| Chinese bayberry is a subtropical fruit native to China and grown in eastern and southern China. The fruit is very popular with consumers for its attractive red to purple color, appealing flavor and rich in vitamins and minerals and other nutritive properties. Chinese bayberry matures and is commercially harvested in the hot and rainy early summer season. After harvest, the fruit is susceptible to mechanical injury and pathogenic attack, with extreme short shelf-life under ambient temperature mainly due to its soft texture, high water content and lack of epicarp protection, as well as high respiration rate. Green mold decay caused by Penicillium citrinum Thom and Verticicladiella abietina (Peak) Hughes is the major postharvest fungal disease on Chinese bayberries and can be effectively controlled by synthetic chemical fungicides. However, with increasing consumer concern over chemical residues on foods and pathogen resistance to fungicides, there is an urgent need to search for effective and safe alternatives to control postharvest diseases of Chinese bayberries.The present study was designed to investigate the effect of hot air treatment (HAT) and methyl jasmonate (MeJA) on inhibiting major fungal disease of green mold decay on Chinese bayberry fruit (Myrica rubra sieb.et Zucc. Cv. Wumei) and the possible mechanisms involved, and to compare the costs and benefits between MeJA and HAT using the flesh column in vivo incubation method. Furthermore, the effectiveness of HAT in combination with MeJA in controlling postharvest disease on Chinese bayberry fruit was also assessed. In addition, the effect of ethanol vapor treatment (EVT) alone or in combination with HAT on postharvest mold and natural decay, as well as microbial loads on Chinese bayberries was investigated. The results were as follows:(1) The effect of hot air treatment (HAT) on postharvest decay and quality of Chinese bayberry fruit (Myrica rubra Seib & Zucc.) was investigated and optimized by response surface methodology (RSM) under different time-temperature combinations. A central composite rotatable design (CCRD) was applied with treatment temperature (ranging from 45 to 55℃) and exposure time (from 105 to 195 min) as independent variables while postharvest decay, firmness, Vc content and TSS/TA of Chinese bayberry after storage at 1℃for 7 days followed by 20℃for 1 day as investigated responses. The results indicated that the response and variables were fitted well to each other by multiple regressions, and overall optimization showed the lowest incidence of decay and highest values of firmness, Vc content and TSS/TA were achieved by an air treatment at treatment temperature of 48℃and exposure time of 180 min. HAT at 48℃for 3 h enhanced the resistance of Chinese bayberry fruit against green mold decay caused by P. citrinum and reduced the severity of the disease. The HAT-treated fruit demonstrated a gradual development of disease resistance against fungus P. citrinum infection and reached a maximum at 6 h after the treatment, being associated with a rapid and strong activation of defense enzymes including chitinase,β-1,3-glucanase, POD and PPO, and an increase in lignin content, and an inhibition of SOD, CAT and APX decrease. Moreover, the results have shown that the HAT treatment could significantly promote accumulation of HSP70 during the whole incubation at 1℃, which acts as a molecular chaperone that played a key role in induced resistance in Chinese bayberry fruit. In addition, the in vitro experiment showed that HAT significantly inhibited spore germination, germ tube elongation and mycelial growth of P. citrinum. These results indicate that HAT can effectively reduce fruit decay possibly by directly inhibiting pathogen growth and indirectly inducing disease resistance.(2) The effects of MeJA on postharvest decay and antioxidant capacity in harvested Chinese bayberry fruit were investigated. MeJA at 10μmol/L was most effective in reducing fruit decay; quality parameters including pH value, total soluble solids, and titratable acidity were not significantly affected by MeJA treatment. Fruit treated with 10μmol/L MeJA exhibited significantly higher levels of total phenolics, flavonoids, anthocyanins and carotenoids as well as individual phenolic compounds including gallic acid, protocatechuic acid, myricetin, quercetin-3-O-rutinoside, and cyanidin-3-glucoside than the controls. These fruits also maintained significantly higher antioxidant activity as measured by scavenging capacity against DPPH, superoxide, and hydroxyl radicals and by the reducing power test compared to the controls. These results indicate that MeJA can effectively reduce fruit decay and improve antioxidant capacity of Chinese bayberry fruit.Polyphenolic biosynthesis is the result of coordinated action of phenylpropanoid related enzymes including PAL, C4H,4-CL、DFR and CHS. The results indicated that both PAL and DFR were key enzymes that determined the polyphenolic biosynthesis in Chinese bayberry fruit. MeJA treatment could induce the activities of a series of phenylpropanoid related enzymes, and thus promote the polyphenolic biosynthesis.