Effects of MeJA Treatment on Skin Browning and Antioxidant Capacity of Postharvest ‘Huangguan’ Pear During Cold Storage

Wang Qingguo, Wang Xuan, Xu Xinxin, Tao Ning, Liu Pei

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Journal of Agriculture ›› 2020, Vol. 10 ›› Issue (2) : 85-91. DOI: 10.11923/j.issn.2095-4050.cjas18100007

Effects of MeJA Treatment on Skin Browning and Antioxidant Capacity of Postharvest ‘Huangguan’ Pear During Cold Storage

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Abstract

To study the inhibiting mechanism of MeJA on skin browning, treatment of 1, 10 and 100 μmol/L MeJA dip were applied to post-harvested ‘Huangguan’ pear fruits and the effects on skin browning spot index, polyphenol content, PPO and PAL activity, antioxidant defense systems including POD, CAT, APX activity, DPPH clearance ratio and the H2O2 content were detected. The results showed that compared with control (without MeJA treatment), treatment of 100 μmol/L MeJA reduced the skin browning spot index markedly, decreased PPO, PAL activity and polyphenol content, increased POD, CAT, APX activity and DPPH clearance ratio, decreased the H2O2 content. Above all, treatment of 100 μmol/L MeJA could reduce skin browning of ‘Huangguan’ pear during the long-term cold storage through inducing antioxidant defense of pear skin, removing H2O2 and inhibiting oxidation of phenols.

Key words

MeJA / ‘Huangguan’ Pear / Skin Browning / Antioxidant Capacity

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Wang Qingguo , Wang Xuan , Xu Xinxin , Tao Ning , Liu Pei. Effects of MeJA Treatment on Skin Browning and Antioxidant Capacity of Postharvest ‘Huangguan’ Pear During Cold Storage. Journal of Agriculture. 2020, 10(2): 85-91 https://doi.org/10.11923/j.issn.2095-4050.cjas18100007

0 引言

‘黄冠’梨果实皮薄、果肉洁白、质地细腻、石细胞少、汁液多、酸甜可口,且结果早、适应性强,目前在中国华北、西北及长江中下游地区广泛种植,具有较大的经济效益[1,2]。‘黄冠’梨果实常温下极易软化,采后通常需要低温贮藏,由于低温下冷害的影响,‘黄冠’梨果皮容易产生果面褐斑,大大降低商品价值[3]。低温胁迫往往会诱导果实内活性氧大量积累,而过多的活性氧物质可造成膜脂过氧化,破坏膜系统;低温贮藏期间‘黄冠’梨果皮褐变的发生往往与细胞膜损伤及酚类物质代谢密切相关,因此提高低温贮藏条件下果实细胞膜稳定性、氧化防护体系等可控制果皮褐变的发生[4,5,6]
茉莉酸甲酯(methyl jasmonate,MeJA)是茉莉酸的类似物,广泛存在于高等植物体内,是一种天然的植物生长调节因子,在植物生长发育、抗逆反应等方面发挥着重要的调控作用[7]。MeJA可以调控植物的抗冷反应,外源喷施MeJA能够显著提高水稻、小麦等植株冷胁迫下的抗性[8,9];作为抗氧化增效剂,MeJA可通过调控酚类成分的合成与代谢有效提高植物生物活性、延长果蔬保质期和提高植物抗冷能力[10]。近年来在采后果蔬保鲜上的研究表明,采后MeJA处理能够有效的诱导增强果实的抗冷性,减轻枣、草莓、桃、番茄、香蕉等采后冷害的发生,并且能够促进营养成分的有效积累,在延长农产品的贮藏期和提高其营养价值等方面起到积极的作用[11,12,13,14,15]。此外,MeJA可以通过诱导果蔬调节活性氧代谢,提高自身抗氧化能力,维护细胞壁代谢平衡,调节脯氨酸和γ-氨基丁酸等物质合成,从而提高果实抗冷性[16,17,18,19,20,21]
前期研究表明,‘黄冠’梨经MeJA处理后即使快速降温,仍能有效抑制采后果皮褐变并保持良好的外观、口味及较高的货架期Vc含量,但对硬度和可溶性固形物含量没有显著影响[22]。在此基础上,笔者从酚类物质代谢、活性氧积累以及抗氧化相关酶活等方面进一步探讨采后MeJA处理对‘黄冠’梨果皮褐变的控制机理,为解决低温贮藏过程中‘黄冠’果皮易褐变提供理论基础。

