XVI International Botanical Congess

Ultrastructure of the shoot and root apical meristem cells, differentiating and mature mesophyll cells of Pinus sylvestris was investigated in annual cycle. Throughout plastid life cycle plastid divisions were observed in active and dormant apical meristem, during mesophyll cell expansion and in mature mesophyll cells at the beginning of winter dormancy release. Plastids divide by binary fission. The initiation of the replication process is first seen as a centripetal invagination. Then by successive narrowing of the central constriction, the dumbbell-shaped plastids are separated to the two daughter plastids. The plastid-dividing rings (PDs) were clearly visible at the final stages of plastid division in the isthmus region of constricted undifferentiated plastids in shoot and root apical meristems. PDs were located outside the inner membrane of plastid envelope. Cells with PDs are much less frequently encountered in active meristem than in dormant. Apparently the final stages of plastid division in active root and shoot meristems are extremely rapid or/and PDs represent an arrested stage in plastid division.