Characteristics and Potential Use of Fruits from Different Varietal Groups of Sechium edule (Jacq.) Sw

Abstract

(1) Background: Chayote [Sechium edule Jacq. (Sw.)] is a non-traditional export product; recently, demand has increased due to its nutritional and functional properties. There is a wide diversity of varietal groups (VGs) within this species. Despite this, only virens levis and nigrum spinosum varieties are commercialized on a large scale, while the rest are underutilized and poorly studied, so the genetic pool of this species is at risk. (2) Methods: The following variables were evaluated in the fruits of 10 chayote groups of varieties: shape, size, weight, stomatal frequency (SF), stoma size, stomatal index (SI), color index (CO*), pigments, titratable acidity (TA), total soluble solids (TSS), total sugars and moisture content. In addition, the postharvest behavior of the ten VGs stored at room temperature and the effect of 1-MCP on fruit quality during cold storage were evaluated. (3) Results: The groups a. minor and n. minor showed rapid weight loss, the albus varieties showed high epidermis oxidation, while v. levis, n. maximum, n. spinosum and n. xalapensis were susceptible to viviparity, blisters and fungal incidence. 1-MCP prevented chilling injury (CI) and weight loss. (4) Conclusions: The diversity of postharvest characteristics allows the use of VGs for different uses such as a fresh fruit, agroindustrial transformation or mixing with other vegetables.

1. Introduction

Chayote [Sechium edule Jacq. (Sw.)] is native to Mexico and Central America but is now cultivated in many tropical and subtropical regions of the world [1]. There is a wide varietal diversity within the S. edule species with fruits of different color, size, presence of spines, shape and phytochemical composition, which influences the taste of the fruits [2]. Currently, twelve varietal groups are recognized within the species; those of commercial importance are virens levis and nigrum spinosum. The high demand for these two varieties has meant that the other varietal groups have been ignored or are only traded in local markets, which puts the genetic richness at risk. The fruit is non-climacteric and is harvested at horticultural maturity [1].

Recently, the consumption of chayote has increased due to its nutritional properties; the fruit is low in calories (19–31 Kcal/100 g) and is a good source of fiber (0.40–7.53%) and minerals such as potassium and calcium. It also contains essential amino acids such as valine, leucine and phenylalanine and vitamins C, E and B9 [3,4]. Chayote fruits also are rich in compounds with functional properties and significant amounts of phenolic acids, flavonoids and different types of cucurbitacins (Cus), which have an antiproliferative activity against cancer cells such as HeLa, P-388 and L-929 [5]. They also have antifungal properties, limiting the germination of Botrytis cinerea conidia [6] and antioxidant activity [7]. For example, the fruits of nigrum xalapensis contain 13.44, 5.60 and 0.62 mg g⁻¹ of Cu types D, I and B, respectively, while the v. levis variety has 0.23, 0.08, 1.09 and 0.11 mg g⁻¹ of rutin, florizidine, myricetin and floretin, respectively. Yellow varieties such as a. levis contain Cu D (4.77 mg g⁻¹), Cu I (3.52 mg g⁻¹), Cu B (0.46 mg g⁻¹) and Cu E (1.73 mg g⁻¹) [2]. Other cucurbits such as cucumber (Cucumis sativus L.) only contain 0.69 to 0.89 mg g⁻¹ of Cu E [8], and in watermelon (Citrullus colocynthis), only Cu E (0.21–0.3 mg g⁻¹ FW) can be detected [9].

Visual appearance, ripeness, weight, size, shape, characteristic color (from ivory to dark green) and no defects and diseases, as well as internal parameters such as flavor, aroma, texture and nutritional composition, are the important quality traits for the commercial acceptability of the fruits (Figure 1). In the case of chayote, size, uniform weight, the presence or absence of thorns and the color of the exocarp are important quality characteristics for the market [10].

The shelf life of chayote is short, and at room temperature, commercial quality can only be maintained for up to seven days [11]. The main problems are rapid weight loss, fungal incidence, blistering and early seed germination (viviparity) [12]. In this respect, refrigeration is an effective method of maintaining quality; however, temperatures below 7 °C can cause chilling injury (CI), presented as brown spots and depressions on the fruit epidermis which generally appear once the fruits are returned to room temperature [12,13].

1-methylcyclopropane (1-MCP) is a good option to prolong the shelf life of non-climacteric fruits. In cherries (Prunus avium L.) stored at 1 °C for 30 days, treatment with 1-MCP (1 μL L⁻¹ for 24 h) reduced physical injury and the incidence of physiological disorders and maintained fruit firmness [14]. In zucchini (Cucurbita pepo), the application of 2.4 μL L⁻¹ of 1-MCP for 48 h reduced the respiration rate and ethylene production by half, a significant reduction in weight loss and cold damage during storage for 14 days at 4 °C, although these effects depended on the variety [15]. Likewise, the application of 625–650 nL L⁻¹ of 1-MCP reduced weight loss, softening and color changes in melon fruits (Cucumis melo L.), prolonging shelf life by 10 days [16].

