Iodine

Toxicity to terrestrial plants

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

toxicity to terrestrial plants: long-term

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: see 'Remark'

Remarks:

This data which uses KI as the test substance, is considered relevant for read across to iodine as iodide is the major existent species of iodine in soil which is available for interactions with soil components. Whilst there is some data that is not specified, the study follows scientific principles and is generally well documented. For further details, refer to the "Conclusions" section.

Qualifier:

no guideline followed

Principles of method if other than guideline:

Three soil types (see details of the soils below) were treated less than 48 days before this experiment. A protocol in a previous publication by the study author was used:

Sheppard, S.C., Evenden, W.G., Abboud, S.A. & Stephenson, M. (1993). A plant life-cycle bioassay for contaminated soil, with comparison to other bioassays: Mercury and Zinc. Arch. Environ. Contam. Toxicol., 25, 27-35

Three replicates of the I-treated soils and five replicates of controls were used. In summary, ten seeds of Brassica rapa were distributed evenly on the soil surface and the effect levels for different parameters were investigated under specific environmental conditions. The parameters investigated were emergence, early bloom, plant height, shoot dry weight, number of pods, pod dry weight and seed dry weight.

Follow the harvesting of B.rapa fifty seeds of leaf lettuce (Lactuca sativa cv. Green Ice) were planted in the soils that were broken up by hand and rewetted. This was done 108 days after the soils had been treated with I, and re-analysis of the soils confirmed the I concentration. Emergence counts were made on several days, and final emergence counts corresponded to when no further plants emerged. The numbers of treatments and replicates were identical to the B. rapa experiment.

GLP compliance:

not specified

Specific details on test material used for the study:

See information under "Test material identity". This data which uses KI as the test substance, is considered relevant for read across to iodine as iodide is the major existent species of iodine in soil which is available for interactions with soil components. For further information, refer to the environmental fate section of the dossier.

Analytical monitoring:

yes

Details on sampling:

The I concentrations were confirmed by neutron activation/gamma spectroscopy (NA/GS).

Vehicle:

yes

Details on preparation and application of test substrate:

Three soils were used by the study authors, due to their "very different properties". The soils are given below:

- SIlty-clay soil: This consisted of 43% clay, pH 7.9, an organic matter content (OMC) of 27 g kg-1 and a cation exchange capacity (CEC) of 18 cmol (NH4) kg-1. The soil shrank when dry to about 80% of its volume when moist.
- Garden soil: This soil had a high humus content, typical of a garden soil. This consisted of 46% clay, pH 7.3, an OMC of 89 g kg-1 and a CEC of 40 cmol (NH4) kg-1.
- Sand: This soil consisted of 3% clay, pH 6.3, negligible OMC and a CEC of 1 cmol (NH4)kg-1.

All of the soils were air-dried to a workable moisture content and sieved to pass a 5 mm mesh and stored in sealed buckets. The moisture-holding capacity (MHC) was determined for each soil and container size by filling the container with soil, wetting to excess and allowing to drain for 24 hours.

Species:

Brassica rapa

Plant group:

Dicotyledonae (dicots)

Details on test organisms:

Ten seeds of B. rapa selection Aaa 1-33 (Crucifer Genetics Cooperative, 1985) were distributed evenly on the soil surface. Three replicates of the I-treated soils and five replicates of controls were used.

Species:

Lactuca sativa

Plant group:

Dicotyledonae (dicots)

Details on test organisms:

fifty seeds of leaf lettuce (Lactuca sativa cv. Green Ice) were planted in the soils that were broken up by hand and rewetted.

Test type:

other: The study in Brassica rapa is a plant life cycle bioassay. The study in lettuce (Lactuca sativa) is a seed germination bioassay.

Study type:

laboratory study

Substrate type:

artificial soil

Limit test:

no

Total exposure duration:

50 d

Remarks:

This duration is for the plant lift cycle bioassay in Brassica rapa. The seed germination bioassay duration is not stated.

