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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. Ca2+ at concentrations of at least 0.2 mM was essential for optimal elongation in control seedlings. Ca2+ also relieved Al toxicity with the net effect that maximum reduction of elongation by 1 μM Al was achieved at 0.2 mM Ca2+. 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 μM 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 mM CaSO4 adjusted to pH 4.5 with H2SO4, variously supplemented with Al2(SO4)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 Mg2+, 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. [ABSTRACT FROM AUTHOR]

Maize (Zea mays L.) is the third most important cereal grown in the world. In South and Central America, maize is mostly grown on acidic soils. On these soils, yields are limited by deficient levels of available P, Ca, and Mg, and toxic levels of Al and Mn. A greenhouse study was conducted with 22 maize genotypes originating from Africa, Europe, and North, Central, and South America on acid, dark red latosol (Typic Haplorthox) at 2%, 41%, and 64% Al saturation at correspondingpH of 5.6, 4.5, and 4.3. With increasing Al levels, the nutrient efficiency ratios (NER = mgs of dry shoot weight / mg of element in shoot) for K, Ca and Mg increased, and NER for P and Zn tended to decrease. Overall, Al-tolerant genotypes produced higher shoot and root weight and had higher NER for P, Ca Mg, and Fe at 41 % Al saturation. Genotypes used in this study showed genetic diversity for growth and NER of essential nutrients. It was concluded that selection of acid-soil-tolerant genotypes and further breeding of acid-soil-tolerant maize cultivars are feasible. [ABSTRACT FROM AUTHOR]