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Toxicity to terrestrial plants

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Endpoint:
toxicity to terrestrial plants: short-term
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Acceptable, well documented publication which meets basic scientific principles.
Principles of method if other than guideline:
Modified Allium test (Fiskesjö 1985 based on Levan 1938):
GLP compliance:
no
Analytical monitoring:
yes
Vehicle:
no
Species:
Allium cepa
Plant group:
Monocotyledonae (monocots)
Details on test organisms:
- Common name: onion
- Plant family: Alliaceae
Test type:
seed germination/root elongation toxicity test
Study type:
laboratory study
Substrate type:
other: natural and artificial soil liquids
Limit test:
no
Total exposure duration:
4 d
Test temperature:
20 °C
Species:
Allium cepa
Duration:
4 d
Dose descriptor:
EC50
Effect conc.:
25 other: µM
Nominal / measured:
meas. (arithm. mean)
Conc. based on:
other: monomeric labile Al
Basis for effect:
other: root elongation

The effect of pH using the Allium Test on synethical soil liquids shows the shortest root length after 4 days at pH 3.5 of approx. 5 mm, with increasing root lengths as pH increases and the longest roots of approx. 30 mm length at pH 4.3, slightly decreasing again towards a pH of 4.8.

Root development of Allium cepa bulbs grown in mor soil liquids and synthetic liquids of different aluminum content show phytotoxicity of aluminum on roots as also presented in the following table:

Mor soil liquid

 

Synthetic liquid

 

Labile monomeric Al (µM)

Root length (mm)

Monomeric Al (µM)

Root length (mm)

<0.5 (tap water)

53.7 ± 4.2

Tap water

47.6 ± 2.4

0.8

35.0 ± 4.9

0

23.0 ± 2.4

 

 

4.1

21.9 ± 2.5

 

 

8.1

22.7 ± 2.1

13.7

31.0 ± 4.5

11.1

21.8 ± 3.7

 

 

15.2

15.4 ± 3.0

21.5

17.6 ± 1.1

20.0

16.3 ± 3.3

63.3

10.4 ± 2.1

34.8

8.3 ± 1.6

85.2

5.8 ± 1.4

69.7

5.3 ± 0.7

The labile monomeric Al plot for soil liquid coincided best with the monomeric Al fraction of the synethetic soil liquid when all Al measurements were plotted against each other. The presence of dissolved organic matter, predominantly fulvic acids, affect the distribution of the different Al fractions in the naturally derived soil liquid. An approximated Al concentration of 25 µM in both liquids caused a root growth reduction of 50% compared to root growth in solutions without any Al added (see EC50 above).

A non-linear relationship between naturally occuring Al concentrations in soil liquid derived from natural sites and the root elongation of Allium cepa in the Allium Test was observed. More organic Al was expected to dominate the soil water in the 15 cm samples than in the 50 cm samples, which therefore harbour more inorganic (more toxic) Al species. The naturally found total Al concentrations ranged from 41 to 381 µM total Al with root lengths of 12.2 to 3.4 mm measured during the Allium Test.

Endpoint:
toxicity to terrestrial plants: short-term
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Acceptable, well documented publication which meets basic scientific principles.
Principles of method if other than guideline:
Short-term test with Zea mays plants grown in culture solution and exposed to AlCL3 at different pH values. Root growth was followed.
GLP compliance:
no
Analytical monitoring:
no
Vehicle:
no
Species:
Zea mays
Plant group:
Monocotyledonae (monocots)
Details on test organisms:
- Common name: Maize
- Plant family: Poaceae
Test type:
seed germination/root elongation toxicity test
Study type:
laboratory study
Substrate type:
other: nutrient solution
Limit test:
no
Total exposure duration:
5 d
Endpoint:
toxicity to terrestrial plants: short-term
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Acceptable, well documented publication which meets basic scientific principles.
Principles of method if other than guideline:
seed germination/root elongation toxicity test
GLP compliance:
no
Analytical monitoring:
no
Vehicle:
yes
Species:
Triticum aestivum
Plant group:
Monocotyledonae (monocots)
Details on test organisms:
- Common name: Wheat
- Plant family: Poaceae
Test type:
seed germination/root elongation toxicity test
Study type:
laboratory study
Substrate type:
other: nutrient solution
Limit test:
no
Total exposure duration:
2 d
Species:
Triticum aestivum
Duration:
2 d
Dose descriptor:
EC50
Effect conc.:
1.2 other: µM
Nominal / measured:
nominal
Conc. based on:
other: Al3+
Basis for effect:
other: root elongation
Reported statistics and error estimates:
Multiple regression analyses using root length as function of Al3+ activity and the activity of the cation added as chloride salt. Tektronix 4052A computer and statistical software (Plot 50, 4050D04, multiple linear regression).

