Bioaccumulation and phytotoxicity of ZnO nanoparticles in soil-grown Brassica chinensis L. and potential risks

The trade-off regulation of arbuscular mycorrhiza on alfalfa growth dose-dependent on gradient Mo exposure

Molybdenum (Mo) is an essential nutrient for leguminous plants, but the effects of Mo exposure on plant growth, especially in relation to soil microorganisms, are not fully understood. This study employed alfalfa (Medicago sativa L.) to evaluate the physiochemical responses to gradient soil Mo variations and explore the potential regulatory role of rhizosphere microorganism - arbuscular mycorrhizal fungi (AMF) in modulating Mo's impact on plant physiology, with a focus on metabolic pathways. The results showed that Mo exerted hormetic effect (facilitation at low doses; inhibition at high doses) on alfalfa growth, promoting biomass (below 90.94 mg/kg, with a 63.98 % maximum increase), root length (below 657.11 mg/kg, with a 39.29 % maximum increase), and plant height (below 304.03 mg/kg, with an 18.4 % maximum increase). Excess Mo (1000 mg/kg) resulted in a reduction in photosynthesis and biomass growth due to increased oxidative stress (p < 0.05). Within the stimulatory zones, AMF enhanced Mo accumulation in alfalfa, augmenting its phytological effects. Exceed the stimulatory zones, AMF enhanced alfalfa Fe uptake and reduced the generation of reactive oxygen species (ROS) induced by excess Mo by shifting the redox homeostasis-controlled enzyme from peroxidase (POD) to superoxide dismutase (SOD), thereby improving alfalfa's tolerance to Mo. Metabolomic analysis further revealed that AMF promoted the biosynthesis of indole acetic acid (IAA) and various amino acids in Mo-stressed alfalfa (p < 0.05), which accelerated alfalfa growth and mitigated Mo-induced phytotoxicity. These insights provide a foundation for developing sustainable management strategies for Mo-exposed soils using AMF inoculants, such as minimizing Mo fertilizer application in Mo-deficient soils and facilitating the reclamation of Mo-contaminated soils.

Effects of particle size on toxicity, bioaccumulation, and translocation of zinc oxide nanoparticles to bok choy (Brassica chinensis L.) in garden soil

The indiscriminate use of zinc oxide nanoparticles (ZnO NPs) in daily life can lead to their release into soil environment. These ZnO NPs can be taken up by crops and translocated to their edible part, potentially causing risks to the ecosystem and human health. In this study, we conducted pot experiments to determine phytotoxicity, bioaccumulation and translocation depending on the size (10 - 30 nm, 80 - 200 nm and 300 nm diameter) and concentration (0, 100, 500 and 1000 mg Zn/kg) of ZnO NPs and Zn ion (Zn2+) in bok choy, a leafy green vegetable crop. After 14 days of exposure, our results showed that large-sized ZnO NPs (i.e., 300 nm) at the highest concentration exhibited greater phytotoxicity, including obstruction of leaf and root weight (42.5 % and 33.8 %, respectively) and reduction of chlorophyll a and b content (50.2 % and 85.2 %, respectively), as well as changes in the activities of oxidative stress responses compared to those of small-sized ZnO NPs, although their translocation ability was relatively lower than that of smaller ones. The translocation factor (TF) values decreased as the size of ZnO NPs increased, with TF values of 0.68 for 10 - 30 nm, 0.55 for 80 - 200 nm, and 0.27 for 300 nm ZnO NPs, all at the highest exposure concentration. Both the results of micro X-ray fluorescence (μ-XRF) spectrometer and bio-transmission electron microscopy (bio-TEM) showed that the Zn elements were mainly localized at the edges of leaves exposed to small-sized ZnO NPs. However, the Zn elements upon exposure to large-sized ZnO NP were primarily observed in the primary veins of leaves in the μ-XRF data, indicating a limitation in their ability to translocate from roots to leaves. This study not only advances our comprehension of the environmental impact of nanotechnology but also holds considerable implications for the future of sustainable agriculture and food safety.

