| Fruit ripening is a genetically programmed event that is characterized by a number of biochemical and physiological processes that alter fruit firmness,fruit colour,flavour,aroma, and texture.Fruit softening is a major factor that determines fruit quality and post-harvest life. In melon(Cucumis melo L.),a typical climacteric fruit,some cultivars show an extremely rapid decrease in flesh firmness during ripening,limiting their transport,storage,and shelf-life.In the factors influencing fruit ripe and quality,both the calcium content and ethylene production have an important role.A number of studies have suggested that calcium is able to inhibit the maturation and senescence of many fruit and the ethylene have promotion this process.However,the report about effect of calcium on fruit maturation and senescence of ethylene-promoted has little systematic research.Moreover,this research is vitally important on both the mechanism of ethylene promoting and the application of calcium suppressing during the maturation and senescence of fruits.The muskmelon was studied by following treatments:pre-harvest spraying plants, post-harvest immersing fruit,exo-ethylene treating different maturity of fruit,and the treating sequence of calcium and ethylene.We investigated the ripe-related physiological parameters, the process of ethylene biosynthesis,the morphology of flesh cell,quality,and the gene expression of soften-related enzyme-polygalacturonase,and so on.The results have determined that both calcium and ethylene regulated the maturity and senescence of muskmelon which provided a reliable base in theory for the regulation technology of delaying senescence and extending shelf life.The more details are the following.1.The suitable concentration of calcium is 0.5 percent base on both pre-harvest and post-harvest.We investigated the flesh firmness,respiratory rate and quality related parameters via two ways of applying calcium.The results evaluated calcium availably delayed the softening of flesh,dropped respiratory rate,improved the content fructose, glucose,sucrose and soluble total sugar and starch,and as well reduced the content of soluble solids.Above results confirmed the calcium played an important role on delaying soften and improving the quality of fruit.2.The effect of pre-harvest and post-harvest calcium on the cell wall hydrolases and senescence-related enzymes was investigated during the process of fruit soften.The results showed that calcium decreased the activity of cell wall hydrolases(PG,PE,CMC-Na), increased the activity of POD,SOD and CAT,and reduced the content of soluble solids, electrical conductivity,the activity of LOX and pH ratio.It turned out that calcium has ability of delaying the fruit soften by regulating the activity of cell wall hydrolases and senescence-related enzymes.3.The ethylene biosynthesis during the softening and senescence of fruit were studied in view of two ways of calcium treatment.The results suggested that calcium postponed the peak of ethylene and reduced the production.This process was implemented by the regulation of ethylene biosynthesis:postponing the peak and production of ACC content and ACS activity,decelerating the transformation from ACC to ethylene by ACO catalysis,and thereby reducing the ethylene production.4.The textural structure of flesh was observed with two ways of calcium treatment.As it turned out,calcium could delay the appearance of water-core,especially for the post-harvest calcium.Moreover,calcium maintained the integrity of cell wall,kept the density among cells, and inhibited regional expansion of cell wall,and thus delayed the softening of fruit.5.The different maturity of fruits was treated by 10μL·L-1 exo-ethylene,which demonstrated ethylene accelerating the softening of fruit.The survey of firmness,respiratory rate and quality revealed that ethylene significantly elevated respiratory rate,dropped the content of soluble solids,fructose,sucrose and starch,and slightly increased the content of sucrose and soluble total sugar.The results of the tests indicated that ethylene accelerated the softening of different maturity fruits and deteriorated in the quality of fruit to the extent.6.The changes of cell wall hydrolases and senescence-related enzymes were investigated after the different maturity of muskmelon being treated by ethylene.We found out that the ethylene were able to increase the activity of cell wall hydrolases,dropped the activity of POD,SOD and CAT,and the content of soluble protein,at the same time elevated the activity of PAL and LOX,electrical conductivity and Ph ratio.Base on these results ethylene could promote the maturity and senescence of fruit by ways of regulation the activity of cell wall hydrolasases and senescence-related enzymes.7.The effect of muskmelon of different maturity