Constitutive and aluminium-induced patterns of phenolic compounds in two maize varieties differing in aluminium tolerance

Aluminium tolerance in maize is mainly due to more efficient AI exclusion. Nonetheless, even in tolerant varieties AI can gain access into the cells. Detoxification by binding to strong organic ligands should therefore play a role also in plants with high AI exclusion capacity. To test this hypothesis in this study the concentrations of soluble, free and bound, phenolics were analyzed in roots of two maize varieties differing in AI tolerance. Exposure for 24 h to 50 mu M AI in nutrient solution strongly inhibited root elongation in the sensitive variety 16 x 36, but not in the AI-tolerant variety Cateto. Cateto accumulated about half the concentration of Al in roots than 16 x 36 (analysis performed after root desorption with citrate). Roots of AI-tolerant Cateto contained higher concentrations of caffeic acid, catechol and catechin than roots of the sensitive variety. Exposure to AI induced the accumulation of taxifolin in roots of both varieties. However. Al-tolerant Cateto accumulated about twice the concentration than Al-sensitive 16 x 36 of this pentahydroxyfavonol. The molar ratio for phenolics with catecholate groups to AI was about unity in roots of Cateto, while in those of 16 x 36 the ratio was ten times lower. Both the fact that these phenolics are strong ligands for Al and their high antioxidant and antiradical activity suggest that these compounds may provide protection against the AI fraction that is able to surpass the exclusion mechanisms operating in the tolerant maize variety. (C) 2009 Elsevier Inc. All rights reserved.