ONLINE IMAGE COLLECTION

About the Image

Tension tissue in transverse hand section of internode 7 from unfixed wild-type alfalfa (Medicago sativa), shown for the first time in an herbaceous perennial using zinc chloro-iodide staining and differential interference contrast to increase resolution of unstained cells (c. 100X). Tension tissue in angiosperms, associated with mechanical strengthening of stems and branches to offset leaning or other mechanical challenges, is sometimes correlated with the presence of gelatinous fibers and/or reduced lignin content. The inner layers of the gelatinous fibers stained dark red to purple, and the lignified outer cell walls stained orange to yellow. Reaction phloem fibers were also identified above the reaction xylem. The tension tissue was discovered not only in the wild type, but also to a greater extent in alfalfa line pC3H-I with c. 64% less lignin. In addition, tension tissue alters cell wall properties to help provide sufficient mechanical support in the lignin downregulated line. Tension wood, however, is often viewed as deleterious in pulp/paper and lumber manufacture; this study raises the possibility of its increased formation in biotechnologically modified lignin-reduced woody plants.