Study On The Differences Of Starch And Sugar Conversion During Fruit Development Of Different Oriental Melon (Cucumis Melo Var. Makuwa Makino) Cultivars

Oriental melon(Cucumis melo var.makuwa Makino)has a long history of cultivation in China,and is favored by consumers for its sweet quality.Sugar content is a decisive factor for fruit quality of melon.The results showed that the starch degrades gradually and leads to the accumulation of soluble sugar,but the specific relationship between the two needs to be further clarified.Therefore,oriental melon fruit with different sucrose content were selected as test materials,namely high-sugar cultivar‘YMR’and‘HDB’,low-sugar cultivar‘XSM’and‘HPM’were selected in this experiment.The contents of starch and sugar,the activities of key enzymes and the expression patterns of genes related to starch-sucrose metabolism were determined during the development of oriental melon fruit.Hoping to clarify the change of starch and its effect on sugar content and accumulation during fruit development of oriental melon,it is of great significance to improve fruit quality.The main results were shown as follows:1.The‘YMR’,‘HDB’and‘XSM’,‘HPM’of oriental melon with different sugar content were sampled at different day after anthesis.The dynamic changes of important parameters during fruit development of oriental melon were measured,and the results showed that,the soluble solid content(SSC)and fruit hardness of the flesh of‘YMR’and‘HDB’during the fruit development of melon are significantly higher than those of‘XSM’and ‘HPM’.And during the fruit ripening process of the two varieties,there was a peak of ethylene release at30 days after anthesis,which were typical respiration climacteric fruit.However,the ethylene release of‘XSM’and‘HPM’fruits during the ripening process is low and there is no obvious peak change.which belonged to non-respiration climacteric fruit.2.Use Transmission Electron Microscope(TEM)to observe the changes of starch grains in the pulp tissue at the developmental stage.It was found that the number of starch grains in the pulp tissue of the‘YMR’gradually increased with the development of the fruit,and the volume gradually increased.Finally,the starch grains disappeared and all were degraded;while the volume of starch grains in the pulp tissue of‘XSM’gradually became smaller,and finally the starch grains in the plastids disappeared.By measuring the starch content,the‘YMR’and‘HDB’fruits accumulated starch in the mature stage,and then decreased rapidly after the peak of starch accumulation.When the fruits were fully ripe,almost all starch was converted into soluble sugar,which was a high-starch accumulation cultivar.The starch of‘XSM’and‘HPM’fruit was at a low level during the whole development,without starch accumulation,which was low-starch accumulation cultivar.3.High performance liquid chromatography(HPLC)was used to determine the type and content of sugar during the development of different cultivars of fruits.The results showed that the content of glucose and fructose did not change significantly during the development process,and there was no difference between the cultivars.The sucrose content gradually increases as the fruit develops.The sucrose content of‘YMR’and‘HDB’fruits is significantly higher than that of‘XSM’and‘HPM’during the ripening period.4.The determination of starch metabolism related enzyme activities showed that the AGPase、AMY and BMY enzyme activities of‘YMR’and ‘HDB’fruit were significantly higher than those of‘XSM’and‘HPM’.AGPase activity is consistent with the changes in starch content during fruit development and promotes starch synthesis.AMY and BMY activities gradually increase during fruit ripening,which is beneficial to the degradation of fruit starch and affects the difference in sucrose content during fruit ripening.The expression of 10 genes involved in starch synthesis and metabolism was analyzed by real-time quantitative PCR.The results showed that the expression of CmAPS2-2,CmAMY2,CmBAM1 and CmBAM9 in‘YMR’and‘HDB’fruit was significantly higher than that in‘XSM’and‘HPM’,which regulated starch synthesis and degradation.5.The results of measuring the activity of sucrose-related enzymes during fruit development of melon showed that acid invertase(AI),neutral invertase(NI)and sucrose synthase(SS)had no difference in the development of four muskmelon cultivars.AI and NI have higher activity in the early stage of melon fruit.As the fruit developing,the activity of invertase is always at a low level,while the sucrose synthase(SS)gradually increases during the fruit ripening period.The activity of sucrose phosphate synthase(SPS)increased rapidly during the fruit ripening stage of‘YMR’and‘HDB’,which was significantly higher than that of‘XSM’and‘HPM’.There was no significant change in‘XSM’and‘HPM’.Therefore,the change of SPS activity is one of the main reasons for the difference in fruit sugar accumulation.CmSPS1 was up-regulated in fruit,and the expression of‘YMR’and‘HDB’was significantly higher than that of ‘ XSM ’ and ‘ HPM ’,which promoted sucrose accumulation.6.The results of correlation analysis between starch,sucrose,enzyme activity and gene expression during fruit development of‘YMR’and‘HDB’were higher than‘XSM’and‘HPM’.Starch content played a positive role in regulating sucrose accumulation.AGPase is a key enzyme in starch synthesis and affects starch accumulation.BMY plays a key role in starch degradation of fruits,and SPS is the main sucrose synthesis enzyme,which promotes sucrose synthesis and accumulation.AI and NI play negative regulatory roles in starch metabolism and sugar accumulation.CmAPS2-2 、 CmAMY2 、 CmBAM1 、 CmBAM9 and CmSPS1 positively regulate starch metabolism and sugar accumulation in‘YMR’and‘HDB’fruits.In summary,the starch metabolism of oriental melon fruits positively regulates sucrose accumulation,leading to differences in sugar accumulation between different varieties,and this process is jointly regulated by ADPase,AMY,BMY,SPS and CmAPS2-2,CmAMY2-1,CmBAM1 and CmBAM9.