The Related Factors Of Night Low Temperature Effect On Development And Sugar Accumulation Of Melon During Fruit Enlarging

Melon is a important kind in horticulture production in north area.However,lowtemperature stress usually occur during fruit enlarging in autumn in north area,which affectsthe development and reduces the quality.As a result,melon production in autumn is limitedseriously.Thus it is very important to study the related factors which cause small fruit and lowsugar of melon fruit and estabolish the planting measure to regulate the yield and qualityunder low temperature stress.Therefore,this paper studied the effects of different night low temperature on melongrowth,espaciallly fruit enlarging,sugar accumulation in fruit and their factors with thematerial of"yumeiren".Furthermore,this paper demonstrated the relationship betweenphotosynthesis,stachyose metabolism in leaves,the transcript of galactinol synthase gene,ultrastructure changes of phloem,the factors of sink strength and fruit enlarging and sugaraccumulation in fruit during fruit enlarging under normal and night low temperaturecircumstance simulated by climate room.Finally,we demonstrated the factors which restrictfruit enlarging and sugar accumulation.In a word,this research will be helpful for controllingcarbohydrate metabolism of melon under low temperature stress.The key objectives werelisted as follows:1.It was clear that 9℃in night distinctly restricted the growth of melon.In detail,9℃in night reduced the height and stem diameter with 4.89% and 2.87% respectively.It wasdisadvantage for photosynthetic dry matter accumulation of melon plants under 9℃in night,and the distribution of dry matter in fruit decreased while other organs increased.Moreover,the single fruit weight,transverse length and vertical length were reduced at 23.85%,12.14%and 10.58% respectively after the treatment of 9℃in night.However,the effect of 12℃innight on melon development was little.2.Carbohydrate contents in melon fruit were affected by the treatment of 9℃in night.Fructose and glucose decreased after treatment of 9℃in night for 3 d while significantlydecreased sucrose content occurred in middle and later period,yet galactinol content had nochange.Meanwhile,total carbohydrate content was lower than control with markedlysignificant difference after treatment of 9℃in night for 3 d,and the difference was moreobvious as the time prolonging.Otherwise,compared with control,starch content increasedobviously at 3～12 d after treatment of 9℃in night,however,after treatment of 12℃innight,starch content increased in evidence while each carbohydrate content was almostinvariable.3.The experiment demonstrated that photosynthesis of melon fell under the treatment of9℃in night.Plants at 9℃exhibited a consistently lower net photosynthetic rate(Pn),stomatal conductance(Gs)and transpiration rate(Tr)relative to control plants for allexperimental times,yet concentration of CO₂(Ci)only decreased at beginning of 9℃in nighttreatment.Compared with control,Ls increased after treatment of 9℃in night for 6～12 d,with significant difference,suggesting that Ls may be the important reason for the decrease of photosynthetic rate.What was more,the change of chlorophyll content may be related withthe decrease of photosynthetic rate.However,after the treatment of 12℃in night,netphotosynthetic rate was only notably affected at 12 d,Ls was bigger than control after thetreatment for 6～9 d,while the other factors related to photosynthesis were not affected.4.Stachyose metaolism in melon leaves was limited after the treatment of 9℃in night.Frist of all,the contents of carbohydrate and starch accumulated largely.A classic linearincrease in RFO concentration,particularly raffinose,was observed after 6 d of treatment at 9℃with markedly significant difference,and exhibited a nearly two times relative to control.In addition,high glucose and fructose accumulation was also observed in leaves subjected to9℃in night,and stachyose content increased obviously compared with control at the end oftreatment,but these changes were smaller than raffinose.Meanwhile,the increase of starchwith 20.28% on average in overall treatment time.Secondly,stachyose-metabolizing enzymesactivities were affected when melon plant obejected to 9℃innight.The treatment of 9℃innight led to a marked decrease in overall SS,SPS after 6 d,and a sharp increase in AI and NI,especially AI.Galactinol synthase and stachyose synthase activity also reduced in evidence,and the change was more stronger as the time prolonging.Finally,a obvious reduction ofCmGAS1 and CmGAS2 transcript was obversed at the end of treatment of 9℃in night.There was no change of stachyose metabolism in melon leaves after treatment of 12℃innight.5.The