1 材料与方法

1.1 实验材料

‘黄冠’梨采摘自河北省藁城马庄果园,采摘后选取大小一致、无机械伤、病虫害、表面颜色均匀的果实,套网套,装箱,常温运回山东农业大学食品科学与工程学院果蔬保鲜实验室。

1.2 仪器与设备

T6新世纪紫外-可见分光光度计(北京普析通用仪器有限责任公司);电热恒温水浴锅(国华仪器有限公司);Allegra 64R高速冷冻离心机(美国Beckman公司);-80℃超低温冰箱(中科美菱低温科技有限公司);微型冷库(济南科达尔实业有限公司);超声波振荡器(上海生析超声仪器有限公司)。

1.3 实验方法

1.3.1 样品处理 分别配制1、10、100 µmol/L的3组MeJA溶液,将梨果浸入溶液(水温20℃)中,处理 3 min,水温20℃清水浸泡3 min为对照,捞出后吸干表面溶液,进行商品化包装后装箱,置于-1~0℃冷库低温贮藏。对照组及3个处理组每组分别处理3箱梨,每箱48个梨,低温贮藏20、40、100天,统计果皮褐变率及褐变指数。对照组及100 µmol/L的MeJA溶液处理组每组处理3箱梨,每箱48个梨,每箱每次取6个梨,低温贮藏30天内,分别于0、5、10、15、20、25、30天进行果皮多酚氧化酶(polyphenol oxidase,PPO)、苯丙氨酸解氨酶(phenylalanine aminolase,PAL)、过氧化物酶(peroxidase, POD)、过氧化氢酶(catalase,CAT)、抗坏血酸过氧化物酶(ascorbate peroxidase,APX)的活力及过氧化氢(hydrogen peroxide,H2O2)含量、总酚含量及1,1-二苯基-2-三硝基苯肼(1-diphenyl-2-picrylhy drazyl,DPPH)自由基清除率等指标测定。
1.3.2 果皮褐变率及褐变指数测定 依据梨果表面褐变面积划分为5个等级。0级,果皮无褐变;1级,出现褐变且褐变面积比例小于15%;2级,褐变面积比例15%~30%;3级,褐变面积比例30%~50%;4级,褐变面积比例大于50%。果皮褐变率及褐变指数通过公式(1)~(2)进行计算[5]
果皮褐变率=果皮褐变果数总果数×100%
(1)
果皮褐变指数=(褐变级别×该级别果数)总果数×最高褐变级数×100%
(2)
1.3.3 总酚含量测定 样品总酚提取、总酚标准曲线制作及样品总酚含量测定参照Folin-酚法测定[23],单位为mg/100 g,以鲜质量计。
1.3.4 PPO和PAL酶活力测定 PPO酶活力测定参考Jiang Yueming等[24]的方法,以1 g‘黄冠’梨果皮(鲜质量)1 min内410 nm波处下吸光度的变化为1个酶活力单位(U)。PAL酶活力的测定参考张福生等[14]的方法,以1 g‘黄冠’梨果皮(鲜质量)1 min内290 nm波长处吸光度的变化为1个酶活力单位(U)。
1.3.5 抗氧化酶活性测定 称取4 g黄冠梨果皮的冷冻样品加入8 mL预冷的50 mmol/L磷酸缓冲液(含5%聚乙烯吡咯烷酮和3 mmol/L乙二胺四乙酸二钠,pH 7.0),4℃条件下充分研磨后,于12000 r/min离心 10 min,上清液用于抗氧化酶活性测定。
POD活性测定采用愈创木酚比色法[25],APX活性测定采用紫外吸光光度法[26],CAT活性测定采用比色法测定[25]
1.3.6 DPPH自由基清除率测定 DPPH自由基清除率的测定参照韦献雅等[27]的方法,结果以清除百分率表示。
1.3.7 H2O2含量测定 H2O2的测定参照刘俊等[28]的方法,单位为nmol/g,以鲜质量计。

