Air Pollution

Guy Padgett  |  8/20/2011 12:23:37 AM

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Ozone

Ozone

Ozone.

Ozone is the most widespread air pollutant that damages plants, including soybeans. Ozone is generated fro the reaction of sunlight with nitrous oxides and oxygen in the presence of hydrocarbons. Such a mixture is commonly referred to as smog and arises primarily from automobile exhausts in urban areas. Most damage occurs when contaminated air stagnates over soybean fields.

Prolonged exposure may result in reduced growth and yields depending on the cultivar and the stage of plant growth. Damage is most severe when plants are growing under optimal conditions. Drought-stressed plants are resistant. Soybeans exposed over a long period (several days or more) to low ozone levels of 0.06 ppm or greater, although damage and symptom development vary depending on the cultivar.

Leaves are most sensistive during the later stages of leaf expansion and symptoms appear 48 hours or more after exposure. For this reason, symptoms are first observed on older leaves. Symptoms often referred to as leaf stippling or bronzing, include small cream to bronze colored lesions on upper leaf surfaces between the veins. Injured leaves may ultimately exhibit chlorosis and die prematurely.

Examination of local air-quality data in addition to symptom appearance will help confirm ozone injury.

Sulfur dioxide is produced primarily from the burning of fossil fuels and is most damaging near coal-fired power plants and ore-smelting operations. Relatively high levels of sulfur dioxide (25 ppm or greater) must occur for symptom development, thus damage is generally confined to localized areas downwind from a point source. Recent installation of emission control devices has reduced the occurrence of sulfur dioxide injury.

The youngest fully-expanded leaves are most sensitive. Plants growing under adequate soil moisture are more sensitive than drought-stressed plants. Yield may be reduced if exposure is prolonged or occurs during early reproductive stages of growth. Sensitivity varies among soybean cultivars.

Symptom development may be chronic from exposure to low levels over a long period, or acute following a brief exposure to high levels of the pollutant. Chronic symptoms consist of leaf chlorosis occurring between veins. Acute symptoms include watersoaked areas between leaf veins that later become necrotic and appear bleached. Veins of severely affected leaves also become necrotic and leaves often drop prematurely. Symptoms often appear on both leaf surfaces.

The proximity of affected fields to a point source of the pollutant, and air quality data are useful for diagnosis.
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