Induction Effect Of Salicylic Acid(SA) On Cucumber Against Powdery Mildew

Salicylic acid,an acknowledged chemical inducer,can induce plant to acquire system resistance against many kinds of pathogens.In this paper,the effects of salicylic acid on inducing cucumber against powdery mildew were studied.Results are as follows:When the seeds of cucumber were treated with a certain concentration of salicylic acid,the germination rate,germ length,radicle length,plant height,root length,fresh weight,dry weight and chlorophyll content of cucumber were improved.At 0.5-1.0 mmol/L,salicylic acid could increase the increment of cucumber slightly.When the content of salicylic acid reached to 1.0 mmol/L,the facilitation effect of it on the radicle length and plant height of cucumber were 30.85%and 26.65%respectively.If the content of salicylic acid were too higher or too lower,it showed no effect,even harm on cucumber.Our results indicated that salicylic acid could not improve the growth of cucumber at content of 0.1 mmol/L,but showed toxic effect on it at 10.0 mmol/L while the inhibitory rate could reach to about 20%.As well,higher content of salicylic acid(at 10.0mmol/L)could show inhibitory effect on the elongation of cucumber germ and radicle,and plant growing.The activities of defensive enzymes in cucumber improved after the seedling being treated with salicylic acid,and all of them reached to the peak value between 24 to 48 hours.Among them,the activity of SOD increased at once after the salicylic acid was sprayed on the surface of cucumber leaf,and the activity could sustain for a long time.After 96 h,the activity of SOD in the treated seedling was similar to the control.In early time,the activities of PPO and POD have no difference between the treatment and control after the cucumber seedlings being treated.After 24 h,the enzyme activity of treated seedlings increased and reached to the maximum at 72 h.The activity of PAL were always higher than the control.Among the resistance-related substances,the content of soluble protein in the treated cucumber seedlings was lower than in the control while soluble sugar and lignin were higher.Nine days after being treated,the soluble sugar content in the cucumber leaf was up by 60%.When the salicylic was added into the water,the germination rate of the spore of cucumber powdery mildew pathogen was reduced.Furthermore,the higher content of salicylic acid added the lower germination rate of the spore was observed.At 10 mmol/L,the germination rate of the treatment was only about half of the control.Salicylic acid could control the cucumber powdery mildew well in the field,and the control effect could reached to 50%when the salicylic acid of 1.0 mmol/L was sprayed on the cucumber leaf.If the salicylic acid was used with the biological source and biocontrol bacterium,the control effects were higher than treated only with one kind of substance.The control effects-could reach to 65.24%and 71.92%respectively.