Interactive Intoxicating and Ameliorating Effects of Tannic Acid, Aluminum, Copper, and Selenate in Wheat Roots.

Tannic acids and tannins are polyphenolic compounds produced by plants.  They sometimes constitute a large fraction of the dry mass of living and dead plant tissues, and they are important components of soil and water organic matter.  Tannic acids and tannins form complexes with proteins, metals, and plant and soil particles and perform several physiological and ecological functions.  A commercial product carrying the labels Tannic Acid, F.W. 1701.18, and C₇₆H₅₂O₄₆ (implying 1,2,3,4,6-penta-O-digalloyl-beta-D-glucopyranose) was actually a mixture of galloyl glucoses, gallic acid, and a lower molecular weight component together having an average molecular weight of 860 g/mol.  This product will be referred to as TA.  Root elongation by wheat seedlings in solution culture was inhibited by TA at concentrations > 4 mg L^-1, but TA alleviated the toxicity of Al³⁺, Cu²⁺, and SeO₄^2-.  Al³⁺ and (possibly) SeO₄^2- alleviated TA toxicity, but Cu²⁺ did not.  Neither Ca²⁺ nor H⁺ alleviated the toxicity of TA, and TA did not alleviate the toxicity of H⁺, but Ca²⁺ did.  The interactions of Al³⁺ and TA (each toxic but each alleviating the toxicity of the other) were stoichiometric.  Growth was affected as though 362 g TA bound one mole of Al so strongly that if (grams TA)/(moles Al) > 362, then free Al ≈ 0, and if (moles Al)/(grams TA) > 0.00276, then free TA ≈ 0.  Using this binding scheme, growth was modeled successfully on the basis of free TA and free Al.  TA enhanced root surface negativity and enhanced root surface adsorption of Al and Cu.