SUBCELLULAR DISTRIBUTION AND CHEMICAL FORMS OF FLUORIDE IN TEA TREE LEAVES (CAMELLIA SINENSIS L.) AND ITS CELL WALLS

Tea tree (Camellia sinensis L.) is an hyper-accumulator of fluorine (F), especially in leaves. Here, the accumulation of different fluorine forms and their subcellular distribution in leaves and cell walls were analyzed in three tea cultivars (F-sensitive Wuniuzao, F-resistant Fudingdabai, and Fuyun No.6). Results indicated that the F content varied significantly in the three cultivars and increased with the maturity of leaves, with most F accumulated in cell walls. Specifically, the greatest amount of F was found in the water-soluble form, about 61.90-87.52% of the total fluorine, followed by the residual-fluorine, about 5.41-27.25%, the organic matter-bound fluorine, about 1.92-8.32%, and, with the lowest proportion, the iron-manganese-bound and exchangeable fluorine. Fluorine in the cell walls was mainly present in the water-soluble form, followed by residual-fluorine. There was a distinct difference between the three different cultivars in F concentration, subcellular distribution, and chemical forms. When compared with the two F-resistant cultivars, the F-sensitive Wuniuzao had higher water soluble F and less residual F in leaves and cell walls. These results suggest that mature leaves are the main F enrichment site, and the retention of F in the cell wall in a bound form might be an important way for tea plants to reduce F toxicity.