ツバキ科植物のフッ素吸収に関する生物地球化学的研究

It is well known that some plants belonging to Theaceae, such as tea plants, selectively accumulate fluorine, whereas fluorine contents of the other plants is very low. But little is known about the accumulation mechanism of fluorine by Theaceae. Some plants belonging to Theaceae also are the accumulator plants of aluminium. For example, tea plants contain high levels of fluorine and aluminium in the leaves. (Leaves usually contain much more fluorine and aluminium than other parts of plants.) It is noticeable that same plants show high fluorine and aluminium contents. Judging from strong chemical reactivity between fluorine and aluminium, it is assumed that fluorine absorption by plants is closely related to aluminium absorption by plants. Therefore, the absorption of fluorine by Theaceae from soils and the chemical state of the absorbed fluorine were studied in relation to absorption of aluminium by the plants. The results are summarized as follows. It was found out that the plants enriched in fluorine always showed high aluminium contents, whereas the plants enriched in aluminium did not always show high fluorine contents. And the fluorine contents of the accumulator plants was always found significantly and positively correlated with the aluminium contents of the plants. The ratios of fluorine to aluminium contents of leaves (F/Al ratio) were about the same at different positions on the stem of the plants. And the F/Al ratio of leaves did not show difference among the varieties of the plants growing at the same site, but showed significantly difference among the growing site. Therefore, in order to explain the results mentioned above, the author proposed the following hypotheses. (a) Soluble fluorine of soils exist as fluoro-aluminium complexes, such as AlF^<2+> and AlF_2^+. (b) Accumulator plants of fluorine absorb fluorine and aluminium in the forms of AlF^<2+>, AlF_2+ and Al^<3+>. To prove the hypothesis (a), the amounts of soluble fluorine and aluminium of tea garden soils and fluoride and aluminium of tea garden soils and fluoride-polluted soils were determined with various extracting solutions. And it was estimated that soluble fluorine in the soils existed in the forms of fluoro-aluminium complexes or hydroxy-fluorapatite Ca_<10>(PO_4)_6F_x(OH)_<2-x>(x<2). When soil pH was high, most of the soluble fluorine existed as hydroxyfluorapatite, and with decreasing soil pH, hydroxyfluorapatite was dissolved by Al^<3+> forming fluoro-aluminium complexes. Subsequently, the absorption of fluoro-aluminium complexes by plants (hypothesis (b)) was confirmed by the water culture of tea plants. From the results described above, the author might conclude that soluble soil fluorine existed as fluoro-aluminium complexes, and the accumulator plants of fluorine absorbed fluorine and aluminium in the forms of Al^<3+>, AlF^<2+> and AlF_2^+ from soils.