Introduction

The production volume of mango in Mexico is estimated at 2 089 041.18 t, mainly of the Ataulfo, Haden, Kent, Keitt, Tommy Atkins and Manila varieties, the latter accounts for about 20 % of this volume (^{Servicio de Información Agroalimentaria y Pesquera [SIAP], 2019}). Due to its sensory quality, ‘Manila’ mango fruits exceed in demand the other varieties at the national level, hence its commercial importance. However, this cultivar has several phytosanitary problems, high sensitivity to mechanical damage and high metabolism, which significantly affects its quality (^{Vargas-Ortiz et al., 2013}). In addition, fruits from forced or early production systems present problems of lack of color and rapid wilting due to high water loss during storage for maturation (^{Siller-Cepeda et al., 2009}).

Physiologically, mango has climacteric behavior, with high respiration rate and low ethylene production (^{León et al., 1997}; ^{Montoya-Zapata et al., 2018}). In addition, because of its high sensitivity to water loss and accelerated wilting, ‘Manila’ mango is classified as a cultivar with short shelf life (^{Saucedo-Veloz et al., 1977}; ^{García-Osuna et al., 2005}).

Different postharvest technologies on ‘Manila’ mango fruits reveal that refrigeration temperatures of 12-13 °C keep fruits for 10-14 days with adequate internal quality, but with less intensity of typical yellow color, an effect associated with chilling injury symptoms (^{Saucedo-Veloz et al., 1977}; ^{Russián-Lúquez & Manzano-Méndez, 2003}; ^{Patel et al., 2015}). The use of controlled and modified atmospheres has been reported as an alternative to prolong the shelf life of mango fruits, as well as mitigate the incidence of chilling injury and rot development (^{Zaharah & Singh, 2011}). However, each cultivar needs a certain combination of gases and cooling temperature to avoid induction of anaerobic respiration and consequent formation of unpalatable aromas. ^{Zaharah and Singh (2011)} report as promising the combination of 3 % O₂ + 6 % CO₂ + 13 °C for mango cultivar Kensington Pride, while for cultivar Keitt the best combination is 6 % O₂ + 10 % CO₂ + 7 °C, both for six weeks.

1-methylcyclopropene (1-MCP), an inhibitor of the mechanism of action of ethylene, has been proposed as an alternative to delay ripening in climacteric fruits (^{Blankenship & Dole, 2003}). Experiments on ‘Keitt’ mango fruits have shown that 300 nL·L^-1 of 1-MCP, combined with a hydrothermal treatment (52 °C for 5 min) and stored at 13±2 °C for 20 days, prolonged their shelf life by five days compared to the control (^{Paull, 1993}; ^{Osuna-García et al., 2007}). Furthermore, ^{Ortiz-Franco et al. (2016)} report that treatment with 1 000 nL·L^-1 of 1-MCP maintained the physicochemical characteristics associated with the ripening of ‘Ataulfo’ mango fruits, this for 20 days at 13 °C in addition to 5 days at 25 °C.

The improper application of harvesting rates, which results in the marketing of ‘Manila’ mango fruits harvested before reaching physiological maturity, represents another factor contributing to the loss of quality, by showing an incomplete ripening process with poor flavor and color (^{Food and Agriculture Organization of the United Nations [FAO], 1987}; ^{Martinez-Gonzalez et al., 2017}). This leads to fruits that are more sensitive to weight loss and have rapid wilting (^{Siller-Cepeda et al., 2009}). The application of ethylene gas treatments, or ethylene releasing compounds, has been proposed as an alternative to homogenize mango fruit maturation; however, in the case of the Manila crop, the effect of these treatments on fruit physiology and quality is still unknown. ^{Romero-Gomezcaña et al. (2006)} have reported that treatment with 0, 15 and 20 g·L^-1 of calcium carbide (acetylene releaser, with analogous action to ethylene), at 20±2 °C and 74±4 % relative humidity, advanced maturation by three days, improved external and internal color and accelerated firmness losses.

Therefore, the objective of this research was to evaluate the effect of postharvest treatment with 1-MCP and Ethephon on maturation and quality of early production of mango cv. Manila fruits. Under the hypothesis that the application of 1-MCP and Ethephon delays and advances, respectively, the ripening of ‘Manila’ mango fruit without significant changes in quality and postharvest life, compared to untreated fruit (control).

