Aluminum-induced organic acids exudation by roots of an aluminum-tolerant tropical maize

Aluminum (Al) tolerant and sensitive plants selected from the tropical maize Variety Taiuba were grown in complete nutrient and simple salt solutions in the presence and absence of phytotoxic concentrations of Al. During the first 20 hr of Al exposure, the root growth rate of both tolerant and sensitive plants was severely inhibited as a consequence of Al infiltration into the root tip cells. After this period, however, roots of ill-treated tolerant plants recovered to a growth rate similar to that of control plants, while the root growth rate of sensitive plants remained severely inhibited. The recovery of the root growth rate of tolerant plants coincided with the extrusion of the Al that had been absorbed in the first 20 hr of Al exposure, When the roots of tolerant and sensitive plants were grown in simple salt solutions containing a series of Al concentrations, a dose-dependent citrate and malate exudation was observed from tolerant but not from sensitive roots. The level of citrate exudation was two- to four-fold that observed for malate. The organic acid exudation was not influenced by the level of phosphate in the growth solution, suggesting a specific A-inducing process involved in the Al tolerance in maize. We concluded from these results that the Al infiltration in the roots at the beginning of Al exposure induces the exudation of organic acids which may exclude the toxic ion from the root tip cells of tolerant plants. (C) 1997 Elsevier Science Ltd.