Paper Reviewed
Jia, X., Zhao, Y.H, Liu, T. and He, Y.H. 2017. Leaf defense system of Robinia pseudoacacia L. seedlings exposed to 3 years of elevated atmospheric CO2 and Cd-contaminated soils. Science of the Total Environment 605-606: 48-57.

Black locust trees (Robinia pseudoacacia) are frequently used for soil phytoremediation due to their high tolerance of heavy metal toxicity. To date, a number of studies have examined the impact of atmospheric CO2 enrichment on heavy metal pollution, finding that elevated levels of CO2 tend to help alleviate the growth-reducing impacts of heavy metal toxicity stress. However, most of these studies have been conducted over time scales of one growing season or less.

In an effort to determine whether such ameliorating CO2-induced benefits persist over longer periods of time, Jia et al. (2017) grew black locust seedlings in open-top chambers for three full years under two atmospheric CO2 concentrations and three levels of soil cadmium (Cd) contamination. CO2 concentration included ambient and enriched, at 385 and 700 ppm, respectively; while soil Cd treatments included concentrations of 0, 1 and 5 mg per Kg of dry soil.

Results of the experiment revealed that trees growing in the elevated CO2 environment experienced a decrease in leaf Cd uptake and an overall reduction in soil Cd content. In addition, under Cd stress, leaf chlorophyll content was greater under elevated CO2 conditions compared with ambient CO2. What is more, analysis of antioxidant enzymes and secondary metabolites revealed that elevated CO2 "enhanced the leaf defense system of R. pseudoacacia exposed to Cd by stimulating antioxidant enzyme activity, osmotic adjustment, and the production of glutathione, flavonoids and phenolic acids."

In light of the above findings, it would appear that the results of longer-term CO2 enrichment studies confirm what is generally found in shorter-term CO2 enrichment studies of heavy metal toxicity, namely, that elevated CO2 helps to alleviate this stress by stimulating protective and defensive responses. And that is great news, especially for regions of the planet that are plagued with heavy metal contamination.