Rare earth elements in plants

Since 1960, the positive effects of rare earth elements (REE) on crop physiology have been observed, and support for photosynthesis, biomass accumulation, secondary metabolites, or enzymes has been reported in 40% of studies. A higher content of chlorophylls a and b as well as carotenoids have been found along with an increased efficiency of photosystem II photochemistry and electron transfer rates. An increased activity of a key photosynthetic enzyme was also found in several plants growing in soil with a higher content of REE. An appropriate amount of REE also activates the antioxidant activity of peroxidase, superoxide dismutase, and catalase. These enzymes, together with a higher content of flavonoids and carotenoids, increase the resistance of plants to oxidative stress caused by different abiotic stresses. The positive effect of REE on biomass accumulation was also confirmed, but their affection on mycorrhizal symbiosis remains ambiguous. On the other hand, an excess of REE leads to damage to plants including the chloroplast ultrastructure. Therefore, the positive and negative effects of REE remain controversial, and the mechanisms of effects of REE in plants remain poorly understood. In addition to physiological processes, the absorption, bioavailability, and translocation of REE in plants as well as their possible ecotoxicology and hyperaccumulation are discussed in this review.