ABSORPTION OF ATMOSPHERIC NO2 BY PLANTS AND SOILS .2. NITRITE ACCUMULATION, NITRITE REDUCTASE-ACTIVITY AND DIURNAL CHANGE OF NO2 ABSORPTION IN LEAVES

NO2 uptake by leaves and reduction systems of nitrite produced were investigated using 3-wk-old kidney bean, sunflower and corn plants, which were grown in vermiculite beds supplied with tap water. The plants were fumigated with 4 ppm NO2 for 6 h in an artificially-lit chamber (30 klx), and the following events were observed. Fumigation in the light at daytime caused severe injury (wilting) in kidney bean leaves and slight injury in sunflower leaves, but corn leaves were tolerant to this treatment. A high concentration of nitrite rapidly accumulated in the kidney bean leaves, but the nitrite reductase activity was only increased to a small extent. In sunflower leaves a high concentration of nitrite was accumulated, and the nitrite reductase activity also increased rapidly. Fumigation in the dark at nighttime caused low concentration of nitrite to accumulate in kidney bean and sunflower leaves, and it was leveled down at the later fumigation time. The nitrite reductase activity of the 2 plants was increased with a slow rate continuously during the NO2 fumigation period. In the corn leaves, nitrite was not detected in the daytime, but only a low concentration of nitrite was detected at night. A high activity of nitrite reductase was detected compared to the other 2 plants. Application of nitrate through the culture solution reduced nitrite accumulation in the light but not in the dark. These results indicate that acute NO2 injury is mainly related to nitrite accumulation. Low NO2 absorption and high nitrite reductase activity (including its rapid induction) may reduce injury in leaves. When kidney bean and sunflower plants were fumigated with 4 ppm NO2 at different times of the day, a high concentration of nitrite accumulated only in the daytime, and not at night even if they were fumigated in the light. Absorption of 15NO2 at night by sunflower leaves was around 14% than at daytime. A diurnal change of NO2 uptake apparently is operating.