Lignification in poplar tension wood lignified cell wall layers

摘要

The lignification process in poplar tension wood lignified cell wall layers, specifically the S₁ and S₂ layers and the compound middle lamella (CML), was analysed using ultraviolet (UV) and transmission electron microscopy (TEM). Variations in the thickness of the gelatinous layer (G-layer) were also measured to clarify whether the lignified cell wall layers had completed their lignification before the deposition of G-layers, or, on the contrary, if lignification of these layers was still active during G-layer formation. Observations using UV microscopy and TEM indicated that both UV absorbance and the degree of potassium permanganate staining increased in the CML and S₁ and S₂ layers during G-layer formation, suggesting that the lignification of these lignified layers is still in progress during G-layer formation. In the context of the cell-autonomous monolignol synthesis hypothesis, our observations suggest that monolignols must go through the developing G-layer during the lignification of CML and the S₁ and S₂ layers. The alternative hypothesis of external synthesis (in the rays) does not require that monolignols go through the G-layer before being deposited in the CML, or the S₁ and S₂ layers. Interestingly, the previous observation of lignin in the poplar G-layer was not confirmed with the microscopy techniques used in the present study.