Cobalt Interactions With Zinc, Cadmium, and Nickel in Lettuce Seedling Tests

We developed a dose-response curve for Co with lettuce seedlings in solution culture, and it indicated four possible phase changes. A plateau below about 7 μeq L−1 changed to a phase 2, which changed to a phase 3 with less slope at 20 μeq L−1, and at about 200 μeq L−1 there was a change to a steeper slope, which progressed to lethal toxicity. When Co was supplied as the fourth element in excess (with Zn, Cd, and Ni), synergistic effects were obtained, even without Co being in the toxic range if it were the only toxicant supplied. Synergistic effects cast some doubt on the ability to fully predict, from models, the potential effect on plant growth of at least some combinations of trace elements.