Postharvest Application of 1-Methylcyclopropene (1-MCP) on Climacteric Fruits: Factors Affecting Efficacy

ABSTRACT Climacteric fruits continue to ripen postharvest, characterized by an increase in respiration and ethylene. Thus, the shelf-life, quality, and marketability of these fruits is reduced rapidly. One of the most important postharvest technology used to reduce the effect of exogenous and endogenous ethylene and extend the shelf-life of climacteric fruits is 1-methylcyclopropene (1-MCP). Currently, the application of 1-MCP on certain climacteric fruits is not commercially feasible due to the variable effects associated with this technology. In this review, factors that influence 1-MCP efficacy in climacteric fruits such as fruit maturity, concentration, genotypes, storage temperature and atmospheres, packaging material and application timing are critically discussed. In addition, current techniques used to mitigate the 1-MCP variability such as combined application with other postharvest treatments are briefly described and discussed. This review will provide an important information on the utilization of 1-MCP to prolong shelf-life in various climacteric fruits and help in the development of protocols for unregistered genotypes.


Introduction
Ripening is the last stage of fruit maturation and is characterized by an increase in respiration rate and ethylene production in climacteric fruits that leads to changes such as color, flavor and organoleptic components triggered by exogenous and endogenous ethylene (Barry and Giovannoni, 2007). As ripening is the last stage of fruit maturation, elevated respiration rate and ethylene production lead to an increased ripening rate, and reduced shelf-life and commercial value of the fruit. Therefore, it is important to control postharvest fruit ripening to prolong shelf-life and maintain quality. The use of ethylene inhibitor 1-methylcyclopropene (1-MCP) has allowed the export of horticultural produce to long distant markets, which has significant economic (Mattheis, 2008;Watkins, 2006) and social impacts on exporting and importing countries, respectively. The beneficial effects such as extended shelf-life and reduced physiological disorders were reported in 1-MCP-treated fruits (Blankenship and Dole, 2003;Watkins, 2006).
Despite these benefits, the effect of 1-MCP has been variable in climacteric fruits such as avocado, banana, and pear (Watkins, 2006). Both preharvest and postharvest factors could make 1-MCPtreated fruits fail to recover their ripening ability post storage (Dong et al., 2014;Wang and Sugar, 2015). Lack of ripening in 1-MCP-treated fruits was described as a major setback for commercialization of the treatment in banana (Harris et al., 2000;Watkins, 2006) and pear (Chiriboga et al., 2011). In apples, 1-MCP induced physiological disorders (Watkins and Nock, 2012) and premature ripening (DeEll et al., 2016). Until these inconsistencies are understood and corrected, 1-MCP will

Fruit Maturity
In the later stages of fruit maturation, there, is an increase in respiration rate and ethylene production prior to ripening initiation in climacteric fruits (Barry and Giovannoni, 2007). This increase in ethylene production is due to higher expression of ethylene receptor genes and an increase in the activity of their related enzymes, which leads to fruit ripening (Chiriboga et al., 2013;Yang et al., 2013). Thus, the ripening of climacteric fruits leads to the reduction of ethylene receptor proteins in cells (Kevany et al., 2007). Generally, the quantities of ethylene receptors are not reported in the literature. However, the expression of their genes and related enzymes are well documented. It was shown that the expression of ethylene receptor genes increases with advanced maturity and ripening (Thongkum et al., 2018), indicating that ethylene receptors in immature and overripe fruits, may be less compared to matured green fruits.
Since 1-MCP reduces fruit ripening by binding ethylene receptors and inhibiting ethylene action in a signaling pathway (Sisler and Serek, 1997), its efficacy may depend on receptor quantity or availability. There may be less 1-MCP binding in immature or overripe fruit. In bananas, it was shown that 1-MCP efficacy depends on maturity (Harris et al., 2000;Moradinezhad et al., 2008). The authors reported that more matured fruit did not respond to 1-MCP compared to less matured ones. Therefore, a commercial consignment of banana will have fruit with different maturities, which will lead to variability in 1-MCP efficacy (Harris et al., 2000). This could be attributed to that matured banana fruit are sensitive to 1-MCP compared to less matured. However, it is known that ripe fruits such as avocado and banana do not respond to 1-MCP. Therefore, 1-MCP should be applied to preclimacteric matured fruit as opposed to ripe or overripe ones.
