The utility of Brachylaena discolor as a bioindicator of air pollution within selected industrial areas in KwaZulu-Natal.

Abstract

The negative impacts of air pollution have made monitoring of air quality increasingly important. This is primarily true for industrial areas such as the South Durban Basin (SDB) within the eThekwini Municipal Area (EMA), South Africa. Bioindicators can complement the process of monitoring air quality. For the establishment of Brachylaena discolor DC. tree leaves as a bioindicator of air pollution, this study investigated the effects of sulphur dioxide (SO₂) pollution on various biochemical (intracellular superoxide [·O₂⁻], hydrogen peroxide [H₂O₂] production, total aqueous [TAA] and enzymic antioxidants [superoxide dismutase and catalase], lipid peroxidation [LPO] and electrolyte leakage), physiological (leaf chlorophyll fluorescence and chlorophyll content) and morphological (leaf area [LA]) biomarkers of stress. Leaves were sampled from (four) trees growing at three industrial sites (Prospecton, Ganges and Southern Works) within the SDB and from greenhouse-grown trees that served as an ex situ control. The sampling (n=24, per parameter) accommodated directional and seasonal effects. Annual [SO₂] measured at all three treatment sites (Prospecton [4.39±3.92 ppb], Ganges [5.10±4.73 ppb] and Southern Works [6.71±5.47 ppb]) during the study were high compared to global standards. Values for all biomarkers did not differ significantly for leaves from different cardinal points within sites but seasonal differences were evident in some cases; ·O₂⁻, LPO, electrolyte leakage, leaf chlorophyll fluorescence, and LA were significantly (p<0.05) correlated with seasonal [SO₂]. Except for ·O₂⁻, superoxide dismutase and catalase, all other biomarkers investigated could differentiate between SO₂ exposed and unexposed leaves. However, only electrolyte leakage was sensitive enough to reflect differences in [SO₂] across the treatment sites. Qualitative data on land-use practices at each site suggests that the pollution sources/pollutants differed across the SDB and that the use of SO₂ as the sole proxy of air pollution may not be ideal. Actually, ·O₂⁻, H₂O₂, lipid peroxidation and LA data suggested that trees at Ganges were exposed to the highest levels of stress, even though annual average [SO₂] was highest at Southern Works. Nevertheless, the investigated biomarkers provide motivation for the establishment of B. discolor a bioindicator of air pollution within the SDB. If the appropriate biomarkers are measured (e.g. LPO, electrolyte leakage, leaf chlorophyll fluorescence and LA in this study), B. discolor leaves can serve as reliable bioindicators complementing current air monitoring techniques within the EMA.