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EC number: 200-543-5
CAS number: 62-56-6
The effect of thiourea on the seedling
germination and growth of the monocot Avena sativa (oats) and the
dicots Brassica rapa (turnip) and Lepidium sativum (garden
cress) was studied in three different bioassays.
A 3-d in-vitro test according to Moewus
(1949) and Neururer (1975) with Lepidium sativum was conducted to
assess the inhibition of germination and seedling development. Nine
concentrations (4 replicates) of thiourea covering 'no reaction' to
'total inhibition' of germination including untreated controls were
tested using moistened filter paper.
Growth test, conducted with oats (Avena
sativa) and turnips (Brassica rapa) for 10 and 14 days,
respectively, in soil and non-sorptive substrate (vermiculite). Plants
were cultivated in self-watering pots containing 70 g dry sandy loam
soil or 70 ml vermiculite. Thiourea was tested at nine concentrations in
4 replicates. After 10 and 14 days the shoot fresh weight was recorded
for turnips and oats, respectively.
A long-term (8 week) bioassay with oats in
hydroponic culture was developed to examine whether extended exposure
has an effect (dose-response-time relationship) on germination and
growth. Concentrations that showed slight or no effects in bioassays (A)
and (B) were used to determine the influence of long-term exposure. Oats
were cultivated in Quickpot plates that were placed in 1 L plastic
containers containing a nutrient-chemical solution. Six plants per
container were transplanted. The nutrient-chemical solution was renewed
once a week to maintain constant chemical and nutrient concentrations.
The total fresh weight (shoot and root) of all plants per container was
recorded weekly, by weighing the whole system. Thiourea was applied at
eight concentrations in six replicates.
The most sensitive results in the different
bioassays are as follows:
- Bioassay A: Lepidium sativum, 3-d EC50
= ca. 1000 mg/L (based on growth)
- Bioassay B: Brassica rapa, 10-d EC50
= 52.1 mg/kg soil dw (based on growth; soil experiment)
- Bioassay C: Avena sativa, 8-wk EC5
= ca. 3 mg/kg soil dw (based on growth)
The test design for bioassay (B) is
considered most relevant as incubation conditions are similar to real
world field conditions. Therefore, the 10-d EC50 for Brassica
rapa of 52.1 mg/kg soil dw is carried forward to the risk
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.
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)).
- 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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