Pre-exposure of calli to ozone promotes tolerance of regenerated Lycopersicon esculentum cv. PKM1 plantlets against acute ozone stress

Studies were performed to evaluate the effects of pre-exposure of calli to ozone in promoting tolerance of the regenerated Lycopersicon esculentum cv. PKM1 (tomato) plantlets against acute ozone stress (AOS). Calli induced from tomato leaf explants were subjected to pre-treatment with ozone: T 1=100 ppb, T 2=200 ppb and T 3=300 ppb. For the control (C) calli, charcoal-filtered air was supplied to test differential sensitivity of regenerated plantlets to acute ozone stress. All treated calli were subsequently transferred to shooting, rooting medium and acclimatized. The plantlets regenerated from the respective ozone ( T 1, T 2, T 3)-treated calli are referred to here as T 1, T 2, T 3 plantlets and the plantlets regenerated from control calli are referred to as control plantlets. The frequencies of regeneration of tomato plantlets from the calli were T 1=86%, T 2=82% and T 3=67%, and 92% regeneration was obtained from control calli. In order to evaluate the ozone tolerance, all the regenerated plantlets were exposed to the acute ozone exposure (AOE). After AOE, the T 2 plantlets endured remarkably well by experiencing reduced ozone stress, which was evident from the lower level of hydrogen peroxide and oxidative stress-related enzymes such as ascorbate peroxidase (EC 1.11.1.11) and superoxide dismutase (EC 1.15.1.1) activities relative to T 3, T 1 and C plantlets. All T 2 plantlets showed enhanced tolerance against AOE by upholding enhanced soluble phenol content, a higher level of foliar and apoplastic ascorbic acid, elevated dehydroascorbate reductase (EC 1.8.5.1) and glutathione content. The present study reveals that the calli pre-exposed to T 2 ozone treatment resulted in an increase in the level of antioxidants and provided the plants greater protection against acute ozone stress.