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Article

Comparative Effect of Melatonin and 1-Methylcyclopropene Postharvest Applications for Extending ‘Hayward’ Kiwifruit Storage Life

by
María Celeste Ruiz-Aracil
,
Fabián Guillén
,
Mihaela Iasmina Madalina Ilea
,
Domingo Martínez-Romero
,
José Manuel Lorente-Mento
and
Juan Miguel Valverde
*
Postharvest Research Group of Fruit and Vegetables, Agro-Food and Agro-Environmental Research and Innovation Center (CIAGRO-UMH), University Miguel Hernández, Ctra. Beniel km. 3.2, 03312 Orihuela, Spain
*
Author to whom correspondence should be addressed.
Agriculture 2023, 13(4), 806; https://doi.org/10.3390/agriculture13040806
Submission received: 2 March 2023 / Revised: 28 March 2023 / Accepted: 29 March 2023 / Published: 31 March 2023
(This article belongs to the Special Issue Effect of Preharvest and Postharvest Technologies on Fruit Ripening and Senescence)
Browse Figures
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Abstract

Kiwifruit, like many other fruits, is susceptible to dehydration, leading to texture changes and a loss of flavour during storage. Exposing kiwifruit to suboptimal temperatures can control these changes but can cause internal browning. Postharvest treatments with substances such as 1-methylcyclopropene (1-MCP) are some of the most successful commercial technologies in the conservation of fruits and vegetables. In recent years, there has been a growing interest among researchers in alternative technologies based in postharvest treatments with plant growth regulators. In this sense, melatonin (MT) has been shown to improve fruit quality, extending shelf life. The aim of this study was to compare these two different technologies applied at postharvest to evaluate the impact on kiwifruit quality. Optimal 1-MCP fumigations and MT solutions were assayed on ‘Hayward’ kiwifruit under similar conditions. Quality parameters were evaluated at 14-day intervals during 84 days of cold storage plus 5 days at 20 °C. The results showed that both treatments were similarly effective in maintaining quality parameters such as weight loss, respiration, firmness, and acidity. Although 1-MCP treatments delayed the evolution of kiwifruit colour and chlorophyll degradation as compared to MT, MT treatments controlled chilling injury better than 1-MCP. This effect was not related to a greater cell membrane integrity since fruit batches treated with 1-MCP were the ones that showed the lowest electrolyte leakage level. In conclusion, both treatments maintained fruit quality and delayed ripening in a similar way. In this sense, the results suggest that MT immersion treatments could act as efficient delaying senescence as fumigations with 1-MCP maintaining kiwifruit quality during refrigerated storage.

