For this reason, symptoms are first observed on older leaves. 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. 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. Veins of severely affected leaves also become necrotic and leaves often drop prematurely.