Yamagishi Shohei, Kojima Miho, Kuroda Katsushi, Abe Hisashi, Sano Yuzou
Forestry and Forest Products Research Institute, Tsukuba, 305-8687, Japan.
Graduate School of Agriculture, Hokkaido University, Sapporo, 060-8589, Japan.
Ann Bot. 2024 Oct 30;134(4):561-576. doi: 10.1093/aob/mcae113.
Pit pairs and their filter-like partition, i.e. pit membranes, play important roles as water pathways, barriers and regulators in the water-conducting system of angiosperms. In Fraxinus species, the intervessel and vessel-parenchyma pit membranes in sapwood are normally encrusted during winter. Although these encrustations inevitably influence the performance of pits, their properties and functions remain unclear. This study aimed to reveal the morphological and chemical characteristics of encrustations in F. mandshurica in order to deepen understanding of the seasonal encrustation of pit membranes.
Seasonal and positional variations in the presence and morphology of encrustations were examined by field-emission scanning electron microscopy (FE-SEM). Cryo-FE-SEM for freeze-fixed greenwood samples was conducted to clarify whether encrustations were present in living trees. Chemical components were examined by histochemical staining using light and electron microscopy, immunofluorescence labelling and ultraviolet microspectroscopy.
Encrustations began to deposit in autumn before leaf senescence and disappeared in spring before bud flushing. They infiltrated within the pit membranes, which suggested that they severely limit the permeation of pits. The encrustations differed in morphology among positions: they entirely filled the pit chambers in latewood, while they covered the pit membranes in earlywood. The encrustations were similarly observed in the samples that were freeze-fixed immediately after collection, indicating that they are present in living trees. The encrustations contained polysaccharides, including xyloglucan and homogalacturonan, and phenolic compounds, possibly including flavonoids and coumarins. These chemical components were also detected in droplets found in the latewood vessels with the encrustations, suggesting that the materials constituting encrustations were supplied through the vessel lumens.
Encrustations undoubtedly cover the pit membranes in living F. mandshurica trees in winter and their morphology and chemical composition indicate that they are impermeable, have positional differences in function and are characterized by elaborate deposition/removal processes.
纹孔对及其类似滤网的分隔结构，即纹孔膜，在被子植物的水分传导系统中作为水分通道、屏障和调节器发挥着重要作用。在白蜡树属物种中，边材中的导管间纹孔膜和导管 - 薄壁细胞纹孔膜在冬季通常会被结壳。尽管这些结壳不可避免地会影响纹孔的性能，但其特性和功能仍不清楚。本研究旨在揭示水曲柳中结壳的形态和化学特征，以加深对纹孔膜季节性结壳的理解。
通过场发射扫描电子显微镜（FE - SEM）研究结壳存在情况和形态的季节及位置变化。对冷冻固定的绿材样本进行低温FE - SEM观察，以明确活树中是否存在结壳。使用光学和电子显微镜组织化学染色、免疫荧光标记及紫外显微光谱法检测化学成分。
结壳在秋季叶片衰老前开始沉积，并在春季芽萌发前消失。它们渗透到纹孔膜内，这表明它们严重限制了纹孔的通透性。结壳在不同位置的形态有所不同：它们完全填充了晚材中的纹孔腔，而在早材中则覆盖在纹孔膜上。在采集后立即冷冻固定的样本中也同样观察到了结壳，表明它们存在于活树中。结壳含有多糖，包括木葡聚糖和同型半乳糖醛酸，以及酚类化合物，可能包括黄酮类和香豆素类。在有结壳的晚材导管中发现的液滴中也检测到了这些化学成分，这表明构成结壳的物质是通过导管腔供应的。
结壳无疑在冬季覆盖了活的水曲柳树中的纹孔膜，其形态和化学成分表明它们是不可渗透的，在功能上存在位置差异，并且具有精细的沉积/去除过程。
