Ultrastructural Changes of Secondary Phloem Cells in the Cambium Annual Activities of Taxodium Ascendens
Abstract
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 indifferent 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.
Keywords
Download Options
Introduction
The secondary phloem derived from the vascular cambium of trees is responsible for the synthesis and storage of organic matter and the transport and distribution of photosynthates (Esau K,1969). At present, there have been a large number of studies on the phloem development of hardwood phloem, with a comprehensive summary of all kinds of cells in the phloem of hardwood (Derry et al. 1967; Zhao 2012), while most of the studies on softwood are focused on xylem (Rossi et al. 2016; Pattathil et al. 2016; Zhang et al. 2018; Zheng et al. 2022) and cambium (Rossi et al. 2006; Prislan et al. 2016; Myśkow et al. 2019), there are few systematic reports on the structure and development of all kinds of cells in the phloem of softwood.. The phloem of softwood is usually composed of axial sieve cells, phloem fibers, phloem parenchyma cells, and radial phloem ray parenchyma cells (Abbe et al. 1939). Indifferent developmental stages, the morphology, contents, and composition of all kinds of cells are different (Gričar et al. 2017). For more than half a century, scholars have carried out a large number of phloem development studies on various tree species and combined them with climatic factors to explore (Gričar et al. 2007; Luis et al. 2007; 2011; Dié et al. 2012; Salmon et al. 2019). However, most studies focus on the seasonal growth pattern and growth of phloem (Cardoso et al. 2019; Kopanina et al. 2022; Ohse et al. 2022). The studies on the seasonal changes of various cell morphology of softwood phloem are mainly focused on the microscopic layer (Mullendore 2010; Prislan et al. 2013; Baba et al. 2019). At present, the ultrastructural layer studies on phloem are mainly focused on sieve cells (Knoblauch et al. 2018; Prislan et al. 2018), the study of other phloem cells is not reported comprehensively. The resolution of modern electron microscopy has gone deep into the most basic structural units of wood cells (Singh et al. 2001; Fromm et al. 2003; Chukhchin et al. 2020). Using electronic microscope to explore the process of wood formation will be the research trend in the future. Taxodium ascendens is a successful fast-growing timber tree species introduced and cultivated in the Yangtze River in China. In this paper, the ultrastructural changes of sieve cells, phloem parenchyma cells, phloem ray parenchyma cells, and phloem fibers were observed by transmission electron microscope, to obtain more information about the seasonal activity of cambium and secondary phloem changes in gymnosperms. The purpose of this paper is to enrich the biological knowledge of the growth and development of coniferous wood, combine the formation of phloem and xylem, and connect with the overall growth of trees. This not only helps to consolidate the theory of cambium development but also helps to provide a basis for practical problems such as forest management measures and regulation of wood and partial bark growth. It also provides a scientific basis for the cultivation of Taxodium ascendens plantations, wood material improvement, and forest resource utilization, and contributes to the efficient utilization of bark resources.
Conclusion
1) The newly differentiated young sieve cells were observed in late March. The cell walls of the newly differentiated young sieve cells were thickening during the developmental period (April to June), and S plastids could be observed in June. Some sieve cells went into programmed death at the end of August. In the process of sieve cell programming, P plastids condensed, and the nuclear changes were very obvious. The nucleus in the sieve cells showed pyknotic degeneration, chromatin aggregates, nucleoli disappeared, and nucleoplasm decreased. At the same time, along with the gradual reduction of nuclear volume, the sieve cells reach their functional stage when they stop functioning and become phloem sieve cells without dredging function. The protoplasts also died, and finally, the whole cell was squeezed and deformed. 2) The phloem parenchyma cells of Taxodium ascendens belong to secretory cells, which contain a lot of resin. Phloem parenchyma cells began to divide and differentiate in mid-April. At the initial stage of cambium activity, that is, from mid-April to early May, the resin was distributed in large droplets, and at the peak of cambium activity (from the end of May to the end of June). The resin was highly dispersed in the cell, and then the resin droplets gathered. During the dormant period, resin droplets were evenly distributed along the inner wall of the cells. 3) 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 entered programmed death, and most of the ray parenchyma cells died in dormancy in December, and there were only a large number of oil droplets and empty vesicles in the cells. 4) The development and maturation of phloem fibers occur in all periods. Phloem fibers can be seen in any period of phloem development. The proportion of mature and developing phloem fibers differs slightly in each period. The development of phloem fiber can be divided into three periods: the transition period between nonfunctional sieve cells and phloem fiber, the phloem fiber development period, and the phloem fiber maturity period. In the nonfunctional sieve cell-phloem fiber transition period, the sieve cell is extruded and the intercellular layer is thickened; at the phloem fiber development stage, the secondary wallis greatly thickened; at the phloem fiber mature stage, the mature phloem fiber cell lumen is very small. AUTHOR CONTRIBUTIONS Conceptualization, Y. X; Methodology, Validation, Formal analysis, Y. X and H.W; Investigation, H.W., and J.T.; resources, H.L.; data curation,Y.X. and H L; writing—original draft preparation, H.W. and Y.X; writing—review and editing, supervision and funding acquisition, Y.X.; project administration and visualization, Y.X and H L.; All authors have read and agreed to the published version of the manuscript.
FUNDING This research was funded by NSFC (31971584, 31570551).
CONFLICTS OF INTEREST The authors declare no conflict of interest.
