3.3 │ Superoxide dismutase (SOD)
Analysis of variance and the activities of SOD in roots of susceptible and resistant cucumber genotypes subjected to pathogen infection are presented in Table 2 and 4 and Fig 3. The SOD activity in the control of each genotype showed little change from day 7 to day 21 (Table 4). However, the SOD activity in the inoculated cucumbers changed dramatically. Before inoculation, the SOD concentrations of all genotypes were relatively low. In resistant and moderately resistant genotypes, the SOD activity increased after inoculation by P. melonis at 14 DAI, remained high in Soheil and Baby and decreased in Ramezz at 24 DAI (Fig 3. A, B and C). The levels of SOD in inoculated Soheil ranged from 0.42 U mg−1 (7 DAI) to 1.68 U mg−1(21 DAI), in Ramezz, it ranged from 0.21 U mg−1 (7 DAI) to 0.35 U mg−1 (21 DAI) and in Baby from 2.19 U mg−1 (7 DAI) to 0.79 U mg−1 (21 DAI). The genotype with the highest activities was the moderately resistant Baby. The SOD activity in ‘Baby’ was 4.09 times greater compared to control at 14 DAI. The induction of SOD in all genotypes found little to no significant differences compared to control plants at 7 DAI, except Baby and Yalda (Table 4). The SOD activity of the susceptible Mini 6-23 also showed increases of 1.47 U mg−1 on day 21 after inoculation, but the susceptible Yalda began to decline on day 21 after inoculation. The maximum amount of SOD activity between the three susceptible genotypes in inoculated plants was observed in Mini 6-23 at 21 DAI (Fig 3.E). The increase in SOD activity caused by inoculation did not correlate with damping-off susceptibility (Supplemental Table 1, varied between −0.56 and -0.14).