Pears can be processed in various ways, including canning the fruit or producing juice or syrup. They are rich in minerals and contain numerous bioactive compounds with anti-diabetic, anti-inflammatory, antibacterial, and anti-cancer properties (Öztürk et al., 2015). The ‘Docteur Jules Guyot’ pear (Pyrus communis L.) is a prominent European pear variety. This variety was introduced to China over a century ago and is primarily grown in the warm and semi-humid region of the Jiaodong area in Shandong province. Consumers favor European pears for their delicate taste and high nutritional value (Hewitt et al., 2020). However, European pears cannot be consumed immediately after picking and need to undergo ripening and softening to obtain a good taste.

During fruit ripening, chlorophyll in the pericarp is gradually broken down, while lutein and carotene in the chloroplast breaks down more slowly, so the pericarp appears red, orange, and yellow. It was discovered that the involvement of abscisic acid (ABA) significantly contributed to the enhancement of peel color modification. Anthocyanin accounts for a large percentage of the total antioxidant content in climacteric fruit figs, and it was found that ABA initiated anthocyanin biosynthesis and improved the color of figs (Lama et al., 2020). For non-climacteric fruit sweet cherry, the upregulation of PavNCED1 and genes related to the ABA signaling pathway was observed during fruit transition from the light green stage to the pink stage (Kuhn et al., 2013). Pre-harvest application of S-ABA enhanced the development of a deep orange fruit color in early maturing M7 sweet orange varieties (Rehman et al., 2018). These findings indicate that the involvement of ABA is pivotal in the fruit ripening process, specifically in facilitating color change.

Alterations in cell wall composition led to increased tenderness in the fruit’s texture. The pectin-degrading enzyme activity influenced the change of pectin content in the cell wall. For climacteric fruit blueberry, the content of water-soluble pectin (WSP) in the ABA-treated group is higher than both the control group and nordihydroguaiaretic acid (NDGA)-treated group, and the pectin content of sodium carbonate soluble pectin (NSP), enzyme-soluble pectin (ESP), cellulose (CEL), and hemicellulose (HCEL), respectively, is lower than that in the control group and NDGA-treated group (Zhou et al., 2021). In the comparison of ‘Jingbai’ and ‘Yali’ pears, it was discovered that the former had a quicker softening rate, a higher pectin solubility, a much lower NSP content, and a significantly higher WSP content, whereas ‘Yali’ pears had a firm texture, and the pectin components were slightly changed during storage (Wei et al., 2015).

Besides the alterations in physiology, the volatile aromatic substances emitted by the fruit can also reflect the ripeness and overall quality throughout the maturation process (Obenland et al., 2012). Most of the aroma compounds in fruit accumulate during the early stages of ripening and reach their peak levels during the ripening stage (Han et al., 2022). The presence of aroma significantly influences the final sensory quality and fruit preferences among consumers (El Hadi et al., 2013). The aroma of pear varieties was attributed to esters, aldehydes, alcohols, ketones, and hydrocarbons, which are responsible for the aroma of different pears. These compounds have been extensively researched, with esters being the most significant (Willner et al., 2013). When kiwifruit was stored at low temperatures, the exogenous ABA treatment could regulate the synthesis of aroma compounds, and the considerable increase in esters’ level was also connected with this ripening phase time (Han et al., 2022). It was found that the synthesis of volatile esters can improve the flavor of bananas (Mukherjee et al., 2022). During the ripening process of the ‘Nanguo’ pear, the levels of esters increase while aldehydes decrease, leading to the grassy aroma of the fruit which gradually declines, allowing its unique aroma to emerge (Li et al., 2012).

ABA plays a role in controlling ethylene production and signaling to trigger the ripening of banana fruit (Thakur et al., 2019). In climacteric fruits such as peach and kiwifruit and in non-climacteric fruits such as grape, ABA was closely associated with the ripening and softening of the fruit, not only promoting the fruit coloration but also participating in the ripening of the fruit by affecting the activity of pectin degradation enzymes, such as polygalacturonase (PG), pectate lyase (PL), pectin methylesterase (PME), and cellulase (Zhang et al., 2009b; Gambetta et al., 2010; Fullerton et al., 2020). In mango, the PG activity was increased after ABA treatment, which resulted in the softening of the fruit (Zaharah et al., 2013). Ethylene started to stimulate PL expression in the early stages of banana ripening, and even after full ripening, the banana continued to express PL at a high level (Pua et al., 2001). Certain climacteric fruits, such as tomatoes, peaches, and avocados, all have high PG activities that correlate with the fruit softening rate (Brummell and Harpster, 2001).