Therefore, the aim of this work was to determine the commercial potential of 10 variety groups of S. edule as a strategy to preserve these genetic resources and promote their commercial cultivation. For this, the morphological and biochemical characterization of chayote fruits was carried out, and their postharvest behavior and the effect of 1-MCP during cold storage were evaluated.

2. Materials and Methods

2.1. Morphological and Physicochemical Characterization of the Fruits

Fruits were harvested from the National Germplasm Bank of Sechium edule in Mexico (BANGESe). Harvesting was carried out at horticultural maturity (18 ± 2 d after anthesis) [17], choosing fruit without any external defects or disease occurrence. The fruits of the 10 chayote varietal groups assessed were albus minor, albus levis, albus dulcis, albus spinosum, albus levis gigante, virens levis, nigrum minor, nigrum xalapensis, nigrum spinosum and nigrum maxima (Figure 1). At least 50 fruits per varietal group were selected in order to realize the characterization.

The shape of chayote fruit was classified as pyriform, elongated pyriform, obovoid and round [18]. The length (cm) was measured from the slit to the apex using a digital vernier. The weight (g) was determined with a digital balance (Model SI-2000S, Setra Systems, Inc., Boxborough, MA, USA) with an accuracy of 0.01 g.

Stomata size, stomata frequency (SF) and stomata index (SI) were reported as µm, stomata/mm² and percentage (%), respectively. For this, the epidermis printing method was used [19]. For this purpose, a thin layer of nail polish was placed on 1 cm² of the equatorial epidermis of the fruits and allowed to dry at room temperature. The print was then peeled off and placed on a slide. Photographs of these preparations were obtained with ImageJ^® 1.45 software adapted to an optical microscope (model B-510PH, Optika S.L.R., Ponteranica, Italy) and a digital camera (Canon, model EOS Rebel T7, Japan). Photographs for SF were taken at 10×, representing 1.552 mm² each, while epidermal cell counts and stomata size were taken at 40×, representing 0.094 mm² each. The SI was calculated considering S = the number of stomata per unit area and E = the number of epidermal cells per unit area, according to the equation [20]:

$\mathrm{SI} (\%)= {\displaystyle \frac{\mathrm{S}}{\mathrm{E}+\mathrm{S}}} \times 100$

Color was measured with a colorimeter (3NH TECNOLOGY Co., Ltd., model NR20XE, Shenzhen, China). L, a* and b values were taken, and the color index (CO*) was calculated [21], where L = lightness, a* = red-green and b* = yellow-blue coordinates:

$\mathrm{CO}\ast = {\displaystyle \frac{{\mathrm{a}}^{\ast } \times 1000}{{\mathrm{L} \times \mathrm{b}}^{\ast }}}$

For total chlorophyll (C_a+b) and total carotenoids (C_x+c), 2 g of the epidermis was taken, 10 mL of 80% (v/v) acetone was added and stored in the dark at room temperature for 24 h. Afterward, absorbance (A) was measured at 470, 646 and 663 nm with a UV spectrophotometer (GENESYS^TM 10UV, Thermo Scientific, Madison, WI, USA); the content was expressed as mg g⁻¹ fresh weight and calculated according to that proposed by Lichtenthaler [22]:

C_a+b = 7.15A₆₆₃ + 18.71A₆₄₆

${\mathrm{C}}_{\mathrm{x}+\mathrm{c}}={\displaystyle \frac{{1000\mathrm{A}}_{663} -{1.82\mathrm{C}}_{\mathrm{a}} - {85.2\mathrm{C}}_{\mathrm{b}} }{198}}$

For the calculation of titratable acidity (TA), 3 g of pulp was macerated with 30 mL of distilled water, the mixture was titrated with NaOH 0.1 N and phenolphthalein was used as indicated, reporting the values as % citric acid. TSS (°Bx) was measured with a digital refractometer (PAL-1, Atago^®, Saitama, Japan). For the measurement of total sugars (%), a flask was boiled with 5 g of pulp and 60 mL of 80% (v/v) ethanol until approximately 10 mL was reduced and the supernatant recovered. Then, 0.3 mL of supernatant–distilled water (1:100), 0.3 mL of 5% phenol (v/v) and 1.5 mL of 95% H₂SO₄ (v/v) were mixed. Subsequently, absorbance was measured at 490 nm; 95% (w/v) glucose was used as the standard [23].

For moisture content (%), 1 cm thick slices were taken from the center of the fruit without seed and skin and placed in a mechanical convection oven (Thermo Scientific, model Imperial V, Greenville, NC, USA) at 50 °C until a constant dry weight was obtained and calculated with the following equation:

$\mathrm{Moisture} \mathrm{content} (\%)= {\displaystyle \frac{\mathrm{Wet} \mathrm{weight} - \mathrm{dry} \mathrm{weight}}{\mathrm{Wet} \mathrm{weight}}} \times 100$

2.2. Postharvest Characterization and 1-MCP Application

Postharvest characterization: The fruits (at least 50 per varietal group) were washed with a 1% sodium hypochlorite solution and dried at room temperature. Postharvest characterization of all groups was carried out under room conditions (21 °C and 70% RH).

