Gómez-Arroyo et al. 2017 — Dandelion as heavy metal biomonitor, Mexico City metropolitan area

Gómez-Arroyo and colleagues used leaves of Taraxacum officinale (common dandelion) as a passive biomonitor to assess atmospheric heavy metal deposition across two contrasting zones of the Mexico City Metropolitan Area (MAMC): an urban campus site (Centro de Ciencias de la Atmósfera, CCA-UNAM) and a peri-urban high-altitude station (Altzomoni, Iztaccíhuatl-Popocatépetl National Park, ~60 km from CCA). Plants were greenhouse-propagated from seed, exposed in groups of 20 for six weeks in each season, then analysed for eight metals (Al, Cd, Cr, Fe, Mn, Ni, Pb, V) by graphite-furnace atomic absorption spectrophotometry. The authors paired the chemical measurements with molecular stress indicators — microRNA-398 (miR398) expression, superoxide dismutase 2 (CSD2) expression, and reduced MTT as a free-radical proxy — to evaluate whether the leaf-tissue metal loads were biologically significant. The combination of chemical measurement and plant stress physiology is the paper’s methodological novelty.

Key numbers

Concentrations are reported in µg/g of leaf tissue (samples were oven-dried and acid-digested before measurement; the figure caption does not explicitly state dry-weight basis but the digestion protocol implies it). All values below are from Fig. 1 and the Results narrative; the paper presents the data primarily as bar charts with error bars rather than tabulated means.

Altzomoni, dry season (peri-urban, high altitude): Al = 932.3 µg/g (highest concentration measured in the study, attributed in part to volcanic dust from nearby Popocatépetl); Mn = 289.1 µg/g; Cr = 241 µg/g; Cd, Ni also elevated significantly versus control (p < 0.001). Table 1 lists Al, Cd, Cr, Mn, Ni as the five metals significantly increased at Altzomoni in the dry season.

Altzomoni, rainy season: three metals predominated (Fe, Ni, Pb per Table 1), with Ni at 385 µg/g the highest of the three. Lead was significantly increased at Altzomoni in both rainy and dry seasons (Fig. 1 Pb panel and Results narrative).

CCA-UNAM, dry season (urban core): three metals significantly increased (Cd, Mn, V per Table 1), with vanadium especially elevated at 2181 µg/g — consistent with petroleum-combustion emissions from urban traffic. Cd at CCA dry was the highest Cd value in the dataset (Fig. 1 Cd panel, ~150 µg/g vs ~75 µg/g at Altzomoni dry).

CCA-UNAM, rainy season: only Mn and V significantly increased versus control (Table 1).

Reported instrument detection limits (from the Validation of the method section; units not explicitly stated, but inferred to be µg/L in the digest solution given calibration curve at 5–20 µg/L): Al = 6.03, Cd = 0.07, Cr = 0.38, Fe = 2.04, Mn = 0.46, Ni = 0.78, Pb = 1.14, V = 3.12.

Molecular stress markers (Table 1):

miR398 expression was reduced at all four station-season combinations relative to control plants (control miR398 = 1.0 by construction); CSD2 expression was sharply upregulated in dry-season samples at both stations (12.0× at CCA dry, 16.25× at Altzomoni dry), tracking the higher metal loads.

Methods (brief)

Plants were propagated from T. officinale seed in sterile soil in a greenhouse at CCA-UNAM at 18.5–31.12 °C; specimens 10–15 cm tall were transferred for exposure. Two groups of 20 plants each were placed at each station in each season; one group went to chemical analysis, the other to molecular analysis. Leaves were washed with deionized water and dried before acid digestion of 0.5 g sub-samples in 15 mL of 3.0 M HNO₃ in a CEM MARS 5 PTFE-flask microwave digestion system (HP-500), diluted to 25 mL final volume in 0.45 N HNO₃. Metals were quantified by graphite-furnace atomic absorption spectrophotometry (GBC AVANTA g coupled to a PAL3000 autosampler) with deuterium-lamp background correction and hollow-cathode lamps; calibration used NIST-traceable standards prepared in five dilutions across 5–20 µg/L. Detection limits were defined as Y_B + 3·S_b (background signal plus three standard deviations of the background).

