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Soil salinity and water stress modify crop sensitivity to SO2 exposure

Ma, Qifu (1993) Soil salinity and water stress modify crop sensitivity to SO2 exposure. PhD thesis, Murdoch University.

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Sulphur dioxide (SO2) is a pnmary gaseous pollutant which has toxic effects on the growth, yield and quality of both agricultural and natural plant species. Although plant responses to SO2 exposure have been extensively studied, much less is understood concerning the influences of other environmental stresses on the expression of effects of gaseous air pollutants. Evaluation of such interactions should be of an economic importance in agriculture and horticulture since plants growing in the field usually encounter air pollution and other stresses simultaneously. Soil water stress and salinity are the common environmental stresses and they have some physiological similarities. This thesis aims to investigate to what extent water stress and salinity modify or amplify the detrimental effects of SO2 on foliar injury, plant growth and yield, and some physiological and biochemical changes in potato (Solanum tuberosum L. cv. Russet Burbank) and soybean (Glycine max L. cv. Buchanan) crops under field conditions.

SO2 exposure induced growth reductions in well-watered potato plants but usually not in the water-stressed plants, indicating a protective function of soil moisture stress in the response of plants to SO2. This could be caused by a reduced SO2 uptake m water-stressed plants, as well-watered plants had much higher leaf sulphur concentrations than did the water-stressed plants at the same SO2 fumigation levels. SO2 also increased leaf sulphur concentrations in soybean, but simultaneous exposure to SO2 and salinity significantly decreased leaf sulphur concentrations when compared with exposure to SO2 alone. As a consequence, SO2-induced foliar injury was more severe in the well-watered or nonsaline plants than in the water-stressed or saline plants.

Exposure conditions can also be important in determining the response of a plant to stress interactions. Contrasts of sequential and simultaneous exposures to SO2 and salinity were made in this project so as to examine stress compensatory mechanisms and predisposition characteristics. It was found that low salinity pretreatment (27 mM NaCl) ameliorated the detrimental effects of SO2 on soybean growth probably by inducing stomatal closure. However, high salinity (48 mM NaCl) treated plants, which also showed high stomatal resistance, were severely injured by subsequent SO2 exposure especially at high SO2 concentrations (300 nl 1-1). It was likely that high salinity pretreatment decreased or even destroyed plant homeostasis due to direct injury of high ion concentrations. By comparison, plants pretreated with SO2 became vulnerable to salt injury and those pretreated with high SO2 were killed after 12 days of high salt stress. This was probably because SO2 altered the patterns of assimilate allocation favouring shoot growth at the expense of root growth and induced other metabolic changes. As a consequence, the resistance of polluted plants to salinity stress was reduced.

SO2 pollutant increased the shoot to root ratios by either reducing root growth or stimulating shoot growth, whereas soil moisture stress had the opposite effect. Exposure to 300 nl 1-1 SO2 under well-watered conditions induced an increase in the shoot to root (including tuber) ratios of potato plants early in the growing season. In contrast, water stress decreased the ratios in the control and 110 nl 1-1 SO2 treatments, but not at 300 nl I-1 SO2 indicating that high SO2 had disrupted this acclimatory response to soil moisture stress. SO2-induced increase in the shoot to root ratios was also observed in the soybean experiments. However, it appeared that soil salinity did not significantly affect the ratios.

High SO2 decreased the number and weight of root nodules, and suppressed nodule nitrogenase activity. Consequently, both shoot and root nitrogen concentrations were reduced. In combination with low salinity, however, the adverse effects of high SO2 on nodule number, specific nodule activity and plant nitrogen concentrations were ameliorated. Biomass was usually not very sensitive to the interactive effects of SO2 and salinity, probably because it is slower to respond to the stresses following physiological and biochemical processes. In the field, stress interactions may become even more complicated due to interactions with other environmental stresses.

In conclusion, moderate soil salinity and moisture stress can modify crop sensitivity to SO2 exposure mainly through stomatal mechanisms. Such interactions, together with the knowledge of interactions of gaseous au pollutants and other environmental stresses (e.g. light, humidity and temperature), are important when we attempt to establish dose or concentration-response relationships for the development of predictive models for the effects of air pollutants on crops or native plants. Environmental factors may readjust the dose thresholds of au pollutants, above which detrimental effect are likely and below which insignificant effects or growth stimulations occur. Therefore, air quality standards designed to protect vegetation may need to· consider variations in regional environmental conditions.

Item Type: Thesis (PhD)
Murdoch Affiliation: School of Environmental Science
Supervisor(s): Murray, Frank
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