Fruit Cracking in Melon

Abstract

Melon (Cucumis melo L.) is an economically profitable crop in temperate and tropical regions. Melons vary in shape, size, and skin texture; they are classified under different varieties. Fruit cracking is a general problem of melon fruit grown worldwide. This physiological disorder intensively affects the production and marketable value of fruits. Studies revealed that fruit cracking causes a 70% loss in the economy of the melon fruit industry. The cracking becomes more visible when the fruits reach maturity; no single factor is known to prevent it effectively. The severity of fruit cracking depends on the nature of the variety, climate zone, where the variety is grown, fruit growing patterns, and cultural practices. It has also been linked to improper irrigation, environmental factors, and nutritional deficiencies, particularly boron, calcium, zinc, and potassium. Horticultural practices, such as spraying growth promoters, micronutrients, antitranspirant, and regular drip irrigation with mulching, have been recommended to avoid fruit cracking in melon. Although fruit cracking is a significant economic risk, research on its cause and management in melon is limited compared to other fruit crops. Hence, the present chapter summarizes the underlying causes of melon fruit cracking and potential control strategies to reduce melon fruit cracking.

1. Introduction

Melon (Cucumis melo L.), a major cucurbit cultivated around the world, exhibits significant morphological, physiological, and molecular variations at the cultivar subgroup level [1, 2, 3]. Considering the most significant character, i.e., ovary harness, that distinguishes the taxa, the species Cucumis melo is divided into subspecies Cucumis melo ssp. melo and Cucumis melo ssp. agrestis at the intraspecific level [4, 5, 6]. This subspecies was further classified into different botanical groups or variants, resulting in quadrinomial nomenclature (e.g., Cucumis melo ssp. agrestis var. momordica) [7] as per the studies done by Hammer et al.; Pitrat et al., Pandey and Anju [8, 9, 10] five melon types are included under Cucumis melo ssp. Agrestis, while 12 botanical variants are enlisted in Cucumis melo ssp melo. The intraspecific classification of melon varieties is depicted in Figure 1 [11].

Figure 1.

Intraspecific classification of Cucumis melo.

Melon cracking is a physiopathy that causes both internal and external changes in the fruit by disrupting the water balance and nutrient homeostasis. Fruit cracking occurs naturally at the end of fruit development, just after ripening and before seed dispersal [12]. It is a physiological disorder that affects the exocarp and mesocarp and can be distinguished from epidermis cracking, which is more superficial and includes the cuticle and epidermal tissue [13]. Deeper cracking is distinguished by an opening to the pulp’s interior, known as splitting [14, 15]. The fruit rind is pertinent for crack resistance, portability, storability, and quality during storage (shelf-life quality) [16]. Thin-rind varieties, such as var. momordica, var. conomon, and var. hami, are more prone to cracking and causing more damage during transportation or retail display. As part of the dehiscent mechanism, climacteric varieties are predisposed to blossom end cracking [17].

Fruit cracking may be caused by high evapotranspiration, low relative air humidity (R.H.), water imbalance, and sharp temperature fluctuations during fruit growth and development [18]. It has also been linked to improper irrigation, environmental factors, and nutritional deficiencies, particularly boron, calcium, zinc, and potash [19, 20]. When the fruits reach maturity, the cracking becomes more visible [21, 22]. Several horticultural practices, such as spraying growth promoters, micronutrients, antitranspirants, and regular drip irrigation and mulching, have been recommended for fruit cracking management [23].

This book chapter reviews the research and information on fruit cracking, an essential physiological disorder of melon varieties, in terms of different patterns of cracking, factors associated with it, and management practices to control its intensity.

