Processes of protoplast senescence and death in 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: determination, extension growth, specialization, senescence, and death. Ultrastructural analysis of these cells has been carried out at the late phases of their development (senescence and dying off) 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 of a vegetation season. The ultrastructural analysis of flax phloem fibers has not revealed any typical apoptosis features. Gradual degradation of the cytoplasm starts during the active thickening of a secondary cell wall and occurs via the intensification of autolytic processes, causing a partial loss of cell content. The rupture of tonoplast is the final stage. The obtained data allow us to suppose that the protoplast dying off occurs during process of the senescence, and its program is similar to the cell death program realized in the xylem fibers of woody plants.