The changes in cell wall architecture during lignification of bamboo, Phyllostachys aurea Carr

The cell wall architecture of bamboo at various developmental stages of culm growth was investigated by rapid-freezing and deep-etching (RFDE) electron microscopy. The unlignified primary wall (ULP) was characterized by the narrow spacing between the cellulose microfibrils in fibres, but not in parenchyma cells. The unlignified secondary wall (ULS) largely consisted of dense cellulose microfibrils with narrow spacing or "slit-like" pores. However, such pores are difficult to observe in the lignified secondary wall (LS) of fibres. The cell wall architecture of the delignified secondary wall (DLS) in fibres showed porosity similar to that of ULS. Pores in the middle lamella and secondary walls of ULS in fibres were reduced considerably or disappeared immediately after lignification. However, the pores reappeared following delignification. These findings suggest that deposition of lignin in ULS immediately proceeds in the pores during maturation to LS. Based on RFDE electron microscopic images, the pore sizes of primary and secondary fibre walls were significantly smaller in bamboo than in either Eucalyptus or Pinus, suggesting a denser arrangement of cellulose microfibrils in bamboo fibre walls than in either tree species. The author hypothesizes that the narrow spacing between cellulose microfibrils in bamboo fibres may be one of the reasons for the deposition of less lignin in bamboo than in tree species. This is the first report on the three-dimensional architecture of unlignified and lignified walls in bamboo.