Effect of different concentrations of ozone on physiological changes associated to gas exchange, fruit ripening, fruit surface quality and defence-related enzymes levels in papaya fruit during ambient storage

• Ozone fumigation delayed ripening of papaya.
• High concentration of ozone (>3.5 ppm) caused surface browning.
• Ozone increased activities of PAL, POD and PPO in papaya.

Papaya fruit (Carica papaya L.) were exposed continuously to ozone fumigation (0, 1.5, 2.5, 3.5 and 5.0 μL L−1) for 96 h prior to ambient storage (25 ± 3 °C and 70 ± 5% RH) for an additional 10 days. The rate of ethylene formation and changes in the activities of the plant defence enzymes, phenylalanine ammonia-lyase (PAL) (EC 4.3.1.5), peroxidase (POD) (EC 1.11.1.7) and polyphenol oxidase (PPO) (EC 1.14.18.1), were measured in ozone treated and untreated papaya fruit after 0, 2, 4, 6, 8, 10, 12 or 14 days. The fruit treated with ozone at concentrations lower than 5 ppm had lower respiration rate and delayed ripening compared to the control. However the fruit that had received high ozone concentrations (>3.5 μL L−1) produced more ethylene and caused injury to fruit tissue. Enzyme activities were higher in ozone treated papaya fruit than in untreated fruit throughout the storage period. The greatest changes in enzyme activities were obtained with the highest ozone dose (5 μL L−1).