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 r adiata cv. Berken) seedlings was measured in dilute complete nutrient solutions to which varying amounts of lanthanum (La) or cerium (Ce) ha d been added. The nutrient solutions were aged for 9 d prior to conduc ting the root elongation experiments and solution samples ultra-filter ed 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 t o 8 mu M remained in solution in the presence of 5 mu M P at pH 4.5, b ut substantial losses of these elements and P occurred at pH 5.5. The relative root elongation of both plant species decreased with increasi ng concentrations of La or Ce in solution. Mungbean was much more sens itive than corn. Cerium was more toxic than La to mungbean, the concen trations 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 be ing 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.