Post exposure observation period:

None stated

Test temperature:

The soil in the growth chamber provided day/night temperatures of 25 degrees C/22 degrees C.

pH:

- SIlty-clay soil: pH 7.9
- Garden soil: pH 7.3
- Sand: pH 6.3

Moisture:

- SIlty-clay soil: Soil was moistened to the moisture holding capacity with deionised water.
- Garden soil: Soil was moistened to the moisture holding capacity with deionised water.
- Sand: Soil was moistened to the moisture holding capacity with deionised water.

Details on test conditions:

- Plant life cycle bioassay, Brassica rapa:
The soils were treated with I less than 48 days before the experiment. A 400 cm3 aliquot of dry soil was placed into a 500 mL plastic container and gently tamped. Ten seeds of B.rapa were evenly spread on the soil surface and were placed at least 1 cm from the edge of the container, and pressed 5 mm into the soil using a marked rod. The soil was moistened to the moisture holding capacity using deionised water and was covered and then placed in the growth chamber. As described above, the growth chamber provided day/night temperatures of 25 degrees C/22 degrees C and a light flux density at the top of the container or plant of about 250 µE m-2 s-1. Once germination began, the cover was removed. After germination was determined to be completed, the number of seedlings was counted and reduced to five per container. To minimise the effects of location, the containers were removed, randomised and replaced in the growth chamber at least every other day. The seedlings were watered through the use of a capillary mat. The number of plants was counted in each container when it was determined that about half of the plants were in bloom. As the flowers opened, manual pollination using a small paint brush occurred daily until the blooming ended. After seeds from the first blooming phase were starting to mature, no water was applied for 2-3 days to encourage determinant growth. The harvest was conducted when the seeds became dark in colour at 50 days after planting. At this point, the height of the individual plants was recorded, and the pods were removed and counted. The stems and pods were dried at 50 degrees C and were weighed separately. The seeds were separated from the pods and were cleaned by hand, and the clean seed was weighed.

- Seed germination bioassay, lettuce:
After the B. rapa was harvested, the soils in the containers were partially dried and broken up by hand. Fifty seeds of leaf lettuce were planted and the soil rewetted. This was done 108 days after the soils had been treated with I, and re-analysis of the soils confirmed the I concentration. Emergence counts were made on several days, and final emergence counts corresponded to when no further plants emerged. The numbers of treatments and replicates were identical to the B. rupa experiment.

Nominal and measured concentrations:

The concentrations of stable I chosen to study were 0, 10, 22, 46, 100, 220, 460 and 1000 mg I kg -1 soil, applied as KI. The KI was applied in solution to the soils, with several cycles of addition and mixing to achieve uniform distribution.

Reference substance (positive control):

no

Species:

Brassica rapa

Duration:

50 d

Dose descriptor:

NOEC

Effect conc.:

10 mg/kg soil dw

Nominal / measured:

meas. (not specified)

Conc. based on:

element

Basis for effect:

other: Seed dry weight

Remarks on result:

other: Clay soil

Species:

Brassica rapa

Duration:

50 d

Dose descriptor:

EC50

Effect conc.:

38 mg/kg soil dw

Nominal / measured:

meas. (not specified)

Conc. based on:

element

Basis for effect:

other: Seed dry weight

Remarks on result:

other: Clay soil

Species:

Lactuca sativa

Duration:

50 d

Dose descriptor:

NOEC

Effect conc.:

1 000 mg/kg soil dw

Nominal / measured:

meas. (not specified)

Conc. based on:

element

Basis for effect:

seedling emergence

Remarks on result:

other: clay, sand and garden soils

Details on results:

The most sensitive and relevant data endpoints have been included in the effect concentrations section above. The full results table from the publication is included in the "Any other information on results incl. tables section" below.

For the parameters shoot dry weight, number of pods, pod dry weight and seed dry weight, these were assessed on both a per plant and a per container basis, and the values which gave lower NOELS are reported. For the clay soil, the data expressed per plant gave lower NOELS. For the garden soil, the data expressed per container gave lower NOELs.

