Effect of plant origin and phenological stage on the allelopathic activity of the invasive species Oxalis pes-caprae

Allelopathy has been invoked to be an invasion driver according to the novel weapon hypothesis (NWH). Polyploidization also can be an important factor in invasion success. However, there are very few studies addressing both topics together in plant invasion. We analyze the allelopathic effect of the polyploid Oxalis pes-caprae, an invader in the Mediterranean-climate areas worldwide, from different origins (native: South Africa; invaded: Chile, Australia, California and the Mediterranean Basin) on Lactuca sativa as a model species. These origins differ in ploidy levels. We measured seed germination, initial plant height, aboveground and belowground biomass of Lactuca grown in a field soil mixed with Oxalis material and with or without activated carbon under common garden conditions. We performed the experiment twice, when Oxalis was green and vigorous and when senescent. We found that vigorous Oxalis tends to reduce Lactuca germination, but both vigorous and senescent Oxalis increase Lactuca biomass probably due to an increase in nutrients provided by plant material. We reported differences in the allelopathic potential of Oxalis depending on their origin. These results highlight the independent evolution of invasiveness traits in distant introduced ranges and the importance of the origin of the plant material used in experiments when testing hypotheses about invasion drivers.