Boron

Endpoint summary

Description of key information

Boron accumulates in aquatic and terrestrial plants but does not magnify through the food-chain. BSAF (biota-to soil accumulation factor) values derived from tests performed in real soils are generally < 100. Data from both laboratory and field observations indicate that body burdens of boron decrease at higher trophic levels. Because boron is incorporated into plant cell walls, a diet rich in plant material is correspondingly high in boron, compared to diets rich in meat or fish. However, data from animals and humans indicate that boron is quickly removed via faeces and urine, so body concentrations do not continually increase. Consequently, the potential for secondary poisoning is not significant.Bioaccumulation studies are therefore scientifically unjustified.

Additional information

Additional information

 

The WHO (1998) review of boron noted that highly water soluble materials are unlikely to bioaccumulate to any significant degree and that borate species are all present essentially as undissociated and highly soluble boric acid at neutral pH. The available data indicate that both experimental data and field observations support the interpretation that borates are not significantly bioaccumulated.

Boron is known to be a critical element for the normal growth and productivity of aquatic and terrestrial plants. Boron is incorporated into plant cell walls, so some accumulation vs. the environment may be anticipated, i.e. active transport. The minimum required level in plants is dependent on the plant species.

While several studies report concentrations on boron in plant tissues, only few provide both soil and tissue concentrations, data required to derive the BSAF values that can be found in section 4.3.1 and 4.3.2. These values are well below the BSAF values used to establish significant bioconcentration (BSAF 3000 to 5000).

Aquatic bioaccumulation

Bioconcentration factors of <0.1 to 10.5 L/kg have been reported from laboratory tests of fish and oysters (Hamilton and Wiedmeyer, 1990; Thompson et al. 1976). Saiki et al. (1993) measured boron levels in aquatic food chains and observed the highest concentrations of boron in detritus and filamentous algae. Invertebrates and fish had lower concentrations, indicating that bioaccumulation was not occurring. Based on these data, boron does not bioaccumulate in the aquatic environment. Note that the freshwater and marine BCF values are of the same order of magnitude.

Terrestrial bioaccumulation

Boron is known to be a critical element for the normal growth and productivity of terrestrial plants. Boron is required in plants for normal metabolic functioning of sugar transport, cell wall synthesis, lignification, carbohydrate metabolism, RNA metabolism, respiration, indole acetic acid (growth regulator) metabolism, phenol metabolism, the integrity of membranes, and the pollination process (Marschner, 1995). There is a certain minimum requirement of boron for a plant. However, there are considerable interspecies differences in the levels required for optimal growth. Monocotyledons generally require less then dicotyledons (Gupta et al, 1985).

Boron uptake varies with stage of growth and the concentration varies among the plant parts (Gupta et al, 1985). Plants also are known to change soil pH locally by root exudates to enhance uptake of essential nutrients (Reimann et al. 2001, WHO 1998).

The uptake mechanism has long been debated. It was first suggested that boron moves to the root surface in the soil solution by mass flow and enters the roots by passive diffusion (Bingham et al, 1970). However this concept has been challenged by Bowen (1968, 1969, 1972), Bowen and Nissen (1977), and Reisenauer et al (1973). They indicated that boron is actively absorbed in ionic form particularly when the boron concentration in soil is low (Gupta et al, 1985). This has been confirmed by more recent studies, which provided evidence for channel- and/or transporter-mediated boron transport systems (Tukano et al, 2005). The isolation of the boron transporter in BOR1-1 mutant plants showed elevated sensitivity to boron deficiency, especially in young growing organs in shoots. BOR1 is a membrane protein that belongs to the bicarbonate transporter superfamily (Takano et al, 2002; Frommer et al 2002).

Takano et al (2005) found that the activity of the BOR1 plasma membrane transporter for boron in plant is regulated (endocytosis and degradation) by boron availability, to avoid accumulation of toxic levels of boron in shoots under high boron supply, while protecting the shoot from boron deficiency under boron limitation. This process is known as homeostasis mechanism.

Once in the plant, boron is passively carried in the transpiration stream to the leaves where the water evaporates and boron accumulates. This explains why boron concentrations are generally lower in roots, stems, and fruits than in leaves (WHO 1998). Once assimilated by the plant, boron becomes one of the least mobile micronutrients (Wolg 1940, Eaton 1944, Dible and Berger, 1952). Since boron is not readily transported from old to young plant parts, the earliest deficiency symptoms are found in young parts while the earliest toxicity symptoms are found in the old plant parts (Gupta et al, 1985).

 

Secondary poisoning

Based on the available information, there is no indication of a bioaccumulation potential, rather evidence demonstrating homeostasis mechanisms for certain trophic levels and hence secondary poisoning is not considered relevant (see CSR chapter 7.6 "PNEC derivation and other hazard conclusions").