Differentiation in a secondary xylem vessel element of Populus deltoides using ultrastructural analysis

Key words: 
• botany /
• Populus deltoides /
• vessel /
• ultrastructure /
• critical stage /
• programmed cell death

Abstract: Using electronic microscopy，ultrastructural changes were observed during differentiation in a secondary xylem vessel element （VE） of Populus deltoides. Results showed that morphological development of VE differentiation was successively divided into three stages. First was prolongation of the primary cell wall （the initial stage），where the VE was highly vacuolated and the protoplasm was distributed along the cell wall. Second was establishment of the secondary cell wall （the pivotal stage） where substances accumulated before the tonoplast broke，and VE organelle structure was distinct. Golgi bodies and vesicles，which were associated closely with synthesis and transportation of secondary cell wall substances，were also abundant. After the tonoplast broke，these substances accumulated faster. Simultaneously，the protoplasm disaggregated，and the agglomerate chromatin was distributed over the margin of the nucleus showing typical characteristics of programmed cell death（PCD）. During secondary cell wall formation，no cell wall substances accumulated between terminal cell walls of neighboring VEs. In addition，terminal cell wall substances were disaggregated in the post secondary cell wall formation. Later，when the remnant terminal cell wall broke the third stage，perforation，occurred. Thus，for these successive stages of VE differentiation，the critical stage，when differentiation was not reversible，was at the start of secondary cell wall formation with succeeding VE differentiation similar to a typical PCD process. ［Ch，2 fig. 13 ref.］