The effects of rare earth elements on the growth and nutrition of plants

The rare earth elements (REEs) have many and varied uses throughout the world. However, the large scale use of REEs in agriculture is confined to China where some beneficial effects of REEs have been reported. Very little is known about the basic physiological effects of REEs on plants. Such information is essential for an understanding of how these elements may influence agricultural crop production. In this paper we summarise results of experiments rigorously conducted over 3 years to examine the effects of lanthanum (La) and cerium (Ce) on the growth and mineral nutrition of plants. Lanthanum and Ce were applied to the foliage or roots of two plant species (Corn (Zea mays) and mungbean (Vigna radiata)) of agricultural importance in the sub-tropical and tropical areas of the world. A commercial REE fertiliser was obtained from China, chemically analysed and found to contain mainly La and Ce nitrates. This fertiliser and comparable synthetic REE solutions were applied at the recommended rates to the leaves of corn and mungbean plants grown on a low-REE medium under well-controlled environmental conditions. Foliar application of REEs did not significantly increase the shoot dry weight of corn or mungbean. Both REE sources applied at 0.5 and 1.0% produced symptoms of foliar damage and reduced shoot dry weight in both plant species. Damage symptoms and growth reductions of plants sprayed with pure La and Ce solutions were similar to those of plants sprayed with commercial REE fertiliser. When REEs were maintained in a soluble form in nutrient solutions comparable in composition to soil solutions, concentrations of La or Ce from 1 to 16 mu M (0.1-2 ppm) were found to be toxic to the root elongation of corn and mungbean. Subsequently, when concentrations of La or Ce below 1.5 mu M (<0.2 ppm) were accurately maintained in solution, concentrations as low as 0.2 mu M (0.03 ppm) were shown to be toxic to mungbean. Thus Ce at 0.2 mu M (0.03 ppm) reduced the total plant dry weight of mungbean by 44%. In another experiment, it was shown that not only did La and Ce at 0.2 - 5.0 mu M (0.03-0.7 ppm) reduce the growth of mungbean, but also the uptake rates of all nutrients were reduced. For corn, growth was little affected by solution La or Ce concentrations up to 5 mu M (0.7 ppm), however, the uptake of Ca was decreased consistent with the known function of REEs as Ca antagonists. The foliar and root applications of REEs did not result in beneficial effects on plant growth; rather, the overwhelming majority of the results demonstrate detrimental effects on plant growth and nutrition. From these results it is concluded that the REEs are very unlikely to have a role to play in increasing agricultural plant production.