Studies on mechanisms of aluminum tolerance in sorghum

The effects of Al on plant growth, nitrogen uptake and assimilation, and organic acid composition of Al-tolerant and Al-sensitive sorghum (Sorghum bicolor (L.) Moench) genotypes were studied. Hypotheses that associate differential Al tolerance with differential (i) nitrate/ammonium uptake, (ii) plant-induced solution pH changes, (iii) nitrate reductase activity, (iv) accumulation of free proline, and (v) accumulation of organic acids were tested under controlled environments in nutrient solutions with different Al levels. The Al-tolerant (SC-283) genotype had higher dry matter production, root length, N content, nitrate- and ammonium-N uptake, and nitrate reduction than the Al-sensitive (ICA-Nataima) genotype when both were grown with Al. The effect of Al on plant growth was not significantly affected by relative nitrate and ammonium supplies. However, the effect of Al on nitrate and ammonium uptake rates was more pronounced when plants were grown with a high nitrate-N supply (195 mg L$\sp{-1}$) compared to lower supplies (180 and 150 mg L$\sp{-1}$). SC-283 plants induced higher solution pH than did ICA-Nataima plants, when grown with Al. These differences in solution pH were caused by differential nitrate and ammonium uptake between the two genotypes. ICA-Nataima had higher root accumulation of free proline than SC-283 when both genotypes were grown with Al. Proline seemed to accumulate with Al treatment as a result of reduced growth and nutrient uptake. Differential nitrate/ammonium uptake was the only trait that appeared to be associated with differential Al-tolerance in sorghum. Differences in nitrate and ammonium uptake accounted for genotypic differences in plant-induced solution pH. Differential nitrate reductase activity in shoots and proline accumulation in roots appeared to be secondary responses to Al toxicity rather than mechanisms for Al tolerance. Aluminum tolerant genotypes (S42884 and SC-283) accumulated larger amounts of organic acids than the Al-sensitive genotype (ICA-Nataima) when plants were grown with Al. These differences in organic acid accumulation may indicate that the Al-tolerant genotypes had a higher potential for chelating and detoxifying Al than the Al-sensitive genotype. However, biochemical studies at the cytosol level are needed before actual Al-binding by the organic acids can be assumed.