Abstract
Harsh environmental conditions of drought and high temperature are characteristic features of the long dry season of the arid desert in southern Saudi Arabia. The leafless stem succulent Caralluma acutangula (Decne.) N.E.Br. (Apocynaceae) possesses structural and functional adaptations necessary for survival in its natural habitat. Morphological adaptations include thick epicuticular wax, thick cuticle, low stomatal frequency and lobed stem that gives the plant a low surface : volume ratio. As the plant water status decreased during the dry season, depth of stem lobes increased resulting in increased turbulent air flow and decreased stem temperature. Anatomical adaptations allow for a hydrenchyma-chlorenchyma water movement that helps the hydrenchyma to serve as a water reservoir for maintaining the chlorenchyma water status and preserving photosynthetic machinery. Functional adaptations include constitutive crassulacean acid metabolism (CAM) and a shift to CAM-idling under protracted water stress. Photo-protective strategies included accumulating anthocyanin pigment during the wet season and shifting towards non-radiative dissipation of excess energy during the dry season as indicated by increased non-photochemical chlorophyll fluorescence quenching, denoting increased heat energy dissipative mechanisms during the dry season.