Cadmium accumulation in rice grains is mitigated by duckweed-like hydrophyte through adsorption and increased ammonia nitrogen

Although increasing attention has been paid to agronomic measures for reducing the heavy metal load in rice grain, the effects of duckweed-paddy co-cropping technology on the accumulation of cadmium (Cd) in rice grains remain unclear. To investigate its specific effects on Cd accumulation in paddy fields, three types of duckweed-like hydrophyte (DH), Azolla imbricata, Spirodela polyrrhiza, and Lemna minor were chosen for study. Their use resulted in a reduction of Cd content in rice grains from 0.40 mg/kg to <0.20 mg/kg, with A. imbricata yielding the best results (0.15 mg/kg). The three types of DH reduced the available Cd content in the soil by 10 % to 35 % after the paddy tillering stage. The reduction of available Cd content was attributed to the absorption, high pH, and increase of relative abundance of special bacteria of immobilizing Cd. In addition, DH could regulate soil nitrogen leading to ammonium nitrogen increased from 75 mg/kg to 100 mg/kg, while nitrate nitrogen decreased from 0.55 to 0.1-0.3 mg/kg. The increase of ammonium nitrogen content might induce the low Cd transfer ability in rice plant and then low Cd content in rice grain. This study demonstrated that DH has a good effect on the reduction of the Cd concentration in rice grains. Consequently, duckweed-paddy co-cropping technology offers a potential solution to heavy metal pollution and agricultural non-point source pollution, as it not only reduces Cd levels in rice plants, but also fixes nitrogen, reducing the need for nitrogen application.

Time Course of Age-Linked Changes in Photosynthetic Efficiency of Spirodela polyrhiza Exposed to Cadmium

Short-term assessment of adverse effects is essential for populations exposed to higher risk of environmental pollution. This study presents the time course of physiological and morphological changes attributed to cadmium, emphasizing age-linked differences in the susceptibility of photosynthetic apparatus of Spirodela polyrhiza fronds exposed to different cadmium concentrations. A four-frond colony represented by mother, daughter, and granddaughter plants was exposed to cadmium concentrations for 6, 24, and 72 h to establish its effect on different generations of the great duckweed. The duration of cadmium exposure accounted for the most variation in chlorophyll content as the most influential variable, and after 72 h, frond responsiveness was a function of cadmium concentration. Carotenoid contents behaved slightly differently in fronds of different ages, with the oldest mother frond exhibiting accelerated senescence. Chlorophyll fluorescence measurements showed that cadmium affects different photosynthetic electron transport segments relative to the frond's chloroplast structure level. Photosynthesis of mother fronds exposed to low cadmium and daughter fronds exposed to high cadmium was determined by the functionality of primary electron acceptance at the PSII level. Mother plants exposed to higher cadmium concentrations were characterized by closed and inactive reaction centers, dissipated energy outflux, and inhibited photosynthesis. Young fronds exposed to low and high cadmium concentrations were characterized by increased non-reducing reaction centers and thermal phase reduction, with activated dissipative mechanisms at high cadmium concentrations. Cadmium-induced changes in the ultrastructure of chloroplasts were visible after 6 h of exposure to lowest concentrations, with gradual degradation of the thylakoid system as the fronds aged. Younger fronds responded to cadmium more dynamically through molecular, physiological, and anatomical changes and tolerated a more reduced electron transport chain under given conditions than older fronds.

Graphene oxide interaction with Lemna minor: Root barrier strong enough to prevent nanoblade-morphology-induced toxicity

The production of graphene oxide (GO) along with its applications in various aquatic environments is vastly increasing thanks to its rapidly expanding range of new GO-based environmental technologies. Therefore, the fate of GO in aquatic environments is an important issue, as it could become an environmental challenge if its potential toxic mechanism is not addressed properly. Number of studies reporting the toxicity of GO to various aquatic organisms is still increasing. However, research data on the possible toxic mechanism of GO towards aquatic plants have yet to be collected, especially regarding GO's surface chemistry. Here, we studied the interaction of three differently oxidized GO systems with model aquatic plant Lemna minor. We found that although none of the three GOs caused lethal phytotoxicity to Lemna after 7 days, the mechanism of action was dependent on the GO's surface oxidation. Based on the amount of functional surface groups, the GO was able to directly interact with the Lemna's root through its edges. However, in this case in contrast to algae and crustaceans, the interaction did not lead to a mechanical damage. Therefore, our results showed that GO is not hazardous to Lemna minor even at very high concentrations (up to 25 mg/L), because the root barrier proved to be strong enough to prevent GO's penetration and its consequent toxicity.

