Effects of abscisic acid on ozone tolerance of rice (Oryza sativa L.) seedlings

Ozone is one of the major gaseous pollutants detrimental to crop growth and metabolism. The objective of this research was to study how ABA ameliorates the effects of ozone on rice seedlings. Seedlings of two rice cultivars with different sensitivities to ozone (Tainting 67, tolerant; and Taichung Native 1, sensitive) were treated with 400 ppb of ozone or ABA and 400 ppb of ozone to determine their effect on growth, stomatal movement, chlorophyll characteristics, and the activity of antioxidant enzymes. Activities of the enzymes SOD, APOD, GR and POD were significantly higher in the sensitive cultivar, TN 1, than in the tolerant cultivar, TNG 67. Seedlings of the sensitive cultivar pretreated with ABA (10 muM) were significantly more tolerant of ozone than control seedlings. Pretreatment with ABA effectively reduced stomatal conductance and the degree of injury. Abscisic acid also increased ascorbate peroxidase and glutathione reductase activity. Ozone increased peroxidase activity in sensitive seedlings, but ABA decreased peroxidase activity. The sensitive cultivar had a higher density of stomata on its leaves than the tolerant cultivar. The results suggest that ABA induced tolerance to ozone may be more associated with its effects on stomatal movement than on the modulation of antioxidant enzyme activity.