Graphical Abstract

1. Introduction

Kiwifruit is a subtropical crop and the species Actinidia deliciosa is the most widely cultivated worldwide, commonly known as green kiwifruit. Kiwifruit contains a large amount of nutrients that provide digestive, immune, and metabolic health benefits to the consumer. It is also an important source of vitamin C and E, dietary fibre, folic acid, potassium, and bioactive compounds such as antioxidants, phytochemicals, and enzymes, which provide functional and metabolic benefits [1,2,3].
The fruit must be carefully harvested since even small damages would affect the production of endogenous ethylene, thus increasing fruit softening. The decrease in texture (hardness or firmness) and the increase in total soluble solids (TSSs) are considered the main indicators of kiwifruit’s ripening stage since, in the case of kiwifruit, the evolution of the ripening process cannot be observed externally since the exocarp does not change colour [4,5]. On the other hand, after harvest, the fruit is susceptible to diseases caused mainly by fungi such as Botrytis cinerea, Penicillium expansum, Alternaria alternata, Botryos-phaeria dothidea, and Diaporthe spp., which cause a shorter shelf life and a decrease in its commercial yield, incurring postharvest losses [6].
Kiwifruit is a climacteric fruit; therefore, the ripening process is affected by the rate of ethylene production. Kiwifruit are harvested at physiological maturity but in an immature state with insufficient endogenous ethylene content to induce kiwifruit ripening. When stored below 10 °C, ethylene production in kiwifruit is almost nil and it is not strictly necessary to use exogenous ethylene treatment to induce ripening, since it has been shown that ripening and softening during cold storage also occur in the absence of detectable ethylene (exogenous or endogenous [7]).
Currently, there are several postharvest technologies with positive effects which maintain the quality of the fruit once it has been harvested. Physical methods such as temperature management and variations in the atmosphere concentration are used to both accelerate fruit ripening to reach commercial maturity and reduce the fruit respiration rate, therefore delaying ripening and extending its shelf life [8,9,10]. In this sense, the effect of ionizing radiation has been applied successfully, reducing spoilage caused by fungal pathogens [11]. Exogenous treatments have also been studied to increase kiwifruit’s shelf life, such as edible coatings [12], methyl jasmonate [13], or γ-aminobutyric acid (GABA) solutions to reduce the chilling injury (CI) associated with the postharvest cold storage of kiwifruit [14]. On the other hand, ozone postharvest treatments were successful in maintaining fruit quality and reducing fungal growth [15,16].
Another widely used method to extend kiwifruit’s shelf life is the use of commercial ethylene inhibitors such as 1-methylcyclopropene (1-MCP) [17,18,19,20]. 1-MCP binds to ethylene receptors and inhibits the ethylene action, reducing the fruit respiration rate, thus delaying fruit ripening and softening processes, and thereby preventing quality loss [21,22,23,24]. The preharvest application of 1-MCP delays fruit ripening, reducing the negative impact of late harvest and its subsequent effects during storage [25]. Postharvest treatment with 1-MCP delays the increase in membrane permeability and lipid peroxidation, preserving the unsaturated fatty acid content, which is related to the maintenance of membrane integrity [26]. It has also been shown that 1-MCP maintains the content of bioactive compounds and total antioxidant capacity [27] as well as ascorbic acid and phenol content [28]. 1-MCP is also able to effectively eliminate off-flavours by suppressing ethanol metabolism in kiwifruit [29], but also, different studies described a decrease in the fruit aroma properties [30] including kiwifruit [31].
Melatonin (MT) is a multifunctional signalling molecule present in plants that plays a biostimulatory, growth-regulatory, and antioxidant role through the direct scavenging of reactive oxygen species (ROS) and reactive nitrogen species (RNS) under abiotic and biotic stress conditions [32]. MT-treated fruit exposed to suboptimal storage conditions maintained an optimal intracellular energy status, reducing physiological disorders during postharvest storage [33]. Exogenous MT treatment delays softening and preserves kiwifruit quality during low-temperature storage by suppressing cell-wall-degrading enzyme activity, reducing lipid peroxidation, and increasing the accumulation of antioxidant compounds [34,35,36,37,38,39]. In this sense, MT contributes to the enhancement of defence responses and, interestingly, reduces the alcoholic off-flavour produced by ethanol fermentation during kiwifruit’s postharvest stage [40,41].
Both MT and 1-MCP have been studied individually in kiwifruit but the effectiveness of these treatments in increasing or maintaining quality traits in kiwifruit has not been compared. For this reason, the aim of this research was to elucidate the potential of both technologies under similar conditions to maintain kiwifruit quality during cold storage.

2. Materials and Methods

2.1. Plant Material and Experimental Design

Kiwifruit (Actinidia deliciosa cv. Hayward) were manually harvested from a commercial plot in Carlet (Valencia) on 10 November 2021 when the fruit reached harvest maturity (6–7 °Brix) based on the literature guidelines [38,39]. A total of 315 kiwifruit without pests, diseases, or mechanical damage were graded and selected for uniform size. After selection, the kiwifruit were grouped into 3 replicates of 5 fruits for each treatment and sampling day. For the control treatment, distilled water immersions were applied for 10 min. For MT treatment, immersions in 0.1 mM solutions were performed for 10 min following optimal conditions in previous studies [36,40]. 1-MCP treatments were applied by mixing commercial tablets releasing 0.5 µL L−1 [21,42] using a commercial activator solution provided by SmartFresh (Agro-Fresh Inc., Philadelphia, PA, USA). All the dips of the different MT and control batches also contained Tween 20 (0.05%). Once the fruits were treated, they were left to dry at 20 °C for 1 h and then placed in individual 130 L plastic containers per treatment, providing the same conditions for the different batches. The containers were then closed and hermetically sealed and, thus, the released 1-MCP was left to act for 24 h at 20 °C while the control batches were exposed in these containers to normal air at the same temperature conditions. All these fruits were then stored for 84 days at 2 °C and 90% RH plus an additional period of 5 days at 20 °C for subsequent shelf life determinations.

