摘""要:果皮褐變是荔枝在采后貯運(yùn)流通環(huán)節(jié)最顯著的品質(zhì)劣變特征之一,嚴(yán)重影響其商品價(jià)值。本研究以糯米糍荔枝為材料,設(shè)置輕度失水(dehydration"treatment,DT)、熱激(heat-shock"treatment,HT)和冷激(cold-shock"treatment,CT)3種逆境脅迫處理,通過(guò)研究荔枝果實(shí)經(jīng)脅迫處理后果皮外觀、褐變指數(shù)、褐變相關(guān)生理指標(biāo)及多酚氧化酶(polyphenol"oxidase,PPO)、過(guò)氧化物酶(peroxidase,POD)和超氧化物歧化酶(superoxide"dismutase,SOD)活性的應(yīng)激變化,探究不同脅迫所誘導(dǎo)荔枝褐變的內(nèi)在差異。結(jié)果表明:3種脅迫處理后,常溫下果皮褐變加劇,亮度下降,含水量降低,膜透性增加,其中DT處理組果實(shí)褐變最迅速,褐變指數(shù)第6天即達(dá)到4.99,HT和CT處理組誘導(dǎo)果皮褐變的速度較DT處理組慢;DT處理組荔枝完全褐變后果皮呈干黃色,脆性高,含水量最低,僅為27.76%,明顯低于HT和CT處理組的45.20%和42.99%。果實(shí)呼吸速率高低處理組表現(xiàn)為CTgt;HTgt;DT,相對(duì)電導(dǎo)率高低處理組順序則是DTgt;HTgt;CT;貯藏前期HT和CT處理組果實(shí)的PPO、POD活性受到抑制,SOD活性則在后期明顯提高。透射電鏡結(jié)果顯示,失水和冷激脅迫引起褐變的果皮細(xì)胞結(jié)構(gòu)被破壞,內(nèi)含物基本降解消失;熱激脅迫的褐變果皮細(xì)胞保持結(jié)構(gòu)完整,含有大量聚結(jié)的沉積物。相關(guān)性分析結(jié)果表明,3種脅迫處理的荔枝果皮褐變指數(shù)均與相對(duì)電導(dǎo)率呈顯著正相關(guān),與L*、果皮含水量和花色素苷含量呈顯著負(fù)相關(guān)(Plt;0.05)。綜上,失水脅迫處理導(dǎo)致荔枝快速褐變,熱激和冷激脅迫可以通過(guò)誘導(dǎo)荔枝自身應(yīng)激提高果皮含水量和果實(shí)呼吸強(qiáng)度,抑制果皮相對(duì)電導(dǎo)率升高,抑制PPO和POD活性,提高貯藏后期SOD活性,從而延緩果皮褐變。
關(guān)鍵詞:糯米糍;脅迫應(yīng)激;果皮褐變;抗氧化中圖分類(lèi)號(hào):S667.1""""""文獻(xiàn)標(biāo)志碼:A
Relationship"Between"Stress-induced"Responses"and"Browning"in"Litchi"under"Different"Stress"Treatments
XUE"Xiaoqing1,2,"LIAN"Xiaowei1,"SUN"Yan1,"XIAO"Zhuolin1,"LUO"Tao2,3,"WU"Zhenxian2,3*
1."College"of"Food"and"Medicine,"Qingyuan"Polytechnic"College,"Qingyuan,"Guangdong"511510,"China;"2."College"of"Horticulture,"South"China"Agricultural"University"/"Guangdong"Provincial"Key"Laboratory"of"Postharvest"Science"of"Fruits"and"Vegetables"/"Engineering"Research"Center"of"Southern"Horticultural"Products"Preservation,"Ministry"of"Education,"Guangzhou,"Guangdong"510642,"China;"3."Guangdong"Litchi"Engineering"Research"Center,"Guangzhou,"Guangdong"510642,"China