Materials and methods

For the experiment, ‘Manila’ mango fruit produced in the region of Costa Chica in the state of Guerrero were harvested from an orchard at the ejido de la Estación (16° 46’ 50’’ LN and 99° 41’ 05’’ LW, at 10 m a. s. l.), with 30-year-old trees. The fruits came from a forced production system with potassium nitrate. A total of 210 fruits were harvested at physiological maturity (external green color, ()20 % yellow flesh color, 10.6 % total soluble solids (TSS), 2.8 % citric acid and flesh firmness 53. 6 of N), which were washed to remove latex residues and divided into three lots of 70 fruits each to establish the following treatments: 1-MCP (SmartFresh® 14 % a.i.), Ethephon 240 LS (21.65 % 2[chloroethyl] phosphonic acid) and control.

For treatment with 1-MCP, fruits were placed in an airtight container (volume 1/8 m³) and the regulator was applied at a concentration in the atmosphere of 600 nL·L^-1 for 24 h at 20 °C. The dose was calculated based on fruit weight and container volume. For the Ethephon treatment, an aqueous solution of 1 000 µL·L^-1 of 2-(chloroethyl) phosphonic acid was prepared at pH 5. The application was by immersion for 5 min. After the three treatments were established, the fruits were stored under ripening conditions (22±2 °C and 55±5 % RH) for 8 days. The variables evaluated were respiration rate, weight loss, firmness, external color, TSS, titratable acidity, vitamin C and β-carotenes.

Respiration rate was quantified by gas chromatography according to the headspace method described by ^{Salveit-Mikal et al. (1992)}. In one out of four containers (2.12 L) per treatment, three pre-weighed fruits were placed and hermetically sealed for 1 h; subsequently, 1 mL of gas was taken from the headspace and injected into a gas chromatograph (model 5890 series II, Hewlett Packard, USA), fitted with an open-type column and a porous silica layer packing connected to a thermal conductivity detector (TCD). The operating conditions were column at 150 °C, TCD at 180 °C, injector at 170 °C and helium as carrier gas (20 mL·min^-1). The CO₂ standard (INFRA®) was injected at a concentration of 500 ppm. Determinations were made daily during the established storage period and data were reported as mL CO₂·kg^-1·h^-1.

The weight of 10 individual fruits was measured daily using a digital scale (ALSEP EY-2200, Japan); the data were reported as percentage loss (%) based on the difference in daily weight compared to the initial weight. A texturometer (Force Five FDV-30, Wagner, USA) with a Magness Taylor prop of 11 mm diameter was used to evaluate pulp firmness. Measurements were made on two opposite sides of the equatorial zone of the fruit, after removing the epicarp, and data were reported in Newtons (N). Epicarp color was determined with a reflection colorimeter (D-25, Hunter Lab, USA) using the parameters L, a, b and hue angle (h*), the latter was determined by the equation h* = tan^-1 (b/a). (^{McGuire-Raymod, 1992}; ^{Arias et al., 2000}).

TSS (°Bx) were measured using a digital refractometer (Pr-100 series A56280, ATAGO®, Japan), according to the methodology described by the Association of Official Analytical Chemists (^{AOAC, 1990}). Titratable acidity, measured as percentage citric acid, was determined in the pulp by titration with 0.01 N NaOH (^{AOAC, 1990}). The ascorbic acid (vitamin C) content in pulp was determined by extraction with oxalic acid and titration with 2,6-dichlorophenolindophenol (^{AOAC, 2007}). Previously, a standard curve was prepared, and data were reported in mg_AA∙100 g^-1. Total carotenoids were determined spectrophotometrically (^{AOAC, 1990}); for this purpose, extraction was carried out with petroleum ether, and absorbance measurements were recorded at 454 nm. Carotenoid content was reported in mg·100 g^-1.

Respiration was monitored daily in all treatments, treating each container as an experimental unit.

Weight loss and external color were measured daily and every two days, respectively, in 10 fruits, and each fruit was taken as the experimental unit. The other variables were measured every two days on four fruits separately, where the experimental unit was one fruit. The mean and standard deviation were calculated with the data on respiration rate, weight loss, carotenoids and ascorbic acid content. An ANOVA was carried out for the other variables under an experimental design with a three-level factorial arrangement, and a Tukey’s test for comparison of means (P ≤ 0.05). The data were analyzed with the statistical program SAS (2002) version 9.0.