2.2. Postharvest Quality Parameters

Three replicates of 5 fruits from each treatment lot were randomly selected at 14-day intervals during cold storage plus 5 days at 20 °C. The weight loss of individual kiwifruit was calculated as a percentage of the weight loss obtained with respect to day 0 and expressed as a % using a KERN 440-35N digital balance (Balingen, Germany).
CO2 and ethylene production were determined in triplicate by placing 5 fruits from each replicate in a 3.4 L plastic container hermetically sealed with a rubber stopper for 60 min using the static method. After that, 1 mL of gas sample was taken in duplicate from the headspace and carbon dioxide was quantified using a Shimadzu 14B (Shimadzu Europa GmbH, Duisburg, Germany), and ethylene production was evaluated with a Shimadzu GC 2010 gas chromatograph according to a previously described method [43]. The respiration rate ethylene production was expressed as mg of CO2 kg−1 h−1 and nL g−1 h−1, respectively.
Firmness was determined individually using a TX-XT2i texture analyser (Stable Microsystems, Godalming, UK) equipped with a flat probe. The rate of descent of the disc was 20 mm min−1 until a deformation of 5% was reached. Two equidistant readings were taken in the equatorial region of each fruit. Fruit firmness was expressed as the ratio of applied force to distance travelled (N mm−1).
The colour was measured with a Minolta colorimeter (CRC400, Minolta Camera Co.; Kantō, Tokyo, Japan); three colour measurements were made for each peeled fruit at three points equidistant from the equatorial zone and expressed as CIE Hue* (arctg b*/a*) according to CIELab coordinates.
The determination of the kiwifruit susceptibility to CI was performed with a visual assessment of the kiwifruit flesh after peeling, similarly to a previously described method [44]. Visual assessment was performed individually on each fruit using a 5-point scale (0–4 scale) according to the surface area of surface pitting and dark watery spots: 0 (no symptoms), 1 (<25%), 2 (25–50%), 3 (51–75%), and 4 (>75%). The CI index was expressed as CI index = Σ [(scale × N)/total fruit number]. N is the number of fruits on the corresponding scale.
Electrolyte leakage (EL) was evaluated in the flesh tissue, following the method described by McCollum and McDonald [45] with some modifications. First, slices of the three replicates per treatment were cut to 2 mm thick in the equatorial zone of the kiwifruit. Fifteen discs were extracted for each replicate using a 0.5 cm diameter cork borer. After 3 rinses of 3 min for each replicate with deionized water, they were subjected to constant shaking with 50 mL of deionized water at room temperature. After 30 min, the initial electrical conductivity (C1) was measured using a Crison conductivity meter. The samples were frozen and then brought to 121 °C for 15 min. Total conductivity (C2) was evaluated with samples at room temperature (20 °C). EL was calculated as (C1/C2) × 100.
The total chlorophyll content (TCC) was evaluated by extracting a homogeneous mixture of 5 kiwifruit halves from each of the 5 fruits of each replicate following the method described by Vu et al. [46]. Pigment extraction was achieved through homogenization in methanol for 2 min; then, the samples were centrifuged, and the supernatant was evaluated using a spectrophotometer (1900 UV/Vis, Shimadzu, Kyoto, Japan) at 652 and 665 nm. The data, expressed as mg 100 g−1 of kiwifruit flesh, are the mean ± SE of 3 determinations.
The TSS and titratable acidity (TA) were determined in duplicate in the filtered juice extracted from the mixture of the remaining 5 kiwifruit halves for each replicate per batch. The TSSs in kiwifruit juice were measured using an Atago PR-101 digital refractometer (Atago Co. Ltd., Tokyo, Japan) at 20 °C, and the TA was determined in each sample using automatic titration (785 DMP Titrino, Metrohm, Herisau, Switzerland). The TSSs and TA were expressed as a%.

2.3. Statistical Analysis

A completely randomized design was used in this study. All data in this document are expressed as mean ± standard error (SE). Data were subjected to an analysis of variance (ANOVA). Mean comparisons were performed using a multiple range test (Tukey’s HSD test) to find significant differences (p < 0.05). Different lowercase letters indicate a significant difference between treatments on the same sampling date. All analyses were performed with the SPSS software package, version 22 (IBM Corp.; Armonk, NY, USA).

