Selenium toxicity stress-induced phenotypical, biochemical and physiological responses in rice plants: Characterization of symptoms and plant metabolic adjustment

Comprehensive physiology and proteomics analysis revealed the molecular toxicological mechanism of Se stress on indica and japonica rice

Selenium pollution can lead to a decrease in crop yield and quality. However, the toxicological mechanisms of high Se concentrations on crops remain unclear. This study aimed to elucidate the physiological and proteomic molecular responses to Se stress in Oryza sativa. The results showed that under selenium stress, enzymatic activities of catalase, peroxidase, and superoxide dismutase in indica rice decreased by 61%, 28%, and 68%, respectively. The contents of non-enzymatic antioxidant substances ascorbic acid, glutathione, cysteine, proline, anthocyanidin, and flavonoids were decreased by 13%, 39%, 46%, 32%, 20%, and 5%, respectively, which significantly inhibited the antioxidant stress process of plants. At the same time, the results of proteomics analysis showed that rice seedlings, under Se stress, are involved in photosynthesis, photosynthesis-antenna proteins, carbon fixation, porphyrin metabolism, glyoxylate, and dicarboxylate. The differentially expressed proteins in metabolism and glutathione metabolism pathways showed a downward trend. It significantly inhibited the anti-oxidative stress, photosynthesis, and energy cycling process in plant cells, destroyed the homeostasis balance of rice plants, and inhibited the growth and development of rice. This finding reveals the molecular toxicological mechanism of Se stress on rice seedlings and provides a possible way to improve Se-resistant rice seedlings.

Selenium Status of Southern Africa

Selenium is an essential trace element that exists in inorganic forms (selenite and selenates) and organic forms (selenoamino acids, seleno peptides, and selenoproteins). Selenium is known to aid in the function of the immune system for populations where human immunodeficiency virus (HIV) is endemic, as studies suggest that a lack of selenium is associated with a higher risk of mortality among those with HIV. In a recent study conducted in Zambia, adults had a median plasma selenium concentration of 0.27 μmol/L (IQR 0.14-0.43). Concentrations consistent with deficiency (<0.63 μmol/L) were found in 83% of adults. With these results, it can be clearly seen that selenium levels in Southern Africa should be investigated to ensure the good health of both livestock and humans. The recommended selenium dietary requirement of most domesticated livestock is 0.3 mg Se/kg, and in humans above 19 years, anRDA (recommended daily allowance) of 55 mcg Se/per dayisis recommended, but most of the research findings of Southern African countries have recorded low levels. With research findings showing alarming low levels of selenium in soils, humans, and raw feed materials in Southern Africa, further research will be vital in answering questions on how best to improve the selenium status of Southern African soils and plants for livestock and humans to attain sufficient quantities.

Selenium-Induced Enhancement in Growth and Rhizosphere Soil Methane Oxidation of Prickly Pear

As an essential element for plants, animals, and humans, selenium (Se) has been shown to participate in microbial methane oxidation. We studied the growth response and rhizosphere methane oxidation of an economic crop (prickly pear, Rosa roxburghii Tratt) through three treatments (Se0.6 mg/kg, Se2.0 mg/kg, and Se10 mg/kg) and a control (Se0 mg/kg) in a two-month pot experiment. The results showed that the height, total biomass, root biomass, and leaf biomass of prickly pear were significantly increased in the Se0.6 and Se2.0 treatments. The root-to-shoot ratio of prickly pear reached a maximum value in the Se2 treatment. The leaf carotenoid contents significantly increased in the three treatments. Antioxidant activities significantly increased in the Se0.6 and Se2 treatments. Low Se contents (0.6, 2 mg/kg) promoted root growth, including dry weight, length, surface area, volume, and root activity. There was a significant linear relationship between root and aboveground Se contents. The Se translocation factor increased as the soil Se content increased, ranging from 0.173 to 0.288. The application of Se can improve the state of rhizosphere soil's organic C and soil nutrients (N, P, and K). Se significantly promoted the methane oxidation rate in rhizosphere soils, and the Se10 treatment showed the highest methane oxidation rate. The soil Se gradients led to differentiation in the growth, rhizosphere soil properties, and methane oxidation capacity of prickly pear. The root Se content and Se translocation factor were significantly positively correlated with the methane oxidation rate. Prickly pear can accumulate Se when grown in Se-enriched soil. The 2 mg/kg Se soil treatment enhanced growth and methane oxidation in the rhizosphere soil of prickly pear.

