Beneficial effect of aluminum on growth of plants adapted to low pH soils

Plants in which growth was reduced by low and high Al applications were designated as Al-sensitive plant (Hordeum vulgare) and Al-medium tolerant plants (Leucaena leucocephala, Ischaemum barbatum, Stylosanthes guianensis, and Fagopyrum esculentum), respectively, while plants in which growth was not affected or was stimulated by Al application were designated as Al-tolerant plant (Brachiaria ruziziensis) and Al-stimulated plants (Melastoma malabathricum, Melaleuca cajuputi, Acacia mangium, Hydrangea macrophylla, Vaccinium macrocarpon, Polygonum sachalinense, and Oryza sativa), respectively. Plants tolerant to or stimulated by Al were further classified based on the criteria of Al accumulation: 1) Al-excluders such as M. cajuputi, A. mangium, L. leucocephala, I. barbatum, S. guianensis, and O. sativa, 2) Al root-accumulators such as V. macrocarpon, B. ruziziensis, and P. sachalinense, and 3) Al-accumulators such as M. malabathricum, H. macrophylla, and F. esculentum. The growth and N, P, and Ii uptake in M. malabathricum, iii. cajuputi, A. mangium, L. leucocephala, H. macrophylla, V. macrocarpon, I. barbatum, P. sachalinense, F. esculentum, and O. sativa were stimulated by Al application, especially P uptake, while in H. vulgare (Al-sensitive plant) they were reduced by Al application. Ca and Mg uptake of many plants was inhibited by Al application, while that of some plants adapted to low pH soils was not affected at all (Ca and Rig: M. cajuputi, H. macrophylla, V. macrocarpon, I. barbatum, and S. guianensis; Mg: B. ruziziensis and P. sachalinense). In M. malabathricum, the relationship between Al and Ca (or Mg) was antagonistic because the Ca and Mg contents decreased by Al application even though dry matter, N, P, and K accumulation was stimulated by Al application. Plants adapted to low pH soils grew poorly in the no-Al treatment. Since the effect of the pH on plant growth was less conspicuous than that of Al, growth stimulation by Al application was ascribed not only to the alleviation of H+ toxicity but also to the increase of root activity such as P uptake.