3. Results and Discussions

3.1. Effect of Exogenous 1-MCP and MT on Weight Loss and Fruit Firmness

Water loss in kiwifruit is related to a degradation of the outer layers, hairiness, and cell death of the skin tissue [47]. As shown in Figure 1A, the weight loss of the postharvest kiwifruit for each treatment steadily increased gradually during the storage period at 2 °C + 5 days at 20 °C. We observed that treatments containing MT (6.59 ± 0.22) or 1-MCP (6.73 ± 0.19) were effective in delaying weight loss significantly (p < 0.05) as compared to the control fruit (7.69 ± 0.28) after 28 days of storage at 2 °C + 5 days at 20 °C. On the other hand, it was proven that MT was equally effective as 1-MCP since there were no significant differences (p > 0.05) between these treatments. Although both 1-MCP and MT delayed weight loss, this effect was reduced at the end of storage, without significant differences (p > 0.05) as compared to the control batch (Figure 1A). In this cultivar, weight losses higher than 8–10% are related to kiwifruit shrivelling and, in this sense, this parameter was delayed for 2 weeks in melatonin- and 1-MCP-treated fruit as compared to the control fruit [48]. Postharvest weight losses are caused by transpiration and fruit respiration processes, so the observed differences could be related to a higher tissue integrity and a reduced respiration in kiwifruit treated with 1-MCP or MT [49]. The 1-MCP treatments delayed the weight loss of different vegetal products as avocado, tomato, and peach [50,51,52], and MT as a postharvest treatment had a positive effect, delaying this parameter in peach, strawberry, and apple [53,54,55]. Similar results have been observed for kiwifruit treated with 1-MCP [18,22,28] or MT [35].
Firmness is a key parameter that reflects fruit quality and influences consumer acceptability [56]. Fruit softening has been linked to fruit dehydration, which consequently results in water loss and reduced turgor pressure [47,57]. As expected, due to fruit ripening and senescence, kiwifruit firmness decreased throughout the cold storage, regardless of the treatment applied (Figure 1B). However, the highest fruit firmness levels were obtained for MT and 1-MCP-treated fruit, maintaining values of 22.72 ± 1.06 and 23.05 ± 1.15 N mm−1, respectively, at day 14 while a measurement of 19.63 ± 1.18 N mm−1 was obtained for the control batch. The 1-MCP and MT treatments were significantly (p < 0.05) effective in delaying fruit softening as compared to the control fruit during storage. In this sense, fruit firmness was maintained at higher firmness levels than those that negatively affect the consumer’s acceptance during cold storage [58]. The MT and 1-MCP treatments did not show significant differences (p > 0.05) between lots in the previously evaluated parameters (weight loss and fruit firmness). In this sense, it is interesting to highlight that MT, being a natural-origin substance, displayed similar results to those observed for 1-MCP-treated fruit for these important fruit quality traits. Therefore, the positive effect of both treatments on reducing weight loss (Figure 1A) would be one of the causes of the firmness maintenance as observed in Figure 1B. Similar studies have previously demonstrated that 1-MCP postharvest applications on different fruit such as kiwifruit [20,21], pears [59], and blueberry fruit [60] delayed weight losses and maintained fruit firmness through a delayed respiration rate and a reduced transpiration process. Although, in this study, ethylene levels were in general detected at lowlevels as basal ethylene production, 1-MCP blocked the perception of these low ethylene levels [7], reducing kiwifruit respiration which also affected transpiration processes [61]. Additionally, improved tissue integrity related to the reduced activity of cell-wall-degrading enzymes was observed in other studies [18,23]. In this sense, 1-MCP applications [26,62,63] and MT postharvest treatments [34,35,40] on different fruit species also reduced ROS species and degrading-enzyme activity, thus maintaining cell membrane fluidity.

3.2. Effect of Exogenous 1-MCP and MT on Respiration Rate and Ethylene Production

A higher respiration rate leads to a shorter fruit shelf life, mainly due to higher metabolic activity in the vegetal product. Once kiwifruits are harvested, an increase in the rate of respiration that accelerates fruit ripening and softening can be observed [23]. When studying the evolution of this parameter during storage, a respiration increase was accompanied by a maximum peak as fruit ripening progressed, showing a typical climacteric pattern (Figure 2A). The respiration evolution was significantly controlled (p < 0.05) in all fruit lots treated with MT and 1-MCP, displaying lower respiration production during storage as compared to the control lot. However, there were also significant differences (p < 0.05) between the MT- and 1-MCP-treated batches, since, at the beginning of the respiration curve, the MT treatment had the lowest respiration levels, while 1-MCP reached the lowest respiration levels at the end of the storage period.
The lower respiration observed for the 1-MCP-treated kiwifruit could indicate that this compound is delaying senescence through reduced fruit metabolism by competitively binding to the ethylene receptor. A significant reduction in CO2 production has been reported for kiwifruit treated with 1-MCP in the presence or absence of exogenous ethylene [18,21,28,64]. In this study, ethylene production was very low, being detected just at basal levels, and did not show significant differences (p > 0.05) between treatments (Figure 2B). In this sense, a significantly higher ethylene level was detected at the end of the storage period only for the control fruit as compared to the rest of the treatments (1-MCP and MT), which did not display significant differences (p > 0.05) between them. A reduced respiration pattern has also been observed in MT-treated kiwifruit [34,41]. In addition, several studies have demonstrated that MT applications in strawberry and mango increase the GABA shunt pathway, leading to increases in this metabolite, which is an immediate substrate for respiration, increasing the energy status of plant cells and leading to an improved metabolic balance [65,66].