Abstract:"The"browning"of"the"fruit"pericarp"is"one"of"the"most"significant"quality"deterioration"characteristics"of"litchi"during"storage,"transportation,"and"circulation"after"harvest,"which"seriously"affects"its"commercial"value."In"this"study,"three"stress"treatments"were"applied"to"litchi,"mild"dehydration"treatment"(DT),"heat"shock"treatment"(HT),"and"cold"shock"treatment"(CT)."We"analyzed"the"changes"in"appearance"quality,"browning"index,"physiological"indices,"and"the"activities"of"polyphenol"oxidase"(PPO),"peroxidase"(POD),"and"superoxide"dismutase"(SOD)"after"treatment"to"explore"a"new"approach"to"delay"pericarp"browning"by"inducing"resistance"in"litchi."The"results"indicated"that"after"the"three"stress"treatments,"the"browning"of"the"litchi"pericarp"was"exacerbated,"accompanied"by"a"decrease"in"pericarp"brightness,"water"content,"and"increased"membrane"permeability"at"room"temperature."The"browning"in"the"DT"group"was"the"most"rapid,"reaching"a"browning"index"of"4.99"on"the"6th"day."In"contrast,"HT"and"CT"significantly"delayed"the"browning"of"the"litchi"pericarp."The"completely"browned"pericarp"in"the"DT"group"was"dry,"yellow,"and"brittle,"with"the"lowest"water"content"(27.76%),"which"was"significantly"lower"than"that"in"the"HT"and"CT"groups"(45.20%"and"42.99%,"respectively)."The"order"of"respiration"rates"was"CTgt;HTgt;DT,"and"the"relative"electrical"conductivity"was"DTgt;HTgt;CT."The"activity"of"PPO"and"POD"in"the"HT"and"CT"groups"was"inhibited"during"the"early"part"of"storage,"while"the"activity"of"SOD"significantly"increased"at"the"later"stage."Transmission"electron"microscopy"results"showed"that"the"cell"structure"of"the"brown"peel"was"destroyed"and"the"contents"were"largely"degraded"in"the"DT"and"CT"groups,"whereas"the"brown"pericarp"cells"in"the"HT"group"remained"structurally"intact"and"contained"a"large"amount"of"coalesced"sediments."Correlation"analysis"results"showed"that"the"browning"index"of"the"litchi"pericarp"under"the"three"stress"treatments"was"positively"correlated"with"relative"conductivity"and"negatively"correlated"with"L*"(brightness),"water"content"in"the"pericarp,"and"anthocyanin"content"(Plt;0.05)."In"conclusion,"mild"dehydration"treatment"led"to"rapid"browning"of"litchi,"while"heat"shock"and"cold"shock"stress"treatments"maintained"higher"water"content"in"the"pericarp"and"respiratory"intensity"of"the"litchi"fruit