Influence of Steaming on Ultrastructure of Bordered Pit Membrane in Loblolly Pine

Although it is known that steaming improves the permeability of a number of species, the mechanism has not been previously studied. This paper presents a study with an electron microscope of the ultrastructure of steamed wood in order to find an explanation for the improved permeability. The wood used consisted of samples that had been previously tested for permeability. Until used in this study, all of these samples had been stored in a 50 percent relative humidity room for a period of 2 years. Material for replicas was obtained from steamed and control samples of four trees. The results indicate that components in the pit membrane are hydrolyzed during steam treatment. The hydrolysis results in a general weakening of the membrane and also a reduction in the strength of the bond between the border and torus of aspirated pits. Consequently, these chemical changes appear to be responsible for the improved permeability because they reduce the effectiveness of aspiration.

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