Effect of Silicon Nutrition on Oxidative Stress Induced by Phytophthora melonis Infection in Cucumber

The effect of silicon nutrition on root rot of cucumber caused by Phytophthora melonis was studied in a greenhouse experiment. Two cucumber cultivars (Cucumis sativus 'Dominus' and 'Super Dominus') fertilized with three concentrations of Si (0.0, 1.0, and 1.7 mM Si in the form of sodium silicate) were not inoculated or were inoculated with P. melonis. The P. melonis isolate significantly decreased root dry weights, although the magnitude of growth reduction varied with cultivar and Si concentration. Silicon nutrition at either concentration significantly reduced disease severity relative to the treatment that received no silicon. There was a significant negative correlation between the extent of root rot caused by P. melonis and the extent of electrolyte leakage of roots. Roots infected with P. melonis had greater root catalase (CAT) and ascorbate peroxidase (APX) activities. There was a positive correlation between silicon concentrations and CAT and APX activities in plants inoculated with P. melonis. Silicon improved activity of antioxidant enzymes, resulting in enhanced crop resistance to oxidative stress induced by P. melonis infection and improved cucumber growth.