1-Methylcyclopropene Maintains Firmness and Peel Color and Reduces Decay Area of Artificially Wounded Fruits in Mature Japanese Pear (Pyrus pyrifolia Nakai ‘Shizukisui’)

Recently, there has been an increasing need to maintain the freshness of fruits and vegetables for longer durations due to expanding distribution networks, differences in cultivars and harvest times, and diversification of sales and processing. Fruits, such as apples and persimmons, are matured and aged with the participation of ethylene, an important plant hormone. Over the years, ethylene production and activity inhibitors to maintain the freshness of fruits during distribution have received considerable attention. The physiological response of plants to ethylene is triggered by the binding of ethylene to its receptors, which suppress the expression of genes involved in maturation. However, they are inactivated due to binding to ethylene, resulting in the rapid progression of fruit maturation and aging. Recently, aminoethoxyvinylglycine (AVG) and 1-methylcyclopropene (1-MCP) have been identified as potent inhibitors of ethylene production and activities. For instance, AVG prolongs the preservation period of apples (Yuan & Carbaugh 2007), and prevents fruit dropping in apple and satsuma mandarin (Kondo & Takahashi 1989; Kondo & Hayata 1995; Ogata et al. 1997). Recently, post-harvest application of acibenzolar-S-methyl has been shown to modulate the glutathione-ascorbate cycle to delay the senescence of ‘Doctor Jules Guyot’ pears (Huang et al. 2022). Postharvest treatments with the ethylene antagonist 1-MCP have been widely studied as a method of delaying ripening (Watkins 2008), which could be effective in maintaining the quality of fruits such as apple, kiwifruit, avocado, banana, nectarine, peach, tomato, and other climacteric fruits.

The effectiveness of 1-MCP differs, ranging from almost compromising the ability of the fruit to ripen properly to relatively small and transitory effects. Although 1-MCP treatment has been extensively applied to apple preservation, studies on its effect on Japanese pear are limited. Presently, there is a need to lengthen the preservation time and quality of Japanese pear (Pyrus pyrifolia Nakai) by maintaining the texture that was at harvest. Moreover, the cultivar and climatic conditions may affect the response of the pear fruit to 1-MCP treatment. For instance, early-maturing Japanese pear cultivars have low storability and shelf life of approximately 5 days, indicating the need for the fruits to be harvested before maturity to enable the distribution of wholesome fruits to consumers because they may get spoiled before they reach the retail market. However, it is believed that 1-MCP treatment can improve and maintain the quality of pear fruits long after harvest, which will promote the shipment of suitable and high-quality fruits. Several studies have examined the effect of 1-MCP treatment on different Pyrus communis pear cultivars (Calvo & Sozzi 2004; Ekman et al. 2004; Bai et al. 2006; Chino et al. 2010; Chiriboga et al. 2013; Huang et al. 2022). Additionally, some studies have reported the effect of 1-MCP on the preservation of Asian pear species, including Japanese cultivars (Li & Wang 2009; Lee et al. 2017). Sakamoto et al. (2021) reported that 1-MCP treatment suppressed the respiration rate and ethanol production rate and preserved the quality of Japanese pear fruit. However, studies on the effect of 1-MCP on rind color and fruit decay, and the effects of pre-cooling temperature at different maturity stages of Japanese pear fruit are limited. Moreover, since the effects of 1-MCP may be affected by the genotype and maturity stage, it is necessary to evaluate different genotypes and maturity stages (Blankenship & Dole 2003; Itai 2004). The Japanese pear ‘Shizukisui’ used in this study was generated by irradiating the ‘Kisui’ cultivar with gamma rays to obtain black spot disease resistance (Sawano et al. 2011).

The objective of this study was to examine the effect of 1-MCP treatment on the firmness and rind color of the Japanese pear ‘Shizukisui’ at two different maturing stages. Additionally, the suppressive effect of 1-MCP on fruit decay area and decay rate in wounded Japanese pear ‘Shizukisui’ and ‘Kosui’ was examined.

In the present study, we examined the responses of immature and mature fruits of Japanese pear to 1-MCP treatment. The results of the study showed that 1-MCP treatment for 24 h before storage suppressed peel color change and maintained the firmness of ‘Shizukisui’ fruit, which was consistent with previous findings in pear (Hiwasa et al. 2003; Kubo et al. 2003). Particularly, it is believed that 1-MCP treatment can extend the freshness, firmness, and peel color of mature fruits, as well as inhibit senescence in several fruits by competing with ethylene for attachment to the enzyme receptor (Kashimura 2005). The findings of the present study show that 1-MCP treatment extended/maintained fruit firmness and peel color in the presence or absence of ethylene treatment, indicating that its effect might be either through inhibition of ethylene or stimulation of some metabolic systems. Presently, both climacteric and nonclimacteric Japanese pear (P. pyrifolia Nakai) cultivars have been identified (Kitamura et al. 1981). Itai (2004) reported that there are significant variations in the ethylene production of Japanese pear cultivars, ranging from low to high. Generally, early-maturing cultivars tend to produce more ethylene than late-maturing cultivars; however, ‘Kosui’ has an ethylene production rate of approximately 1–2 μL·kg⁻¹·h⁻¹. Thus, it seems that ‘Shizukisui’ and ‘Kosui’ are close to nonclimacteric fruits; therefore, it is possible that the nonsignificant effect of exogenous ethylene on peel color and firmness could be due to the characteristics of the fruits used in this study. Moreover, previous findings suggest that nonclimacteric fruits do not require ethylene for ripening. In contrast, some studies indicated that nonclimacteric fruits may also produce ethylene to promote fruit ripening postharvest (Tatsuki 2007). Perkins-Veazie et al. (1996) reported that ethylene production by strawberry, which is a nonclimacteric fruit, is extremely low compared to that of apples, and the exposure of detached green strawberry fruit to 5,000 μl·L⁻¹ propylene failed to stimulate respiration or ethylene production, but advanced pigmentation change and fresh weight. Additionally, it is believed that the ethylene receptors in Japanese pear may differ from those of other fruits, such as apple, pear, mango, and persimmon.

