Rare earth elements and plant growth: II. Responses of corn and mungbean to low concentrations of lanthanum in dilute, continuously flowing nutrient solutions.

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Abstract
Corn (Zea mays cv. Hycorn 82) and mungbean (Vigna radiata cv. Berken) plants were grown for 14 d in dilute nutrient solutions containing constant lanthanum (La) concentrations from 0 to 1.37 μM. Solutions were maintained at pH 4.5 to prevent precipitation of La. Lanthanum at 0.63 μM increased the root growth of corn by 36% and 0.19 μM La increased mungbean root growth by 21% relative to controls. However, no beneficial effects of La on the total dry matter yield of either plant species were demonstrated; that of corn was unaffected, whilst that of mungbean was reduced by over 30% at solution La concentrations greater than 0.19 μM. Roots of both plant species accumulated 20 to 150 times higher concentrations of La than the shoots. The highest La concentrations in roots were 1775 mg/kg in corn and 2955 mg/kg in mungbean. Where La was added to the nutrient solutions, concentrations of La in the shoots ranged from 9 to 16 mg/kg for corn and from 34 to 52 mg/kg for mungbean. The oldest leaves of both plant species accumulated higher La concentrations than found in the remainder of the shoots. Both plant species demonstrated an ability to restrict the uptake of La into the shoots, as the concentrations of La in the shoots increased only slowly with increasing concentrations of La in the roots and in the nutrient solution. The data suggest critical shoot and root La concentrations of the order of 34 and 775 mg/kg respectively, for toxicity in mungbean. Critical La concentrations for toxicity in corn must be grsater than 16 mg/kg in shoots and 1775 mg/kg in roots.