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. / Thesis (M.Sc. (Botany))--North-West University, Potchefstroom Campus, 2006.
| Identifer | oai:union.ndltd.org:NWUBOLOKA1/oai:dspace.nwu.ac.za:10394/1110 |
| Date | January 2006 |
| Creators | Swanepoel, Jacoba Wilhelmina |
| Publisher | North-West University |
| Source Sets | North-West University |
| Detected Language | English |
| Type | Thesis |
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