."The"treatments"inhibited"the"increase"of"relative"electrical"conductivity"and"the"activities"of"PPO"and"POD"enzymes"and"increased"the"activity"of"SOD"in"the"later"storage"period"by"stimulating"the"litchi’s"self-resistance,"thereby"inhibiting"the"browning"of"litchi"fruits"and"delaying"the"decline"in"fruit"quality.
Keywords:"litchi"cv."Nuomici;"stress"treatment;"stress-induced"response;"pericarp"browning
DOI:"10.3969/j.issn.1000-2561.2025.06.011
荔枝是廣東省產(chǎn)量最大的水果之一,味道鮮美,營(yíng)養(yǎng)豐富,備受消費(fèi)者歡迎。在夏季,荔枝集中上市以鮮食為主,常溫銷(xiāo)售荔枝極易出現(xiàn)果皮褐變現(xiàn)象,迅速失去商品價(jià)值,容易出現(xiàn)“果賤傷農(nóng)”的現(xiàn)象,進(jìn)而影響其經(jīng)濟(jì)價(jià)值,制約產(chǎn)業(yè)健康發(fā)展。因此,研究采后荔枝保鮮防褐變技術(shù)具有重要意義。
失水、熱、冷等逆境脅迫會(huì)打破細(xì)胞內(nèi)活性氧產(chǎn)生和清除的動(dòng)態(tài)平衡,大量活性氧物質(zhì)累積會(huì)使脂類(lèi)、蛋白質(zhì)、核酸等物質(zhì)遭受破壞,導(dǎo)致細(xì)胞凋亡,引起果皮褐變,果實(shí)衰老。因此,為了確保自身正常的生理活動(dòng),植物會(huì)激活自身生理及抗氧化機(jī)能,產(chǎn)生多酚氧化酶(polyphenol"oxidase,PPO)、過(guò)氧化物酶(peroxidase,POD)和超氧化物歧化酶(superoxide"dismutase,SOD)等防御性抗氧化酶,幫助細(xì)胞消除這些活性氧物質(zhì)造成的傷害,從而應(yīng)對(duì)逆境脅迫。
失水脅迫是影響荔枝采后褐變的主要不利因素,大量失水會(huì)破壞果皮細(xì)胞完整性,導(dǎo)致細(xì)胞內(nèi)物質(zhì)大量外滲,氧氣、酶類(lèi)與外滲出的酚類(lèi)物質(zhì)、花色素苷接觸發(fā)生聚合反應(yīng),導(dǎo)致褐變[1-2]。李奕星等[3]的研究表明,荔枝采收后不進(jìn)行包裝處理,在常溫放置12~24"h,果實(shí)即出現(xiàn)褐變癥狀,果皮失水加劇,褐變指數(shù)迅速升高,二者呈顯著正相關(guān);但輕度失水可以保持軟棗獼猴桃果實(shí)品質(zhì),降低腐爛率,保持高活性氧清除能力,抑制成熟進(jìn)程,緩解采后運(yùn)輸機(jī)械傷[4]。因此推測(cè)輕度失水脅迫可能對(duì)荔枝保鮮品質(zhì)也產(chǎn)生重要影響。
采后熱激處理是指用一定的高溫對(duì)果蔬進(jìn)行短時(shí)間脅迫處理,通過(guò)抑制病原菌,改變酶活性調(diào)節(jié)果實(shí)生理生化代謝,誘導(dǎo)果實(shí)產(chǎn)生抗逆性以消減或消除脅迫帶來(lái)的不利影響,從而達(dá)到貯藏保鮮的目的[5]。熱激處理使用方便,簡(jiǎn)潔高效,能延遲果蔬褐變、同時(shí)保持質(zhì)地,REHMAN等[6]研究發(fā)現(xiàn),熱激處理后,甜椒可以顯著提高抗氧化酶活性,失重率、活性氧和丙二醛(MDA)含量明顯降低,降低冷害程度。李彥麗等[7]的研究也表明,熱激能有效抑制PPO、POD活力,減緩相對(duì)電導(dǎo)率升高,降低百合褐變度和腐爛率。鮮切大白菜經(jīng)55"℃熱水處理3"s貯藏10"d后仍能保持良好的色澤,感官品質(zhì)明顯高于對(duì)照組[8]。
冷激處理是指將果蔬置于不會(huì)造成冷害和凍害的低溫條件進(jìn)行短時(shí)間脅迫處理,通過(guò)冷激脅迫誘導(dǎo)果蔬提高自身生理抗性,從而達(dá)到延長(zhǎng)貯藏期和提高品質(zhì)的目的[9]。李冰茹等[10]的研究表明,冷激處理可以緩解脅迫引起的氧化過(guò)程,抑制鮮切火龍果果肉的褐變。通過(guò)維持細(xì)胞結(jié)構(gòu)完整性,降低過(guò)氧化程度,調(diào)控活性氧代謝,冷激還可維持鮮切桃子[11]、木瓜[12]的貯藏品質(zhì),延長(zhǎng)貨架期。
