Peduncle vulnerability to embolism is related to conduit dimensions of the critically endangered slipper orchids in Southwest China

Members of the family Orchidaceae are amongst the most ornamental and protected plants in the world. The understanding of physiological and ecological characteristics of orchids is of great significance for predicting their response to environmental changes and establishing orchid conservation strategies. The physiological characteristics of flowers affect their relationship with pollinators and successful plant reproduction, but research on water physiology of flowers is still lacking. Cypripedium and Paphiopedilum are closely related phylogenetically in Orchidaceae, but they are different in floral longevity, life history, and habitat. In this study, we used the optical vulnerability method to investigate differences in flower peduncle drought resistance in the four slipper orchids of Cypripedium and Paphiopedilum, which are critically endangered orchids in Southwest China, and to test whether peduncle conduit dimensions determine embolism resistance. We found that the peduncles of Paphiopedilum species had greater xylem embolism resistance than that of Cypripedium. Across species, the time of 50% embolism of the peduncles was strongly correlated with the mean ratio (t/b)³ in peduncles. Our results are in line with habitat-based expectations that flower peduncle embolism resistance and conduit traits in Paphiopedilum species has helped them adapt to seasonal drought stress in karst habitats. However, the peduncle traits of Cypripedium reflect adaptations to an environment characterized by abundant soil water during the flowering period. Moreover, drought resistance of the flower peduncle was related to conduit dimensions. The results of this study contribute to our understanding of the divergent adaptation of flower traits in slipper orchids, which is beneficial for the conservation of endangered orchids.