Chemical modification of secondary xylem under tensile stress in the stem of Liriodendron tulipifera

Tension wood is a specialized tissue that develops in the upper side of leaning stem and branches in angiosperm. In yellow poplar, tension wood does not form a G-layer, which is one of the most characteristic features of typical tension wood. In order to determine whether the chemical modification associated with tension wood formation in yellow poplar is consistent with those of G-layer-forming angiosperm species, tension wood was induced via mechanical bending treatment for 7 and 14 days in the stem of 2-year-old yellow poplars, and its major cell wall components were analyzed. Whereas the cellulose contents of the tension wood were higher than those in opposite wood, its lignin contents were lower; the result is consistent with that seen in other angiosperm tension wood. Interestingly, the lignin contents differed between tension wood and opposite wood sides at 7 days in the bent sample, but became similar levels to each other at later stages of tension wood formation. The S/G ratio of tension wood was also higher than that of opposite wood. Additionally, the composition of hemicelluloses (glucomannan and xylan) was altered significantly under tensile stress conditions in the yellow poplar stem. Therefore, the majority of the cell wall polymers were altered significantly in the tension wood relative to those in opposite wood, and these changes appear to be developmentally regulated, regardless of the existence of the G-layer in the fiber.