Biochemical and molecular responses of the ascorbate-glutathione cycle in wheat seedlings exposed to different forms of selenium

Biofortification aims to increase selenium (Se) concentration and bioavailability in edible parts of crops such as wheat (Triticum aestivum L.), resulting in increased concentration of Se in plants and/or soil. Higher Se concentrations can disturb protein structure and consequently influence glutathione (GSH) metabolism in plants which can affect antioxidative and other detoxification pathways. The aim of this study was to elucidate the impact of five different concentrations of selenate and selenite (0.4, 4, 20, 40 and 400 mg kg-1) on the ascorbate-glutathione cycle in wheat shoots and roots and to determine biochemical and molecular tissue-specific responses. Content of investigated metabolites, activities of detoxification enzymes and expression of their genes depended both on the chemical form and concentration of the applied Se, as well as on the type of plant tissue. The most pronounced changes in the expression level of genes involved in GSH metabolism were visible in wheat shoots at the highest concentrations of both forms of Se. Obtained results can serve as a basis for further research on Se toxicity and detoxification mechanisms in wheat. New insights into the Se impact on GSH metabolism could contribute to the further development of biofortification strategies.

Selenium enhances ROS scavenging systems and sugar metabolism increasing growth of sugarcane plants

Selenium (Se) beneficial effect on plants is related to an increase in nitrogen (N) assimilation and its role as an abiotic stress mitigator by reactive oxygen species (ROS) scavenging enhanced by antioxidant metabolism. This study aimed to evaluate sugarcane (Saccharum spp.) growth, photosynthetic and antioxidant responses, and sugar accumulation in response to Se supply. The experimental design was a factorial scheme 2 × 4: two sugarcane varieties (RB96 6928 and RB86 7515) and four Se application rates (0; 5; 10 and 20 μmol L-1) applied as sodium selenate in the nutrient solution. Leaf Se concentration increased under Se application in both varieties. The enzymes SOD (EC 1.15.1.1) and APX (EC 1.11.1.11) showed increase activities under Se application on variety RB96 6928. Nitrate reductase activity increased in both varieties resulting in the conversion of nitrate into higher total amino acids concentration indicating an enhanced N assimilation. This led to an increased concentration of chlorophylls and carotenoids, increased CO2 assimilation rate, stomatal conductance, and internal CO2 concentration. Selenium provided higher starch accumulation and sugar profiles in leaves boosting plant growth. This study shows valuable information regarding the role of Se on growth, photosynthetic process, and sugar accumulation in sugarcane leaves, which could be used for further field experiments. The application rate of 10 μmol Se L-1 was the most adequate for both varieties studied considering the sugar concentration and plant growth.

Integrated agronomic, physiological, microstructure, and whole-transcriptome analyses reveal the role of biomass accumulation and quality formation during Se biofortification in alfalfa

Se-biofortified agricultural products receive considerable interest due to the worldwide severity of selenium (Se) deficiency. Alfalfa (Medicago sativa L.), the king of forage, has a large biomass, a high protein content, and a high level of adaptability, making it a good resource for Se biofortification. Analyses of agronomic, quality, physiological, and microstructure results indicated the mechanism of biomass increase and quality development in alfalfa during Se treatment. Se treatment effectively increased Se content, biomass accumulation, and protein levels in alfalfa. The enhancement of antioxidant capacity contributes to the maintenance of low levels of reactive oxygen species (ROS), which, in turn, serves to increase alfalfa's stress resistance and the stability of its intracellular environment. An increase in the rate of photosynthesis contributes to the accumulation of biomass in alfalfa. To conduct a more comprehensive investigation of the regulatory networks induced by Se treatment, the transcriptome sequencing of non-coding RNA (ncRNA) was employed to compare 100 mg/kg Se treatment and control groups. The analysis identified 1,414, 62, and 5 genes as DE-long non-coding RNAs (DE-lncRNA), DE-microRNAs (DE-miRNA), and DE-circular RNA (DE-circRNA), respectively. The function of miRNA-related regulatory networks during Se biofortification in alfalfa was investigated. Subsequent enrichment analysis revealed significant involvement of transcription factors, DNA replication and repair mechanisms, photosynthesis, carbohydrate metabolism, and protein processing. The antioxidant capacity and protein accumulation of alfalfa were regulated by the modulation of signal transduction, the glyoxalase pathway, proteostasis, and circRNA/lncRNA-related regulatory networks. The findings offer new perspectives on the regulatory mechanisms of Se in plant growth, biomass accumulation, and stress responses, and propose potential strategies for enhancing its utilization in the agricultural sector.

