Root structure and cell-wall extensibility of adventitious roots of tea (Camellia sinensis cv. Yabukita)

In spite of the importance of white root development in the formation of adventitious root system in tea trees, little is known about the structure and cell wall properties of growth zone of the root. To characterize growth zone of tea roots in terms of tissue structure and cell wall extensibility, the development of endodermis and exodermis and the quantitative determination of cell wall extensibility along root axis were investigated in the apical part of white roots growing in hydroponics. This is the first report of tissue structure and mechanical properties of cell walls in the growing adventitious roots of tea. Endodermis with Casparian bands (stained with berberine) and exodermis with suberin lamellae (stained with Fluorol yellow) were found in the apical part of the root, at 4 mm behind the tip of the main adventitious roots and at 1 mm behind the tip of lateral roots, respectively. The gradient of cell wall extensibility along the root axis of lateral roots was measured every one-mm intervals, and those of hydroponics-grown roots (HR) and compared with those of soil-grown roots (SR). The extensible cell walls are located at an extremely apical part of the root compared with herbaceous plants such as pea plants. The decline of the extensibility towards the base of SR was steeper than that of HR. It has been suggested that the apical location of extensible cell walls or early ageing of longitudinal cell files was the basic traits of tea trees, and that the ageing process was accelerated in the soil compared to that in hydroponics solution. Correlation between the endodermis and exodermis development and the ageing of cell walls or loss of cell wall extensibility is discussed in terms of viscoelastic parameters.