Molybdenum-mediated nitrogen accumulation and assimilation in legumes stepwise boosted by the coexistence of arbuscular mycorrhizal fungi and earthworms

Molybdenum (Mo) is a critical micronutrient for nitrogen (N) metabolism in legumes, yet the impact of Mo on legume N metabolism in the context of natural coexistence with soil microorganisms remains poorly understood. This study investigated the dose-dependent effect of Mo on soil N biogeochemical cycling, N accumulation, and assimilation in alfalfa under conditions simulating the coexistence of arbuscular mycorrhizal fungi (AMF) and earthworms. The findings indicated that Mo exerted a hormetic effect on alfalfa N accumulation, facilitating it at low concentrations (below 29.98 mg/kg) and inhibiting it at higher levels. This inhibition was attributed to Mo-induced constraints on C supply for nitrogen fixation. Concurrently, AMF colonization enhanced C assimilation in Mo-treated alfalfas by promoting nutrients uptake, particularly Mg, which is crucial for chlorophyll synthesis. This effect was further amplified by earthworms, which improved AMF colonization (p < 0.05). In the soil N cycle, these organisms exerted opposing effects: AMF enhanced soil nitrification and earthworms reduced soil nitrate (NO3--N) reduction to jointly increase soil phyto-available N content (p < 0.05). Their combined action improved alfalfa N assimilation by restoring the protein synthesis pathway that is compromised by high Mo concentrations, specifically the activity of glutamine synthetase. These findings underscored the potential for soil microorganisms to mitigate N metabolic stress in legumes exposed to elevated Mo levels.

Integrative chemical, physiological, and metabolomics analyses reveal nanospecific phytotoxicity of metal nanoparticles

The increasing application of metal nanoparticles (NPs) via agrochemicals and sewage sludge results in non-negligible phytotoxicological risks. Herein, the potential phytotoxicity of ZnO and CuO NPs on wheat was determined using integrative chemical, physiological, and metabolomics analyses, in comparison to Zn2+ and Cu2+. It was found that ZnO or CuO NPs had a stronger inhibitory effect on wheat growth than Zn2+ or Cu2+. After exposure to ZnO or CuO NPs, wheat seedlings accumulated significantly higher levels of Zn or Cu than the corresponding Zn2+ or Cu2+ treatments, indicating the active uptake of NPs via wheat root. TEM analysis further confirmed the intake of NPs. Moreover, ZnO or CuO NPs exposure altered micronutrients (Fe, Mn, Cu, and Zn) accumulation in the tissues and decreased the activities of antioxidant enzymes. The metabolomics analysis identified 312, 357, 145, and 188 significantly changed metabolites (SCMs) in wheat root exposed to ZnO NPs, CuO NPs, Zn2+, and Cu2+, respectively. Most SCMs were nano-specific to ZnO (80%) and CuO NPs (58%), suggesting greater metabolic reprogramming by NPs than metal ions. Overall, nanospecific toxicity dominated the phytotoxicity of ZnO and CuO NPs, and our results provide a molecular perspective on the phytotoxicity of metal oxide NPs.

An Innovative Approach to Alleviate Zinc Oxide Nanoparticle Stress on Wheat through Nanobubble Irrigation

The extensive utilization of zinc oxide nanoparticles in consumer products and the industry has led to their substantial entry into the soil through air and surface runoff transportation, which causes ecotoxicity in agro-ecosystems and detrimental effects on crop production. Nanobubbles (diameter size < 1 µm) have many advantages, such as a high surface area, rapid mass transfer, and long retention time. In this study, wheat seedlings were irrigated with a 500 mg L-1 zinc oxide nanoparticle solution delivered in the form of nanobubble watering (nanobubble-ZnO-NPs). We found that nanobubble watering improved the growth and nutrient status of wheat exposed to zinc oxide nanoparticles, as evidenced by increased total foliar nitrogen and phosphorus, along with enhanced leaf dry mass per area. This effect can be attributed to nanobubbles disassembling zinc oxide aggregates formed due to soil organic carbon, thereby mitigating nutrient absorption limitations in plants. Furthermore, nanobubbles improved the capability of soil oxygen input, leading to increased root activity and glycolysis efficiency in wheat roots. This work provides valuable insights into the influence of nanobubble watering on soil quality and crop production and offers an innovative approach for agricultural irrigation that enhances the effectiveness and efficiency of water application.

