Study On Protective Effects Of KHCO₃ Spraying On Powdery Mildew In Melon

Melon（Cucumis melo L.）is a fruit vegetable in Cucurbitaceae family,which is widely cultivated in China.Powdery mildew is not only the main disease of melon plants,but also the main obstacle for its green production.In this study,melon（Cucumis melo L.cv.‘Yangjiaomi’）was used to explore the regulatory and mitigating effects of potassium bicarbonate（KHCO₃）on powdery mildew in melon plants,and the underlying physiological mechanisms was further invetigated.The main results are as follows:1.KHCO₃spraying regulated melon resistance to powdery mildew.To unveil the most effective treatment,melon seedlings were first speayed with four concentation levels of KHCO₃ solution including 0,2.5,5 and 10 g·L^-1.At 2 days after spraying（DAS）,P.xanthii inoculation was carried out for the seedlings.The results showed that powdery mildew was well preveted by the spraying of 2.5 g·L^-1 KHCO₃ solution for melon seedlings without obvious damages,demonstrating the effectiveness of this KHCO₃ concentration.Then the spraying of 2.5 g·L^-1 KHCO₃ solution was carried out for melon seedlings with the distilled water（H₂O）spraying as a control.At 2 DAS,the sprayed seedlings were inoculated with P.xnathii to to explore the regulatory effects of KHCO₃ on powdery mildew.The results showed that,plant height,stem diameter,fresh and dry weight of aboveground and underground tissues,the contents of nitrogen,phosphorus and potassium,and photosynthetic rate were significantly decreased for the H₂O-sprayed melon seedlings under powdery mildew.Compared with the control,the accumulation of superoxide anion（O₂.^-）,hydrogen peroxide（H₂O₂）and malondialdehyde（MDA）was significantly decreased via the stimulation of superoxide dismutase（SOD）,peroxidase（POD）and catalase（CAT）activities for KHCO₃ sprayed seedlings under powdery mildew.Meanwhile,the activities of phenylalanine ammonia lyase（PAL）and polyphenol oxidase（PPO）in phenylpropane metabolic pathway was significantly increased and the contents of some secondary metabolites in this pathway was dramatically accumulated in the KHCO₃ sprayed seedlings,thus leading to the reduction of disease incidence and the alleviation of growth and photosynthetic inhibition.2.KHCO₃ spraying alleviated the damages of powdery mildew to melon plants.At 7 days after inoculation（DAI）,the spraying of 2.5 g·L^-1 KHCO₃ solution was carried out for the infected seedlings with H₂O spraying as a negative control to explore its mitigating effects on powdery mildew.The results showed that,compared with the control,the accumulation of superoxide anion（O₂.^-）,hydrogen peroxide（H₂O₂）and malondialdehyde（MDA）caused by powdery mildew was significantly reducded for the KHCO₃-sprayed seedling via the stimulation of superoxide dismutase（SOD）and peroxidase（POD）activities.The activities of phenylalanine ammonia-lyase（PAL）and polyphenol oxidase（PPO）together with some secondary metabolites was greatly induced,and the disease incidence was thus significantly decreased.Additionally,the enhancement in the contents of soluble proteins,particularly some disease-related ones,were observed.As a result,the phytosynthetic capacity was recovered to some extents,and the inhibitory effects of powdery mildew on the growth performance of melon seedlings were alleviated.3.HCO₃^-is the key factor of KHCO₃ regulating melon tolerance to powdery mildew.Melon seedlings were first sprayed with distilled water（H₂O）,KOH solution（p H=8.54）,1.86 g·L^-1 KCl solution and 2.5 g·L^-1 KHCO₃（p H=8.54）solution,respectively.At 2 DAS,the effects of KHCO₃ spraying on powdery mildew was explored in melon seedlings.The results showed that,in comparison to the other sprayings,the decreased disease index was only reveaded for the KHCO₃-treated seedlins together with the increased activities of superoxide dismutase（SOD）,peroxidase（POD）and catalase（CAT）and the decreased contents of hydrogen peroxide（H₂O₂）and malondialdehyde（MDA）.In addition,the activities of phenylalanine ammonia lyase（PAL）and polyphenoloxidase（PPO）,as well as increased the contents of total phenols and flavonoids,were dramatically increased in the KHCO₃-sprayed seedlings upon P.xanthii infection.This evidence indicated that the significant roles of KHCO₃might be closely associated with the HCO₃^-anion in the protection of melon plants from powdery mildew damages.