Comparison of root and foliar applications of potassium silicate in potentiating post-infection defences of melon against powdery mildew

The application of silicon to the roots or leaves reduces the severity of powdery mildew (Podosphaera xanthii) in melon but the latter treatment is less effective. This study compared key biochemical defence responses of melon triggered by P. xanthii after root or foliar treatment with potassium silicate (PS). Treatments consisted of pathogen-inoculated or mock-inoculated plants supplied with PS via roots or foliarly, as well as a non-treated control. The activity of defence enzymes and the concentration of phenolic compounds, lignin and malondialdehyde were determined from leaf samples at different time points after inoculation. Pathogen-inoculated plants irrigated with PS showed both an accumulation of silicon and primed defence responses in leaves that were not observed in pathogen-inoculated plants either sprayed with PS or not treated. These responses included the anticipated activity of peroxidase and accumulation of soluble phenols, the activation of chitinase and repression of catalase, and the stronger activation of superoxide dismutase, peroxidase and beta-1,3-glucanase. Moreover, the lignin concentration increased in response to inoculation, whereas the malondialdehyde concentration decreased. For the foliar treatment, however, only an increase in lignin deposition was observed compared with the control plants. The results show that silicon strongly plays an active role in modulating the defence responses of melon against P. xanthii when supplied to the roots as opposed to the foliage.