Relative effectiveness of calcium and magnesium in the alleviation of rhizotoxicity in wheat induced by copper, zinc, aluminum, sodium, and low pH

Abstract

Root elongation in short-term experiments with wheat (Triticum aestivum L.) seedlings demonstrated that the following ions were rhizotoxic in the order Cu²⁺>Al ³⁺≫ 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 (ψ₀). 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.