Oxidative Damage Of Sulfur Dioxide Derivatives To Plants

Sulfur dioxide (SO₂) is a common air pollutant, present in low concentrations in the urban air, and in high concentrations under occupational exposures. It produced by combustion and processing of sulfur-containing fossil fuels and was a main component of the gaseous pollutants contributing to the formation of acid rain in China. Since SO₂ is highly soluble in water, SO₂ absorbed by plants changes into SO₃^2- rapidly. Some part of SO₃^2- is used for the biosynthesis of Cys, and the rest is changed into SO₄^2- by oxidation in a light-dependent manner. SO₄^2- is stored in vacuoles and subsequently used for the biosynthesis of Cys. The previous studies showed that SO₂ or its hydrated form, mixture of HSO₃^- and SO₃²-, caused visible foliar damage such as chlorosis and necrosis, inhibited seedling growth, cell division and sister chromatid exchange (SCE), injured photosynthetic process and also influenced the activities of enzymes for scavenging reactive oxygen species (ROS) in plant cells. But the mechanism is not clear, how and when the damage occurs in plant cells keep unknown.The high plant cytogenetic assays, which are considerably less expensive and simplicity, have been validated in the international collaborative study and proven to be efficient tests for genotoxicity monitoring of environmental pollutants.The Vicia faba test was essential carried out according to Kanaya et al. The Vicia faba seedlings divided into seven groups were used for SO₂ exposure with 28 mg/m³. Then the cytogenetic damage induced by SO₂ was investigated in root and leaf meristematic cells. The results showed that mitotic index decreased, but the frequencies of micronuclei increasedsignificantly in both root and leaf meristematic cells after seedlings exposed to SO2, and the longer the treatment duration was, the higher the degree of the effects was. After incubated in tap water, the mitotic index increased compared to those still growing in sand. At the same time, during paraffin isolated SO2 from tap water, the frequencies of micronuclei in root tip cells was lower than in leaf tip cells. It is suggested that sulfur dioxide was genotoxic at higher concentrations, and the Viciafaba test could be used to detect the toxicity of SO2 pollution.As for barley test, dry seeds (Jinke 571) were sterilized with 10% NaCIO and then germinated on moist filter paper. The control group seedlings were incubated in distilled water. Solutions of the mixture of NaHSO3and Na2SO3 (1:3) from 0.1 to 10 mmol/L were used for treatments. Our experimental results showed that activities of SOD, CAT (0.1 mmol/L group) and GSH-PX, and the contents of GSH significantly increased in barley leaves after seedlings exposed to the chemicals at concentrations of 0.1 to 10 mmol/L for 6 days. However, the activities of SOD, CAT and GSH-PX, and contents of GSH decreased after barley seedlings exposed to the chemicals for 8 days and 10 days, while the inhibition of seedlings growth enhanced. Comparing to the control, total protein contents increased after treated for 8 days, but decreased in those samples exposed for 10 days. The ratio of water to leaf fresh weight decreased with duration time in all treated groups and control, and more the relative content of water in 10 mmol/L treatment group is lower. Our results indicated that the activities of anti-oxidative enzymes changed with the concentrations and duration time after seedlings exposed to bisulfite-sulfite mixture, as well as the growth of seedlings. In a word, bisulfite-sulfite solutions is toxic to growth and physiology in barley, and this effect might come from its oxidative damage to barley cells.The inorganic sulfur in the environment (e.g. SO42" in the soil and SO2 in the air) is assimilated into Cys mainly by the Cys biosynthetic pathway in plants. Cys is incorporated into proteins and GSH or serves as the sulfurdonor of Met and sulfur-containing secondary products in plants. GSH levels respond to the availability of Cys. Therefore, the Cys biosynthetic pathway may be considered as the sole detoxification mechanism of SO2, and the enhancement of the ability of Cys synthesis in plants may be the best way to produce the transgenic plant tolerant to SO2. Effects of external Cys on the toxicity of bisulfite-sulfite solutions were investigated in barley seedlings. Results showed that the activities of SOD increased in barley leaves after seedling roots exposed to the mixture of Cys and bisulfite-sulfite solutions. At the same time, the contents of GSH significantly increased with treated concentrations, while the contents of chlorophyll and carotenoid increased, but MDA decreased in the same groups. Our results indicated that external Cys could have protective effect against the toxicity of bisulfite-sulfite solutions, and this effect might come from its anti-oxidative effects in barley cells.And external ASC can increase the mitotic index of the root tip cells in barley;meanwhile it can also improve the dry weight and the length of roots and leaves. Comparing to the control group, the activities of POD and SOD> and the contents of chlorophyll and protein in barley leaves were also increased. However, the content of MDA was decreased. It was suggested that exogenous ASC could release the toxicity of bisulfite-sulfite solutions in barley seedlings, mainly because of its anti-oxidative function.