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

Abstract

PREMISE Invasion processes involve several mechanisms, some of which have received little attention. Allelopathy has been invoked as an invasion driver according to the novel weapon hypothesis (NWH), and polyploidization can also be an important factor for invasion success. However, very few studies have addressed both topics together. We analyzed the allelopathic effect of the polyploid Oxalis pes‐caprae, an invader in mediterranean‐climate areas worldwide, from different origins and ploidy levels (native: South Africa; invaded: Chile, Australia, California, and the Mediterranean Basin) on Lactuca sativa as a model species. METHODS We measured seed germination, initial plant height, and aboveground and belowground biomass of Lactuca grown in field soil mixed with Oxalis material and with or without activated carbon in a common garden experiment. We performed the experiment twice, when Oxalis was vigorous and when senescent. RESULTS Vigorous plants of Oxalis tended to reduce Lactuca germination, but both vigorous and senescent Oxalis plants increased Lactuca biomass, probably due to an increase in nutrients provided by plant material. The highest increase in Lactuca traits occurred with Oxalis plants from the Iberian Peninsula. Allelopathy only happened when Oxalis was senescent and was especially strong with plants from Chile. CONCLUSIONS Although we did not find broad evidence for the NWH considering all areas together, we reported differences in the allelopathic potential of Oxalis plants depending on their origin. These results highlight the independent evolution of invasiveness traits in distant introduced ranges and the importance of the invaders’ origin when testing hypotheses about invasion drivers.

The other way around: the utility of a plant invader

“…Also, a clear negative effect of AC on plant biomass and survival was also reported in a previous study (Yuan et al ., 2021). Although AC was proven to be valid in allelopathic studies by blocking allelopathic effects (e.g., Sturm et al ., 2018; Kheirabadi et al ., 2020; Lorenzo et al ., 2021), experimental designs as the one used in this study must allow the identification of possible isolated and interactive effects of AC on target plants. As AC may have species-specific effects on plant performance through changes in chemical and biological properties of soils, it is essential to account for these effects in allelopathy studies.…”

“…For instance, we used dried instead of fresh plant material because it is a useful way to conserve and store the surplus plant material before its use, preventing decomposition and keeping allelopathic potential (Bonanomi et al ., 2018; Gatto et al ., 2021). We also used activated carbon (AC), as an allelopathy neutralizer, commonly used to discriminate if the effects of plants are driven by allelopathic compounds (Sturm et al ., 2018; Lorenzo et al ., 2021). Moreover, as the use of U. europaeus materials is relevant in invaded areas, because there is already an enormous quantity of undesirable U. europaeus plants needed to be removed, the plants used in invaded areas need to be cut before seeding, and even before flowering, to avoid any possibility to spread seeds or add invasive seeds to the crop field, which could increase the invasion problem and affect the crop yield.…”