Overview
This MDPI Agriculture review synthesises laboratory and greenhouse evidence on how exogenous selenium (Se) — applied as selenite (SeO₃²⁻), selenate (SeO₄²⁻), selenocysteine (SeCys), selenomethionine (SeMT), or selenium nanoparticles (SeNPs) — modulates uptake, translocation, antioxidant metabolism, osmolyte production, phytohormone signalling, secondary-metabolite biosynthesis, and mineral-nutrient acquisition in crop and model plants exposed to cadmium, lead, arsenic, chromium, nickel, copper, zinc, aluminium, and mercury. The paper is a narrative review with no independent measurements, no primary data extraction, and no quantitative meta-analysis; quantitative entries in Tables 1–3 are experimental treatment doses and qualitative response statements drawn from cited primary studies rather than occurrence values. It is ingested as an out-of-core-scope agronomic-mitigation-context reference following the Safety Evaluation of Absorbent Hygiene Pads: A Review on Assessment Framework and Test Methods precedent for methodology / context reviews without primary metal-occurrence data; metals: [], ingredients: [], and products: [] are correct because the paper contributes no measured contamination values to any HMI ingredient, product, or metal page. The review is potentially useful as a starting point for an HMI mitigation-lever entry on Se biofortification / Se co-application as an agronomic intervention to reduce heavy-metal accumulation in cereal and vegetable crops, but each specific mechanistic claim should be sourced from its underlying primary citation rather than from this review.
Key numbers
This is a narrative review with no primary measurements. The numerical content consists of (a) bibliometric counts of publications on Se / HM / Se+HM 2010–2022 from Dimensions software, (b) experimental treatment doses of Se and heavy metals tabulated from cited primary studies, and (c) the review’s own qualitative statements about Se concentration thresholds for beneficial vs toxic effects in plants.
Bibliometric framing (Figures 1–2, pages 3–4)
- Search platform: Dimensions software 2.6.0 (https://www.dimensions.ai/). Search terms: “selenium”, “heavy metal stress”, “stress tolerance”, “plant”. Export date: 10 May 2023. Year filter: 2010–2022. Country filter: past eight years.
- Figure 1: publications per year 2010–2022, with separate counts for Se-only, HM-only, and HM+Se. Selected year-totals as reported: 2010 (Se 1, HM 62, HM+Se 2); 2017 (Se 5, HM 164, HM+Se 2); 2021 (Se 19, HM 392, HM+Se 13); 2022 (Se 18, HM 423, HM+Se 28).
- Figure 2: country-wise totals over the past eight years (counts as reported, Se vs HM): USA (Se 62, HM 208), China (Se 53, HM 184), Russia (Se 33, HM 141), Bangladesh (Se 12, HM 85), Pakistan (Se 15, HM 126), India (Se 21, HM 138), Poland (Se 16, HM 59), Others (Se 34, HM 315).
Author-stated Se concentration thresholds for plants (pages 2, 6–7)
- “The low concentration (<1000 mg Se/kg DW) of Se can reduce metals’ toxic effects on plants and develop tolerance against stress by inducing different metabolic processes” (cited to ref [20]).
- “However, at higher concentration (>1000 mg Se/kg DW), Se affects plant growth and decrease the level of organogenesis, nucleic acid synthesis and protein synthesis” (cited to ref [20]).
- “Selenium might be useful for plant growth and development at low concentrations under optimum and unfavorable conditions” (cited to ref [17]); at high doses “it generates ROS and impairs several cellular activities” (cited to ref [99]).
