Soil Contamination with Europium Induces Reduced Oxidative Damage in Hordeum vulgare Grown in a CO 2 -Enriched Environment.

The extensive and uncontrolled utilization of rare earth elements, like europium (Eu), could lead to their accumulation in soils and biota. Herein, we investigated the impact of Eu on the growth, photosynthesis, and redox homeostasis in barley and how that could be affected by the future CO 2 climate (eCO 2 ). The plants were exposed to 1.09 mmol Eu 3+ /kg soil under either ambient CO 2 (420 ppm, aCO 2 ) or eCO 2 (620 ppm). The soil application of Eu induced its accumulation in the plant shoots and caused significant reductions in biomass- and photosynthesis-related parameters, i.e., chlorophyll content, photochemical efficiency of PSII, Rubisco activity, and photosynthesis rate. Further, Eu induced oxidative stress as indicated by higher levels of H 2 O 2 and lipid peroxidation products, and lower ASC/DHA and GSH/GSSG ratios. Interestingly, the co-application of eCO 2 significantly reduced the accumulation of Eu in plant tissues. Elevated CO 2 reduced the Eu-induced oxidative damage by supporting the antioxidant defense mechanisms, i.e., ROS-scavenging molecules (carotenoids, flavonoids, and polyphenols), enzymes (CAT and peroxidases), and ASC-GSH recycling enzymes (MDHAR and GR). Further, eCO 2 improved the metal detoxification capacity by upregulating GST activity. Overall, these results provide the first comprehensive report for Eu-induced oxidative phytotoxicity and how this could be mitigated by eCO 2 .