Synergistic effect of flyash and SO2 on development of cucumber (<i>Cucumis sativus</i> L.) leaf injury

Abstract In a two-factorial experiment, sulfur dioxide alone at concentration of 131 μg/m3 over long exposures or flyash from either coal or oil combustion alone applied onto cucumber (Cucumis sativus L.) foliage causes no visible injury. However, if both flyash and sulfur dioxide were presented simultaneously, a specific mosaic-chlorosis symptom developed on leaves. A synergistic effect was thus demonstrated. Histopathology of the mosaic-chlorosis symptom was studied. An unusual pattern of tissue abnormally initiated from the collapse of the anticlinal cell walls of the upper epidermal cells and progressed along cell walls into mesophyll tissues. Cellular injury at the lower spongy tissues typical of that caused by SO2 was also observed. A significant elevation of sulfate-sulfur in the leaves treated with a combination of SO2 and flyash was demonstrated. Significant interactions between the two factors were found for both injury and sulfur chemistry. This suggests that flyash deposits on the leaf surface play a catalytic role in oxidizing atmospheric sulfur dioxide continuously to form sulfuric acid (H2SO4) in situ. This acid could infiltrate into mesophyll damage cells and tissues en route.