Changes In Cell Wall Architecture Of Ray Parenchyma In Populus Tomentosa During The Xylem Differentiation

Ray parenchyma cells in tree xylem not only play an important role in water transport and nutrient storage,but also significantly affect the physical and mechanical properties of wood,as well as wood impregnation modification and other applications.However,ill now the cell wall architecture of ray parenchyma cells has been still unclear because of the long differentiation period of wood rays.Poplar is an important plantation species and the main raw material for wood processing and utilization.Herein,Populus tomentosa was chosen as the research object to explore the anatomical characteristics and cell wall composition changes of ray parenchyma cells in different differentiation stages.Firstly,the differentiation period of ray parenchyma cells was studied through analyzing the radial changes of the nuclei morphology,the number of nuclei and the number of starch grains of ray parenchyma cells from the xylem to the heartwood by the histochemical staining techniques.Subsequently,focusing on different differentiation stages of two types of ray parenchyma cells,five radial positions including xylem（R1）,middle sapwood（R2）,near transition zone（R3）,transition zone（R4）,and heartwood（R5）were chosen.Transmission Electron Microscope（TEM）,Polarizing Microscope,Scanning Electron Microscope（SEM）technology,cell maceration and other techniques were used to study the changes of cell wall ultrastructure,cell wall microstructure.Finally,the cell wall components of two kinds of ray parenchyma cells at five radial positions were analyzed by histochemical staining and Fourier transform infrared spectroscopy imaging system（FTIR imaging）.Through the above research,the differentiation of ray parenchyma cells and the changes of cell wall structure of Populus tomentosa were obtained.It could provide a theoretical basis for understanding the structure and performance of ray parenchyma cells in the future,and ultimately laid a scientific foundation for achieving high-quality cultivation of trees and high-value utilization of wood.The main conclusions of this study are summarized as follows:（1）The ray parenchyma cells of Populus tomentosa can be divided into contact ray parenchyma cells and isolation ray parenchyma cells according to the different differentiation period.Contact ray parenchyma cells completed the secondary wall thickening and lignification at the current-year xylem,while isolation ray parenchyma cells began to lignify from the sapwood（R2）,and lignified significantly in the transition zone（R4）and completed secondary wall thickening and lignification in the heartwood（R5）.Contact ray parenchyma cells died at the 27^thannual ring away from the cambium,but isolated ray parenchyma cells died at the 33^thannual ring away from the cambium.There was no starch grain in the contact ray parenchyma cells,and the content of starch grains in the isolation ray parenchyma cells gradually decreased from the current-year xylem to the transition zone,and completely disappeared in the heartwood.（2）Contact ray parenchyma cells and isolation ray parenchyma cells of Populus tomentosa displayed different changes of cell wall structure during the differentiation.Contrast with the cell wall structure of wood fiber cells,ray parenchyma cells had protective layers on the inner of secondary cell wall.The protective layers are high electronic contrast.Moreover,the secondary wall of ray parenchyma cells is a multilayer structure,over three layers.The middle lamella of isolation ray parenchyma cells is lower electronic contrast than that of contact ray parenchyma cells for each differentiation stage.While,the electronic contrast of middle lamella of isolation ray parenchyma cells became darker in the heartwood（R5）.Moreover,two protective layers rather than single protective layers were present in the isolation ray parenchyma cells in the heartwood（R5）.The quantitative analysis of cell wall thickness indicated that the horizontal wall and tangential wall of contact ray parenchyma cells are both thinner than those of isolation ray parenchyma cells from R1 to R5.And the thickness of the horizontal wall of isolation ray parenchyma cells is higher than that of their tangential wall.（3）The structures and dimensions of pits in contact ray parenchyma cells and isolation ray parenchyma cells of Populus tomentosa during the differentiation were significantly different.Contact ray parenchyma cells had ray pits and ray-vessel pits,and sieve pits can be observed on some tangential walls.On the contrary,isolation ray parenchyma cells only had ray pits,and sieve pits were present on most of tangential walls.Meanwhile,the single pit and slightly bordered pits were present on the horizontal wall and tangential wall of the two kinds of ray parenchyma cells.The density and dimensions of pits in contact ray parenchyma cells and isolation ray parenchyma cells of Populus tomentosa were significantly affected by the horizontal wall,tangential wall and the differentiation stage.The density of pits on the tangential wall was higher than that of the horizontal wall for two types of ray parenchyma cells.Moreover,the density of pits on the tangential wall of isolation ray parenchyma cells was higher than that of contact ray parenchyma cells.Besides,the pit width,the pit membrane thickness and the pit membrane diameter on the horizontal wall of the two types of ray parenchyma cells were higher than those on the tangential wall in R1-R5.And accompanied with the secondary cell thickening,the pit width on the horizontal wall of the contact ray parenchyma cells decreased more significantly than the pit membrane diameter from the current year xylem（R1）to the heartwood（R5）.（4）The changes of wall composition in the differentiation process of contact ray parenchyma cells and isolation ray parenchyma cells were different.The 1660 cm^-1lignin absorption peak and 1315 cm^-1crystalline cellulose absorption peak appeared earlier in the contact ray parenchyma cells.The cell wall of contact ray parenchyma cells deposited more lignin near the transition zone（R3）and transition zone（R4）than hemicellulose and cellulose;The cell wall of isolation ray parenchyma cells deposited more lignin than hemicellulose near the transition zone（R3）to heartwood（R5）,and more lignin than cellulose in the middle of sapwood（R3）and heartwood（R4）.The deposition ratio of lignin to hemicellulose in the cell wall of the contact ray parenchyma cells in the current year xylem（R1）and transition zone（R4）was higher than that of the isolation ray parenchyma cells.The deposition ratio of lignin to cellulose in the contact ray parenchyma cells of the current year xylem（R1）was higher than that in the isolation ray parenchyma cells.