RARE-EARTH ELEMENTS AND PLANT-GROWTH .2. RESPONSES OF CORN AND MUNGBEAN TO LOW CONCENTRATIONS OF LANTHANUM IN DILUTE, CONTINUOUSLY FLOWING NUTRIENT SOLUTIONS

Corn (Zea mays cv. Hycorn 82) and mungbean (Vigna radiata cv. Berken) plants were grown for 14 d in dilute nutrient solutions containing con stant lanthanum (La) concentrations from 0 to 1.37 mu M. Solutions wer e maintained at pH 4.5 to prevent precipitation of La. Lanthanum at 0. 63 mu M increased the root growth of corn by 36% and 0.19 mu M La incr eased mungbean root growth by 21% relative to controls. However, no be neficial effects of La on the total dry matter yield of either plant s pecies were demonstrated; that of corn was unaffected, whilst that of mungbean was reduced by over 30% at solution La concentrations greater than 0.19 mu M. Roots of both plant species accumulated 20 to 150 tim es higher concentrations of La than the shoots. The highest La concent rations in roots were 1775 mg/kg in corn and 2955 mg/kg in mungbean. W here La was added to the nutrient solutions, concentrations of La in t he shoots ranged from 9 to 16 mg/kg for corn and from 34 to 52 mg/kg f or mungbean. The oldest leaves of both plant species accumulated highe r La concentrations than found in the remainder of the shoots. Both pl ant species demonstrated an ability to restrict the uptake of La into the shoots, as the concentrations of La in the shoots increased only s lowly with increasing concentrations of La in the roots and in the nut rient solution. The data suggest critical shoot and root La concentrat ions of the order of 34 and 775 mg/kg respectively, for toxicity in mu ngbean. Critical La concentrations for toxicity in corn must be greate r than 16 mg/kg in shoots and 1775 mg/kg in roots.