Sulfur in Plants
Sulfur is an essential element in plants. The organic fraction in the leaves of different plants is fairly consistent in amount. Sulfate may be present in the leaves in a wide range of concentrations, without appreciably affecting the organic sulfur level. When sulfur dioxide is absorbed into the leaves of the plant in small amounts, it is primarily oxidized to sulfate or reduced to organic sulfide.
Effects of Sulfur Dioxide on Plants
Toxicity in the plants is largely due to the reducing properties of the gas. If the concentration is limited, it can be tolerated in the cells and the plant won’t be affect much. But when the concentration limit is exceeded, the cells are first inactivated with or without plasmolysis and then killed. When an extensive area of these cells are killed, the tissues dry up, leaving interveinal and marginal acute injury in the form of foliar necrosis. A slow oxidation of sulfite to sulfate occurs in the cells when only a few of the cells in an area are injured. This reduces the toxicity of the sulfite by about a factor of 30. Consequently, if the sulfur dioxide is not added to the system too quickly, a large amount may be added without causing injury before sulfate toxicity, a form of chronic injury, occurs as chronic injury results from long-term exposure to much lower concentrations of the gas. Chronic injury in plants is essentially cumulative in nature, taking the form of reduced growth and yield and increased senescence, often with no clear visible symptoms or else with some degree of chlorosis. Sulfate toxicity is manifested by white or brownish-red areas on a leaf caused by the rupture of some cells or chloroplasts within them. Sulfur dioxide can also modify the response of plants to other environmental stresses, both biotic and abiotic, often exacerbating their adverse impacts.
Stomata of the Plant
According to Black (1985), the stomata responds to many toxic chemicals because, first of all, the stomata control the major proportion of water loss from plants, thus any agent that influences this function may alter the water relations and energy balance of leaves, affecting the cellular turgor, growth and the operation of important physiological and biochemical processes of the plant. Secondly, pollutant-induced changes in the stomata aperture will influence the exchange of other gases between the interior of the leaf and the atmosphere. Plants exhibit greater damage from pollutants when they are exposed with their stomata open. Control of the pollutant uptake is largely affected by the stomata but may be influenced by other resistances that flow in the pathway into the plant.
Early work on the effects of pollutants on plant metabolism revealed severe depressions of photosynthesis and the appearance of visible injury that resulted when plants were exposed to high concentrations on sulfur dioxide (SO2). Changes in photosynthesis and Carbon Dioxide (CO2) concentration in the leaf were thought to be responsible for the reductions in transpirational water loss that were associated with the visible damage. SO2 can have a direct effect on stomatal behavior that is independent of changes in photosynthesis or appearance or visible injury (a). SO2 increased stomatal opening in beans but also occurred when the pollutant concentration is relatively low and they still retained their capacity to respond to other environmental conditions like light and CO2.
Concentration and duration of pollutant exposure may determine stomatal responses and prevailing environment conditions may influence whether stomata show enhanced opening and closure in response to exposure and not only do different species differ in their response, but even different varieties and individuals grown under different conditions may differ in their stomatal sensitivity to SO2.
Thomas (1961) said the same conditions that enhance the rate of photosynthesis and respiration of gas intake also enhances the absorption of the sulfur dioxide. These factors that cause the stomata to open include high light intensity, especially in the morning hours, high relative humidity, adequate moisture supply and moderate temperatures. Most plants close their stomata at night and are therefore much more resistant in the dark than in light. Plants that do not close their stomata at night are about equally sensitive in dark as in light.
Materials and Methods
Yield responses of 28 cultivars exposed to simulated H2SO4 rain were studied. Plants tested were grown in pots in stationary, close top field exposure chambers and exposed the three H2SO4 rain simulates of pH levels 4.0, 3.5 and 3.0 and to a control simulant at pH 5.6.
Rain stimulants were delivered using stainless steel nozzles at an average of 6.7 mm/hr, 1.5 hours per day, 3 days per week for a total of 30 mm/week.
Blowers were attached to the small round and square chambers to control the heat load in the chambers and complete chamber air exchange with ambient air at a rate of 1.5 air changes per minutes.
More than half of the crops tested on were first exposed to rain stimulants within a day of seeding. The remainder received first exposure several weeks after seeding because they were germinated in a greenhouse to provide uniform seedlings and later transplanted to study pots. Exposure to simulated rain continued until final harvest for all the crops (Cohen, 1981).
Black, V. J. (1985). SO2 Effects on Stomatal Behavior. Winner, W. E., Mooney, H. A., Goldstien, R. A. Sulfur Dioxide and Vegetation : Physiology, Ecology, and Policy Issues. (pp. 96-117). San Francisco, CA, America: Stanford University Press
Cohen, J. C. (1981). Effects of Simulated Sulfuric Acid on Crop Plants. Covallis, OR, U.S.A. Oregon State University.
Thomas, M. D. (1961). Effects of Air Pollution on Plants. http://whqlibdoc.who.int/monograph/WHO_MONO_46_(p233).pdf
WHO Regional Office for Europe, Copenhagen, Denmark, 2000