3.6 │ Activity of polyphenol oxidase (PPO)
As shown in Table 7, comprehensible difference in PPO activity was not observed in the control of each genotype from day 7 to day 21. After inoculation with P. melonis , the PPO activity in the cucumber plants changed notably. Changes in the PPO activity of the roots of the six cucumber genotypes with different levels of resistance after inoculation with P. melonis are shown in Table 7 and Fig 6. The PPO activities in the resistant and moderately resistant genotypes were significantly higher than those in the three highly susceptible genotypes. In Soheil and Baby, the activity of PPO continuously displayed high activity until 21 DAI (Fig 6.A and B). In the resistant genotype Ramezz, the significant increase in the activity of POX occurred at 7 DAI and then began to decrease at 14 and 21 DAI after inoculation (Fig 6. C). The maximum amount of PPO activity was found to be 6-fold higher in roots of inoculated Ramezz at 7 DAI than the respective control plants. The CAT activity of the three highly susceptible genotypes Extrem, Mini 6-23 and Yalda revealed a similar trend after inoculation: a slow increase at 7 DAI followed by a decrease at 14 and 21 DAI (Table 7). In the present study, PPO activity showed no significant differences in inoculated susceptible genotypes, as compared to non-inoculated plants. There was no correlation between damping-off susceptibility and PPO activity in the cucumber genotypes (Supplemental Table 1). Furthermore, the activity of PPO significantly was correlated with the POX (r = 0.23) and PAL (r = 0.53) activities in the six genotypes of cucumber with different susceptibilities to damping-off (Supplemental Table 2).