Development of distinct cell wall layers both in primary and secondary phloem fibers of hemp (Cannabis sativa L.)

Formation of thickened cell wall allows plant fibers to obtain the strength necessary to the realization of their function as mechanical tissue. It is due to such cell wall that fibers of textile crops, like flax, hemp, and ramie, acquire characteristics that make possible to use these fibers in textile and technical applications. In the hemp stem, primary and secondary phloem fibers originating from the different type of meristems are formed. Analysis of ultrastructure coupled with immunolabelling demonstrated distinct layers within thickened cell wall in the fiber of both types: the outer layer is built as typical secondary cell wall of xylan-type, while the major portion was identified as the layers of tertiary (gelatinous or G-layer) cell wall. A newly deposited layer of the tertiary cell wall (Gn) is transformed into mature G-layer by the post-synthetic modification. The general design of cell wall in primary and secondary phloem fibers is similar, but with xylan layer being considerably thicker in secondary fibers than in primary ones. The formation of the tertiary cell wall in primary and secondary hemp fibers was associated with the synthesis of pectin component - rhamnogalacturonan I together with beta-(1,4)-D-galactan, which has been detected and characterized both in the buffer-extractable fraction and among the strongly retained within cell wall polysaccharides. Comparison of the obtained results with data on the flax fiber cell wall development permitted to find similarities, as well as some differences of G-fiber cell wall organization in different plant species.