Morphological features of petals of Nerium oleander L.

Nerium oleander L. (oleander) is a very desirable ornamental, widely diffused throughout the Mediterranean region that exhibits a prolonged flowering period during the dry season. The structure of white, pink and red petals of single flowers of N. oleander was examined using light microscopy, scanning electron microscopy and atomic force microscopy. The petals consist of one-layered adaxial epidermis with conical-papillate cells, a mesophyll with loosely arranged cells and large intercellular spaces, and one-layered abaxial epidermis with cuboid cells. The adaxial and the abaxial epidermises of petals are further ornamented by wrinkled reliefs, which increase the cell surface area. Roughness of the extracellular relief varies between adaxial and abaxial surfaces of white and red petals; the opposite holds true for the pink petals of the wild plants of N. oleander. Roughness was linearly correlated with vertical distance, and surface distance was linearly correlated with horizontal distance of the reliefs. Micromophology of petals of N. oleander revealed sub-micron features that may be particularly important for the performance of the nectarless flowers under ambient conditions, affecting energy exchange with the surrounding environment and adhesive properties of pink flowers during a seven-day flower lifespan, and white and red flowers during a five-day lifespan.