Results and discussion

The fruits of the three treatments showed significant differences in respiration rate at the maximum climacteric, which was reached after four days of storage at 22±2 °C, with values of 109.2±10.1, 89.4±6.1 and 73.1±7.3 mL CO₂·kg^-1·h^-1, for the Ethephon, control and 1-MCP treatments, respectively (Figure 1). ^{Leon et al. (1997)} report values of 134.4 mL CO₂·kg^-1·h^-1 after six days at 25 °C in ‘Manila’ mango. This contrasts with values of 75.8 mL CO₂·kg^-1·h^-1 in ‘Kent’ mango after five days at 25 °C found by ^{García-Martínez et al. (2015)}. The above reveals the high metabolic activity of ‘Manila’ mango. The results obtained show the effect of Ethephon, which, by releasing ethylene, increases respiration rate, an effect that has been reported by ^{Salveit-Mikal et al. (1992)}. The response to 1-MCP is associated with the blockage of the ethylene binding site with the receptor at the site of action, which results in a lower respiration rate during maturation stage (^{Jiang-Weibo et al., 2004}).

Figure 1. Respiration rate of mango cv. Manila fruits treated with Ethephon (1000 μL·L^-1) and 1-MCP (600 nL·L^-1), stored at 22±2 °C. Each value represents the mean ± standard deviation (n = 4). 

After four days of storage, weight losses were between 6-8 %, with no wilting symptoms, which manifested after eight days with losses of 10-12 % (Table 1). These results show the high sensitivity of ‘Manila’ mango fruits to water losses due to transpiration. According to ^{Barbosa-Martínez et al. (2009)}, these losses are due to the reduced thickness of the epicarp and cuticle, in addition to a thinner epidermal cell layer compared to cultivars from Florida; this translates into a lower resistance to gas and water vapor transport. Regarding the control, fruits treated with 1-MCP presented significantly higher weight losses, with 7.9 and 12.1 % at four and eight days of storage, respectively. ^{Manganaris et al. (2008)} point out that this effect is related to changes in the quantity and composition of epicuticular waxes, which regulate the transport of water vapor around the fruit epicarp (Figure 2).

Figure 2. Weight loss (%) of mango cv. Manila fruits treated with Ethephon (1 000 μL·L^-1) and 1-MCP (600 nL·L^-1), stored at 22±2 °C. Each value represents the mean ± standard deviation (n = 10). 

As part of the changes associated with the maturation process, pulp firmness decreased as storage time progressed (^{Yashoda-Hosakote et al., 2006}); however, the process was more abrupt in fruits with Ethephon, which showed a significantly lower firmness (33.4 N) compared to the initial value (53.6 N) after two days. The control (30.6 N) and 1-MCP (32.9 N) treatments showed significant differences until after four days of storage (Table 1). Values between 10 and 20 N were found in all three treatments at six days, and between 2 and 3 N after eight days. ^{Gil et al. (2006)} report, for ‘Tommy Atkins’ mango, that a firmness of 5.88 to 13.75 N places the fruit in a low-quality range, which suggests that the firmness of fruit from the three treatments after six days of storage is acceptable.

No significant differences were observed between treatments (Table 1), except on the fourth day, where fruit with Ethephon had significantly lower firmness (18.6 N) than the control and 1-MCP. The response of the Ethephon treatment is based on its effect as an ethylene releaser by breaking the 2-chloroethylphosphonic acid molecule (^{Bondad, 1976}; ^{Osuna-Enciso et al., 2012}), which brings forward the phenomenon of autocatalysis and the activation of enzymes related to cell wall degradation and loss of firmness (^{Ali-Zainon et al., 2004}).

In relation to the control, the fruits treated with 1-MCP showed no significant differences in pulp firmness during the established storage periods, although a lag effect was observed after six days, when the fruits showed the greatest firmness (16.1 N). ^{Cin-Dal et al. (2006)} mentioned that the response to 1-MCP applications varies, among other factors, with concentration and exposure time, which suggests the need to test other combinations that will optimize the treatment for this quality parameter.

Epicarp color showed changes according to the storage time (Table 1). As for the initial value (126.5°), the hue angle decreased in the three treatments, but more rapidly in fruits with Ethephon (117.1°) after four days of storage, while the control (114.8°) and 1-MCP (117.7°) reached similar values after six days. Luminosity increased in the three treatments, with significant differences from the fourth day of storage in fruits with Ethephon, and up to the eighth day in the control and 1-MCP. These results show a greater color change in ‘Manila’ mango fruits treated with Ethephon, as reported by ^{Osuna-Enciso et al. (2012)} in Keitt cultivar mangoes.