Appraisal of a novel extraction technique for estimation of cadmium content in pea seedlings based on human health risk assessment

In this study, a novel extraction and safety evaluation method for heavy metals based on different functions of plants was proposed, and an edible plant (pea) was used as the research material to explore the feasibility of the novel method. Pea sprouts were cultured in cadmium (Cd) concentrations of 0, 1.0, 3.0, and 5.0 mg L-1, respectively. The Cd in pea sprouts was continuously extracted with 100 °C distilled water, 60% ethanol, 6% acetic acid, and simulated gastric juice. It was observed that highest amount of Cd (48.65-58.87%) was found in the extraction of roots with 6% acetic acid, followed by 100 °C distilled water (28.68-37.61%). While in stems, most of the Cd (70.73-85.39%) was extracted by 6% acetic acid. The recovery rate of the sequential chemical extraction technique employed in this experiment was between 93 and 106%. Compared with traditional methods, this study has its development potential in two aspects. First, it can determine which steps of sequential extractions of heavy metals in plants are the most harmful to humans. Secondly, corresponding measures can be taken to reduce heavy metals in vegetables used daily, such as soaking edible vegetables in vinegar for a short time. Novelty statement: In this study, a novel extraction and safety evaluation method for heavy metals based on different functions of plants was proposed, and an edible plant (pea) was used as the research material to explore the feasibility of the novel method. Compared with the commonly used extraction methods, the novel method is more reasonable and has greater development potential.

Differential Bioaccumulation of Select Heavy Metals from Wastewater by Lemna minor

The capacity of Lemna minor to remediate toxic heavy metals from wastewater is reasonably well documented. In view of the pivotal role of this species in the environmental clean-up, here we evaluated the bioaccumulation potential of L. minor for cadmium (Cd), lead (Pb), and nickel (Ni) through a controlled experiment. L. minor tolerated the metals Cd, Ni, and Pb up to 0.5, 5, and 8 mg/L, respectively, and beyond these concentrations the toxicity symptoms appeared. Bio-concentration factor varied at different concentrations of heavy metals tested. Overall, L. minor showed good phytoremediation potential for all the three tested heavy metals (Cd, Ni, and Pb), though in relative terms it was more effective in extracting Ni and Cd, as compared to Pb, both in single and mixed concentrations. In view of the growing pollution in Kashmir Himalayan aquatic habitats the phytoremediation by invasive species such as L. minor promises to be one of the best choices than other native plants for cleaning up of polluted soils/water because of its fast growth rate, high abundance, easy handling, and wide distribution in Kashmir Himalayan aquatic ecosystems.

Intraspecific variations in cadmium tolerance and phytoaccumulation in giant duckweed (Spirodela polyrhiza)

Duckweeds are widely recognized for the heavy metal phytoremediation. However, the intraspecific variations in biological responses of duckweeds to heavy metal remain largely unknown. Here, the toxicity and phytoaccumulation of cadmium (Cd) were synchronously evaluated in 30 accessions of giant duckweed (Spirodela polyrhiza) collected from different provenances in Southern China. Exposure to 1 μM Cd decreased relative growth rates of dry weight, fronds number and fronds area, as well as photosynthetic pigment contents, while it increased H₂O₂ accumulation, lipid peroxidation and activities of anti-oxidant enzymes in the majority of accessions. Cd treatment led to remarkable Cd accumulation but little changes in the starch content in giant duckweed. The biological responses to Cd varied among the accessions. Further correlation analysis indicated that growth traits and Cd concentration were positively correlated with Cd accumulation, while the contents of chlorophyll, H₂O₂ and MDA were negatively associated with Cd accumulation. Our results proved the great intraspecific variation in Cd tolerance of giant duckweed, suggesting a valuable natural resource for Cd phytoremediation. Moreover, different mechanisms may be exploited by S. polyrhiza for phytoaccumulation, but growth maintenance, Cd uptake and antioxidative enzyme-independent ROS-scavenging under Cd exposure are the common mechanisms contributing to Cd accumulation ability.