Total RNA was extracted with Trizol and quantified by NanoDrop ND-100 (260/280 > 1.8 for all samples). miR398 was reverse-transcribed using the Takara Mir-X miRNA First-Strand Synthesis Kit and quantified by SYBR Green real-time PCR on a Stratagene Mx300SP thermocycler with U6 snRNA as constitutive reference. CSD2 was reverse-transcribed using the Thermo Scientific First Strand cDNA Synthesis Kit and quantified by SYBR Green qPCR (Thermo Maxima kit) on a BIORAD MJ Mini thermocycler with KNOX-1 of Taraxacum as constitutive reference; relative expression was computed by the 2^(-ΔΔCt) algorithm (Livak & Schmittgen 2001). MTT-reduction assay used 1 cm² of leaf homogenised in 0.9% saline, centrifuged at 10,000 rpm, incubated with 5 mg/mL MTT in PBS for 24 h at 37 °C, dissolved in HCl/NP40/isopropanol, and quantified at 590 nm with 620 nm background correction.

The paper does not separate Cr-VI from total Cr (frontmatter therefore records Cr, not Cr-VI). Arsenic and mercury were not measured. Because the concentration data are presented primarily as figures rather than tabulated values, exact means below the values quoted above must be read off Fig. 1 with attendant precision loss; this limits the paper’s utility for quantitative occurrence synthesis but does not undermine its biomonitoring conclusions.

Relevance to this wiki

This paper is not a food occurrence study. Dandelion leaves consumed as food (which is a practice in parts of Europe and Mexico) would be one application of the data, but the paper does not frame it that way. The primary relevance to this wiki is threefold. First, it provides atmospheric-deposition context for the supply-chain framing: urban and peri-urban growing areas in heavily polluted basins like MAMC can expose leafy crops to substantial metal loads from atmospheric sources independently of soil metal status. Second, the dry-season versus rainy-season variation provides empirical support for seasonal-variance language on ingredient pages for leafy vegetables grown in highly urbanised airsheds. Third, the molecular stress-response data (miR398 reduction paired with CSD2 upregulation, and elevated reduced-MTT in dry-season samples) illustrate the plant-side oxidative stress that accompanies metal accumulation, relevant to mechanism framing on the leafy-vegetables and fresh-herbs pages.

The geographic signal in the data — V dominance at the urban CCA site (gasoline/petroleum combustion), Al/Mn/Cr/Cd/Ni dominance at the Altzomoni high-altitude site (volcanic dust plus long-range transport) — is the more specific contribution. The Pb signal is notable: Pb was significantly elevated only at Altzomoni in both seasons, not at the urban CCA site, which the authors attribute to the rough/pubescent leaf surface of dandelion preferentially trapping airborne Pb particles at the higher-altitude site.

Implications

Supply chain: supports soil and any future atmospheric-deposition page with evidence that atmospheric deposition can be a significant, geography-specific metal source for leafy crops independent of soil metal status. The V signal at CCA dry (2181 µg/g) is a particularly clean illustration of petroleum-combustion provenance for a metal that is otherwise uncommon in soil-only literature.

Ingredients: marginal direct relevance to leafy-vegetables and fresh-herbs for the urban-Mexico geographic context; not a food occurrence study, so should not contribute to contamination_profile values.

Microbiome: none.

Verification notes

This page was originally drafted 2026-05-15 from a thinner pass over the source. Re-ingested 2026-05-18 with the full PDF re-read to correct the following defects:

• Frontmatter metals previously listed only [Pb, Cd, Al, Cr, Ni, Mn]; Fe and V were measured and reported (Fig. 1, eight-metal panel) and have been added.
• Previous body text stated “CCA showing higher Pb in the rainy season … attributed to atmospheric re-suspension of street dust.” The paper does not say this. Table 1 lists Pb as significantly increased only at Altzomoni in both seasons; Pb at CCA was not significantly above control. The atmospheric-re-suspension attribution is not in the source. Corrected.
• Previous body text listed the five metals elevated at Altzomoni in the dry season as “aluminum, cadmium, chromium, nickel, and lead.” Table 1 of the source lists Al, Cd, Cr, Mn, Ni (manganese, not lead). Pb was elevated at Altzomoni but in both seasons, separately. Corrected.
• Previous body text framed CCA as showing “high vanadium and cadmium concentrations” — Table 1 actually lists three metals elevated at CCA dry (Cd, Mn, V). Corrected and Mn added.
• Previous version omitted reported instrument detection limits, which the source does provide (Validation of the method section). Added.
• Previous version lacked a ## Key numbers section as required by the source-page template. Added with values from Fig. 1, Table 1, and the Results narrative.
• sample_n previously null; set to 20 (plants per group per station per season, per Materials and methods).

Wiki pages updated on ingest

• soil

Page history

The five most recent substantive edits to this page. The full version history lives in git; when DOI minting comes online (see schema docs), each entry below will also link to a version-pinned DataCite DOI.