Effects of sulphur dioxide on photosynthesis in the succulent Augea capensis Thunb

The impacts of sulphur dioxide (SO2) 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 C3-mode CO2 assimilation. We investigated the effects of SO2 treatments (1.3 and 0.6 ppm), administered under controlled growth conditions, on potted plants of Augea capensis Thunb., a succulent exhibiting C3-mode photosynthesis. The effects on photosynthesis were assessed by means of CO2 assimilation and chlorophyll a fluorescence measurements. Surprisingly, the only inhibitory effects on photosynthesis were induced when SO2 fumigation occurred in the dark. An inhibition of 38% and 62% in carboxylation efficiencies and CO2 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 SO2 detoxification, is proposed for the observed diurnal (day versus night) differences in SO2 sensitivity in this species.