Cobalt-Zinc Interactions in Bush Beans Grown in Solution Culture

We grew bush beans (Phaseolus vulgaris L. cv. Improved Tendergreen) in nutrient solutions with two elevated levels of Zn and two of Co singly and in factorial combination. Levels were high enough to result in some toxicity in comparison with a control in which no Co and an adequate amount of Zn were added. In relationship with the control treatment, the low Co treatment increased vegetative yield, while the high Co decreased yield. The ability of the low Co level to increase yield was a function of the applied Zn level. Yield increases for low Co were 31, 11, and 0%, for increasing levels of Zn. Both elevated Zn levels were toxic. When the elevated levels of Co and Zn were interacted, synergistic, additive, and protective effects were all observed. Phosphorus, Fe, Co, and Zn concentrations in plant parts were involved in the interactions. The combination of highest Zn and Co resulted in most depression of Fe in leaves. Cobalt somewhat suppressed the ability of high Zn to depress uptake of K + Ca + Mg, which effect decreased synergism. The protective effect was the result of Zn depressing leaf concentration of Co.