Abstract: [Objective] Reaction wood is one of the limiting factors for efficient cultivation and wood utilization of fast-growing timber species. There are various types of tension woods (TWs) in Betula luminifera plantation, but the research on their characteristics and forming causes is absent. In this study, microscopic structure, physical and chemical characteristics of B. luminifera TW were determined, and distributions of endogenous hormones were analyzed during the early period of its formation to provide knowledge of the TW formation mechanism in birch. [Method] B. luminifera clone 1V25-2 was used for artificially inducing tension wood by bending. Anatomical characteristics of xylems were observed, and fiber characteristics, microfibril angle, and contents of cellulose and lignin were determined. The distributions of four endogenous hormones were analyzed using the ELISA during the early period of TW formation (from 6 h to 7 d of a tension time course). [Result] TW fiber presented a conspicuously thickened inner layer of the cell wall after bending for 12 months, and the fiber of the double wall was averagely 1.8 times more thickness than the opposite wood (OW) fiber. The double-stained results with Safranin-Astra blue showed that a gelatinous layer deposited in the lumen side of secondary cell wall of TW. Fiber lengths and cellulose contents of TW were both obviously greater than those of OW, but lignin contents exhibited the converse tendency. At the early stages of bending treatment, four endogenous hormones showed the distinct distributions in different xylem regions. The IAA content of TW was all lower than that of OW, but the difference between the two regions reached to significant level only after 7 days bending. The distributions of GAs (GA₁+GA₃) were same as IAA. Except for the sample of 6 h bending, the BR contents of OW were significantly higher than those of TW, and showed a rising tendency as bending progressed. Although ZR content showed a rising tendency as bending progressed, their differences between TW and OW were insignificant for each sampling time. However, the ZR content of TW and OW was significantly higher than that of the control after 7-day of bending. [Conclusion] After 12 months bending, cell walls of B. luminifera were obviously thickened in TW xylem. A typical gelatinous layer was deposited in the lumen side of secondary cell wall of this region. And specific physical and chemical characteristics, such as the longer fibers and higher cellulose content, were found in TW. At the early stages of bending, four endogenous hormones showed the different distribution patterns in the xylems, implying that re-distributions of these hormones may be associated with the TW formation in B. luminifera.

Key words: Betula luminifera, tension wood, anatomical characteristics, physicochemical properties, endogenous hormones, distribution regulations