Sexual dimorphism in water and nitrogen use strategies inHonckenya peploides: timing matters

Aims Sexes of dimorphic species often differ in ecophysiological traits and display spatial segregation. These differences have been interpreted as an evolved response of the sexes to meet the specific resource demands associated with reproduction. Sexes may differ not only in the amount of resources allocated to reproduction but also in the timing of allocation to reproduction. In this study, we hypothesize that as a consequence of their specific resource demands for reproduction, the sexes of the dune plant Honckenya peploides differ in terms of temporal patterns of water use efficiency and nitrogen use and acquisition. Methods Water use efficiency, as inferred from leaf carbon isotope discrimination (Δ13C), nitrogen use, estimated by leaf nitrogen isotope composition (δ15N), and the foliar carbon and nitrogen contents were measured in males and females at three different points in time. Important Findings Females had greater water use efficiency than males, regardless of time. The ratio of N15 to N14 did not change with time in males, but significantly decreased in August for females. The total N content in the leaf tissues of females decreased as the season progressed, while in males a decrease was only found from April to June and then it remained constant from June to August. A similar pattern, but reversed, was followed by the foliar C/N ratio. Additionally, negative relationships between leaf Δ13C and N content were found at all times for males and only at the end of the season for females. Thus, our hypothesis that sex-specific patterns of nitrogen and water use efficiency will depend on time was supported. Overall, our results highlight the importance of including time in studies of sexual dimorphism, and also the role that physiological specialization plays in meeting the specific demands associated with reproduction.