In the seed germination bioassay in lettuce (Lactuca sativa), there was no significant decrease in lettuce emergence as a result of soil I at any of the tested concentrations.

As indicated by the results table below, Brassica rapa was particularly affected by I in the sand soil. No plants survived at concentrations above 10 mg kg-1. Although the control results are not individually specified, the publication states that the emergence in the controls (0 mg kg-1) of the sand soil was similar to the controls of the other soils. However, the EC50/LC50 for seedling emergence in the sand was about 16 mg kg-1 and the NOEL was < 10 mg kg-1. There were only two treatments with living plants, with the study author noting that interpolation by regression for the other response parameters was inappropriate for sandy soil. The reasons for greater toxicity in the sand soil compared to the other soils were considered to be due to the low sorptive capacity and possibly because the plants (and other organisms) were already under stress related to nutrient supply and physical matrix strength. Therefore the results observed in sandy soil have not been considered as the most sensitive and relevant values for risk assessment. However, the effects of the sandy soil were not noted in lettuce, which was unaffected by I in the sandy soil or in any of the other soils. This observation was not thought to be due to the delay prior to planting the lettuce, so was concluded by the study author to be due to the lettuce being less sensitive than B.rapa to I.

In the bioassay in B.rapa this study investigated a full life cycle through to the harvest of seed, and therefore a number of toxicity endpoints were investigated.

In all of the measured endpoints, the study author noted statistically deleterious effects at soil I concentrations above 100 mg/kg. The most sensitive NOEC in clay soil was 10 mg kg-1 (seed dry weight) and the most sensitive EC50 was 38 mg kg -1 (seed dry weight). The plant height endpoint was also nearly as sensitive to I in the clay soil (NOEL 24 mg kg-1/EC50 65 mg kg-1) and was more sensitive in garden soil (NOEL 32 mg kg-1/EC50 79 mg kg-1) than seed dry weight. The study author noted that the plant height is related to seed production in B. rapa as much of the height comes from the growth of the inflourescence.

The rationale for I being less toxic in garden soil than in clay was provided in the publication. The study author stated that this is likely to be related to the high organic matter content of the soil. The soil organic matter increases the sorption of I, which is considered by the study author to lessen its toxicity. The sorption was considered from a bioassay using Daphnia magna (as part of this publication) where 11% of the I was sorbed in the sand soil, 30% in the clay and 76% in the garden soil. The corresponding solid/liquid concentration values were 0.067, 2.1 and 18 litre kg-1. The level of sorption noted appears to be generally consistent with the toxicity values noted.

Results with reference substance (positive control):

Not applicable

Reported statistics and error estimates:

The general linear model and non-linear regression routines were used. Significance was assigned to the 0.05 probability level. The statistical contrasts with controls were conducted by single degree of freedom, two tailed tests. From results that showed statistically significant effects, a segmented regression was iteratively fit to the data. This allowed for the determination of the No-observed-effect-level (NOEL) and the concentration that reduced performance to 50% of that of the controls (the Effect Concentration to 50%, EC50, or the Lethal Dose to 50%, LD50). The regression described the response as a constant value at soil I concentrations below the NOEL and as linearly decreasing values at soil concentrations above the NOEL.

The most sensitive and relevant data endpoints have been included in the effect concentrations section above. The full results table from the publication is included below.

Summary of Soil I Concentrations Corresponding to No-Observed Effect Concentrations (NOEC) and levels where Performance was 50% of Controls (EC₅₀) forBrassica rapa, Derived from the Segmented Regression Model (table adapted from publication)

 

Bioassay and endpoint	Sand^a		Clay		Garden soil
	NOEC	EC50	NOEC	EC50	NOEC	EC50
Emergence	<10	16	40	150	40	150
Early bloom	-	-	40	60	40	60
Plant height	-	-	24	65	32	79
Shoot dry weight	-	-	15	51	40	100
Number of pods	-	-	20	51	40	110
Pod dry weight	-	-	19	52	48	98
Seed dry weight	-	-	10	38	47	97
Lettuce emergence	No significant decrease as a result of soil I

 

a The emergence of B. rapa was severely affected in the sand soil so that subsequent measurements were not meaningful.

b These parameters were assessed on both a per plant and a per container basis, and values which gave lower NOECs are reported. For the clay soil, the data expressed per plant gave lower NOECs, and for the garden soil, the data expressed per container gave lower NOECs.   

