Responses of Trichilia dregeana leaves to sulphur dioxide pollution: A comparison of morphological, physiological and biochemical biomarkers

Industrial zones in eThekwini, South Africa, such as the South Durban Basin (SDB) are often characterised by extremely poor air quality owing to industrial emissions. This study investigated the effects of SO2 pollution on selected stress biomarkers in the leaves of Trichilia dregeana to assess the utility of this species as a bioindicator of this pollutant. Leaves were sampled from trees growing at three industrial (treatment) sites (Prospecton, Ganges and Southern Works) within the SDB and from greenhouse-located trees (ex situ control). Sampling accommodated for directional and seasonal wind effects and yielded a sample size of n = 24 for all four seasons. Ground-level SO2 concentrations ([SO2]) measured at each site were positively correlated with leaf sulphate contents and both [SO2] and leaf sulphate levels of the control were lower than the treatment sites. Values for the various biomarkers did not differ significantly for leaves from different cardinal points within sites, but seasonal variations were evident. Except for leaf chlorophyll fluorescence, all biomarkers could discriminate between treatment and control leaves. Though seasonal data for a number of these biomarkers were significantly correlated with leaf sulphate content, none of the biomarkers were sensitive enough to reflect differences in leaf sulphate levels across the treatment sites. Furthermore, the results suggest that leaf area and relative chlorophyll content should be measured in combination with each other. Leaf sulphate content is a reliable proxy for SO2 pollution. All biomarkers, except for chlorophyll fluorescence, can be used to establish T. dregeana leaves as a bioindicator of SO2 pollution.