| Abstract: | The stomatal resistance, measured with a ventilated diffusion porometer at various times before, during, and after exposure to 20–25 pphm ozone, was followed in water-stressed or well-watered beans, beans exposed at either low (37%) or high (73%) atmospheric humidity, and two tobacco cultivars exposed at the same two humidities. The two tobacco cultivars that were compared were the 03-susceptible Bel W-3 and the 03-resistant Consolidated L. The stomata of the water-stressed but unwilted bean plants closed quickly from a resistance of 2.9 ± 0.3 sec/cm to 8.4 ± 1.0 sec/cm when exposed to O3 whereas those in the unstressed plants closed slowly from a resistance of 2.5 ± 0.6 sec/cm to 5.2 ± 0.8 sec/cm after exposure to O3 for 10 min. Exposure to 03 for 30 min in the moist atmosphere caused no change in stomatal resistance of the bean plants whereas in the dry atmosphere the stomata closed from a resistance of 3.7 ± 0.4 sec/cm to 6.7 ± 0.6 sec/cm, but opened again when ozonation was terminated. With tobacco exposed to O3 in a dry atmosphere the stomata of the 03-resistant cultivar closed more rapidly than the 03-susceptible variety, whereas in a moist atmosphere the stomata of both cultivars closed slowly and equally during the 60 min of ozonation. |