Copper and mercury induced oxidative stresses and antioxidant responses of Spirodela polyrhiza (L.) Schleid

Duckweed is recognized as a phytoremediation aquatic plant due to the production of large biomass and a high level of tolerance in stressed conditions. A laboratory experiment was conducted to investigate antioxidant response and mechanism of copper and mercury tolerance of S. polyrhiza (L.) Schleid. To understand the changes in chlorophyll content, MDA, proline, and activities of ROS-scavenging enzymes (SOD, CAT, GPOD) during the accumulation of Cu⁺² and Hg⁺², S. polyrhiza were exposed to various concentrations of Cu⁺² (0.0–40 μM) and Hg⁺² (0.0–0.4 μM). antioxidant activity initially indicated enhancing trend with application of 10 μM Cu⁺²; 0.2 μM Hg⁺² (SOD), of 20 μM Cu⁺²; 0.2 μM Hg⁺² (CAT) and of 10 μM Cu⁺²;0.2 μM Hg⁺² (GPOD) and then decreased consistently up to 40 μM Cu⁺² and 0.4 μM Hg⁺². In the experiment chlorophyll and frond multiplication initially showed increasing tendency and decreased gradually with the application of increased metal concentration. Application of heavy metal has constantly enhanced proline and MDA content while the maximum increase was observed with the application of 40 μM Cu; 0.4 μM Hg for proline and MDA respectively. The upregulation of antioxidant enzymes and proline reveals that S. polyrhiza has strong biochemical strategies to deal with the heavy metal toxicity induced by the accumulation of Cu⁺² and Hg⁺².

•

Biochemical responses of copper and mercury in Spirodela polyrhizawere investigated.

•

The threshold tolerance level for Cu and Hg was found < 20 μM and < 0.2 μM respectively.

•

The results reveal that S. polyrhiza is biochemically well adapted to cope with HM toxicity induced by Cu and Hg.

Effects of ZnO nanoparticles on the toxicity of cadmium to duckweed Lemna minor

Release of nanoparticles into the aquatic environment will inevitably influence the behavior and toxicities of other existing pollutants. In the present study, 10 mg/L of nano-ZnO (diameter 20-30 nm) was used to evaluate its impacts on cadmium (Cd) toxicity on duckweed Lemna minor based on IC₅₀ values and four biological parameters including percent inhibition of growth rate (I_r), ratio of chlorophyll/pheophytin (D665/D665a), antioxidant enzymes, and H⁺-ATPase. Results of the 96-h IC₅₀ values of Cd with or without nano-ZnO indicate no additional toxicological effects of nano-ZnO to plants. Further examinations using two Cd concentrations (0.1 and 1 mg/L) showed that nano-ZnO did not influence the inhibitory effect of 0.1 mg/L Cd, but significantly (P < 0.05) reduced the stress of 1 mg/L Cd to the duckweed. The index D665/D665a reflected that the toxic effect of 1 mg/L Cd was significantly (P < 0.05) suppressed by nano-ZnO. H⁺-ATPase was also sensitive to reveal the protective effects of nano-ZnO on the duckweed under Cd exposure. However, the responses of the antioxidant enzymes SOD and CAT failed to reflect the effects of nano-ZnO on Cd toxicity. Hysteretic addition of nano-ZnO for 24 h showed that the protective effects of nano-ZnO were weakened. Our results suggest that the adsorption of Cd to nano-ZnO may result in lower Cd uptake by L. minor, thus reducing its toxicity.

Real-time CO2 uptake/emission measurements as a tool for early indication of toxicity in Lemna-tests

This paper presents an application of continuous monitoring of the emission and uptake rate of CO₂ in Lemna toxicity test. On a real-time basis, the CO₂ concentration data were collected by the Arduino platform-based respiratory activity measuring system (ResTox) and reported as CO₂ concentration dynamic curves. The results of CO₂ measurements demonstrated that tested metals (Co, Cu, Hg, and Cd), as well as herbicides (nicosulfuron, diquat, and tembotrione), stimulated the CO₂ exchange rates at low doses, while at high doses CO₂ exchange rates were inhibited. The addition of higher concentrations of clopyralid stimulated photosynthetic activity and caused a higher increase in respiration rates indicating its mode of action as auxin mimic herbicide. The results obtained underline the necessity of considering other biological endpoints like continuous measurements of gas exchange from the very beginning of exposure to toxicants. Simultaneous measurements of real-time CO₂ concentrations, as the primary effect of toxicant mode of action, and processes that are supported by carbon flux, as the secondary effect or endpoint, are needed to relate actual and substrate-induced or inhibited respiration and photosynthesis to those processes. Therefore, continuous measurements of CO₂ exchange rates can be implemented for the initial screening of potential toxicity to give valuable information that is needed for further examination of toxicity mechanisms and risk assessment.