Adaptive Strategies of the invasive aquatic plant, Ludwigia grandiflora subps. hexapetala: Contrasting Plasticity Between Aquatic and Terrestrial Morphotypes

Abstract Biological invasion is the fifth biggest threat to biodiversity and ecosystem processes. Invasive species are able to colonise new environments rapidly due to their phenotypic plasticity. Ludwigia grandiflora subsp. hexapetala (Lgh), an aquatic invasive plant, invades various aquatic habitats, such as rivers, ponds, and more recently, especially in France, wet meadows. More recently, a terrestrial morphotype, emergent for half the year, has been identified. The behaviour of aquatic and terrestrial morphotypes in aquatic and terrestrial conditions was analysed through observations of morphological traits, and assays of metabolomics and phytohormones 14 and 28 days after the beginning of the experiment. The phenotypic plasticity was evaluated by calculating the relative distance plasticity index (RDPI) in response to environmental changes (terrestrial versus aquatic). RDPI were measured for morphological traits, metabolic and phytohormonal contents. Our results revealed that the terrestrial morphotype showed higher morphological trait values than the aquatic morphotype, independently of conditions, suggesting a pre-adaptation of Lgh to terrestrial habitats. The aquatic and terrestrial conditions shaped the Lgh metabolomic responses. However, the aquatic morphotype displayed higher phenotypic plasticity indexes than the terrestrial one. In addition, plasticity indexes evolved during the acclimatisation process, leading to increased RPDI values in most cases. The two morphotypes present distinct responses to aquatic and terrestrial conditions (especially in aquatic conditions for the terrestrial morphotype), highlighting the capacity of Lgh for invading new habitats due to its phenotypic plasticity, and a potential local adaptation. This study contributed to the understanding of how invasive species mobilise their phenotypic plasticity in new habitats, and the time required for local adaptation to occur.