Salicylic acid alleviates postharvest chilling injury of sponge gourd (Luffa cylindrica)

Journal of Integrative Agriculture 2017, 16(3): 735–741 Available online at www.sciencedirect.com

ScienceDirect

RESEARCH ARTICLE

Salicylic acid alleviates postharvest chilling injury of sponge gourd (Luffa cylindrica) HAN Cong1, 2, 3*, ZUO Jin-hua1*, WANG Qing1, DONG Hai-zhou2, GAO Li-pu1 1

National Engineering Research Center for Vegetables, Beijing Academy of Agriculture and Forestry Sciences, Beijing 100097, P.R.China 2 College of Food Science and Engineering, Shandong Agricultural University, Tai’an 271018, P.R.China 3 College of Food Science and Technology, Nanjing Agricultural University, Nanjing 210095, P.R.China

Abstract Effect of salicylic acid (SA) on chilling injury (CI) of sponge gourd during storage (9 days, 9°C) plus shelf life (2 days, 20°C) was evaluated in this study. SA treatment at the concentration of 1.5 mmol L–1 significantly reduced postharvest CI of sponge gourds. Besides, the application of SA could effectively decrease the electrolyte leakage, reduce the accumulation of malondialdehyde (MDA) and total phenolics, enhance the activities of antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT) and ascorbate peroxidase (APX), and inhibit the activities of phenylalanine ammonia-lyase (PAL) and polyphenol oxidase (PPO). The beneficial effects of SA could be attributed to preserved membrane integrity, inhibited membrane peroxidation, enhanced antioxidant system and suppressed activities of browning related enzymes. In a sense, SA as a postharvest tool may be commercially used in alleviating CI of sponge gourd. Keywords: sponge gourd, salicylic acid, chilling injury, quality, antioxidant enzyme

1. Introduction Sponge gourds, the fruit of Luffa cylindrical, are widely known and/or used for their fibers (Oboh and Aluyor 2009). The young immature fruits, harvested before internal fibers have started to harden, are edible and consumed much like summer squash (Porterfield 1955; Zong et al. 1993). As a

Received 11 January, 2016 Accepted 2 April, 2016 HAN Cong, E-mail: [email protected]; ZUO Jin-hua, E-mail: [email protected]; Correspondence GAO Li-pu, Tel: +86-10-51503051, E-mail: [email protected] * These authors contributed equally to this study. © 2017, CAAS. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/) doi: 10.1016/S2095-3119(16)61390-4

summer season vegetable, sponge gourd is a good source of nutrients and contains multiple beneficial elements especially saponin and mucus (Porterfield 1955). However, the fruit is highly perishable and its shelf-life is no more than one week at ambient temperature because of wilting and yellowing in appearance and lignification and fibrosis in texture. Low temperature storage has been demonstrated the main postharvest strategy applied in extending postharvest life of fruits and vegetables, while the beneficial effects may be limited in sponge gourd because of chilling injury (CI). The visible chilling-induced symptoms in sponge gourd include discoloration, watery black or brown spots on and under the skin, and serious black-colored rot after removal of fruit from low temperature storage (Zong et al. 1993). Damage caused by low temperature severely reduced the quality and storage life of sponge gourd. Salicylic acid (SA), a natural and safe phenolic compound, has been found to generate a wide range of metabol-

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ic and physiological responses in plants and exhibited a high potential in controlling postharvest losses of horticultural crops (Asghari and Aghdam 2010). In recent years, the application of exogenous SA at non-toxic concentrations increases resistance to postharvest CI in fruits, vegetables and flowers such as peach (Cao et al. 2010; Yang et al. 2012), tomato (Zhang et al. 2011; Aghdam et al. 2012), plum (Luo et al. 2011), pomegranate (Sayyari et al. 2009), pineapple (Lu et al. 2011), bamboo shoot (Luo et al. 2012) and anthurium (Promyou et al. 2012). The mechanisms of SA treatment in alleviating CI were extensively, which could be attributed to enhancing membrane integrity and antioxidant system activity, increasing arginine pathways and heat shock proteins (HSPs) gene expression, activation of C-repeat binding factor (CBF) pathway and alteration in phenylalanine ammonia-lyase (PAL) and polyphenol oxidase (PPO) enzymes activities (Aghdam and Bodbodak 2013). To the best of our knowledge, the effect of SA treatment on physiological and biochemical changes associated with CI and postharvest quality of sponge gourd is not yet fully understood. The objectives of this study were to examine the effects of SA on controlling postharvest CI of sponge gourd, and to investigate the underlying mechanisms by SA in reducing damage on cell membranes integrity, inhibiting oxidative stress and inducing alterations in tissue browning related substances and enzymes.

