Abstract :
Ozone (O3) is the most important gaseous air pollutant in the world because of its adverse effects on vegetation in general and crop plants in particular. Since nitrogen dioxide (NO2) is a precursor of ozone, studying the implication of sequences of these two gases is very important. Hence, the effects of sequences of fluctuating levels of ozone (O3) and/or nitrogen dioxide (NO2) on growth and stomatal diffusive resistance were studied in Radish (Raphanus sativus L.). Nitrogen dioxide was applied either alone early in the day (06:00 to 10:00 hr), late (18:00 to 22:00 hr), both early and late (06:00 to 10:00 hr and 18:00 to 22:00 hr) or early with ozone (O3 from 10:00 to 18:00 hr), late with ozone, and both early and late with ozone. Ozone was also applied alone (10:00 to 18:00 hr), while control plants (C) were not exposed to either gas. The exposure profiles for both gases pproximatedsine waves with peak concentrations of 120 parts per billion by volume; nl l-1. In the case of O3, this is close to the reported threshold for adverse effects, while with NO2 it is below the reported threshold. Ozone alone had no effect on growth after 21 days; while NO2 caused a significant increase in growth only when applied alone early. Exposures to NO2 in sequence with O3 had negative effects on growth. In the control treatment, stomata tended to be relatively closed in the morning and late afternoon. Nitrogen dioxide alone enhanced this closure, while O3 alone or with NO2 countered the closure.
