[Senescence and apoptosis of protoplasts from flax fibers: an ultrastructural analysis].

Plant fibers represent specialized cells that perform a mechanical function. Their development includes the following phases, typical for the most plant cells: anlage, extension growth, specialization, senescence, and apoptosis. Ultrastructural analysis of these cells has been carried out at the late phases of their development (senescence and apoptosis) using flax phloem fibers, a classical object for the analysis of sclerenchyma fiber formation. The results of the performed analysis show that flax fiber protoplasts remain viable until the end ofa vegetation season. The ultrastructural analysis of flax phloem fibers has not revealed any typical apoptosis manifestations. Gradual degradation of the cytoplasm starts during the active thickening of a secondary cell wall and manifests via the intensification of autolytic processes, causing a partial loss of cell content. The final stage represents the breaking of tonoplast integrity. The obtained data allow us to suppose that the apoptosis of flax fibers occurs during their senescence, and its program is similar to the cell death program realized in the xylem fibers of woody plants.