ABA facilitates fruit ripening by stimulating the activation of genes associated with fruit softening. In peach fruit, virus-induced gene silencing of PpPG21 and PpPG22 downregulated the expression of these genes to affect the solubilization and depolymerization of peach pectin to achieve the purpose of delaying fruit softening (Qian et al., 2021). In persimmons, ABA can induce the expression of DkACS2 and DkACO1, promote ethylene synthesis, and accelerate fruit softening, while NDGA delays their expression (Duan et al., 2015). ABA biosynthesis and catabolism in fruit cells involve both feed-forward and feedback pathways. The overexpression of SlNCED1 can increase the endogenous ABA content, and the silencing of this gene can inhibit epidermal coloring and softening in tomato fruit (Taylor et al., 2000; Zhang et al., 2009a). CYP707As can inhibit the synthesis of ABA, and the SlCYP707A1, SlCYP707A2, and pear PpcYP707A1 genes in tomatoes are engaged in the metabolic process of ABA during fruit ripening (Nitsch et al., 2009; Dai et al., 2014; Ji et al., 2014). For the non-climacteric fruit strawberry, feed-forward and feedback are closely associated with promoting FveNCED expression and inhibiting FveCYP707A4a expression (Liao et al., 2018).

The ‘Docteur Jules Guyot’ pear cannot be eaten immediately after picking. After softening, the storage time is greatly reduced, and the quality of the fruit decreases. Therefore, elucidating the role of ABA in fruit softening is crucial for promoting and extending the fruit’s shelf-life. To the best of our knowledge, the molecular mechanism underlying the softening process of ‘Docteur Jules Guyot’ pear fruit induced by ABA was limited. This research investigated the impact of ABA and NDGA on the ‘Docteur Jules Guyot’ pear, determining the fruit firmness, activity of pectin degradation-related enzymes, content of substance, and aroma compounds, thus illuminating the role of ABA in the postharvest ‘Docteur Jules Guyot’ pear softening.

Research has shown that abscisic acid (ABA) can influence the epidermal color shift that occurs during fruit ripening in various fruit species. The changes in fruit epidermal color are closely correlated with the accumulation of anthocyanins. In climacteric fruit tomato, treatments with ABA and nordihydroguaiaretic acid (NDGA) also affected this process. The exogenous application of ABA resulted in an increase in ABA content, whereas NDGA inhibited this elevation. Compared to the control group, ABA treatment induced the coloration of tomato, while NDGA treatment delayed the coloring process (Mou et al., 2016). In the non-climacteric fruit strawberry, ABA promotes fruit ripening by reducing the time required to reach the fully red stage (Lu et al., 2023). Furthermore, the h value of citrus fruit color index indicated that exogenous ABA can accelerate the coloring of citrus fruit, while NDGA treatment can inhibit fruit coloring (Wang et al., 2016). This study investigated the application of ABA, which accelerated pear fruit coloration by 4 days over the control group. This indicates that ABA can influence the epidermal color change in pear fruit ( Figure 1 ).

The degree of connection between cell walls will ultimately affect the texture of the fruit. During the maturation process, changes in pectin, hemicellulose, and cellulose are considered to be the reasons for the changes in cell wall structure during the period of firmness loss related to maturity (Chen et al., 2015). Research shows that water-soluble pectin (WSP) increases, indicating that the change of pectin was related to the long-term stored quality of fruit (Murayama et al., 2006). The modifications of the fruit cell wall vary according to species (Forlani et al., 2019). For some climacteric fruits such as kiwifruit, tomato, and avocado, pectin is depolymerized during fruit ripening, whereas in the climacteric fruit apple and non-climacteric fruit watermelon, pectin solubilization and depolymerization have a low degree of pulp-melted fruit, indicating that the firmness of the fruit depends on the integrity of the cellular connections (Brummell, 2006). After analyzing the composition of cell wall components in ‘Docteur Jules Guyot’ pear, it was observed that there was an increase in the levels of WSP and ionic-soluble pectin (ISP) content, and the content changes were the largest under the promotion of ABA ( Figure 2 ).

The associated metabolic enzymes can affect the texture of fruit through the cell wall. It has previously been reported that polygalacturonase (PG) is an essential factor related to the change of pectin components (Wakabayashi, 2000). PG activity and cell wall modification in hard-tasting fruits are lower than softer fruits. After MdPG1 was silenced, the fruit softening was slower, and the pectin content was reduced (Atkinson et al., 2012). Research has found that the transcriptional accumulation of the PG gene was well correlated with softening in pear fruit (Sekine et al., 2006). Furthermore, pectate lyase (PL) activity appears to be more widely recognized in ripe fruits (Al Hinai et al., 2021). The findings indicated that during the ripening and softening of the ‘Docteur Jules Guyot’ pear, the activities of PG and PL under ABA treatment remained high, contrasting with the fruit firmness trend ( Figure 2 ).