Phytopathogenic fungi were isolated in the fruits with obvious symptoms such as mycelial growth or rot; isolation techniques and taxonomic keys were used [24]. Fruits were considered to lose commercial quality when viviparism (early seed germination) appeared and evident dehydration showed. The following scales were developed for this purpose: Viviparism: Level 0 = no seed present, 1 = seed visible and basal opening and 2 = seed fully exposed; Dehydration: Level 0 = none, 1 = mild, 2 = moderate and 3 = severe; Chilling injury: Level 0 = none, 1 = mild, 2 = moderate (light brown spots) and 3 = severe (brown and soaked spots); Blisters: Level 0 = any blister, 1 = 1 to 5 blisters, 2 = 6 to 15 blisters, 3 = 16 to 25 blisters and 4 = 26 to 50 blisters. The scales are according to Ramirez-Rodas [12].

The α-amylase activity was determined in those varieties with a high incidence of viviparity such as v. levis, n. maxima, n. xalapensis and n. spinosum. The Alpha-amylase SD kit (Megazyme, UK, Wicklow, Ireland) was used, following the manufacturer’s recommended instructions. Six fruits per variety were evaluated, taking tissue from the basal part of the fruit and the terminal part of the seed, which in previous studies showed the highest enzyme activity. The tissue was freeze-dried (model Freezone 4.5, Labconco, Co., Kansas City, MO, USA) and then finely ground for analysis; 250 mg of tissue was used for the test, and the results were presented as amylase units SD g⁻¹ of dry tissue.

The weight loss (%) was determined with a digital balance (Model SI-2000S, Setra Systems, Inc., Boxborough, MA, USA) with an accuracy of 0.01 g. For the postharvest characterization, fifteen fruits per variety were used; the measures were taken every day until fruit decay. For the second experiment (1-MCP), 15 fruits per variety were also used and measured at harvest (day 0) and every day after cold storage. One fruit was considered as a replicate. The following equation was used:

$\mathrm{Weight} \mathrm{loss} (\%)= {\displaystyle \frac{\mathrm{initial} \mathrm{weight} - \mathrm{final} \mathrm{weight}}{\mathrm{initial} \mathrm{weight}}} \times 100$

1-MCP application: The effect of 1-MCP on the cold storage of chayote was only investigated in the fruit of some variety groups. For this purpose, the application of 1-MCP (600 nL L⁻¹) was performed at room temperature by placing the fruit in a hermetically sealed acrylic box (0.125 m³) and leaving a vial containing the 1-MCP concentration (SmartFresh^®; 14%, Rohm and Haas, Philadelphia, PA, USA). The exposure time was 24 h at 24 ± 1 °C and 60% RH. After application, fruits treated with 1-MCP and their controls were stored at 8.7 °C and 95% relative humidity. Storage period was 2 weeks for the varieties of a. minor, n. minor and v. levis (B) and 3 weeks for a. dulcis, a. levis and v. levis (A).

2.3. Statistical Analysis

Morphological and biochemical characterization data were expressed as the mean ± standard error. A principal component analysis (PCA) and dendrogram were performed with the data, using the ade4 and cluster.datasets packages, respectively. The hierarchical map of postharvest problems was drawn up using the Pheatmap package of R software (version 4.0.2). The physicochemical characteristics and postharvest variables were analyzed using Kruskall–Wallis (α = 0.05) due to the measurements not meeting normality.

3. Results and Discussion

3.1. Morphological and Phytochemical Characterization of the Fruits

Phenotypic and chemical variations within and between populations of the same species can occur in the flowers, clusters, leaves and fruits [25]. S. edule varietal complexes have diversified over time [26], and this diversity can be explained by processes of microevolution, where the combination of genetic plasticity, phenotypic drift and environmental factors result in phenotypes with specific morphological and chemical characteristics [27]. However, diversity is also associated with the process of domestication through natural and artificial selection of a species, as producers focus on the selection of fruits with certain characteristics such as plant productivity, fruit size, quality, flesh content and flesh color [1].

According to Table 1, the most common shape of chayote fruits is pyriform; however, some varieties of the albus group are round, and n. maxima is elongated pyriform. On average, we can classify chayote varieties as very large (n. maxima), medium to large (v. levis, a. levis gigante, n. spinosum, n. xalapensis), medium (a. spinosum, a. dulcis), small (a. levis) and very small (a. minor, n. minor). Fruit weights ranged from 23.44 g for a. minor to 511.32 g for n. maxima. Currently, the most traded variety is v. levis, which is exported with an approximate weight of 310 g [12].