3.3. Effect of Exogenous 1-MCP and MT on TSS and TA

Total soluble solids (TSSs) are one of the main quality parameters for assessing the ripening stage of kiwifruit and this content increases after harvest even when kiwifruits are stored at 0 °C [67]. Figure 3A shows how the TSSs increased gradually throughout storage for all treatments. Overall, the amount of TSSs throughout storage was not significantly affected by the MT and 1-MCP treatments. However, the MT-treated fruit delayed TSS accumulation, displaying significant differences (p < 0.05) at the end of storage with respect to both the control and 1-MCP-treated fruit (Figure 3A). The minimum TSSs content at harvest for ‘Hayward’ kiwifruit has been previously described as 6.2 % and has been used for many years [68]. This level matched with that observed at harvest in the kiwifruit used in this research. However, Goldberg et al. [69] found that the appropriate value for TSSs in order to increase the potential for longer storability in this cultivar should be 7 %. Our results are in consonance with other studies on kiwifruit in which a nonsignificant effect on TSS content was observed in general when 1-MCP was applied (p > 0.05) [23,34,36], and even lower values were obtained as compared to control fruit when MT postharvest treatments were assayed [18,26,29]. In this sense, the additive effect exhibited by MT applications in delaying TSS accumulation could indicate a stronger antisenescent effect than that observed in 1-MCP-treated fruit.
With respect to TA, gradual decreases in this parameter were observed as storage progressed (Figure 3B) since organic acids are substrates for the enzymatic reactions of the respiration process [20,70,71]. TA evolution in fruit batches treated separately with 1-MCP or MT displayed a significant (p < 0.05) effect, reducing the acidity losses as compared to the control batches. On the other hand, at the end of the storage period, the TA mean values for the control fruit did not differ significantly (p > 0.05) from those of the treatments applied after 56 days of cold storage (Figure 3B). The higher TA observed in fruit treated with 1-MCP [18,21,29] or with MT [34] could be related to a lower degradation metabolism of organic acids since 1-MCP reduced ethylene perception, thus delaying kiwifruit ripening. On the other hand, MT could be increasing the energy cell status, in turn being responsible for a higher maintenance of mitochondrial activity, delaying the catabolism of primary substrates such as sugars and organic acids, and, thus, delaying ripening and senescence [34,66].

