Dilute acid pretreatment differentially affects the compositional and architectural features of Pinus bungeana Zucc. compression and opposite wood tracheid walls

Compression wood represents a unique challenge for biochemical processing of sustainable biomass to produce biofuels and bioproducts on account of its highly lignified tracheids relative to opposite wood. In the current work, differentiating response of Pinus bungeana Zucc. compression wood and opposite wood tracheid walls to dilute acid pretreatment was elucidated by combining chemical and microscopic approaches. Dilute acid pretreatment released greater amount of hemicelluloses from opposite wood (91.0%) than from compression wood (31.2%). The dissolution of hemicelluloses was not uniform and varied across the tracheid wall. In addition, for both compression wood and opposite wood the heterogeneity in lignin distribution was enhanced, probably attributed to the lignin migration and relocalization. The ultrastructural arrangement of the cell wall was also altered with the pretreated opposite wood tracheid wall exposing more apparent cellulose microfibril bundles and less abundant globular structures compared to compression wood. The dilute acid pretreatment demonstrated that the greater recalcitrance of compression wood probably originated from the higher lignin content and the resulting more compact structure of the cell wall.