Changes Of Structure And Performance Of Cell Walls During The Transformation From Sapwood To Heartwood Of Cunninghamia Lanceolata

Transformation from sapwood to heartwood is a unique biological process in trees and anessential part of wood secondary growth after cambial initials are differentiated to xylem cells.Structural and property changes during heartwood formation were investigated on Chinese fir(Cunninghamia lanceolata) at cellular level by means of light microscopy, electron microscopy,X-ray diffraction, imaging FT-IR spectroscopy, UV-vis microspectrophotometry, dynamicmechanical analysis and dynamical vapor sorption, in order to elucidate the transformationfrom sapwood to heartwood and promote the utilization of heartwood and provide a new viewfor the design of biomimetic materials.Cellular changes of the cell wall and protoplasm in tracheids and ray parenchyma cellsduring the transformation were observed at micro-and ultrastructure level. In the cambial zone,the thickness of fusiform and ray cell walls were thin and their radial walls were much thickerthan tangential walls. Fusiform cells were highly vacuolated with the protoplasm confined tothe periphery of cell lumen. At the time of wood cell differentiation, differentiating xylemmother cells began to lose the protoplasm. Concomitantly, their walls also became thickenedand showed characteristically distinct wall layers. In comparison, the ray cell walls appearedtypical polylamellate structure and thinner walls than the tracheids. In the current year’sgrowth, xylem mother cells finished their differentiation within the first growth ring and lefthollow dead tracheary elements, while the xylem ray cells remained alive for at least13years.They contained cell protoplasm, although the amount, shape and size altered when shiftingtoward intermediate wood. Subsequently, the ray cells disintegrated their protoplasm, includingthe nuclei, their organelles and reserve materials, which marked the formation of heartwood.The structure of chemical components in wood cell walls changed during heartwoodformation. No major changes in microfibril angle of wood cell walls were examed fromsapwood to heartwood. For the matrix of cell walls, there was no significant variations inhemicelluloses, but the molecular structure of lignin altered. The amount of lignin in cell walls increased from sapwood to heartwood, and lignin of heartwood had a higher degree ofcross-linking than that of sapwood and transition wood. There were no obvious differences inlignin structure between sapwood and transition wood.Correlated well with the chemical variations, hygroscopic and viscoelastic propertychanges were also studied by dynamic mechanical analysis and dynamic vapor sorption. Ligninsoftening temperature from sapwood to heartwood increased with the rise of frequency andheartwood lignin had a higher softening temperature and apparent activation energy, which wasattributed to higher crosslink densities heartwood cell walls than sapwood. There were nosignificant differences in lignin softening temperature between sapwood and transition wood.Meantime, the hygroscopic properties of wood cell walls from sapwood to heartwoodvaried. The heartwood had the same moisture sorption as sapwood and transition wood at lowrelative humidity (RH<40%), but adsorbed fewer water at high relative humidity (RH>40%).Applying the model of Hailwood-Horrobin water sorption, it was suggested that the differencesin moisture sorption between sapwood and heartwood at higher relative humidity came fromchanges of absorbed polymolecular water in cell walls. The extractives in cell walls might bethe reason for its less hydroscopic properties at higher relative humidity.In conclusion, the structure of cell walls and their chemcial, physical and mechanicalproperties changed during the transformation from sapwood to heartwood. The variation oflignin strucutre in the matrix of wood cell walls during heartwood formation might be themajor cause for the changes of viscoelastic and hygroscopic properties of cell walls.