2. Patterns of melon fruit cracking

Cracking is a notable disorder that can result in significant losses of marketable yield and revenue in the fruit industry. Variations can also be seen in the cracking pattern. Longitudinal cracking, burst cracking, ring or concentric cracking, crazing or russeting, star or radial cracking, and core failure are the different types of fruit cracking in melon. Snap melon has a wide variety of cracking patterns (Figure 2a and b) [24]. Melon fruit cracking or splitting around the distal (blossom-end) is thought to be a seed dispersal mechanism associated with post-climacteric senescence [17, 25, 26, 27]. These include both longitudinal and random cracking patterns that begin around the fruit’s equatorial region.

Figure 2.

(A) Pattern of fruit cracking in Indian snapmelon. (B) Fruit cracking in culinary melon and water melon.

3. Factors affecting fruit cracking in melon

The severity of melon fruit cracking depends on the nature of the variety, the climate zone where the variety is grown, fruit growing patterns, cultural practices, etc. It has also been linked to improper irrigation, environmental factors, and nutritional deficiencies, particularly boron, calcium, zinc, and potassium (Figure 3).

Figure 3.

Factors affecting fruit cracking.

3.1 Plant factors

3.1.1 Plant morphological characteristics

Fruit cracking is influenced by the morphology of the fruit’s surface. Characteristics such as netting, netting density, and sutures all play essential roles in melon fruit cracking. Deeply sutured melon cultivars are more vulnerable than non-sutured melon cultivars. The deep, sutured, and thin rind of Canary melon causes it to crack early. Thick cuticle deposition on the netted region reduces rind elasticity during fruit expansion, whereas rapid water intake raises turgor pressure in the flesh and makes the fruit more susceptible [28].

The shape of melon fruits impacts cracking. Martinez et al. [17] reported that round melons are less susceptible to cracking than oblong melons. Many morphological characteristics influence cracking sensitivity, including cuticle thickness and physical properties, the number of hypodermal layers [29], fruit shape, and fruit size [30, 31, 32]. However, calcium content and pectin value, cell wall structure and component [33], and the quantity and volume of intercellular spaces could all influence these properties [34]. Skin’s biomechanical properties play an important role in maintaining internal pressure and resisting fruit cracking. According to Lane et al. [35], the fruit skin’s role in cracked resistance is related to the calcium content of epidermis cells, which increases cell integrity.

Many physical characteristics contribute to fruit cracking resistance. The sensitive cultivar had thinner skin, a higher seed-to-skin weight ratio, and was significantly smaller in size. According to Aloni et al. [36] & Matthews et al. [37], when exposed to water stress, peel mechanical properties change, with decreased peel extensibility leading to thicker and stiffer peel. These changes may account for the cracking of pomegranate fruits [38].

3.1.2 Varietal influence on melon fruit cracking

Fruit cracking susceptibility is thought to be inherited and the severity of cracking varies greatly between varieties; Cuartero et al. [39] discovered that genetic characteristics influence fruit cracking. Fruit cracking is more or less common in different cultivars [40]. Multiple genes regulate fruit cracking [41]. Intraspecific classification of melon species revealed about 16 varieties of melon, with significant variation in their cracking pattern. Thin rind melon types, such as Hami melon, snapmelon, and conomon group, were found to be more prone to cracking than thick rind melon types such as reticulatous indorous and group. The high intensity of cracking may also be due to Hami melon’s high sweetness (total soluble solids 11–15%). Cracking disorder in thin-rind cultivars is exacerbated when cracks form after harvest, during transportation, or during retail display. As a result, such cultivars require special handling during transportation, in the laboratory and at retail.

Snapmelon, an Indian native, is also known as ‘Phoot,’ which means ‘to split,’ and has a variety of cracking patterns. In this variety, the presence of incipient fruit cracking or splitting indicates harvest readiness or fruit maturity [42]. Cracking occurs in the Charentais-type cultivar Vedrantais as a result of high ethylene production as part of the seed dispersal mechanism [25]. During harvest or transportation, an explosive type of cracking was observed in an indorous variety, known as Piel de Sapo-type cultivar T111. It is a cultivar with deep sutures that cracks easily [43].

