Speciation and phytotoxicity of mono- and polynuclear aluminum in dilute hydroxy-aluminum solutions

• Main Author: Parker, David R.
• Other Authors: Agronomy
• Format: Others
• Language: en_US
• Published: Virginia Polytechnic Institute and State University 2018
• Subjects: LD5655.V856 1988.P375; Plants > Effect of aluminum on
• Online Access: http://hdl.handle.net/10919/82626

Despite long-standing interest in the toxicity of aluminum (Al) to plants, relationships between phytotoxicity and the chemistry of Al in hydroxyl-dominated systems remain unresolved. First, the ferron (8-hydroxy-7-iodo-5-quinoline-sulfonic acid), aluminon (aurintricarboxylic acid, triammonium salt), and 8-hydroxyquinoline methods were evaluated and compared for their ability to yield estimates of the mononuclear fraction, ƒ_m, in Al solutions of varying basicity. Reactions of ferron or aluminon with Al were kinetically modeled as two parallel irreversible reactions, while 8-hydroxyquinoline required use of an arbitrary cutoff time. Estimates of ƒ_m were in the order 8-hydroxyquinoline > ferron > aluminon at basicities ≤ 1, while at a basicity of 2.25 the general order was aluminon ≥ 8-hydroxyquinoline > ferron. Aging solutions up to 32 days resulted in decreased estimates of ƒ_m using aluminon, but estimates using ferron and 8-hydroxyquinoline were only minimally affected. All three methods yielded results of adequate precision for most purposes, although the ferron procedure was only about one-half as sensitive than the other two. Additional studies using ferron demonstrated its utility for characterizing the nonmononuclear Al fraction using kinetic analyses. The ferron method was selected for continued use based on its simplicity, level of precision, and moderate reaction rate with Al. Next, 2-day old wheat (Triticum aestivum L.) seedlings were transferred to 500-mL test solutions for 2 days of additional growth at 298 K in the dark, and final root lengths used as the measure of Al toxicity. Quantities of polynuclear hydroxy-Al were estimated using ferron, and activities of mononuclear species were calculated using the computer program GEOCHEM. In the absence of polynuclear hydroxy-Al, raising solution pH consistently ameliorated Al toxicity suggesting that Al³⁺ is the primary toxic species. The results did not confirm other reports that the sum of the mononuclear species activities is a reliable predictor of Al phytotoxicity. Polynuclear hydroxy-Al was also demonstrably toxic, supporting isolated reports of the same finding. Last, the toxicity of the polynuclear species as a function of experimental conditions was investigated. Increasing solution age, rate of base addition, or levels of added phosphate decreased the quantity of reactive polymers ([Al_b]) as measured by ferron. Inhibition of root growth of both wheat and soybean (Glycine max (L.) Merr.) was well-correlated with [Al_b], and these polymers were considerably more toxic than Al³⁺. The wheat cultivars 'Tyler' and 'Seneca' exhibited differential tolerance to Al³⁺, but not to [Al_b], further suggesting differences in the physiological bases of mono- and polynuclear Al toxicity. Characterization of Al_b by several methods indicated that the toxic species are relatively small, highly charged, and principally the so-called Al₁₃ polymer. These findings have practical implications for the accurate prediction of toxic Al levels in both solution culture media and soil solutions. === Ph. D.