Duckweeds (Lemnaceae species) are extensively used models in ecotoxicology, and chlorophyll fluorescence imaging offers a sensitive and high throughput platform for phytotoxicity assays with these tiny plants. However, the vast number of potentially applicable chlorophyll fluorescence-based test endpoints makes comparison and generalization of results hard among different studies. The present study aimed to jointly measure and compare the sensitivity of various chlorophyll fluorescence parameters in Spirodela polyrhiza (giant duckweed) plants exposed to nickel, chromate (hexavalent chromium) and sodium chloride for 72 h, respectively. The photochemistry of Photosystem II in both dark- and light-adapted states of plants was assessed via in vivo chlorophyll fluorescence imaging method. Our results indicated that the studied parameters responded with very divergent sensitivity, highlighting the importance of parallelly assessing several chlorophyll fluorescence parameters. Generally, the light-adapted parameters were more sensitive than the dark-adapted ones. Thus, the former ones might be the preferred endpoints in phytotoxicity assays. Fv/Fm, i.e., the most extensively reported parameter literature-wise, proved to be the least sensitive endpoint; therefore, future studies might also consider reporting Fv/Fo, as its more responsive analogue. The tested toxicants induced different trends in the basic chlorophyll fluorescence parameters and, at least partly, in relative proportions of different quenching processes, suggesting that a basic distinction of water pollutants with different modes of action might be achievable by this method. We found definite hormetic patterns in responses to several endpoints. Hormesis occurred in the concentration ranges where the applied toxicants resulted in strong growth inhibition in longer-term exposures of the same duckweed clone in previous studies. These findings indicate that changes in the photochemical efficiency of plants do not necessarily go hand in hand with growth responses, and care should be taken when one exclusively interprets chlorophyll fluorescence-based endpoints as general proxies for phytotoxic effects.

The biosorption and recovery of ionic gadolinium (Gd) from contaminated water by the free-floating duckweed Lemna gibba was studied. The highest non-toxic concentration range was determined as (6.7 mg L^-1). The concentration of Gd in the medium and in the plant biomass was monitored and a mass balance was established. Tissue Gd concentration of Lemna increased with increasing Gd concentration of the medium. The bioconcentration factor was up to 1134 and in nontoxic concentrations up to 2.5 g kg^-1 Gd tissue concentration was reached. Lemna ash contained 23.2 g Gd kg^-1. Gd removal efficiency from the medium was 95%, however, only 17-37% of the initial Gd content of the medium accumulated in Lemna biomass, an average of 5% remained in the water, and 60-79% was calculated as a precipitate. Gadolinium-exposed Lemna plants released ionic Gd into the nutrient solution when they were transferred to a Gd-free medium. The experimental results revealed that in constructed wetlands, L. gibba is able to remove ionic Gd from the water and can be suitable for bioremediation and recovery purposes.

In this study, we exposed a commonly used duckweed species—Lemna gibba L.—to twelve environmentally relevant metals and metalloids under laboratory conditions. The phytotoxic effects were evaluated in a multi-well-plate-based experimental setup by means of the chlorophyll fluorescence imaging method. This technique allowed the simultaneous measuring of the growth and photosynthetic parameters in the same samples. The inhibition of relative growth rates (based on frond number and area) and photochemical efficiency (F_v/F_o and Y(II)) were both calculated from the obtained chlorophyll fluorescence images. In the applied test system, growth-inhibition-based phytotoxicity endpoints proved to be more sensitive than chlorophyll-fluorescence-based ones. Frond area growth inhibition was the most responsive parameter with a median EC₅₀ of 1.75 mg L⁻¹, while F_v/F_o, the more responsive chlorophyll-fluorescence-based endpoint, resulted in a 5.34 mg L⁻¹ median EC₅₀ for the tested metals. Ag (EC₅₀ 0.005–1.27 mg L⁻¹), Hg (EC₅₀ 0.24–4.87 mg L⁻¹) and Cu (EC₅₀ 0.37–1.86 mg L⁻¹) were the most toxic elements among the tested ones, while As(V) (EC₅₀ 47.15–132.18 mg L⁻¹), Cr(III) (EC₅₀ 6.22–19.92 mg L⁻¹), Se(VI) (EC₅₀ 1.73–10.39 mg L⁻¹) and Zn (EC₅₀ 3.88–350.56 mg L⁻¹) were the least toxic ones. The results highlighted that multi-well-plate-based duckweed phytotoxicity assays may reduce space, time and sample volume requirements compared to the standard duckweed growth inhibition tests. These benefits, however, come with lowered test sensitivity. Our multi-well-plate-based test setup resulted in considerably higher median EC₅₀ (3.21 mg L⁻¹) for frond-number-based growth inhibition than the 0.683 mg L⁻¹ median EC₅₀ derived from corresponding data from the literature with standardized Lemna-tests. Under strong acute phytotoxicity, frond parts with impaired photochemical functionality may become undetectable by chlorophyll fluorometers. Consequently, the plant parts that are still detectable display a virtually higher average photosynthetic performance, leading to an underestimation of phytotoxicity. Nevertheless, multi-well-plate-based duckweed phytotoxicity assays, combined with chlorophyll fluorescence imaging, offer definite advantages in the rapid screening of large sample series or multiple species/clones. As chlorophyll fluorescence images provide information both on the photochemical performance of the test plants and their morphology, a joint analysis of the two endpoint groups is recommended in multi-well-plate-based duckweed phytotoxicity assays to maximize the information gained from the tests.

The Special Issue “Duckweed: Research Meets Applications” of the journal Plants (ISSN 2223-7747) presents a comprehensive update of the current progress in the field

Submerged and free‐floating macrophytes are the two main vegetation types that can alternatively dominate in small shallow ponds and ditches. Submerged macrophytes provide more aquatic ecosystem services and cause fewer problems with anoxia and greenhouse gas emissions than free‐floating plants, but are inferior competitors for light. High pH values have been proposed as a contributor to submerged plant dominance, but threshold values for pH‐induced growth reduction of floating plants by submerged macrophytes are not known. In this study, we conducted laboratory experiments to test whether submerged plants (Ceratophyllum demersum) can effectively limit the growth of free‐floating competitors (Lemna gibba) by exceeding a critical threshold pH value. We used field data to test if and when such pH values can be achieved within dense submerged macrophyte stands. The pH values that caused 50% reduction in fresh weight‐ and chlorophyll‐based growth of L. gibba were 9.6 and 9.8, respectively, and the growth was negligible above pH 10.0. The photochemical efficiency of photosystem II and chlorophyll content of the floating plants declined rapidly above pH 9 and 9.5, respectively. Similarly, phosphorus concentration decreased and dry matter content increased sharply in L. gibba fronds above pH 9.5, respectively. We thus expect a critical pH threshold for L. gibba at around 9.5. Our mesocosm and field data show that the critical pH threshold for L. gibba growth is regularly exceeded within dense stands of bicarbonate‐using submerged macrophytes in lentic water‐bodies. Such conditions can be prevalent in the upper water layer during most of the day (12 h) in summer. The literature showed that many duckweed species (and other common free‐floating plants) in temperate and tropical water bodies show growth inhibition above pH 8. Therefore, high pH created by submerged macrophytes that use HCO₃⁻ in photosynthesis can be a major mechanism by which dominance over free‐floating competitors is maintained in densely vegetated lentic eutrophic water bodies. We propose that alternative stable states occur between free‐floating and submerged macrophytes along a pH gradient. Active management of pH may, thus, help in maintaining the desired state.