Superoxide dismutase responses of strawberry cultivars to infection by Mycosphaerella fragariae

In controlled conditions, the effect of leaf infection by Mycosphaerella fragariae on total superoxide dismutase (SOD, EC 1.15.1.1) activity and induction of SOD isozymes was studied in three different strawberry cultivars, i.e. "Joliette" (resistant), "Honeoye" (partially resistant) and "Kent" (susceptible). Infection of the strawberry leaves with M. fragariae resulted in increase in SOD activities in all three cv. Total SOD increased 1d after inoculation in Joliette and Kent, and 2d after inoculation in Honeoye and reached the highest level in all three cv, at the 2nd day after inoculation, then slowly declined afterward. Total SODs in Joliette and Honeoye at the 2nd day after inoculation were 4516 and 4947Ug(-1) FW, respectively, which were significantly higher than that of Kent (3255Ug(-1)FW). Banding pattern of SOD isozymes in all three cv was also affected by infection. Electrophoresis profile of infected cv revealed two newly synthesized isozymes in Joliette and Honeoye, in which one band, i.e. R(f) = 0.53 was observed exclusively in inoculated Joliette and Honeoye. Therefore, it is considered to be associated with leaf spot resistance.