Psidium guajava L. is among the most cultivated and consumed tropical fruits in the world, both fresh and processed (Gill, Dhaliwal, Mahajan, Paliyath, & Boora, 2016). According to official data, in 2018 the production volume of this fruit in Mexico amounted to 324,665 t, with a planted area of 22,561 ha. The main producing states are Aguascalientes, State of Mexico, Michoacán and Zacatecas, among which Michoacán stands out with 11,079 ha and an average yield of 14.6 t∙ha-1 (Servicio de Información Agroalimentaria y Pesquera [SIAP], 2018). About 99.2 % of guava production in Mexico destined for export is marketed in the United States, Canada, United Kingdom, Germany and Spain (SIAP, 2018), where it is considered an exotic fruit.
Guava fruit quality is defined by the genotype and agronomic management of the trees (Damián-Nava et al., 2004; Salas-Araiza & Romero-Nápoles, 2012), in which the inclusion of harvest indices is important because they guarantee the producer fruit with the minimum quality for post-harvest handling and marketing (Azzolini, Jacomino, & Urbano, 2004). These indices include physical (color, firmness and characteristic shape) and chemical ones (titratable acidity, total soluble solids and ascorbic acid) (Herman-Fisher et al., 2011), and it is essential to know their response to the presence and attack of pests and diseases (Delgado & Sáenz-Aponte, 2016).
Pests that attack guava include the guava fruit fly (Anastrepha striata), temolillo (Cyclocephala lunulata), glasshouse whitefly (Trialeurodes vaporariorum), green shield scale (Pulvinaria psidii), aphids (Aphis gossypii and Myzus persicae), thrips (Frankliniella spp.), mealy bug (Pseudococcus spp.), potato leafhopper (Empoasca fabae) and mites (Eotetranychus sp., Panonychus sp., Tetranychus urticae and Oligonychus sp.) (Salas-Araiza & Romero-Nápoles, 2012). However, in recent years the guava weevil (Conotrachelus spp.) has become one of the phytosanitary management problems with the greatest economic impact within the main guava production areas in Mexico (Vargas-Madríz, Martínez-Damián, & Mena-Nevárez, 2017).
It is important to highlight that within the genus Conotrachelus, the species C. dimidiatus is one of the most aggressive, especially when it is in larval state because it feeds on the fruit and seeds causing petrification and premature ripening of the fruit, giving it an unpleasant appearance that prevents its marketing (Gill et al., 2016; Augusto-Monroy & Ildefonso-Insuasty, 2006). Infestation levels can reach 70 % of the planted area and generate losses of 60 % of the production value (Aragón-García, Pérez-Torres, Vera-Cano, Trejo-Vázquez, & Mota-Nava, 2015; Delgado & Sáenz-Aponte, 2016), which makes it necessary to carry out research aimed at the efficient control of Conotrachelus spp. in its larval stage.
The study of the damage of Conotrachelus spp. larvae on the quality of guava fruit is recent, since there is little or no information on it, which includes aspects such as the reproductive habits of Conotrachelus spp. (Aragón-García et al., 2015), rearing (Iglesias, Lucia & Machado, 2014), hydrothermal control treatments (Vargas-Madríz et al., 2017) and biological control in the field and greenhouse (Delgado & Sáenz-Aponte, 2016). Therefore, the objective of this research was to evaluate some physicochemical quality parameters in guava fruit with presence of C. dimidiatus (Champion) in larval stage.
Materials and methods
Location of the experiment and plant material. The experiment was carried out during August and September 2016 in the Fruit Physiology Laboratory of the Department of Plant Science at the Universidad Autónoma Chapingo, Mexico. Guava fruit of the commercial variety Media China were used, infested with C. dimidiatus larvae, from the "Rancho La Abuela" orchard located in the municipality of Tolimán, Querétaro de Arteaga, Mexico (20° 53’ 38.8’’ NL and 99° 54’ 59.9’’ WL, at 1,167 masl), with a mean annual temperature of 24.9 °C.
