Stem pitting has been found by the authors in such species of citrus as sweet lime (Citrus limettioides TANAKA), Hassaku (C. hassaku HORT ex TANAKA), Banpeiyu (C. grandis OSBECK), Matopeiyu (C. grandis OSBECK), Tanigawabuntan (C. grandis OSBECK), Matobuntan (C. grandis OSBECK), grapefruit (C. paradisi MACF.), Kotokan (C. kotokan HAYATA), Parson Brown (C. sinensis OSBECK), Wasington Navel orange (C. sinensis OSBECK, var. brasilliensis TANAKA), Funadoko (C. funadoko HORT. ex Y. TANAKA), Kikudaidai (C. canaliculata HORT. ex Y. TANAKA), and Yuzu (C. junos SIEB. ex TANAKA). In these species, stem pititng of Hassaku trees has been known as one of the symptoms of Hassaku dwarf disease, and it is closely associated with stuntiog of tree in the fields. But, stem pitting of other species has not so awared, because of the less cultivation of these species in Japan.
To see whether a certain factor involved in stem pitting initiation of citrus can be transmitted by tissue graft to healthy trees of other citrus or not, the following experiments of transmission of stem pitting were carried out. 1) Budwood from 11 species of citrus (Sweet lime, Banpeiyu, grapefruit, Hassaku, Kotokan and Washington Navel orange) which showed stem pitting in the fields, were inoculated to seedlings of Mexican lime (Citrus aurantifolia SWINGLE) by means of tissue graft. 2) Budwood from Hassaku tree affected by Hassaku dwarf disease, was inoculated to seedlings of sweet lime. 3) Budwood from Hassaku tree affected by Hassaku dwarf disease, was inoculated to two-year old Mediterranean sweet orange trees (Citrus sinensis OSBECK) grafted on Kabusu daidai rootstock (Citrus aurantium LINN.).
The results show that within one or two years after inoculation, stem pitting was formed in Mexican lime (Fig. 3, B, C) and sweet lime seedlings (Fig. 4, B), however, no stem pitting was observed in the wood surface of sweet orange trees. But, in the case of sweet orange trees, abnormal parenchymatous tissue was recongnized in the wood differentiated after inoculations (Fig. 5, B. the arrow).

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It was found by the authors10) that if citrus trees were cultivated under the alternate high temperature (day: 25-30°C and night: 15-20°C) during winter season, the differentiation of fibrous tissue in wood was taken place even in winter season.
Two year old Hassaku twigs (Citrus hassaku) which had stem pitting on wood were grafted on sweet lime (Citrus limeettioides) stocks. Ten months later, they were used as materials in the experiments. That is, after dividing them into two sets, one set was cultivated under the alternate high temperature (day: 25-30°C and night: 15-20°C) during winter season (from the beginning of October till the end of April) and another in natural condition of the same winter. As a consequence of the experiments, the formation of new stem pitting was not detected under the alternate high temperature (Fig. 3A₁ and A₂), but it appeared on wood in natural conditions (Fig. 3B₁ and B₂).
This observation provided that the stem pitting phenomenon on wood of citrus tree depends on the cambial condition which has been influenced by thermoperiodicity.

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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.
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).
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.
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.
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.
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.

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日本原生のものを含めたカンキツ属35, キンカン属5種•品種について, その42形質を調査し, 大型計算機を用いた多変量解析を行った. 多変量解析は, Qモードクラスター分析法, 主成分分析法併用クラスター分析法,非線形マッピング法及び数量化理論第3類の計4法について各計算条件を決定した後に行った.
非線形マッピング法では良い結果は得られなかった.また, 数量化理論第3類による分析が最も良い結果を示し, クラスター分析2法も条件を決定したことにより良い結果が得られた.
シトロン, マンダリン及びブンタンは, カンキツ属の中で比較的初期に分化したと考えられる. グレープフルーッとハッサクは, 極めてブンタンに近い形態であった. マンダリン類は, 形態的な幅広い変異性を示した. さらに, ナツダイダイ, キクダイダイ, イヨ, ヒュウガナツ及びカブスは, スイートオレンジに近い傾向を示した. ヒメレモンは, レモンやラフレモンとは遠い位置になった. ベルガモットは, ライムよりもサワーオレンジやシトロン, ブンタンに近かった. シキキツは, 明らかにマンダリンとキンカンの中間に位置した. マメキンカンは他のキンカンとはかなり違った形質であった.

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The rays in the xylem of citrus trees were classified into two types (Fig. 1 and 3). One was uniseriate and the number of cells in the height (the length of axial direction) of tangential section of the ray was usually 1 or 2 through successive stages of the secondary growth. Another was uniseriate in the earlier stage of annual ring development and underwent change to multiseriate in the later stage. Many papillae were found not only on surface of the walls of the ray cells, but also on surface of the walls of parenchymatous cells, fibers and pith cells (Fig. 9, 10 and 13). Such cells appeared to be interconnected with each other morphologically by the papillae. The number of papillae on the outer or inner tangential walls of such cells were more than those on the upper or lower transverse walls. From these results it may be estimated that ray cells have some physiological interconnection in radial direction of secondary wood by the papillae.
As mentioned in the previous paper18), abnormal tissues associated with the stem pitting were formed in ray initials of the cambium instead of differentiation into normal multiseriate ray cells. Furthermore the abnormal tissues were found to be more remarkably in parenchymatous tissues than in fibrous tissues of the wood. From these results, it may be assumed that the formation of abnormal tissues is related with the differentiation of parenchymatous cells during the secondary growth. Concerning the general pattern of variation in annual wood ring, it was clarfied that during secondary growth the cambium produced almostly fibrous cells in summer season and differentiated parenchymatous cells in winter season.
These facts and assumption suggest that the abnormal tissues and parenchymatous tissues are formed in the same season and may be influenced by physiological condition of the cambium. In other words, the stem pitting phenomenon on the wood of citrus trees might be depend on cambial condition in a given season of a year.

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