Postharvest applications of n-butanol increase greasiness in apple skins by altering wax composition via effects on gene expression
Introduction
Like the organs of most land plants, apple (Malus domestica Borkh.) fruits are covered with hydrophobic organic compounds called cuticular waxes (Pollard et al., 2008). The physical and chemical properties of the cuticular wax determines the appearance of an apple fruit and so significantly affect consumer preference and thus commercial value (Glenn et al., 1990). In apples, the C29 homologues are the most abundant constituents of the wax layer and exist in solid form (Belding et al., 1998; Curry, 2008; Veraverbeke et al., 2001). However, the fluidity of the surface waxes of some apple cultivars, such as ‘Jonagold’ (Veraverbeke et al., 2001), ‘Royal Gala’ (Curry, 2008) and ‘Cripps Pink’ (Yang et al., 2017b) increase during storage, ultimately causing an unpleasant greasy feeling. Skin greasiness can appear either on the tree or later, during storage. Greasiness is associated with an increase in the oil fraction of the cuticular lipids (Dadzie et al., 1995; Fan et al., 1999). It has recently been suggested that skin greasiness may be associated with the accumulation of the long-chain unsaturated fatty acid esters of farnesol (Christeller and Roughan, 2016). Our subsequent work confirmed that significant accumulations of the fluid wax constituents result in a phase change from solid to liquid in the surface waxes of the apple cultivars ‘Cripps Pink’ and ‘Jonagold’ (Yang et al., 2017b). The fluid wax compounds comprise mainly oleate and linoleate esters of (E,E)-farnesol and short-chain alcohols (C3-C5) and lesser amounts of free unsaturated fatty acids.
Various studies have shown that the provision of exogenous substrates to the seeds and fruits of some plants promotes the synthesis of the corresponding esters and that the synthesis of a particular ester is selective to the substrate (Bartley et al., 1985; Berger and Drawert, 1984; Yu et al., 2018). For example, the synthesis of volatile esters can be enhanced by supplying alcohols (C2-C8) as a substrate, in which n-butanol (NBA) and n-pentanol are converted maximally into their corresponding esters, with ester production increasing with increasing concentrations of the exogenous alcohols (Bartley et al., 1985; Berger and Drawert, 1984). Our recent studies show that the unsaturated fatty acid esters of short-chain alcohols accumulate extensively during the development of skin greasiness in ‘Cripps Pink’ apples, in which the butyl ester content is particularly high (Yang et al., 2017a, 2017b). Moreover, studies have shown that NBA is the most abundant volatile alcohol during postharvest storage of ‘Cripps Pink’ apples (Carmen et al., 2008; Villatoro et al., 2008). Accordingly, we speculated that volatile biosynthesis affects the development of skin greasiness by providing the volatile alcohol substrates for greasy ester production (Yang et al., 2017b). Hence, it is worthwhile to conduct a thorough study of the effects of NBA on the development of apple skin greasiness.
Here, we compare wax composition and the expression levels of genes related to wax biosynthesis and wax export in control and NBA-treated ‘Cripps Pink’ apples stored at 20 °C. The aim is to gain new insights into skin greasiness by analyzing the effects of applications of exogenous volatile alcohol substrates on skin greasiness in apple fruit during storage at ambient temperatures.
Section snippets
Plant materials and treatments
Mature ‘Cripps Pink’ apple fruit was picked from an orchard in Fufeng County, Shaanxi Province, China, on 30 October 2017. Apples of uniform size (60 mm in diameter) without any physical damage were selected and randomly divided into three groups. One group was the control, and the other two groups were treated with NBA vapor using the method reported by Berger and Drawert (2010) with some modifications. Apples and a glass vessel with filter paper soaked with liquid NBA were enclosed in a
Effects of NBA on respiration rate, ethylene production and fruit firmness during postharvest ripening at 20 °C
The respiratory rate of control and NBA-treated fruit showed a similar trend (Fig. 1A). During the first days of storage, from day 0 to day 7, there was a slight increase in the respiration rates of all three groups of fruit. After a slight decline, the respiration rate then began to rise rapidly on day 14 and reached a climacteric on day 21. Subsequently respiration showed a downward trend until the end of storage. Changes in the ethylene production of the NBA-treated fruit during storage
NBA treatment promotes the accumulation of liquid esters in waxes, thus accelerating the development of skin greasiness
Certain apple cultivars develop a greasy surface during ripening, such as ‘Jonagold’ (Veraverbeke et al., 2001), ‘Royal Gala’ (Curry, 2008) and ‘Cripps Pink’ (Yang et al., 2017b). Skin greasiness is commercially undesirable. Although there has been significant research on the cuticular waxes of apples (Curry, 2008; Veraverbeke et al., 2001), the causes of skin greasiness have not been fully explained. Previously, Christeller and Roughan (2016) presumed that the accumulations of the liquid
Conclusion
The accumulation of liquid waxes in ‘Cripps Pink’ apples during storage was promoted by NBA treatment, which also accelerated the development of skin greasiness. Meanwhile, neither earlier ripening nor senescence were affected. Chemical analyses show that the content of fluid wax components contributing to skin greasiness (free linoleic and oleic acids and their corresponding esters, especially butyl esters) increased significantly in NBA-treated fruit. The results suggest that the level of
Acknowledgment
This work was supported by the National Industry Technology System of China for Apple (CARS-27).
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