Table 1 — Role of Se under HM stress conditions (page 6)
Treatment doses and qualitative response statements as tabulated by the authors from cited primary studies:
| Species | HM treatment | Se treatment | Reported response | Ref. |
|---|---|---|---|---|
| Brassica napus | 5 µM Cd + 500 µM Pb | 15 µM | Enhanced SOD and GSH-Px to minimise Cd/Pb-induced oxidative stress | [87] |
| Brassica juncea | 300 µM Cr | 4 µM | Strengthened SOD, CAT, APX, GPOX, GR, GST, DHAR, MDHAR, AsA, GSH against Cr stress | [31] |
| Brassica juncea | 100 or 200 µM Cd | 50 µM | Increment in CAT, APX, GR activity | [88] |
| Brassica spp. | 50 µM Cd | 3 µM | Induced ROS detoxification; maintained SOD, CAT, POD | [89] |
| Glycine max | 25 µM As | 25 µM | Reduced As toxicity by improving photosynthesis, antioxidants, regulation of defence genes | [27] |
| Vicia faba | 50 µM Pb | 6 µM | Up-regulated CAT, GPOX, GSH-Px | [86] |
| Phaseolus aureus | 10 µM As | 5 µM | Reduced As-induced oxidative stress by increasing CAT, APX, GR, AsA, GSH | [85] |
| Satureja hortensis | 150 µM Cd | 40 µM | Elevated CAT, POD; reduced Cd toxicity | [90] |
| Cucumis sativus | 25 µM Cd + 200 µM Ni + 100 µM Pb | 8 µM | Stimulated antioxidant system by enhancing CAT, APX, GPOX | [83] |
| Triticum aestivum | 50 µM Cd | 5 and 10 µM | Down-regulation of genes of Cd uptake and transport | [91] |
| Oryza sativa | 100 µM As | 25 µM | Up-regulated As-tolerant genes; induced antioxidant expression | [92] |
| Oryza sativa | 20 µM Cd | 1 µM | Up-regulated CAT and GSH-Px activity; reduced lipid oxidation | [93] |
| Oryza sativa | 25 µM As | 25 µM | Positively enhanced CAT, APX, GSH-Px, GR, GST, GSH | [94] |
| Lolium perenne | 0.2 µM Al | 5 µM | Reduced lipid peroxidation by increasing SOD and APX activity | [84] |
Table 2 — Role of phytohormones under heavy-metal stress (pages 9–10)
Selected entries as tabulated (HM dose / phytohormone dose / response, cited primary reference):
| Species | HM treatment | Phytohormone treatment | Response | Ref. |
|---|---|---|---|---|
| Brassica juncea | 24 µM As | 200 µL/L ethephon | Improved photosynthetic attributes, reduced As and ABA accumulation | [123] |
| Sedum alfredii | 100 µM Cd | 0.2 mg/L ABA | ABA enhanced endogenous ABA production; regulated Cd-tolerance genes | [119] |
| Populus × Canescens | 3 µM Pb | 10 µM ABA | Ameliorated Pb toxicity by minimising oxidative stress | [124] |
| Oryza sativa | 150 µM As | 3 µM IAA | Improved growth; accumulated more amino acids and proteins | [120] |
| Cajanus cajan | 5 µM Cu²⁺ | 1 nM JA | Improved photosynthesis, antioxidative system | [125] |
| Solanum lycopersicum | 3 µM Cd | 10 nM HBL | Improved overall growth; positively regulated N-metabolism | [126] |
| Zea mays | 50 µM Cd | 10⁻⁹ M IBA | Reduced Cd toxicity by inducing ROS detoxification; improved nutrient status | [127] |
| Vigna radiata | 60 µM Ni | 10⁻⁴ M GA₃ | Improved growth and biomass; reduced Ni uptake | [113] |
| Helianthus annuus | 4 µM U/Cd | 500 mg/L IAA | ROS detoxification; increased U/Cd uptake from soil | [122] |
| Brassica juncea | 50 µM Cd | 200 µL/L ethephon | Improved growth; induced antioxidants and amino-acid accumulation | [128] |
| Brassica juncea | 1.2 µM Cr | 200 µL/L ethephon | Mitigated Cr stress; reduced oxidative stress; enhanced proline accumulation | [15] |
| Hordeum vulgare | 10 µM Cd | 1 µM GR24 (strigolactone analogue) | Reduced Cd toxicity; improved photosynthesis and uptake of essential nutrients | [121] |
| Panicum virgatum | 10 µM Cd | 1 µM GR24 | Increment in photosynthetic parameters; stimulation of antioxidant system | [129] |
| Solanum lycopersicum | 150 µM Cd | 100 µM NO | Positive effect on photosynthesis; improved growth; osmoprotectants accumulation | [130] |
| Oryza sativa | 5 µM Cr | 0.1 nM 24-EBL | Induced Cd detoxification by enhancing photosynthesis | [131] |
Table 3 — Effect of Se on secondary-metabolite production (pages 12–13)
| Plant species | Se dose | Secondary metabolite | Response | Ref. |
|---|---|---|---|---|
| Mentha suaveolens | 10 µM | Essential oils (piperitenone oxide, limonene, jasmone) | Enhanced growth and SMs production | [158] |
| Brassica juncea | 4 µM | Phenols, flavonoids, anthocyanins | Improved photosynthesis via antioxidants/SMs against Cr stress | [31] |
| Brassica oleracea | 25 µM | Glucoraphanin | Reduced glucosinolate precursors; suppressed biosynthetic genes | [32] |