Fruit aroma can reflect the flavor quality of fresh fruit and has become an important factor influencing consumer choice. Fruit flavors are made up of a complex mixture of substances, and the level of different substances affects the variation of fruit flavors. It has been found that, in most fruits, volatile compounds consist mainly of esters, alcohols, aldehydes, and ketones (El Hadi et al., 2013). During the ripening of ‘Pink Lady’ apples, esters such as hexyl acetate, hexyl 2-methylbutyrate, hexyl caproate, and hexyl butyrate accounted for a relatively large proportion of the fruit volatiles (Villatoro et al., 2008). In European pear, esters have also been identified as the main volatile compound substances (Rapparini and Predieri, 2002). From our experimental results, we can see that the ester content of the fruits increased significantly after ABA treatment ( Figure 3 ). Previous studies had also found that ABA can promote the production of short-chain ester compounds in apple (Wang et al., 2018). In addition, ABA treatment also significantly increased the production of esters in fruit aroma in low-temperature-treated kiwifruit (Han et al., 2022). Therefore, ABA treatment not only promoted the softening of the pear fruit but also promoted the pear fruit to emit a stronger fruit flavor.

In European pear, the expression of genes associated with cell walls during the softening process was also studied (Fonseca et al., 2005). In non-climacteric fruit strawberry, silencing both of the PL gene FaPLC and the PG gene FaPG1 slowed the softening of the fruit (Jiménez-Bermúdez et al., 2002; Quesada et al., 2009). Similarly, we found that under the induction of ABA, the expression of PcPG1, PcPG2, and PcGAL1 increased quickly ( Figure 4 ). 9-cis-Epoxy carotenoid dioxygenase (NCED) plays a key role in ABA synthesis and has been isolated from various fruits (Burbidge et al., 1999). At the transcriptional level, the ABA content in climacteric fruit tomato was regulated by SlNCED1 (Zhang et al., 2009a). ABA accumulation was enhanced in SlNCED1 over-expressed in tomato (Ji et al., 2014). After silencing SlNCED1 in transgenic tomato, endogenous ABA content decreased and cell wall depolymerization degree and fruit ripening were also inhibited (Thompson et al., 2000). The results showed that during the ripening process of ‘Docteur Jules Guyot’ pear, under the regulation of ABA, the gene expression of PcNCED1 and PcNCED2 was upregulated ( Figure 5 ). It has been found that ABA promotes fruit ripening through a signaling cascade pathway (Leng et al., 2013). It has been predicted that NAC TFs can directly target genes encoding enzymes that catalyze changes associated with degradation of cell wall components or lignification pathways, indirectly affecting fruit texture by regulating endogenous ABA and ethylene levels, which promotes the expression of downstream genes (Liu et al., 2022). In tomato fruit, SlNOR promotes cell wall degradation by directly binding to the promoter region of the cell wall degradation-related gene SlPL and activating its expression (Gao et al., 2020). SlNOR-like 1 promotes tomato fruit softening by activating SlPG2a, SlPL, SlCEL2, and SlEXP1 (Gao et al., 2018).

The post-harvest ‘Docteur Jules Guyot’ pear was treated with abscisic acid (ABA) and nordihydroguaiaretic acid (NDGA), and the results showed that ABA treatment accelerated the reduction of fruit firmness. The changes in the contents of water-soluble pectin (WSP), ionic-soluble pectin (ISP), and covalent binding pectin (CBP) destroyed the ligation properties in the cell wall and improved the activity of related metabolic enzymes. The volatile ester content associated with pear fruits increased substantially. On the other hand, NDGA treatment delayed the softening rate of the fruit, and the content of ester compounds in volatile aromatics was also low. Meanwhile, the activity of pectin degradation-related enzymes was reduced. In addition, ABA treatment also upregulated the essential pectin degradation enzyme genes PcPG1, PcPG2, PcPL, PcPME2, PcPME3, and PcGAL1 and the ABA biosynthetic encoding genes PcNCED1 and PcNCED2. In summary, the results showed that ABA treatment could shorten the post-ripening and softening period of ‘Docteur Jules Guyot’ pear fruit and advance the eating time of the fruit.