3.4. Effect of Exogenous 1-MCP and MT on Chlorophyll Content and Internal Colour

Chlorophyll is the main pigment responsible for the colour of kiwifruit flesh and is considered an important index of kiwifruit maturity and senescence [72]. When we studied the chlorophyll content evolution, a decrease in this parameter throughout storage was observed for all kiwifruit studied (Figure 4A). After 14 days of cold storage + 5 days at 20 °C, the 1-MCP treatment was effective in delaying chlorophyll loss significantly (p < 0.05) (6.18 ± 0.07 mg 100 g−1) as compared to the control fruit (5.39 ± 0.09 mg 100 g−1) and the kiwifruit treated with MT (4.95 ± 0.08 mg 100 g−1). Although, at the beginning of the experiment, the 1-MCP-treated fruit displayed a positive effect on delaying chlorophyll loss in kiwifruit, this effect was reduced as the storage progressed until the end of the study. At this point, the mean values of chlorophyll losses were similar between the different fruit batches (Figure 4A). Treatments with 1-MCP have been shown to maintain chlorophyll content by delaying ripening and senescence in both climacteric and nonclimacteric fruit [18,49,73]. In kiwifruit, 1-MCP fumigations reduced the ethylene perception in ‘Hayward’ kiwifruit even at the basal levels observed in this study (Figure 2B) but also in a previous study designed under similar conditions [7]. On the other hand, photosynthetic pigments such as chlorophylls are the main targets of ROS which increase their content in the plant cells as senescence accelerates [18,74]. Treatments with 1-MCP improved the levels of protective enzymes (APX, POD, SOD, and CAT) against reactive oxygen damage, delaying fruit senescence and maintaining kiwifruit’s shelf life [75]. Likewise, it has been documented that exogenous MT delays tissue degradation through an antioxidant effect on free radicals, also protecting chlorophyll-related proteins [32,37]. However, the chlorophyll content of the MT-treated kiwifruit in this experiment (Figure 4A) did not differ from the control batch. In this sense, although chlorophyll content has been linked to fruit ripening and softening, in this case, chlorophyll degradation in the MT-treated fruit was not coincident with a greater weight loss (Figure 1A), higher firmness losses (Figure 1B), or higher ethylene production (Figure 2B) as compared to the control fruit.
In contrast to other fruits, visible changes in kiwifruit skin colour do not occur during development but do occur in the flesh. Colour fruit changes in kiwifruit are mainly due to the chlorophyll content evolution [69], which decreases during storage, as we evaluated previously (Figure 4A). Colour evaluation was expressed as CIE Hue* which represents the evolution to darker fruit shades, and for this reason, this parameter is useful for assessing fruit senescence. Figure 4B shows that the CIE Hue* decreased during storage in kiwifruit. In this sense, the lower the CIE Hue*, the darker the external pulp shade is. The MT and 1-MCP treatments were effective in delaying colour evolution significantly (p < 0.05) as compared to the control fruit batch. However, no significant differences (p > 0.05) were found between the MT- and 1-MCP-treated fruit, although at the beginning of the experiment, the 1-MCP treatment controlled the decrease in CIE Hue* (Figure 4B), probably due to a higher chlorophyll content, as observed in Figure 4A. The chlorophyll content and CIE Hue* were not in consonance, probably due to the fact that not only chlorophylls are related to CIE Hue* evolution, but carotenoids also are present in all green tissues of plants, and the evolution of these carotenoids in kiwifruit also affects the CIE Hue* colour evolution at immature green stages, as they do for yellow flesh cultivars [76]. MT applications on different fruit species such as strawberry and sweet cherry [54,77] also displayed a similar pattern, maintaining different colour parameters as compared with control fruit. In kiwifruit, the carotenoid content shows a decreasing trend during the beginning of the storage, whereas the chlorophyll content remains stable and starts to decrease once ripening is more advanced [78]. The major colour changes in kiwifruit are caused by the enzymatic and nonenzymatic development of brown-pigmented substances and by decolouration due to chlorophyll and carotenoid degradation decreasing the CIE Hue* values in the flesh, but this degradative evolution is directly related to membrane integrity [79]. For this reason, the effect of these treatments maintaining the chlorophyll content and CIE Hue* values could be affected by the maintenance of fruit firmness and membrane structure provided by both 1-MCP and MTtreatments, as was shown in our results.

3.5. Effect of Exogenous 1-MCP and MT on EL and Chilling Injury

Cold storage induces an increase in ROS in kiwifruit, which ultimately cause oxidative damage to cell membranes, increasing the levels of EL due to the loss of cellular integrity [80]. EL increased throughout cold storage for all treatments. However, the 1-MCP treatment was significantly effective (p < 0.05) in delaying the electrolyte outflow as compared to the control fruit. On the other hand, although a delay in the evolution of this parameter was also observed in the MT-treated fruit, the differences displayed as compared to the control fruit were smaller (Figure 5A). Thus, as described in this study, 1-MCP and MT, by maintaining membrane integrity [22,26,34] and probably through an improvement in the oxidative balance [40,63,81], could be the main causes for reducing oxidative stress through maintaining the structure of the cell membranes and, thus, delaying EL evolution in fruit treated with these substances.
Chilling injury is a physiological disorder that manifests itself in fruit after being exposed to temperatures close to the freezing point. Cold tolerance depends on several factors such as the species, cultivar, harvest time, temperature, and time of exposure to cold storage [19,82]. Usually, chilling injury symptoms increase after fruit are moved from cold storage to an ambient temperature. Chilling injury in kiwifruit appeared after 42 days of cold storage plus an additional period of 5 days at 20 °C, increasing until the end of storage. As expected, the impact of CI during storage was, in general, low at 2 °C for kiwifruit, but small watery spots appeared in a small percentage of the tested fruit. A recent study has confirmed that although a 2.5 °C storage temperature prevents chilling injury in kiwifruit [83], lower temperatures such as 2 °C may affect kiwifruit CI, especially when the fruit are stored at this temperature rapidly after harvest [58], as was observed in this experiment. However, MT and 1-MCP increased cold tolerance significantly (p < 0.05) as compared to the control fruit during storage (Figure 5B). The MT-treated fruit displayed a lower CI impact as compared to 1-MCP after 56 and 70 days of storage; however, this additional effect on MT-treated fruit was not related to EL evaluations. MT and 1-MCP postharvest treatments may increase cold tolerance by protecting the cell membranes, as has been observed in other studies with these substances in other plant species [58,84,85,86]. In kiwifruit, both MT treatments [40] and 1-MCP fumigations [64] have been shown to reduce the symptoms of CI caused by suboptimal temperatures. A higher fruit firmness and membrane integrity could be the main factors increasing kiwifruit’s cold tolerance, as observed in the MT and 1-MCP-treated fruit in this study.