Stomata play a major role in water loss through transpiration and are the gateway for infections caused by plant pathogens [28]. In cherry, apple, pomegranate or litchi, the frequency and size of stomata is related to physiological problems such as fruit cracking [29,30]. Likewise, the agglomeration of pathogenic bacteria such as Salmonella occurs close to the stomatal opening of the fruit [31]. Stomatal density decreases as the fruit expands, even losing functionality as it becomes covered with cuticular waxes [32]. Stomatic size, aperture and stomatal index are influenced by external factors such as CO₂ concentration and temperature [33] and by endogenous signaling via the likes of phytohormones and proteases, which are fundamental for the physiological adaptations of plants to biotic and abiotic stresses [28,34].

The presence of stomata in the fruit epidermis suggests an important role in gas exchange, with the potential to assimilate CO₂ and optimize photosynthesis [35]. In this regard, Sui et al. [36] showed that cucumber fruit can contribute 9.4% of carbon fixation, although the stomatal frequency of the fruit is only 1.58 and 0.91% compared to the adaxial and abaxial parts of the leaves, respectively. In this study, the stomatal frequency of the VGs varied between 5.73 and 39.26 stomata/mm²; other authors confirmed that there are less than 40 stomata/mm² in the fruits and leaves [13,37].

The highest stomata index (SI) was found in n. xalapensis with 0.66%, and the lowest was 0.14% for a. spinosum, although some morphotypes of n. xalapensis and n. spinosum can reach values higher than 0.90%. Fruit stomata measure on average 28.76 µm, being very similar between varieties, but n. spinosum (35.24 µm) has significantly larger stomata and v. levis (22.14 µm) the smallest. The size is quite similar for n. xalapensis and n. spinosum at 30.79 and 34.45 µm, respectively [12]. While there are differences in SI, SF and stomatal size between varieties, these values are known to be specific to a species grown in a given environment. For these reasons, it is very difficult to use the SI as an identification parameter [38]. The presence of chlorophylls and stomata in chayote suggests that, like other fruits, there is a photosynthetic activity, which may be differential depending on the stage of development.

The color of the epidermis is one of the most important quality characteristics. The yellow albus varietal groups had CO* values between −1.071 and 0.744, the light green virens levis varietal group was −4.793 and the dark green nigrum varieties values were between −14.322 and −8.87 (Table 1). Total chlorophyll and carotenoid content have a strong correlation with fruit color [39]. In this study, the content of total chlorophyll and carotenoids in the epidermis of chayote had an R² correlation of 0.92 and 0.86 with the color index, respectively. According to Fu et al. [40], the transcription of genes such as HCAR and CHL1 is involved in chlorophyll synthesis, while CHY2 influences carotenoid content.

Chlorophylls a and b are the main photosynthetic pigments; however, the role of carotenoids in photosynthesis is also important because they can absorb light at wavelengths where chlorophylls cannot (400–550 nm) and provide photoprotection to tissues under conditions of high light intensity [41]. The highest amount of chlorophylls and total carotenoids was found in n. minor with 0.227 and 0.044 mg g⁻¹, respectively, while the albus groups had at most 0.001 mg g⁻¹ of both. Cadena-Iñiguez et al. [17] mentioned that the varieties a. minor, a. dulcis and a. levis have up to 10 times less chlorophyll than the varieties nigrum and levis. The C_a content was higher than that of C_b, indicating that the fruits grew under conditions of higher light intensity [42]. C_b is synthesized from the oxidation of C_a by the action of chlorophyllide oxidase [43].

Low acidity is a characteristic of cucurbits, for example, melon (0.11% acidity) [44], cucumber (0.098%) and watermelon (0.096%) [45]. In all the chayote varieties, the acidity was around 0.13%. The variety a. dulcis had the highest acidity with 0.18% and n. maxima the lowest with 0.10%. Riviello-Flores et al. [7] determined an acidity of 0.085 and 0.10% for v. levis and n. spinosum juices, respectively, slightly lower than those obtained in this study.

A high TSS content is associated with a longer shelf life because these reserves allow for the maintenance of respiration intensity. In cucumber, a 5.4% increase in TSS increased the probability of marketability by 1.8 times, i.e., fruits had less chilling damage, wilting, color loss and disease incidence. On average, chayote contains 5.11 °Bx, slightly higher than cucumber with 3.0 to 4.0 °Bx [46] and zucchini (Cucurbita pepo L.) with 3.76 °Bx [47]. In this study, the variety a. dulcis had the highest amount with 5.56 °Bx and n. maxima the lowest with 4.78 °Bx.

Fructose and glucose are the major components of total sugar in chayote [13]. Verma et al. [48] characterized 74 chayote accessions, which contained 1.09 to 2.94% total sugars, and these values are similar to those obtained in this study, which ranged from 1.51 to 2.55%. Other cucurbits are reported to have higher sugar content, such as melon with 3.85–8.5% [49], pumpkin 9.39–10.49% [50] and watermelon 7.27–11.38% [51], but cucumbers are similar to chayote with 2.87–4.72% [52].

