Interactions between sulphur dioxide and soil salinity in wheat plants

Abstract

Increasing sulphur dioxide (SO2) concentration in the atmosphere with expansions of electricity production and mineral processing in the world has led to numerous studies and understandings of adverse impacts of SO2 on growth and physiology of plants in Although there have been many studies on SO2 dose-response (physiology, growth, and (or) yield) relationships in plants, there is a lack of understanding of influences of environmental stress such as NaCl salinity, on plant responses to elevated SO2 concentrations in the air, which may occur in agricultural areas such as India, China and Australia with agricultural and natural ecosystems. low rainfall, or artificial irrigation and elevated SO2 levels in the air. Since stomatal conductance predominantly controls SO2 uptake and then its toxicity in leaf cells, salinity induced-stomatal closure could decrease the SO2 uptake and protect plants against SO2 toxicity. On the other hand, SO2 itself may induce changes in stomatal conductance, this may in turn, influence leaf transpiration, then salt (NaCl) uptake from the transpiration stream, and eventual salt toxicity in NaCl salinity-stressed plants, Therefore, simultaneous exposure to soil NaCl salinity and SO2 may modify each other's effects on the responses of physiology and growth in the plants. The studies in this thesis examined responses of growth and physiology in wheat plants exposed to SO2, NaCl salinity and their combination under near ambient conditions. Wheat (Triticum aestivum cv. Wilgoyne (Ciano/Gallo)) plants were exposed to factorial treatments of SO2 and NaCl salinity in fumigation chambers with rain-exclusion tops under near-ambient conditions. In experiment 1, wheat plants were exposed to a factorial combination of three levels of salinity (with high Na/Ca ratio): control, 50 and 100 mM NaCl, and three levels of SO2: < 10 (ambient), 44 and 107 nl 1-1 for 4 hours per day for up to 110 days. In experiment 2, wheat plants were exposed to a factorial combination of two levels of salinity (with low Na/Ca ratio): control and 50 mM NaCl, and three levels of SO2: < 10 (ambient), 231, and 441 nl 1-1 for 4 hours per day for up to 51 days. Low concentrations of SO2 (44 and 107 nl 1-1) stimulated vegetative growth in early growth phase, but had no effect later. Exposure to higher SO2 concentration (441 nl 1-1) significantly decreased plant growth. Low SO2 concentrations increased grain yield, but higher SO2 concentration (441 nl 1-1) decreased ear yield. Shoot to root ratios in plants were not changed by the low SO2 concentrations (44 and 107 nl 1-1), but increased by the relatively higher SO2 concentrations (231 and 441 nl 1-1). Increasing NaCl concentrations significantly reduced plant growth and grain yield. Severe NaCl salinity caused a substantial decrease in root growth, due to the effect of high Na+/Ca2+ ratio, resulting in an increase in Mild NaCl salinity initially decreased shoot growth, resulting in a decrease in shoot to root ratio, but later a shoot to root ratio. significant decrease in root growth developed, resulting in an increase in shoot to root ratio. The effects of SO2 fumigation and NaCl salinity on plant dry weights and shoot to root ratios were mostly additive, except that dry weight in plants subject to mild NaCl salinity was decreased less by 441 nl 1-1 SO2 fumigation than the nonsaline plants. Exposure to 231 nl 1-1 SO2 increased shoot to root ratio in plants with mild NaCl. To understand physiological mechanisms behind the negative growth responses to SO2 fumigation, NaCl salinity and their combinations, stomatal conductance, sulphur and salt accumulation, photosynthesis, carbohydrate concentrations, and nitrogen metabolism parameters were examined in long-term fumigation experiments under near-ambient conditions. In the long-term exposure to increased SO2 concentrations, the responses of plant growth and yield were negatively correlated with SO2 concentrations, but not with concentrations of sulphur in plant tissues. Plants exposed to low (107 nl 1-1) and relatively higher concentrations of SO2 (441 nl 1-1) had similar concentrations of sulphur in leaves. Reductions in growth and yield in plants exposed to higher concentrations of SO2 resulted from SO2 toxicity to physiological processes, such as nutritional and ionic balances, photosynthesis, and nitrogen metabolism. Exposure to 441 nl 1-1 SO2 significantly decreased stomatal conductance, net photosynthesis rate, and carbohydrate availability, which contributed an increase in shoot to root ratio. SO2 increased sulphate anion concentration which significantly disturbed ionic balance, causing increased K+ and decreased Na2 + , Ca2+ and CP concentrations and the nitrogen/sulphur balance in plant tissues. Although SO2 fumigation did not affect nitrogen uptake rate, it changed nitrogen distribution, nitrate reductase activity in leaves, soluble protein content and concentrations of total amino acids in plant tissues. Salt toxicity and secondary physiological damages contributed to growth and yield reductions in plants subject to prolonged NaCl Increasing NaCl concentrations caused excessive salt (C1-, Na+) accumulation in plant tissues, and decreases in Although net photosynthesis rates in the youngest fully expanded leaves were little affected by NaCl salinity, long-term exposure to NaCl salinity decreased nonstructural carbohydrate concentrations in plant Cumulative salt injury may have caused a significant salinity stress. concentrations of K+ and sulphate anion. tissues. photosynthetic loss in old leaves and a decrease in total carbon gain in plants. Increasing NaCl salinity also significantly disturbed nitrogen metabolism in plants, by decreasing nitrogen uptake, altering nitrogen distribution, and increasing accumulation of total amino acids. The nature of interactions between SO2 fumigation and NaCl salinity appeared to depend on their effects on the uptake and toxicity of SO2 and NaCl salt to physiological processes. Although severe NaCl salinity significantly decreased sulphur concentration in leaves of plants exposed to SO2, due to NaCl-induced increase in stomatal resistance, it decreased growth and yield in plants exposed to low SO2 concentrations and severe salinity. However, mild NaCl salinity that induced a slight increase in stomatal resistance, only slightly decreased leaf sulphur concentration in plants exposed to higher concentrations of SO2. Therefore mild NaCl salinity could not effectively protect plants against SO2 toxicity through reducing SO2 uptake in the long term, In contrast, exposure to 441 nl l-1 SO2 significantly decreased Cl' and Na+ concentrations, but increased K+ concentrations in leaves of plants subject to long-term mild salinity stress, which decreased salt toxicity to physiological processes. Fumigation with SO2 concentrations of less than 441 nl 1-1, additively affected photosynthesis, carbohydrate production, and nitrogen metabolism parameters in plants subject to long-term NaCl salinity. Exposure to 441 nl 1-1 SO2 and mild NaCl salinity had antagonistic interactions on physiological responses, such as carbohydrate concentrations and nitrogen metabolism parameters. Plant tolerance to the long-term NaCl salinity was enhanced by 441 nl 1-1 SO2 fumigation by decreasing Na+ and CF concentration and increasing K+ concentration in leaves, and salt toxicity to physiological processes, resulting in antagonistic interactions on growth and yield. It is concluded that increasing NaCl salinity could not effectively protect plants against SO2 toxicity on physiology and growth by increasing leaf stomatal resistance, in the wheat plants exposed to a long-term simultaneous SO2 fumigation under ambient conditions. The two stresses are most likely to additively affect physiological responses, growth and yield in wheat plants in the long term. However, relatively higher SO2 fumigation appeared to protect the saline plants from salt toxicity to physiological processes and eventual growth, through decreasing stomatal conductance and salt concentrations in plant tissues. The effects of SO2 fumigation, NaCl salinity and their combination on the physiological responses to eventual growth in plants are discussed in the context of conceptual models