4. Conclusions

1-Methylcyclopropene and melatonin postharvest treatments delayed kiwifruit ripening, reducing weight losses and fruit respiration and maintaining higher fruit firmness and acidity level in a similar way. 1-Methylcyclopropene fumigations were more effective in maintaining higher values when the colour evolution and total chlorophyll content were evaluated as compared to the rest of the batches in this study. However, these parameters are only visible when the kiwifruit peel was removed. In this sense, kiwifruit chilling injury was lower when melatonin treatments were applied as compared to 1-methylcyclopropene. Therefore, alternative postharvest treatments based on melatonin solutions could be considered as effective as 1-methylcyclopropene fumigations which are applied commercially nowadays. However, further research including the effect of the combination of these treatments at different concentrations and the impact of these technologies on advancing the ripening stages at harvest should be conducted.

Author Contributions

Conceptualization, F.G. and J.M.V.; methodology, F.G., D.M.-R. and J.M.V.; software, J.M.V.; validation, M.C.R.-A., F.G. and J.M.V.; formal analysis, M.C.R.-A., F.G., M.I.M.I. and J.M.V.; investigation, M.C.R.-A., F.G., M.I.M.I., D.M.-R., J.M.L.-M. and J.M.V.; resources, F.G., D.M.-R. and J.M.V.; data curation, M.C.R.-A., F.G., M.I.M.I., J.M.L.-M. and J.M.V.; writing—original draft preparation, M.C.R.-A.; writing—review and editing, F.G. and J.M.V.; visualization, M.C.R.-A., F.G. and J.M.V.; supervision, F.G. and J.M.V.; funding acquisition, F.G., D.M.-R. and J.M.V. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Spanish Ministry of Science, Innovation and Universities and European Commission with FEDER funds through Project RTI2018-099664-B-100.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. Evolution of weight losses (%) (A), and fruit firmness (N mm−1) (B) of ‘Hayward’ kiwifruit treated with distilled water (Control), 1-MCP at 0.5 µL L−1 or melatonin at 0.1 mM (MT) during cold storage plus 5 days at 20 °C. Data are the mean ± SE (n = 3). Different lowercase letters show significant differences (p < 0.05) among treatments for each sampling date.
Figure 1. Evolution of weight losses (%) (A), and fruit firmness (N mm−1) (B) of ‘Hayward’ kiwifruit treated with distilled water (Control), 1-MCP at 0.5 µL L−1 or melatonin at 0.1 mM (MT) during cold storage plus 5 days at 20 °C. Data are the mean ± SE (n = 3). Different lowercase letters show significant differences (p < 0.05) among treatments for each sampling date.
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Figure 2. Evolution of respiration rate (mg CO2 kg−1 h−1) (A) and ethylene production (nL g−1 h−1) (B) of ‘Hayward’ kiwifruit treated with distilled water (Control), 1-MCP at 0.5 µL L−1, or melatonin at 0.1 mM (MT) during cold storage plus 5 days at 20 °C. Data are the mean ± SE (n = 3). Different lowercase letters show significant differences (p < 0.05) among treatments for each sampling date.
Figure 2. Evolution of respiration rate (mg CO2 kg−1 h−1) (A) and ethylene production (nL g−1 h−1) (B) of ‘Hayward’ kiwifruit treated with distilled water (Control), 1-MCP at 0.5 µL L−1, or melatonin at 0.1 mM (MT) during cold storage plus 5 days at 20 °C. Data are the mean ± SE (n = 3). Different lowercase letters show significant differences (p < 0.05) among treatments for each sampling date.
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Figure 3. Evolution of total soluble solids (TSSs) (%) (A) and titratable acidity (TA) (%) (B) of ‘Hayward’ kiwifruit treated with distilled water (Control), 1-MCP at 0.5 µL L−1, or melatonin at 0.1 mM (MT) during cold storage plus 5 days at 20 °C. Data are the mean ± SE (n = 3). Different lowercase letters show significant differences (p < 0.05) among treatments for each sampling date.