Fresh fruits and vegetables are highly perishable due to their high water content and active metabolism after harvest [53]. Tissue turgidity and a high rate of transpiration make them susceptible to cuticle damage and conidia germination causing disease [54]. On average, chayote had moisture contents above 90%; the highest content was found in n. maxima with 96.25%, v. levis with 95.34% and some morphotypes of n. spinosum and n. xalapensis with amounts above 95.0%. The lowest content was found in a. dulcis with 81.23%. The humidity of chayote is very close to that of other cucurbits such as cucumber, which contains 95.10 to 96.26% [47] and watermelon with a content of 90.35 to 92.41% [55].

According to the dendrogram (Figure 2), there were two main groups for chayote fruits: one group was formed by the light–dark green varieties and the other by yellow albus varieties. Also, there were four distinguished groups. In group I, there is only the varietal group n. maxima of very large size and very elongated shape, with the lowest color, acidity and TSS and the highest moisture content. Group II consists of the varieties v. levis, n. minor, n. spinosum and n. xalapensis, generally pyriform in shape, light and dark green in color, higher pigment content and medium-high moisture content. In group III, a. dulcis can be observed, which differs from the other albus varietal groups because it has the lowest moisture, slightly elongated pyriform shape and the highest TSS content and acidity. Finally, group IV includes the varieties a. levis gigante, a. levis, a. spinosum and a. minor, characterized by their yellow color and a color index close to 0, medium moisture content, very low chlorophyll and total carotenoids, generally higher TSS and total sugar content, and low stomatal index.

3.2. Postharvest Characterization

Chayote is a non-climacteric fruit with no significant changes in organoleptic characteristics after harvest and during storage. In Figure 3, the varieties are grouped according to the main postharvest disorders; a. minor and n. minor are similar due to rapid weight loss and dehydration of the fruit. The varieties a. levis, a. dulcis, a. levis gigante and a. spinosum had a high disease incidence, which causes darkening of the exocarp, while n. xalapensis and n. spinosum were susceptible to viviparism and fungal incidence, and v. levis and n. maxima were highly susceptible to blistering and viviparism.

At room temperature, the shelf life of chayote was short, from 3 days for a. minor to 11 days for v. levis. Díaz-Perez et al. [56] determined that when cucumber loses between 5 and 6% of its weight, the probability of being marketed is reduced by 50%, and when it loses more than 10%, this probability is 0%. In this work, it was determined that a weight loss of more than 10% was critical because at this level the fruits showed signs of evident dehydration and oxidation of the epidermis, although in a. minor and n. minor, these affectations were evident with losses of 6%. Weight loss during the storage of chayote in general was very high and increased with time (p < 0.001), losing between 6.29 and 22.53% in 8 days and from 11.48 to 35.57% in 14 days with a daily loss ranging from 0.82 to 2.38% (Table 2). Lower weight losses were found for other non-climacteric fruits like mandarin (Citrus reticulata Blanco) [57] or prickly pear (Opuntia Ficus-indica (L.) Mill.) [58]. On the contrary, pumpkin, due to its hard epicarp, lost 2.33% after 42 d of storage at 23 °C and 45% HR [59].

Weight loss is caused by metabolic processes such as respiration and transpiration. Larger fruits lose less weight than smaller ones [60]; in this case, the fruits of n. maxima and a. levis gigante and v. levis lost significantly (p <0.01) less weight than very small fruits such as n. minor and a. minor. The storage at 21 °C and 70% RH investigated in this study corresponds to the conditions under which the chayote fruits are marketed by local producers; changes in relative humidity can affect the quality of the fruits, due to water vapor pressure deficit. For example, pomegranate fruits (Punica granatum) stored at 20 °C and 65% RH lost more than 29% of their original weight, while fruits stored at 95% RH lost only 5.79% because this reduces the water vapor pressure deficit between the fruit and the environment [61].

High temperature and rainfall during chayote harvest season favors fungal attack and the presence of blisters, which affect the quality of the fruit. The infection can occur in the field or in the packinghouse; fungi such as Colletotrichum, Fusarium, Geotrichum, Phytophthora, Didymella and Chaetomium have been reported [62]. In this study, Phoma and Alternaria were also identified in most of the infected fruits (Table 2). In chayote fruits, it has been reported that brown rot is caused by Fusarium citri [63]. Blisters or bladders significantly affect the fruits of v. levis, a. levis gigante, n. maxina and a. dulcis; spines help to hide blisters in spinosum genotypes. It is generally accepted that blisters are caused by Colletotrichum sp. [62,64].