In 1-MCP-treated avocado, early harvested fruit (firmness, 200 N) ripened slower compared to late-harvested (firmness, 180 N), indicating longer shelf-life (Pereira et al., 2014). A similar effect was reported in 1-MCP-treated pears (Escribano et al., 2016;Villalobos-Acuna et al., 2011). These findings suggest that the activities of ethylene biosynthesis enzymes are high in lateharvested fruit, concomitant with high ethylene production (Bulens et al., 2012), leading to reduced 1-MCP efficacy. Due to 1-MCP and ethylene competitively binding the same sites or receptors (Sisler et al., 2006;Sisler and Serek, 1997;Watkins, 2006), high ethylene production indicates fewer free receptors (Kevany et al., 2007) for 1-MCP binding. Therefore, partial response to 1-MCP may suggest that ethylene receptors were less at the time of treatment application due to ripening.
Although the role of maturity on 1-MCP efficacy is extensively documented, the interaction between internal ethylene concentration (IEC) and 1-MCP in these studies is limited. An internal ethylene concentration, 0.3 μL L −1 , was able to reduce the effectiveness of 1-MCP in pear fruit (Villalobos-Acuna et al., 2011). Furthermore, Yu and Wang (2017) indicated that 1-MCP combined with ethylene at the ratio of 1:1 (0.3:0.3 µLL −1 ) allowed the fruit to recover their ability to ripen compared to those treated with 1:5 (0.3:1.5 µLL −1 ) in pear fruit. In banana, 1-MCP and ethephon (liquid form of ethylene) at 50 µLL −1 : 400 nL L −1 recovered their ability to ripen uniformly compared to those treated with 100 µLL −1 : 400 nL L −1 (Zhu et al., 2015). The same trend was reported in tomato when 100 µLL −1 ethylene was combined with 500 nL L −1 1-MCP compared to 10 µLL −1 :500 nL L −1 . This indicated that the ripening of 1-MCP-treated fruit at shelf-life depends on the initial level of endogenous ethylene prior to storage. However, the level of ethylene that would simultaneously allow for 1-MCP efficiency in prolonging storage and uniform ripening at shelf-life is unclear. Therefore, the internal ethylene concentration during fruit maturation needs to be monitored to predict 1-MCP responses (Villalobos-Acuna et al., 2011) and the variable effect. This will be important in fruits such as avocado and banana, and any other intended for long-term cold storage (apple and pear), which are harvested mature-green, unripe, however, once the ripening process has started it cannot be slowed. In other climacteric fruits, especially those harvested ripe like peaches and mangos, it is possible that the 'optimum' or 'commercial' harvest stage does not mean that IEC is not enough to compete with 1-MCP. Therefore, considering the internal concentration of ethylene at harvest will help fruit industries to reconsider the 'recommended' 1-MCP concentration for the fruits and establish the correct concentration based on fruit maturity.
In a banana study, 500 nL L −1 1-MCP effectively prolonged the shelf-life of the cultivar 'William,' however, the treatment-induced uneven chlorophyll degradation (Harris et al., 2000). In a different study, 400 and 600 nL L −1 concentrations delayed fruit ripening in 'Brazil' banana, compared to 200 nL L −1 , however with uneven color change (Zhu et al., 2015). At lower concentration (30 nL L −1 1-MCP) banana fruit had shortened shelf-life compared with higher concentration. However, higher concentrations reduced fruit quality by inducing uneven ripening (Zhu et al., 2015). The aqueous 1-MCP (50-600 μg L −1 ) delayed ripening in tomato . It was reported that exposure of 200 μg L −1 1-MCP is enough to bind ethylene receptors completely and rapidly, indicating concentrations of 400 and 600 μg L −1 will be in excess and may impede the ripening capacity of the fruit . In the pear study, Escribano et al. (2017) observed the least softening inhibition in fruit treated with 250 μg L −1 1-MCP compared with 500, 750 and 1000 μg L −1 .
