Possible involvement of hydrogen peroxide in the development of resistance mechanisms in ‘Golden Delicious’ apple fruit

Apples (Malus domestica L. cv Golden Delicious) were picked 7 days before (harvest 1), or after (harvest 2) the commercial harvest time. Changes in H2O2 levels and in the activity of the H2O2-generating enzyme superoxide dismutase (SOD) and H2O2-scavenging enzymes catalase (CAT) and unspecific peroxidase (POX) were estimated immediately after harvest and during the first 24 h following wounding and inoculation with Penicillium expansum. Fruit from harvest 1 had lower decay incidence and severity of infection than those from harvest 2. Immediately after wounding or inoculation, the less susceptible fruit (harvest 1) showed a significant increase in H2O2 levels concomitantly with higher activity of SOD. No significant changes in CAT and POX were found. In susceptible fruit (harvest 2), both H2O2 levels and SOD activity in wounded/inoculated fruit remained similar to those in control fruit. In contrast, CAT and POX activity significantly increased as a consequence of wounding and inoculation. Collectively, these results provide evidence that harvest date is important in determining the susceptibility of ‘Golden Delicious’ fruit to P. expansum, and that H2O2 and associated metabolism induced by wounding could play a role in defence mechanisms in this fruit.