Nickel effects on two maize (Zea mays) cultivars: Growth, structure, Ni concentration, and localization

The toxic effects of nickel on maize growth and structure and the accumulation and distribution of Ni in the plant were investigated. Two cultivars of maize, each with a different sensitivity to Ni, were grown on nutrient solution with different nickel concentrations for a period of 8 days. The sensitive and tolerant cultivars exhibited reduction in root and shoot growth with Ni concentrations greater than 20 mu M and 40 mu M, respectively. The toxic effects of nickel on the structure and ultrastructure of maize roots and leaves were studied by light microscopy and transmission and scanning electron microscopy. With 60 mu M Ni, the sensitive cultivar showed a strong reduction in root mitotic activity (80%). Statoliths were absent in cells of the root cap, while leaves contained large amounts of starch in the chloroplasts of their bundle sheath cells. This suggests a decrease in carbohydrate transport between the leaves and the roots. The tissue localization of Ni by the dimethylglyoxim and silver sulphide methods showed important accumulation in the root apex and in the chloroplasts of the bundle sheath cells. These results suggest that Ni reduces maize growth by a reduction in root mitotic activity, probably because of direct action on the meristem. Starch accumulation in the leaves could come directly from a decrease in the root sink effect caused by the reduced mitotic activity or indirectly from an inhibition of carbohydrate transport.