Morphological changes of Podosphaera xanthii and induced biochemical defenses of cucumber after treated by (+)-(S)-ar-turmerone

Cucumber powdery mildew (PM) is a destructive fungal disease which has a substantial impact on cucumber yield and quality. Our previous work has demonstrated that (+)-( S )-ar-turmerone (ATM) is an effective compound for controlling PM infected by Podosphaera xanthii in cucumber. In this study, the effect of ATM on cucumber PM was histologically investigated by observations on the morphology of the pathogenic fungus at different time points after inoculation. It was found that the ATM caused the collapse of the pathogen's conidia and mycelia structure, the cytoplasm coagulation, the reduction of the conidia germination rate, and the inhibition of the conidiophore growth, therefore, prevented the re-infection of the P. xanthii . Evaluation of the content of total phenols and flavonoids in cucumber ATM-treated leaves showed that the ATM-treatment could increase the content of total phenols and flavonoids in cucumber leaves significantly in comparison to the control. A further analysis of the contents of the compounds showed that ATM-treatment could increase the content of rutin , quercetin , hesperetin , and gallic acid , syringic acid , vanillic acid and salicylic acid. Meanwhile, the activity of phenylalanine ammonia-lyase (PAL) and polyphenol oxidase (PPO) in cucumber leaves were increased slightly, but the activity of peroxidase (POD) and superoxide dismutase (SOD) were significantly increased through ATM treatment. It should be estimated from this study that the ATM not only directly affected the growth of P. xanthii but also enhanced the cucumber plant resistance to the disease.