2. Materials and methods 2.1. Plant materials, treatments and storage Sponge gourds Bangbang gua (Luffa cylindrical (L.) Roem. Bang banggua) were hand-harvested at commercial maturity from an organic orchard in Fangshan, Beijing, then wrapped with mesh bag (avoid bruise) and packed in sealed foam boxes (hold moisture), and transferred at ambient temperature to the laboratory within 3 h. Fruits were selected for uniformity of size (length: about 25 cm), color, firmness, shape and free from blemishes, then randomly divided into 2 lots (150 fruits per lot). The first group of fruits was immersed in a 1.5-mmol L–1 SA solution for 15 min, while distilled water was used as control. The SA concentration was chosen as being optimal from preliminary experiments of CI evaluation. Subsequently, all fruits were air-dried for approximately 30 min and stored at 9°C. Following 1, 3, 5, 7 or 9 days of cold storage, fruits were moved to a controlled environment chamber and maintained at 20°C for 2 days (shelf life). Fruit CI, electrolyte leakage, malondialdehyde (MDA) and H2O2 content were assessed after 2 days of shelf-life. Samples of fruit flesh (approx. 50 g) were frozen in liquid nitrogen and stored at –80°C for measurements of other parameters.

2.2. CI evaluation Upon removal from cold storage, the chilling symptoms of sponge gourd developed rapidly. It appeared as surface browning at the first several days of storage period, and then serious black-colored rot was observed on and under the skin. The severity of the symptoms was individually assessed in each fruit with the score from 0 to 4: 0 (no symptom), 1 (1–25% surface browning), 2 (26–50% surface browning), 3 (black-colored rot appeared on and under the skin), 4 (serious black-colored rot appeared on and under the skin). CI index was calculated by using the following formula: CI index=∑(The level of CI scored in sponge gourd×Number of fruit with the corresponding degree)/Total number of fruit in the sample

2.3. Electrolyte leakage Electrolyte leakage was determined according to Huang et al. (2012) with some modifications. Discs were removed with a cork borer (10 mm in diameter) from 10 fruits. Thirty discs were rinsed twice and then incubated in 25 mL of 0.3 mol L–1 mannitol in distilled water at 25°C, and shaken for 30 min. Electrolyte leakage was determined with a conductivity meter (Model DDSJ-308A, Shanghai Scientific Instruments, China). Another 30 discs were boiled for 15 min in 25 mL of 0.3 mol L–1 mannitol in distilled water and then cooled to 25°C to assess total electrolytes. The relative leakage was expressed as a percentage of the total electrolytes.

2.4. Membrane oxidation MDA concentration was measured according to Luo et al. (2011). Tissue samples (2.0 g) were ground in liquid nitrogen and extracted in 5 mL 10% (w/v) trichloroacetic acid (TCA). After centrifugation at 10 000×g for 15 min, 2 mL aliquot of the supernatant was mixed with 2 mL 10% (w/v) TCA containing 0.6% (w/v) TBA. The mixture was heated to 100°C for 20 min, quickly cooled and centrifuged at 10 000×g for 10 min. The supernatant was collected and absorbances at 532, 600 and 450 nm were then measured by a Hitachi U-1800 spectrophotometer (Hitachi High-Technologies Corp., Tokyo, Japan). The MDA concentration was calculated according to the formula: 6.45×(OD532−OD600)−0.56× OD450. All measurements were performed in triplicate.

2.5. Enzyme activity Frozen tissue samples (2 g) from 10 fruits were homogenized with 10 mL of ice-cold extraction buffers containing 0.2 g

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2.6. Total phenolics For analysis of total phenolics content, frozen tissue samples (4 g) from 6 fruits were extracted using a mortar and pestle with 35 mL of cold 70% methanol, and the samples were then centrifuged at 13 000 r min–1 at 4°C for 15 min. A sample of the crude extract (500 μL) was added to 1.5 mL of distilled water and 1 mL of Folin-Ciocalteu reagent; 1 mL of 10% (w/v) Na2CO3 was added. After incubating for 2 h at 25°C, the absorbance of the resulting standard (10, 20, 50, and 100 mg L–1) was measured. The total phenolic content was expressed as milligrams of gallic acid per kilogram of fresh weight.

2.7. Statistical analysis All statistical analyses were performed with SPSS ver. 13.0 (SPSS Inc., Chicago, IL, USA). Data at each time point were analyzed by one-way ANOVA, and mean separations were performed by Duncan’s new multiple range test. Differences at P<0.05 were considered significant. There were no significant interactions between treatments and experiments, the values were recorded as means±SD.