 

Validity criteria fulfilled:

not specified

Conclusions:

The most sensitive NOEC occurred in clay soil was 10 mg kg-1 (seed dry weight) and the most sensitive EC50 was 38 mg kg -1 (seed dry weight). Although this study was not conducted to any test guidelines, this study investigated three different soil types with known compositions which gave an indication on the effect of soil parameters on the soil toxicity values derived. The soil concentrations have also been confirmed by chemical analysis and KI was added in solution with several cycles of addition to achieve uniform distribution.

The main parameter which appeared to effect the toxicity was the organic matter content as this enhances the sorption of I which reduces its toxicity. The three soil types used had varying organic matter content which gave an indication of reasonable worst case toxicity values. The plants were also exposed to I after taking into account the sorption of I reaching a steady state. A steady state was assumed to be reached, as the soils were treated less than 48 days before the experiment - previous work by the study author suggests that it took at least 20 days for the sorption of I to reach a steady state. Although the control results are not individually specified, the test results were compared with the negative controls using statistical contrasts. It is therefore assumed that the concentrations tested and the replicates were sufficient in number to allow the determination of statistical significance.

Whilst there is no data regarding the validity criteria used for this study, it is considered that this study follows scientific principles and is well documented. This study has considered different soils of known composition, tested 8 concentrations of I (including 0 mg I kg-1 soil) with three replicates of the I treated soils and five replicates of controls. The 50 day study duration in B. rapa is considered meet the duration and recommended test species for a chronic plant study and investigates effects such as the number of pods, pod dry weight and seed dry weight which are chronic measurements (Kalsch et al, 2006) . This study goes beyond the requirements of Annex IX in REACH, which requires a short term toxicity study in plants. This study is therefore considered to meet the criteria of Klimisch score 2 - reliable with restrictions.

Kalsch W, Junker T and Römbke J - A Chronic Plant Test for the Assessment of Contaminated Soils. Part 1: Method development Journal of Soils and Sediments Publisher Springer Berlin / Heidelberg Issue Volume 6, Number 1 / February, 2006 (Abstract) - This publication was used to develop the ISO method - Chronic toxicity in higher plants – ISO 22030.

Executive summary:

This study investigated the toxicity of inorganic iodine (applied as KI) to the terrestrial plants, Brassica rapa, (life cycle bioassay), and Lactuca sativa (seed germination bioassay) in three soil types, clay soil, sand soil and garden soil. This use of KI as the test substance, is considered relevant for read across to iodine as iodide is the major existent species of iodine in soil which is available for interactions with soil components. For further information, refer to the environmental fate section of the dossier. The protocol is not conducted to any test guidelines (e.g. OECD Guidelines) but follows scientific principles and is well documented, supporting a Klimisch score of 2 - reliable with restrictions. The most sensitive NOEC in clay soil was 10 mg kg-1 (seed dry weight) and the most sensitive EC50 was 38 mg kg -1 (seed dry weight).

Reference 2

Endpoint:

toxicity to terrestrial plants: long-term

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

comparable to guideline study with acceptable restrictions

Qualifier:

no guideline followed

Principles of method if other than guideline:

The goals of the present study are to investigate the effect of low dose rates of iodine sources in the nutrient solution on lettuce biomass production and plant quality. Two trials were conducted with butter head lettuce to study the effect of iodine form and concentration in the nutrient solution on growth, development and iodine uptake of lettuce, grown in water culture.

GLP compliance:

not specified

Analytical monitoring:

yes

Details on sampling:

Samples for iodine analysis were taken from the nutrient solution at the start and at the end of the experiment. The sub-samples from two and three containers per treatment were pooled together, respectively, resulting in two analytical results per treatment per sampling date. Similar bottles were used as for the routine test.

