Differences of growth response to aluminum excess of two Melaleuca trees differing in aluminum resistance

Factors that inhibit the growth of plants in strongly acidic soils include low pH and aluminum excess. We evaluated two Myrtaceae species (Melaleuca cajuputi and Melaleuca bracteata), which are useful trees in tropical regions due to their resistance to low pH and excessive aluminum, to determine their response characteristics to environmental stresses. The results revealed that M.cajuputi, the growth by the aluminum concentration was not inhibited. However, the root growth of M.bracteata, by the aluminum treatment was inhibited remarkable, and 83.2% inhibited the maximum more than aluminum free. The pH of rhizosphere of both plants has decreased by the BCP plate. However, the pH decrease has decreased as for M. bracteata with high concentration of aluminum, but M.cajuputi is not changed. Next, it was able to be confirmed of aluminum accumulated in the root M.bracteata more than M.cajuputi by hematoxylin and aniline blue dyeing. At the result, it was able to be confirmed of aluminum accumulated in the root M.bracteata more than M.cajuputi. As for M.bracteata, aluminum was absorbed in the inner-cell of the root tissue. On the other hand, M.cajuputi had aluminum only in the surface of the root. We concluded that M.cajuputi was an aluminum tolerance because the mechanism that aluminum doesn't invade the internal tissue of the root was possessed.