現(xiàn)階段,熱激、冷激等保鮮技術(shù)因生產(chǎn)損耗大、品種差異大等原因,未能在實(shí)際生產(chǎn)中得到大規(guī)模推廣應(yīng)用,利用荔枝自身在脅迫條件下產(chǎn)生的應(yīng)激反應(yīng)來(lái)探究褐變的研究也較少。因此,本研究用糯米糍荔枝為研究材料,選擇3種不同脅迫處理荔枝,以荔枝果皮外觀、褐變指數(shù)、細(xì)胞微觀結(jié)構(gòu)、生理指標(biāo)、抗氧化指標(biāo)等為評(píng)價(jià)依據(jù),探討不同脅迫誘導(dǎo)荔枝抗逆性與褐變相關(guān)物質(zhì)的變化,研究荔枝在采后生理生化水平對(duì)失水、熱激和冷激脅迫的響應(yīng)與褐變的關(guān)系,旨在為開(kāi)發(fā)綠色環(huán)保、成本適中、適于大規(guī)模商業(yè)化生產(chǎn)的保鮮技術(shù)提供參考依據(jù)。
1.1""材料
選取荔枝品種糯米糍(Litchi"chinensis"Sonn."cv."Nuomici)果實(shí)為試驗(yàn)材料,采自廣東深圳商業(yè)果園。果實(shí)采收后用泡沫箱加冰袋密封,立即運(yùn)回實(shí)驗(yàn)室,去枝葉,保留果蒂,選取成熟度一致(八成熟),大小均一,無(wú)病、蟲(chóng)、傷、褐的果實(shí)作為試驗(yàn)材料。
1.2""方法
1.2.1""試驗(yàn)設(shè)計(jì)""將選取的材料分為3組分別進(jìn)行如下處理,每個(gè)處理約200個(gè)果實(shí):(1)輕度失水處理(DT),果實(shí)使用0.01"mm厚薄膜封裝后在薄膜表面打孔;(2)熱激脅迫處理(HT),果實(shí)60"℃熱水浸泡2"min,晾干,薄膜封裝;(3)冷激脅迫處理(CT),果實(shí)?20"℃冷凍1"h,晾干,薄膜封裝。本試驗(yàn)處理方法已經(jīng)進(jìn)行多次試驗(yàn)重復(fù)、篩選,各處理的方法基于前期試驗(yàn)效果,選取可以對(duì)果實(shí)形成脅迫,但不會(huì)形成徹底的破壞性的處理強(qiáng)度進(jìn)行后續(xù)試驗(yàn)。每個(gè)處理取20個(gè)小托盤(pán)作為重復(fù),每個(gè)托盤(pán)約10個(gè)果實(shí)。操作完成后,將果實(shí)轉(zhuǎn)移至(25±1)℃,相對(duì)濕度為70%~75%的觀察室進(jìn)行貯藏。
1.2.2""指標(biāo)測(cè)定""在0、1、3、5、6"d分別測(cè)定荔枝果實(shí)相關(guān)指標(biāo),同時(shí)對(duì)果皮取樣備用。取樣時(shí),用液氮將果皮充分浸泡,徹底凍結(jié)后立即用錫箔紙包裝,放入自封袋中于?80"℃冰箱中保存,用于后續(xù)試驗(yàn)。
(1)外觀品質(zhì)測(cè)定。每組處理選取大小一致、無(wú)機(jī)械傷的糯米糍,每天取樣拍照。
(2)果皮顏色測(cè)定。使用Minolta"CR-300全自動(dòng)色差儀對(duì)每個(gè)荔枝果皮取3個(gè)點(diǎn),測(cè)定L*值。
(3)果皮褐變指數(shù)測(cè)定。參照LIU等[13]的方法,隨機(jī)選取30個(gè)果實(shí)進(jìn)行分級(jí),評(píng)定褐變等級(jí)后計(jì)算褐變指數(shù)。
(4)果皮結(jié)構(gòu)透射電鏡觀察。參照李葉[14]、尚衛(wèi)娜等[15]的方法,略做修改,制作荔枝果皮透射電鏡樣品。樣品制作完成后利用UCT超薄切片機(jī)將樣品切成50~70"nm的薄片,用Tecnai"12分析型透射電子顯微鏡在100"kv下進(jìn)行觀察,拍攝。
(5)果皮細(xì)胞膜透性測(cè)定。參考LIN等[16]的方法,測(cè)定相對(duì)電導(dǎo)率。取10個(gè)荔枝果實(shí),用打孔器(直徑0.5"mm)制取30個(gè)圓片(3片/果),蒸餾水漂洗3次,從中隨機(jī)取出10個(gè)小圓片轉(zhuǎn)入含25"mL蒸餾水的試管中靜置30"min,然后用DDS-307精密電導(dǎo)率儀測(cè)定浸出液的電導(dǎo)率。將試管放入沸水中水浴15"min,流水冷卻至室溫,測(cè)定全透后浸出液的電導(dǎo)率。測(cè)定以雙蒸水為空白,重復(fù)3次。以前后電導(dǎo)率比值作為相對(duì)電導(dǎo)率來(lái)表示荔枝果皮的膜透性。計(jì)算公式:
A=×100%
式中,A表示相對(duì)電導(dǎo)率(%);M1表示浸泡30"min后浸出液測(cè)定值;M2表示煮沸15"min后冷卻的浸出液測(cè)定值。