3. Results and discussion As a key signaling molecule, SA has been reported to play an important role in triggering the plant response to adverse environmental conditions, such as UV light, drought, salinity, chilling stress and heat shock (Ding et al. 2001; Ding and Wang 2003; Asghari and Aghdam 2010). This is the first report on the effect of SA treatment on low temperature storage of sponge gourd. In the present study, the severity of chilling symptoms of sponge gourd was evaluated through the CI index parameter. No visible CI symptoms were observed in both control and SA treated sponge gourds in the first 3 days of storage (Fig. 1). However, since the 5th day, the control samples developed chilling symptoms severely, and the index was 3.4 after 9 days at 9°C plus 2 days at 20°C. SA treatment significantly inhibited the increase of CI index, and the largest reduction was obtained when the concentration reached 1.5 mmol L–1. On account of this, we determined the effective dose of SA was 1.5 mmol L–1. It is well established that electrolyte leakage is a measurement of loss of semipermeability of cell membranes, and has been

4 CK 0.5 mmol L−1 SA 1.0 mmol L−1 SA 1.5 mmol L−1 SA 2.0 mmol L−1 SA

3 CI index

of PVPP and ground at 4°C. For superoxide dismutase (SOD), the extraction buffer was 100 mmol L–1 sodium phosphate (pH 7.8). For catalase (CAT) and ascorbate peroxidase (APX), the extraction buffer was 100 mmol L–1 sodium phosphate (pH 7.0) while the buffer of APX containing 0.1 mmol L–1 EDTA, 1 mmol L–1 ascorbic acid and 1% polyvinyl-pyrrolidone. For lipoxygenase (LOX), the extraction buffer was 100 mmol L–1 Tris-HCl (PH 8.0). For the assay of polyphenol oxidase (PPO), the extraction buffer was 100 mmol L–1 sodium phosphate (pH 6.4), while the 50 mmol L–1 sodium borate buffer (pH 8.8, containing 5 mol L–1 β-mercaptoethanol) was used for PAL. The homogenate was centrifuged at 15 000×g for 30 min at 4°C and the supernatant was used for the enzyme assay. Each treatment contained three replicates and the experiment was repeated three times. Superoxide dismutase (SOD) activity was determined by the method of Jin et al. (2009). One unit of SOD activity was defined as a change of 1 per min in OD560 and results were expressed as U mg–1 FW. Catalase (CAT) activity was analyzed based on Wang and Tian (2005) with certain modifications. The reaction mixture consisted of 1 mL sodium phosphate buffer (50 mmol L–1, pH 7.0), 1 mL H2O2 (40 mmol L–1) and 1 mL enzyme extract. One unit of CAT activity was defined as a change of 0.01 per min in OD240 and results were expressed as U g–1 FW. Ascorbate peroxidase (APX) activity was determined by the method of Nakano and Asada (1989). One unit of APX was defined a change of 0.01 per min in OD290, and results were expressed as U g–1 FW. Polyphenol oxidase (PPO) activity was analyzed according to Liu et al. (2007). One unit of PPO was defined a change of 0.001 per min in OD460, and results were expressed as U g–1 FW. Phenylalanine ammonia-lyase (PAL) activity was assayed following the method of Meng et al. (2008). One unit of PAL was defined a change of 0.01 per min in OD290, and results were expressed as U g–1 FW.

a

a ab bc c

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Fig. 1 Effect of salicylic acid (SA) on chilling injury (CI) index of sponge gourd during storage (9 days, 9°C) plus shelf life (2 days, 20°C). Data are means±SD (n=10). The same as in Fig. 2. Different letters at each storage time represent significant differences at P<0.05. The same as below.

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widely used as an indicator of CI (McCollum and McDonald 1991). As shown in Fig. 2, the increase of electrolyte leakage in SA-treated fruits was obviously lower compared with control ones, which suggested that SA can decrease cold damage to cell membranes and therefore preserve cell membrane integrity of sponge gourd. Thus, our results demonstrated that SA as a postharvest tool can be used to alleviate CI of sponge gourd. One of the main cause of CI is the overproduction of reactive oxygen species (ROS, such as O2-., H2O2 and .OH), if not effectively and rapidly removed, the redundant ROS could cause lipid peroxidation (Chen and Li 2001; Liu et al. 2010). MDA, produced from the peroxidation of membrane 80

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Fig. 2 Effect of SA on electrical conductivity of sponge gourd during storage (9 days, 9°C) plus shelf life (2 days, 20°C).

