High tolerance to zinc but no evidence for local adaptation in the aquatic plant Lemna minor

Duckweeds are a widely distributed and economically important aquatic plant family that have high potential for phytoremediation of polluted water bodies. We collected four ecotypes of the common duckweed Lemna minor from the four corners of Switzerland and assessed how their home versus away environments influenced their growth. Additionally, we investigated their response to a metal pollutant (zinc, Zn) in both their home and away environments. Zn is found in freshwater systems and can become harmful to plants at elevated concentrations. We hypothesized that growing in their home environment would help the plants buffer the negative effect of the metal pollutant. To test this, we measured Lemna growth in a common garden experiment in a glasshouse where the four ecotypes were grown in each of the water environments, as well as in three different concentrations of Zn. To investigate whether interactions between Lemna and its microbial community can enhance or reduce tolerance to heavy metal pollution, we sampled chlorophyll-a as a proxy for algal biomass. Finally, we measured total nitrogen and total organic carbon to describe the abiotic environment in more detail.The four Lemna ecotypes exhibited significantly different growth rates across the water treatments. This difference in fitness was matched with DNA sequencing revealing genetic differentiation between the four ecotypes. However, the effect of the water and Zn treatment on Lemna growth was the same for all ecotypes. We did not find evidence for local adaptation; instead, we observed strong plastic responses. Lemna growth rates were higher under higher Zn concentrations. This positive effect of Zn on Lemna growth could be in part due to reduced competition with algae. We conclude that L. minor ecotypes may exhibit large differences in growth rate, but that the species overall has a high Zn tolerance and strong plastic adaptive potential in novel environments.