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Toxicity to terrestrial plants

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Description of key information

The most sensitive reliable EC50 value for thiourea plant toxicity is reported to be 52.1 mg/kg soil dw (Günther & Pestemer, 1990). This value is carried forward to the risk assessment. 

Key value for chemical safety assessment

Short-term EC50 or LC50 for terrestrial plants:
52.1 mg/kg soil dw

Additional information

Two reliable studies (Günther & Pestemer, 1990 - key study; Pestemer & Auspurg, 1986) are available for the assessment of thiourea. In addition, two further studies with a Klimisch 3 rating are reported as supportive data.

Günther & Pestemer (1990) published results on the effects of thiourea on seedling germination and growth of the monocot Avena sativa (oats) and the dicots Brassica rapa (turnip) and Lepidium sativum (garden cress) that were studied in three different bioassays (A: 3-d growth on moistened filter paper; B: 10–14 days growth in soil and vermiculite; C: 8-wk growth in hydroponic culture (Avena sativa only)).

The most sensitive results in the different bioassays are as follows:

- Bioassay A: Lepidium sativum, 3-d EC5 = ca. 100 mg/L

- Bioassay B: Brassica rapa, 10-d EC5 = 21.1 mg/kg soil dw (soil experiment)

- Bioassay C: Avena sativa, 8-wk EC5 = ca. 3 mg/kg soil dw

Respective EC50 values (where applicable) are as follows:

Bioassay A: Lepidium sativum: EC50 (3 d) = 1000 mg/L

Bioassay B: Brassica rapa (soil experiment): EC50 (10d) = 52.1 mg/kg soil dw (soil experiment); Avena sativa (soil experiment): EC50 (14 d) = 339.4 mg/kg soil dw

Pestemer and Auspurg (1986) investigated the effect of thiourea on seedling emergence and growth of the monocot Avena sativa and the dicots Brassica napus, Brassica rapa, Brassica campestris var. chinensis, and Lepidium sativum at nominal concentrations ranging between 0 and 1000 mg thiourea/kg soil dw in a 14-d study. The study design was similar to OECD 208 (1984). Depending on plant species, seed emergence and growth were affected by treatment with thiourea at concentrations of 100 mg/kg soil dw and/or 1000 mg/kg soil dw. The % inhibition in seed emergence and growth in the treated species as compared to the control ranged from ≤ 30 to ≥ 50 %. The only monocot species used in the test was Avena sativa with an EC50 of < 1000 mg thiourea/kg soil dw.

For most dicot species no effects were observed at 1 mg/kg thiourea/kg soil dw (Brassica napus, Lepidium sativum), or even stimulation (Brassica rapa). Negative effects (≤ 30–50 % inhibition) could be observed at concentrations of 100 mg thiourea/kg soil dw. Therefore, in a worst case approach, the 14-d EC50 is considered to be 100 mg thiourea/kg soil dw.

As the 10-d EC50 value of 52.1 mg/kg soil dw for Brassica rapa determined by Günther & Pestemer (1990) is more sensitive than the 14-d EC50 of 100 mg/k soil dw in the study by Pestemer & Auspurg (1986), the former is carried forward to the risk assessment.

Further supporting studies are available (Friesel et al., 1984; Korte and Freitag, 1984) which are, however, considered to be unreliable, and thus are disregarded in the hazard assessment (PNEC derivation).

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