EFFECTS OF SILICON ON THE ACTIVITIES OF DEFENSE-RELATED ENZYMES IN CUCUMBER INOCULATED WITH PSEUDOPERONOSPORA CUBENSIS

The relationship between silicon (Si) and plant disease has been investigated for many years, and there is an increasing interest in the use of Si for induction of defense mechanisms in response to fungal attacks. A confirmative role of Si in protecting plants against disease, however, has not yet been documented. The effects of silicon application on major defense-related enzymes activities were investigated in cucumber inoculated with downy mildew (Pseudoperonospora cubensis). The results showed that addition of Si to the nutrient solution significantly increased leaf Si content. Silicon supply to cucumber plants inoculated with P. cubensis significantly stimulated the activities of several defense-related enzymes. In particular, guaiacol peroxidase and polyphenol oxidase showed more intense and rapid activation. The optimum responses of enzyme activities were obtained at 3.6 mM Si. At this level of Si, the downy mildew disease index was reduced by more than 60% compared to control plants not receiving Si. It is concluded that the concentration of Si plays an important role in inhibiting P. cubensis via stimulating the activities of major defense-related enzymes.