Assessing phytotoxicity and tolerance levels of ZnO nanoparticles on Raphanus sativus: implications for widespread adoptions

The escalating release of zinc oxide nanoparticles (ZnO NPs) into the environment poses a substantial threat, potentially leading to increased concentrations of zinc (Zn) in the soil and subsequent phytotoxic effects. This study aimed to assess the effects of ZnO NPs on Raphanus sativus (R. sativus) concerning its tolerance levels, toxicity, and accumulation. ZnO NPs were synthesized by the wet chemical method and characterized by powder X-ray diffraction (PXRD), Fourier-transform infrared (FTIR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy, dynamic light scattering (DLS), and scanning electron microscopy (SEM). The effect of ZnO NPs (70 nm) on R. sativus grown in coir was evaluated. The application of 1,000 mg/L of ZnO NPs resulted in a significant increase (p < 0.05) in soluble protein content, carbohydrates, chlorophyll a (Chl-a), chlorophyll b (Chl-b), total chlorophylls, carotenoids, and antioxidants by 24.7%, 58.5%, 38.0%, 42.2%, 39.9%, 11.2%, and 7.7%, respectively. Interestingly, this dose had no impact on the indole acetic acid (IAA) content. Conversely, the use of 2,000 mg/L of ZnO NPs in the same medium led to a significant reduction (p < 0.05) in soluble protein content by 23.1%, accompanied by a notable increase in IAA by 31.1%, indicating potential toxicity. The use of atomic absorption spectroscopy confirmed the internalization of zinc in seedlings, with a statistically significant increase (p < 0.05). In control plants without ZnO NPs, Zn concentration was 0.36 mg/g, while at the highest ZnO NPs tested dose of 10,000 mg/L, it significantly rose to 1.76 mg/g, causing leaf chlorosis and stunted seedling growth. This suggests potential health risks related to Zn toxicity for consumers. Given the adverse effects on R. sativus at concentrations above 1000 mg/L, caution is advised in the application and release of ZnO NPs, highlighting the importance of responsible practices to mitigate harm to plant life and consumer health. The study demonstrated the tolerance of R. sativus to high Zn levels, classifying it as a Zn-tolerant species.

Microplastic pollution in terrestrial ecosystems: Global implications and sustainable solutions

Microplastic (MPs) pollution has become a global environmental concern with significant impacts on ecosystems and human health. Although MPs have been widely detected in aquatic environments, their presence in terrestrial ecosystems remains largely unexplored. This review examines the multifaceted issues of MPs pollution in terrestrial ecosystem, covering various aspects from additives in plastics to global legislation and sustainable solutions. The study explores the widespread distribution of MPs worldwide and their potential antagonistic interactions with co-occurring contaminants, emphasizing the need for a holistic understanding of their environmental implications. The influence of MPs on soil and plants is discussed, shedding light on the potential consequences for terrestrial ecosystems and agricultural productivity. The aging mechanisms of MPs, including photo and thermal aging, are elucidated, along with the factors influencing their aging process. Furthermore, the review provides an overview of global legislation addressing plastic waste, including bans on specific plastic items and levies on single-use plastics. Sustainable solutions for MPs pollution are proposed, encompassing upstream approaches such as bioplastics, improved waste management practices, and wastewater treatment technologies, as well as downstream methods like physical and biological removal of MPs. The importance of international collaboration, comprehensive legislation, and global agreements is underscored as crucial in tackling this pervasive environmental challenge. This review may serve as a valuable resource for researchers, policymakers, and stakeholders, providing a comprehensive assessment of the environmental impact and potential risks associated with MPs.