(6)果實(shí)呼吸強(qiáng)度測(cè)定。取20個(gè)果實(shí)放入密封罐中,于貯藏溫度下密閉2"h后用一次性注射器取密封罐中氣體1"mL,用HITACHI"G-3900型氣相色譜儀測(cè)定。
氣相色譜工作條件為:色譜柱為Molecular"sieve"5"A和Poraoak"Q同心雙層柱,柱溫為50"℃,載氣為He氣,熱導(dǎo)檢測(cè)器(TCD)溫度為120"℃,進(jìn)樣口溫度為120"℃,載氣流速度為30"mL/min。計(jì)算公式:
B=
式中,B表示呼吸強(qiáng)度CO2[mg/(kg·h)];b表示氣相色譜檢測(cè)氣樣的CO2含量(%);V1表示密封罐體積(L);V2表示荔枝果實(shí)體積(L);M表示二氧化碳摩爾質(zhì)量(44"g/mol);W1表示密封罐中荔枝果實(shí)質(zhì)量(kg);H1表示密封時(shí)間(h);T表示果實(shí)貯藏溫度(℃)。
(7)荔枝果皮含水量測(cè)定。從3個(gè)荔枝果實(shí)上共取果皮1"g,用水分測(cè)定儀測(cè)定,測(cè)定完畢后,直接從儀器上讀取數(shù)值并記錄。每個(gè)測(cè)定重復(fù)3次。
(8)荔枝果皮花色素苷含量測(cè)定。(a)花色素苷的提?。喝?"g的荔枝果皮樣品,裝入小燒杯或量程為50"mL的大離心管中,用0.5%"HCl浸提液分6、5、4"mL"3次浸泡,每次浸泡時(shí)間均為約1"d,直至泡至樣品無(wú)色,最后將3次的收集液混合,定容至15"mL,過(guò)濾。(b)花色素苷含量的測(cè)定:取上面所得的浸提液1"mL"2份,分別用KCl-HCl的緩沖液(0.4"mol/L,pH"1.0)和檸檬酸/磷酸氫二鈉緩沖液(0.4"mol/L,pH"5.0)稀釋定容至5"mL,混勻后,用蒸餾水作對(duì)照,分別測(cè)定OD510。
D=
式中,D表示單位鮮重樣品花色素苷含量(mg/mL);△OD510=OD510(pH"1.0)-OD510(pH"5.0)。
(9)果皮PPO活性測(cè)定。取適量荔枝果皮混合凍樣,液氮冷凍后高速研磨成粉末,準(zhǔn)確稱(chēng)取1"g果皮凍粉于離心管中,加入已經(jīng)預(yù)冷的PBS緩沖液(0.05"mol/L、pH"7.0)5"mL,冰浴浸提20"min,期間上下顛倒數(shù)次,10"000"r/min,4"℃離心20"min,上清液即為粗酶液。參考TANG等[17]的方法,以每克果皮在398"nm下吸光值增加0.01為1個(gè)酶活性單位(U),酶活性以U/g表示。測(cè)定重復(fù)3次。
(10)果皮POD活性測(cè)定。粗酶液提取同PPO。參照J(rèn)AYACHANDRAN等[18]的方法,以每克果皮在470"nm下吸光值增加0.01為1個(gè)酶活性單位(U),酶活性以U/g表示。測(cè)定重復(fù)3次。
(11)果皮SOD活性測(cè)定。取適量荔枝果皮液氮冷凍后高速研磨成粉,準(zhǔn)確稱(chēng)取1"g粉末,加入預(yù)冷的5.0"mL"PBS緩沖液(0.05"mol/L、pH"7.8)和0.1%聚乙烯吡咯烷酮(PVP),快速震蕩混勻。1000"r/min,4"℃離心30"min,上清液用于試驗(yàn)的測(cè)定。測(cè)定方法依照南京建成的試劑盒說(shuō)明書(shū)進(jìn)行。
1.3""數(shù)據(jù)處理
使用SPSS"26.0軟件進(jìn)行數(shù)據(jù)統(tǒng)計(jì)分析,用Origin"2021軟件作圖,Adobe"Photoshop"CS"3軟件進(jìn)行圖片編輯。相關(guān)分析的統(tǒng)計(jì)量用樣本相關(guān)系數(shù)R表示,當(dāng)|R|≥R0.05臨界值時(shí),若Rgt;0,則認(rèn)為變量之間存在正相關(guān)關(guān)系;若Rlt;0,可認(rèn)為變量之間呈負(fù)相關(guān)關(guān)系。
2.1""不同脅迫處理對(duì)荔枝果皮色澤的影響
荔枝果皮色澤是判斷其商品價(jià)值的重要因素,是消費(fèi)者購(gòu)買(mǎi)商品的直接衡量標(biāo)準(zhǔn)[19],剛采收的荔枝果皮顏色鮮紅,但短時(shí)間內(nèi)果皮顏色即迅速由鮮紅色變成棕褐色,失去商品價(jià)值。
圖1顯示,0"d時(shí)果實(shí)表皮完整,顏色鮮紅飽滿(mǎn)。隨著貯藏時(shí)間延長(zhǎng),各處理果實(shí)果皮逐漸褐變,失去其鮮艷色澤,6"d時(shí)DT處理組果實(shí)已經(jīng)完全褐變,果皮干黃色,失去商品價(jià)值。HT及CT處理組果實(shí)貯藏末期也失去果實(shí)鮮紅色,出現(xiàn)了大面積褐變,但尚未達(dá)到全果褐變。
2.2""不同脅迫處理對(duì)荔枝果皮亮度和褐變指數(shù)的影響
