Within-photoperiod differences in ozone sensitivity of soybean and tomato

Tropospheric ozone (O3) is produced photochemically resulting in daily patterns of concentrations that normally peak in mid-photoperiod, but timing of the peak concentration may vary according to atmospheric conditions. This study was conducted to determine if there are within-photoperiod differences in plant sensitivity to O3. Soybean (Glycine max Merr.) and tomato (Lycopersicon esculentum Mill.) plants were grown in controlled environment chambers for 14 d and exposed to O3 concentrations of 0.08, 0.12, 0.16, 0.20 and 0.24 μL L−1 for 1 h. Exposures occurred beginning at 10:30, 13:30 or 16:30 h, during a daily photoperiod of 06:00–22:00 h. Differences in sensitivity (as indicated by significant (P ≤ 0.05) growth differences) depending on time of O3 exposure were observed for both species. The order of plant sensitivity for exposure times was 10:30 > 13:30 > 16:30 h. Sensitivity to O3 was concluded to depend on the time of exposure within the photoperiod. Key words: Glycine max Merr., Lycopersicon esculentum Mill., ozone injury, within-photoperiod, controlled exposures, plant growth