Elevated CO2(eCO2) is one of the climate changes that may benefit plant growthunder emerging soil contaminants such as heavy metals. In this regard, the morpho-physiological mechanisms underlying the mitigating impact of eCO2on beryllium(Be) phytotoxicity are poorly known. Hence, we investigated eCO2and Be interactiveeffects on the growth and metabolism of two species from different groups: cereal(oat) and legume (alfalfa). Be stress significantly reduced the growth and photosyn-thetic attributes in both species, but alfalfa was more susceptible to Be toxicity. Bestress induced reactive oxygen species (ROS) accumulation by increasing photorespi-ration, subsequently resulting in increased lipid and protein oxidation. However, thegrowth inhibition and oxidative stress induced by Be stress were mitigated by eCO2.This could be explained, at least partially, by the increase in organic acids (e.g., citricacid) released into the soil, which subsequently reduced Be uptake. Additionally,eCO2reduced cellular oxidative damage by reducing photorespiration, which wasmore significant in alfalfa plants. Furthermore, eCO2improved the redox status anddetoxification processes, including phytochelatins, total glutathione and metallothio-neins levels, and glutathione-S-transferase activity in both species, but to a greaterextend in alfalfa. In this context, eCO2also stimulated anthocyanin biosynthesis byaccumulating its precursors (phenylalanine, coumaric acid, cinnamic acid, and narin-genin) and key biosynthetic enzymes (phenylalanine ammonia-lyase, cinnamatehydroxylase, and coumarate:CoA ligase) mainly in alfalfa plants. Overall, this studyexplored the mechanistic approach by which eCO2alleviates the harmful effects ofBe. Alfalfa was more sensitive to Be stress than oats; however, the alleviating impactof eCO2on Be stress was more pronounced in alfalfa.