Coal dust pollution effects on wetland tree species in Richards Bay, South Africa

摘要

In this study, the effects of coal dust on four, sympatric, wetland tree species in Richards Bay Harbour were investigated. We tested the hypothesis that leaf micromorphology influenced dust accumulation and that coal dust occluded stomata and reduced photosynthetic performance of three mangroves, Avicennia marina, Bruguiera gymnorrhiza and Rhizophora mucronata, and a mangrove associate, Hibiscus tiliaceus. To investigate leaf micromorphology, leaf blade material of the four species was prepared following standard procedures and viewed under scanning electron microscopy. Gas exchange and chlorophyll fluorescence measurements were made at saturating light (> 1000 μmol m~(-2) s~(-1)) and high temperature (> 25℃) on leaves that were either covered or uncovered with coal dust. There was no evidence of occlusion of stomata by dust. Dust accumulation in A. marina and H. tiliaceus was exacerbated by the presence of a dense mat of trichomes on the undersurface of the leaves, as well as by the sticky brine secreted by salt glands in the former species. Coal dust significantly reduced CO_2 exchange, Photosystem II (PS II) quantum yield and electron transport rate (ETR) through PS II in A. marina and H. tiliaceus but not in the other two mangroves. Reduction in photosynthetic performance was attributed to reduction in light energy incident on the photosynthetic tissues.