Study On Regulation And Mechanism Of Quality Deterioration Of Loquat Fruit After Harvest

Loquat fruit is an important native fruit representing an endemic species of China. The rich red to orange color and appealing flavor make this juicy fruit popular with consumers.The harvest season in China lasts only from the middle of May to the middle of June.After harvest,loquat fruit are susceptible to decay,mechanical damage,moisture and nutritional losses.Low temperature storage is commonly used to inhibit decay incidence and extend loquat storage periods,but it increases the severity of flesh leatheriness.Therefore,it is crucial to study the regulation and mechanism of quality deterioration of loquat fruit after harvest for transportation and storage.In this paper,the effect of MeJA and 1-MCP on the major physiological disorder flesh leatheriness and the major fungal disease anthracnose rot in loquat fruit and the possible mechanisms involved were investigated.In addition,the effect of antagonistic yeast Pichia membranefaciens alone or in combination with 2%CaCl₂ (w/v) or 10μmol/L MeJA on controlling anthracnose rot and the modes of action in loquat fruit were investigated.The results were as follows:(1):Loquat fruit were pretreated with 10μmol/L MeJA for 24h and then stored at 35 days at 1℃to investigate the effect of MeJA treatment on chilling tolerance in loquat fruit and the influence on the cell wall substances was also studied.Fruit firmness,internal browning index,decay index and weight loss increased while extractable juice rate,total soluble solids(TSS) and total titratable acidity(TA) contents decreased during the storage.After 21 days of storage at 1℃,the control fruit exhibited chilling injury symptoms including stuck peel,firm and juiceless flesh,and internal browning,which were collectively referred as flesh leatheriness.Treatment with 10μmol/L MeJA inhibited the increases in fruit firmness,internal browning index and weight loss,maintained higher extractable juice rate and TSS and TA contents, thereby delaying the development of flesh leatheriness.10μmol/L MeJA treatment decreased the activities of PAL,PPO,POD,CAD and 4-CL and inhibited the accumulation of lignin in loquat fruit during cold storage.Meanwhile,the treatment also maintained the balance of changes in PME and PG activity,inhibited the increases in AIR,hemicelluose and cellulose.In addition,a higher level of water-and CDTA-soluble pectin and a lower level of Na₂CO₃-soluble pectin were observed in MeJA-treated fruit.These results indicated that MeJA treatment could significantly enhance the chilling tolerance of loquat fruit after harvest and alleviate the chilling symptoms of lignification.(2):The effect of 10μmol/L MeJA on activities of active oxygen detoxifying enzymes and the contents of membrane fatty acid in cold-stored loquat fruit was investigated.Control fruit showed severe chilling symptoms of flesh leatheriness after 21days cold storage which was alleviated by applying MeJA during storage.This MeJA treatment enhanced the activities of SOD,CAT and APX in loquat fruit. Meanwhile,the accumulation of O₂ and H₂O₂ was inhibited by the MeJA treatment. Palmitic acid,stearic acid,oleic acid,linoleic acid and linolenic acid were the major fatty acids in the loquat fruit.During storage,contents of palmitic acid,stearic acid and oleic acid were increased while contents of linoleic acid and linolenic acid were decreased.MeJA treatment inhibited the decrease in the contents of linoleic acid and linolenic acid.These results suggest that 10μmol/L MeJA could significantly enhance the chilling tolerance of loquat fruit due to increased SOD,CAT and APX activities and linoleic acid and linolenic acid contents.In addition,the decreased levels of O₂ and H₂O₂ were also correlated with the induced chilling tolerance by MeJA treatment.(3):The effect of 50nL/L 1-MCP on reducing chilling injury in cold-stored loquat fruit and the possible mechanisms involved were investigated.After 21 days of storage at 1℃,the control fruit exhibited chilling symptoms.Treatment with 50nL/L 1-MCP inhibited the increases in fruit firmness,internal browning index,maintained higher extractable juice rate,thereby delaying the development of flesh leatheriness. 