Changes in Parenchyma Cells of Poplar Xylem during Transition from Growing to Wintering Stages

Ultrastructural changes in xylem parenchyma cells during transitionfrom growing to wintering stages were investigated. Changesin the fine structures of parenchyma cells in differentiatedxylem started in mid-August. At this time, dictyosomes werefrequently found and vesicles were abundant in the cells inwhich active formation of cellular structures took place. Thecytosol was filled with polysomes. In mid-September, the cytoplasmof the parenchyma cells gradually began to fill up with microbodies,and the endoplasmic reticulum cisternae had already startedto decrease. Dictyosomes and vesicles were still abundant atthis stage and microbody-associated vesicles could be seen,indicating participation of the vesicles in the formation ofmicrobodies at this stage. A direct structural connection withrespect to the biogenesis of the microbodies remained unclear.In the early stage, the microbodies showed a dense center surroundedby a less dense outer surface, and in the latter stage in November,the outer surface exhibited an increased accumulation of osmiophilicmaterials. There were few further changes in the overall patternof the fine structures until mid-winter. (Received July 2, 1987; Accepted November 30, 1987)