RARE-EARTH ELEMENTS AND PLANT-GROWTH .1. EFFECTS OF LANTHANUM AND CERIUM ON ROOT ELONGATION OF CORN AND MUNGBEAN

Root elongation of corn (Zea mays cv. Hycorn 82) and mungbean (Vigna radiata cv. Berken) seedlings was measured in dilute complete nutrient solutions to which varying amounts of lanthanum (La) or cerium (Ce) had been added. The nutrient solutions were aged for 9 d prior to conducting the root elongation experiments and solution samples ultra-filtered to 0.025 mu m before chemical analyses for La or Ce, and phosphorus (P). Concentrations of La up to 10 mu M and concentrations of Ce up to 8 mu M remained in solution in the presence of 5 mu M P at pH 4.5, but substantial losses of these elements and P occurred at pH 5.5. The relative root elongation of both plant species decreased with increasing concentrations of La or Ce in solution. Mungbean was much more sensitive than corn. Cerium was more toxic than La to mungbean, the concentrations associated with a 50% reduction in root elongation being 0.9 mu M Ce and 3.1 mu M La. Lanthanum was more toxic to corn than Ce, the concentrations corresponding to a 50% reduction in root elongation being 12.2 mu M Ce and 4.8 mu M La at pH 5.5, or 7.1 mu M La at pH 4.5.