Leaf micromorphology in Rosaceae tribe Spiraeeae (subfamily Amygdaloideae) and its systematic and ecological implications

We conducted a comparative micromorphological study on the leaves of all nine genera of the Rosaceae tribe Spiraeeae, including the monotypic Korean genus Pentactina. Spiraeeae possess amphistomatic and hypostomatic leaves. Leaf epidermal characteristics varied with the morphology of stomatal complexes, epidermal cells, trichomes, and epicuticular waxes. Specific leaf epidermal features may be systematically relevant for identifying lower taxonomic levels, genera, and/or species. For example, papillate epidermal cell types were consistently observed in Spiraea sect. Calospira. Hirtellous trichomes are diagnostic characteristics of the genera Holodiscus and Kelseya among this tribe. Short-stalked capitate glandular and crispate villous trichomes were only observed in Holodiscus microphyllus and Spiraea douglasii, respectively. We also confirmed that leaf epidermal characteristics strongly support the independent taxonomic position of the Korean endemic Pentactina and the transfer from Physocarpus to Spiraea of Physocarpus insularis. Moreover, we identified xeromorphic features in the studied taxa and inferred their ecological functions, such as water repellence and anti-transpirant activities. This study presents new perspectives for future research on character evolution and xeromorphic adaptation on the basis of leaf epidermal characteristics.