Among dioecious plants, patterns of sexual dimorphism in vegetative and reproductive traits are usually associated with contrasting strategies in life history of sex morphs. Due to sex-specific differences in reproductive costs, effects of changed environmental conditions may affect sexes differently and cause changes to reproduction success and consequently long-distance dispersal and colonization. However, the influence of aquatic conditions on the plasticity of sexual dimorphism has received less attention, particularly for aquatics that exhibit rich phenotypic plasticity and great diversity in their sexual systems. Using a common garden experiment with dioecious submerged plant Vallisneria natans grown at various water depths simulating different light availability, we measured variations in 20 traits for females and 19 traits for males (total = 540 plants from 30 seed families) including components of morphological, reproductive and photosynthetic traits. We investigated sex-specific plastic responses and performance of sexual dimorphism in response to water depth change.
Results/Conclusions
Females displayed much greater leaf length, vegetative size and resource allocation towards reproductive biomass than males at all depths, whereas spathe number and gamete production per spathe displayed reverse pattern. Besides most traits in each sex (16 in female and 12 in male) showing striking phenotypic plasticity, the degree of sexual dimorphism increased significantly for total biomass and investment in reproduction, but decreased for leaf length, spathe number and flowering ramet percentage in low light, deep water. Females varied more than males in total biomass, leaf length, reproductive effort, sex organ sizes and rate of photosynthesis in response to decreased underwater light availability, suggesting that female has greater plasticity than male. These findings illustrated considerable plasticity in the degree of sexual dimorphism in a variety of vegetative and reproductive traits across different environments driven by the contrasting reproductive functions of the sexes in relation to pollen and seed dispersal. Females of V. natans responded more sensitively than males to light stress resulted from water depth variation in either aboveground vegetative growth or reproduction. This study provides novel insight into adaptive strategies of dioecious macrophytes to maintain function and fitness under severe environmental conditions.