(3) The effect of MeJA on postharvest decay of Chinese bayberry fruit caused by P. citrinum, and its ability to induce biochemical defense responses in fruits, were assessed. The results demonstrated that treatment with 10μmol/L MeJA could significantly inhibit the green mold decay caused by P. citrinum on Chinese bayberry fruit. MeJA treatment in reducing the decay on Chinese bayberries could be related to the priming for defense responses, which indirectly triggered the new biosynthesis of pathogenesis-related (PR) proteins in Chinese bayberries and stimulated biosynthesis of tatol phenolics, lignin and phytoalexin when the fruit was challenged or infected by pathogen, rather than a direct activation of defense responses. In addition, the in vitro experiment showed that MeJA was not directly capable of inhibiting spore germination and germ tube elongation of P. citrinum. Thus, these results MeJA can effectively reduce fruit decay possibly by priming action for "preserve the disease resistance", rather than a direct inhibitory effect on pathogen growth.(4) After 3h treatment of the HAT at 48℃, the biosynthesis of total protein as well as chitinase and PAL protein in flesh column of Chinese bayberry were markedly promoted. However, after heating for 3 h, the intensity of the immune signal of chitinase and PAL protein in fruit column incubated in CHX (inhibiting protein synthesis) or actidione D (inhibiting mRNA transcription) began to get weaker, indicating the heat treatment might confer disease resistance through the accumulation of PR protein and HSP70 at the transcriptional and translational level. Furthermore, the development of disease resistance was accompanied by significantly lower glucose, sucrose and aimino acid contents, possibly indicating the induced resistance was an irreversible process of consuming substrates and energy.On the other hand, the in vivo incubation experiment showed 10μmol/L MeJA treatment could not directly induced biosynthesis of total protein as well as chitinase and PAL protein in flesh column of Chinese bayberry. Meanwhile, MeJA did not affect the oxalic acid, tartaric acid, and malic acid as well as soluble sugars and aimino acid contents, but promoted the content of citric acid. To take the MeJA-induced Priming mechanism mentioned in Chapter 5 into account, between benefits of MeJA-mediated resistance and the costs of substrates and energy in Chinese bayberry fruit brought into a state of equilibrium. (5) The effect of HAT (48℃for 3h) alone or in combination with MeJA (10μmol/L) on postharvest decay and physiological response, and their effects on fruit quality in harvested Chinese bayberries were investigated. The results showed that the combined treatment of HAT and MeJA resulted in a remarkably improved control of fruit decay compared with treatment of HAT or MeJA alone during the storage at 1℃. HAT alone or its in combination with MeJA could promote the respiration rate over the earlier period of the storage, inhibit the ethelyne production and membrane lipid peroxidation, thus retard the senescence process as confirmed by lower MDA content. Meanwhile, the combined treatment induced higher activities of the defense-related enzymes including chitinase,β-1,3-glucanase and PAL in Chinese bayberry fruit than applying either alone, which were closely related to the enhanced disease resistance. In addition, HAT in combination with MeJA showed an improved effect on inhibiting the decrease of firmness and Vc content during the storage. Moreove, the combined treatment maintained TSS and TA contents as well as pH value, further higher scavenging capacity against DPPH radicals. Thus, these results suggest that the combination of HAT treatment and MeJA might be a more useful technique to reduce fruit decay and maintain quality in Chinese bayberries during postharvest storage compared with the HAT or MeJA alone.(6) Results demonstrated that EVT at 500μL/L for 3 h was found most effective in reducing natural decay without impairing quality. Furthermore, the effect of EVT (500μL/L for 3 h) alone or in combination with hot air treatment HAT (48℃for 3 h) on postharvest decay and microbial loads in Chinese bayberries was investigated. Treatment with ethanol vapor or hot air alone both resulted in significantly lower decay incidence caused by V. abietina, P. citrinum or Trichoderma viride compared with the control. The combined treatment showed the lowest incidence of fruit decay caused by these pathogens. This treatment also significantly inhibited spore germination and germ tube elongation of the pathogens in vitro than EVT or HAT alone. Meanwhile, the combined treatment exhibited the lowest natural decay incidence and microbial loads on Chinese bayberries without impairing fruit quality. These results suggest the usefulness of the combined treatment for reducing decay and maintaining quliaty in harvested Chinese bayberries. |
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