Multiple regression analyses using root length as function of Al3+ activity and the activity of the cation added as chloride salt. Tektronix 4052A computer and statistical software (Plot 50, 4050D04, multiple linear regression).

Endpoint:
toxicity to terrestrial plants: short-term
Type of information:
migrated information: read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Acceptable, well documented publication which meets basic scientific principles.
Principles of method if other than guideline:
Seeds germinated for 3 days in darkness, then transferred into 90-L circulating culture systems for 4 days under light. On day 5 in the nutrient solution, aluminum was added as AlCl3. Roots of intact plants were exposed for 30 minutes or 18 hours. After exposure, the roots were rinsed in ice-cold potassium citrate to remove loosely bound aluminum.
GLP compliance:
no
Analytical monitoring:
no
Vehicle:
no
Species:
Glycine max (G. soja)
Plant group:
Dicotyledonae (dicots)
Details on test organisms:
- Common name: Soybean
- Plant family: Fabaceae
Test type:
seed germination/root elongation toxicity test
Study type:
laboratory study
Substrate type:
other: nutrient solution
Limit test:
no
Total exposure duration:
24 h

Glycine max seeds were germinated for 3 days in darkness in 0.1 mM CaSO4 and transferred into four 90-L circulating culture systems for an additional four days (light provided by incandescent and fluorescent light source with PPFD of 400 µmol/m2/s. The nutrient solution consisted of KH2PO4, KNO3, CaSO4, MgSO4 and Fe2(SO4)3. The pH was maintained at 4.2 ± 0.2 by addition of H2SO4. Solution temperature was 26 ± 1 °C during the 12/12 day-night cycle. The calculated free Al3+ activity in the exposed conditions was 38 µM with no aluminum in the controls. Roots of intact plants were exposed to the 38 µM aluminum concentration for 30 minutes and 18 hours.

Pattern of Al accumulation

After exposure, the whole plants were rinsed in ice-cold 10 mM potassium citrate for 30 minutes to remove loosely bound Al from the root surface and cell walls. Cryosections of freeze-dried root tips were made, with consequent SEM (scanning electron microscope) and SIMS (secondary ion mass spectrometer) analyses.

Root growth

The primary roots were inserted into open-ended plastic tubes which tapered from a 10 -mm inside diameter at the top to 4 mm near the root apex. The tubes were returned to the two solutions (38 µM Al and control) and given a 2 hour equilibration period. The position of the apex was marked on the outside of the tube at the begin of the study and the length recorded after 2, 4, 6 and 24 hours to the nearest 10 µm using a stereomicroscope.

Endpoint:
toxicity to terrestrial plants: long-term
Data waiving:
other justification
Justification for data waiving:
other:

Description of key information

Key value for chemical safety assessment

Additional information

There are no studies available on terrestrial plants for the registered substance reaction mass of aluminium hydroxide and aluminium nitrate and aluminium sulphate. Nevertheless, aluminium is the most abundant metallic element in the Earth's crust. Based on its ubiquitous occurrence the present natural background concentration far outweighs anthropogenic contributions of aluminium to the terrestrial environment. As detailed in the endpoint summary on terrestrial toxicity in general further toxicity testing on terrestrial organisms is considered unjustified and waiving based on exposure consideration is applied.

However, for reasons of completeness existing data on the terrestrial toxicity of aluminium are provided in addition and summerised here.

