Differential Regulation of Superoxide Dismutase Activity in Selected Strawberry Lines Exposed to Mycosphaerella fragariae

The effect of leaf infection by the fungus Mycosphaerella fragariae, on total superoxide dismutase activity and induction of SOD isozymes was studied under controlled conditions using four selected strawberry cultivars: Kent (HS, highly susceptible), Joliette (HR, highly resistant) and two advanced strawberry lines, SJ8976-1 and APF029-4 (MR, moderately resistant). Observations were made of conidia morphology of M. fragariae grown in strawberry leaf agar (SLA) at different stages of conidia development and of infective symptoms of the leaves after inoculation. Inoculation of strawberry leaves with M. fragariae increased protein content and SOD activities in all four cultivars. In all cases, total SOD increased 1 day (d) after inoculation, reaching a peak 2 d after inoculation, and slowly declining thereafter. Total SOD activity in Joliette, SJ8976-1 and APF029-4 two days after inoculation was 4376, 4433 and 4283 U g-1 FW, respectively, and significantly was lower for Kent (3656 U g-1 FW). From the electrophoresis profile of the strawberry cultivars, 2 newly synthesized isozymes were found in infected Joliette and SJ8976-1, (Rf = 0.31 and 0.34), which are believed to be associated with leaf spot resistance.