morphological observation using SEM showed that there were two circles ofvascular bundle on transverse section of side branches,and the vascular bundle in inner circlenamed initial vascular bundle was bigger while the vascular bundle in outer circle namedsubsequent vascular bundle was smaller,and each vascular bundle had two phelom.Thetreatment of 9℃in night postponed the development period of vascular bundle.Whenmelon plants were treated under 9℃in night for 12 d,five vascular bundles in inner werebig,while five vascular bundles in inner circle were smaller than control.These changes canmake against sugar transport.Moreover,12℃in night did not affact the development ofvascular bundles.6.The ultrastructure of inner phloem of melon under condition of different nighttemperature were observed using TEM.The results showed that 9℃and 12℃in nightchanged ultrastructure of inner phloem on different degree.In detail,plasmalemmainvagination appeared in common companion cells and intermediary cells.Mitochondrianumber decreased steadily,the arrangement direction of grana changed,chloroplasts lamellawas blurry and multivesicular structures increased in common companion cells.A large ofstach grains appeared in phloem parenchyma cells and the character of nacreous wall wasmore obvious than control.Althrough these changes were related to cold acclimation,deleterious to sugar transport.7.It was found that the treatment of 9℃in night led to sugar accumulation in vascularbundles of side branches and fruit stem.The contents of fructose and raffinose were higher than control after treatment for 3 d,while the increase of glucose,sucrose and stachyoseoccured after treatment for 9 d in vascular bundles of side branches.The content of galactinoldrcreased compared with control,yet other carbohydrates were opposite in vascular bundlesof fruit stem.What was more,the amount of accumulated sugar in vascular bundles of fruitstem was more than that in vascular bundles of side branches,in paticular sucrose,suggestingthat the restriction degree of sugar transport in vascular bundles of fruit stem may be biggerthan vascular bundles of fruit stem and further verification was obtained that vascular bundlesof fruit stem may be more sensitive to low temperature.After treatment of 12℃in night for 6d,the content of galactinol was lower than control in vascular bundles of side branches andother carbohydrate had no change.Meanwhile,the contents of glucose,fructose and raffmosewere higher than control with significant difference at the end of treatment and othercarbohydrate had no change in vascular bundles of fruit stem.8.It was found that the sink strength was affected by different night low temperature ondifferent degree,especially 9℃in night.The factors related to sink strength were sugar-metabolizing enzymes activities,endogenous hormone and polyamine contents.The changeof sugar-metabolizing enzymes activities in fruit indicated that acid invertase(AI),netutralinvertase(NI)andα-galactosidase activities enhanced,but sucrose synthase(SS)activities andsucrose phosphate synthase(SPS)activities declined with extraodinary significant differenceafter treatment of 9℃in night.What was more,the effects of 12℃in night onsugar-metabolizing enzymes activities was small,with no significant difference.9.The content of endogenous hormone had a great change after treatment of night lowtemperature.After the treatment of 9℃in night,GA₃,ZT and IAA reduced in evidence.GA₃/ABA and ZT/ABA was also reduced significantly,and ABA increased in the last stage offruit enlarging period with the extraordinary significant difference,but after the treatment of12℃in night,GA3,IAA and ABA added until the later of fruit enlarging period.In addition,the analysis of the relationship between fruit endogenous hormone and the increment rate ofsingle fruit indicated that whether treatments or control,there exists extreme positivesignificant linear relationship between GA₃/ABA and increment rate of single fruit,withdetermination coefficient bigger.However,except for control,there was no significant linearrelationship between ZT/ABA and increment rate of single fruit.10.The effect of different night low temperature on polyamine content in melon fruitwas observed in this paper.The contents of total PA,Spd and Put decreased at the beginningof treatment of 9℃in night,and the content of Spm reduced overall.Moreover,the contentof Put was lower than control with significant difference at the beginning of treatment of 12℃in night,whereas Spd and total PA were almost invariable.From the analysis of relatinshipbetween polyamine and increment rate of single fruit,it was indicated that the contents of Put,Spd and total PA contents in fruit were positively correlated to fruit enlarging.