| As research on biocontrol of postharvest diseases of citrus fruits has developed, many biological agents have been discovered by scientists at home and abroad. But only a few have been put into application. This is mainly because that their effects on controlling postharvest diseases are not as good as chemical fungicide.Therefore, in order to improve their biocontrol efficiency and put them into practical use, there is an urgent need to research on the mechanism of the action of these biocontrol agents.Kloeckera apiculata (34-9) has been found to have great inhibition effect on Penicillium italicum of citrus in vitro and in vivo.And the inhibition effects are different when the biocontrol agent is inoculated before or after the pathogen. This research is aimed to find out the mechanism of its action, including whether it competes with pathogen for nutrient, induces the resistance of citrus fruits, and secrets fungistatic compounds to hurt or kill the pathogen. And the results are as followed:(1)By using orange juice to simulate natural environment in vitro,we found that more than 94% condial of the pathogen could germinate when they were incubated in 0.5%,5%,10% orange juice without biocontrol agent 34-9.But in the presence of biocontrol agent 34-9,condial germination was inhibited. And as the concentration of orange juice increased, the inhibition effect became weaker. In 0.5%,5% and 10% orange juice, the inhibition rates are 80.9%,40.3% and 4.3% respectively. And in each concentration of orange juice, the population size of antagonistic yeast 34-9 was significantly increased.The population sizes are respectively 1.70,2.6,32.5 times of the original size.And the inhibited condial could regain the ability to germinate in new fresh orange juice.Condial germination increased significantly in 0.5% and 5% orange juice after an additional incubation of 24 h, but it showed little change in 10% orange juice. And the biocontrol effect in vivo could be weakened by adding exogenous nutrient to the wound of fruits, especially by adding amino acid.(2) To some extent, biocontrol agent 34-9 induced the resistance of fruits. The longer the fruit were treated by biocontrol agent 34-9 and the nearer the site was to the spot treated by biocontrol agent 34-9, the resistance of fruits was stronger. Within 24 hours, the systemic resistance of fruits couldn't be strengthened by the antagonist, but it became much stronger after 48 hours.Antagonistic yeast cell induced an increase in the activity of PPO,β-1,3-glucanase and PAL.But its secreting substance could induce an increase in the activity of SOD,POD,CAT andβ-1,3-glucanase.The fungistatic compounds produced by biocontrol agent 34-9 made the peak of the activity of POD,β-1,3-glucanase and SOD appear at 24 h,24 h and 48 h, respectively, and they were 1.8 times,1.2 times and 1.4 times of control.After being treated for more than 48h, the activity of CAT kept higher than control.(3) Crude extract could effectively control P. italicum in vitro and in vivo.In vitro, the minimum inhibitory concentration of crude extract to P. italicum of citrus was 3.75μL/mL, and the minimum fungicidal concentration was 10μL/mL.The plasma membrane integrity could be hurt by the crude extract. The longer it was treated by the crude extract, the more seriously it was hurt. After being treated for 2 h,5% condial was hurt; for 4 h, it increased to 20%;and between 4 h and 8 h, there was another 10%.The decay incidence of fruits could be reduced to 50% and 89% respectively when they were treated by 5μL/mL and 10μL/mL crude extract. |
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