Samples for dry matter analysis were taken as a sub-sample from the freshly harvested plant material. All heads were cut in half and subsequently from the remaining leaf mass, the inner part, being the folded yellow leaves of the head, was separated from the outer part, being the unfolded green mature leaves. All samples were washed with demineralised water and some detergent. Dry matter was determined from all replicates. The fresh material was dried for 48 h at 80℃, using stainless steel containers with paper cover. After the drying process, for each treatment three samples were prepared for the dry matter analysis. This was done by pooling the replicates 1 and 3, replicates 4 and 5, and 2 (solely).

Vehicle:

no

Species:

Lactuca sativa

Plant group:

Dicotyledonae (dicots)

Details on test organisms:

- Common name: butter head lettuce

Substrate type:

other: without substrate

Limit test:

no

Total exposure duration:

14 wk

Remarks:

14 weeks (winter trial) and 7 weeks (summer trial)

Test temperature:

a day and night ventilation temperature of 15 and 10 ℃

pH:

5.5

Details on test conditions:

The experiments were carried out in a greenhouse compartment of 96 m2 in the Bleiswijk facility of Wageningen UR Greenhouse Horticulture in the Netherlands, equipped with automatic vents and heating systems, controlled by a climate computer.
Lettuce was sown in vermiculite, rinsed with rain water, on 19 October 2007 and 26 May 2008 for the winter and summer trial, respectively. No nutrient solution was added. On 20 November 2007 and 16 June 2008 (winter and summer trial, respectively) the plants were placed in growth containers, after removing the vermiculite from the roots by washing them with clean water. Plants were grown in a hydroponic system, without substrate. Polyethylene growth tubs were used, dimensions: 50×30×20 cm3, filled with 28 L of nutrient solution, provided with a PVC cover with plant holes. The covers were concealed with a black and white polyethylene sheet to prevent evaporation from the root environment. Plants were placed in small slits made in the foil, causing the roots to hang in the nutrient solution. Each container carried 10 plants in the winter trial and six plants in the summer trial. The containers were provided with air nozzles, which continuously supplied air into the nutrient solution to provide oxygen to the roots.

The standard nutrient solution for lettuce in water culture was used (Macronutrients (mmol /L): NH4 1.6, K 13.8, Ca 5.6, Mg 1.3, NO3 24.1, Cl 0.0, SO4 1.3, H2PO4 2.5; Micronutrients (μmol /L): Fe 40.0, B 30.0, Mn 5.0, Zn 4.0, Cu 0.8, Mo 0.5). A concentrated stock solution was prepared with standard commercially available liquid fertilisers, except for KNO3 as the major K source, for which a chemically pure source was used. All containers were filled with this standard nutrient solution with an electrical conductivity (EC) of 2.5 dS/m. The pH was set at 5.5. Then, each individual container was supplemented with the required quantity of KIO3 and KI. A mixture of rainwater and desalinated water, obtained by reversed osmosis, was used in the winter trial. As these sources appeared to have a non-negligible background concentration of iodine, in the summer trial laboratory quality demineralised water was used.
The EC and pH of the nutrient solution were measured regularly throughout the whole experiment with routine testing equipment. As the EC remained very stable during the trial period, no adjustments had to be made. When necessary, the pH was corrected by adding either 1 mol HNO3 solution or 1 mol KOH or KHCO3 solution.
After 14 weeks (winter trial) and 7 weeks (summer trial) the plants were harvested and the total fresh weight was determined. Crop quality in terms of the appearance of the heads, colour and visible symptoms was judged visually.
Samples for control of the nutrient solution in the root environmentwere taken at the start, half-way and at the end of the experiment. The samples were gathered by sub-sampling from 20 containers. As the concentrations of macro- and micronutrients were within the standard targets throughout both experiments, no adjustments were made during the experiments.
The climate was controlled automatically, with a day and night ventilation temperature of 15 and 10℃, and for the winter trial a set point for additional heating at 10 ℃. CO2 was applied at levels of 900 ppm. In the winter trial additional assimilation light was used, at levels of 43.4W/m2 from 07 : 00 to 18 : 00 h, from 10 December 2007 onwards.
In the winter trial on 3 January 2008, four out of ten plants per container were harvested in order to give the remaining plants more space. The final yield was on 25 January 2008. The summer trial was harvested at once, on 17 July 2008.