果蔬褐變程度可以通過(guò)采后色澤變化體現(xiàn),由圖2A可知,3個(gè)脅迫處理組果皮的L*值隨著貯藏時(shí)間的延長(zhǎng)出現(xiàn)了明顯下降,即荔枝果皮的亮度變暗,這與果實(shí)褐變加重及紅色褪去有關(guān)。HT處理組荔枝果皮的L*值明顯低于其他處理,說(shuō)明熱激脅迫使荔枝果皮亮度明顯降低,而CT處理組的數(shù)值在貯藏后期較高,認(rèn)為冷激處理在貯藏后期維持果皮亮度效果較好,對(duì)維持果實(shí)感官品質(zhì)有一定的作用。
褐變直接影響荔枝果實(shí)的商品價(jià)值,褐變指數(shù)可以直觀評(píng)價(jià)果皮褐變程度。由圖2B可知,不同脅迫處理的荔枝果皮褐變指數(shù)在貯藏期間均呈上升趨勢(shì)。6"d時(shí),DT處理組果實(shí)褐變指數(shù)達(dá)到4.99,HT和CT處理組果實(shí)分別4.47和3.89,CT處理組褐變程度明顯較低,可見(jiàn)冷激脅迫處理延緩果皮褐變程度較好,這與本研究中對(duì)荔枝果皮色澤的觀察結(jié)果相印證。
2.3""不同脅迫處理對(duì)荔枝果皮超微結(jié)構(gòu)的影響
果皮組織結(jié)構(gòu)與果實(shí)采后生理、耐貯性和抗病性密切相關(guān)[20-21],本研究中,荔枝鮮果果皮細(xì)胞排列緊密,結(jié)構(gòu)完整,細(xì)胞間隙小,細(xì)胞膜緊貼在細(xì)胞壁上,有發(fā)達(dá)的大液泡和豐富的細(xì)胞器(圖3)。DT和CT處理組褐變果皮細(xì)胞排列疏松,細(xì)胞壁結(jié)構(gòu)逐漸分解,部分細(xì)胞解體融合,導(dǎo)致失水和透氣能力增加,果實(shí)衰老生理活動(dòng)加劇,內(nèi)容物基本消失,最終走向腐爛。HT處理組褐變果皮細(xì)胞結(jié)構(gòu)基本完整,排列較整齊,細(xì)胞內(nèi)含有大量聚結(jié)成塊的內(nèi)容物。
2.4""不同脅迫處理對(duì)荔枝貯藏相關(guān)指標(biāo)的影響
相對(duì)電導(dǎo)率是衡量脅迫處理下果皮膜透性的重要指標(biāo),通常相對(duì)電導(dǎo)率越大,細(xì)胞衰老和破壞程度越重。由圖4A可知,相對(duì)電導(dǎo)率隨貯藏時(shí)間的延長(zhǎng)逐漸增大,說(shuō)明隨著貯藏時(shí)間的延長(zhǎng),荔枝果皮細(xì)胞膜受損加劇。6"d時(shí),DT處理組果皮的相對(duì)電導(dǎo)率為初始值的3.73倍,表明果皮完全褐變后果皮膜透性大幅增加,細(xì)胞失去活性。HT和CT處理組果實(shí)處理完成后,相對(duì)電導(dǎo)率分別為18.79%和27.23%,是由于HT和CT過(guò)程對(duì)組織造成了一定的損傷,但果實(shí)仍可進(jìn)行一定的修復(fù),常溫1"d后其值有明顯下降,隨后隨著果實(shí)的衰老,相對(duì)電導(dǎo)率開(kāi)始持續(xù)上升,但后期貯藏期間二者持續(xù)低于DT組。
呼吸作用是果蔬采后仍具有生命力的重要表現(xiàn),呼吸強(qiáng)度越高,果蔬受損程度越高[22]。呼吸強(qiáng)度通常是通過(guò)CO2呼出量或O2吸入量進(jìn)行衡量,如圖4B所示,貯藏初期荔枝果實(shí)呼吸強(qiáng)度迅速下降,隨著貯藏時(shí)間延長(zhǎng),3個(gè)脅迫處理的荔枝果實(shí)呼吸強(qiáng)度繼續(xù)緩慢下降。HT和CT處理果實(shí)的呼吸強(qiáng)度稍高,認(rèn)為冷激和熱激脅迫處理對(duì)果實(shí)造成了一定的損傷,因此荔枝果實(shí)通過(guò)加速呼吸應(yīng)對(duì),從而減緩品質(zhì)劣變。
荔枝果皮失水與褐變密切相關(guān),果皮含水量的變化對(duì)褐變的進(jìn)程有一定影響,伴隨著含水量的下降,荔枝果皮迅速褐變。圖4C表明,DT處理組果實(shí)含水量迅速下降,低于其他處理組,果實(shí)含水量由最初的72.72%迅速降低為27.76%。HT和CT處理組果實(shí)的含水量貯藏期間明顯較高,表明熱激和冷激脅迫處理均可以有效抑制水分散失,從而在一定程度上延緩果皮褐變。
花色素苷是荔枝褐變過(guò)程中的重要物質(zhì),在貯藏過(guò)程中容易降解,因此果皮呈色發(fā)生變化。在PPO和POD作用下,花色素苷進(jìn)一步降解,加速褐變[23]。圖4D表明,在整個(gè)貯藏期間,3組脅迫處理的荔枝在貯藏過(guò)程中花色素苷含量總體呈現(xiàn)逐漸下降趨勢(shì),說(shuō)明隨著花色素苷逐漸被降解,褐變加劇。其中,HT處理組果實(shí)果皮的花色素苷含量低于其他2個(gè)處理組,可能與花色素苷耐熱性差,熱激脅迫處理使花色素苷迅速降解有關(guān)。
