On the Phytotoxicity of Polynuclear Hydroxy‐Aluminum Complexes

Abstract

AbstractIn a previous study we demonstrated the phytotoxicity of polynuclear hydroxy‐Al, a finding that disagrees with a number of existing reports. The objectives of this study were to examine how toxicity of polynuclear Al might vary with experimental conditions or with choice of test plants, and to identify and characterize the toxic polynuclear species. Wheat (Triticum aestivum L. cv. Tyler or Seneca) or soybean (Glycine max (L.) Merr. cv. Stafford) seedlings were cultured for 2 d in dilute CaCl2 solutions containing 15 µM total Al at a basicity (initial molar OH/Al ratio) of 2.0. Increasing rate of base addition, solution age, or levels of added phosphate decreased the fraction of reactive polymers (Alb) and increased that of precipitated Al (Alc) as determined by the ferron method. Mononuclear Al was consistently ≤3 µM and did not contribute to toxicity. Inhibition of root growth was well correlated with [Alb] for both plant species. That only 3 to 4 µM [Alb] was required to fully inhibit wheat root growth suggested that detection of toxic polynuclear Al may at times be difficult in chemically complex media. All plants tested were considerably more tolerant of Al3+ than of Alb, suggesting fundamental physiological differences in the toxicity of the two types of Al. This conclusion was further supported by the finding that the two wheat cultivars exhibited differential tolerance to Al3+, but not to Alb. Several analyses suggested that Alb was composed primarily of the so‐called Al13 polymer, which may form as a consequence of synthesis conditions. Given uncertainties in the occurrence of this polymeric species, both in the laboratory and in nature, several lines of future investigation are suggested.