Aluminum (Al) toxicity usually occurs in acidic soils with a pH of 5.5 or lower. The selection and breeding of Al-tolerant cultivars is a useful approach for protecting maize from Al toxicity. Rapid and reliable screening methods must be developed to discriminate Al-tolerant and Al-sensitive maize genotypes. The relative root growth (RRG) of the longest root in a toxic Al solution was used to classify 141 maize germplasm lines into three groups with varied Al sensitivity: Al-sensitive, moderately Al-tolerant, and Al-tolerant. Among these lines, the cultivars HZ85 and 178 had the highest RRG values and therefore the highest Al tolerance. Further root assessment of six representative lines using other methods, such as digital imaging analysis of total root length and superficial area or volume, biomass measurement, and hematoxylin staining, was roughly consistent with the classification based on RRG. These results indicated that the RRG of the longest root could be used as a reliable and reproducible phenotypic index for the evaluation of Al tolerance in maize genotypes. Cultivars with different Al tolerances can be used to improve breeding and explore the mechanism of Al tolerance in maize.
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