Reaction mass of ethanol and propan-2-ol

Toxicity to terrestrial plants

Description of key information

Ethanol:
No data is available that expresses results in terms of mg/kg soil - all tests carried out using exposure to plants as aqueous solutions.
Allium cepa (6 day, growth).  EC50=11800mg/l, EC10=790mg/l
Lactuca sativa (3 day, germination), EC50=5382mg/l
Zea mays (9hr, growth and respiration), EC50>11800mg/l, EC10=790mg/l
Avena sativa, saccharum officinarum: no growth inhibition
Solanum tuberosum: no respiration inhibition
Pisum sativum: no germination inhibition
Isopropanol:
IC50 (germination) = 2104 mg/kg, 3 day, Lactuca sativa (Reynolds, 1977)

Additional information

Ethanol:

In a root growth inhibition test using alliums (allium cepa) proposed for use as a screening test for toxicity assessment of effluents, rivers etc, ethanol was found to have an EC50 value of 11800mg/l and an estimated EC10 of 789mg/l when the sets were placed in aqueous ethanol solutions. The dose response curve was relatively shallow and effectively linear over the concentration range examined.  This study is used as the key for deriving a no effect level (EC10).

The toxicity towards lettuce seeds (Lactuca sativa) was reported for a large number of aliphatic alcohols and glycols, including ethanol. Measurement of toxicity was in terms of inhibition of germination. The EC50 (50% inhibition) for ethanol at 30C was found to be 117mM or 5382mg/l.  The result from this study is used to derive the critical EC50 value for terrestrial plants.  Lactuca sativa germination was also examined in another study that examined the use of solvents as prospective carriers of other substances, such as herbicides, into seeds  The study also examined peas (Pisum sativum). Seeds were treated for 24 and 44 hours with pure absolute ethanol or water as a control and the impact of this treatment on subsequent germination was assessed.  The study found that ethanol inhibited germination in lettuce to about 30% of the level found in controls but there was no effect in peas. The study cannot be used in quantitative risk assessment but it does confirm that lettuce seeds are a relatively sensitive species but also that the dose response curve is very shallow and that ethanol is actually of very low toxicity to seeds.

In a study that measured the growth and respiratory response of maize seedlings (Zea mays), ethanol was found to have significant inhibitory effects. The seedlings were exposed to ethanol solutions up to 1.5% volume for a period of 9 hours. Growth of coleoptiles and roots and overall respiration was reduced by around 10% in solutions of 1000ppm (789mg/l) compared to controls. The dose response curve was however very shallow; 50% inhibition by any measure was not reached at the maximum tested dose of 15000ppm (11800mg/l).

In a study to examine the effects on growth of oat seedlings (Avena sativa) of of exposure to ethanol solution through the roots, seedlings were exposed to ethanol solution at levels of 0.2% and 0.3% under different combinations of light and dark and co-exposure to CO2.  A mixed response was seen with growth of both mesocotyls and coleoptiles depressed by ethanol exposure during the first 3 days pretreatment but with mesocotyl promoted by around 60 -70% during further exposure under darkness but a lesser depression of 10 -20% of coleoptiles with exposure to ethanol.  With this mixed result, interpretation of the information is unclear.

In a study to assess the effect of ethanol as a prospective stimulant of root production in sugar cane sets, sugar cane (Saccharum officinarum) cuttings were pre-soaked in ethanol aqueous solutions up to 10% in strength for 24 hours before planting in soil. After 4 weeks, the ethanol was noted to produce a marked increase in root production an increase in the number of root primordia with a effect thresholds of 1% and 4% respectively. This endpoint is however of uncertain toxicological significance and the concentrations used not of environmental relevance.

A number of studies have looked at the impact of ethanol exposure on potato tubers (Solanum tuberosum).  In a study designed to look for alternative respiratory stimulants to ethylene, potato tubers were exposed to ethanol vapour at concentrations up to 20000ppm in air and oxygen and the rate of respiration (CO2 production) determined and used as a measure of effect. Ethanol exposure stimulated respiration at all concentrations although the peak in air was with 5000ppm ethanol.  In a similar older  study designed to look at the effect solvent vapour on metabolism of potatoes, tubers were exposed to ethanol vapour at concentrations up to 368mg/l in air and the rate of respiration (CO2 production) determined and used as a measure of effect. Ethanol exposure again stimulated respiration with increasing concentrations to around a peak at 40mg/l after which respiration rates fell again.  In another very old study designed to look at the effect of ethanol as a candidate substance to break the dormancy of potatoes, tubers were exposed to ethanol solutions in water up to concentrations of 8% (v/v) and degree of sprouting/growth and the level of measured enzymes (catalase and peroxidase) used as a measure of effect. Ethanol exposure stimulated growth and levels of peroxidases with increasing concentration and no maximum seen. These studies confirm that ethanol has an effect on potatoes but because the importance of this end point is unclear (“Positive effect” seen) and exposures were much higher than environmentally relevant, the results from these studies cannot be used for quantitative risk assessment although it they do show qualitatively that ethanol is of low toxicity.

In a study that followed the induction of cytochrome P450 in Helianthus tuberosus following exposure to a number of test substances, ethanol at a dose of 300mM in water was found to induced enzyme concentrations by 2 -3x. This endpoint is of uncertain toxicological significance and therefore cannot be used for risk assessment purposes.

All of the available studies report results in mg/ml, since none actually used test soil as the growth medium. Assuming the composition of soil to be 20% air (density 1.3kg/m3), 20% water (density 1000kg/m3) and 60% soil (density 2500mg/m3) and for soil to therefore have a dry bulk density of 1.875kg/l, the EC50 for the aqueous phase can be estimated as 5382mg/l (from above)*20%/dry bulk density, which is 633mg/kg dw.

Overall, ethanol can be considered as of very low toxicity to plants based on this result. This is supported by the fact that ethanol is recommended as a potential vehicle in OECD guideline 227 on assessment of vegetative vigour.

Isopropanol:

The toxicity of 2-propanol to the seed germination of the dicotyledonLactuca sativa(garden lettuce) was assessed in a published non-guideline study that predates GLP requirements for ecotoxicity studies (Reynolds, 1977). This laboratory study used Great Lakes garden lettuce seeds sown in aluminium boxes (4.2 x 2.1 x 1.6 cm) with glass windows in the lids, containing 0.5% agar (8 mL) in which appropriate weights of 2-propanol were dissolved. Germination at 30ºC was recorded after 3 days. The results indicate that 35 mM of 2-propanol caused 50% inhibition of the germination of lettuce seeds. The molecular weight of 2-propanol is 60.1 g/mol and thus, the 50% inhibition value can be calculated to be 2104 mg/kg in the agar.

 

This study did not investigate any monocotyledon species and with only one dicotyledon species reported, it is difficult to establish an effect level that will be representative of the effects of 2-propanol on seedling emergence of higher plants.

Although the reliability of this study could not be sufficiently assessed, the result based on a 96 hours test supports that IPA also beyond 24h exposure does not pose significant hazard to invertebrates. Also, these results are consistent with observations on acute fish studies performed over 48 hours in a static test as well as in a flow through test, proving that the substance does not volatilize from test water within 48 hours.