The distribution and deposition pattern of lignin in alfalfa stem tissue

This study tested the occurrence, distribution, and deposition of lignin in stem tissue of the alfalfa variety ‘Gannong No.5’ using the histochemical staining method. The research results showed that the distribution of lignin in the stem of alfalfa is closely related to the vascular tissue in different development stages. At the apex of the stem tissue, lignin is only found in the primary xylem. Then, lignin begins to deposit in the primary phloem fibers, secondary xylem, and pith rays, with the internode moving down.Both guaiacyl (G) lignin and syringyl(S) lignin are present in alfalfa stems, and S lignin occurs later than G lignin. Between the vascular bundles, the deposition pattern of the two kinds of lignin is “vascular bundle near the collenchyma→vascular bundles between the collenchyma”. However, the deposition of lignin shows the heterogeneity within the vascular bundle. The deposition pattern of G lignin is “primary xylem vessels→primary phloem fibers→secondary xylem and pith rays”, while the deposition pattern of S lignin is “primary phloem fibers→secondary xylem and pith rays→pith parenchyma”. It is suggested that the special deposition pattern of lignin in the stem of alfalfa is a strategy that is probably appropriate to the growing environment in the north temperate zone.