Integrating Chlorophyll a Fluorescence and Enzymatic Profiling to Reveal the Wheat Responses to Nano-ZnO Stress

It has been shown that increased concentrations of zinc oxide nanoparticles (nano-ZnO) in the soil are harmful to plant growth. However, the sensitivity of different wheat cultivars to nano-ZnO stress is still unclear. To detect the physiological response process of wheat varieties with different tolerance to nano-ZnO stress, four wheat cultivars (viz., cv. TS1, ZM18, JM22, and LM6) with different responses to nano-ZnO stress were selected, depending on previous nano-ZnO stress trials with 120 wheat cultivars in China. The results found that nano-ZnO exposure reduced chlorophyll concentrations and photosynthetic electron transport efficiency, along with the depressed carbohydrate metabolism enzyme activities, and limited plant growth. Meanwhile, the genotypic variation in photosynthetic carbon assimilation under nano-ZnO stress was found in wheat plants. Wheat cv. JM22 and LM6 possessed relatively lower Zn concentrations and higher leaf nitrogen per area, less reductions in their net photosynthetic rate, a maximum quantum yield of the PS II (Fv/Fm), electron transport flux per cross-section (ETo/CSm), trapped energy flux per cross-section (TRo/CSm), and total soluble sugar and sucrose concentrations under nano-ZnO stress, showing a better tolerance to nano-ZnO stress than wheat cv. TS1 and ZM18. In addition, the chlorophyll a fluorescence parameters Fv/Fm, ETo/CSm, and TRo/CSm could be used to rapidly screen wheat varieties resistant to nano-ZnO stress. The results here provide a new approach for solving the issues of crop yield decline in regions polluted by heavy metal nanoparticles and promoting the sustainable utilization of farmland with heavy metal pollution.

Combined effects of zinc oxide nanoparticles and melatonin on wheat growth, chlorophyll contents, cadmium (Cd) and zinc uptake under Cd stress

Zinc oxide nanoparticles (ZnONPs) and melatonin (MT) have been known to regulate heavy metal toxicities in plants in some studies, the effect of their combined use on cadmium (Cd) uptake by wheat (Triticum aestivum L.) and underlying mechanisms is largely unknown. Thus, plant growth, uptake and translocation of Cd mediated by soil applied ZnONPs and foliar applied MT were investigated in wheat grown in Cd polluted soil under ambient conditions. The results depicted that ZnONPs stimulated the growth, chlorophyll contents, and yield of wheat in a dose additive way and this effect was further increased with foliar application of MT. 100 mg/kg of ZnONPs alone enhanced the grain yield by 60.5 % and this increase was about 177.5 % under combined ZnONPs and 100 μM MT treatment. ZnONPs treatments decreased Cd concentration whereas increased zinc (Zn) concentrations in shoots, roots, husks and grains and the effect was further increased with exogenous MT combined with NPs in a dose-additive way. 50 and 100 mg/kg ZnONPs treatments alone decreased grain Cd by 6.5 %, and 20 % and increased the Zn concentration by 20.1 % and 24 % than control. 100 mg/kg ZnONPs +100 μM MT treatment decreased the grain Cd by 63.5 % and increased grain Zn by 51 % than control treatment. Total Cd uptake (tissues biomass × Cd concentration in respective tissues) in shoots, roots, husks and grains increased with ZnONPs alone or combined with MT than control whereas soil post-harvest bioavailable Cd concentration decreased with treatments than control. The Cd reduction in grains was due to increase in biomass and Zn concentration thereby decreasing bioavailable Cd in soil and its accumulation in plants. This study suggested that combined use of ZnONPs and MT may provide new approaches for minimizing Cd and biofortification of Zn in edible parts of plants.

Modulations in protein phosphorylation explain the physiological responses of barley (Hordeum vulgare) to nanoplastics and ZnO nanoparticles

To address the knowledge gap on the effects of the co-existence of nanomaterials on plant growth, barley (Hordeum vulgare L.) plants were irrigated with zinc oxide nanoparticles (0.5 g L^-1), nanoplastics (1 g L^-1), and the combination of these two nanomaterials for 10 days. The co-existence of nanoplastics and ZnO nanoparticles increased H₂O₂ concentration by 12.76% and 38.30%, compared with the ZnO nanoparticles and nanoplastics exposure. The concentration of abscisic acid (ABA) in plants under the co-existence of nanoplastics and ZnO nanoparticles was 29.53% and 10.42% higher than that in ZnO nanoparticles treated plants and nanoplastics treated plants. The global analysis of phosphoproteomics identified 132 phosphorylated proteins and 173 phosphorylation sites in barley leaves exposed to the nanomaterial combination, which were related to photosynthesis, carbon fixation, nitrogen metabolism, and arginine and proline metabolisms. Further physiological analysis indicated that the combination of ZnO nanoparticles and nanoplastics caused larger damage to the systems of antioxidant and carbohydrate metabolisms as exemplified by decreased activities of apoplastic peroxidases (25.10%-48.60%), glutathione reductase (91.07%-94.94%), and sucrose synthase (53.59%-61.19%) in roots and increased cell wall invertase activity (12.97%-17.61%) in leaves, compared with the single nanomaterial treatments. These results indicate that the modulations in protein phosphorylation were closely related to the physiological responses to nanomaterial exposure, suggesting that the co-existence of nanomaterials may lead to greater impacts than single ones.

