Abstract:Global climate change is predicted to increase average temperatures, alter geographical patterns of rainfall, and increase the frequency of extreme climatic events. Such changes are likely to alter the timing and magnitude of drought stresses experienced by plants. Drought and heat events are occurring more frequently with climate change; therefore, plants must develop morphological and physiological characteristics that confer tolerance to different drought conditions (ecotypes). The objective of this study was to explore leaf epidermal micromorphology of Ziziphus jujuba var. spinosa in response to different natural drought conditions. The plant samples were collected from Yantai, Shijiazhuang, Ningxia, and Xinjiang provinces, China, which form a gradient of environmental drought according to soil moisture, annual precipitation, and humidity. We found that hairs developed in both the upper and lower epidermis, which was thicker in leaf veins and functioned to reduce evaporation from the leaf surface. Cuticles and wax composed of carbon (C), oxygen (O), and calcium (Ca) were distributed in the leaf epidermis and formed protuberances. Large numbers of stomata (approximately 40 actual unit area were arranged in regular patterns on the leaf epidermis. Numerous crystals were also observed on the epidermis in each ecotype, and the shape and composition of the crystals differed among the ecotypes. The crystals were categorized according to shape and included six-surfaced, irregularly schistose, cubic, rhabdolith, and raphide forms. Single crystals or crystal clusters were the common crystal types. As the natural drought gradient increased from Yantai to Xinjiang, the density of leaf epidermal hairs in Z. jujuba var. spinosa increased, the number of stomata decreased, and stoma were more shrunken, which could help plants to reduce transpiration and tolerate drought. The numbers of crystals increased and crystal volume diminished, which would enhance leaf rigidity and drought resistance. Crystals mainly included C, O, and Ca; however, silica (Si) was detected in the Ningxia ecotype, in which it enhanced leaf mechanical properties. With increasing drought severity, carbon content increased but oxygen and calcium decreased. In summary, changes in morphological structure, biological function, and leaf epidermal micromorphology (including epidermal hairs, cuticles, wax, and crystals) represented adaptive ecological characteristics for long-term drought tolerance.