To reduce the effect of higher 1-MCP concentration in banana, fruit were fumigated with 400 nL L −1 1-MCP after dipping in 50 μL L −1 ethephon for 1 min, which prolonged fruit shelf-life and ensured full ripening with uniform color change (Zhu et al., 2015). Similarly, the combined application of 0.3 µLL −1 1-MCP and 0.3 µLL −1 ethylene allowed the pear fruit to recover their ability to ripen (Yu and Wang, 2017). These results indicated that ethylene can be used to reduce 1-MCP concentration that will bind to ethylene receptors. How, more research is required to establish application timing of ethylene and 1-MCP. This will indicate which treatment between 1-MCP and ethylene to apply first.

Genotypes
The effect of genotypes on 1-MCP efficacy has been widely investigated (Dong et al., 2013;Pan et al., 2016). The studies showed that genotypes differ in their sensitivity to 1-MCP. In plum (Prunus salicina), 500 μL L −1 1-MCP inhibited skin color change in 'Gaixian,' with no effect in 'Aozhou 14' and 'Yuhuang' while there was a delayed ethylene production and softening in 'Gaixian' and 'Aozhou 14,' compared to 'Yuhuang' (Pan et al., 2016). These results indicated that 1-MCP delayed ripening rate in 'Gaixian' and 'Aozhou 14' more than in 'Yuhuang' plum. In a tomato study by Sabir et al. (2012), they compared the efficacy of 1-MCP in different genotypes and reported that 'Target' responded better to the treatment compared to '601,' '602,' and '603' cultivars. Different genotypes might have a different number of ethylene receptors (Blankenship and Dole, 2003;Jung and Watkins, 2014) and internal ethylene concentration (Watkins and Nock, 2012) which affect the efficacy of the 1-MCP (Jung and Watkins, 2014;Zhang et al., 2011).
In addition, distinct quality characteristics in various genotypes might attribute to variability in 1-MCP efficacy (Wei et al., 2010). Genotypes with thin peel may have a good 1-MCP sorption capacity compared to those with thick peel Chope et al., 2007). Dong et al. (2013) observed that 1-MCP diffusion was rapid through spinach and bok choi leaves compared to tomato and avocado pericarp tissues.
The chemical composition of cultivars may also modulate the influx of 1-MCP into the tissue (Chope et al., 2007). The applied and diffused 1-MCP may differ in respect of cultivars due to different insoluble dry matter components (Cools et al., 2011;Nanthachai et al., 2007), indicating that 1-MCP is also bound by non-target sites (Huber et al., 2010;Nanthachai et al., 2007). Moreover, the sorption rate and capacities differ based on fruit tissues (Choi et al., 2009;Nanthachai et al., 2007). 1-MCP was highly sorped in oily fruit such as avocado compared to tomato Dong et al., 2013). The nonspecific targets such as lipids modulate the efficacy of 1-MCP (Nanthachai et al., 2007). Therefore, 1-MCP efficiency may be compromized due to free ethylene receptors as the recommended concentration for ethylene binding will be reduced.
In banana studies, 1-MCP induced uneven ripening in 'Williams' (Harris et al., 2000), 'Brazil' (Zhu et al., 2015), 'Prata' (de Melo Silva et al., 2004) bananas. This probably suggests that the uneven ripening phenomenon in 1-MCP-treated bananas is not linked to genotypes. To the best of our knowledge, no data exist to suggest that chlorophyll degradation or metabolism in different banana genotypes differ in an optimum or average ripening condition. However, it has been shown that chlorophyll degradation in banana (Musa, AAA group) and plantains (Musa, ABB group) differ under elevated CO 2 (Song et al., 2015) and temperature (Yang et al., 2009). Therefore, it will be interesting to test whether chlorophyll metabolism in banana and plantain treated with 1-MCP is similar. This will also indicate whether the sensitivity of the two genotypes to 1-MCP are the same, which could be an interesting model to understand 1-MCP physiology in banana fruit.

