The effect of ozone on photosynthesis and respiration of Scenedesmus obtusiusculus Chod., with a general discussion of effects of air pollutants in plants

In the present study the mode of action of the air pollutant ozone was investigated by studying its effects on photosynthesis, respiration and some biochemical and structural properties of the unicellular alga, Scenedesmus obtusiusculus CHOD.In chapter 1, an effort was made to review the extensive literature on the effects of the most important air pollutant gases SO 2 , HF, NO 2 , PAN and O 3 on the processes of photosynthesis and respiration, while possible modes of action of each of these gases are discussed.In chapter 2, section 2.1., the procedure for ozone exposure is extensively described; a lot of time was required to obtain a reliable experimental design.In chapter 3, section 3.1., it is shown that photosynthesis and dark respiration are inhibited by previous exposure of the cell suspensions to ozone; the inhibition is increased with the duration of exposure and with temperature during exposure. The effect of an ozone treatment is increased by light during the exposure, and, consequently, photosynthesis appears more inhibited in as much as the light- intensity during the preceding exposure to ozone was higher. Photosynthesis is most inhibited in the light saturated part of the light intensity curve, which indicates that the inhibition of dark reactions of photosynthesis is more pronounced than that of the primary photoreactions. The degree of inhibition of photosynthesis is practically independent of cell density during a preceding exposure to ozone in the dark. The inhibition of photosynthesis, induced by an ozone exposure in the light, can be somewhat recovered by an after-treatment with pure air; however, the recovery effect was not alway very clear.In chapter 3, section 3.2., it is shown that the chlorophyll content decreases by exposure to ozone; the decrease is more pronounced after exposure in the light than after exposure in the dark. Preferentially, chlorophyll a is destroyed, which implies that the ratio chlorophyll a/b decreases.In chapter 3, section 3.3., it appeared in a preliminary study that there is a substantial loss of lipid in ozone-exposed algae, and there are indications that this is mainly due to oxidation of the unsaturated fatty acid, linolenic acid. Malondialdehyde production, often accompanying lipid peroxidation, was not detected in the ozone exposed algae.In chapter 3, section 3.4., it is shown in a preliminary study that there is a very small loss of protein content in ozone-exposed algae, which may be ascribed mainly to oxidation of the acid component serine, and of an unidentified alkaline compound.In chapter 3, section 3.5., in a preliminary study, it is described that, in ozone exposed algae, ultrastructural changes are observed, e.g., a disruption of the plasmalemma and the chloroplast envelope, a widening of the chloroplast membrane system, and the formation of electron-thin ring systems around starch granules.