Nguyen Tran Nguyen (1) , Kazuo Nakabayashi (2) , Julian Thompson (3) and Kounosuke Fujita (1, 4)
1. Graduate School of Biosphere Sciences, Hiroshima University, Hiroshima 739-8528, Japan / 2. Institute for International Cooperation, Japan International Cooperation Agency, Tokyo 162-8433, Japan / 3. Department of Geography, University College London, 26 Bedford Way, London WC1H 0AP, U.K. / 4. Author to whom correspondence should be addressed (fujiko@hiroshima-u.ac.jp) / Received December 11, 2002; accepted March 29, 2003; published online September 15, 2003
Summary
Responses of Melaleuca leucadendra (L.) L., Melaleuca cajuputi Powell, Acacia auriculiformis A. Cunn. ex Benth. and Eucalyptus camaldulensis Dehnh. to aluminum (Al) toxicity at low pH are poorly understood. We investigated effects of low pH and exudation of ligands by roots on Al tolerance of these species. Seedlings were grown hydroponically in nutrient solutions at pH 4.2 or 3.5 containing AlCl3 at concentrations ranging from 0 to 4 mM Al. The presence of 4 mM Al at pH 3.5 depressed growth in all species. Growth depression was greatest in E. camaldulensis, least in A. auriculiformis. In the low Al treatment (0.5 mM Al), roots of M. cajuputi tended to have the highest Al concentration among species, whereas in the 4 mM Al treatment, the highest Al concentration was found in roots of E. camaldulensis. Aluminum application enhanced root exudation of citrate in all species, with the enhancement in M. cajuputi, M. leucadendra and A. auriculiformis being similar and much greater than in E. camaldulensis. Exudation of oxalate and phenolic compounds was greater in E. camaldulensis than in the other species. The presence of Al enhanced phosphate exudation in all species, particularly in A. auriculiformis. Acacia auriculiformis was tolerant to low pH, probably because the presence of an unknown substance increased the pH. Application of 0.38 mM Al alleviated the toxicity of the pH 3.5 treatment in E. camaldulensis and M. cajuputi, whereas low pH alleviated Al toxicity in A. auriculiformis. We conclude that exudation of ligands such as citrate and phosphate only partly accounts for interspecific differences in Al tolerance among the tropical woody plants studied, whereas the reciprocal alleviation of Al toxicity and low pH differed considerably among the species.
Keywords : Acacia auriculiformis, Eucalyptus camaldulensis, low pH tolerance, Melaleuca cajuputi, Melaleuca leucadendra .
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