Packaging Material
For gaseous and aqueous applications, 1-MCP is effective at the lower concentrations of 2.5 nL L −1 −1 μL L −1 (Nanthachai et al., 2007;Watkins, 2006) and 50-600 μg L −1 , respectively. Although the optimum concentrations for gaseous 1-MCP differ among fruits and from country to country (Ambaw et al., 2013a), its diffusion to fruit depends on the container design, fruit arrangement and placement within cool a room (Ambaw et al., 2014). Additionally, fruit packaging material may influence the concentration delivered to fruit and consequently 1-MCP efficacy. In fact, packaging materials adsorb 1-MCP concentration (Ambaw et al., 2013a(Ambaw et al., , 2014Rodríguez-Pérez et al., 2009) with corrugated fiberboard boxes adsorbing 1-MCP more compared to the corrugated high-density polyethylene (HDPE) boxes (Rodríguez-Pérez et al., 2009). Similarly, wooden boxes adsorbed 1-MCP more compared to plastic bins (Ambaw et al., 2013a). The 1-MCP adsorption by wooden boxes is attributed to the fact that they are made from cellulose fiber and glucose-based compounds in the cell walls of plant adsorb 1-MCP. This suggests that not all the applied 1-MCP concentrations diffuse into the fruit when applied on fruit packaged with wooden-boxes. Therefore, non-wooden containers such as plastic bins must be used when 1-MCP is applied (Ambaw et al., 2013a(Ambaw et al., , 2014 to mitigate the variable effect.

Application Method
The effect of 1-MCP on fruit depends on the application used, which can either be in an aqueous or gaseous form (Escribano et al., 2017;Pongprasert and Srilaong, 2014). In the gaseous phase, 1-MCP gas is released from aqueous dissolution in a sealed container with fruits (Ambaw et al., 2013a(Ambaw et al., , 2013b. In this phase, a small amount of water is added to 1-MCP powder to allow for its ingress and diffuse to fruit (Ambaw et al., 2013a(Ambaw et al., , 2013bDong et al., 2013). Whilst in an aqueous mode, fruits are submerged in a solution of 1-MCP formulation . The application of 625 μg L −1 aqueous 1-MCP was comparable to 500 nL L −1 gaseous 1-MCP, which effectively delayed the ripening rates and preserved avocado and tomato quality . In pear fruit, 250-1000 μg L −1 and 0.6 μL L −1 aqueous and gaseous 1-MCP, respectively, reduced ripening rate and prolonged shelf-life (Escribano et al., 2017). This suggests that aqueous mode requires more 1-MCP concentration compared to gaseous mode to be effective. This difference was ascribed to differences in diffusion rate. Since the immersion time in aqueous mode (1-5 min) is less than that in gaseous mode (6-24 h), the diffusion rate is likely to be slow in aqueous mode (Choi et al., 2009). Therefore, higher concentrations probably increase the rate. Notwithstanding these differences, an aqueous mode was more consistent in delaying ripening rate compared to gaseous 1-MCP in avocado (Pereira et al., 2014), pear (Escribano et al., 2017) and banana (Pongprasert and Srilaong, 2014). There is, probably, a greater potency in 1-MCP applied in an aqueous mode compared to gaseous. The limited information on aqueous 1-MCP effect on fruit postharvest quality and comparative studies between the modes make it difficult to exploit the mode in the fruit industry. However, the application of 1-MCP in aqueous mode is fast and easy and does not require a sealed room or tent for application (Escribano et al., 2017). However, facilities like water tanks are needed, which can make the mode less practical, especially in fruits that are not hydrocooled. This could be one of the reasons why aqueous is not the dominant mode of application in fruit industries.
The application of 1-MCP in both modes requires a certain exposure time, sometimes longer depending on the genotype, for it to be effective in its ingress or diffusion to fruit. Submerging pear fruit in 250 μg L −1 1-MCP for 1 min or 45 seconds ripened later compared to 15 and 30s (Escribano et al., 2017). In avocado and tomato, submerging fruits in a 1-MCP solution for 1 min reduced ripening rate Pereira et al., 2014;Zhang et al., 2011). In the gaseous application method, treatment duration takes about 12 to 24 h (Blankenship and Dole, 2003). There is limited information on the optimum immersion time in aqueous mode, however, Escribano et al. (2017) concluded that 'Bartlett' pear be treated with 500 μg L −1 for 1 min to slow ripening rate.
The previous study by Singh and Pal (2008) reported that the efficacy of gaseous 1-MCP to prolong shelf-life and preserve guava quality depends on exposure time. In their results, these authors found that exposing the fruit to 300 nL L −1 1-MCP for 24 h prolonged shelf-life, whilst 600 nL L −1 was effective when subjected for 12 h. Commercially, gaseous 1-MCP is used due to its known protocol in various climacteric fruits and simplicity when dealing with a large bulk of fruit. However, there are limited studies that investigated the effect of the application method on fruit ripening recovery. Aqueous 1-MCP has potential in industries where fruit are sanitized before packaging, as it would be easy and practical due to sanitization solutions such as calcium chloride being applied in combination with 1-MCP. For example, aqueous 1-MCP was found to be compatible with chloride in tomatoes (Choi et al., 2009). However, more research is warranted to establish a proper protocol for aqueous 1-MCP.