1-MCP treatment decreased the activities of PAL,PPO,POD,CAD and 4-CL and inhibited the accumulation of lignin in loquat fruit during cold storage.Meanwhile, the treatment also maintained the balance of changes in PME and PG activity, inhibited the increases in protopectin and maintained a higher level of water-soluble pectin.SOD and CAT were higher in 1-MCP-treated fruit during the storage period. In contrast,the accumulation of MDA and the increase in electrolyte leakage and LOX activity were inhibited by the 1-MCP treatment.These results indicated that MeJA treatment could significantly enhance the chilling tolerance of loquat fruit after harvest and alleviate the chilling symptoms of lignification.(4):The effect of 10μmol/L MeJA on reducing anthracnose rot caused by Colletotrichum acutatum infection in postharvest loquat fruit and the possible mechanisms involved were investigated.Treatment of loquat fruit with MeJA resulted in significantly lower disease incidence rate and smaller lesion diameter compared with the control fruit.The MeJA-treatment enhanced the activities of chitinase andβ-1,3-glucanase in loquat fruit,which is considered to play important roles in disease resistance.Meanwhile,MeJA treatment significantly inhibited activities of CAT and APX in early infection while SOD activity was not significantly affected,thus resulting in a higher level of H₂O₂ in the earlier period of incubation.The enhanced H₂O₂ generation by MeJA treatment might serve as a signal to induce resistance against C.acutatum infection.In addition,MeJA treatment inhibited the activities of PAL,PPO and POD enzymes and lignin synthsis,thereby alleviated the symptoms of flesh leatheriness in loquat fruit,which suggest that lignin and activities of those three enzymes may not a determining factor of disease resistance in loquat fruit.However, the content of total phenolic compounds in loquat fruit was increased by MeJA treatment and the phenolic compounds induced by the treatment might also be related to the higher resistance of the MeJA-treated fruit to infection by the pathogen.The in vitro experiment showed that MeJA at 10μmol/L inhibited mycelial growth,spore germination and germ tube length of C.acutatum.These results indicated that MeJA treatment can effectively inhibit anthracnose rot caused by C.acutatum in postharvest loquat fruit.It is postulated that the control of the disease is directly because of the inhibitory effect of MeJA on pathogen growth,and indirectly because of the induced disease resistance triggered by enhanced H₂O₂ level and activities of chitinase andβ-1,3-glucanase.(5):The effect of P.membranefaciens applying alone or in combination with 2% CaCl₂(w/v) or 10μmol/L MeJA on reducing anthracnose rot in loquat fruit were investigated.The results showed that the concentrations of antagonist P. membranefaciens had significant effects on biocontrol effectiveness of loquat anthracnose rot caused by C.acutatum.The higher concentrations of the antagonist, the lower disease incidence and the smaller lesion diameter.Washed cell suspensions of yeast provided control of anthracnose rot better than yeast in culture broth. Treatment of wounds with autoclaved cell cultures or cell-free culture filtrate did not prevent decay.Combining P.membranefaciens with 2%CaCl₂(w/v) or 10μmol/L MeJA provided a more effective control on C.acutatum than applying the antagonistic yeast,CaCl₂ or MeJA alone.The fruit treated with 1×10⁸CFU/ml washed cell suspension of P.membranefaciens alone showed higher levels of chitinase andβ-1,3-glucanase than the control fruit,CaCl₂ or MeJA and antagonistic yeast in combination induced higher activities of these two enzymes than the yeast,CaCl₂ or MeJA alone.The experiment in vitro showed 1×10⁸CFU/ml washed cell suspension of P.membranefaciens inhibited spore germination and germ tube length of C. acutatum,which could be enhanced when combined with CaCl₂ or MeJA.These results can probably explain why P.membranefaciens could reduce anthracnose rot in loquat fruit and why the biocontrol efficiency of the yeast could be enhanced by the application of CaCl₂ or MeJA.