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
vs. away environments influenced their growth. Additionally, we
investigated their response to a metal pollutant (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 their 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 zinc 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 have a high Zn tolerance and strong plastic adaptive
potential in novel environments.