Partial immersion of fruit in aqueous 1-MCP led to asynchronous ripening in avocado and tomato , indicating that 1-MCP efficacy is influenced by exposure time or surface contact. Moreover, full fruit immersion in aqueous 1-MCP was effective in reducing ripening and extending shelf-life compared to partial immersion in avocado and tomato . Similarly, banana fruit fumigated with gaseous 1-MCP had uneven ripening compared to those treated with 1-MCP microbubble technology, a technique that delivers a dissolved gas in an aqueous solution, due to uneven contact with fruit surface (Pongprasert and Srilaong, 2014). Therefore, to avoid the variable effect, care must be taken to ensure the exposure of 1-MCP to the whole fruit surface. In general, the application of 1-MCP in a liquid phase and using microbubble technology allows for uniform surface contact. However, costs associated with microbubble and its complexity regarding installation might be expensive for commercial applications.

Application Time
The time between harvest and application is crucial for the efficacy of 1-MCP treatment (DeEll et al., 2008). This could be attributed to the fact that taking too long before the 1-MCP application might allow the fruit to accumulate internal ethylene leading to fruit partial response to 1-MCP. To mitigate variable effect or lack thereof, researchers sought to manipulate the application timing of 1-MCP onto the fruits.
Little is known about the exact time for postharvest 1-MCP application on most fruits (DeEll et al., 2016). In 'Cortland' and 'Law Rome' apples, delaying 1-MCP treatment by 7 and 14 d led to a severe incidence of superficial scald compared to 3 d (Jung and Watkins, 2008), indicating that early application maximizes the control of superficial scald. In 'Barlett' pear, application of 1-MCP on the fruit 1 and 3 d after harvesting, reduced the development of superficial scald compared to 7 d application (DeEll and Ehsani-Moghaddam, 2011). Lu et al. (2013) showed that delaying 1-MCP application by 4 and 7 d increased the superficial scald incidence in 'Cortland' and 'Delicious' apples. Application of 1-MCP on 'Booth 7' avocado in both aqueous and gaseous form within 48 h postharvest was effective in delaying ethylene production (Pereira et al., 2014). Moreover, 'McIntosh' apple ripening can be effectively controlled if 1-MCP is applied no later than 3 d postharvest (DeEll et al., 2008). This indicates that early postharvest 1-MCP application helps maintain fruit quality. However, it must be noted that, at a commercial level, 1-MCP application is delayed until storage rooms are filled with fruit during the harvesting period (DeEll et al., 2016;Nock and Watkins, 2013). In pear fruit, it might take approximately 7-12 d to fill storage room (Wang and Sugar, 2015), which makes harvesting and 1-MCP exposure on the same day impossible (DeEll et al., 2016;Nock and Watkins, 2013). It is worth noting also that due to different metabolic rate and ethylene production in various fruits, the timing of 1-MCP application might differ based on the genotype or commodity .
Multiple application techniques were used to prolong shelf-life and minimize premature ripening in apple fruit (DeEll et al., 2016;Watkins and Nock, 2012). In this technique, a lower concentration of 1-MCP is applied and as the treatment begins to dissipate, 1-MCP is applied again. DeEll et al. (2016) reported higher firmness retention in apples that received 1-MCP prior and post storage, indicating that multiple applications reduce fruit softening. The same trend was reported in apples after 1, 4 and 8 d of application . Although multiple or repeated 1-MCP application improves its efficacy, there is limited information on the mode of action of this practice. However, 1-MCP-treated climacteric fruits regain their ethylene sensitivity for ripening initiation after a certain storage duration by synthesizing ethylene receptors (Watkins, 2006). Therefore, multiple applications of 1-MCP during the storage period might block the synthesis of new receptors. In addition, this practice has been mostly reported on pome fruit, with limited information on fruits such as banana, avocado and papaya which lose their ability to ripen uniformly after 1-MCP application. This technique could be used in these fruits as they are highly sensitive to 1-MCP and the recommended concentrations appear to affect their ripening ability during shelf-life.