The importance of selenium in food enrichment processes. A comprehensive review

Selenium is an essential element that determines the proper life functions of human and animal organisms. The content of selenium in food varies depending on the region and soil conditions. Therefore, the main source is a properly selected diet. However, in many countries, there are shortages of this element in the soil and local food. Too low an amount of this element in food can lead to many adverse changes in the body. The consequence of this may also be the occurrence of numerous potentially life-threatening diseases. Therefore, it is very important to properly introduce methods that condition the supplementation of the appropriate chemical form of this element, especially in areas with deficient selenium content. This review aims to summarize the published literature on the characterization of different types of selenium-enriched foods. At the same time, legal regulations and prospects for the future related to the production of food enriched with this element are presented. It should be noted that there are limitations and concerns with the production of such food due to the narrow safety range between the necessary and the toxic dose of this element. Therefore, selenium has been treated with special care for a very long time. For this reason, the presented mechanisms of production processes related to increasing the scale of selenium supplementation should be constantly monitored. Appropriate monitoring and development of the technological process for the production of selenium-enriched food is very important. Such food should ensure consumer safety and repeatability of the obtained product. Understanding the mechanisms and possibilities of selenium accumulation by plants and animals is one of the most important directions in the development of modern bromatology and the science of supplementation. This is particularly important in the case of rational nutrition and supplementing the human diet with an essential element such as selenium. Food technology is facing these challenges today.

Foliar Application of Selenium Associated with a Multi-Nutrient Fertilizer in Soybean: Yield, Grain Quality, and Critical Se Threshold

Selenium uptake and its content in soybean grains are affected by Se application methods. This study evaluated the impact of Se foliar application combined with a multi-nutrient fertilizer (MNF) on soybean, establishing a Se threshold to better understand the relationship between Se content in grains and yield of two genotypes (58I60 Lança and M5917). Two trials were conducted in a 4 × 2 factorial design: four Se rates (0, 10, 40, 80 g Se ha-1) and two methods of foliar Se application (Se combined or not with MNF). Foliar fertilizers were applied twice, at phenological stages of beginning of pod development and grain filling. Grain yield increased with the application of MNF, yet Se rates increased Se contents linearly up to 80 g Se ha-1, regardless of the use of MNF. Lança and M5917 genotypes had grain Se critical thresholds of 1.0 and 3.0 mg kg-1, respectively. The application of Se favored higher contents of K, P, and S in grains of genotype Lança and higher contents of Mn and Fe in grains of genotype M5917. Our findings highlight the importance of addressing different Se fertilization strategies as well as genotypic variations when assessing the effects of Se on soybean yield and grain quality.

Advances in Research on the Involvement of Selenium in Regulating Plant Ecosystems

Selenium is an essential trace element which plays an important role in human immune regulation and disease prevention. Plants absorb inorganic selenium (selenite or selenate) from the soil and convert it into various organic selenides (such as seleno amino acids, selenoproteins, and volatile selenides) via the sulfur metabolic pathway. These organic selenides are important sources of dietary selenium supplementation for humans. Organoselenides can promote plant growth, improve nutritional quality, and play an important regulatory function in plant ecosystems. The release of selenium-containing compounds into the soil by Se hyperaccumulators can promote the growth of Se accumulators but inhibit the growth and distribution of non-Se accumulators. Volatile selenides with specific odors have a deterrent effect on herbivores, reducing their feeding on plants. Soil microorganisms can effectively promote the uptake and transformation of selenium in plants, and organic selenides in plants can improve the tolerance of plants to pathogenic bacteria. Although selenium is not an essential trace element for plants, the right amount of selenium has important physiological and ecological benefits for them. This review summarizes recent research related to the functions of selenium in plant ecosystems to provide a deeper understanding of the significance of this element in plant physiology and ecosystems and to serve as a theoretical basis and technical support for the full exploitation and rational application of the ecological functions of selenium-accumulating plants.

