Comprehensive evaluation of acid-aluminum tolerance of white clover germplasm resources

White clover is a high-quality protein forage and is widely cultivated in the Yangtze River basin and the south of China where most soils are acidic. It was found that white clover has some acid and aluminum tolerance; hence, screening acid and aluminum tolerant white clover germplasm is important for developing high quality resources of white clover and breeding new varieties with acid and aluminum tolerance. The aim of this study was to evaluate the acid-aluminum (Al³⁺) tolerance of different white clover germplasm lines and select the tolerant lines. To this end, we evaluated the growth characteristics of 30 white clover germplasm lines under Al³⁺ stress at germination stage. A subordinate function method was used to evaluate the resistance of white clover to acid-aluminum toxicity. It was found that, as the Al³⁺ concentration increased, the germination rate, germination potential and germination index gradually decreased, and the radicle became shorter and thicker. The germination rate and radicle length of white clover were significantly inhibited at Al³⁺ concentration ≥ 1.0 mmol·L⁻¹ compared with that of the control. Other major traits of white clover, except for radicle diameter, also differed significantly under different Al³⁺ concentrations (P < 0.05). The values for each trait differed extremely significantly among white clover germplasm materials at the germination stage (P < 0.01). The acid-aluminum tolerance of different white clover germplasm materials could be evaluated at the germination stage at a Al³⁺ concentration of 2 mmol·L⁻¹; at this concentration, the majority of traits of white clover exhibited high values of coefficients of variation and high statistically significant differences. The 30 white clover germplasms were divided into three groups with different acid-aluminum tolerance by cluster analysis. The acid-aluminum tolerance of the germplasm among different clusters was found to be consistent with the comprehensive evaluation value. The results of the multivariate evaluation revealed the white clover germplasms EM196 and EM085, from Netherland and Australia, respectively, to be the most Al³⁺ tolerant lines.