Application and Storage Temperatures
Depending on the fruit and cultivar, 1-MCP is applied at either lower or higher temperatures (Blankenship and Dole, 2003). For instance, the application of 1-MCP onto banana fruit at a higher temperature (25°C), had a pronounced effect in prolonging its storage and shelf-life (Zhu et al., 2015). In contrast, 1-MCP applied at a lower temperature (0°C) reduced the ripening rate compared to 8°C in plum (cv. Sungold) (Velardo-Micharet et al., 2017). However, 1-MCP applied at 0°C had no pronounced effect on 'Bartlett' pear (Villalobos-Acuna et al., 2011). This contradictory effect might be due to the ability of genotypes to withstand cold stress and therefore chilling-susceptible fruits may not respond to 1-MCP at lower temperatures (Qiu et al., 2009). However, this probably depends on storage duration (Villalobos-Acuna et al., 2011). It has been suggested that lower temperatures might influence the interaction of 1-MCP with receptors by slowing its movement to the receptors or binding sites and therefore reducing affinity (Villalobos-Acuna et al., 2011). However, to the best of our knowledge, there is no data to support this hypothesis. What influences 1-MCP to be efficient when applied at lower or higher temperatures is unclear. Depending on genotype and storage duration, it appears that for the optimum response, 1-MCP must be applied at higher temperatures. More research is warranted to investigate 1-MCP sorption under cold storage.
The storage temperature is used in combination with 1-MCP to extend postharvest life and preserve fruit quality. Although optimum storage temperatures to extend postharvest life in climacteric fruits are generally known, there is limited information on the optimum storage temperature for 1-MCP-treated fruits. Storage temperatures influence 1-MCP efficacy in various fruits. In a previous study, Qiu et al. (2009) investigated the impact of 1-MCP on 'Sunrise' apple at different temperature regimes. They reported that 1-MCP was effective in reducing ethylene production and softening at higher temperatures (22°C) compared to lower temperatures (5°C). Similarly, Yang et al. (2013) reported that at 20°C, 1-MCP reduced respiration rate, ethylene production, ethylene biosynthesis and perception genes in 'Golden Delicious' apple. Likewise, softening and the expression of cell wall metabolism genes were inhibited in 1-MCP-treated 'Fuji' and 'Golden Delicious' apples at 20°C (Wei et al., 2010). In 'Fuji' apple, 1-MCP reduced softening, organic acids, and the expression of organic acids metabolism genes at 20°C (Liu et al., 2016). A similar effect of 1-MCP at higher or holding temperatures between 20 and 24°C was reported in avocado (Pereira et al., 2015(Pereira et al., , 2013 In contrast to higher storage temperatures, 1-MCP reduced ethylene production and the activities of 1-aminocyclopropane-1-carboxylic synthase (ACS) and oxidase (ACO) enzymes at 1°C in 'Golden Smoothe' apple (Vilaplana et al., 2007). The effects of cold storage on 1-MCP efficacy were investigated in 'D' Anjou' pear (Xie et al., 2014). It was reported that storage of 1-MCP-treated fruit at −1.1°C effectively reduced respiration rate, ethylene production and chlorophyll loss compared to 1.1°C. However, higher firmness was retained at 1.1 compared to −1.1°C. Moreover, 1-MCP was effective in reducing the incidence and prevalence of superficial scald at −1.1 compared to 1.1°C. Likewise, the accumulation of α-farnesene and conjugated trienols was effectively inhibited by 1-MCP at −1.1 compared to 1.1°C (Xie et al., 2014).
The application of 1-MCP onto fruit induces uneven recovery during ripening in avocado and banana fruits (Blankenship and Dole, 2003;Watkins, 2006). It seems storage temperature influences the fruit ripening recovery from 1-MCP. For instance, Xie et al. (2014) showed that rate-limiting enzymes (ACS and ACO) in ethylene biosynthesis were inhibited in 1-MCP-treated fruit at −1.1 compared to 1.1°C. It was proposed that to avoid uneven recovery in 1-MCP-treated pear, fruit should be stored at 1.1 instead of commercially recommended −1.1°C (Xie et al., 2014). At a higher temperature (25°C), 1-MCP-treated banana (cv. Brazil) fruit recovered unevenly from 1-MCP treatment (Zhu et al., 2015). However, few studies are reporting the efficacy of 1-MCP on banana fruit at cold storage (13-15°C). Therefore, to increase 1-MCP efficacy in various treated fruit or cultivars, the optimum holding or storage temperature should be determined. Such knowledge will help packagers in planning postharvest treatments to prolong storage life and maintain fruit quality.