Iron oxide nanoparticles impart cross tolerance to arsenate stress in rice roots through involvement of nitric oxide

The growth and development patterns of crop plants are being seriously threatened by arsenic (As) contamination in the soil, and it also acts as a major hurdle in crop productivity. This study focuses on arsenate As(V) mediated toxicity in rice plants. Further, among the different type of NPs, iron oxide nanoparticles (FeO NPs) display a dose-dependent effect but their potential role in mitigating As(V) stress is still elusive. FeO NPs (500 μM) play a role in imparting cross-tolerance against As(V) induced toxicity in rice. Growth attributes, photosynthetic performance, nutrient contents and biochemical parameters were significantly altered by As(V). But FeO NPs rescued the negative consequences of As(V) by restricting its entry with the possible involvement of NO in rice roots. Moreover, results related with gene expression of NO(OsNoA1 and OsNIA1) and proline metabolism were greatly inhibited by As(V) toxicity. But, FeO NPs reversed the toxic effect of As(V) by improving proline metabolism and stimulating NO mediated up-regulation of antioxidant enzymes particularly glutathione-S-transferase which may be possible reasons for the reduction of As(V) toxicity in rice roots. Overall, it can be stated that FeO NPs may act as an As(V) barrier to restrict the As(V) uptake by roots and have the ability to confer cross tolerance by modulating various morphological, biochemical and molecular characteristics with possible intrinsic involvement of NO.

Nano-selenium enhances the antioxidant capacity, organic acids and cucurbitacin B in melon (Cucumis melo L.) plants

Pesticides are widely used in melon production causing safety issues around the consumption of melon and increasing pathogen and insect tolerance to pesticides. This study investigated whether a nano-selenium (Nano-Se) spray treatment can improve resistance to biological stress in melon plants, reducing the need for pesticides, and how this mechanism is activated. To achieve this, we examine the ultrastructure and physio-biochemical responses of two melon cultivars after foliar spraying with Nano-Se. Nano-Se treatment reduced plastoglobulins in leaf mesophyll cells, thylakoid films were left intact, and compound starch granules increased. Nano-Se treatment also increased root mitochondria and left nucleoli intact. Nano-Se treatment enhanced ascorbate peroxidase, peroxidase, phenylalanine ammonia lyase, β-1,3-glucanase, chitinase activities and their mRNA levels in treated melon plants compared to control plants (without Nano-Se treatments). Exogenous application of Nano-Se improved fructose, glucose, galactitol, stachyose, lactic acid, tartaric acid, fumaric acid, malic acid and succinic acid in treated plants compared to control plants. In addition, Nano-Se treatment enhanced cucurbitacin B and up-regulated eight cucurbitacin B synthesis-related genes. We conclude that Nano-Se treatment of melon plants triggered antioxidant capacity, photosynthesis, organic acids, and up-regulated cucurbitacin B synthesis-related genes, which plays a comprehensive role in stress resistance in melon plants.

