ZINC RHIZOTOXICITY IN WHEAT AND RADISH IS ALLEVIATED BY MICROMOLAR LEVELS OF MAGNESIUM AND POTASSIUM IN SOLUTION CULTURE

Authors: PEDLER, JUDITH - UNIV OF MELBOURNE, AUST; Kinraide, Thomas; PARKER, DAVID - UNIV OF CALIFORNIA

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/26/2003
Publication Date: 2/4/2004
Citation: Pedler, J.F., Kinraide, T.B., Parker, D.R. 2004. Zinc rhizotoxicity in wheat and radish is alleviated by micromolar levels of magnesium and potassium in solution culture. Plant and Soil. 259:191-199.

Interpretive Summary: A large number of soil elements may be toxic to plants. These elements may be ordinary soil constituents or even essential elements that are present in excessive concentrations, or they may be pollutants. In order to enhance crop productivity and to ensure food safety, crops and soils must be managed to counter the effects of potentially toxic elements. We have long known that the addition of calcium, and to a lesser extent, magnesium alleviates toxicities and reduces the presence of potentially dangerous elements in food supplies. The present study reports a remarkable and new observation. Zinc toxicity may be alleviated by both calcium and magnesium, but the latter element is 170 times more effective than calcium. This alleviation appears to entail the intracellular detoxification of zinc rather than its exclusion. These observations raise potentially important possibilities for agriculture. Namely, the alleviation of intoxication by one element (including elements other than zinc) by fertilization with small amounts of another element (including elements other than magnesium). This topic is considered further in a companion article.

Technical Abstract: 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 micomolar were able to significantly alleviate rhizotoxicity caused by Zn concentrations as high as 60 micomolar. In the split-root studies, 100 micomolar 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 micomolar) 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 micomolar), 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.