Functional role of wood fibersin water transpor in vessel-bearing hardwoods

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It is widely accepted that in vessel-bearing hardwoods wood fibers have specialized in mechanical support of plant body during long-term xylem evolution. To date, it is unclear to what extent wood fibers may contribute to water transport in xylem of vessel-bearing angiosperm species. In this study we conducted dye-injection experiments using acid fuchsin in order to examine the water transport of various types of wood fibers. In addition, distribution and structure of pits were examined by scanning electron microscopy in common walls between vessel elements and each type of wood fibers. As a result, specialized wood fibers such as libriform fibers and living fibers did not play any functional role in water transport, while less specialized wood fibers such as fiber-tracheids and vasicentric tracheids considerably contribute to water transport. Pit pairs were commonly present in common walls between vessel elements and the less specialized wood fibers. In contrast, pit pairs were absent from common walls between vessel elements and the specialized wood fibers. In some species with specialized fibers, no pits were found in the common walls. In others species with specialized fibers, blind pits were found on either side of the common walls. In conclusion, such differences in cell wall modifications are responsible for the differences in the function of the wood fibers in water transport. During the course of this study, interesting findings were obtained as by-products. In some species, vessels that were located in a particular layer of growth ring were simultaneously cavitated soon after maturation without severe water stress despite the fact that surrounding wood fibers retained water in their lumina. This put a new problem to researchers in some fields of tree physiology, wood science and functional plant anatomy.