Selenium Effect Threshold for Soil Nematodes Under Rice Biofortification

Crop biofortification with inorganic selenium (Se) fertilizer is a feasible strategy to improve the health of residents in Se-deficient areas. For eco-friendly crop Se biofortification, a comprehensive understanding of the effects of Se on crop and soil nematodes is vital. In this study, a rice pot experiment was carried out to test how selenite supply (untreated control (0), 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 100, or 200 mg Se kg^-1) in soil affected rice growth, rice Se accumulation, and soil nematode abundance and composition. The results showed that selenite supply (5-200 mg kg^-1) generally increased the number of rice tillers, rice yield, and Se concentrations in rice grains. In soil under 10 mg kg^-1 Se treatment, the genus composition of nematodes changed significantly compared with that in the control soil. With increased Se level (> 10 mg kg^-1), soil nematode abundance decreased significantly. Correlation analysis also demonstrated the positive relationships between soil Se concentrations (total Se and bioavailable Se) with rice plant parameters (number of rice tillers, rice yield, and grain Se concentration) and negative relationships between soil Se concentrations (total Se and bioavailable Se) with soil nematode indexes (nematode abundance and relative abundance of Tobrilus). This study provides insight into balancing Se biofortification of rice and soil nematode community protection and suggests the effective concentrations for total Se (1.45 mg kg^-1) and bioavailable Se (0.21 mg kg^-1) to soil nematode abundances at 20% level (EC20) as soil Se thresholds. At Se concentrations below these thresholds, rice plant growth and Se accumulation in the grain will still be promoted, but the disturbance of the soil nematodes would be negligible.

Selenium alleviates toxicity in Amaranthus hypochondriacus by modulating the synthesis of thiol compounds and the subcellular distribution of cadmium

As a beneficial element, Selenium (Se) reduces toxic cadmium (Cd) absorption in many crops, but the effects of Se on Cd hyperaccumulator plants are unclear. This study examined the effects of Se on Amaranthus hypochondriacus (K472). The results showed that Se increased antioxidant enzyme activities, reduced Cd concentrations and toxicity, restored cell viability, and enhanced photosynthesis; these effects increased the biomass of roots, stems, and leaves by 59.87%, 53.85%, 44.19%, respectively, and these values exceeded the biomass of roots and stems in untreated control plants by 56.69% and 15.37%, respectively. Moreover, Se promoted PC synthesis, stably chelated Cd in the form of PC3 and PC4 and transported PC-Cd to vacuoles. Furthermore, Se protected organelles and reduced Cd migration by increasing Cd levels in cell walls and vacuoles. Interestingly, although the Cd content in K472 was decreased, Se maintained the total extracted Cd concentrations and its remediation efficiency by improving biomass and increased tolerance to Cd by approximately 5 times. The experimental results provide novel insights and methods for mitigating toxicity, promoting growth, and broadening the engineering application scope of K472; these results also provide a theoretical basis for further application of Se in soil with high Cd concentrations.

Advances in "Omics" Approaches for Improving Toxic Metals/Metalloids Tolerance in Plants

Food safety has emerged as a high-urgency matter for sustainable agricultural production. Toxic metal contamination of soil and water significantly affects agricultural productivity, which is further aggravated by extreme anthropogenic activities and modern agricultural practices, leaving food safety and human health at risk. In addition to reducing crop production, increased metals/metalloids toxicity also disturbs plants' demand and supply equilibrium. Counterbalancing toxic metals/metalloids toxicity demands a better understanding of the complex mechanisms at physiological, biochemical, molecular, cellular, and plant level that may result in increased crop productivity. Consequently, plants have established different internal defense mechanisms to cope with the adverse effects of toxic metals/metalloids. Nevertheless, these internal defense mechanisms are not adequate to overwhelm the metals/metalloids toxicity. Plants produce several secondary messengers to trigger cell signaling, activating the numerous transcriptional responses correlated with plant defense. Therefore, the recent advances in omics approaches such as genomics, transcriptomics, proteomics, metabolomics, ionomics, miRNAomics, and phenomics have enabled the characterization of molecular regulators associated with toxic metal tolerance, which can be deployed for developing toxic metal tolerant plants. This review highlights various response strategies adopted by plants to tolerate toxic metals/metalloids toxicity, including physiological, biochemical, and molecular responses. A seven-(omics)-based design is summarized with scientific clues to reveal the stress-responsive genes, proteins, metabolites, miRNAs, trace elements, stress-inducible phenotypes, and metabolic pathways that could potentially help plants to cope up with metals/metalloids toxicity in the face of fluctuating environmental conditions. Finally, some bottlenecks and future directions have also been highlighted, which could enable sustainable agricultural production.