Fruit were collected according to harvest indices used by producers in the Queretaro region, i.e. with firm pulp and the onset of the change from dark green to light green (yellowish). In order to ensure the presence of C. dimidiatus larvae, each fruit was visually recorded and corroborated with an optical microscope in the laboratory.
Experimental design. A completely randomized design was established, where the experimental unit was a set of nine fruit with 15 replicates in the determinations of fruit weight, firmness, length, diameter and color, and three fruit as the experimental unit with 24 replicates for the titratable acidity, total soluble solids and vitamin C analyses.
Fruit weight (g). It was determined by means of a portable electronic scale (Scout Pro SP202, Ohaus®, USA) with a precision of 0.01 g.
Firmness (N). It was measured in the equatorial zone of the fruit with a digital texturometer (Compact Gauge, Mecmesin CE®, USA) fixed on a table, with a conical strut of 9 mm in diameter and height.
Fruit length and equatorial diameter (mm). They were obtained with a digital Vernier caliper (model 122200, Surtek®, Mexico).
Color: It was determined on the epidermis of the equatorial part of the fruit by means of a portable sphere spectrophotometer (X-Rite, SP-62®, USA), in which the CIE 1976 color coordinates (L*a*b) (Voss, 1992) were obtained; later, the chromaticity [C* = (a2+b2)1/2] and hue [arctan-1 (b/a)] values were determined.
Titratable acidity (% citric acid). It was determined according to the methodology proposed by the Association of Official Analytical Chemists (AOAC, 1990), with slight modifications. Ten g of fruit were homogenized with 50 mL of distilled water; from this mixture, 10 mL were taken to be neutralized with a 0.1 N NaOH solution, in which 1 % phenolphthalein was used as indicator.
Total soluble solids (°Brix). The concentration of soluble solids was quantified with a portable digital refractometer (PAL-1®, Atago, EUA), which uses a scale of 0-53°.
Vitamin C (ascorbic acid). It was estimated according to the Tillman method (AOAC, 1990), known as DFI-2, 6 dichlorophenol-indophenol, for which an aliquot of 10 mL was taken from a mixture of 5 g of finely chopped fruit homogenized with 50 mL of oxalic acid.
The data obtained were analyzed with Student's t-test (P ≤ 0.05) for two populations, using Sigma Stat 4.0 (Systat Sofware, Inc., San José California, USA).
Results and discussion
Determining the commercial value of any fruit and vegetable product begins with a subjective inspection of its size and morphology; therefore, adequate nutritional management and phytosanitary control is important (Damián-Nava et al., 2004; Herman-Fisher et al., 2011). According to the statistical analysis, guava fruit infested with C. dimidiatus larvae showed a weight decrease of 61.2 % with respect to those not infested, with an average value of 20.73 g (Table 1). Aragón-García et al. (2015) report that the female C. dimidiatus oviposits larvae in the early stages of fruit development, and that this behavior could suggest an alteration of the continuous flow of sap and nutrients through the phloem and xylem, which alters fruit development and growth (Tafoya, Perales-Segovia, González-Gaona, & Calyecac-Cortero, 2010). Also, Rajan, Yadava, Kumar, and Saxena (2008) indicate that the high concentration of auxins, gibberellins and cytokines in immature seeds is directly related to the final size of the fruit. Additionally, it is known that most varieties of guava are diploid (2n=22) and have a very variable number of seeds (2-463) (Gill et al., 2016), and that the proportion of pulp in the fruit is closely correlated with its weight and number of seeds (Pandey, Jain, Sharma, & Tiwari, 2002).
|Treatment||Fruit weight (g)||Firmness (N)||Fruit length (mm)||Equatorial diameter (mm)|
Changes in fruit firmness are associated with the degree of pectin solubilization due to the enzymatic activity of pectinesterase, polygalacturonase and pectate lyase (Azzolini et al., 2004; Gill et al., 2016), and it manifests itself during the ripening process with the weakening of the primary cell wall and middle lamina (Parra, 2014). In this case, the firmness showed a statistically different behavior (P < 0.05) between fruit with and without the presence of larvae, with values of 23.32 and 16.55 N, respectively (Table 1).
