A rapid assay for aluminium phytotoxicity at submicromolar concentrations

Investigations of Al phytotoxicity, including the identification of the Al species responsible for toxicity, require a rapid assay procedure employing very low concentrations of Al and a chemically simple rooting medium. Root elongation in newly germinated red clover ( Trifolium pratense L. cv. Kenland) was inhibited by submicromolar concentrations of Al. Ca 2+ at concentrations of at least 0.2 m M was essential for optimal elongation in control seedlings. Ca 2+ also relieved Al toxicity with the net effect that maximum reduction of elongation by 1 μ M Al was achieved at 0.2 m M Ca 2+ . Elongation in control seedlings was at least 90% of maximum from pH 4.5 to 5.7. Increases in pH relieved Al toxicity so that maximum sensitivity to 1 μ M Al occurred at pH 4.7. As a consequence of these experiments and other considerations we chose for our basic assay a medium composed of 0.2 m M CaSO 4 adjusted to pH 4.5 with H 2 SO 4 , variously supplemented with Al 2 (SO 4 ) 3 . Day‐old seedlings were incubated in this aerated medium in the dark at 23°C for one day. No additions of other solutes increased the sensitivity of the assay, but amelioration of Al toxicity was effected by Mg 2+ , F ‐ , phosphate and citrate. Increases in ionic strength per se had comparatively little effect on the toxic effects of Al. Two barley cultivars ( Hordeum vulgare L. cv. Dayton and Kearney) and two wheat cultivars ( Triticum aestivum L. cv. Hart and Thorne) known to differ in sensitivity to Al were reliably separated at submicromolar Al concentrations by the assay procedure, which was slightly modified. Suggestions for the improvement of the assay and for applications to future research are offered.