Aluminum Effect on Organic Acid Production and Accumulation in Sorghum

Seedlings of two cultivars of sorghum (Sorghum bicolor) differing in aluminum (Al) tolerance grown in a pH 4.0 Clark's nutrient solution were exposed to Al at concentrations of 0 and 185 μ M for 7 days. The organic acid concentrations in the roots and leaves and in the external solution were determined, as well as the activities of the enzymes related to the biosynthesis and degradation of these acids. Contents of the main organic acids found in the roots and leaves increased with plant exposure to Al and were always higher in the aluminum (Al)-tolerant cultivar. Malic and t-aconitic acids were the most abundant and showed the highest absolute changes in the presence of Al in both cultivars, especially the Al-tolerant cultivar. Aluminum also changed the activities of most of the enzymes related to organic metabolism studied here. The more important effects of Al on the enzymes of the malic-acid metabolism were found on the fumarase (EC 4.2.1.2) and malate dehydrogenase (EC 1.1.1.37) in the roots, in addition to the fumarase in the leaves and on the enzymes of the trans-aconitic acid on the citrate dehydratase (EC 4.2.1.4), especially in the roots. Aluminum-induced changes in these enzymes in the two sorghum cultivars seem to favor a higher production, accumulation, and exudation of these two organic acids in the Al-tolerant cultivar compared with the Al-sensitive one. Organic acids, especially malic acid, probably play an important role in these sorghum cultivar's, tolerances to high levels of Al. Our data support the hypothesis that Al tolerance in sorghum is controlled mainly by an internal detoxification mechanism, probably involving organic acid metabolism.