## 3 Results and Discussion
### Antioxidant Activity, Total Phenolic and Ascorbic Acid Content  
Application of ozonation process in controlled conditions is associated with the occurrence of oxidative stress in the plant cell structures as a result of which enzymatic systems are activated, leading to the production of greater quantities of secondary metabolites affecting the final quality of the raw material [25].  
The ozone treatment applied to rhubarb petioles immediately after harvest produced a change in antioxidant potential determined in relation to ABTS and DPPH radicals (Figure 6), and in the total contents of polyphenols (Figure 7) as well as the contents of vitamin C (Figure 8). Short exposure (5 min) of rhubarb petioles to gaseous ozone at a rate of 10 ppm led to an increase in antioxidant activity  
Figure 4: Mean values of destructive force identified for the specimens \(\varphi=4\) mm cut out transversely to the axis of rhaponic rhubarb petioles. Differences in the results between the values of the destructive force for individual variants are marked with different small letters, \(p<0.05\).  
Figure 5: Destructive force identified for the specimens \(\varphi=4\) mm cut out along the axis of rhaponic rhubarb petioles. Differences in the results between the values of the destructive force for individual variants are marked with different small letters, \(p<0.05\).  
(Figure 6A,B). On the other hand, the ozone treatment applied at the same rate and prolonged to 15 or 30 min resulted in a decreased antioxidant potential of the plant material investigated (Figure 6B). A similar relationship was observed when ozone was applied at a higher rate of 100 ppm. However, if the ozone treatment was continued for a duration of 30 min, there was a significant increase in the antioxidant potential of the raw material, compared to the control variant (Figure 6A). Likewise, 30 min exposure of rhubarb petioles to ozone applied at a rate of 100 ppm resulted in the highest increase in total polyphenols (Figure 7) and vitamin C (Figure 8), compared to the control.  
Figure 6: Antioxidant activity. ABTS test (part **A**), DPPH test (part **B**) in _Rheum rhaponticum_ L. petioles, depending on the duration of ozone treatment (n = 3). The differences in the results between the ozonation time for individual variants are marked with different small letters and the difference between the measurement values for the tested variants is marked with different capital letters, \(p<0.05\).  
Figure 7: Total polyphenolic content in _Rheum rhaponticum_ L. petioles, depending on the duration of ozone treatment (n = 3). The differences in the results between the ozonation time for individual variants are marked with different small letters, and the difference between the measurement values for the tested variants is marked with different capital letters, \(p<0.05\).  
The use of the ozonation process on the petioles of rhubarb gives them higher concentrations of bioactive compounds than in natural conditions. After this process, this raw material has new properties that affect its usefulness in processing, including the production of functional food with a high content of bioactive compounds, especially antioxidants [26].  
Increased contents of bioactive compounds in raw plant material subjected to ozone treatment were also reported by [PERSON] and [PERSON] [25]. Their study showed that exposure of raspberries to ozone resulted in higher contents of polyphenols, vitamin C and antioxidant potential when the fruit were stored at room temperature. A similar relationship between the contents of selected bioactive compounds and ozone treatment was demonstrated for raspberries by [PERSON] et al. [27] and for high blueberry by [PERSON] et al. [8]. [PERSON] et al. [28] investigated effects of gaseous ozone in papaya fruit. They found that exposure to the gas produced increase in polyphenol contents and in antioxidant potential of the plant material.