Phenotypic plasticity of vegetative and reproductive traits in monoecious and dioecious populations of Sagittaria latifolia (Alismataceae):: a clonal aquatic plant

1 The phenotypic plasticity of vegetative traits is a characteristic feature of aquatic plants, promoting survival and growth in the heterogeneous environments typical of wetlands. Less is known about plastic responses of life-history and reproductive traits, particularly patterns of sex allocation. 2 We investigated the plasticity of vegetative and reproductive traits in Sagittaria latifolia, a clonal aquatic plant whose populations are either monoecious or dioecious. Plants of the two sexual systems exhibit divergent life-history characters associated with the disturbed vs. competitive habitats in which monoecious and dioecious populations occur, respectively. We evaluated the prediction that populations of the two sexual systems would have different patterns of phenotypic plasticity because of the contrasting habitats in which they occur. 3 We grew four clonal replicates of 10 genotypes from seven monoecious and five dioecious populations (total = 480 plants) in two fertilizer treatments under glasshouse conditions and measured components of life history, leaf and flower morphology, and sex allocation. 4 The two sexual systems displayed divergent patterns of plasticity for four life-history traits but only flowering time and ramet production showed the expected pattern of greater plasticity in monoecious populations, and the reverse was true for flower production. Fertilization had opposite effects in the two sexual systems for corm production (increased in monoecious populations) and time to flowering (delayed in dioecious populations). 5 Leaf size generally increased due to the addition of fertilizer; however, this increase was substantially greater in dioecious populations. Larger leaf size in dioecious populations was associated with more convex leaves and greater surface area, potentially increasing light capture in the shaded and more competitive habitats in which these populations occur. 6 We found significant plasticity for female sex allocation in monoecious populations, with more female flowers at higher nutrient levels. However, the majority of populations had a significant genetic component to variation in sex allocation and/or significant genotype x environment interactions. These patterns are consistent with monoecy representing a flexible reproductive strategy for regulating mating opportunities in heterogeneous habitats.