Ethylene Regulation Fruit Cell Wall Metabolism And Its Structure Change In Ripening Muskmelon

Melon(Cucumis melo L.)is an important horticulture crop.It is popular because of its good appearance,rich nutrient and unique flavor.However,Muskmelon were harvested in hot and rainy season,physiological metabolism showed increased drastically and flesh firmness decreased sharply at room temperature,which limited their transportation,storage and shelf-life of fruits.The melon variety ‘Xiangmier’ was used in this experiment,the fruits were harvested 43 days after pollination(DAP),experimental treatments as followings:1.fruits sprayed with 200mg·L-1AVG at 35 DAP;2.fruits fumigated with 1μL·L-1 1-MCP for 24 hours at room temperature;3.fruits Soaked with 2000 μL·L-1 ethephon for 10 min;4.fruits sprayed with water as the control(CK).All of the above treated fruits were storied at room temperature(22℃~24℃)and with 80~85% RH;5.fruits sprayed with water,and then stored at 10±1℃.The characteristics of fruits were determined,including physiological properties,cell wall component,cell wall metabolism-related enzyme activity and its genes expression,cellular structure changes during ripening.The regulation mechanism of ethylene factors(1-MCP,AVG,low temperature)on fruit softening were studied,which will guide the postharvest handling for muskmelon.The results are as follows:1.The change of physiological characteristic were investigated during ripening,the results showed that respiration rate and ethylene production increased significantly accompanied by the decline of firmness.AVG,1-MCP,and low temperature treatment significantly inhibited respiration rate and ethylene production.But ethephon treatment significantly accelerated them at earlier storage(9 d).The efficiency of each ethylene factors on fruit hardness keeping was AVG>1-MCP>CK>Ethephon,and low temperature was beneficial to keeping flesh hardness.Fruit texture change was closely related with respiration and ethylene production.It showed that the ripening and softening of fruit were affected by ethylene.2.The cellular structure changes in ripening fruit.The size of parenchymal cell increased and the density of cell decreased,and cell gap grew bigger.Parenchyma cells and sponge tissue fusion occurred at later storage,showed as the cell scattered respectively,cell wall distorted partly and fused each other.The AVG treatment could keep cell integrity and restrain plasmatorrhexis occurrence,whereas ethephon treatment promoted the cell rupturing.The low temperature treatment maintained the cell integrity in the earlier storage,but occurrence of cell dissolution presented in the later storage.3.The change of cell wall composition was studied during ripening.Flesh hardness,cell wall materials(CWM)and cellulose decreased gradually at room temperature,but the water-soluble pectin(WSP)contents increased.Covalent pectin(CSP)content was declined at earlier storage,rebounded slightly at late storage.Ionic pectin(ISP),loose hemicellulose,closely hemicelluloses was rising first,and then downward.AVG,1-MCP significantly inhibited the decomposition of CWM,CSP,delayed the WSP increasing during ripening.AVG,1-MCP also could inhibit the decomposition of ISP,hemicellulose,cellulose at earlier storage.At room temperature,ethephon significantly accelerated the CWM,ISP,CSP,loose hemicellulose,cellulose transformation and decomposition,promoted the transformation of various soluble pectin to WSP at later storage,speed up degradation of closely hemicellulose.Low temperature significantly inhibited the conversion of other soluble pectin to WSP,delayed the decomposition of ISP,CSP,hemicellulose and cellulose,and reduced decomposition of CWM,loose hemicellulose at later storage.The change of cell wall components is closely related to firmness and ethylene production.It showed that pectin components and cellulose degradation promoted the decreasing of flesh hardness,affected by ethylene factors.4.The enzyme activity related cell wall metabolism and its gene expression showed that PG enzyme activity was similar to its gene expression,and has no obvious relation with hardness.AVG significiantly suppressed PG enzyme activity,followed with Cm-PG1,2,3 down-regulated expression.1-MCP significiantly inhibited Cm-PG1,2,3 expression,but the enzyme activity increased.Ethephon promoted enzyme activity rise and Cm-PG1,3 gene expression in early stage.On the contrary,low temperature depressed enzyme activity and gene expression at earlier storage.PME activity showed increasing first and decreasing at the later storage,which were according with pectin change tendency,demonstrated PME participated in pectin decomposition.At normal temperature,effect of 1-MCP treatment were superior to that of AVG treatment in depressing PME activity,and ethephon promoted PME activity,whereas the low temperature depressed it.All the above indicated that PG and PME activity affected by ethephon and temperature.β-GAL activity were consistent with Cm-GAL1,2 expression.At the earlier storage,β-GAL activity and its gene expression increased gradually,but pectin hydrolyzing sped up following with reducing of fruit firmness,and vice versa appeared.Those demonstrated that β-GAL were close related to fruit softening.AVG and 1-MCP treatment remarkly depressed β-GAL activity and genes expression at earlier storage.Ethephon treatment enhanced increasing of enzyme activity at later storage,but depressed the genes expression.The enzyme activity of fruits stored in low temperature significantly were lower than that of fruits at room tempetature(CK),its gene expression was depressed at earlier and promoted at later storage,which indicated β-GAL activity and gene expression was affected by ethylene.At room temperature,Cm-XTH1,2,3 expression gradually increased at earlier stage,decreased in the middle stage,rose again at later stage.At the harvested day,the Cm-XTH1,2,3 expression of fruits treated with AVG were highly notably,which indicated AVG treatment delayed fruit ripening before harvest,and also depressed Cm-XTH1,2,3 expression during storage.1-MCP inhitited Cm-XTH1,2 expression,but promoted Cm-XTH3 expression.Ethephon promoted Cm-XTH1,3 expression,but depressed Cm-XTH2 expression.Cm-XTH1,2,3 expression of fruits stored at low temperature were depressed at earlier,but promoted in the later.All above demonstrated that Cm-XTH1,2,3 expression was independent of exogenous ethephon,but were related to storage temperature.