Aluminium 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 to Al may be achieved by the detoxification (complexation) of Al by organic ligands present in the rhizosphere. The Al-complexing ability of seven organic ligands, citric, oxalic, gluconic, glucuronic, mucic, galacturonic and polygalacturonic (pectin) acids, was investigated. The proportion of organically-complexed Al was determined using colorimetric methods based on differences in reaction rate with pyrocatechol violet or aluminon. The colorimetric methods confirmed that citric acid forms a strong complex with Al at pH 4.2. In contrast, pectin and related organic ligands weakly complexed Al in acidic conditions. In an additional study, the Al-binding ability of pectin and Ca-pectate was compared at a biologically significant concentration of 32 µM Al. Only 29% of free Al remained in solution in the presence of Ca-pectate, while 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. Root growth of mungbean (Vigna radiata (L.) Wilczek) confirmed differences in the ability of citrate, oxalate and galacturonate to complex Al.