Delgado and Sáenz-Aponte (2016) observed that the presence of C. psidii Marshall larvae causes growth to stop and the volume of fruit juice to decrease, negative aspects that contribute to the reduction of its final size and the appearance of petrified fruit, without this meaning its senescence; i.e., it increases the synthesis and autocatalytic accumulation of ethylene that leads to a heterogeneous ripening process, and in most cases it concludes with the presence of fruit that are not suitable for consumption (Aragón-García et al., 2015). In this regard, Tafoya et al. (2010) suggest that this is attributable to a decrease in the photoassimilate translocation rate of the leaves through phloem, which initially affects the hardening of the constituent polymers of the cell walls (Farenczi, Gambetta, Franco, Arbiza, & Gravina, 1999; Parra, 2014) and culminates with the fall of the fruit and the exit of the larva for its later concealment in the soil, where it continues with its development as a pupa (Iglesias, Lucia, & Machado, 2014).
Fruit length and equatorial diameter
Values related to the equatorial diameter of fruit infested with larvae were significantly (P < 0.001) lower (36.96 mm) compared to healthy fruit (48.73 mm); similar behavior was presented in fruit length with 31.76 and 44.75 mm, respectively (Table 1). Montoya-Holguin, Cortés-Osorio, and Chaves-Osorio (2014) point out that the relationship between the two parameters (length and diameter) is closely related to the characteristic shape pattern of the fruit of each variety, so if this relationship is <1 or >1 they are flattened or elongated shapes, respectively. In this case, the evaluated fruit had an elongated shape and only an important effect on fruit size was observed as a consequence of the feeding and development process of the C. dimidiatus larva. Farenczi et al. (1999) indicate that characters such as fruit diameter and length, in addition to being determined by the genotype, are regulated by environmental and internal factors of the plant, where the growth is based on the level of contribution of photoassimilates and the capacity of the developing organs to exert sufficient demand on the points of production (leaves).
Statistical analysis did not reveal variation (P ≤ 0.05) between treatments for color components related to brightness (L) and °Hue; however, significance was found for the chromaticity values determined in fruit with and without the presence of larvae with values of 38.75 and 35.56, respectively (Table 2), which was manifested in physical form with fruit of greater yellow coloration. Damián-Nava et al. (2004) report that the variety Media China has a climacteric behavior, and that one of its most outstanding quality characteristics is its distinctive yellow color, whose development and stability on the epidermis of the fruit already harvested is influenced by the agronomic management and environmental conditions present during its growth and onset of physiological maturity (Gill et al., 2016; Mercado-Silva, Benito-Bautista, & García-Velasco, 1998).
|L (brightness)||C (purity)||°Hue|
Herman-Fisher et al. (2011) and Parra (2014) emphasize that any mechanical damage suffered by the fruit during its pre- and post-harvest handling affects its ethylene autocatalysis, which prematurely activates the activity of various enzymes such as chlorophyllases, which cause the appearance of secondary colors (yellow) to the original (Azzolini et al., 2004). In this sense, the results observed in chromaticity or color purity can be explained, to a large extent, by the serious damage caused by the C. dimidiatus female when drilling a 2-mm-deep hole over the equatorial area of the fruit (Aragón-García et al., 2015; Tafoya et al., 2010), this without considering that the accumulated excrement of the larvae causes a slight fermentation of the fruit, negatively affecting the synthesis and accumulation of ethylene, which prevents its use in industry or as animal feed (Salas-Araiza & Romero-Nápoles, 2012).
