VISUALIZATION OF CAVITATED VESSELS IN WINTER AND REFILLED VESSELS IN SPRING IN DIFFUSE-POROUS TREES BY CRYOSCANNING ELECTRON-MICROSCOPY

Xylem cavitation in winter and recovery from cavitation in the spring were visualized in two species of diffuse-porous trees, Betula platyph ylla var. japonica Hara and Salix sachalinensis Fr. Schm., by cryo-sca nning electron microscopy after freeze-fixation of living twigs. Water in the vessel lumina of the outer three annual rings of twigs of B. p latyphylla var. japonica and of S. sachalinensis gradually disappeared during the period from January to March, an indication that cavitatio n occurs gradually in these species during the winter. In April, when no leaves had yet expanded, the lumina of most of the vessels of both species were filled with water. Many vessel lumina in twigs of both sp ecies were filled with water during the period from the subsequent gro wth season to the beginning of the next winter. These observations ind icate that recovery in spring occurs before the onset of transpiration and that water transport through twigs occurs during the subsequent g rowing season. We found, moreover, that vessels repeat an annual cycle of winter cavitation and spring recovery from cavitation for several years until irreversible cavitation occurs.