The ecological and evolutionary significance of heterophylly in aquatic plants: A case study in Proserpinaca (Haloragaceae)

Plants of freshwater wetland habitats display a wide variety of putatively adaptive physiological, morphological, and phenological responses to seasonal and local fluctuations in their environment. The considerable plasticity exhibited by aquatic and amphibious plant species suggests myriad ways in which these plants may have adapted to the aquatic environment, and provide ideal circumstances in which to further our understanding of phenotypic plasticity and its role in ecological niche breadth. In this dissertation, I review one of the most well-characterized patterns of phenotypic plasticity exhibited by semi-aquatic plant species---variation in leaf form, or heterophylly---and examine what is currently known about the environmental mediation of this trait in aquatic plant taxa. I also present a comparison of patterns of seasonal variation in leaf morphology among species of the amphibious (semi-emergent) plant genus Proserpinaca (Haloragaceae) under field conditions, in an explicit test of the hypothesis that plasticity in leaf morphology is adaptive. In this study I found that plants with greater plasticity in leaf morphology (i.e., more heterophyllous) exhibited higher relative biomass and greater relative flower and fruit production across two consecutive field seasons, indicating that this pattern of plasticity is adaptive and is currently being maintained by natural selection. I also compared patterns of plasticity in traits commonly associated with flooding tolerance among these Proserpinaca species in a greenhouse experiment. I investigated if patterns of plasticity in response to contrasting flooding regimes differed among species, whether these patterns of plasticity were adaptive, and if genetic variation for plasticity was detectable for any of these traits. I found that selection in favor of heterophylly was stronger under consistently flooded conditions. Species that exhibited greater morphological plasticity in leaf shape also exhibited the greatest plasticity in stomate density across submerged, transitional and aerial leaves. These results are discussed in the context of the hypothesis that phenotypic plasticity may play a key role in the adaptation of organisms to changing environmental conditions.