The discrepancies on the efficacy of 1-MCP-treated fruit stored at various temperatures might be that the formation of new receptors or binding sites is temperature-dependent (Watkins, 2006), although there is limited information on the influence of temperature on the synthesis of new receptors (Villalobos-Acuna et al., 2011). It is unclear whether temperature allows for the release of 1-MCP from the receptors or allows for the synthesis of new receptors (Villalobos-Acuna et al., 2011). However, the available literature suggests that the synthesis of new receptors in 1-MCP-treated fruit may be rapid at higher (in tropical and subtropical climacteric fruits) or sub-zero (temperate climacteric fruits) temperatures. This indicates that the synthesis of new receptors in 1-MCP-treated fruit is storage-duration dependent. This area needs to be investigated to gain insight into the role temperature plays on the efficacy of 1-MCP.

Storage Atmosphere
The use of storage atmospheres such as controlled atmosphere (CA), modified atmosphere packaging (MAP) and hypobaric storage has been shown to complement 1-MCP in extending storage life and preserving produce quality ( Table 1). The use of 1-MCP-treated fruit under the storage atmosphere, especially CA, has been limited to apple and pear fruits. In fact, CA is one of the mostly used and tested storage atmospheric in 1-MCP-treated fruit. Application of 1-MCP-treated apple (cv. Jonagold) under CA at 1°C reduced the respiration rate and ethylene production (Bekele et al., 2015). In 'Abbe Fetel' pears, treatment with 1-MCP under CA, reduced softening and color change when stored at both 0.5 and 1°C (Rizzolo et al., 2015).
It appears that 1-MCP efficacy under CA is influenced by the levels of oxygen (O 2 ) and carbon dioxide (CO 2 ), with the levels dependent on fruit type and cultivars. For instance, CA with 1 kPa O 2 + < 0.5 kPa CO 2 inhibited 1-MCP-treated 'Fuji Suprema' apple ripening rate, however with increased cavities compared to 1-MCP-non-treated (Weber et al., 2017). In 1-MCP-treated 'Empire' apple under CA with 2 kPa O 2 + < 2 kPa CO 2 and 3 kPa O 2 + < 2 kPa CO 2 , the fruit had a slightly similar softening rate at 9-month-storage (Fawbush et al., 2009). However, the same authors reported that at 4.5-month-storage, 1-MCP-treated fruit softened faster under CA with 3 kPa O 2 than with 2 kPa O 2 . In 'Cripples Pink' apple, 1-MCP-treated fruit maintained the levels of phenolics and antioxidant capacity constant under CA with 2 kPa O 2 + < 2 kPa CO 2 during storage in both peel and flesh at 0°C compared to non-treated (Hoang et al., 2011). In contrast, 1-MCP-treated 'Empire' apple under CA with 2 kPa O 2 + < 2 kPa CO 2 had reduced total antioxidants and flavonoids in a peel at 0.5°C, whereas the variables increased in flesh (Fawbush et al., 2009). Therefore, although CA combined with 1-MCP controls postharvest disorders, the effect could be altered due to oxygen and carbon dioxide levels. The CA; controlled atmosphere; DCA; dynamic controlled atmosphere; MAP; modified atmospheric packaging optimum or correct levels of O 2 and CO 2 under CA for 1-MCP-treated fruit is limited. More work is needed to investigate the influence of O 2 and CO 2 levels in CA on the efficacy of 1-MCP for different genotypes.
The effect of dynamic controlled atmosphere (DCA) storage on 1-MCP efficacy was reported in apple fruit (Weber et al., 2017). The reduced ethylene production and the activity of the ACC oxidase enzyme were reported in 1-MCP-treated 'Fuji Suprema' apple under DCA with respiratory quotient (RQ) of 1.5 and 2 at −0.5°C (Weber et al., 2017). However, 1-MCP-treated fruit under DCA respiratory quotient 2 (1-MCP + DCA-RQ2) had increased ethylene and lower titratable acidity compared to DCA with a respiratory quotient of 1.5 (1-MCP + DCA-RQ1.5) at shelf-life (20°C). This was ascribed to higher ethanol, acetaldehyde and ethyl acetate produced in 1-MCP+DCA-RQ2 treated fruit (Weber et al., 2017), suggesting that 1-MCP efficacy is reduced under anaerobic conditions.
