Wood is the product of cambial activity in trees, and the seasonal activity style of cambium directly influences wood biomass production, structures and properties. The seasonal changes in the ultrastructure of the vascular cambium activity of Populus tomentosa Carr. planted in Beijing area were examined in shoot tissues collected during 15 months by means of transmission electron microscopy. Before xylem mother cells reactivated completely, the dividing fusiform cells in cambium and new phloem cells had appeared at the same time. The initiation of cambial activity may be related to the bud sprouting and the young leaf growth in shoots. More details about the ultrastructural changes of cambial cells at the onset of cambial activity have been gained. When the large vacuole in active cambial cells divided into smaller ones during the dormant phase, proteinaceous material that disappeared in active cambial cells refilled many of these small vacuoles. In addition, lipid droplets and starch granules had the same cycles as proteinaceous material. The plasmalemma invaginations of fusiform cells were observed not only in active phase but also in dormancy. The endomembrane system consisting of nuclear membrane, endoplasmic reticulum (ER), dictyosomes and their secretory vesicles, changed in form and distribution at different phases during a cycle and performed important roles at the onset of active cambium and during the wall formation process of secondary xylem cells. The tangential walls remained relatively thin throughout the year but the radial walls thickened markedly when the cambium was dormant. During the transition from dormancy to activity, a partial autolysis occurred in the radial walls of the cambial cells, especially at the cell wall junctions. A notable feature of the cells at the onset of cambial activity was the thinning of the radial walls.
毛白杨维管形成层带细胞超微结构的季节性变化
殷亚方1 姜笑梅1* 崔克明2
（1. 中国林业科学研究院木材工业研究所，北京100091；2. 北京大学生命科学学院，北京100871）
摘要： 木材（次生木质部）是树木形成层细胞分化的产物，形成层的活动方式不仅影响木材的产量，而且影响木材的结构和性质。利用透射电子显微镜观察了生长在北京地区的毛白杨（Populus tomentosa Carr.）枝条形成层带细胞一个完整活动周期的超微结构变化。在木质部母细胞完全恢复活动之前，形成层纺锤状原始细胞的分裂和韧皮部细胞的分化已经开始。枝条上芽的展开和幼叶的生长可能决定了形成层带细胞的这种活动方式。透射电镜观察更清楚地揭示了树木形成层细胞在活动初期的分化特点。活动期形成层细胞中的大液泡在进入休眠期后逐渐分成许多小液泡分散在细胞质中。随着液泡融合逐渐消失的深色蛋白类物质又重新充满了大部分液泡。油滴和淀粉颗粒的年变化情况同液泡中的蛋白类物质基本相似。无论在活动期还是休眠期，形成层纺锤形细胞的质膜上都发现有许多可能与物质运输有关的小泡状内折。由核膜、内质网和高尔基体及其分泌小泡组成的细胞内膜系统，在形成层活动周期的不同阶段，其形态和分布明显不同，尤其在形成层细胞的恢复活动及其衍生木质部细胞次生壁的沉积过程中发挥着重要作用。整个活动周期中，形成层纺锤形细胞的径向壁都比弦向壁厚，处在休眠期的形成层带细胞，其径向壁与弦向壁的差别则更明显。形成层恢复活动时，径向壁上特别是与弦向壁相连的角隅处出现部分自溶现象。细胞壁特别是径向壁的变薄是形成层细胞恢复活动的重要特征。
关键词： 维管形成层；超微结构；内膜系统；细胞壁；季节性变化；毛白杨
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