## 3 Discussion  
The biocontrol efficiency of BCN2 and BCN10 on loquat fruit was better than in the double petri-dish experiment. We speculated that the treatment enhanced activities of defense-related enzymes including chitinase, \(\beta\)-1,3-glucanase, phenylalanine ammonialyase, peroxidase, and polyphenoxidase, and promoted accumulation of H\({}_{2}\)O\({}_{2}\). With the release of volatile compounds of bacteria, total phenolic content and 2,2-diphenyl-1-picrylhydrazyl radical scavenging activity in loquat fruit might also increase, which changed the antioxidative activity and reduced the disease incidence in fruit. In this study, since _Bacillus_ strains did not directly contact with loquat fruit, that means VOCs produced were responsible for the biocontrol efficiency. We did test some enzymes in loquat fruits after bacterial VOC's treatment, though the data were instable, the trends were consistent with our speculations. Previous work has already reported that _Bacillus_ spp. could enhance the activity of defense enzymes in rice [22], tomato [23], and peach [24], etc. There are a few reports showing that bacterial VOC can influence the activity of some defense enzymes of fruit and antioxidant enzymes of plant pathogens significantly [25; 26; 27]. Although the mechanisms by which microbial antagonists control postharvest diseases have not been clearly elucidated, induced disease resistance has been inferred to be one of the major modes of their actions [4]. Chitinase hydrolyzes the \(\beta\)-1-4-linkage in chitin, which is an essential cell wall component of fungi, while \(\beta\)-1,3-glucanase directly degrades cell walls of pathogens or indirectly releases oligosaccharides and elicits defense reactions; therefore both enzymes are thought to be involved in plant defense mechanisms against fungal infection [28]. PAL is the first enzyme in the phenylpropanol pathway leading to the biosynthesis of phenolics, phytoaleoxins, lignins and many other compounds associated with localized disease resistance in plants [29], while PPO and POD are both involved in lignification of host plant cells and considered as key enzymes related to defense reaction against pathogen infections [30]. The induction of these defense-related enzymes by different biocontrol agents has been observed in harvested apple, Chinese bayberry, loquat, and peach fruit, and is correlated to increased disease resistance and reduced disease severity [31; 32; 33; 34]. In order to understand the nature of VOC produced by BCN2 and BCN10, SPME and GC-MS were used in couple. This simple and rapid technology for sampling volatile compounds at low concentrations in headspace analysis has been successfully used to characterize VOCs. VOCs are chemicals with low molecular weights (\(<\)300 Da), high vapor pressure and low water solubility, and terpen, alkanes, and aromatic hydrocarbons are mostly identified by GC-MS analysis. These compounds include myrcene, limonene, geraniol, undecane, cyclohexane, ethylbenzene, toluene, and so forth, and many antifungal volatiles come from plant essential oils [35; 36; 37]. The headspace analysis showed that 29 and 30 organic compounds were detected from BCN2 and BCN10, respectively, but not in the control. Although most of the compounds detected have already been reported in different _Bacillus_ strains [38; 39], methodologies applied to collect and detect VOCs should be taken into account, because it can also influence the results and often confuse the comparison between different studies. Moreover, these data not only depend on compound concentration but also on the fiber affinity and the detector sensing to the different analytes. As proposed by [PERSON] [40], the activities of antifungal VOCs decrease in the following order: organic acids \(>\) aldehydes \(>\) alcohols \(>\) ethers \(>\) ketones \(>\) esters \(>\) lactones. However, in other works, when a single molecule was tested on molds, this order was not absolute.