Physiological Mechanism And Regulation Of Chilling Injury In Postharvest Kiwifruit

Kiwifruit as an important fruit is popular for its thin skin, juiceful texture and rich innutrient. The kiwifruit, as a typical kind of respiratory climactic fruit, undergoes rapidsoftening with severe decay after harvest, which shortens its shelf life. Cold storageeffectively inhibits fruit softening and extends the postharvest life of kiwifruits. However,kiwifruit is susceptible to chilling injury when protractedly stored at inaptitude temperatures.CI symptoms of kiwifruit are characterized by the appearance of concave spot in the skin,grainy tissue in the outer pericarp and the development of diffuse pitting in association with awater-soaked appearance. These symptoms become most severe after fruit is transferred toambient temperature. CI increase susceptibility to decay, which become a major obstacle topostharvest storage and transportation. At present, most study focus on storage of kiwifruit.Little information is available on the CI of kiwifruit. Thus, it is crucial to study themechanism and regulation of chilling injury in postharvest kiwifruit for storage andtransportation.In this paper，it was investigated that the effect of different harvested maturity, gradualcooling, low temperature conditioning （LTC） and exogenous putrescine （Put） on chillinginjury in kiwifruit and the possible mechanisms. The results were as follows:1. The effects of different harvested maturities on chilling injury, fruit quality，respirationand ethylene production, relative membrane permeability, malonaldehyde （MDA） content andthe activities of antioxidant enzymes in kiwifruit （Actinidia deliciosa cv.‘Xuxiang’） duringcold storage were investigated. The results showed as follows:（1） The storage qualities and chilling injury of kiwifruit were much difference amongⅠ （TSS:4.5%～5.5%）, Ⅱ（TSS:6.5%～7.5%）, Ⅲ（TSS:8.0%～9.0%） in low temperaturestorage. Compared to fruits of Ⅱ and Ⅲ, the fruits of Ⅰmaturity showed early CI symptomscharacterized by flesh grainy and water-soaked, maintained higher weight loss ratio, decayrate, chilling injury index and chilling injury incidence. In addition, the fruits ofⅠ maturityshowed a decrease in firmness, total soluble solids （TSS）, titratable acidity （TA） and VCat theend of storage and shelf life. However, the fruits of Ⅲmaturity maintained higher weight lossratio, decay rate, and lower firmness, TA and VCthan these of Ⅱ maturity. Thus，the best harvest maturity grade for kiwifruit storage performed well was Ⅱmaturity.（2） The respiration rate, ethylene production and the relative membrane penetrabilityabnormally increased under chilling stress. However, the fruits of Ⅱm aturity maintainedlower respiration and ethylene production, induced the increase in the activities of superoxidedismutase （SOD）, ascorbate peroxidase （APX）, catalase （CAT） and peroxidase （POD）,reduced lipid peroxidation and alleviated chilling injury. These results indicated thatⅡmaturity retard chilling injury in ‘Xuxiang’ fruit which may be related to enhancing theactivities of antioxidant enzymes.2. The ‘Xuxiang’ Kiwifruit were treated by direct cooling and gradual cooling todeterminate the effect of cooling pattern on chilling injury, weight loss ratio, decay rate andfruit quality. The gradual cooling treatment significantly reduced chilling injury index,chilling injury incidence, weight loss ratio and decay rate, maintained higher firmness, TSS,VCand TA. It was found that treatment2（10℃�'5℃2d�'2℃2d�'0±0.5℃） was mosteffective in ‘Xuxiang’ fruit.On the basis of above results, the gradual cooling2in ‘Xuxiang’ fruit was chosen toinvestigate the effects of gradual cooling on chilling injury and the influence on active oxygenmetabolism. The results showed that the respiration rate, ethylene production, relativemembrane permeability and malonaldehyde （MDA） contents increased, while the activities ofCAT, SOD, POD and APX decreased during the development of CI symptom. However，thegradual cooling treatment significantly alleviated chilling injury, reduced respiration andethylene production, and inhibited the accumulation of MDA and the increase in relativemembrane permeability. Moreover，the gradual cooling treatment maintained higher activitiesof SOD, CAT, APX and POD than direct cooling fruit during the storage, and kept lowerlevels of superoxide anion production rate (O₂^·—) and H₂O₂content. These results indicatedthat gradual cooling treatment retard chilling injury in ‘Xuxiang’ fruit which may be related toenhancing the activities of active oxygen scavenging enzymes and reducing the accumulationof active oxygen.3. The effects of LTC on chilling injury, weight loss ratio, decay rate and fruit quality inkiwifruit （Actinidia deliciosa cv.