Boron toxicity in plants is most likely to occur following the continued use of boron-contaminated irrigation waters, however, direct exposure from airborne emissions may also produce localized toxic effects (Howe 1998). Symptoms of boron toxicity in plants include yellowing, spotting or drying of leaf tissues, especially on the tips and sides of older leaves (Gupta et al. 1985).
Ecosystem specific information
Arable habitats - In arable areas, boron deficiency is likely to be more common than boron toxicity. However, toxicity may occur following the continued use of boron-contaminated irrigation water, or excessive applications of boron-rich fertlisers, sewage sludges or fly ashes (Gupta et al. 1985; Howe 1998).
Bogs, wetland and heath - the main threat to wetland ecosytems is phytotoxicity due to exposure to boron-contaminated wastewater (Howe 1998). A significant reduction in frond production was reported in duckweed, Spirodella polyrrhiza, exposed to 3.55 mg B / litre for 10 days, with the corresponding EC50 calculated to be 14.3 mg/l. Significantly growth reduction was reported at 18.9 mg/l (Davis et al. 2002).
Coastal and rocky habitat - inhibition of photosynthesis has been reported in marine phytoplankton exposed to boron concentrations of 30mg/l, with growth of marine algae inhibited at 50 mg/l. No effect on sea urchin embryo (Anthocidaris crassispina) development was reported at 37 mg/l, but exposure to 75 mg/l was fatal (Eisler 1990). LC50 values in marine fish range from 12.2 to 74.0 mg/l (for 283 and 96 hour exposures, respectively) (WHO 1998).
Freshwater - many freshwater aquatic organisms are relatively tolerant to boron. Effects on bacteria have been reported at concentrations greater than 18 mg/l. No effect concentrations for freshwater algae (including green & blue green algae) ranged from 10 to 24 mg/l for chronic exposures. Toxicity to aquatic plants is variable, with some species able to tolerate boron concentrations up to 60 mg/l without any effect, whereas chlorosis, necrosis and death have been reported in others following exposure to 20 mg/l (Davis et al. 2002). Acute toxicity values for invertebrates are generally in the range of 100 to 200 mg/l, with no effect concentrations for chronic exposures of Daphnia magna ranging from 6 to 10 mg/l. Acute toxicity to fish (96hour-LC50 values) range from 10 to 300 mg/l, with rainbow trout (Oncorhynchus mykiss) and zebrafish (Brachydanio) the most sensitive species (WHO 1998). Adverse effects have also been reported in embryonic and early life stages of fish, following exposure from fertilisation to up to 8 days post hatching, with LC50 values for these lifestages ranging from 22 to 155 mg /l (Howe 1998).
Grassland - Grassland species have been shown to accumulate boron. Legume species, clustered clover (Hordeum murinum) and cotton clover (Trifolium tomentosum) accumulated more boron than the grasses, wall barley (Hordeum murinum) and soft brome (Bromus hordaceus) (Adarve et al. 1998).
Woodland and hedgerow – direct exposure from airborne emissions may produce localized toxic effects in endemic tree species (Howe 1998). Clorosis and needle necrosis have been reported in Monterey Pine (Pinus radiata), maple (Acer sp.) growing near emission sources. The threshold boron concentration in foliage of trees with visible damage ranged from 200 to 700 mg/kg. (Smidt and Whitton 1975; Temple and Linzon 1976; Lan g et al. 1986).
Boron is an essential trace element for plants, therefore any adverse effects must be balanced against its essentiality. The range of optimal boron concentrations is generally quite narrow; boron concentrations outwith this range will result in adverse effects, due to either boron deficiency or toxicity.
|Critical Load/ Level|
No estimate available