Colonization, regeneration potential and growth rates of fragments of the exotic aquatic macrophyte Hydrilla verticillata

Hydrilla verticillata is an aquatic macrophyte recently found in the Upper Paraná River floodplain. This species has a high potential to reproduce and disperse to new sites, mainly through stem fragments. In this study, we evaluated the colonization (the ability of roots to develop), regeneration (the ability of lateral shoots to develop) and growth rates of this species using fragments of different lengths that originated from different plant parts. We selected parts with and without apical tips, each 5 and 10 cm long, and the following hypothesis was tested: the fragments with apical tips have greater colonization and regeneration abilities than fragments without apical tips, and these abilities depend on the fragment length. Our results showed that both fragments with and without tips presented the same number and dry weight of roots and shoots and thus did not differ in their colonization or regeneration abilities. The length was important for the colonization ability, as 10 cm fragments produced more roots than 5 cm fragments. The high colonization and regeneration abilities from different fragments coupled with the high growth rates exhibited by H. verticillata may account for its rapid spread in novel habitats and its competitive success over native species. Thus, we suggest that the inadequate management of this species, resulting in the formation of fragments, especially of large lengths, may have important implications with regard to its infestation of new sites.