2.5""不同脅迫處理對(duì)荔枝果皮PPO、POD和SOD活性的影響
多種現(xiàn)象表明PPO與植物抵御外界脅迫相關(guān),但其中關(guān)聯(lián)機(jī)制并未解開(kāi),通常PPO可以在有氧條件下引起酚類(lèi)物質(zhì)的酶促褐變,活性越高則褐變?cè)娇靃24]。由圖5A可知,貯藏期間3種脅迫處理果實(shí)的PPO活性均呈現(xiàn)先上升后下降的趨勢(shì),在1"d時(shí)達(dá)到峰值,隨后下降,伴隨著荔枝果實(shí)褐變逐漸加重。HT和CT處理組果實(shí)在處理結(jié)束后PPO活性大幅提高,在1"d時(shí)達(dá)到峰值后即迅速下降,推測(cè)荔枝果實(shí)在經(jīng)過(guò)熱激和冷激脅迫后,細(xì)胞受到一定的損傷,通過(guò)提高PPO的活性緩解初期損傷。通常認(rèn)為70~90"℃能破壞PPO催化活性,但所需時(shí)間取決于原料[25],本研究中的HT處理溫度較低,時(shí)間短,不足以滅活PPO活性,因此荔枝果實(shí)依然繼續(xù)褐變。
POD是可以引起酚類(lèi)物質(zhì)酶促褐變的重要物質(zhì)之一[26],圖5B顯示,3個(gè)脅迫處理的荔枝果皮中POD活性呈先升后降趨勢(shì),DT處理組果實(shí)在1"d時(shí)即達(dá)到峰值884.00"U/g。HT和CT處理組果實(shí)的POD活性峰值推遲至3"d時(shí),且峰值低于DT處理組果實(shí),認(rèn)為熱激和冷激脅迫都能抑制POD活性,從而延緩褐變。TH和CT處理組果實(shí)處理完成后,POD活性立刻降低,認(rèn)為POD對(duì)溫度敏感,高溫和低溫都抑制其活性。
3個(gè)處理組果皮SOD活性整體呈先降后升趨勢(shì)(圖5C),PPO和POD呈現(xiàn)先上升后下降的趨勢(shì),認(rèn)為SOD活性是在抗氧化作用的后期才逐漸占據(jù)主導(dǎo)。熱激和冷激脅迫處理完成后,二者的SOD活性立刻上升,表明荔枝果實(shí)可以通過(guò)提高SOD活性緩解初期熱激和冷激對(duì)果實(shí)的損傷。
2.6""不同脅迫處理荔枝果皮褐變相關(guān)指標(biāo)之間的相關(guān)性分析
荔枝褐變指數(shù)與相關(guān)生理生化指標(biāo)之間的相關(guān)性分析如圖6所示,3個(gè)脅迫處理的荔枝果皮褐變指數(shù)均與相對(duì)電導(dǎo)率呈顯著正相關(guān);與果皮L*值、含水量和花色素苷含量呈顯著負(fù)相關(guān),表明在荔枝經(jīng)失水、熱激和冷激脅迫逆境下,這些
顯著相關(guān)指標(biāo)均可對(duì)褐變產(chǎn)生直接影響。
3.1""不同脅迫處理對(duì)荔枝外觀性狀的影響
荔枝果皮色澤、亮度、褐變指數(shù)等果實(shí)外在品質(zhì)形狀可以直接影響果實(shí)商品價(jià)值,本研究中3個(gè)脅迫處理均導(dǎo)致荔枝果皮迅速褐變,但HT和CT處理組果實(shí)的褐變速度明顯低于DT處理組,表明熱激和冷激脅迫處理對(duì)抑制果皮褐變有積極作用,馬鈺晴[27]的研究也證實(shí)冷激處理可以減輕桃果實(shí)褐變,保持良好的果實(shí)品質(zhì)。其中,熱激脅迫導(dǎo)致荔枝果皮亮度降低,這與其在卷心菜[28]、大白菜[29]、花椒芽[30]上的表現(xiàn)一致,熱激處理雖然對(duì)褐變有作用,但對(duì)果實(shí)外觀有一定的影響,這可能與熱激溫度有關(guān),后續(xù)可以通過(guò)設(shè)置多組熱激溫度比較來(lái)探究。
3.2""不同脅迫處理對(duì)荔枝果皮細(xì)胞結(jié)構(gòu)的影響
已有研究表明,熱激處理可以鈍化細(xì)胞壁降解酶,降低細(xì)胞壁降解速率,從而保持細(xì)胞結(jié)構(gòu)完整,維持貯藏期間萵筍[31]、百合鱗莖片[32]的硬度,黃瓜[33]和蜜柚[34]在熱激處理后,細(xì)胞壁降解也出現(xiàn)顯著減緩,這與本研究中HT處理組褐變果皮細(xì)胞結(jié)構(gòu)基本完整,排列較整齊結(jié)果一致。細(xì)胞膜完整性可以通過(guò)相對(duì)電導(dǎo)率衡量,本研究中熱激和冷激處理均明顯抑制了荔枝果皮相對(duì)電導(dǎo)率的增加,有助于維持細(xì)胞膜完整性,這與熱激和冷激在黃瓜[35]、桃[27]、火龍果[10]中的研究結(jié)果相符。
3.3""不同脅迫處理對(duì)荔枝果皮生理生化指標(biāo)的影響