The premature germination of the seed inside the fruit is called viviparism, and in chayote, it is a major problem during storage and retail. It is especially important in n. spinosum and n. xalapensis, where around 50% of the fruits showed an exposed seed before day 10 after harvest and was higher than in v. levis (Figure 4). Previous studies showed that at harvest, the seed in v. levis is about 25% of its development and reaches physiological maturity on day 10 with the growth of the embryonic axis, leading to fruit opening [65]. During this stage, there may have been an increase in the activity of enzymes related to cell wall degradation metabolism such as α-galactosidases, β-1,3-glucanases, polygalacturonases and endotransglycosylases, which weaken the cellulose microfibrils and allow radicle protrusion. However, in this study, the germination of small fruits such as a. minor and n. minor did not appear during storage. Cucurbitaceae such as Lagenaria siceraria (Molina) also show viviparity at harvest, which increases to 97.84% during a season of heavy rainfall [66]. According to the above, as in fungal infections and blistering, viviparism also increases during the rainy season [63].

Increased α-amylase activity allows the hydrolysis of stored starch to form α-maltose and α-glucose, necessary to provide energy for germination [67]. During the first three days of storage, α-amylase activity remained constant in n. maxima and v. levis, decreased in n. spinosum and increased significantly in n. xalapensis. From day 3 onward, activity increases as viviparity increases. Interestingly, the activity in n. xalapensis, which showed the highest increase on day 3, was reduced on days 5 and 7 (Figure 5).

There is a complex hormonal relationship in the regulation of viviparity, abscisic acid (ABA) being a negative regulator and gibberellins a positive regulator. The expression of α-amylase-related genes is activated in the presence of gibberellic acid and negatively regulated by the presence of sugars in the embryo [68]. Similarly, the increase in α-amylase activity is related to an increase in ethylene production. In banana, transcription factors involved in the ethylene signaling pathway, such as MaEIL2, induce the transcription of the MaAMY3 and MaISA2 genes coding for α-amylases and isoamylases, increasing their activity and thus starch degradation [69].

3.3. Effect of 1-MCP on Chayote Fruits

The variables of weight loss and chilling injury are shown in Table 3 and Figure 6. Temperatures less than 10 °C usually cause CI in tropical and subtropical crops. Cucumber fruits are susceptible to CI at 5 °C [70], while zucchini fruits show symptoms of chilling injury from day 3 at 1 °C [71]. In this study, storage at 8.7 °C and 95% RH for 2 weeks caused CI in every varietal group. This agrees with previous studies that mention that chayote fruits v. levis, n. spinosum and n. xalapensis are susceptible to chilling injury when stored at low temperatures [12,13]; nevertheless, no reports are found for the other varieties.

1-MCP significantly reduced weight loss in all the chayote varieties except a. levis (Table 3). It has been proven that 1-MCP reduces fruit metabolism, such as respiration, transpiration and ethylene production reducing enzyme activity and starch degradation [72]. In tomato, 1 μL L⁻¹ 1-MCP induced wax and cutin biosynthesis affecting the gene expression for cuticle formation, reducing fruit transpiration [73]. Recent research reports that the effectiveness of 1-MCP is based not only on the fact that it blocks the ethylene receptors but also affects biosynthesis due to its affinity to the active site of ACC oxidase [74].

The symptoms of CI in chayote fruit appear as dark brown spots and slight sinking and drying of the epidermis, which occur in the first week in albus varieties. In the fruits stored for 2 weeks, v. levis (B) had a lower CI than n. minor and a. minor, but after 3 weeks of storage, this disorder was similar within varieties. CI is associated with the accumulation of ROS (reactive oxygen species), which leads to lipid peroxidation, causing damage to the cell membrane and higher weight loss [75]. The composition of the plasma membrane of cells is related to cold tolerance, with saturated fatty acids solidifying faster than unsaturated fatty acids when exposed to low temperatures. Changes in membrane fluidity lead to biochemical disorders such as ion leakage, oxidative stress and energy imbalance [76]. The reduction in CI and weight loss by 1-MCP can be explained by the maintenance of cell wall integrity. Lin et al. [77] showed that applications of 1.2 μL L⁻¹ of 1-MCP for 12 h delayed the softening of plums (Prunus salicina Lindl. cv. Younai), by decreasing the activity of pectinesterase, polygalacturonase, cellulase and β-galactosidase, preventing the wall disassembly. In addition to cell wall strengthening, 1-MCP stimulated stress tolerance mechanisms by maintaining increased activity of the enzymatic antioxidant system, such as superoxide dismutase, catalase, ascorbate peroxidase and glutathione reductase, reducing chilling injury in nectarine [Prunus persica (L.) Batsch, var. nectarine] [78].

Due to the high content of chlorophylls, the nigrum chayote groups are a good option for markets that demand colorful fruits with health benefits. Larger VGs such as n. maxima, a. levis gigante and v. levis and morphotypes of n. spinosum can be used for agroindustrial process to obtain purees. On the other hand, the more sweet-tasting albus groups can be used as a minimally processed vegetable or desserts.

