柑橘樹における stem pitting の形成に関する研究 (第2報)

In Japan, the symptom of stem pitting of citrus has been found on trees suffered from dwarf virus disease of Hassaku (Citrus hassaku HORT. EX. TANAKA), Buntan (C. grandis OSBECK), Washington navel orange (C. sinensis OSBECK) and etc. In the present studies, stem pitting of one and two years old mexican lime (C. aurantifolia SWINGLE) seedlings, inoculated by bud graft infected with Hassaku dwarf virus taken from twenty five or twenty seven years old Hassaku trees suffering from Hassaku dwarf virus disease (at Wakayama prefecture), were observed anatomically and histochemically.<br>Under the investigation of serial transverse sections of stem, hypertrophic and hyperplastic parenchymatous cells as previously noticed, were observed in the xylem and the phloem. These cells formed the disorganized abnormal tissue in the secondary vascular tissue (Fig. 1 and 2).<br>In order to study the developmental change of this abnormal tissue, serial tangential sections of wood were made (Fig. 4 and 7). Examination of such material revealed that the ray initials in cambium reacted to virus initially, and these affected cells failed to differentiate into ray tissue, that is, elementary tissue of the xylem and the phloem. As the secondary thickening growth proceeded, these abnormal cells missed their normal organization, divisioned hyperplastically, and then formed abnormal tissue or lesions as previously described.<br>Furthermore, as the thickness of wood increased, the walls of these abnormal tissue cells were delayed in lignification as compared with surrounding normal tissue cells (Fig. 2). Because of this delaying in lignification of cell wall, pitting was formed on the wood and remain in evidence if the bark was removed. However, in the case of abnormal tissues which composed only of lignified abnormal cells (Fig. 6 and 8), stem pitting were not shown even if the bark was peeled, and such examples were observed frequently in Hassaku trees growing in outdoor condition.<br>Although annual rings in wood of Hassaku tree could not be identified surely, growth rings were recongnized. These rings were distinguished into two kinds. One was consisting mainly of fiber cells of fiberous tissues of vasicentric type (Fig. 5B and C). And the other was consisting mainly of parenchymatous cells or parenchymatous tissues of aliform type (Fig. 5D), diffused type (Fig. 5E), or terminal type (Fig. 5F). And lignified abnormal tissues in the wood of Hassaku trees, were recognized mostly lesions in growth rings of the latter kind.<br>From the above anatomical observations the following hypothesis may put on: the abnormal initial cells in cambium underwent severe division by the time, when the cambium formed parenchymatous growth ring. On the other hand, at such time as the cambium separated fiberous cells, these abnormal initial cells in the cambium ceased their severe hyperplastic divisions. In other words, the formation of stem pitting of citrus trees might be under the control of the physiological situation of cambium.