1.4 实验设计及数据统计分析

平均数以及标准方差通过Excel 2010进行数据整理分析,以平均数±标准偏差表示。数据经过Sigmaplot 12.5进行主成分分析,SPSS 17.0进行显著性分析,显著水平为P<0.05。

2 结果与分析

2.1 不同浓度MeJA处理对贮藏期间‘黄冠’梨果皮褐变的影响

‘黄冠’梨在低温贮藏过程中,随着贮藏时间延长,果皮褐变逐渐加重,如图1所示,对照组贮藏至40天果皮褐变率已达69%,褐变指数达31%;之后增幅不大。与对照组相比,1、10、100 µmol/L的MeJA处理的‘黄冠’梨果皮褐变率及褐变指数均有所下降,其中 100 μmol/L MeJA处理对果皮褐变的抑制效果最显著(P<0.01)。
图1 不同浓度MeJA处理对贮藏期间‘黄冠’梨果皮褐变率及褐变指数的影响

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2.2 100 μmol/L MeJA处理对‘黄冠’梨果皮总酚含量及PPO、PAL酶活的影响

‘黄冠’梨在低温贮藏过程中,果皮PPO酶活通常会增强,导致酚类物质氧化大幅度降低,PPO和酚类物质的代谢是导致‘黄冠’梨果皮褐变的直接原因[29]。PAL是苯丙氨酸途径第一步限速酶,它可以特异性催化苯丙氨酸合成反式肉桂酸,PAL活性的增加可以促进黄酮类及多酚类成分含量的增加[10,14]。而MeJA可有效透过植物细胞膜发挥信号分子作用调节众多次生代谢产物包括酚类物质等合成[30,31]
由2.1节可知,100 μmol/L MeJA处理能极显著降低‘黄冠’梨低温贮藏期间的果皮褐变(P<0.01)。因此,分别测定对照组和100 μmol/L MeJA处理组的‘黄冠’梨果皮中PPO、PAL酶活力和总酚含量,结果见图2~3。
图2 100 μmol/L MeJA处理对贮藏期间‘黄冠’梨果皮PPO酶活力及总酚含量的影响

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图3 100 μmol/L MeJA处理对贮藏期间‘黄冠’梨果皮PAL酶活力的影响

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图2A所示,对照组‘黄冠’梨低温贮藏30天期间,PPO酶活力一直持续增加;与对照相比,贮藏5天后,100 μmol/L MeJA处理后的‘黄冠’梨果皮PPO酶活力逐渐降低,贮藏20天时略有增加,但整个贮藏期间PPO酶活均一直显著低于对照组(P<0.01)。
图2B所示,对照组‘黄冠’梨在贮藏30天内,前期(10天内)果皮总酚大幅度下降,随后略有波动,贮藏20天后大幅度上升;而经100 μmol/L MeJA处理后,‘黄冠’梨在贮藏20天内,果皮总酚含量下降明显减缓,贮藏20天后含量上升,贮藏期间含量一直显著高于对照组(P<0.01)。
图3可知,对照组‘黄冠’梨低温贮藏5天内PAL酶活大幅度降低,5~15天持续增加,随后降低但贮藏20天后又逐渐增加;与对照相比,贮藏5天后, 100 μmol/L MeJA处理后的‘黄冠’梨果皮PAL酶活性也出现增加、降低又增加的趋势,但PAL活性均一直显著低于对照组(P<0.01)。
图2~3结果表明,采后100 μmol/L MeJA处理降低了‘黄冠’梨贮藏期间果皮PAL酶的活性,抑制了PPO酶活性的上升,减缓了果皮总酚的消耗。