lipids, is often used as an indicator of oxidative stress and damage (Ohkawa et al. 1979). As shown in Fig. 3, the increase of MDA content was closely associated with the development of CI, and significant difference (P<0.05) was observed since the 5th day. In order to maintain the concentration of ROS at relatively low levels, plants have evolved mechanisms to scavenge these toxic and reactive species by antioxidant compounds and by enzymatic antioxidant systems, such as SOD, CAT and APX (Wise 1995). SOD, the major O2-. scavenging enzyme, catalyzes the disproportionation of O2-. radicals into H2O2 and O2, while, both CAT and APX catalyze the degradation of H2O2 to H2O and O2. In consistent with the previous findings in peach (Wang et al. 2006; Yang et al. 2012) and cucumber (Cao et al. 2009) fruit, SA treatment maintained higher activities of SOD, CAT and APX in sponge gourd compared with control samples (Figs. 4–6). This observation indicated that the beneficial effect of SA in alleviating CI of sponge gourd is involved in elevating antioxidant enzymes activities. Interestingly, unlike with the decreasing trend of CAT and APX, the activity of SOD had a slight increase at the 5th day in control ones and 7th day in SA treated samples, respectively. Such a result may be due to the defence mechanism of plant which induced the activity of SOD, and thereby detoxified superoxide ion to H2O2. With the extension of low temperature storage, fresh browning of sponge gourd was occurred under the skin. PAL and PPO are two important enzymes related to the browning process of fruits and vegetables. PAL participates in the biosynthesis of phenolic compounds (Rösler et al. 1997), while PPO oxidizes these phenols to quinines (Martinez and Whitaker 1995) and thereby causes tissue 1.8

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Fig. 3 Effect of SA on malondialdehyde (MDA) content of sponge gourd during storage (9 days, 9°C) plus shelf life (2 days, 20°C). Data are means±SD (n=6). The same as below.

SOD activity (U mg–1 FW)

MDA (nmol L–1 g–1 FW)

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Fig. 4 Effect of SA on superoxide dismutase (SOD) activity of sponge gourd during storage (9 days, 9°C) plus shelf life (2 days, 20°C).

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32

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Fig. 5 Effect of SA on catalase (CAT) activity of sponge gourd during storage (9 days, 9°C) plus shelf life (2 days, 20°C).

Fig. 7 Effect of SA on total phenolics content of sponge gourd during storage (9 days, 9°C) plus shelf life (2 days, 20°C).

3.5

800 a a

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Fig. 6 Effect of SA on ascorbate peroxidase (APX) activity of sponge gourd during storage (9 days, 9°C) plus shelf life (2 days, 20°C).

Fig. 8 Effect of SA on phenylalanine ammonia-lyase (PAL) activity of sponge gourd during storage (9 days, 9°C) plus shelf life (2 days, 20°C).

browning. In the present study, the content of phenolic compounds and activities of PAL and PPO in both control and SA treated samples increased throughout the whole storage period. However, the application of SA significantly restrained this increasing trend and brought lower values of these three parameters in SA treated sponge gourds (Figs. 7–9). This observation was in agreement with the previous researches of plum (Luo et al. 2011), pineapple (Lu et al. 2011), pomegranate (Sayyari et al. 2009) and bamboo shoot (Luo et al. 2012), that the suppression of tissue browning by SA was usually associated with diminished total phenolics content and inhibited PAL and PPO activities.

4. Conclusion Low-temperature induced CI in sponge gourd. This is the first study reporting on the use of SA treatment in alleviating postharvest CI of sponge gourd. The date obtained in the present study showed that the application of SA alleviated CI symptoms, reduced electrolyte leakage, inhibited the increase of MDA and total phenolics contents, enhanced the antioxidant enzymes activities of SOD, CAT and APX, and suppressed PAL and PPO activities. The beneficial effects of SA may be attributed to maintaining membrane integrity, inhibiting membrane peroxidation, enhancing antioxidant system and suppressing browning related

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800

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PPO activity (U g–1 FW )

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Fig. 9 Effect of SA on polyphenol oxidase (PPO) activity of sponge gourd during storage (9 days, 9°C) plus shelf life (2 days, 20°C).

enzymes. These results suggested that SA as a postharvest tool could effectively alleviate CI and enhance chilling tolerance of sponge gourd. However, further studies are needed to explore the commercial use of SA in alleviating CI of sponge gourd.

Acknowledgements This work was supported by the Ministry of Agriculture of China (CARS-25), the Special Fund for Agro-scientific Research in the Public Interest, China (201203095), the National Natural Science Foundation of China (31401536), the Natural Science Foundation of Beijing, China (6144020), the Young Investigator Fund of Beijing Academy of Agricultural and Forestry Sciences of China (201404).

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