Growth regulators on quality traits and volatile organic compounds profile of ‘Royal Gala’ apple at harvest and after dynamic controlled atmosphere storage
Introduction
‘Gala’ apples and mutants have fruit drop before fruit has developed an optimum red color, size and maturity, which causes expressive economic losses (Yuan and Carbaugh, 2007; Unrath et al., 2009). Thus, apple growers apply growth regulators to reduce fruit drop, such as naphthalene acetic acid (NAA) and aminoethoxyvinylglycine (AVG) (Steffens et al., 2006; Brackmann et al., 2014, 2015; Arseneault and Cline, 2016). There is a lack of information about the effects of these growth regulators on quality and volatile profile of the apples stored under controlled atmosphere (CA) and dynamic controlled atmosphere (DCA).
Different from the CA storage process, which employs static O2 partial pressure (pO2), DCA changes pO2 according to a physiological response of the fruit (Zanella, 2003; Weber et al., 2015), which allows storage of the fruit near to the lowest oxygen limit (LOL) tolerated by apples. To keep apples near the LOL, it is necessary to monitor stress by low pO2 to avoid damages caused by excessive fermentation metabolism. For this, there are some methods: based on the ethanol measurement in the air inside the storage room (DCS – dynamic control system) or in fruit (ILOS-Plus) (Veltman et al., 2003); based on chlorophyll fluorescence (DCA-CF) (Prange et al., 2007); and based on respiratory quotient (DCA-RQ) (Weber et al., 2015; Bessemans et al., 2016). The least used method is DCS, because ethanol does not show a precise correlation with fermentative metabolism, since this alcohol may be metabolized to esters (Brackmann et al., 1993; Wright et al., 2015). DCA-CF method measures the fluorescence emitted by chlorophyll when fruit is submitted to stress by low pO2 (Prange et al., 2007; Wright et al., 2012, 2015), which was the first widely used DCA method. Recently, the DCA-RQ method was developed and studied in Brazil and other countries (Weber et al., 2015; Bessemans et al., 2016; Both et al., 2017; Stanger et al., 2018; Anese et al., 2019). RQ is defined as the ratio between CO2 production and O2 consumption by the stored fruit (Weber et al., 2015; Bessemans et al., 2016). Both et al. (2017) and Thewes et al. (2017a) found that DCA-RQ1.5 and DCA-RQ2.0 showed higher levels of key volatile organic compounds (VOCs) in ‘Royal Gala’ and ‘Galaxy’ apples, respectively, such as butyl acetate, 2-methylbutyl acetate and hexyl acetate, than CA. The main key VOCs are synthetized from amino acids and fatty acids by the action of enzymes, such as lipoxygenase (LOX) and alcohol acyltransferase (AAT), which are affected by ethylene concentration (Defilippi et al., 2005a; Contreras and Beaudry, 2013; Contreras et al., 2016; Xiaotang et al., 2016). Some growth regulator used on apple orchard affect ethylene biosynthesis, such as naphthalene acetic acid (NAA) and aminoethoxyvinylglycine (AVG).
NAA is a synthetic auxin that reduces fruit abscission, even though it accelerates postharvest fruit ripening by increase of ACC (1- aminocyclopropane-1-carboxylate) synthase, consequently increasing ethylene production and fruit softening, reducing storage potential (Yuan and Carbaugh, 2007; Li and Yuan, 2008; Unrath et al., 2009; Brackmann et al., 2014). Moreover, NAA increases gene expression of the ethylene biosynthesis (MdACS1 and MdACO1), ethylene perception (MdERS1) and of apple cell wall degradation (MdPG1) (Li and Yuan, 2008; Yuan and Li, 2008). Shin et al. (2016) found that auxin could be critical in determining the time of activation of ethylene biosynthesis genes (MdACS3 and then MdACS1) in maturing apple fruit. Brackmann et al. (2014) found that NAA applied in the field, increases the ripening rate and reduces gas diffusion of ‘Brookfield’ apple after storage under CA. We hypothesize that NAA can increase the concentration of VOCs in ‘Royal Gala’ apples stored under DCA-CF and DCA-RQ due to ripening advancing effect of NAA even in fruit treated with AVG.
Another tool used by apple growers is AVG, which can reduce fruit drop, delay fruit ripening during storage and increase fruit size. AVG suppresses the gene expression of ACC synthase (MdACS5A and MdACS5B), ACC oxidase (MdACO1), and indirectly reduces gene expression of enzymes such as polygalacturonase (MdPG2) and cellulase (MdEG1) (Li and Yuan, 2008). Salas et al. (2011) found that the application of AVG suppresses VOCs, including alcohols and esters, in ‘Golden Delicious’ after 35 d at 8 °C. It can be possible that apples treated with AVG during pre-harvest and stored under DCA-RQ will not show lower VOCs concentration since DCA-RQ1.5 enhances the concentration of VOCs (Both et al., 2017; Thewes et al., 2017a).
The aim of this study was to evaluate the effects of NAA and AVG application, isolated and combined, on the quality and VOCs profile of ‘Royal Gala’ apples at harvest and after 9 months of storage under CA, DCA-CF and DCA-RQ1.3 conditions.
Section snippets
Plant material, orchard location, harvest maturity, sample preparation and applied treatments
Apples of the cultivar Gala, strain Royal Gala, were harvested in a commercial orchard located in Vacaria, RS, Brazil. The ‘Royal Gala’ apples were grafted on M9 rootstocks. A density of 3,575 plants ha−1 was used in the orchard. During the growing season, the following fertilization was carried out: 80 kg ha-1 of nitrogen and 120 kg ha-1 of potassium. Pre-harvest treatments were: [1] control, water only, applied 7 d before harvest, [2] NAA (0.04 kg ha-1 of NAA - Fruitone™) applied 7 d before
Metabolism and quality parameters at harvest
There was no stimulation of maturation with the application of NAA at harvest. Fruit with NAA, harvested at the same date of control, did not show differences in ethylene production, internal ethylene concentration (IEC), ACC oxidase, respiration and starch index compared to control fruit (Table 2). Other researchers found that naphthalene acetic acid (NAA) accelerated apple postharvest ripening (Li and Yuan, 2008; Yuan and Li, 2008; Brackmann et al., 2014).
Fruit with AVG application were
Conclusion
Pre-harvest AVG + NAA application caused an increase in some important VOCs such as 2-methylbutl acetate, hexyl acetate and butyl acetate in ‘Royal Gala’ apples after long-term storage under CA, when compared to DCA-CF and DCA-RQ1.3 storage. AVG and NAA applied isolated maintain higher total esters after CA storage.
Apple with AVG + NAA application stored under DCA-CF or DCA-RQ1.3 is a promising tool to maintain higher flesh firmness, reduce physiological disorders and maintain higher healthy
Author statement
All the authors: Rogerio de Oliveira Anese, Fabio Rodrigo Thewes, Auri Brackmann, Erani Eliseu Schultz, Roger Wagner, Bruna Klein, Magno Roberto Pasquetti Berghetti, Lucas Mallmann Wendt, participated equally by the project: conceptualization, methodology, data curation, visualization, investigation supervision, validation writing, reviewing and editing.
Declaration of Competing Interest
None.
Acknowledgements
To Conselho Nacional de Desenvolvimento Científico e Tecnológico, Coordenação de Aperfeiçoamento de Pessoal e de Nível Superior and Fundação de Amparo à Pesquisa do Estado do Rio Grande do Sul, for financial support.
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