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The impacts of sulphur dioxide (SO 2 ) pollution on succulents in arid environments are relatively unknown and the few studies that have considered these effects focused primarily on species exhibiting crassulacean acid metabolism (CAM). However, the unique metabolic adaptations in CAM species make it difficult to extrapolate these findings to succulents with typical C 3 -mode CO 2 assimilation. We investigated the effects of SO 2 treatments (1.3 and 0.6 ppm), administered under controlled growth conditions, on potted plants of Augea capensis Thunb., a succulent exhibiting C 3 -mode photosynthesis. The effects on photosynthesis were assessed by means of CO 2 assimilation and chlorophyll a fluorescence measurements. Surprisingly, the only inhibitory effects on photosynthesis were induced when SO 2 fumigation occurred in the dark. An inhibition of 38% and 62% in carboxylation efficiencies and CO 2 saturated rates of photosynthesis were observed. However, these effects only occurred at the highest concentration and were fully reversible, indicating no permanent metabolic damage. Only minor effects on photosystem II electron transport were observed, indicating that photochemical reactions were not the primary site of inhibition. An explanation, based on possible diurnal differences in cellular capacity for SO 2 detoxification, is proposed for the observed diurnal (day versus night) differences in SO 2 sensitivity in this species. |
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