Garden cress was grown hydroponically in molybdenum-spiked tap water and in pooled groundwater from farm wells in La Pampa, Argentina, where molybdenum, arsenic and vanadium occur naturally at high levels. Cress molybdenum rose with the concentration in the growth solution, with toxicity signs only at very high doses. Plants grown in the real groundwater took up much less molybdenum than plants grown in synthetic solutions of similar strength, which the authors attribute to competing elements in the groundwater.
Key numbers
- Pooled La Pampa groundwater: 65.0 ± 0.6 µg/L (GW1) and 92.5 ± 2.4 µg/L (GW2) molybdenum, mean ± SD, n = 3 (Table 4, p. 48936).
- The same groundwater carried 969 and 23.7 µg/L arsenic and 432 and 278 µg/L vanadium (Table 4, p. 48936).
- Cress grown in the groundwater: 1.89 ± 0.20 mg/kg (GW1) and 4.59 ± 0.48 mg/kg (GW2) molybdenum, dry weight, n = 2 (Table 4, p. 48936).
- Cress grown in tap water spiked to 150 µg/L molybdenum at pH 7: 33.0 ± 3.9 mg/kg dry weight (Table 4); 35.2 mg/kg dry weight average in the control experiments (Abstract, p. 48929).
- Across spiked solutions from below 0.1 to 7000 µg/L, cress molybdenum rose from 1.29 to 821 mg/kg dry weight (r = 0.9531); stunted roots appeared at 5000 and 7000 µg/L (Results, p. 48932).
- Cress grown in synthetic solutions of 50 and 100 µg/L molybdenum contained 8.2 and 16.6 mg/kg dry weight (Discussion, p. 48938).
- Reference values cited by the authors: typical plant range 0.7 to 2.5 mg/kg dry weight; former WHO drinking-water guideline 70 µg/L; FAO irrigation-water limit 10 µg/L (pp. 48929, 48937; secondary).
- Limit of detection for molybdenum in solution: 0.1 µg/L (Methods, p. 48931).
Methods (brief)
Cress seeds were grown for 12 days in hydroponic vials with tap water, a commercial plant nutrient solution, spiked solutions (with and without iron, manganese and aluminum) and two pooled groundwater samples. Dried plants were acid-digested and analyzed by ICP-MS, validated against NIST SRM 1640a. Partial least squares models related solution chemistry to plant molybdenum.
Evidence fitness
Supports statements that some La Pampa groundwater is close to or above the former WHO value for molybdenum and that co-occurring elements, especially vanadium and arsenic, can lower plant uptake. Cress is a model plant grown hydroponically for 12 days, so the values are not field-crop or food-market concentrations. Only two pooled groundwater samples were used.
Limitations
Small replicate numbers (n = 2 for plant tissue). Laboratory hydroponic conditions differ from soil. The groundwater results are not a survey of La Pampa wells. Funding was partly from a Malaysian ministry; the authors declare no competing interests.
Related evidence
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