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Environmental fate & pathways

Bioaccumulation: terrestrial

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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.

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).