Aluminium sodium dioxide

Toxicity to terrestrial plants

Additional information

There are no studies available on terrestrial plants for sodium aluminate. 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 terrestrail 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. Increassing 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 inhbition of root growth (root elongation) of 60% after 6 hours of exposure to 38 µM Al³⁺. They showed that a substantial aluminum acculmulation in the root tip region after 30 minutes of exposure and report 20 to 25 layers of undiferrentiated 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 disturbe cell devision 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 al 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.