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Toxicity to terrestrial plants

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Calcium is well known as an essential nutrient for higher plants and has important roles for cell wall formation, cell division and cell elongation. Chloride is an essential micronutrient for plants and has an important role in regulating osmotic pressure of cells (SIDS, 2002).

Demands for calcium and chloride in plants/crops

The calcium content of plants varies between 0.1 and > 0.5% of the dry weight depending on the growing conditions, plant species, and plant organ. In well-balanced growing nutrient solutions with controlled pH, maximal growth rates were obtained at calcium supply levels of 2.5-100 µM. Also, calcium can be supplied at higher concentrations and might reach more than 10% of the dry weight without symptoms of serious inhibition of plant growth, at least in calcicole plant species.

A typical symptom of calcium deficiency is the disintegration of cell walls and the collapse of the affected tissues, such as the petioles and upper parts of the stems. Lower calcium contents in fleshy fruits also increase the losses caused by enhanced senescence of the tissue and by fungal infections.

In plant species with relatively low chloride requirement (<1 mg Cl/g leaf dry wt) the demand for chloride can be covered by a concentration of 100 µM Cl-in the nutrient solution. At the supply of 10 µM Cl-the shoot dry weight drops to 50%, indicating that chloride uptake is not so efficient as phosphorus uptake, because the demand for phosphorus in the leaf, which is much higher than that for chloride, can be fulfilled by the supply at a phosphorus concentration lower than 10 µM. In most plant species the Cl-requirement for optimal growth is in the range of 0.2-0.4 mg/g dry matter. The principal effect of chloride deficiency is a reduction in leaf surface area and thereby plant dry weight. With severe deficiency, curling of the young leaves followed by shrivelling and necrosis might occur (SIDS, 2002).

Biological effects monitoring

High proportion of the total calcium in plant tissues is often found in the cell walls. This unique distribution is mainly due to an abundance of binding sites for calcium in the cell walls and the restricted transport of calcium into the cytoplasm.

The proportion of calcium pectate in the cell walls is of importance for the susceptibility of the tissue to fungal and bacterial infections and for the ripening of fruits. Calcium has a significant effect on reducing the toxicity of soluble organic acids in the protoplasm of many plants. A soluble organic acid such as oxalic acid combines with Ca to form the very insoluble salt, calcium oxalate, which is not toxic to plants. Calcium has the role in counterbalancing the harmful effects of high concentrations of other cations at the plasma membrane. In the absence of an exogenous calcium supply, root extension quickly ceases. On the other hand, the reduction in plant growth under heavy salinisation is also suppressed by the supply of calcium.

Calcium competes with Na+for binding to the exchangeable sites in soil. Since soil clay has a higher affinity for Ca2+than Na+, more Na+is likely to be leached out to lower soil layers where it will become less available for plants roots.

Chloride is essential for the photosynthetic O2evolution and the proton-pumping ATPases. Chloride has important functions in osmoregulation at different levels. At the high plant contents it is a main osmoticum in the vacuoles of the bulk tissue (50-150 mM Cl-), together with potassium. At low contents that are in the range of micronutrient (~1 mM Cl-or below), these osmoregulatory functions of chloride are presumably confined to specialized tissues or cells, such as the extension zones of roots and shoots. Chloride also plays an essential role in stomatal regulations through mediating opening and closure of the stomata (SIDS, 2002).

Biotransformation and kinetics

Calcium is always present in the external solution in order to fulfil its functions at the plasma membrane, where it regulates the selectivity of ion uptake and prevents solute leakage from the cytoplasm. In the apoplasm, a part of calcium ions are firmly bound to its structures. Another part of the ions are exchangeable at the cell walls and at the exterior surface of the plasma membrane. A high proportion of intracellular calcium might be sequestered in vacuoles whereas the concentration in the cytosol is extremely low. The same is true for the mobility of calcium in the symplasm from cell to cell and in the phloem. Most of the functions of calcium as a structural component of macromolecules are related to its capacity for coordination, by which it provides stable but reversible intermolecular linkages, predominantly in the cell walls and at the plasma membrane.

Chloride is readily taken up by plants and its mobility in short- and long-distance transport is high. In plants chloride occurs mainly as a free anion or is loosely bound to exchange sites (SIDS, 2002).

Toxicity of calcium chloride for terrestrial organisms

Damage to roadside vegetation has been reported and is attributed largely to the absorption of salt splashed foliage. Sugar maples (Acer saccharum) were exposed to runoff of sodium chloride and calcium chloride for 6 winters (total treatment of 11.2 tonnes /ha per treatment and 15 treatments per winter at weekly intervals, equalling 11.2 kg/m2in total and 1.87 kg/m2in one season). Leaves of these maple trees contained 3 to 6 times the chloride concentration compared to a control stand. Damage to the maples varied but could be correlated with the chloride concentration in the leaf (EPSO, 1984).

From two field experiments with spruce tree (Piceasp.) carried out for ten weeks during a winter season, and a total dose of 1.5 kg/m2NaCl, CaCl2or a 75/25 NaCl/CaCl2mixture, it was found that in the presence of calcium chloride the uptake of Cl-in the root was inhibited (Bogemanset al., 1989). Thus effects of calcium chloride are present but it depends on the amount of accumulated Cl-.

A tentative “PNEC”, a so-called “no-effect-deposition” (NEdep) was derived for the exposure route for deposition of calcium via road salts or dust suppressors.It should be noted, that although the units refer to exposure via air, this value reflects effects caused by CaCl2deposited from air into soil or onto a plants’ surface.An assessment factor of 10 is taken into account.This factor is deemed appropriate as two chronic field studies areavailable in which plants were exposed for one or more seasons.The total dose is tentatively used as the effect concentration, resulting in a NEdep of 150 g/m2.

According to theCanadian Environmental Protection Act (ECHC, 2001),sensitive terrestrial plants may be affected by soil concentrations greater than about 68 mg sodium/kg and 215 mg chloride/kg. Areas with such soil concentrations extend linearly along roads and highways or other areas where road salts are applied for de-icing or dust control. The impact of aerial dispersion extends up to 200 m from the edge of multi-lane highways and 35 m from two-lane highways where de-icing salts are used. Salt injury to vegetation also occurs along watercourses that drain roadways and salt handling facilities (ECHC, 2001).