荔枝果皮褐變與果實(shí)呼吸速率、果皮含水量及花色素苷含量密切相關(guān)。通常認(rèn)為較低呼吸速率能延緩營(yíng)養(yǎng)成分的損耗,有利于果實(shí)的長(zhǎng)期保存[36],但本研究結(jié)果與其有差異。CT處理組果實(shí)呼吸強(qiáng)度在3"d和6"d時(shí)呼吸強(qiáng)度分別為21.29"mg/(kg·h)和18.25"mg/(kg·h),高于DT處理組果實(shí)1.75"mg/(kg·h)和4.8"mg/(kg·h),但褐變程度CT處理組低于DT處理組,因此認(rèn)為輕微的刺激荔枝呼吸反而延緩了果實(shí)衰老,這與高兆銀等[37]的研究結(jié)果一致,認(rèn)為貯藏后期適當(dāng)提高荔枝果實(shí)呼吸強(qiáng)度,使EC(energy"charge)保持在較高水平,可能對(duì)抑制果皮衰老更有利。如何有效刺激并控制荔枝呼吸速率在一定范圍內(nèi)達(dá)到延緩褐變和衰老的目的,需要在后續(xù)實(shí)驗(yàn)中繼續(xù)展開(kāi)研究,為今后的保鮮工作提供更科學(xué)的基礎(chǔ)支持。
隨著貯藏期延長(zhǎng),荔枝果皮水分逐漸減少,花色素苷逐漸被分解,褐變加劇。馬玉芯等[38]指出,熱燙可以有效抑制鮮食玉米水分散失,本研究熱激和冷激脅迫都能較好保持果皮水分,通過(guò)對(duì)果皮水分的保持,進(jìn)而延緩褐變。FANG等[39]研究證實(shí)經(jīng)果梗給荔枝果實(shí)單果供水可以提高果皮含水量,明顯抑制果皮褐變,與本研究結(jié)果相符。TANG等[17]認(rèn)為荔枝采后隨著花色素苷的不斷降解,抗氧化性逐漸減弱,褐變加速,本研究中,HT處理組果實(shí)果皮的花色素苷含量低于其他2個(gè)處理組,推測(cè)可能是花色素苷耐熱性差,熱激脅迫處理溫度使花色素苷迅速降解,李奕星等[40]的研究也指出紅毛丹花色素苷在不同貯藏溫度下,隨著溫度升高,保存率大幅下降,與本結(jié)果一致。
PPO、POD和SOD是植物抗氧化系統(tǒng)中的重要酶類(lèi),通過(guò)協(xié)同作用來(lái)保護(hù)細(xì)胞膜系統(tǒng),減少自由基的毒害,降低生物或非生物脅迫引起的氧化損傷。本研究中熱激和冷激處理完成后,荔枝果皮的PPO和SOD活性大幅上升,隨后迅速下降。NIAN等[12]研究顯示冷激處理可以提高SOD活性,邵登魁等[41]、潘翠萍等[42]的研究也證實(shí)一定程度的低溫處理可以誘導(dǎo)抗氧化物酶活性的提高,從而緩解低溫脅迫初期的傷害。本研究中,熱激和冷激處理整體上抑制了PPO和POD活性,增強(qiáng)了貯藏后期SOD活性和抗氧化防御系統(tǒng),從而延緩衰老和褐變。
3.4""不同脅迫處理荔枝果皮褐變相關(guān)指標(biāo)之間的相關(guān)性分析
已有研究指出,荔枝褐變相關(guān)指標(biāo)可以分為褐變指標(biāo)和抗褐變指標(biāo),因此將本研究中與果皮褐變呈顯著正相關(guān)的果皮L*、含水量和花色素苷含量3個(gè)指標(biāo)歸為褐變指標(biāo),與果皮褐變呈顯著負(fù)相關(guān)的果皮相對(duì)電導(dǎo)率歸為抗褐變指標(biāo),后續(xù)研究中可以通過(guò)抑制褐變指標(biāo)升高,提高抗褐變指標(biāo)含量方向進(jìn)行驗(yàn)證實(shí)驗(yàn),探究有效的荔枝防褐方法。
本文研究了3種不同脅迫誘導(dǎo)糯米糍荔枝果實(shí)抗逆性與褐變相關(guān)物質(zhì)的變化規(guī)律,結(jié)果表明:3種脅迫逆境下,荔枝在常溫貯藏時(shí)均出現(xiàn)亮度下降,果皮褐變加劇現(xiàn)象,其中DT處理組果實(shí)褐變最迅速,HT和CT處理組果實(shí)通過(guò)提高荔枝果皮含水量和果實(shí)呼吸強(qiáng)度,抑制果皮相對(duì)電導(dǎo)率升高,抑制PPO和POD活性,提高貯藏后期SOD活性,延緩了果皮褐變和果實(shí)衰老。與新鮮荔枝果皮細(xì)胞結(jié)構(gòu)完整,排列整齊不同,HT處理能保持褐變果皮細(xì)胞結(jié)構(gòu)完整,內(nèi)容物大量沉積,其他處理細(xì)胞結(jié)構(gòu)分解,內(nèi)容物基本消失。此外,HT和CT處理組果實(shí)還可以通過(guò)激活抗氧化系統(tǒng)來(lái)應(yīng)對(duì)處理初期果實(shí)造成損傷,從而延緩果實(shí)衰老。3種逆境脅迫導(dǎo)致的荔枝褐變均與果皮L*、含水量和花色素苷含量呈顯著正相關(guān),與果皮相對(duì)電導(dǎo)率呈顯著負(fù)相關(guān)。
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