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

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 (psi(0)). 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.