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).