Physiological response of the succulent Augea capensis (Zygophyllaceae) of the southern Namib desert to SO2 and drought stress
Swanepoel, Jacoba Wilhelmina
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The main aim of this study was to investigate the effects of water availability and SO2 pollution, imposed separately or simultaneously, on the photosynthetic metabolism of Augea capensis Thunb., a succulent of the Namib Desert in the region of Skorpion Zinc mine, Namibia. The main driver for this investigation was the need to distinguish between the effects of water availability on plants native to a desert environment, where water availability dominates plant response, but where the possibility of anthropogenic SO2 pollution poses a new threat to the unique succulent vegetation. Fifteen measuring sites were selected in the vicinity of the mine to determine how rainfall influenced the physiological status of the vegetation. Chlorophyll a fluorescence measurements, and analysis of recorded OJlP fluorescence transients with the JIP-test, were used for this purpose. A series of laboratory experiments were also conducted on A. capensis to determine the precise physiological response that water deprivation and SO2 pollution had under controlled growth conditions. Potted plants were exposed to water deprivation or SO2 fumigation in the light or dark. Besides chlorophyll a fluorescence, photosynthetic gas exchange and Rubisco activity were also measured. Changes in fast fluorescence rise kinetics observed under field conditions suggest considerable modulation of photosystem II function by rainfall with concomitant involvement of a heat stress component as well. In both the field and laboratory experiments, one of the JIP-test parameters, the so-called performance index (PIABS), was identified as a very sensitive indicator of the physiological status of the test plants. Moreover, under laboratory conditions, a good correlation existed between the water deprivation-induced decline in CO2 assimilation rates and the decline in PIABS values. The JIP-test in general, and the PIABS in particular, shows considerable potential for application in the investigation of water availability influences on desert ecosystems. In the laboratory experiments, water deprivation caused stomatal closure but also a slight elevation in intercellular C02 concentration and inhibition of Rubisco activity, suggesting that mesophyll limitation was the dominant factor contributing to the decrease in C02 assimilation rates. Following re-watering, A. capensis showed remarkable recovery capacity. Fumigation of A. capensis with 1.2 ppm SO2 in the dark or light revealed relatively small effects on C02 assimilation. The inhibitory effects on photosynthesis were also fully reversible, indicating no permanent metabolic/structural damage. The effects on photosynthesis were more pronounced when fumigation occurred in the dark. This phenomenon might be related to diurnal differences in cellular capacity for SO2 detoxification. When long-term moderate water deprivation was combined with simultaneous SO2 fumigation, there was no additional inhibitory effect on photosynthesis. These findings suggest that water deprivation do not increase sensitivity towards SO2 pollution in A. capensis. Fumigation with SO2, singly or in combination with water deprivation also had no major effect on chloroplast ultrastructure. It appears that A. capensis is remarkably resistant to SO2 pollution even in the presence of low water availability, which is a common phenomenon in desert ecosystems. Since A, capensis seems to be highly tolerant to S02, its suitability as an indicator species for the detection of SO2 pollution effects at Skorpion Zinc mine is questionable. Because water availability dominates the physiological/biochemical response in this species, subtle SO2 pollution effects might be difficult to detect against this dominant background. The high water content of A. capensis and similar succulents might act as a substantial sink for SO2 and could convey considerable tolerance against this form of air pollution.