4. Conclusions

The postharvest behavior of the chayote varies depending on the cultivar. The smallest fruits like a. minor and n. minor showed high weight loss and exocarp wrinkling, while the albus chayotes like a. levis, a. dulcis, a. levis gigante and a. spinosum had a high oxidation index. On the other hand, in n. xalapensis and n spinosum, viviparity and fungal attack are the main problems, while v. levis and n. maxima are highly susceptible to blistering and viviparism. 1-MCP managed to reduce chilling injury, weight loss and evident dehydration during cold storage. This treatment can be practically used to preserve the commercial quality of S. edule fruit varieties.

Author Contributions

E.A.R.-P.: Methodology and laboratory analysis; M.d.L.A.-G.: Conceptualization and writing; J.C.-I.: plant material and experimental design; M.S.-H.: laboratory analysis. Y.R.-R.: discussion and writing. C.G.-O.: fruit quality analysis. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Data is available upon request to the corresponding author.

Conflicts of Interest

The authors declare there are no conflicts of interest.

Footnotes

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Figures and Tables

Figure 1. Morphological characteristics of 10 varietal groups of chayote [Sechium edule Jacq. (Sw.)]: (1) a. minor; (2) a. levis; (3) a. dulcis; (4) a. spinosum; (5) a. levis gigante; (6) v. levis; (7) n. minor; (8) n. xalapensis; (9) n. spinosum; (10) n. maxima. The letters (a, b, c, d, e, f) represent different morphotypes within the same varietal group. Bar size 3 cm.

Figure 2. Cluster dendrogram of the physicochemical and morphological characteristics of 10 varietal groups of chayote [Sechium edule Jacq. (Sw.)]. Roman numerals I–IV indicate the groups similarity among chayote varieties.

Figure 3. (a) Hierarchical map of postharvest disorders that initially affect the loss of quality of 10 varietal groups of chayote [Sechium edule Jacq. (Sw.)]; (b) Fruit chayote varieties with different symptoms and disorders that can affect their organoleptic quality.

Figure 4. Percentage of germinated fruits of chayote [Sechium edule Jacq. (Sw.)] var. v. levis, n. xalapensis, n. spinosum and n. maxima at days 0, 3, 5, 7 and 14 in storage at room temperature (21 °C and 70% RH) (n = 15).

Figure 5. Activity of α-amylase of chayote [Sechium edule Jacq. (Sw.)] var. v. levis, n. xalapensis, n. spinosum and n. maxima at days 0, 3, 5, and 7 in storage at room temperature (21 °C and 70% RH). Different letters on the same day indicate significant differences between means according to Kruskall–Wallis (α = 0.05) (n = 6 ± SE).

Figure 6. Effect of 1-MCP on the commercial quality of chayote fruits after refrigerated storage (8.7 °C, 95% RH). Letters show the treatments: (a) control and (b) 1-MCP treatment. The cold storage for 2 weeks are (A) a. minor and (B) n. minor. The storage for 3 weeks are (C) a. dulcis, (D) v. levis (A,E) a. levis.

Table 1

Phytochemical and morphological characterization of 10 varietal groups of chayote [Sechium edule Jacq. (Sw.)].

Varietal Group	Shape	Length (cm)	Weight (g)	SF(Stomata/mm2)	Stomatal Size (µm)	SI(%)	CO*	Ca+b(µg g−1)	Cx+c(µg g−1)	TA (%)	TSS (°Bx)	TotalSugars (%)	Moisture (%)
a. minor	round	3.05–4.27	23.44 ± 0.64	11.30 b ± 1.37	28.70 bc ± 0.73	0.504	−0.184 ± 0.099	1.0 a ± 0.0	Nd	0.14 bc ± 0.004	5.42 bc ± 0.106	2.38 cd ± 0.12	90.97 bc ± 0.48
a. levis	pyriform	5.77–8.50	114.37 ± 11.30	6.89 ab ± 0.50	29.48 bc ± 0.81	0.190	−1.832 ± 0.165	Nd	Nd	0.11 ab ± 0.004	5.26 abc ± 0.104	2.39 cd ± 0.20	91.99 bcd ± 0.18
a. dulcis	pyriform	7.74–9.56	87.20 ± 1.74	19.52 cd ± 1.03	28.80 bc ± 0.79	0.444	−1.973 ± 0.081	Nd	1.0 a ± 0.0	0.18 e ± 0.007	5.56 c ± 0.160	2.55 d ± 0.28	81.23 a ± 0.78
a. spinosum	pyriform	7.30–7.90	214.08 ± 6.58	5.73 a ± 0.54	31.44 cd ± 0.92	0.142	0.341 ± 0.034	1.0 a ± 0.0	1.0 a ± 0.0	0.14 bc ± 0.004	5.02 abc ± 0.30	2.51 d ± 0.10	90.82 ab ± 0.19
a. levis gigante	pyriform	10.14–13.16	415.32 ± 21.13	17.35 cd ± 0.54	24.01 a ± 0.67	0.248	−1.071 ± 0.181	1.0 a ± 0.0	Nd	0.11 ab ± 0.005	5.18 abc ± 0.237	1.51 a ± 0.05	93.02 cde ± 0.19
v. levis	pyriform	7.68–12.50	197.56 ± 21.28	16.71 c ± 1.55	22.14 a ± 0.98	0.380	−4.793 ± 0.173	33 bc ± 4.0	5 bc ± 1.0	0.13 bc ± 0.005	4.82 a ± 0.216	2.17 bcd ± 0.15	95.34 ef ± 0.025
n. minor	pyriform	5.02–7.18	36.15 ± 1.67	7.23 ab ± 0.60	28.03 b ± 0.85	0.272	−8.87 ± 0.440	227 d ± 16.0	44 d ± 2.0	0.16 de ± 0.004	5.44 bc ± 0.104	2.25 cd ± 0.15	90.58 ab ±0.37
n. spinosum	round–pyriform	7.62–15.82	310.00 ± 33.59	24.66 d ± 1.93	35.24 e ± 0.76	0.579	−5.553 ± 0.538	68 bc ± 17.0	13 bc ± 3.0	0.12 ab ± 0.004	5.05 abc ± 0.283	2.34 cd ± 0.13	93.86 def ± 0.020
n. xalapensis	pyriform	7.27–10.75	252.16 ± 16.61	23.50 cd ± 1.88	32.47 de ± 0.76	0.666	−11.080 ± 0.416	166 cd ± 16.0	30 cd ± 3.0	0.14 bc ± 0.005	5.01 ab ± 0.276	1.76 ab ± 0.12	92.14 cde ± 1.31
n. maxima	elongated pyriform	12.35–15.99	511.32 ± 13.82	39.26 e ± 1.83	27.30 b ± 0.71	0.632	−14.322 ± 3.893	149 cd ± 9.0	23 cd ± 3.0	0.10 a ± 0.004	4.78 a ± 0.11	2.20 bcd ± 0.09	96.25 f ± 0.18
Mean	-	-	227.47	17.21	28.76	0.406	-	0.065	0.012	0.13	5.11	2.21	91.62

