Abstract: Sex ratio plays a critical role in population survival. Dioecious plants may simultaneously display sexual dimorphism and deviations from the equilibrium expectation of 1:1 sex ratio in populations, i.e., biased sex ratio. However, whether biased sex ratios can be attributed to sexual dimorphism has not been well investigated. Here, we conducted experiments using a three-water depth gradient to examine sexual dimorphism performance and population sex ratios for Vallisneria natans, and investigated the variations of natural population sex ratios for it and other two congeners V. spinulosa and V. densesenulata along water depth changes. Females of V. natans had higher reproductive investment than males, and the degree of sexual dimorphism increased with increasing water depth. At greater water depths, increased reproductive expenditure led to higher mortality in females, resulting in a male-biased sex ratio. The sex ratios of natural V. natans and V. spinulosa populations did not significantly deviate from 1:1. In contrast, populations of V. densesenulata exhibited markedly a female-biased sex ratio, which may be attributed to female plants producing more clonal ramets. Furthermore, no evidence was found for variation in sex ratios along water depth gradients in natural populations of any of the three species. These findings highlight the plasticity both of sexual dimorphism and sex ratios in dioecious plants, indicating that sex ratios are species-specific and depend on sex-specific life-history strategy shaped by their growth environments.

Key words: Life-history strategy, sex ratio, sexual dimorphism, submerged macrophyte, Vallisneria