Quantitative Estimation of Physiological Functions of Various Roots with Different Diameters in the Root System of the Tea Tree.

Abstract

The root system of the tea (Camellia sinensis L.) tree consists of various types of roots with different diameters or ages. In order to control the growth of the root system, it is necessary to know the physiological functions of various types of roots within a root system quantitatively. Tea trees, two years after transplanting, were dug out from the field at the bud break stage of the first flush, then the roots were classified into four groups according to their diameter. Top/root ratio of the examined trees was around 1.5. Dry weight ratios of white rootlet (diameter 5.0mm) in root systems were 30%, 10%, 15% and 45%, respectively. Rates of respiration and nitrogen uptake per unit dry weight were higher in the younger rootlets, while the content of total available carbohydrate (TAC) was higher in the lignified thick roots. Quantitatively, 75% of the respiration and 90% of nitrogen uptake in the root system was conducted by the rootlets less than 2.0mm in diameter. Contribution of the white rootlets to the total nutrient uptake of the root system was especially large. On the other hand, 84% of TAC in the root system was localized in thc lignified roots more than 2.0mm in diameter. A higher level of theanine, a main palatable substance of tea, was detected in the white rootlet as compared to the lignified roots, indicating that the synthesis of this substance occurred in the newly developing roots. In the lignified roots, a large amount of arginine accumulated instead of theanine. From these results, ideotype of the root system in the tea tree was considered to be different according to the purpose of tea cultivation. A root system with higher proportion of rootlets would be desirable for increasing the yield and quality of the leaves. On the contrary, existence of well-developed lignified roots would be necessary for tolerating environmental stresses.