ALUMINUM REACTIONS WITH POLYGALACTURONATE AND RELATED ORGANIC-LIGANDS

Aluminium (Al), in inorganic monomeric forms, has been recognised as a limiting factor for root growth in many acid soils. Plant tolerance t o Al may be achieved by the detoxification (complexation) of Al by org anic ligands present in the rhizosphere. The Al-complexing ability of seven organic ligands, citric, oxalic, gluconic, glucuronic, mucic, ga lacturonic and polygalacturonic (pectin) acids, was investigated. The proportion of organically-complexed Al was determined using colorimetr ic methods based on differences in reaction rate with pyrocatechol vio let or aluminon. The colorimetric methods confirmed that citric acid f orms a strong complex with Al at pH 4.2. In contrast, pectin and relat ed organic ligands weakly complexed Al in acidic conditions. In an add itional study, the Al-binding ability of pectin and Ca-pectate was com pared at a biologically significant concentration of 32 mu M Al. Only 29% of free Al remained in solution in the presence of Ca-pectate, whi le 54% remained when pectin was present. This suggests that Ca-pectate , rather than pectin, is responsible for binding Al in root cell walls and consequently plays an important role in Al toxicity to plants. Ro ot growth of mungbean (Vigna radiata (L.) Wilczek) confirmed differenc es in the ability of citrate, oxalate and galacturonate to complex Al.