The effects of cadmium and zinc interactions on the accumulation and tissue distribution of zinc and cadmium in lettuce and spinach.

The interactions between Zn and Cd on the concentration and tissue distribution of these metals in lettuce and spinach were studied at levels corresponding to background and Zn-Cd contaminated sites. Plants were grown in nutrient solutions containing 0.398-8.91 microM Zn and 0.010-0.316 microM Cd. Cadmium accumulated more in old than in young leaves of both crops at any solution Cd level, whereas Zn followed that pattern only at Zn levels > or = 3.16 microM. Increasing solution Cd increased Zn concentrations in young leaves of lettuce but not of spinach, regardless of Zn levels. Cadmium concentrations in young leaves of both crops decreased exponentially with increasing solution Zn at low (0.0316 microM) but not at high (0.316 microM) solution Cd. The Zn: Cd concentration ratios in young leaves of lettuce and spinach grown at 0.316 microM Cd became greater as the solution Zn increased. Cadmium and Zn concentrations in young leaves were related more closely to the relative concentrations of Zn and Cd in solution than were the concentrations in old leaves, especially in lettuce. Studies of Zn-Cd interactions and Cd bioavailability should differentiate between basal and upper leaves of lettuce and spinach. Compared to Cd-only pollution, Zn-Cd combined pollution may not decrease Cd concentrations in lettuce and spinach edible tissues, but because it increases their Zn concentrations it lowers plant Cd bioavailability.