*Article* **Lignin Distribution on Cell Wall Micro-Morphological Regions of Fibre in Developmental** *Phyllostachys pubescens* **Culms**

**Bo Liu , Lina Tang, Qian Chen, Liming Zhu, Xianwu Zou, Botao Li, Qin Zhou, Yuejin Fu \* and Yun Lu \***

Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing 100091, China; liubo@criwi.org.cn (B.L.); 18211090798@163.com (L.T.); chenqian0610@126.com (Q.C.); izhulm@caf.ac.cn (L.Z.); xwzou@caf.ac.cn (X.Z.); botaoLi@163.com (B.L.); zhouqin567@sina.com (Q.Z.)

**\*** Correspondence: bj-fyj@163.com (Y.F.); y.lu@caf.ac.cn (Y.L.)

**Abstract:** Bamboo is a natural fibre reinforced composite with excellent performance which is, to a certain extent, an alternative to the shortage of wood resources. The heterogeneous distribution and molecular structure of lignin is one of the factors that determines its performance, and it is the key and most difficult component in the basic research into the chemistry of bamboo and in bamboo processing and utilization. In this study, the distribution of lignin components and lignin content in micro-morphological regions were measured in semi-quantitative level by age and radial location by means of visible-light microspectrophotometry (VLMS) coupled with the Wiesner and Maule reaction. There as guaiacyl lignin and syringyl lignin in the cell wall of the fibre. Lignin content of the secondary cell wall and cell corner increased at about 10 days, reached a maximum at 1 year, and then decreased gradually. From 17 days to 4 years, the lignin content of the secondary cell wall in the outer part of bamboo is higher than that in the middle part (which is, in turn, higher than that in the inner part of the bamboo). VLSM results of the micro-morphological regions showed that bamboo lignification developed by aging. Guaiacyl and syringl lignin units can be found in the cell wall of the fibre, parenchyma, and vessel. There was a difference in lignin content among different ages, different radial location, and different micro-morphological regions of the cell wall. The fibre walls were rich in guaiacyl lignin in the early stage of lignification and rich in syringyl units in the later stage of lignification. The guaiacyl and syringyl lignin deposition of bamboo green was earlier than that of the middle part of bamboo culm, and that of the middle part of bamboo culm was earlier than that of bamboo yellow. The single molecule lignin content of the thin layer is higher than that of thick layers, while the primary wall is higher than the secondary cell wall, showing that lignin deposition is consistent with the rules of cell wall formation. The obtained cytological information is helpful to understand the origin of the anisotropic, physical, mechanical, chemical, and machining properties of bamboo.

**Keywords:** *Phyllostachys pubescens*; fibre; lignification; micro-morphological regions of cell wall; syringyl lignin; guaiacyl lignin; visible-light spectrophotometry
