Application of neutron radiography to investigate changes in permeability in bacteria treated pinus radiata timber

Abstract

The permeability of softwoods can be enhanced by selective bacterial attack of the pit membranes. In this paper, green flat-sawn Pinus radiata sapwood boards were sprinkled for various exposure times with a nutrient solution containing a mixed bacterial population. The timber samples were subsequently dried and the tangential absorption of water was measured using neutron radiography to track the movement of moisture within the wood. There was a significant increase in water absorption after only two days of bacterial exposure, and the quantity of water absorption nearly doubled after a fortnight of bacterial exposure. These measurements showed that dried timber that has had a greater extent of bacterial exposure in the green condition has more void space available to store and conduct water. Scanning electron micrographs showed that bacteria had colonised the pits near the surface of the wood after only two days bacterial exposure. There was also clear evidence of damage to the margo-fibrils and tori of the pit membranes, which was attributed to enzymatic degradation by the bacteria. This study confirms the conclusions drawn by other investigators that bacteria degrade the pits in the green sapwood, so that many of the pathways for moisture flow remain unblocked once the wood is dried.