Microscopic mechanism of contraction of tension wood G-fiber due to boiling

Abstract

Many woody eudicot plants form a secondary xylem composed of gelatinous fibers (G-fibers) called "tension wood" (TW) along the upper side of the tilted stem or branch. TW generates a large tensile growth stress in the longitudinal direction, allowing the tilted stem or a branch to develop negative-gravitropism in response to the strong gravitational stimulus. This is because the G-fiber tends to contract in the longitudinal direction as it matures. The matured G-fiber also contracts upon boiling in water (= hygrothermal treatment, i.e., HT-treatment), and moisture desorption (= drying treatment). These contractions occur in the cellulose-rich gelatinous layer (G-layer) as an innermost layer of the G-fiber. It is still an unsolved mystery how the G-layer, which is composed of highly crystallized and longitudinally oriented cellulose microfibrils (CMFs), contracts during maturation, boiling, and drying. In the present study, TW specimen of Konara oak (Quercus serrata L.) was subjected to HT-treatment under different temperature and time conditions, and strain due to treatment was followed. Besides, the mass loss due to HT-treatment was also followed. Obtained results are summarized as follows. (1) Green TW specimen of Konara oak contracted in the longitudinal direction when subjected to the HT-treatment at a treatment temperature higher than 40 °C, which eventually converged to a constant value according to each treatment temperature. Magnitude of the longitudinal HTR-strain in the TW specimen was positively correlated with the treatment temperature in the range from 40 to 120 °C, whereas in the normal wood (NW) specimen, it does not occur explicitly when the temperature is less than 100 °C. (2) Both TW and NW specimens showed mass loss when subjected to the HT-treatment. The mass loss rate increased rapidly by the HT-treatment at 120 °C, while it was only slight below 100 °C. There was no significant difference between the mass loss behavior of TW and NW by the HT-treatment. From analyzing those results, physical behavior of CMF and other non-cellulosic matrix components in the G-layer during the HT-treatment was estimated. The discussion was further developed to associate HT-contraction with microscopic mechanisms of the other two characteristic contractions of the G-fiber, i.e., maturation strain and drying shrinkage.