2.3 100 μmol/L MeJA处理对贮藏期间‘黄冠’梨果皮抗氧化酶活性的影响

图4A所示,贮藏期间,对照组‘黄冠’梨果皮POD酶活力总体呈先下降后上升的趋势。而 100 μmol/L MeJA处理显著增加了POD酶活力,从而在贮藏30天期间‘黄冠’梨果皮POD酶活力一直显著高于对照组(P<0.01)。如图4B所示,对照组‘黄冠’梨低温贮藏10天内CAT酶活力大幅度降低,10~20天内酶活力有所上升但随后又大幅度降低;而100 μmol/L MeJA处理显著抑制了‘黄冠’梨低温贮藏期间CAT酶活力的降低,从而使处理组CAT保持在较高水平,并一直显著高于对照组(P<0.05)。如图4C所示,对照组果皮APX酶活力在贮藏25天内缓慢降低,后期有所上升,但100 μmol/L MeJA处理显著增加了果皮APX酶活力,整个贮藏期间显著高于对照组(P<0.01)。
图4 100 μmol/L MeJA处理对贮藏期间‘黄冠’梨果皮抗氧化酶活的影响

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2.4 100 μmol/L MeJA处理对贮藏期间‘黄冠’梨果皮H2O2含量和DPPH自由基清除率的影响

图5A可知,对照组‘黄冠’梨贮藏5天内,H2O2含量快速上升,可见‘黄冠’梨低温下很快发生脂质过氧化反应;随着果实对低温逐渐适应,H2O2含量开始下降,随着贮藏时间延长,贮藏15天后又开始增加。100 μmol/L MeJA处理后,‘黄冠’梨果皮H2O2含量快速下降,贮藏10天后虽有所上升,但整个贮藏期间显著低于对照组。由结果可知,100 μmol/L MeJA处理显著降低了‘黄冠’梨贮藏期间果皮H2O2的积累,从而有效降低了H2O2对细胞膜的氧化伤害。由图5B可知,‘黄冠’梨果皮DPPH自由基清除率均呈下降趋势,100 μmol/L MeJA处理显著抑制了DPPH自由基清除率的下降,有利于维持细胞抗氧化能力。
图5 100 μmol/L MeJA处理对贮藏期间‘黄冠’梨果皮H2O2含量及DPPH清除率的影响

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‘黄冠’梨采后入冷库低温贮藏,贮藏过程中极易遭受冷害,而细胞膜低温下损伤是产生冷害的根本原因[5]。POD、CAT和APX作为活性氧的主要清除酶,在抑制膜脂过氧化、维持膜系统的稳定性中起重要作用[13]。由图4~5可知,100 μmol/L MeJA处理采后‘黄冠’梨果实,通过显著增加POD、APX酶活力、有效抑制CAT酶活力和DPPH自由基清除率的降低,显著降低了H2O2的积累,有利于维持低温贮藏期间果皮中的活性氧代谢的平衡,保护细胞膜的完整性。

3 结论

(1)1、10、100 μmol/L MeJA处理对‘黄冠’梨低温贮藏期间果皮褐变率及褐变指数的影响程度不同,其中100 μmol/L MeJA处理效果显著(P<0.01)。且 100 μmol/L MeJA处理显著降低了PAL、PPO酶活力,有效地减缓了果皮总酚的消耗,从而减少了果皮褐变的发生。
(2)MeJA处理诱导了采后‘黄冠’梨果皮的防御反应,增加了低温贮藏过程中抗氧化酶包括POD、APX、CAT酶活力,有效地抑制了DPPH自由基清除率的降低,保持了较高的抗氧化能力,从而增强了细胞清除自由基以及低温胁迫下的修复与抵抗能力,果皮H2O2含量的降低表明MeJA处理可以降低低温下冷害引发的自由基对细胞产生的伤害。
(3)MeJA处理可以通过影响活性氧代谢和酚类物质代谢来降低采后‘黄冠’梨低温贮藏期间果皮褐变的发生,并能够达到减少‘黄冠’梨鸡爪病、延长贮藏期和保持新鲜品质的目的。

