Responses to drought stress among sex morphs of Oxyria sinensis (Polygonaceae), a subdioecious perennial herb native to the East Himalayas.

It is generally accepted that dioecious plants occur more frequently in dry and nutrient-poor habitats, suggesting that abiotic stress factors could contribute to evolution of dioecy from hermaphrodite. Therefore, experimental investigations on the responses of subdioecious species, a special sexual system comprising male, female, and hermaphrodite plants, to abiotic stress factors could quantify the contribution of selective pressure on the evolution of dioecy. In this study, we evaluated the physiological responses of different sex morphs of Oxyria sinensis Hemsley, a perennial herb native to the East Himalayas, to drought stress. Male, female, and hermaphrodite plants of O. sinensis were subjected to low, moderate, and high drought stress conditions in a glasshouse. Generally, with increasing water stress, the values of most measured variables slightly decreased, whereas water-use efficiency slightly increased. Furthermore, there were no significant differences in most of the measured parameters among the sex morphs under each drought stress treatment, indicating that O. sinensis might be well-adapted to drought stress conditions as its typical habitat is the dry and hot habitats of xerothermic river valleys. However, nitrogen-use efficiency was significantly higher in male and female plants than in hermaphrodite plants under high drought stress conditions, suggesting that that nitrogen-use efficiency under conditions of drought stress might have contributed to the evolution of dioecy from the hermaphrodite to some degree.