Wood Decay by Brown-Rot Fungi: Changes in Pore Structure and Cell Wall Volume

Sweetgum (Liquidambar styraciflua L.) wood blocks were decayed by Postia (= Poria) placenta in soilblock cultures. Decay was terminated at various weight losses, and the pore volumes available to four low molecular weight molecules, (water, 4 Å,; glucose, 8 Å,; maltose, 10 Å; and raffinose, 128,) and three dextrans (Mr 6,000, 38 Å; 11,200, 51 Å; and 17,500, 61 Å) were determined by the solute exclusion technique (Stone and Scallan 1968b). The volume in sound (undecayed) wood that was accessible to the seven probes varied from 1.0 ml g–1 for the three largest to 1.35 ml g–1 for water. Thus, the volume in sound wood attributable to lumens, pits, and other large openings was 1.0 ml g–1 and that accessible to water in the cell wall was 0.35 ml g–1. Of this volume, 80% was inaccessible to molecules > 12 Å in diameter. As the wood was decayed, the volume of pores in the cell wall increased steadily to 0.7 ml g–1 at 35% weight loss. New cell wall volume was accessible to the four low molecular weight probes but not to molecules of Mr ≥ 6,000. The increase in accessible pore volume to the four smallest probes was gradual. Most of the new cell wall volume created by removal of components during decay was in the pore size range of 12 Å, to 38 Å. Within experimental error, no pores of > 38 Å, were observed in sound or decayed wood. Our results are consistent with the hypothesis that the initial depolymerization of cellulose, characteristic of brown rot, is caused by a diffusible agent. The molecular diameter of the agent is apparently in the range 12 Å, to 38 Å, and it causes erosion and thus enlargement of the pores to which it has access.