4 讨论

4.1 外源MeJA处理能诱导‘黄冠’梨果皮提高自身抗氧化胁迫能力,增强抗冷性

低温贮藏过程中‘黄冠’梨果皮褐变的产生,与低温胁迫下ROS的增加引起的细胞膜损伤,进而导致PPO酶促反应增强、果皮酚类物质加剧氧化等密切相关;而通过提高低温贮藏条件下果实自身抗氧化能力的方法,往往可以减轻梨果冷害、降低果皮褐变的发生[4,5,6]。MeJA作为植物体内天然存在的生长调节剂,不仅参与调控果实发育、成熟和衰老,参与多种植物病虫害、机械伤害等抗逆反应,而且能够通过诱导植物抗氧化防护酶基因转录水平的表达,诱导果蔬调节脯氨酸和γ-氨基丁酸等物质合成调节自身抗氧化系统等,来增强植物在低温胁迫下的抗氧化防护能力,提高植物的抗冷性。MeJA处理后的‘黄冠’梨,在低温贮藏过程中,果皮POD、CAT、APX等抗氧化防护酶酶活力显著高于对照组,MeJA处理显著增强了‘黄冠’梨果皮自身的抗氧化防护能力,从而增强了其低温贮藏过程中的抗冷能力。

4.2 外源MeJA处理对‘黄冠’梨果皮褐变抑制的机制探讨

外源MeJA作为一种信号因子,可通过作用于植物受体细胞,激活并诱导胁迫应答基因的表达等,参与植物防御信号的转导过程,提高植物的生物及非生物胁迫的抗性。研究表明,外源MeJA可诱导菠菜游离钙离子浓度升高并激活抗冷转录因子CBF的表达,提高其抗冷害能力[32]。外源MeJA能够诱导小麦冷特异家族WCS120、WCS19基因的表达,提高了超氧化物歧化酶(superoxide dismutase, SOD)和过氧化物酶(peroxidase, PO)的活性,增强了小麦抗冷性[33]。外源MeJA处理显著降低了番茄果实MeJA信号转导途径中的关键转录因子SlMYC2的表达,增加了脯氨酸、番茄红素含量,显著提高了SOD、PO、CAT、APX等抗氧化酶的活性,抑制了电导率和MDA含量并显著增强了番茄的抗冷性;这与笔者通过外源MeJA处理‘黄冠’梨提高抗氧化活性,降低果皮H2O2含量进而降低果皮氧自由基伤害的结果相一致;该研究中还探讨了利用病毒诱导的基因沉默(VIGS)方法获得SlMYC2沉默型的番茄果实,而外源MeJA对SlMYC2沉默型的番茄果实处理后并不能提高其抗性,也进一步说明了外源MeJA作为信号分子,是通过影响受体细胞中转录因子以及相关基因的表达,来提高果实抗冷性的[34]。后期笔者可通过外源MeJA处理对梨果转录组进行测序分析,寻找差异表达基因,分析并研究MeJA对抗氧化酶基因表达、冷胁迫相关蛋白基因表达、细胞膜稳定相关蛋白基因表达的影响来进一步探讨MeJA调控梨果抗冷、果皮褐变的分子机制。

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MYC2, a basic helix-loop-helix transcription factor, is a master regulator in Jasmonic acid (JA) signaling pathway. However, the functions of SlMYC2 in methyl jasmonate (MeJA)-mediated fruit chilling tolerance are far from being clearly understood. Thus, in the present work, we constructed SlMYC2-silenced tomato fruit by virus-induced gene silencing (VIGS) and investigated the function of SlMYC2 in MeJA-induced tomato fruit chilling tolerance. The results showed that MeJA treatment markedly induced the SlMYC2 expression; increased proline content, lycopene content, and antioxidant enzyme activities, including superoxide dismutase, peroxidase, catalase, and ascorbate peroxidase; inhibited the increase of electrical conductivity and malondialdehyde content; and effectively reduced the chilling injury (CI) incidence and CI index. However, these effects of MeJA treatment were partially counteracted in SlMYC2-silenced tomato fruit, and the CI incidence and CI index in ( SlMYC2-silenced + MeJA)-treated fruit were higher than those in MeJA-treated fruit. Our results indicated that SlMYC2 might be involved in MeJA-induced chilling tolerance, possibly by ameliorating the antioxidant enzyme system of fruit and increasing proline and lycopene levels.

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