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

在双子叶树木中，木材纤维通常在成熟时或成熟后不久脱水。迄今为止，这种脱水现象的机制尚未被理解。因此，本研究旨在可视化硬木纤维的脱水过程，并为阐明脱水机理提供线索。低温扫描电镜显示，具有微小边缘的小凹坑的木材纤维在成熟时或成熟后几乎同时脱水。相比之下，具有明显边界的较大凹坑的木纤维（典型的纤维管胞和vascentric管胞）在成熟后在其腔内保留水分。场发射扫描电镜显示，前一种类型的纤维之间的坑膜经常穿孔，而后一种类型的纤维之间的坑膜密集堆积。这些发现表明，纤维间孔膜穿孔的存在是纤维细胞在成熟时或成熟后不久脱水的条件。此外，我们研究了存在于木质部组织最外层的气体组成。结果，我们无法重复分析气体成分，因为我们用于采集样品的工具（注射器）中发生了人为气体样品泄漏。然而，木质部组织中CO_2的浓度很可能比大气中高得多。木纤维细胞死亡前发生的主动呼吸作用可能参与了木纤维细胞的脱水过程。

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.