involving the use of transgenic plants revealed a two- fold increase in H+ influx, which resulted in increased rhizosphere pH. So, in addition to internal or external Al detoxification by organic acids (Larsen et al., 1998; Zheng et al., 1998), an increase in rhizosphere pH can also reduce Al toxicity. Plant genotypes however dif- fer in their handling of excess Al. Calba and Jaillard (1997) showed that a maize genotype that was most tolerant of Al toxicity caused a lesser rhizosphere acid- ification because NO3− uptake was less affected than for the Al-sensitive genotype.