The Progression of Cavitation in Earlywood Vessels of Fraxinus mandshurica var japonica during Freezing and Thawing.

For an examination of the progression of cavitation in large-diameter earlywood vessels of a deciduous ring-porous tree, potted saplings of Fraxinus mandshurica var japonica Maxim. were frozen and then thawed. The changes in the amount and distribution of water in the lumina of the current year's earlywood vessels during the course of the freezing and thawing were visualized by cryo-scanning electron microscopy. When samples were frozen, most of the current year's earlywood vessels were filled with water. After the subsequent thawing, the percentage of cavitated current-year earlywood vessels gradually increased with time. All of the current year's earlywood vessels were cavitated within 24 h, and only limited amounts of water remained in the lumina of earlywood vessels. Similar cavitation of earlywood vessels was observed after thawing of frozen, excised stem pieces. In contrast, many vessels of the current year's latewood retained water in the lumina during freezing and thawing. These observations indicate that the cavitation of the current year's earlywood vessels is not produced during freezing but progresses during rewarming after freezing in F. mandshurica var japonica.