Nitrate and oxalate contents of tea plants (Camellia sinensis L.) with special reference to types of green tea and effect of shading

Abstract

To analyze the utilization of nitrate and synthesis of oxalate in tea plants Camellia sinensis L., differences in nitrate and oxalate contents in different parts (flushes, leaves, stems, and roots) and in four types of Japanese green tea (normally steamed and heavily steamed Sencha, Gyokuro, and Tencha) were examined. Nitrate content was high only in fibrous roots (0.25 g NO3 − kg−1 DW) and very low in other parts (< 0.06 g NO3 − kg−1 DW). The highest content of water-soluble oxalate was detected in flushes, whereas that of crystal oxalate (calcium oxalate) in mature leaves. Oxalate was also detected in the xylem sap. These findings suggest that oxalate, synthesized at the time of nitrate reduction in fibrous roots, is transported to flushes and leaves and then accumulates in mature leaves as crystal oxalate. There were no significant differences between the nitrate and oxalate contents of the normally steamed and heavily steamed types of Sencha, which were produced by steaming for 30 to 60 s and for 60 to 180 s, respectively. The contents of nitrate and oxalate of Gyokuro and Tencha, which were cultivated with shading, were higher than those of Sencha, which was cultivated without shading. These results suggest that shading affects the nitrate and oxalate contents of tea flushes. Nitrate and oxalate contents gradually increased in the flushes with shading but remained almost constant in the flushes without shading. At the end of the shading treatment, the nitrate content per unit field area in the flushes with shading was approximately 4-times higher than that in the flushes without shading. On the other hand, the oxalate amounts per unit field area in both flushes with and without shading increased at the same rates, indicating that the oxalate content per dry weight decreased along with growth. The ratio of oxalate content to reduced N content in the flushes under shading was higher than that in the flushes without shading. These results indicate that shading increased the nitrate content by the repression of the nitrate reductase activity and that oxalate was synthesized not only by nitrate reduction but also via other pathways.