Ultrastructural Changes of Secondary Phloem Cells in the Cambium Annual Activities of Taxodium Ascendens
Ultrastructural Changes of Secondary Phloem Cells in the Cambium Annual Activities of Taxodium Ascendens
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发表时间：
2023
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通讯作者：
Han Lin
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作者：
Youming Xu;Huiling Wang;Jieyun Tao;Han Lin
Tree bark is an important part of tree trunk and it derives from the activities of cambium. More studies were focused on the xylem cells development of trees at the ultrastructural level and not on phloem cells. To explore the ultrastructural changes of all kinds of secondary phloem cells in the cambium annual activities of Taxodium ascendens, and to study the mechanism of its wood formation. The samples of Taxodium ascendens were collected in different growth periods. The ultrastructural changes of all kinds of cells in the secondary phloem of Taxodium ascendens and the process of wood formation were observed by transmission electron microscope. The Results show as follows: .The sieve cells began to differentiate in late March and their cell walls thickened continuously during the periods of April to June. At the end of August, part of the sieve cells entered programmed death, with nucleus pyknosis, chromatin agglomerated, nucleoli disappeared, and the nucleoplasm decreased. At the same time, the size of the nucleus became smaller, and when the sieve cells stopped functioning, the protoplasts died. The whole cell was squeezed and deformed. Phloem parenchyma cells began to differentiate in mid-April. In the early stage of cambium activity (from mid-April to early May), the resin in the parenchyma cells was distributed in the form of large droplets. At the peak of cambium activity (from the end of May to the end of June), the resin was dispersed in the cells as fog, and the resin droplets were evenly distributed along the cell wall during the dormant period. In mid-April, newly differentiated young ray parenchyma cells were observed in the phloem near the cambium. From mid-late April to early June, the young phloem ray cells developed to mature and remained in this state until the middle of November. After that, the cells went into programmed death, and the cells contained only a large number of oil droplets and empty vesicles. The development and maturation of phloem fibers occur in all periods. Phloem fibers can be watched in any period of phloem development. The proportion of mature and developing phloem fibers differs slightly in each period. .The active period of cambium cells lasted from the beginning of March to the end of November. Newly differentiated sieve cells could be observed in March, but newly differentiated phloem parenchyma cells and ray parenchyma cells were not observed until the middle of April. Therefore, the sieve cells differentiated earliest, while the phloem parenchyma cells and ray parenchyma cells were about half a month later than the sieve cells. During the period from the beginning of phloem differentiation to the end of cambium activity, the division and differentiation of secondary phloem cells can be observed continuously.
