Interaction between SO₂ fumigation and drought stress on growth, photosynthesis and symbiotic nitrogen fixation in soybean, studies in an Open-top chamber facility
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Air pollution emissions from South Africa are expected to increase in the short to medium term because of increased industrial activities and expected growth in socio-economic development. The developing world faces a more significant challenge regarding air pollution control with its growing efforts at promoting industrial development. Agriculture plays a critical role in food security and in the economic growth of developing countries. The Highveld region is responsible for more than 90 % of South Africa's air pollution emissions (Rorich & Galpin, 1998). Also, the Northeastern Free State, the North-West Province and the Mpumalanga Highveld region can be considered collectively as the "breadbasket" of South Africa. It is therefore extremely important to establish evidence for the threat that S02 may pose to crop production (Marshall et al., 1998). While the impacts of air pollution have received considerable attention in North America and Europe, there has been little recognition of this issue in developing countries - including South Africa (Marshall et al., 2000). To date, no detailed research to this end has been undertaken in South Africa. The study of the physiological and biochemical basis of S02 impacts on crop plants and the determination of critical levels under South African conditions is of paramount importance. Without this, the quantification of impacts and the rational management of air pollution will remain bogged down by speculation and uninformed perceptions. The current research wishes to make a contribution in this regard. For this study, a battery of open top chambers was successfully commissioned and optimised in which a series of elevated levels of S02 (0, 50, 150 and 300 ppb S02) could be administered to assess the effects of S02 on plants as these would occur under natural conditions in the field. An external (outside) plot was included to assess the chamber effect. It is known that there is a close relationship between air quality guidelines and dose-response relationships (Sanders et al., 1995). By following the development of visual injury symptoms in this study, it could be concluded that low concentrations of S02 can reduce photosynthetic capacity without any accompanying visual injury being apparent after as much as 14 days of exposure. S02 exposure brought about large decreases in the accumulation of biomass in well watered and drought stressed Brassica oleracea and Glycine max plants. Besides biomass in drought stressed plants was reduced even more than in well watered plants, and showed that shoot growth were inhibited more than root growth. Yield reduction of up to 57 % was calculated for plants exposed to the highest S02 concentration and simultaneously subjected to drought stress. The yield reduction probably arose as a result of the increased stomatal conductance in drought stressed plants; drought stress thus not only caused a greater flux of S02 into the leaf, but also resulted in a reduction in the water use efficiency of the plants. Reductions in water use efficiency were also apparent in S02 exposed Brassica oleracea, despite the decreased stomatal conductance and transpiration rate these plants presented. Even though the test plants displayed distinctive stomatal responses toward S02, further analysis of the gas exchange data demonstrated that the S02-induced inhibition of photosynthesis in both Brassica oleracea and Glycine max, was mainly due to limitation of mesophyll (biochemical) processes and, to a lesser extent, due to stomatal limitations. Dose response relationships plotted from photosynthetic gas exchange data emphasised that S02 is a potent inhibitor of photosynthesis and can be described as highly phytotoxic. Analysis of the C02 response of the test plants exposed to different S02 concentrations revealed a severe inhibition in the maximal rate of C02 assimilation (Jmax; regeneration capacity of RuBP) and carboxylation efficiency (CE; Rubisco activity). In vitro measurement of Rubisco activity in Glycine max corroborated this finding. The photosynthetic light response data indicated that the maximum quantum yield and light-saturated rate of photosynthesis in intact leaves of plants exposed to S02 were severely reduced. Although the reduction occurred at all S02 levels applied, the magnitude of these reductions could be ascribed for the most part to the S02 concentration administered. The chlorophyll content decreased significantly at all treatment levels after 35 days' fumigation with S02 in well watered and drought stressed Brassica oleracea and Glycine max. Root nodule ureide content decreased drastically in Glycine max at all S02 concentration levels. It could be assumed that the decreased chlorophyll content and the sharp reduction in the ureide content contributed to the decreases in growth and yield production. Analysis of the chlorophyll a fluorescence OKJIP transients measured in parallel with gas exchange showed that several biophysical parameters of photosystem II were affected in the test plants exposed to S02. The parameters most severely affected were the density of active reaction centres, electron transport and the performance index. Chlorophyll a fluorescence data showed an initial stimulation in well watered plants that were exposed to the lowest S02 concentration. No stimulations of any fluorescence parameters were evident in drought stressed plants. The chlorophyll a fluorescence data supported the gas exchange data, confirming that the inhibition of C02 assimilation is due to impaired formation of end electron acceptors, ATP and NADPH. Chlorophyll a fluorescence also revealed inhibition on the electron acceptor side of PSIl, namely the oxygen evolving complex. Ultrastructural investigation of the leaf material of the Glycine max plants revealed that S02-induced damage occurred mainly in the chloroplasts. Changes in chloroplast shape, damage to internal membranes and damage to the thylakoids were the most notable changes that occurred after 35 days' fumigation with 300 ppb S02. In each case, the effect of S02 in combination with drought was of great severity, indicating that drought-induced partial stomatal closure does not relieve damage caused by S02. The implication of the data obtained has for crop production in highly industrialised regions of the Mpumalanga Highveld of South Africa is discussed.