Abstract:

Phloem is the woody tissue for the storage and long-distance transport of organic matter in vascular bundles. To reveal the process of secondary phloem development and differentiation in Pinus massoniana, the structure of the secondary phloem and the distribution of cell inclusions were observed by histochemical staining, spontaneous fluorescence of phenolic substances and cell segregation. Based on tissue development and differentiation characteristics of P. massoniana secondary phloem, the secondary phloem development was divided into seven stages: the functional phloem stage; the sieve cell lignification stage; the phloem ray bending stage; the parenchyma cell dedifferentiation and division stage; the dedifferentiated parenchyma cell population formation stage; the periderm alteration stage, and the rhytidome stage. An analysis of cell morphology and inclusion distribution characteristics showed that the sieve cells were deformed during lignification, the quantities of parenchyma and resin ducts increased with development and the crystal content in cells, as well as the levels of sugars and lipids in phloem parenchyma cells, increased with development. The results indicate that the P. massoniana phloem first lost longitudinal transport function and then increased its secretory, storage and mechanical functions. Ultimately, the parenchyma differentiated into the cortex and periderm, and the tissue outside the new periderm lignified to form the rhytidome, which fully developed into the protective tissue of the stem.

Key words: P. massoniana, Phloem, Periderm, Lignification, Inclusion