Zinc rhizotoxicity in wheat and radish is alleviated by micromolar levels of magnesium and potassium in solution culture

The effects of excess zinc (Zn) on solution-cultured wheat (Triticum aestivum L., cv. Yecora Rojo) and radish (Raphanus sativus L., cv. Cherry Belle) were studied, using both short-term root elongation studies and longer term split-root experiments. Alleviation of Zn rhizotoxicity by Mg and K was observed, with especially dramatic alleviation of root stunting by Mg. In the short-term studies using a simple medium (2 mM CaCl2, pH 6.0), Mg concentrations of 1-5 muM were able to significantly alleviate rhizotoxicity caused by Zn concentrations as high as 60 muM. In the split-root studies, 100 muM Mg was sufficient to abolish Zn toxicity in both wheat and radish. Paradoxically, Mg enhanced uptake and translocation of Zn while simultaneously alleviating toxicity in these longer-term experiments. In short-term experiments, additions of K (0 to 200 muM) to the basal medium alleviated Zn rhizotoxicity to a more limited extent. In split-root experiments, however, the absence or presence of K in test solutions did not affect plant growth or Zn uptake. When increased from a physiological minimum (e.g., 200 muM), Ca also alleviates Zn toxicity, but the effect is very modest in comparison to that of Mg. The results are discussed in relation to the use of short-term assays of metal tolerance in simple salt solutions, and in relation to possible roles of Mg in the physiology of Zn toxicity.