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Possible Reasons Why Aluminum is a Beneficial Element for Melastoma malabathricum, an Aluminum Accumulator

  • Author(s): Watanabe, Toshihiro
  • Osaki, Mitsuru
  • et al.
Abstract

Melastoma malabathricum is an aluminum (Al) accumulator woody species growing in tropical acid sulfate soils, and highly tolerant to Al stress. On the contrary, its growth is often enhanced by Al application. In this study, two possible reasons why Al has beneficial effects on the growth of M. malabathricum were proposed. The first one is the suppression of iron (Fe) toxicity by Al. Although Fe availability is also high in acid sulfate soils, little is known about its effect on the growth of native plant species growing in acid sulfate soils. When 100 µM Fe was applied to a nutrient solution, the growth of M. malabathricum was severely inhibited because of Fe-induced oxidative stress, but application of 500 µM Al completely ameliorates Fe toxicity. This amelioration is associated with a decrease of Fe concentration in shoots and roots. Thus, one of the primary reasons for the Al-induced growth enhancement in M. malabathricum seems to be the Al-induced reduction of Fe accumulation. The second possible reason is an adverse effect of internal Al tolerant mechanism in M. malabathricum when grown in the absence of Al. M. malabathricum synthesized high concentration of oxalate, an internal chelator for Al detoxification, irrespective of Al concentration in the medium. In the absence of Al, the synthesized oxalate could not make complex with Al but makes insoluble precipitation with alkaline earth metals, such as Ca and Mg in plant, possibly resulting in deficiency of these nutrient cations.

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