on ethylene biosynthesis was studied after the ethylene treatment.The exo-ethylene significantly increased the endo-ethylene production by means of heightening the content of ACC and the activities of ACS and ACO.8.The change of flesh structure was observed during the softening process of ethylene promoting different maturity of muskmelon.Ethylene could induce the phenomenon of water-core and make it even worse.This is probably the result that exo-ethylene has capability of promoting the production of endo-ethylene,stimulated the disassembly of cell wall and/or maintained better cell membrane integrity,and then enlarged intercellular space which lead to the appearance of water-core.In terms of light microscope,the ethylene was able to accelerate the deterioration of cell wall and destroy of structure of cell membrane.9.The flesh firmness,respiratory rate and fruit quality in relation to the reduction of calcium regulating ethylene were investigated during the process of muskmelon soften.The results were as follows:exogenous applied ethylene in calcium condition has higher respiratory ratio than single calcium treatment.Meanwhile,compared to single ethylene treatment,the time sequence of calcium and ethylene further reduced the flesh firmness and elevated the respiratory ratio.In addition,the treatments of time sequence of calcium and ethylene have lower contents of soluble solids,fructose,glucose and soluble starch,as well higher sucrose and soluble total sugar than the single ethylene and calcium.10.The effect of calcium on activities of cell wall hydrolases and senescence-related enzymes is different in ethylene condition.The treatments of time sequence of calcium and ethylene have further advanced above two enzymes than single calcium and ethylene treatments.11.During the process of muskmelon softening,the ethylene biosynthesis was investigated after calcium and ethylene treated in time sequence.The treatments of time sequence of calcium and ethylene elevated the ethylene produciton than single calcium and ethylene treatments by means of adjusting ethylene biosynthesis.This result indicated that there was a relation between the promoter of fruit senescence(Ca) and the inhibitor of fruit senescence(ethylene).Namely,calcium can enlarge the reduction of ethylene during the fruit maturity and senescence.12.Light microscope analyzed the texture structure of flesh during the calcium regulation of ethylene-induced in fruit maturity and senescence.All other treatments of different parts appeared the phenomenon of water-core and visibly flowed juice besides the single calcium treatment.In terms of the extent of water-core,the results of this comparison are as follows: Eth-Ca＞Eth＞CK＞Ca-Eth.Moreover,light microscope analysis that the ethylene destroyed the integrity of cell membrane of muskmelon,and the calcium further enlarged the influence of ethylene.Exogenous applied ethylene in calcium condition almost completely deteriorated the cell wall.Furthermore,although calcium effectively stabilized the structure and maintained the integrity of cell,interacting sites of ethylene was the cell membrane leading to lipid peroxidation.Therefore,exogenous calcium applied in ethylene condition destroyed to the extent the flesh cell,weaken the protection of calcium on cell membrane,and then lead to partly cell mergence.13.Northern blot analysis showed that the expression of MPGs in the four segments of fruit of various storage stages(earlier,middle and final stage) was different after calcium and ethylene treatments.The expression pattern of two MPGs was similar during the storage.With the extension of storage life,the expression of levels was heightened.The expression of both MPG1 and MPG2 were significantly inhibited by calcium,but it was various to express in different calcium treatments.Moreover,there is different expression of both MPG1 and MPG2 in different parts of flesh.The expression of MPGs started in placenta,and frequently weakened from the seed cavity to outside.Furthermore,during the storage,the expression of MPG1 was promoted by ethylene.The expression level of MPG1 in ripe fruit was evidently higher than the maturity fruit,but the sitimulation of ethylene on MPG2 was less.In addition, the response of different segments in different maturity was also various.The expression of MPG1 in mesocarp and exocarp of fruit increased significantly,but the expression of MPG2 in placenta and endocarp was elevated obviously under the ethylene condition.Moreover,the expression of both MPG1 and MPG2 in four segments made an evident raise after ethylene treatment.When the exogeous calciuam and ethylene were applied at the same time,the expression of MPG1 and MPG2 were the hightest in the calcium treatment under ethylene condition.
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