ZnO nanoparticles as potential fertilizer and biostimulant for lettuce

Zn is an indispensable nutrient for crops that usually presents low bioavailability. Different techniques have been proposed to improve the bioavailability of Zn, including the use of nanofertilizers. The objective of the study was to evaluate the applications of drench (D) and foliar (F) ZnO nanoparticles (NZnO) compared to those of ionic Zn²⁺ (ZnSO₄) in lettuce. The plants cv. Great Lakes 407 was produced in pots of 4 L with perlite-peat moss (1:1) under greenhouse conditions. The treatments consisted of NZnO applications that replaced the total Zn provided with a Steiner solution, as follows: Zn²⁺ (100%D) (control); Zn²⁺ (50%D+50%F); NZnO (100%D); NZnO (50%D+50%F); NZnO (75%D); NZnO (50%D); NZnO (75%F) and NZnO (50%F). Four applications of Zn were made with a frequency of 15 days. 75 days after transplant (DAP), the fresh and dry biomass, chlorophyll a, b, and β-carotene, phenolics, flavonoids, antioxidant capacity, vitamin C, glutathione, H₂O₂, total protein, and enzymatic activity of PAL, CAT, APX, and GPX were evaluated. The mineral concentrations (N, P, K, Ca, Mg, S, Cu, Fe, Mn, Mo, Zn, Ni, and Si) in the leaves and roots of plants were also determined. The results showed that, compared to Zn²⁺, NZnO promoted increases in biomass (14-52%), chlorophylls (32-69%), and antioxidant compounds such as phenolics, flavonoids, and vitamin C. The activity of enzymes like CAT and APX, as well as the foliar concentration of Ca, Mg, S, Fe, Mn, Zn, and Si increased with NZnO. A better response was found in the plants for most variables with foliar applications of NZnO equivalent to 50-75% of the total Zn²⁺ applied conventionally. These results demonstrate that total replacement of Zn²⁺ with NZnO is possible, promoting fertilizer efficiency and the nutraceutical quality of lettuce.

The analysis of lysine succinylation modification reveals the mechanism of oxybenzone damaging of pakchoi (Brassica rapa L. ssp. chinensis)

Oxybenzone (OBZ), one of a broad spectrum of ultraviolet (UV) absorbents, has been proven to be harmful to both plants and animals, while omics analysis of big data at the molecular level is still lacking. Lysine succinylation (Ksuc) is an important posttranslational modification of proteins that plays a crucial role in regulating the metabolic network in organisms under stress. Here, we report the changes in intracellular Ksuc modification in plants under OBZ stress. A total of 1276 succinylated sites on 507 proteins were identified. Among these sites, 181 modified proteins were hypersulfinylated/succinylated in OBZ-stressed pakchoi leaves. Differentially succinylated proteins (DSPs) are distributed mainly in the chloroplast, cytoplasm, and mitochondria and are distributed mainly in primary metabolic pathways, such as reactive oxygen species (ROS) scavenging, stress resistance, energy generation and transfer, photosynthetic carbon fixation, glycolysis, and the tricarboxylic acid (TCA) cycle. Comprehensive analysis shows that Ksuc mainly changes the carbon flow distribution, enhances the activity of the antioxidant system, affects the biosynthesis of amino acids, and increases the modification of histones. The results of this study first showed the profiling of the Kusc map under OBZ treatment and proposed the adaptive mechanism of pakchoi in response to pollutants and other abiotic stresses at the posttranslational level, which revealed the importance of Ksuc in the regulation of various life activities and provides a reference dataset for future research on molecular function.

