Abstract
DRY habitats of southwestern Saudi Arabia are home to many stem-succulents, of which Cynanchum forskaolianum inhabit a wide range in these harsh environments. In this study, some anatomical and ultrastructure features of stem in C. forskaolianum were examined by using light microscopy, scanning electron microscopy and transmission electron microscopy to investigate adaptational aspects that enable this plant to cope with its stressful habitats. The stem of the plant has multiple morpho-anatomical adaptations of which low surface/volume ratio is an indication of voluminous capacity to store water. Thin boundary layer results in reduced heat-storage capacity. Papillose shape of epidermal cells together with thick epicuticular wax with thin cuticle reduces water loss. Sunken stomata with deep encryption as well as low stomata' frequency decrease transpiration and enhance water use efficiency. The large substomatal cavities with intercellular spaces in chlorenchyma promote gas exchange and diffusion of CO2. Ultrastructure of chlorenchymatous cells showed that chloroplasts have many plastoglobuli and low stacking of thylakoids, which corresponds to high light stress and leads to chioroplast protection against photoinhibition. Cortex cells have highly undulated walls that lend flexibility during drought. Ultrastructure of these walls revealed light packed fibrillar material, which can play a major role in such flexibility, enabling cells to easily lose and refill with water to support activity of adjacent chlorenchyma cells. Xylem characteristics increase safety of water transport with less vulnerability to cavitation. These anatomical and ultrastructure features of stem show C. forskaolianum as a highly adaptable species in its dry habitats.