Mechanism for the detoxification of aluminum in roots of tea plant (Camellia sinensis (L.) Kuntze).

To determine the mechanism of aluminum (Al) detoxification in the roots of tea plants (Camellia sinensis (L.) Kuntze), the amounts of Al and Al-chelating compounds (fluoride (F), organic acids and catechins) were measured and the chemical forms of Al in root cell extracts were identified by the application of 27Al-nuclear magnetic resonance (NMR) spectroscopy. Tea plants were cultivated in nutrient solutions containing 0, 4, 1.0 and 4.0 mM of Al at pH 4.2 for approximately 10 weeks. The levels of soluble Al, water-soluble oxalate and citrate, but not F, malate or catechins in young roots increased with an increase in the concentration of Al in the treatment solution. The 27Al NMR spectra of root tips and cell sap extracted from root tips that had been treated with Al were almost identical and had four signals, with two (11 and 16 ppm) apparently corresponding to the known chemical shifts of Al-oxalate complexes. In the spectra of cell sap, the resonances at 11 and 16 ppm increased with an increase in the Al contents. These results suggest that the levels of Al-oxalate complexes increased in response to an increase in the Al level, implying that oxalate is a key Al-chelating compound in the mechanism of Al detoxification in the tea root.

Chemical forms of aluminum in xylem sap of tea plants (Camellia sinensis L.).

To identify the chemical forms of aluminum (Al) transported from roots to shoots of tea plants (C. sinensis L.), 27Al-nuclear magnetic resonance and 19F NMR spectroscopy were used to analyze xylem sap. The concentration of Al in collected xylem sap was 0.29 mM, twice as high as that of F. Catechins were not detected in xylem sap. The concentration of malic acid in xylem sap was higher than that of citric acid, whereas the concentration of oxalic acid was negligible. There were two signals in the 27Al NMR spectra of xylem sap, a larger signal at 11 ppm and a smaller one at -1.5 ppm. The former signal was consistent with the peak for an Al-citrate model solution, suggesting that an Al-citrate complex was present in xylem sap. Although the latter signal at -1.5 ppm was thought to indicate the presence of an Al-F complex (at 1.7 ppm) in xylem sap, there was only one signal at -122 ppm in the 19F NMR spectrum of xylem sap, indicating that the main F complex in xylem sap was F-. These results indicate that Al might be translocated as a complex with citrate, while Al-malate, Al-oxalate and Al-F complexes are not major Al complexes in xylem sap of tea plants.