EVIDENCE OF PHYTOALEXINS IN CUCUMBER LEAVES INFECTED WITH POWDERY MILDEW-FOLLOWING TREATMENT WITH LEAF EXTRACTS OF REYNOUTRIA-SACHALINENSIS

Phenolic compounds extracted from cucumber (Cucumis sativus L.) leaves were separated and analyzed for their differential presence and fungi toxicity in relation to a prophylactic treatment with Milsana (Compo, Munster, Germany) against powdery mildew (Sphaerotheca fuliginea). Bas ed on our extraction and purification procedures, at least eight separ ate phenolic compounds with antifungal activity were identified as int rinsic components of cucumber plants. Of these compounds, six displaye d a significant increase in concentration as a result of elicitation w ith Milsana, this being particularly evident when the plant was stress ed by the pathogen. The combined amounts of these antifungal compounds in treated plants was nearly five times the level found in control pl ants. One week after Milsana application, some of the antifungal compo unds obtained through hydrolysis of their glycosidic links were also d etected in their free form, indicating that they are likely liberated from conjugated phenolics by enzymatic hydrolysis in planta. To our kn owledge, these results provide the first direct evidence that cucumber plants produce elevated levels of phytoalexins in response to an elic iting treatment after infection.