| Melissa officinalis | 5 µM | Essential oils (z-citral, citral, geranyl acetate) | Positive effect on growth at low concentration | [20] |
| Brassica oleracea | 10 µM | Phenolic compounds and glucosinolates | Improved growth and yield via antioxidants and SMs | [30] |
| Zea mays | 10 µM | Phenols and flavonoids | Induced accumulation of proteins, sugars, SMs | [159] |
| Oryza sativa | 25 µM | Phenolics (gallic, protocatechuic, ferulic acids) | Increased nutrient uptake; regulated SMs production; reduced As toxicity | [160] |
| Brassica juncea | 50 µM | Phenolic content | Reduced Cd stress via antioxidants and SMs | [88] |
| Allium sativum | 4 µM | Total phenolic content | Improved salt-stress tolerance via PAL regulation and membrane stability | [161] |
| Valerianella locusta | 5 µM | Flavonoids and phenolics | Improved growth and antioxidant activity | [162] |
Author-stated single-point quantitative claims (selected, pages 5–14)
- 2 µM Se “improved root growth and decreased oxidative stress in Al-treated Lolium perenne” (cited to ref [84], page 5 prose). Source-internal discrepancy: Table 1 (page 6) reports the same Lolium perenne / ref [84] entry as 5 µM Se reducing lipid peroxidation under 0.2 µM aluminum (Al), not 2 µM. The wiki preserves both figures as the review reports them; the underlying primary study (ref [84], Cartes et al. 2010 in Ann. Appl. Biol.) would need to be consulted directly to resolve which dose corresponds to which endpoint.
- 4 µM Se “alleviated Zn stress by improving growth and photosynthesis in Billbergia zebrina” (cited to ref [96], page 6).
- SeMT 3 µM “mitigated cadmium (Cd) toxicity in mustard species by enhancing enzymatic antioxidants and preventing Cd aggregation in cell organelles” (cited to ref [89]).
- Leaf spraying of Se (10, 20, 40 µM) “reduced Cd toxicity by limiting Cd uptake by roots and improved photosynthetic attributes, osmoprotectants and antioxidant levels in Satureja hortensis” (cited to ref [90]).
- Foliar Se in wheat leaves “inhibited metal uptake and enhanced nutritional status of wheat by accumulating more Se in the grains” (cited to ref [38]).
Methods (brief)
Narrative literature review covering 1982–2023. Search platform: Dimensions software 2.6.0; supplementary searches via PubMed, Google Scholar, Science Direct, and Scopus. Authors did not declare a PRISMA protocol, did not report inclusion/exclusion criteria beyond topical relevance to Se / HM / plants, and did not perform quantitative meta-analysis or independent data extraction. Bibliometric counts in Figures 1–2 were generated by Dimensions filters on year (2010–2022) and country (past eight years) for the search terms “selenium”, “heavy metal stress”, “stress tolerance”, and “plant”. Tables 1–3 reproduce treatment doses and qualitative response statements from cited primary studies as the authors interpreted them; concentration values reflect what the original experiments applied to plants rather than measured occurrence in food.
Implications
- Certification (HMTc): This review contributes no occurrence values to any HMTc product-category threshold. It is potentially useful as background reading for an HMTc educational note on agronomic interventions (Se biofortification, Se co-application, SeNP foliar treatments) that have been investigated as ways to reduce heavy-metal accumulation in cereal and leafy-vegetable crops. Any HMTc-side use of the specific mechanistic claims should be traced through this review to the underlying primary references rather than cited from the review itself.
- Courses: Useful as a reference for an educational module on agronomic mitigation levers — covering inorganic Se (selenite, selenate) versus organic Se (SeCys, SeMT) versus SeNP application routes, foliar versus root application trade-offs, dose ranges associated with beneficial versus toxic effects in plants (<1000 mg Se/kg DW beneficial; >1000 mg Se/kg DW toxic, per cited ref [20]), and the cross-talk among Se, phytohormones, osmolytes, and secondary metabolites. The tables provide a starting catalogue of crop / metal / Se-dose / response combinations reported in the primary literature.
- App: Not applicable. No per-product or per-ingredient occurrence values; no consumer-facing exposure estimates.
- Microbiome: Not addressed. Out of scope for Microbiome federation.
- Synthesis: No synthesis triggers fire from this ingest. The paper does not contribute primary occurrence values to any ingredient
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