Statistical analysis did not detect differences between fruit with and without larvae, whose values fluctuated between 9.11 and 11.90 % citric acid, respectively (Table 3). In contrast, Mercado-Silva et al. (1998) indicate a decreasing behavior in the acidity values of guava fruit in the growth stages (I and II), corresponding to the intervals of 13-43 and 43-98 days to flowering, respectively, and an increase in acidity at the beginning of the natural maturity of the fruit (stage III), in the interval of 98-133 days to flowering. In this context, it is important to point out that as part of the weevil's reproductive habits, its attack is accentuated in the early stages of fruit development (I and II) (Iglesias, Lucia & Machado, 2014; Tafoya et al., 2010), which coincides with the fact that some larvae abandon the fruit, and those that do not move towards the inner part where they feed on seeds and pulp; this causes malformation and premature and inhomogeneous ripening of the fruit (Vargas-Madríz et al., 2017). Salas-Araiza and Romero-Nápoles (2012) point out that it is common for stage III development to conclude with the abscission of the fruit.
|Treatment||TA1 (% citric acid)||TSS (°Brix)||VC (mg ascorbic acid·100 g-1)|
Total soluble solids (TSS)
The polysaccharides that make up the cell wall function as substrates for obtaining energy during the metabolic respiration process (Parra, 2014), which generates simple sugars such as sucrose, glucose and fructose, which are considered essential components of the characteristic sweetness in the fruit (Herman-Fisher et al., 2011). As shown in Table 3, the TSS concentration did not vary (P ≤ 0.05) between fruit with and without larvae, whose values were 8.33 and 9.12 °Brix, respectively. It has been reported that the physiological behavior of the genus Psidium is variable (Damián-Nava et al., 2004) due to a varietal effect and at harvest time (Gill et al., 2016). In the first case, Damián-Nava et al. (2004) mention that the TSS concentration in genotypes with white and red pulp varies between 5.0 and 13.2 °Brix, while in the second case there is no consensus regarding the ideal maturity stage of this fruit, since its harvest is carried out considering commercial rather than physiological aspects (Vargas-Madríz et al., 2017), i.e., when the fruit is still firm and the change from dark green to yellowish tones begins. In this sense, the values obtained could be associated with the degree of maturity with which the fruit were harvested.
Vitamin C (ascorbic acid)
Guava fruit has several bioactive compounds of nutraceutical interest such as vitamin A, group B vitamins (thiamine and niacin), pectins and minerals (phosphorus, calcium, iron and potassium) (Delgado & Sáenz-Aponte, 2016); it is also an excellent source of antioxidants (such as phenolic compounds and ascorbic acid) (Restrepo-Sánchez, Narváez-Cuenca, & Restrepo-Sánchez, 2009). These compounds play an important role in the prevention of chronic and degenerative diseases (Rajan, Yadava, Kumar, & Saxena, 2008). Therefore, the determination of ascorbic acid content (vitamin C) was of vital importance in this work, and it was found that fruit with C. dimidiatus larvae showed a significant decrease in vitamin C (2.8 mg ascorbic acid·100 g-1) in relation to what was observed in fruit without the presence of this insect (11.71 mg ascorbic acid·100 g-1) (Table 3).
The dissimilarity in the concentration of this vitamin may be linked to the disorganization of the cell membrane caused by the damage produced by the larva. This is because the enzymatic system of ascorbic acid oxidation is controlled (Herman-Fisher et al., 2011; Mercado-Silva et al., 1998), and the damage caused by the larva activates the oxidation of this compound at a higher speed. Gill et al. (2016) indicate that the content of this acid varies depending on the state of maturity, storage conditions of the fruit and analysis method of the fruit, where temperature is an important factor of degradation.
Fruit with the presence of C. dimidiatus (Champion) showed a significant decrease in weight, size and vitamin C content, whereas titratable acidity and total soluble solids remained unchanged. However, an increase in firmness (23.32 N) (petrified fruit) and color purity was observed. Considering the above, it is suggested to conduct research to generate efficient control methods of this larva, due to the implications related to its development and growth.