The combined toxicity and mechanism of multi-walled carbon nanotubes and nano zinc oxide toward the cabbage

The natural environment is a complex system, and there is never only one kind of nanomaterial entering the environment. However, many studies only considered the plant toxicity of one kind of nanomaterial and do not consider the influence of two or more kinds of nanomaterials on plant toxicity. Multi-walled carbon nanotubes (MWCNTs) and zinc oxide nanoparticles (ZnO NPs) are two common and widely used nanomaterials in water environment, so these two kinds of nanomaterials were chosen to explore the effects of their combined toxicity on cabbage. This study investigated the toxicity of MWCNTs combined with ZnO NPs on cabbage by measuring the length of roots and stems, chlorophyll content, oxidative stress, antioxidant enzyme activity, metal element content, and root scanning electron microscopy. The toxicity of single MWCNTs toward cabbage was attributed to direct oxidative damage, while the toxicity of single ZnO NPs toward cabbage was due to the high level of zinc concentration. Moreover, ZnO NPs (10 mg/L) ameliorated MWCNTs toxicity toward cabbage by improving the activity of antioxidant enzymes. ZnO NPs (50 and 100 mg/L) because of the high content of zinc disrupted the balance of other metals in the plant and increased the toxicity of MWCNTs. In conclusion, the combined toxicity of different concentrations and types of nanomaterials should be considered for a more accurate assessment of environmental risks.

Selenium Biofortification of Soybean Seeds Influences Physiological Responses of Seedlings to Osmotic Stress

Climate change poses a serious threat to agricultural production. Water deficit in agricultural soils is one of the consequences of climate change that has a negative impact on crop growth and yield. Selenium (Se) is known to be involved in plant defense against biotic and abiotic stress through metabolic, structural, and physiological activity in higher plants. The aim of this study was to investigate the physiological response of Se-biofortified soybean (Glycine max (L.) Merrill) seedlings under osmotic stress. For this research, we used biofortified soybean grain obtained after foliar Se biofortification in 2020. The experiment was conducted in a growth chamber with two cultivars (Lucija and Sonja) grown on filter paper in three replicates. The experiment was carried out with two watering treatments: distilled water (PEG-0) and 2.5% polyethylene glycol 6000 (PEG-2.5) on Se-biofortified seeds (Se) and nonbiofortified seeds (wSe). Contents of lipid peroxidation product (LP), free proline (PRO), total phenolic content (TP), ferric reducing antioxidant power (FRAP), and ascorbic acid (AA) were analyzed in 7-days-old seedlings. Significant differences were detected in the Se content of soybean grains between the two cultivars. A milder reaction to PEG-2.5 was observed in cultivar Lucija in both Se and wSe treatments, which might represent the mitigating effects of Se on osmotic stress in this cultivar. Contrarily, in cultivar Sonja, Se adversely affected all analyzed traits in the PEG-2.5 treatment. Ultimately, Se is a pro-oxidant in Sonja, whereas it represents an anti-oxidant in Lucija. In conclusion, different soybean cultivars show contrasting physiological reactions to both osmotic stress and Se. However, the activation of antioxidant pathways in Sonja can also be interpreted as added value in soybean seedlings as a functional food.

Antioxidant Enzymatic Activities and Growth Response of Quinoa (Chenopodium quinoa Willd) to Exogenous Selenium Application

Selenium is a trace element essential to many organisms, including higher plants. At low concentrations, it enhances growth and development; however, it is toxic at high concentrations. The development of crops with proper levels of selenium will be worth for both nutrition and Se-based therapeutics. This study aimed to investigate the morphological, physiological, and biochemical responses of the quinoa plant to 0, 2.5, 5, 10, and 20 mg/L of Na2SeO3·5H2O. Selenium at low concentrations (2.5 and 5 mg/L), quinoa plant showed a significant increase of growth parameters, relative water content, photosynthetic pigments, proline, total soluble sugars, and antioxidant enzymes activities as (superoxide dismutase (SOD), catalase (CAT), peroxidase (POD, ascorbate peroxidase (APX), and glutathione reductase (GR)), and contents of malondialdehyde (MDA) and H2O2 were reduced. However, high concentrations (10 and 20) mg/L caused a decrease in plant growth parameters, relative water content, and photosynthetic pigments. In contrast, excess selenium increased the oxidative stress monitored by hydrogen peroxide and lipid peroxidation levels. The enzymatic antioxidant system responded to the selenium supply significantly increased. Osmolytes compounds, such as total sugars and proline, increased in selenium-treated plants. The increase in these osmolytes compounds may show a defense mechanism for the osmotic readjustment of quinoa plants to mitigate the toxicity caused by selenium. This study shows the morphological and physiological responses that must be considered for success in the sustainable cultivation of quinoa plants in environments containing excess selenium.