The success of MAP technology to improve 1-MCP efficacy has been established in banana (Ketsa et al., 2013) and mango (Vázquez-Celestino et al., 2016). Ketsa et al. (2013) reported 100-day storagelife of banana fruit stored in MAP+1-MCP treatment. These authors reported that ethylene production and softening were delayed in this treatment. Similarly, softening was delayed in mango treated with MAP+1-MCP (Vázquez-Celestino et al., 2016). These studies demonstrated that 1-MCP+MAP technology is useful in extending shelf-life of climacteric fruits. However, more work is needed to investigate the effect of 1-MCP in combination with MAP in other genotypes. In addition, the investigation into the mechanisms underlying the synergy of MAP+1-MCP is still obscure, especially in banana fruit.

Future Prospects
Due to the demand for high-quality fruit in the market, the use of ethylene antagonists such as 1-MCP is significant in these markets. However, the efficacy of 1-MCP depends on factors such as maturity, genotypes, application time and methods, storage temperature and atmosphere and packaging material. Moreover, these factors seem to modulate or regulate the response of 1-MCP in produce. In various 1-MCP-treated fruit, it is apparent that the most important factor to consider in improving the efficacy of 1-MCP is phytohormone ethylene induced by harvest maturity. Due to a wide range of ethylene concentrations in various fruits or specific cultivars, it is important to determine the appropriate maturity for a specific species or cultivar. Although a significant amount of research has been done in understanding the appropriate harvest time of fruits using various indices, there is still a need to develop harvest model using the internal ethylene concentration to understand the 1-MCP response. This would allow optimum response and the fruit the ability to recover after certain storage time. It is further reported that 1-MCP blocks ethylene receptors or binding sites irreversibly (Watkins, 2006), allowing the fruit to synthesis new receptors to recover their ripening ability after storage time. However, there is still little information on how this recovery of 1-MCP or synthesis of new binding sites comes about. Therefore, future research should focus on understanding the biochemical factors that allow for the synthesis of new receptors. One such study will be to understand whether fruit reserves play a significant role in the recovery of fruit from 1-MCP and integrate that assessment with the 'optimum harvest maturity.' Such information will help in developing fruit cultivars with improved reserve accumulation capacity.
In addition, one of the most critical factors that affect the commercialization of 1-MCP in banana fruit and some cultivars of pears and avocado in the world is the color change and softening asynchrony. This is an urgent unresolved problem in the fruit industry. Although there is a growing understanding that simultaneous application of 1-MCP with ethylene has the potential to mitigate uneven ripening, the mechanism these gases use to rather allow a transitory effect of 1-MCP in fruit is unclear. The in-depth understanding of the mechanisms these gases use may supply important knowledge advancement, ultimately solving the problem in various other fruits in the industry. Importantly, it may also help to avoid the problem at preharvest stage. Furthermore, the role of 1-MCP on chlorophyll degradation is less explored and could be one of the most interesting opportunities for researchers. Most of the work focused on the external assessment of color in 1-MCPtreated fruit without integrating it with chlorophyll degradation and pigment metabolism.

Conclusion
The objective of this review of the literature was to critically discuss factors that cause variable effects in 1-MCP technology. To present techniques that have the potential in reducing variable effects in 1-MCP technology. This review showed that factors such as harvest maturity, genotypes, storage temperature, storage atmosphere, application and exposure time, and fruit packaging material affect 1-MCP efficacy. The increase in the understanding of these factors will help fruit industries in optimizing 1-MCP technology and increasing their efficiency and commercialization. The role of 1-MCP in fruit postharvest physiology, biochemistry, ripening and senescence etiology is important. However, the lack of responsiveness and recovery poses a serious threat to the commercialization of this technology. This review showed that techniques such as manipulating harvest time and combined application of 1-MCP, and some postharvest technologies maximizes 1-MCP efficiency.

Disclosure Statement
No potential conflict of interest was reported by the author(s).