Mechanisms on the Al tolerance and accumulation of plants adapted to low pH soils

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The effects of N-source and Al on the growth of seedlings of Melastoma malabathricum, Acacia mangium and Melaleuca cajuputi, which are tropical woody plants and are very tolerant to Al, and barley (Hordeum vulgare), which is a typical Al-sensitive plant, were investigated. The Al and N treatments consisted of the application of either 0 or 0.5 mM Al, and 2 mM NH_4^+ or NO_3^-, respectively. Growth of the tropical plants was enhanced by Al and NH_4 application. Excised roots of M.malabathricum dissolved insoluble Al with NH_4 application and absorbed Al mainly from root tips. Al did not affect the leaf N concentration except in the case of barley. Roots of M.cajuputi exuded a large amount of citrate, which slightly increased by the +Al treatment. In A.mangium, the reactivity of soluble Al to PCV (pyrocatecholviolet) decreased in the culture solution of the +Al+NH_4 treatment and Al concentration of roots in this treatment was very low. Roots of M.malabathricum released H+ along with Al uptake as well as NH_4^+ uptake. It is concluded that Al and NH_4^+ exert beneficial effects on the growth of tropical tree seedlings.Al appeared mostly in apoplastic fraction in excised melastoma roots treated with 0.1 mM Al for 180 min, and CCCP did not affect the Al uptake, indicating that Al uptake in the excised roots occurs mostly through passive accumulation in the apoplast. The Al ion displaced 3H^+ on carboxyl groups in the melastoma root apoplast when it was taken up, a finding not observed in barley. This adsorption site had a higher affinity to the Al ion compared with Ca^<2+> or La^<3+>. On the other hand, three months after Al exposure, Al concentration in the symplastic and residual fractions of the melastoma roots was very high and surpassed that in the apoplastic fraction. From the results in the present and previous studies, a hypothesis of the Al uptake mechanism in melastoma roots was proposed.