Open-top chamber facility to study air pollution impacts in South Africa. Part II: SO2–drought interactions on yield, photosynthesis and symbiotic nitrogen fixation in soybean

Abstract

South Africa has an extremely energy-intensive economy, resulting in substantial air pollution through its coal-fired power stations. Modelled sulphur dioxide (SO2) concentrations on the central Highveld mostly range between 10 and 50 ppb, exceeding 50 ppb in source areas. Well-watered and drought-stressed soybean (Glycine max) plants were exposed to different SO2 concentrations in open-top chambers to study the physiology of SO2 injury by measuring in parallel growth, biomass accumulation, photosynthetic gas exchange (as a function of internal CO2 concentration and photon flux density), chlorophyll a fluorescence, in vitro Rubisco activity and symbiotic nitrogen fixation. A strong concentration-dependent SO2-induced inhibition was displayed in all variables. After fumigation for only 7 d, photosynthesis was reduced without any accompanying visual injury symptoms, even at the 50 ppb treatment level. Exposure to SO2 also resulted in large decreases in biomass accumulation of both well-watered and drought stressed plants. Seed yield reduction of up to 57% occurred in plants exposed to the highest SO2 concentration and simultaneously subjected to drought stress. Root nodule ureide content was lowered at all treatment levels, but was lowered more in the SO2-treated plants subjected to drought stress. The photosynthetic gas exchange data showed a severe decrease in carboxylation and quantum efficiency pointing at increasing mesophyll limitation. The chlorophyll a fluorescence data, pointing at impaired electron transport and formation of end electron acceptors as well as the in vitro activity of Rubisco, supported the gas exchange data. Inhibition of photosynthesis proved to be the main constraint imposed by SO2. SO2 stress was aggravated by simultaneous drought stress.