Tomato response to concurrent and sequential NO2 and O3 exposures

In the ambient environment, concentrations of air pollutants vary on a diurnal cycle, resulting in various patterns of concurrent and sequential exposures of plants. The response of tomato plants to sequential and concurrent NO2 and O3 exposures was determined using pollutant levels equal to the maximum acceptable levels recommended by the National Ambient Air Quality Objectives of Environment Canada for a 1 h average. The concurrent treatment, 1 h of NO2 + O3, was compared to 1 h of NO20, O3 or control in plants at the 4 to 6 or the 9 to 11 leaf stage. At the 4 to 6 leaf stage, leaf and stem fresh weights were significantly reduced by the NO2 + O3 treatment relative to control, whereas these growth parameters were not reduced relative to control by the single pollutants indicating a coalitive response. Leaf area was significantly smaller as a result of the NO2 + O3 treatment relative to the NO2 treatment. A main effect of O3 was observed on leaf dry weight. The sequential treatments were: NO2 followed by O3 (NO2-O3); O3 followed by NO2 (O3-NO2); NO2 at night followed by O3 during the daytime (NO2(N)-O3(D)). Each gas exposure was 1 h; only plants at the 4 to 6 leaf stage were treated. Only the O3-NO2 treatment significantly reduced leaf area, leaf fresh weight and stem fresh and dry weights relative to control plants. Inconsistencies among treatments occurring at different time periods of the day suggest that time period of exposure should reflect ambient time periods. The coalitive action, and the sequential treatment response, of these pollutants indicated that criteria based on single pollutants may not be adequate to establish air quality objectives when these pollutants occur together.