Ascorbic acid (AA) can directly scavenge ROS and can act as a
reducing substrate for APX to detoxify H2O2 (Noctor and Foyer,
1998). During the present study, APX activity in leaves decreased
under ACO2 + EO3 to scavenge ROS formed as products of O3 break
down, but could not avoid visible O3 injury on leaves with
advancing age (Kumari and Agrawal, 2014). Significant decrease in
APX activity was also reported in tomato genotypes under O3
exposure (Castagna et al., 2007). Enhanced CAT and POX activities
under ACO2 + EO3 and ACO2 + AO3 also suggest a defense response
in plants to prevent damage against increased H2O2 content in the
leaves. The antioxidant enzymes under ACO2 + EO3 did not
counteract the ROS production to the extent it was achieved
under ACO2 + AO3 where cell membrane integrity and chlorophyll
loss were much less than ACO2 + EO3.