Modulation mechanism of phytotoxicity on Ipomoea aquatica Forssk. by surface coating-modified copper oxide nanoparticles and its health risk assessment

To evaluate the influence of surface coatings on nano-fertilizers uptake and their phytotoxicity to crops and its health risk to Chinese adults, trisodium citrate (TC) and polyethylene glycol (PEG) coatings were prepared on the surface of copper oxide nanoparticles (CuO NPs), respectively, with 100 and 500 mg/L of bare CuO NPs, TC-CuO NPs, and PEG-CuO NPs were exposed to soil-grown Ipomoea aquatica Forssk. Combined bio-transmission electron microscopy and micro-CT observed cellular migration of coated CuO NPs in symplastic and apoplastic pathways, as well as nanoparticles transported through vascular tissues to the above-ground parts. Since TC-CuO NPs had less inhibition on vascular phylogeny of I. aquatica roots which was determined by RT-qPCR, their migration in plants was more efficient, thus exhibiting greater phytotoxicity to shoots. Meanwhile, coatings significantly reduced the phytotoxicity of CuO NPs by stimulating plant antioxidant defense. The risk of CuO nano-fertilizers on human dietary safety was evaluated, the HQ > 1 in the 500 mg/L CuO NPs treatment indicated a potential health risk to Chinese adults, which was reduced by the coatings. This work explored for the first time the mechanism of coating effects on nanoparticles migration efficiency and phytotoxicity at the molecular level and demonstrated that the migration of nanoparticles between tissues could have an impact on phytotoxicity. It implied that coating can be tailored to target nanoparticles to specific regions of the plant. In addition, this study highlights the potential health risks associated with the consumption of I. aquatica fertilized with CuO NPs and provides valuable insights into the environmental applications of nano-fertilizers.

Effect of ZnO and CuO nanoparticles on the growth, nutrient absorption, and potential health risk of the seasonal vegetable Medicago polymorpha L

Background

Medicago polymorpha L., a seasonal vegetable, is commonly grown in China. The increasing use of nanoparticles (NPs) such as ZnO and CuO NPs in agriculture has raised concerns about their potential risks for plant growth and for human consumption. There is a lack of research on the effects of ZnO and CuO NPs on agronomic performance of Medicago polymorpha L. and their potential risks for human health.

Methods

In this study, different treatment concentrations of ZnO NPs (25, 50, 100, and 200 mg kg^-1) and CuO NPs (10, 25, 50, and 100 mg kg^-1) were used to determine their effects on the growth and nutrient absorption of Medicago polymorpha L., as well as their potential risk for human health.

Results

The results showed that ZnO and CuO NPs increased the fresh weight of Medicago polymorpha L. by 5.8-11.8 and 3.7-8.1%, respectively. The best performance for ZnO NPs occurred between 25-50 mg kg^-1 and the best performance for CuO NPs occurred between 10-25 mg kg^-1. Compared with the control, ZnO and CuO NPs improved the macronutrients phosphorus (P), potassium (K), magnesium (Mg), and calcium (Ca). The following micronutrients were also improved: iron (Fe), nickel (Ni), copper (Cu), zinc (Zn), and manganese (Mn), with the exception of nitrogen (N) accumulation. Low treatment concentrations exhibited more efficient nutrient uptake than high treatment concentrations. A comprehensive analysis showed that the optimum concentrations were 25 mg kg^-1 for ZnO NPs and 10 mg kg^-1 for CuO NPs. The potential non-carcinogenic health risk of Medicago polymorpha L. treated with ZnO and CuO NPs was analyzed according to the estimated daily intake (EDI), the hazard quotient (HQ), and the cumulative hazard quotient (CHQ). Compared with the oral reference dose, the EDI under different ZnO and CuO NPs treatments was lower. The HQ and CHQ under different ZnO and CuO NPs treatments were far below 1. This indicated that Medicago polymorpha L. treated with ZnO and CuO NPs did not pose any non-carcinogenic health risk to the human body. Therefore, ZnO and CuO NPs were considered as a safe nano fertilizer for Medicago polymorpha L. production according to growth analysis and a human health risk assessment.