1-MCP delayed the softening of kiwifruit when treated at harvest (Cantin et al. 2011). However, 1-MCP had no effect on kiwifruit during cold storage, even at high concentrations (Kim et al. 2001). By contrast, Murakami et al. (2016) reported that precooling at temperatures below 10 °C for 2 days prior to 1-MCP treatment maintained the hardness of kiwifruit ‘Rainbow Red’ up to 4 months after harvest. However, the present study's findings showed that precooling did not increase the effects of 1-MCP treatment. The differing results can be attributed to differences in the ethylene content in Japanese pear and kiwifruit. Further studies are necessary to elucidate the reasons for these differences.

Furthermore, the results of the present study showed that a combination of 1-MCP and exogenous ethylene treatment significantly increased the SSC of fruits, which was similar to the findings of Itai and Tanahashi (2008) in Japanese pears ‘Gold Nijisseiki’ and ‘Hosui’ (late-maturing cultivars). However, a previous study showed that 1-MCP treatment did not significantly affect the SSC and titratable acidity of climacteric fruits (Egea et al. 2010), whereas Lurie (2007) reported inconsistent titratable acidity levels of 1-MCP-treated fruits.

Additionally, the effect of 1-MCP treatment on the decay rate of Japanese pear was examined. Fruit decay is discussed in terms of cell wall digestion and fungal infection resistance. In pears, not all maturation-related events are necessarily ethylene dependent; however, some important cell wall-modifying enzymes for the softening process are regulated by ethylene. Hiwasa et al. (2003) indicated that 1-MCP treatment decreased the expression of the pear polygalacturonase genes. However, the expression pattern of pear endo-1,4-β-D-glucanase gene was not affected by propylene and 1-MCP treatments. The findings of the present study showed that 1-MCP treatment probably had some effect on cell wall-modifying enzymes, as evidenced by the decrease in the decay rate of 1-MCP-treated ‘Kosui’ fruits. However, it is necessary to examine the expression of cell wall modifying genes and enzymes in response to 1-MCP treatment in future studies. Although pathogen infection has been shown to facilitate fruit decay, the fruits were not inoculated with specific pathogens in this study, thus the involvement of the pathogen and the disease resistance could not be clarified. 1-MCP treatment may play a role in disease control for postharvest fruit; however, this is yet to be examined. Generally, an increase in fruit decay rate is highly correlated with storage duration. Ergun et al. (2005) reported that electrolyte leakage from 1-MCP-treated melon fruit was maintained at 20% during the first 5 days of storage, and it never exceeded 27% thereafter, whereas electrolyte leakage from fruits in the control group increased up to 35% during the storage period.

To the best of our knowledge, the present study is the first to report the effect of 1-MCP on firmness, peel color, moisture loss, SSC, and decay rate of wounded fruits of ‘Shizukisui’ pear. Overall, it is more economical to use 1-MCP in mature fruits rather than in immature ones to prolong fruit firmness and peel color. Additionally, the ability of 1-MCP to maintain fruit firmness may vary depending on the maturity stage during processing. For instance, apricot softening can be suppressed by 1-MCP treatment when the fruit is mature (Botondi et al. 2003), whereas 1-MCP treatment could not delay the ripening of early maturing tomato fruit (Hurr et al. 2005). This study revealed that the effect of 1-MCP on Japanese pear fruits at different maturation stages was similar to that of other crops (apricot and tomato). 1-MCP may affect other metabolites in addition to ethylene receptors since 1-MCP maintained fruit quality regardless of ethylene treatment. The effects of 1-MCP on antioxidant activity and other secondary metabolites need to be investigated.

In conclusion, the findings of this study showed that 1-MCP treatment maintained the firmness and peel color of Japanese pears and reduced the decay rate of wounded fruit. Additionally, 1-MCP treatment increased the SSC of the fruits. However, it is possible that 1-MCP treatment may affect other properties of Japanese pears, such as titratable acidity. Therefore, further studies are necessary to comprehensively examine the effect of 1-MCP treatment on the chemical properties of the Japanese pear and its mechanisms. Additionally, it is necessary to determine the optimal 1-MCP concentration and fruit storage conditions, such as storage temperature, to prolong fruit freshness and quality. Although the findings of the present study showed that 1-MCP treatment, among other positive effects, reduced the decay rate of wounded Japanese pears, the mechanisms of action are poorly understood. Therefore, further studies should examine the expression profiles of cell wall-modifying and ethylene synthesis genes in response to 1-MCP treatment to understand the mechanisms for preventing fruit senescence.