Visualization of dehydration from hardwood fibers at maturation and an analysis of the mechanisms

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In dicotyledonous trees, wood fibers are generally dehydrated at or soon after their maturation. The mechanism of this dehydration phenomenon has not been understood, to date. Thus, this study was carried out to visualize the dehydration from hardwood fibers as well as to gain clues to the elucidation of the mechanism. Cryo-scanning electron microscopy demonstrated that wood fibers with minutely bordered smaller pits were almost simultaneously dehydrated at or soon after maturation. In contrast, wood fibers with distinctly bordered larger pits (typical fiber-tracheids and vasicentric tracheids) retained water in their lumina after their maturation. Field-emission scanning electron microscopy revealed that pit membranes between the former type of fibers were often perforated, whereas those between the latter type of fibers were densely packed. These findings suggest that the presence of perforated interfiber pit membranes is a condition to allow the dehydration from the fiber cells at or soon after maturation. In addition, we investigated the composition of gases that are present in the outermost layer of xylem tissues. As a result, we failed to analyze the gas composition reproducibly because artificial leakage of gas samples took place in a tool (syringe) that we used for sample collection. However, it was very likely that the concentration of CO_2 was much higher in the xylem tissue than in atmosphere. Active respiration which would occur just before cell death of wood fibers might be involved in the dehydration from the fiber cells.