Primary phloem fibres (PPFs) have higher fibre quality and are economically more important for the textile sector than secondary phloem fibres. Both the chemical composition and mechanical structure of the secondary cell wall mainly influence the quality of bast fibres. We investigated the thickening of the galactan-enriched (Gn) layer and its modification process into a gelatinous (G)-layer, which is the largest portion of the secondary cell wall, during the development of the PPF in Cannabis sativa. Stem segments of hemp collected at 17, 29, 52 and 62 days after sowing were comparatively examined using light microscopy, scanning electron microscopy and transmission electron microscopy. The initial cells of PPF started the proliferation and differentiation at 17 days, but the secondary cell wall thickening had already commenced before the 29 days. Both the G- and Gn-layer were rapidly added onto the S-layer of PPFs; thus, the secondary cell wall thickness increased approximately 2-fold at 52 days (from the 29-day mark), and 8-fold at 62 days. The cortical microtubule arrays appeared adjacent to the plasma membrane of PPF cells related to the cellulose synthesis. Additionally, cross-sectioned microfibrils were observed on Gn-layer as the cluster of tiny spots. At 62 days, the specific stratification structure consisting of several lamellae occurred on the G-layer of the secondary cell wall. The secondary cell wall thickened remarkably at 52 days through 62 days so that the mature secondary cell wall consisted of three distinctive layers, the S-, G- and Gn-layer. Cortical microtubule arrays frequently appeared adjacent to the plasma membrane together with cellulose microfibrils on secondary cell wall. The G-layer of PPF at 62 days exhibited the characteristic stratification structure, which demonstrates the modification of the Gn-layer into the G-layer.

中文翻译：

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大麻初级韧皮部纤维发育过程中半乳聚糖富集层的增厚和改性

初级韧皮部纤维 (PPF) 比次级韧皮部纤维具有更高的纤维质量，并且对纺织部门而言在经济上更重要。次生细胞壁的化学成分和力学结构主要影响韧皮纤维的质量。我们研究了在大麻中 PPF 的开发过程中，富含半乳聚糖 (Gn) 层的增厚及其向凝胶状 (G) 层的改性过程，这是次生细胞壁的最大部分。采用光学显微镜、扫描电镜和透射电镜对播种后17、29、52和62天采集的大麻茎段进行对比检查。PPF的初始细胞在第17天开始增殖和分化，但在第29天之前，次生细胞壁已经开始增厚。G 层和 Gn 层都迅速添加到 PPF 的 S 层上；因此，次生细胞壁厚度在 52 天（从 29 天标记）增加了大约 2 倍，在 62 天增加了 8 倍。皮质微管阵列出现在与纤维素合成相关的PPF细胞质膜附近。此外，在 Gn 层上观察到横截面的微纤维作为小点簇。62天时，次生细胞壁的G层出现由数个薄片组成的特定分层结构。次生细胞壁在第 52 天到第 62 天明显增厚，因此成熟的次生细胞壁由三个不同的层组成，即 S-、G-和 Gn-层。皮质微管阵列经常与次生细胞壁上的纤维素微纤维一起出现在质膜附近。PPF的G层在62天时呈现出特征性的分层结构，这表明Gn层向G层的改性。