Iodine was added to the containers only once at transplanting. In both trials a zero treatment without any iodine application was included. Additional treatments consisted of containers without plants, which received the complete nutrient solution plus the highest iodine dose rate of 129 μg iodine/L (both IO3- and I- treatments) and 90 μg iodine/L (I- treatment only) in the winter and summer trial, respectively, in order to check for any possible volatile iodine losses. The treatments were randomly laid out in five replicates as separate containers (with a total of 50 containers) with 10 (winter) and six (summer) plants per container.

Nominal and measured concentrations:

Nominal:
Winter trial: 13, 39, 65 and 129 μg iodine/L, applied as KIO3 and as KI
Summer trial: 13, 39, 65 and 90 μg iodine/L, applied as KIO3 and as KI.

Reference substance (positive control):

no

Remarks on result:

not determinable

Remarks:

The effect of iodine on plant growth and development is not unequivocal.

Details on results:

- Plant biomass and crop quality:
In both the winter and the summer trial, plant biomass and crop quality were not affected, neither by the iodine dose rates in the nutrient solution nor by the iodine form.

Validity criteria fulfilled:

not specified

Conclusions:

No growth response whatsoever to iodine form or concentrationwas found. The effect of iodine on plant growth and development is not unequivocal.

Executive summary:

The goals of the present study are to investigate the effect of low dose rates of iodine sources in the nutrient solution on lettuce biomass production and plant quality. Two trials were conducted with butter head lettuce to study the effect of iodine form and concentration in the nutrient solution on growth, development and iodine uptake of lettuce, grown in water culture.

In both the winter and the summer trial, plant biomass and crop quality were not affected, neither by the iodine dose rates in the nutrient solution nor by the iodine form. No growth response whatsoever to iodine form or concentrationwas found. The effect of iodine on plant growth and development is not unequivocal.

Description of key information

50 day chronic plant study (Brassica rapa) NOEC (seed dry weight) 10 mg I kg -1 soil (clay soil)

50 day chronic plant study (Brassica rapa) EC50 (seed dry weight) 38 mg I kg -1 soil (clay soil)

Key value for chemical safety assessment

Short-term EC50 or LC50 for terrestrial plants:

38 mg/kg soil dw

Long-term EC10, LC10 or NOEC for terrestrial plants:

10 mg/kg soil dw

Additional information

The reviewed study (S.C. Sheppard, 1994) investigated the toxicity of inorganic iodine (applied as KI) to the terrestrial plants, Brassica rapa, (life cycle bioassay), and Lactuca sativa (seed germination bioassay) in three soil types, clay soil, sand soil and garden soil. This use of KI as the test substance, is considered relevant for read across to iodine as iodide is the major existent species of iodine in soil which is available for interactions with soil components. For further information, refer to the environmental fate section of the dossier. The protocol is not conducted to any test guidelines (e.g. OECD Guidelines) but follows scientific principles and is well documented, supporting a Klimisch score of 2 - reliable with restrictions. The most sensitive NOEC in clay soil was 10 mg/kg soil dw (seed dry weight) and the most sensitive EC50 was 38 mg/kg soil dw (seed dry weight).

There is another study (Voogt, 2010), which is to investigate the effect of low dose rates of iodine sources in the nutrient solution on lettuce biomass production and plant quality. Two trials were conducted with butter head lettuce to study the effect of iodine form and concentration in the nutrient solution on growth, development and iodine uptake of lettuce, grown in water culture.

In both the winter and the summer trial, plant biomass and crop quality were not affected, neither by the iodine dose rates in the nutrient solution nor by the iodine form. No growth response whatsoever to iodine form or concentrationwas found. The effect of iodine on plant growth and development is not unequivocal.