Effects On Physiological And Biochemical Changes Of Cucumber Leaf Tissues Infected By Pseudoperonospora Cubensis Under Different Temperatures

Cumber downy mildew is the main damage of cucumber production in solar greenhouse,which seriously affects the yield and quality of cucumber.In the process of pathogen infection,β-glucosidase and Cx（carboxymethyl cellulose）are produced as pathogenic factors to degrade cytoderm of host cells,so that the haustorium produced by hyphae could invade host cells to complete symbiotic process.The successful infection downy mildew of cucumber resulted in a propagation of hyphae between the intercellular spaces of cucumber leaf tissue and a change of leaf tissue structure.At the same time,the temperature of cucumber leaves also changed with the infection of downy mildew.In order to control downy mildew of cucumber,this study used“Changchun honey thorn”as experimental material,and the effects of downy mildew infection on cucumber leaves at 15℃,20℃and 25℃were studied.It revealed the infection process of downy mildew from the aspects of leaf tissue structure,pathogenic enzyme activity and leaf temperature.The optimal concentration of 104 sporangia·m L^-1 and 10 mg·L^-1 phytohormone solution was selected to screen the best preservation time of isolated cucumber leaves.The main findings are as follows:1.In the study of cucumber inoculated leaves’tissue structure change at different culture temperatures of 15℃,20℃and 25℃,CT scanning technique（x-ray three-dimensional microscope）was applied to the study for the first time.The infection of downy mildew caused a gradual expansion of cucumber leaf intercellular space,leaf porosity（intercellular space volume/leaf volume）increased gradually with the increasing of culture time during the incubation period of downy mildew.After the leaf spots appeared,some high-density substances have growth in the corresponding mesophyll tissue,which is consistent with the previous studies on downy mildew of cucumber from the perspective of histology.The change law of leaf tissue structure was the same at different temperature,the time when it changed was different,the influence of leaf tissue structure was the least at 15℃and the maximum value of leaf intercellular space changed fastest at 20℃,which indicated that 20℃was suitable for the growth of downy mildew hypha.2.The activity of pathogenic enzymes in inoculated leaves at 15℃,20℃and 25℃were determined.The results showed that Cx activity changed in the same way,which showed a"M"double peak curve with an increased-decreased-increased-decreased variation tendency,Cx activity of inoculated leaves at 20℃was different from that of 15℃and 25℃.The activity ofβ-glucosidase increased gradually after inoculation.During the incubation period of downy mildew,the maximum enzyme activities of Cx andβ-glucosidase at 15℃,20℃and 25℃increased with the increasing of culture temperature,the maximum enzyme activities were3095.63、4138.8、4764.7 U·g^-1 and 2219.37、2581、3165.18 U·g^-1.3.The temperature differences between inoculated cucumber leaves and normal leaves can be shown in infrared thermographic images.The temperature of inoculated cucumber leaves at15℃,20℃and 25℃was monitored by FLUCK infrared thermography,which revealed the temperature change law of inoculated cucumber leaves.The temperature of inoculated cucumber leaves was lower than that of healthy cucumber leaves during the latent period,the temperature of inoculated cucumber leaves was higher than that of healthy cucumber leaves when disease spots occurred.With the increase of culture temperature,the temperature differences of inoculated leaves（normal leaf temperature-inoculated leaf temperature）increased gradually.The lowest temperature difference-0.67℃appeared at the fourth day for15℃,the lowest temperature difference-1.5℃appeared at the third day for 20℃,and-1.5℃appeared at the second day for 25℃.Infrared thermography can be applied to the early warning of cucumber downy mildew.In summary,the results showed that the activities of pathogenic enzymes increased,cell gap became sparse and temperature difference increased with the increase of culture temperature,which laid a foundation for the prevention of cucumber downy mildew in solar greenhouse.CT scans and infrared thermography can measure and continuously monitor the internal structure of crops and indicators related to downy mildew changes.This study has a certain application value for the control and management of downy mildew in greenhouse vegetable production process,which has a significant sense to improve the yield and quality of protected vegetables.