Occurrence of unidentified bodies that resemble “plastid initials” in poplar cells after breaking of dormancy in midwinter

Summary

Electron-microscopic studies of plastids in cortical cells of poplar (Populus euramericana cv. gelrica) were carried out to examine whether any structural changes were initiated after breaking of dormancy in midwinter under non-growing conditions. After the breaking of dormancy, ultrastructural changes became evident and the profiles of plastids became heterogeneous. Organelles resembling the “plastid initials” proposed by Mühlenthaler and Frey-Wyssling in 1965 were frequently observed concomitant with changes in the plastid envelope. The formation of “plastid initials” appeared to be initiated by the formation of septa in pre-existing plastids. After this stage, narrow connections appeared between the “initials” and the parent plastids. Approximately 50 days after the breaking of dormancy in late March, further heterogeneity in the profiles of plastids was observed. At this stage, young plastids (plastids without starch granules) were frequently observed and the formation of “plastid initials” was hardly ever observed. These observations suggest that the “plastid initials” may be present for only a limited period in the cortical cells of the poplar and may be the precursors of the proplastids. Similar ultrastructural profiles were found in cortical cells of mulberry and in leaf buds of apple trees, suggesting that such changes in the ultrastructure of plastids are a general feature of perennials.