Sexual reproduction of two alien invasive Ludwigia (Onagraceae) on the middle Loire River, France

Ludwigia peploides (Kunth) Raven ssp. montevidensis (Spreng.) Raven and Ludwigia grandiflora (Michaux) Greuter and Burdet ssp. hexapetala (Hooker and Arn.) Nesom and Kartesz, are expanding their geographic range due to clonal reproduction; these taxa are considered the most important nuisance aquatic plants in French river habitats. During 1970s, these two species have colonized former channels and river banks of the middle Loire River, previously scarcely covered by aquatic vegetation. Now, the exotic Ludwigia build up dense and continuous stands with potentially high impact on biodiversity and sedimentation rates.The potential for sexual reproduction in both Ludwigia was assessed with locally collected seed material, from nine populations of L. peploides and five of L. grandiflora. 120 fruits were collected from each population. On 30 fresh fruits of each population (i) fruit lengths, (ii) number of seeds per fruit and (iii) the length of 30 randomly chosen seeds were measured. 45 fruits of each population were used to evaluate duration of fruit buoyancy. Seed viability under different treatments was tested for each population on seeds of 30 water-stored and 30 dry-stored fruits. The tested treatments were: impact of low temperatures (4 °C, −15 °C vs 22–24 °C) and storage conditions (dry storage or storage within tap water). Viability was assessed with the Tetrazolium test. For each treatment (temperature/moisture) and each population, three replicates of 30 seeds were used.Both species produced high numbers of fruits. Buoyancy duration was significantly longer in L. grandiflora than in L. peploides (11.4 ± 2.3 weeks vs 2.2 ± 1.6 weeks), while viability was higher in L. peploides than in L. grandiflora (77 ± 23% vs 47 ± 28%). Low temperatures (4 °C), comparable to winter conditions on the middle Loire River, had no impact on viability rates. Only freezing of seeds that were stored under water reduced seed viability in both species (by about 40% in L. peploides and more than 50% in L. grandiflora).The study thus suggests that sexual reproduction could become an additional mechanism for winter survival and spread of Ludwigia, especially over long distances.