Effects of silver(I) toxicity on microstructure, biochemical activities, and genic material of Lemna minor L. with special reference to application of bioindicator

Comparative Phytotoxicity of Metallic Elements on Duckweed Lemna gibba L. Using Growth- and Chlorophyll Fluorescence Induction-Based Endpoints

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 (Fv/Fo 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 Fv/Fo, 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.

Differential phytotoxic effect of silver nitrate (AgNO3) and bifunctionalized silver nanoparticles (AgNPs-Cit-L-Cys) on Lemna plants (duckweeds)

Duckweeds are aquatic plants often used in phytotoxic studies for their small size, simple structure, rapid growth, high sensitivity to pollutants and facility of maintaining under laboratory conditions. In this paper, induced phytotoxic effects were investigated in Lemna minor and Lemna minuta after exposition to silver nitrate (AgNO₃) and silver nanoparticles stabilized with sodium citrate and L-Cysteine (AgNPs-Cit-L-Cys) at different concentrations (0, 20 and 50 mg/L) and times (7 and 14 days). Lemna species responses were evaluated analyzing plant growth (mat thickness, fresh and dry biomass, relative growth rate - RGR) and physiological parameters (chlorophyll - Chl, malondialdehyde - MDA, ascorbate peroxidase - APX and catalase - CAT). Ag content was measured in the fronds of the two Lemna species by inductively coupled plasma optical emission spectrometry. AgNO₃ and AgNPs-Cit-L-CYs produced phytotoxic effects on both duckweed species (plant growth and Chl reduction, MDA increase) that enhanced in response to increasing concentrations and exposure times. AgNPs-Cit-L-Cys caused much less alteration in the plants compared to AgNO₃ suggesting that the presence of bifunctionalized AgNPs-Cit-L-Cys have a reduced phytotoxic effect as compared to Ag⁺ released in water. Based on the physiological performance, L. minuta plants showed a large growth reduction and higher levels of chlorosis and stress in respect to L. minor plants, probably due to greater Ag⁺ ions accumulation in the fronds. Albeit with some differences, both Lemna species were able to uptake Ag⁺ ions from the aqueous medium, especially over a period of 14 days, and could be considered adapt as phytoremediation agents for decontaminating silver ion-polluted water.

Nano-Ag: Environmental applications and perspectives

Silver nanoparticles (AgNPs) are promising bactericidal agents and plasmonic NPs for environmental applications, owing to their various favorable properties. For example, AgNPs enables reactive oxygen species (ROS) generation, surface plasmon resonance (SPR), and specific reaction selectivities. In fact, AgNPs-based materials and their antimicrobial, optical, and electrical effects are at the forefront of nanotechnology, having applications in environmental disinfection, elimination of environmental pollutants, environmental detection, and energy conversions. This review aims to comprehensively summarize the advanced applications and fundamental mechanisms to provide the guidelines for future work in the field of AgNPs implanted functional materials. The state-of-art terms including (photo)(electro)catalytic reactions, heterojunction formation, the generation and attacking of ROS, genetic damage, hot electron generation and transfer, localized surface plasmon resonance (LSPR), plasmon resonance energy transfer (PERT), near field electromagnetic enhancement, structure-function relationship, and reaction selectivities have been covered in this review. It is expected that this review may provide insights into the rational development in the next generation of AgNPs-based nanomaterials with excellent performances.