Sclerified Parenchyma Differentiation in Hydrangea Veins is Essential for Robust Decorative Sepals (Hydrangea spp.)

摘要

Information about mechanical reinforcement of decorative organs could help development of a novel technique that would give flowers with robust floral organs and broadly contribute to postharvest flower preservation. Hydrangeas (Hydrangea spp.) exhibit remarkable characteristics in terms of mechanical reinforcement of decorative sepals. Although decorative sepals at the flowering stage shrink when they are desiccated, decorative sepals after flowering maintain their shape even after desiccation. In this study, the lignifications of the vein cells in decorative sepals were analyzed using phloroglucinol/HCl-staining. The microstructure of the cell wall was analyzed using transmission electron microscopy (TEM). The three-dimensional structure of vein cells was analyzed using serial block-face scanning electron microscopy (SBF-SEM). Tubular- and spindle-shaped dead cells with a lignified pitted secondary cell wall were observed around the vessel elements in decorative sepals after flowering. These cells were observed as living cells without a secondary cell wall in the veins of decorative sepals at flowering and in fully expanded leaves. Further, 10 hydrangea cultivars were analyzed for development of mechanical reinforcement in vein cells, and some of them were compared by desiccation testing. Decorative sepals of a cultivar lacking those cells exhibited shrinkage after flowering when they were desiccated. In conclusion, dead cells with a lignified pitted secondary cell wall contribute to the reinforcement of veins in decorative sepals of hydrangeas and become sclerified parenchyma cells. Axial parenchyma sclerifying in veins after flowering is essential for robust hydrangea floral organs and represent a new type of mechanical reinforcement tissue in plant decorative floral organs.