Phenolic exudates from Ludwigia peploides and Azolla sp. enhance germination of Polygonum ferrugineum seeds

The release of allelochemicals by aquatic macrophytes can inhibit or benefit other plants. Here we studied allelopathic interactions between syntopic species native to the Neotropical region. First, we quantified the phenolic exudation in monospecies cultures of Ludwigia peploides (L) and Azolla sp. (A) and in co-cultures of both species growing together (A + L) compared to control water. Then, we studied the germination of Polygonum ferrugineum seeds exposed to the exudates and control water. Incubation water from L had a significant lower pH than the other treatments, and a lower conductivity than the control. L alone or in co-culture with A reduced the dissolved oxygen content of the incubation water. Phenolic compounds were undetectable in control water (C) and the A treatment. In water of L and A + L treatments, phenolic concentration increased during the 5-day bioassay, with significant differences from day 3 on compared to treatments C and A. On day 5, more phenolic exudates were found in the A + L than in the L treatment. The germination rate but not germination speed was significantly higher in P. ferrugineum seeds treated with A + L exudates, and correlated positively with the phenolic concentration. Our results suggest that the allelopathic potential of aquatic macrophytes such as L. peploides can be higher in multispecies patches than in monospecies ones. Furthermore, phenolic exudates of macrophytes might stimulate the germination of plants native to the same region, which may have implications in the formation of vegetation patterns in aquatic ecosystems.