Soraya Rouifed

Achene buoyancy and germinability of the terrestrial invasive Fallopia × bohemica in aquatic environment: a new vector of dispersion?

Abstract: Seed dispersal along watercourses can favour the long-distance migration of invasive species, not only for aquatic or wetland species, but also for terrestrial wind-dispersed plants. It is crucial to investigate the role of watercourses in the dispersal of the knotweed hybrid complex (Fallopia × bohemica) due to its frequent occurrence on riverbanks and production of fertile achenes. For this purpose, we experimentally studied buoyancy and germinability of F. × bohemica achenes in stirred water, over 28 d. We also measured the long-term survival and growth of seedlings according to exposure time to water. After nearly 2 d in the water, 50% of achenes were still afloat. After 3 d, germination occurred in water and the seedlings also floated. Moreover, the exposure of achenes to water, for however long, significantly favoured their germination rate, without affecting seedling survival, compared to a direct planting in soil. Furthermore, a maximum seedling dry mass was reached following exposure to water for 277 h (11.5 d), surpassing significantly the dry mass of seedlings planted directly in soil. Water exposure strongly favours achene germination and seedling survival. Our results demonstrate a high potential for the seeds and seedlings of Fallopia to be dispersed successfully by water.

Invasive Fallopia × Bohemica Interspecific Hybrids Display Different Patterns in Secondary Metabolites

Abstract Plant secondary metabolites are important traits that can benefit an invasive plant in its new environment. In the case of rapid evolution in the invaded area, the chemical weapons of introduced plants may diversify, and novel combinations or extreme concentrations of these secondary metabolites may be expressed. The invasive Fallopia species complex (F. japonica, F. sachalinensis, and the F. × bohemica interspecific hybrids) is a good model to assess how chemical traits can vary during post-introduction evolution. We analyzed and compared the composition of secondary metabolite extracts in F. × bohemica hybrids and in the parental species grown in the introduced area. HPLC-DAD profiles were obtained for each Fallopia species, and the main peaks of the HPLC chromatograms represent phenolic compounds. Analyses based on secondary metabolite profiles showed that F. × bohemica hybrids are closer to F. japonica. The F. × bohemica hybrids expressed the compound families described in F. japonica and F. sachalinensis, with quantitative variations between them. Hybrid chemical cocktails showed a diversification of chemical weapons. Furthermore, transgressive segregation was observed. Three dianthrones were identified for the first time in a Fallopia species and were more highly expressed in F. japonica and F. × bohemica hybrids. These results suggest an evolution in the chemical traits of Fallopia taxa in invaded areas such that certain genotypes may well have acquired new chemical cocktails resulting from post-introduction hybridizations.

Contrasting response to clipping in the Asian knotweeds Fallopia japonica and Fallopia × bohemica.

Abstract: Invasive species are often managed with the objective of population control or eradication. Here we tested whether clipping techniques influenced the aboveground and belowground structures of 2 closely related invasive taxa: Fallopia japonica and Fallopia × bohemica. We compared the aboveground response (plant height, number of stems, and number of leaves) of rhizome fragments to clipping of both taxa. The second experiment was aimed at comparing the belowground response of both taxa to the same clipping disturbance. Rhizome fragments were planted in rhizoboxes and subjected to a clipping disturbance after 4 weeks of growth. Regrowth, belowground biomass, specific root length, total root length, and distribution of roots in 3 soil layers were measured. Aboveground plant traits of both taxa were unaffected by clipping. On the other hand, clipping induced a reduction in belowground dry mass and an increased proportion of roots in the upper layer of soil in F. japonica. Both taxa of Asian knotweeds are highly tolerant to clipping because they are able to cope with aerial biomass loss. Nevertheless, this tolerance seems to be lower in F. japonica. The potential implications for the impact of these invasive species are discussed.