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

在双子叶乔木中，木材纤维通常在成熟时或成熟后不久脱水。到目前为止，这种脱水现象的机制还不清楚。因此，本研究旨在可视化阔叶木纤维的脱水过程，并为阐明其脱水机理提供线索。冷冻扫描电子显微镜显示，木材纤维在成熟时或成熟后不久，几乎同时脱水，纤维上有细小的凹坑。相反，具有明显边缘较大凹陷的木纤维(典型的纤维管胞和血管中心管胞)成熟后在其管腔内保持水分。场发射扫描电子显微镜显示，前一类纤维之间的凹坑膜通常是穿孔的，而后一类纤维之间的凹坑膜通常是密集堆积的。这些发现表明，纤维间孔膜的存在是允许纤维细胞在成熟时或成熟后不久脱水的条件。此外，我们还研究了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，因为气体样本在我们用于样本采集的工具(注射器)中发生了人为泄漏。但木质部组织中的CO_2浓度很可能远高于大气中的CO_2浓度。木材纤维细胞死亡前发生的活跃呼吸可能参与了纤维细胞的脱水。

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.