Kinraide & Parker (1987) in a test with Triticum aestivum determined an EC50 of 1.2 µM referring to the inhibition of root elongation of shoots after 2 days exposure. In a study investigating the same effect but using Alium cepa as test organism and a four days exposure period by Berggren & Fiskesjo (1987) an EC50 of 25 µM was determined based on monomeric labile aluminum. Increasing labile monomeric aluminum from 0 to 85.2 µM and increasing monomeric aluminum from 0 to 69.7 µM resulted in a decreasing root length from 53.7 to 5.8 mm and 47.6 to 5.3, respectively. Lazof et al. (1994) demonstrated an inhibition of root growth (root elongation) of 60% after 6 hours of exposure to 38 µM Al3+. They showed that a substantial aluminum accumulation in the root tip region after 30 minutes of exposure and report 20 to 25 layers of undifferrentiated cells around 0.3 to 0.8 mm from the cell apex. In this region they found an aluminum signal up to 60 µM inwards from the surface. Root growth in aluminum sensitive and tolerant cultivars was compared by Calba et al. (1999). These others found that second order root growth was affected by Al for both the sensitive and the tolerant cultivar. The relative growth of second order roots of the tolerant cultivar remained relatively constant in the range of pHs studied, while the sensitive ones declined when the pH was below 4.2. The relative root length of the sensitive species was approx. 20% at pH 3.8 (final pH in the rhizosphere) and approx. 60% at pH 4.5 (final pH in the rhizosphere after 5 days). With the tolerant cultivar, growth ranged from 75% (in comparison to control) at final rhizosphere pH 3.6 and approx.60% at final rhizosphere pH 4.5.

Although results are diverse as a result of various test designs it might be concluded on a strongly generalised basis, that both decreasing pH and/or increasing concentrations of aluminum pose negative effects to roots of terrestrial plants. However, several factors need to be considered in detail, e.g. knowledge of aluminum species responsible for effect, pH regime, soil characteristics, organic matter present, plant species and tolerance mechanisms, in order to assess aluminum toxicity appropriately.

The toxicity of aluminum to vascular plants against the background of such factors was reviewed by Andersson (1988). According to this author soil acidification has the potential to induce aluminum toxicity in plants, as the solubility of aluminum increases exponentially as the pH decreases below 4.5. From the different species of aluminum found it is mainly the labile, monomeric, inorganic species that constitutes the toxic fractions. In terms of measuring toxic concentrations the sum activity of monomeric aluminum species in the soil solution is a better measure than the total concentration of soluble or exchangeable aluminum. A toxicity of aluminum can primarily be expected in mineral soils which have a low content of organic matter and organic acids since these are capable of complexing aluminum, thus reducing the bioavailability of aluminum to plants. The availability of aluminium is also depending on the mineral and soil characteristics. For instance, soils rich in clay have large aluminum fraction, which can be mobilised during an acidification event.

Symptoms of toxicity are first observed in the roots, the development of which is some way hampered, e.g. the elongation of the main root axis diminishes and laterals roots often fail to develop. Roots might also show deformations, they might become stubby, short, swollen, gnarled, or brittle with bent, brown tips. Vascular bundles may not develop properly and the root system can be restricted to soil horizons low in soluble aluminum. As a consequence, the absorption of water and nutrients is often strongly reduced and adds to the adverse influence of aluminum. Seed germination is not as strong affected by aluminium than the survival of seedlings. Higher concentrations and longer exposure times are necessary in order to cause effects to shoots than to roots. Effects observed for shoots include weight decrease and delayed leaf development; more severe effects are wilting, shedding of leaves and death. Such symptoms might be caused by nutrient deficiencies, e.g. inhibition of phosphorous transport by aluminium or uptake and distribution of calcium and other nutrients. On the other hand high concentrations of calcium in the soil can reduce aluminium activity. In general in nutrient-rich soils, plants can cope better with high concentrations of soluble aluminum. On a subcellular level aluminum may disturb cell division and DNA replication, membrane flexibility and permeability is affected, coagulation of proteins occurs, enzymes are influenced negatively. All these effects result in hampered transport mechanisms, decreases sugar phosphorylation and root respiration.

However, not all plant species are affected to the same extent and a variety of tolerance strategies is found. Species adapted to acid conditions are more Al tolerant than others. Evolved strategies include active exclusion mechanism, immobilization of aluminum in roots, tolerance to high tissue levels of aluminium due to inactivation and storing at specific sites, the ability to absorb and use phosphorous and calcium in the presence of aluminum, or low requirement for these nutrients.