Lectin is the major protein in the phloem tissue of S. japonica. By immunohistochemistry using anti-seed lectin antibody it was demonstrated that the lectin was localized in the ray and the axial parenchyma. Neither lectin nor other cross-reactive materials were observed in the cambium, sieve tubes and companion cells. The distribution and localization changed in relation to tissue development. Lectin content in the bark changed during the year, the average in summer being about 50% of that in winter. The distribution of lectin in the bark in winter was similar from the innermost (youngest) to the outermost (oldest) region. In contrast, in summer the innermost region hardly contained any lectin, and the outermost region contained less lectin than the middle. Lectin localization in tissues and cells differed also depending on tissue age. In new tissue, produced in the current year, lectip was absent in summer, was located in the cytoplasmic layer between cell wall and vacuole in autumn, and sequestered in the vacuoles in winter. On the other hand, lectin in old tissue (formed in the previous year) was located throughout the year mainly within the vacuoles, with only very small contents in the cytoplasmic layer in autumn. Within the outermost (oldest) region, in which the lectin content was low in summer, the cells which bordered the outer bark never contained any lectin in summer. The intracellular localization in autumn in new tissue, determined by immunogold electron microscopy, was in the lumen of the endoplasmic reticulum and vesicles, with gold particles hardly present in the cytoplasm. From these findings we conclude that lectin is synthesized on the endoplasmic reticulum and most vigorously in the new tissue in autumn, and that it is mainly consumed in the outermost bark regions, where dilatation occurs and-or where cork cambium is differentiated.
