The role of vascular bundles in reducing drying collapse of parenchyma cells in oil palm wood

To investigate the effect of the structural gradient of oil palm wood (OPW) on the variation in drying collapse, OPW specimens prepared from different locations along the trunk radius and trunk height were kiln-dried at 80 °C to a final equilibrium moisture content of 4%. The effects of anatomical characteristics and the fraction of vascular bundles on the cross-sectional strain due to drying collapse were investigated. Along the trunk radius, there is a critical fraction of vascular bundles above which the drying collapse of OPW becomes exceedingly small. Below the critical value, the drying collapse of OPW linearly increases with a decrease in the fraction of vascular bundles following a rule of mixtures. Microstructural examination reveals that the thickness of the cell wall and the arrangement of cells between fiber cells in the vascular bundles and parenchyma cells at the interface area are effective against parenchyma cell collapse during drying. The decrease in cell wall thickness and the increase in space at the interface between these cells with trunk height correspond to an increase in the critical value of the fraction of vascular bundles from the lower bottom to the upper top zones of the trunk height.