The content of carotenoids in pulp increased as the maturation process progressed, which occurs in fruits with a high content of these pigments (^{Gross, 1987}). The Ethephon treatment favored carotenoid biosynthesis, reaching a significantly higher content on the fourth and sixth day, the latter with 4.81±0.45 mg·100 g^-1. The control reached its highest content on the sixth day (1.91±0.39 mg·100 g^-1), while the 1-MCP treatment had its highest content on the eighth day (3.08±0.2 mg·100 g^-1) (Figure 3). ^{Pott et al. (2003)} observed that the content of total carotenoids in ‘Kent’ mango varied from 0.9 to 12.5 mg·100 g^-1, while ^{Ornelas-Paz et al. (2008)} reported values lower than 1 mg·100 g^-1 for ‘Manila’ mango. This variation explains the effect of different factors affecting the metabolism of these isoprenoids, such as the species, cultivar and production technology applied (^{Gross, 1987}).

Figure 3. Changes in carotenoid content during maturation of mango cv. Manila fruits treated with Ethephon (1 000 μL·L^-1) and 1-MCP (600 nL·L^-1), stored at 22±2 °C. Each value represents the mean ± standard deviation (n = 4). 

Based on the results obtained, it can be assumed that ethylene released by Ethephon stimulates the activity of key enzymes of carotenoid biosynthesis. It has been reported that DXS (1-deoxy-D-xyluloxa-5-phosphate synthase) (E.C.2.2.1.7.) increases its activity in parallel with the increase of carotenoids (^{Botella-Pavía et al., 2004}).

Ascorbic acid content decreased during maturation in all three treatments (Figure 4), although more slowly in the fruits treated with 1-MCP, which allowed them to maintain a significantly higher content (26.8±1.9 mg·100 g^-1) Ascorbic acid content decreased during maturation in all three treatments (Figure 4), although more slowly in the fruits treated with 1-MCP, which allowed them to maintain a significantly higher content (20±1.0 mg·100 g^-1) after 8 days. final losses of ascorbic acid were 58.4, 50.3 and 62.9 % for the control, 1-MCP and Ethephon, respectively. This effect is related to the oxidation of ascorbic acid to dehydroascorbic acid by ascorbate oxidase (^{Lee & Kader, 2000}). Similar losses have been reported by ^{Anowar et al. (2014)} in ‘Ashwina’ mango (58.6 %) and ^{Garcia-Martinez et al. (2015)} in ‘Kent’ (48.8 %). However, ^{Lakshminarayana (1975)} report lower losses in ‘Irwin’ (12.5 %) and ‘Haden’ (18.9 %) mangoes, which showed the effect of cultivar on postharvest metabolism of this vitamin.

Figure 4. Changes in ascorbic acid content during storage of mango cv. Manila fruits treated with Ethephon (1 000 μL·L^-1) and 1-MCP (600 nL·L^-1), stored at 22±2 °C. Each value represents the mean ± standard deviation (n = 4). 

TSS content increased significantly compared to the initial value from day 6 onwards, but without significant differences between treatments. However, the highest TSS content (20.3 %) was seen in fruits with Ethephon, while the lowest was in fruits with 1-MCP (19.2 %) (Table 1). ^{León et al. (1997}) reported values of 22.0 % for ‘Manila’ mango at eating maturity, which is higher than those reported for other cultivars, such as Haden (15.2 %), Tommy Atkins (13.9 %) and Kent (13.7 %) (^{Siller-Cepeda et al., 2009}).

Citric acid content decreased significantly on the sixth day in fruits treated with Ethephon, while in the control and 1-MCP the decline happened after eight days. This effect is associated with the participation of citric acid in the respiration process (^{Medlicott et al., 1990}). After six days of storage, fruits with Ethephon were significantly lower than the control and 1-MCP, with 0.4 % citric acid, which shows a greater progress in the maturation process. ^{León et al. (2005)} report a content of 0.3 % citric acid in ‘Manila’ mango fruit at the consumption maturity stage.

Conclusions

1-MCP reduced the respiration rate during maturation and decreased the loss of ascorbic acid, so it is considered acceptable to be used on ‘Manila’ mango, which has a high metabolic activity, as it can extend shelf life by delaying maturation. However, more research should be carried out on the use of this product for firmness since the concentration and exposure time could lead to positive results in this variable.

The use of Ethephon increased the content of carotenoids in ‘Manila’ mango, up to values that have not been reached in other studies, as well as total soluble solids, but decreased the concentration of ascorbic and citric acids.

The use of both products in ‘Manila’ mango is recommended for technological management. However, further research should be carried out to determine the optimal time of application to propose appropriate technological management for the Manila variety.