Fruit length (range); weight (mean ± SE, n = 20); stomatal frequency (SF) and stomatal size (mean ± SE, n = 30); stomatal index (SI) (mean, n = 20); color index (CO*) (mean ± SE, n = 20); total chlorophyll (C_a+b); total carotenoids (C_x+c); titratable acidity (TA); total soluble solids (TSS); total sugars and moisture (mean ± SE, n = 9). Different letters in the same column indicate significant differences (Kruskall–Wallis, α = 0.05).

Table 2

Weight loss (%) and shelf life (days) of 10 varietal groups of chayote [Sechium edule Jacq. (Sw.)] in storage at room temperature (21 °C and 70% RH) (n = 15 ± SE).

Varietal Group	Weight Loss		Shelf Life (Days)	Pathogen Isolated
	Daily (%)	Level
n. minor	2.38 ± 0.13	High	4	Colletotrichum sp., Fusarium sp.
a. levis gigante	0.82 ± 0.03	Low	10	Colletotrichum sp., Phoma sp.
n. spinosum	1.62 ± 0.08	Medium	7	Colletotrichum sp., Fusarium sp., Phoma sp., Alternaria sp., Aspergillus niger
n. xalapensis	1.29 ± 0.06	Medium	9	Phoma sp., Colletotrichum sp., Fusarium sp., Alternaria sp., Aspergillus niger
a. levis	1.34 ± 0.08	Medium	7	Phoma sp.
a. dulcis	1.56 ± 0.09	Medium	7	Phoma sp., Colletotrichum sp.
n. maxima	1.07 ± 0.06	Low	9	Phoma sp., Colletotrichum sp.
a. minor	2.54 ± 0.15	High	3	No incidence
a. spinosum	1.45 ± 0.10	Medium	7	No incidence
v. levis	0.87 ± 0.3	Low	11	Colletotrichum sp., Phoma sp., Alternaria sp., Epicoccum sp.

Table 3

Effect of 1-MCP on weight loss (% WL) and CI index (0–3) after 2 weeks of cold storage (8.7 °C, 95% RH) for n. minor, a. minor and v. levis (B) and 3 weeks for a. levis, a. dulcis and v. levis (A).

Treatment		n. minor	a. minor	v. levis (B)	a. levis	a. dulcis	v. levis (A)
Control	%WL	17.20 ± 0.77 b *	18.99 ± 0.819 b	13.29 ± 0.55 b	14.34 ± 1.62 a	17.91 ± 1.324 b	10.22 ± 0.59 b
	CI	2.2	2.2	1.3	2.3	2.2	2.2
1-MCP	%WL	10.97 ± 0.76 a	13.52 ± 0.600 a	9.25 ± 0.50 a	11.36 ± 0.91 a	14.40 ± 0.598 a	6.62 ± 0.32 a
	CI	0.9	1.1	0.7	1.6	1.2	1

* Means ± SE with different letters in the same column for % WL are significantly different according to Kruskall–Wallis (α = 0.05) (n = 15 ± SE).

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