2 content could well improve the safety of these forage crops in decades to come, much as was demonstrated by Guo et al. (2006), who according to Jia et al. "reported decreased Cd accumulation in leaves, stems, roots and grains of rice at elevated CO2, by Zheng et al. (2008), who "showed that Pteridium revolutum and Pteridium aquilinum grown on Cu-contaminated soils accumulated less Cu in plant tissues at elevated levels of CO2 than at ambient CO2," and by Li et al. (2010), who working with rice also "found that elevated levels of CO2 diluted grain Cd concentration."

References
Guo, H.Y., Jia, H.X., Zhu, J.G. and Wang, X.R. 2006. Influence of the environmental behavior and ecological effect of cropland heavy metal contaminants by CO2 enrichment in atmosphere. Chinese Journal of Geochemistry 25: 10.1007/BF02840155.

Li, Z.Y., Tang, S.R., Deng, X.F., Wang, R.G. and Song, Z.G. 2010. Contrasting effects of elevated CO2 on Cu and Cd uptake by different rice varieties grown on contaminated soils with two levels of metals: implication for phytoextraction and food safety. Journal of Hazardous Materials 177: 362-361.

Zheng, J.M., Wang, H.Y., Li, Z.Q., Tang, S.R. and Chen, Z.Y. 2008. Using elevated carbon dioxide to enhance copper accumulation in Pteridium revolutum, a copper-tolerant plant, under experimental conditions. International Journal of Phytoremediation 10: 161-172.