Figure 3. Evolution of total soluble solids (TSSs) (%) (A) and titratable acidity (TA) (%) (B) of ‘Hayward’ kiwifruit treated with distilled water (Control), 1-MCP at 0.5 µL L−1, or melatonin at 0.1 mM (MT) during cold storage plus 5 days at 20 °C. Data are the mean ± SE (n = 3). Different lowercase letters show significant differences (p < 0.05) among treatments for each sampling date.
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Figure 4. Evolution of total chlorophyll content (mg 100 g−1) (A) and CIE Hue* (B) of ‘Hayward’ kiwifruit treated with distilled water (Control), 1-MCP at 0.5 µL L−1, or melatonin at 0.1 mM (MT) during cold storage plus 5 days at 20 °C. Data are the mean ± SE (n = 3). Different lowercase letters show significant differences (p < 0.05) among treatments for each sampling date.
Figure 4. Evolution of total chlorophyll content (mg 100 g−1) (A) and CIE Hue* (B) of ‘Hayward’ kiwifruit treated with distilled water (Control), 1-MCP at 0.5 µL L−1, or melatonin at 0.1 mM (MT) during cold storage plus 5 days at 20 °C. Data are the mean ± SE (n = 3). Different lowercase letters show significant differences (p < 0.05) among treatments for each sampling date.
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Figure 5. Evolution of electrolyte leakage (%) (A) and chilling injury index (0–4 scale) (B) of ‘Hayward’ kiwifruit treated with distilled water (Control), 1-MCP at 0.5 µL L−1, or melatonin at 0.1 mM (MT) during cold storage plus 5 days at 20 °C. Data are the mean ± SE (n = 3). Different lowercase letters show significant differences (p < 0.05) among treatments for each sampling date.
Figure 5. Evolution of electrolyte leakage (%) (A) and chilling injury index (0–4 scale) (B) of ‘Hayward’ kiwifruit treated with distilled water (Control), 1-MCP at 0.5 µL L−1, or melatonin at 0.1 mM (MT) during cold storage plus 5 days at 20 °C. Data are the mean ± SE (n = 3). Different lowercase letters show significant differences (p < 0.05) among treatments for each sampling date.
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Ruiz-Aracil, M.C.; Guillén, F.; Ilea, M.I.M.; Martínez-Romero, D.; Lorente-Mento, J.M.; Valverde, J.M. Comparative Effect of Melatonin and 1-Methylcyclopropene Postharvest Applications for Extending ‘Hayward’ Kiwifruit Storage Life. Agriculture 2023, 13, 806. https://doi.org/10.3390/agriculture13040806

AMA Style

Ruiz-Aracil MC, Guillén F, Ilea MIM, Martínez-Romero D, Lorente-Mento JM, Valverde JM. Comparative Effect of Melatonin and 1-Methylcyclopropene Postharvest Applications for Extending ‘Hayward’ Kiwifruit Storage Life. Agriculture. 2023; 13(4):806. https://doi.org/10.3390/agriculture13040806

Chicago/Turabian Style

Ruiz-Aracil, María Celeste, Fabián Guillén, Mihaela Iasmina Madalina Ilea, Domingo Martínez-Romero, José Manuel Lorente-Mento, and Juan Miguel Valverde. 2023. "Comparative Effect of Melatonin and 1-Methylcyclopropene Postharvest Applications for Extending ‘Hayward’ Kiwifruit Storage Life" Agriculture 13, no. 4: 806. https://doi.org/10.3390/agriculture13040806

APA Style

Ruiz-Aracil, M. C., Guillén, F., Ilea, M. I. M., Martínez-Romero, D., Lorente-Mento, J. M., & Valverde, J. M. (2023). Comparative Effect of Melatonin and 1-Methylcyclopropene Postharvest Applications for Extending ‘Hayward’ Kiwifruit Storage Life. Agriculture, 13(4), 806. https://doi.org/10.3390/agriculture13040806

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