We tested for local adaptation in duckweed, a widely distributed aquatic plant family that reproduce fast and clonally. These economically important plants have high potential for phytoremediation of polluted water bodies. We collected four Lemna gibba/minor populations from across Switzerland and assessed how their home vs. away environments influenced their growth. Additionally, we also investigated their response to a metal pollutant (zinc sulfate, ZnSO4). Zinc is commonly found in freshwater systems at low concentrations, but can become harmful at elevated concentrations. We hypothesized that growing in their own (home) environment would help buffer the negative effect of the metal pollutant.
To test our hypothesis, we did a fully reciprocal transplant experiment in a glasshouse where plants from the four collected populations were grown in each of the four collected environments, as well as in three different concentrations of ZnSO4. We ran the experiment for three weeks and assessed the growth rates of all populations. Because duckweed compete with co-occurring algae for nutrients, we sampled total chlorophyll-a as a proxy for algal biomass at the end of the experiment and also measured total nitrogen (TN) and total organic carbon (TOC).
Results/Conclusions
The four Lemna gibba/minor populations exhibited significantly different growth rates across environments. However, the effect of the environment on duckweed growth was the same for all populations. For example, in the source water collected from the Eastern part of Switzerland, all populations grew consistently slower, likely because this environment was low in TN and high in TOC. We did thus not find evidence for local adaptation, and instead observed strong plastic responses in the populations to the test environments. We suggest that local adaptation plays no major role in the success of L. gibba/minor, at least under the conditions of our study.
Unexpectedly, we found that the addition of ZnSO4 increased duckweed growth rates. In contrast, algal growth was inhibited by the metal pollutant. Thus, the positive effect of zinc on duckweed growth could be in part via reducing the competition with algae. We conclude that L.gibba/minor is a good candidate for phytoremediation due to its high tolerance of zinc. However, the origin of the populations used in phytoremediation projects may not be critical due to the plastic adaptive potential of Lemna in novel environments.
