Ecophysiological traits of paphiopedilum and cypripedium

Paphiopedilum has a very close phylogenetic relationship to Cypripedium, but both genera undergo considerable divergence in the aspects of leaf traits and habitats. Little is known about the divergences of leaf functional traits in Paphiopedilum and Cypripedium and their adaptive significances. To understand the differences in leaf traits and adaptability of Paphiopedilum and Cypripedium, we studied leaf anatomy, stomatal anatomy and photosynthesis in Paphiopedilum and Cypripedium. From the cross sections, cuticles could be seen on both sides of a Paphiopedilum leaf and the cuticle on the adaxial surface is especially thicker, but no obvious cuticle exists on the leaves of Cypripedium. Guard cell chloroplasts in the functional stomata are found in Cypripedium, but not in Paphiopedilum. Lack of guard cell chloroplast may slow the photosynthetic induction of Paphiopedilum. Compared with Paphiopedilum, Cypripedium showed a higher maximum electron transport rate, maximum carboxylation rate, light-saturated photosynthetic rate, stomatal conductance and mesophyll conductance, but lower relative stomatal limitations and leaf dry mass per unit area. The leaf structures and physiological functions of Paphiopedilum and Cypripedium reflect the adaptation to their habitats, especially the leaf morphological and physiological evolution of Paphiopedilum is related to water-conserving traits in the karst habitat. Our results provide evidence of divergent evolution of congeneric orchids under natural selection. The results would be beneficial for the development of conservation and cultivation strategies for Paphiopedilum and Cypripedium.