SILICON-MEDIATED ACCUMULATION OF FLAVONOID PHYTOALEXINS IN CUCUMBER

The controversial role of silicon in plant disease resistance, describ ed mostly as a passive mechanical protection, has been addressed. Conc lusive evidence is presented that silicon is involved in the increased resistance of cucumber to powdery mildew by enhancing the antifungal activity of infected leaves. This antifungal activity was attributable to the presence of low-molecular-weight metabolites. One of these met abolites, described here as a phytoalexin, was identified as a flavono l aglycone rhamnetin (3,5,3',4'-tetrahydroxy-7-O-methoxyflavone). This is the first report of a phytoalexin for this chemical group in the p lant kingdom and of a flavonol phytoalexin in cucumber, a chemical def ense long believed to be nonexistent in the family Cucurbitaceae. The antifungal activity of leaf extracts was better expressed after acid h ydrolysis, extending to another plant species the concept that some ph ytoalexins are synthesized as glycosylated phytoalexins or their precu rsors.