The Effect of Foliar Selenium (Se) Treatment on Growth, Photosynthesis, and Oxidative-Nitrosative Signalling of Stevia rebaudiana Leaves

Selenium (Se) enrichment of Stevia rebaudiana Bertoni can serve a dual purpose, on the one hand to increase plant biomass and stress tolerance and on the other hand to produce Se fortified plant-based food. Foliar Se spraying (0, 6, 8, 10 mg/L selenate, 14 days) of Stevia plantlets resulted in slightly decreased stevioside and rebaudioside A concentrations, and it also caused significant increment in stem elongation, leaf number, and Se content, suggesting that foliar Se supplementation can be used as a biofortifying approach. Furthermore, Se slightly limited photosynthetic CO₂ assimilation (A_N, g_sw, C_i/C_a), but exerted no significant effect on chlorophyll, carotenoid contents and on parameters associated with photosystem II (PSII) activity (F_V/F_M, F₀, Y(NO)), indicating that Se causes no photodamage in PSII. Further results indicate that Se is able to activate PSI-cyclic electron flow independent protection mechanisms of the photosynthetic apparatus of Stevia plants. The applied Se activated superoxide dismutase (SOD) isoenzymes (MnSOD1, FeSOD1, FeSOD2, Cu/ZnSOD1, Cu/ZnSOD2) and down-regulated NADPH oxidase suggesting the Se-induced limitation of superoxide anion levels and consequent oxidative signalling in Stevia leaves. Additionally, the decrease in S-nitrosoglutathione reductase protein abundance and the intensification of protein tyrosine nitration indicate Se-triggered nitrosative signalling. Collectively, these results suggest that Se supplementation alters Stevia shoot morphology without significantly affecting biomass yield and photosynthesis, but increasing Se content and performing antioxidant effects, which indicates that foliar application of Se may be a promising method in Stevia cultivation.

Assessment of selenium spatial distribution using μ-XFR in cowpea (Vigna unguiculata (L.) Walp.) plants: Integration of physiological and biochemical responses

Low concentrations of selenium (Se) are beneficial for plant growth. Foliar Se application at high concentrations is toxic to plants due to the formation of reactive oxygen species (ROS). This study characterized Se toxicity symptoms using X-ray fluorescence (XRF) technique in response to foliar Se application in cowpea plants. Five Se concentrations (0, 10, 25, 50, 100 e 150 g ha^-1) were sprayed on leaves as sodium selenate. The visual symptoms of Se toxicity in cowpea leaves were separated into two stages: I) necrotic points with an irregular distribution and internerval chlorosis at the leaf limb border (50-100 g ha^-1); II) total chlorosis with the formation of dark brown necrotic lesions (150 g ha^-1). Foliar Se application at 50 g ha^-1 increased photosynthetic pigments and yield. Ultrastructural analyses showed that Se foliar application above 50 g ha^-1 disarranged the upper epidermis of cowpea leaves. Furthermore, Se application above 100 g ha^-1 significantly increased the hydrogen peroxide concentration and lipid peroxidation inducing necrotic leaf lesions. Mapping of the elements in leaves using the XRF revealed high Se intensity, specifically in leaf necrotic lesions accompanied by calcium (Ca) as a possible attenuating mechanism of plant stress. The distribution of Se intensities in the seeds was homogeneous, without specific accumulation sites. Phosphorus (P) and sulfur (S) were found primarily located in the embryonic region. Understanding the factors involved in Se accumulation and its interaction with Ca support new preventive measurement technologies to prevent Se toxicity in plants.