Interactive intoxicating and ameliorating effects of tannic acid, aluminum (Al3+), copper (Cu2+), and selenate (SeO42-) in wheat roots: a descriptive and mathematical assessment

Tannic acids and tannins are produced by plants and are important components of soil and water organic matter. These polyphenolic compounds form complexes with proteins, metals and soil particulate matter and perform several physiological and ecological functions. The tannic acid (TA) used in our study was a mixture of gallic acid and galloyl glucoses ranging up to nonagalloyl glucose. TA inhibited root elongation in wheat seedlings (Triticum aestivum L. cv. Scout 66) at concentrations >4 mg l(-1); but TA alleviated the toxicity of Al3+, Cu2+ and SeO42-; and Al3+ and SeO42- alleviated the toxicity of TA. The interactions of Al3+ and TA (each toxic but each alleviating the toxicity of the other) were stoichiometric. Growth was affected as though 1 kg TA bound 2.76 mol Al so strongly that if (mol Al)/(kg TA) <2.76, then free Al approximate to 0, and if (mol Al)/(kg TA) >2.76, then free TA approximate to 0. This stoichiometry is consistent with one mole of galloyl groups binding approximately 0.5 mol Al. Using this binding scheme, growth was modeled successfully on the basis of free TA and free Al. TA enhanced the negativity of root surfaces and enhanced the binding of Al and Cu there without enhancing their toxicity. These and other interactions among TA, Al3+, Cu2+, SeO42-, Ca2+, Na+ and H+ were quantified with a comprehensive non-linear equation with statistically significant coefficients.