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Toxicity to terrestrial plants

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Endpoint:
toxicity to terrestrial plants: short-term
Type of information:
experimental study
Adequacy of study:
other information
Study period:
Not reported
Reliability:
4 (not assignable)
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Remarks:
The study assessed the effects of exposure to methyl silic acids on growth and development of wheat seedlings and on susceptibility of the seedling to infection by powdery mildew. The methods used were not in accordance with any standard guideline. It is difficult to assess the significance of the results for the standard REACH endpoints. The study did not include analysis of exposure concentrations and it is unlikely that it was conducted under GLP.
Qualifier:
no guideline followed
Principles of method if other than guideline:
Wheat seedlings were grown under peat-based and hydroponic systems and exposed to methyl silanols.The silanols were assessed for their potential to protect against powdery mildew infection but in the course of the tests the harmful effects of the substances on the plants were also assessed.
GLP compliance:
not specified
Analytical monitoring:
no
Vehicle:
no
Details on preparation and application of test substrate:
Testing was carried out in two substrates

Peat-based substrate: Premier Pro-Mix BX, Premier Horticulture, Quebec, Canada

Hydroponic system: Vermiculite
Species:
Triticum aestivum
Plant group:
Monocotyledonae (monocots)
Details on test organisms:
Wheat: Triticum aestivum L., cultivar AC Drummond
Test type:
other: The study involved the study of plant growth and development in a peat-based system and an assessment of the susceptibility to mildew infection in a hydroponic system
Study type:
laboratory study
Substrate type:
other: Peat-based substrate and hydroponic system using vermiculite substrate
Limit test:
no
Post exposure observation period:
Peat-based system: the exposure period commenced 18-days after sowing and lasted for a further 10 days

Hydroponic system: the exposure period commenced three weeks after sowing and continued for 16 days.
Test temperature:
22°C for 16-h in the light and 18°C for 8-h in the dark
pH:
6.0
Details on test conditions:
Peat-based system: Seeds were sown in peat in 8 cm pots with 10 seeds per pot. The pots were kept in growth chambers (22°C for 16-h in the light and 18°C for 8-h in the dark, 80% humidity) and watered three times per week with distilled water (lees than 5 mg/L Si) until seedling emergence and then with a Hoagland solution. From the 18th day after sowing onward plants were watered (1 L/week) with Hoagland solutions containing 1.7 mM of the test substances. The effect of these treatments on wheat plant growth and development was assessed by visual observation of phytotoxicity symptoms on plants 10 days after the start of the treatments (i.e. day 28). Leaf samples were also taken for Si presence by X-ray microanalysis mapping.

Hydroponic system: Seeds were sown in 9cm pots in vermiculite at a rate of 3 seeds per pot. Each hydroponic system comprised of 10 pots and was designed to immerse roots in nutrient system for 15 mins every 30 mins. The plants were grown in a greenhouse (22°C for 16-h in the light and 18°C for 8-h in the dark, 80% humidity). Three independent replicates of the experiment were performed. seeds were watered with double-distilled water (lees than 5 mg/L Si) at the start of the experiment. One week after seedling emergence (day 10) plants were fed with Hoagland solution that was made with distilled water. Three weeks after sowing (day 21) plants were fed with Hoagland solutions containing either no silicon (Control) or 1.7 mM of ortho silicic acid or 1.7 mM of the monomethyl- or trimethyl silicic acids. The dimethyl silicic acid was not included in the treatments because of extensive phytotoxicity observed in the peat-based exposure system. The silicic acids were added to the the nutrient twice per week. Seven days after the commencement of the silicic acid treatments the plants were inoculated with mildew spores.

Scanning electron microscopy was used to determine Si deposition in the leaves.

Silicon content of plants was determined by ICP-optical emission spectrometry.

Disease development was assessed visually each day after inoculation.

Nominal and measured concentrations:
Peat-based system: 1.7 mM for each substance (other treatments as low as 0.1 mM were also used but the results are not reported in detail).

Hydroponic system: 1.7 mM for monomethyl and trimethyl silic acid. Dimethyl silic acid was not included in this part of the study.
Reference substance (positive control):
no
Reported statistics and error estimates:
Statistical analyses were performed using SAS software version 9.1.

Peat-based system: Visual observations of plants treated with ortho-, monomethyl- and trimethyl silicic acids in the peat-based systems showed no impairment of growth or development compared to the controls. However the dimethyl silic acid induced the appearance of yellow streaks on leaves which subsequently dried out and died. Furthermore, concentrations of the substance as low as 0.1 mM still induced symptoms of phytotoxicity (results not detailed in the publication).

Hydroponic system: X-ray analysis of the plants grown using Si-free Hoagland solution showed no deposition of Si in the leaves. Plants grown with the addition of orth silicic acid showed the presence of silicaphile cells in the leaves. Treatment with monomethyl silicic acid resulted in Si deposits in small pockets that were localised to the areas of mildew presence. Those treated with trimethyl silicic acid showed no visible deposits of Si within the leaves.

Analysis of silicon uptake by the plants showed that the highest concentrations were found in the ortho silicic acid treatment. Concentrations in the Control and trimethyl silic acid treatments were similar while those in the monomethyl silicic acid treatment were intermediate.

Orthosilicic acid provided the highest level of protection against infection by powdery mildew while monomethyl- and trimethyl silicic acids treatments were actually associated with higher levels of infection compared to the Control.

Validity criteria fulfilled:
not applicable
Conclusions:
Exposure of wheat plants to aqueous solutions of silicone degradation products (monmethyl-, dimethyl- and trimethyl silicic acids), even at a concentration of 0.1 mM in the case of dimethyl silicic acid, can induce phytotoxic symptoms and interfere with plant growth.
Endpoint:
toxicity to terrestrial plants: long-term
Data waiving:
study scientifically not necessary / other information available
Justification for data waiving:
other:

Description of key information

There is no reliable study available with the registered substance and its hydrolysis products. However, a read-across study is available with aqueous solutions of silicone degradation products. In a short-term toxicity to terrestrial plants study with the aqueous read-across solutions of silicone degradation products, not conducted according to any guideline and not specified if in compliance with GLP, it was concluded that the test substance could induce phytotoxic symptoms and interfere with plant growth (Côté-Beaulieu et al., 2009).

Key value for chemical safety assessment

Additional information

There is no reliable study available with the registered substance or its hydrolysis products, and the endpoint is waived.

However, a read-across study is available with aqueous solutions of silicone ortho- monomethyl- dimethyl- and trimethylsilicic acids, which correspond to silicic acid, methylsilanetriol, dimethylsilanediol and trimethylsilanol respectively. In the study wheat plants Triticum aestivum L. were exposed with Hoagland solutions containing the test substances in peat based systems and in hydroponic systems. During the hydroponic exposures, plants were infected with fungus mildew. The study was not conducted according to any guideline, and there is no indication it was conducted according to GLP. The study concluded the dimethylsilanediol could induce phytotoxic symptoms and interfere with plant growth. The methods used were not in accordance with any standard guideline, exposure was to aqueous solutions and silica deposits were also detected in the shoots of the controls in the peat-based studies. It is difficult to assess the significance of the results for the standard REACH endpoints. Therefore, the study is considered reliability 4 and the data are not considered to be relevant for the risk assessment.