3.1.3 Hormonal influence on melon fruit cracking

Ethylene is a naturally occurring plant growth regulator that has a variety of effects on the growth, development, and storage life of many fruits. Climacteric fruit ripening in melon is associated with induced ethylene biosynthesis. Ethylene peak within 2 hours in the senescent overripe fruits of snapmelon and cantaloupe melon fruit in the open field is usually associated with stem end cracking in. According to Fernandez Trujillo [44], a small burst of ethylene production more significant than 1–4 Pmol/kg/g in melon is a sign of fruit splitting or other mechanical damage.

Fruit cracking has also been linked to an imbalance of auxins, gibberellins, and cytokinins in various crops [45]. Normal fruits contain more gibberellins and less ABA [41, 42], and an imbalance between the two causes fruit cracking [46]. Gibberellin (GA3) can increase cell wall plasticity, promoting cell growth and cell extension and possibly preventing fruit creasing [47]. Yilmaz and Ozguven [48] discovered that the ABA content of the peel was higher in cracked fruits than in healthy (non-cracked) fruit.

3.2 Environmental factor

According to Ikram et al. [49], direct sunlight raises the temperature and evapotranspiration of the fruit surface, resulting in high moisture loss and increased cracking susceptibility. Fruit cracking susceptibility increases in the field when the day temperature exceeds the light temperature at the end of the ripening process [50]. Intense solar radiation significantly raises the temperature of the fruit, which raises the internal turgor pressure of the pulp inside the rind. Landg and During [51] suggested the effect of intense solar radiation as a cause of melon fruit cracking.

A high relative humidity (R.H.) prevents transpiration, which causes a high pressure inside the fruit, causing the epidermis to crack [52], especially during prolonged periods of 99–100% R.H., alone or in combination with rain. Due to transpiration-inhibiting effects, high relative humidity conditions in the air have been observed to cause fruit cracking, both in the field and in storage [15]. This effect becomes more pronounced whenever nighttime temperatures drop [53]. Fruit cracking can occur during storage due to abrupt changes in R.H. or temperature [15], mainly when the R.H. is high.

Temperature increases, hot, dry winds, a downpour after a dry season, and significant differences in day-night temperatures with temperatures greater than 38°C combined with an R.H. of 60% all favor fruit cracking in most of the crops [54].

3.3 Relative water content on the fruit surface and soil

Fruit cracking is caused primarily by high water content in fruit, especially during the monsoon season or after the first rains after a dry season [15]. Excessive water can cause classic watermelon fruit split incidents. This is common in melons, as well as tree fruits such as plums. Irrigation must be applied evenly, especially during the last 2 weeks of growth. Too much water at this stage may cause the fruit to crack.

Fruit cracking in apples and sweet cherries occurs as a result of excessive cell enlargement of the fruits following a significant increase in soil moisture [55]. The strength of the skin is affected by changes in soil moisture during fruit development. When the moisture content of the soil is reduced, skin strength improves. Skin strength, on the other hand, decreases with increasing soil moisture content [56, 57]. The plant water gradient induces the process of water loss from the plant to the atmosphere via the fruit surface or leaf area surface in the soil-plant-atmosphere continuum, which dramatically influences water relations, and thus leads to the incidence of cracking in fruits because the high demand for water during fruit growth and development is met by the leaf via the source-sink relationship [58]. The development of high hydrostatic pressure in the fruit (turgor pressure) more remarkable than the tensile strength of the cell walls under conditions of high-water availability and low evaporative demand has been attributed to the cracking or splitting of fruits caused by unseasonal rainfall [59].

3.4 Nutritional factors

Fruit cracking is caused by excessive nitrogen fertilization combined with insufficient potassium or calcium in the field [60]. Premature cracking in melon cultivars is caused by an insufficient supply of nitrogen and potassium in the open field (Figure 4) [39]. The phosphorus (P) content of peel during the fruit cracking period is positively related to the rate of fruit cracking. The fruit peel becomes thinner as the P level rises. When citrus fruit lacks phosphorous, it shrinks, and the peel becomes rougher.