‘Hayward’） during cold storage were investigated. Thetreatment with LTC was more effective in reducing chilling injury index and chilling injuryincidence, while maintaining higher level of firmness, TSS, TA and VC. We found that LTC5（12℃，3d） treatment was the optimal treatment to alleviate chilling injury and maintainhigher quality of kiwifruit. Thus LTC5treatment was chosen to to test the effects of LTCtreatment on chilling injury and the possible mechanisms in kiwifruit. The results showed thatLTC treatment inhibited the accumulation of MDA and the increase in relative membrane permeability. Furthermore, LTC treatment decreased accumulation of H₂O₂and O₂^·—and theactivity in lipoxygenase （LOX）, while increased the activities of CAT, SOD, POD and APX inkiwifruit. The treatment also maintained higher levels of the endogenous ABA, IAA and ZR,lower GA₃levels, and higher ratio of ABA/GA₃and ABA/IAA. These results indicated thatthe effect of LTC treatment on alleviating CI of kiwifruit may be due to its ability to improveantioxidant enzyme activities and reduce ROS accumulation, maintain higher levels of theendogenous ABA, IAA and ZR, lower GA₃levels, and higher ratios of ABA/GA₃andABA/IAA.4. Kiwifruit （Actinidia chinensis cv.‘Hongyang’） were treated with differentconcentrations of Put (1mmol·L^-1，2mmol·L^-1，4mmol·L^-1) to investigate the effect onchilling injury, weight loss ratio, decay rate and fruit quality. We found that treatment with2mmol·L^-1Put treatment was most effective in reducing in CI symptoms of grainy andwater-soaked, decreasing weight loss ratio and decay rate and maintaining higher level offirmness, TA and VC. Thus，2mmol·L^-1Put treatment was chosen to investigate the possiblemechanisms of alleviating chilling injury. The results showed as follows:（1） Chilling injurywas related to the reduction of endogenesis Spd and Spm of kiwifruit. The accumulation levelof endogenesis Spm and Spd and chilling injury index appeared remarkable negativecorrelation, R=–0.9255and R=–0.9133. However, exogenous Put treatment increased thecontents of endogenesis Put、Spd and Spm. These results indicated that exogenous Puttreatment reducing in chilling injury may be related to improving endogenous polyaminecontent.（2） Exogenous Put treatment alleviated chilling injury, inhibited respiration andethylene production, maintained higher level of GSH and ASA and lower level of GSSG andDHA, and kept higher ratio of GSH/GSSG and ASA/DHA. The activities of SOD、POD、CAT、APX、DHAR、MDHAR、GR were also induced by exogenous Put treatment. In addition,this treatment inhibited the accumulation of MDA, H₂O₂and O₂^·—and the increase inrelative membrane permeability. These results suggested that the effect of exogenous Puttreatment on alleviating chilling injury of kiwifruit may be attributed to its ability to enhanceantioxidant enzyme activities and the content of antioxidants and decrease lipid peroxidation.（3） Chilling injury was related to increase of LOX activity and the decrease of linoleicacid （C18:2） and linolenic acid （C18:3）. Exogenous Put treatment inhibited increased of LOXactivity, maintained higher relative contents of linoleic acid （C18:2） and linolenic acid（C18:3）, and showed the lower relative contents of palmitic acid （C16:0） and stearic acid（C18:0）. In addition, this treatment inhibited the decrease in index of unsaturated fatty acidsand unsaturated degree of fatty acids. These results suggested that the enhancement of chilling tolerance and the alleviation of chilling injury in kiwifruit by exogenous Put treatment may bedue to the decrease of LOX activity, inhibiting decrease in relative content of unsaturated fattyacids of membrane lipids and maintaining higher unsaturated degree of fatty acids.