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.