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Title: RELATIVE EFFECTIVENESS OF CALCIUM AND MAGNESIUM IN THE ALLEVIATION OF RHIZOTOXICITY IN WHEAT INDUCED BY COPPER, ZINC, ALUMINUM, SODIUM AND LOW PH

Author
item Kinraide, Thomas
item PEDLER, JUDITH - UNIV OF MELBOURNE, AUSTR
item PARKER, DAVID - UNIV OF CALIFORNIA

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/9/2003
Publication Date: 2/4/2004
Citation: Kinraide, T.B., Pedler, J.F., Parker, D.R. 2004. Relative effectiveness of calcium and magnesium in the alleviation of rhizotoxicity in wheat induced by copper, zinc, aluminum, sodium and low ph. 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 relates to a companion paper that reports that zinc toxicity may be alleviated by both calcium and magnesium, but the latter element is 170 times more effective than calcium. This study dissects three mechanisms by which calcium and magnesium alleviate toxicity from the elements copper, zinc, aluminum, sodium, and hydrogen ion, each of which respond differently to calcium and magnesium. 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: Root elongation in short-term experiments with wheat (Triticum aestivum L.) seedlings demonstrated that the following ions were rhizotoxic in the order Cu2+ > Al3+ >> H+ > Zn2+ >> Na+. Additions of Ca2+ and Mg2+ alleviated the toxicity, but the relative ameliorative effectiveness of Ca2+ and Mg2+ depended upon the toxicant. The effectiveness of Mg2+ relative to Ca2+ was 0.098 for Na+, 0.37 for H+, 1.0 for Al3+, 2.1 for Cu2+, and 170 for Zn2+. The mechanisms of inhibition are mainly unknown, but the mechanisms of alleviation are better understood. Mechanism I entails ameliorant-induced reduction of the negativity of the plasma membrane (PM) surface electrical potential. The consequence is a reduced activity of the toxicant at the PM surface because of reduced electrostatic attraction. Ca2+ and Mg2+ are equally effective agents of Mechanism I alleviation but are less effective than H+ and more effective than Na+ for reasons described by electrostatic principles. Mechanism II alleviation is specific for Ca2+ and entails the restoration of Ca2+ at the PM surface if surface Ca2+ has been reduced by the toxicant to growth-limiting levels. This occurs more commonly in Na+ and H+ toxicities than in the others, though in no case is it the principal mechanism of alleviation. Mechanism III alleviation is the collective ameliorative effect of an ion beyond Mechanisms I and II. Differences between Ca2+ and Mg2+ in ameliorative effectiveness are mainly attributable to Mechanism III which, in the case of Zn2+, may entail an internal detoxification and, in the case of Na+, may entail the blockade of a Na+ uptake channel. This study demonstrates that appropriate nonlinear equations incorporating cell-surface ion activities enable the dissection of multiple toxic and ameliorative effects of the ions.