Figure 4.

Premature cracking in var. momordica and var. acidulus.

Potassium can maintain high osmotic and turgor pressures, which can provide energy for cell division, cell wall extension, and cell expansion, thereby accelerating cell growth. A high potassium content can increase fruit size and make the peel thick and smooth; on the other hand, a low potassium content can cause fruit cracking and dropping, resulting in smaller fruit, thinner peel, and a reduction in soluble solids content (SSC), organic acids, and vitamin C [61]. However, some studies have found that the potassium content of cracked fruit peels is higher than that of regular fruit peels [62].

Boron is essential for the promotion of cell division and the synthesis of cell walls [63]. Because of the association of cell wall composition, boron may aid in the maintenance of cell wall integrity and toughness [64, 65, 66]. According to Wu et al. [67], boron can enhance the regulatory function of endogenous growth regulators. Increased boron content in the fruit peel reduces fruit cracking.

Calcium is required for proper plant growth and development because it performs metabolic functions in nutrient uptake, as well as abiotic and biotic stress resistance. Calcium, an important component of cell walls, contributes to cell wall cohesiveness and strength [64], and its concentration was higher in normal fruit than in cracked fruit [68]. Calcium is most likely involved in the increased elasticity, strength, and thickness of epidermal cell walls. Calcium also aids in the deposition of pectin, making fruit more resistant to cracking under the higher rates of turgor pressure seen during water stress. Calcium and magnesium are in charge of strengthening the bonds between epidermal and other fruit cells, resulting in increased strength and decreased cracking [69]. Furthermore, Ca’s role in preventing the formation of an abscission zone between fruit pedicles and bearing branches, as well as regulating enzyme activity and photosynthesis [70, 71, 72], could result in fruit splitting percentage control.

Zinc is also essential in regulating water absorption by plant roots. Fruit cracking has been reduced by a concurrent antitranspirant. Antitranspirants reduce transpiration and regulate fruit skin elasticity by reducing stomatal opening and increasing leaf resistance to water vapor diffusion without affecting carbon dioxide uptake. It also reduces the absorption of radiant energy, lowering leaf temperatures, and transpiration rates; wax emulsions on fruit form thin transparent films that hinder the escape of water vapor from the fruit surface and prevent stomata from opening fully and lowering transpiration [73]. These may be possible reasons for lowering the incidence of fruit cracking in pomegranates.

4. Managemental practices to control fruit cracking

• Orchard floor management should be implemented to prevent rapid changes in soil moisture to protect susceptible fruits from cracking [74]. Drip irrigation, mulching with organic and inorganic sources, spreading compost or manure over the soil, green manuring, and growing cover crops are all good ways to conserve soil moisture by reducing evaporation, protecting the soil from heat and sunlight, and making good use of the available water.

• Cracking occurs due to excess solar radiation, and sunburn can be minimized by shade nets, bagging of individual fruits, or spraying the entire crop with particle films such as kaolin, aluminum silicate clay, or calcium carbonate [75]. These shade nets and particle film reduces radiation stress in crops by protecting the foliage and fruit from ultraviolet and infrared radiation. Excessive evapotranspiration from the fruit surface results in excessive moisture loss when exposed to direct heat. Bagging also aids in the prevention of cracking, the protection of fruit from insects or pests, and the improvement of fruit quality [76].

• Growth regulators, such as forchlorfenuron (CPPU), a cytokinin, and benzothiadiazole, are effectively used in watermelon and Hami melon, respectively, in China in order to control premature cracking.

• Balanced nutrition: Adequate potassium and calcium can promote the development of thicker rinds that are less susceptible to cracking because potassium and calcium play a crucial role in membrane and cell wall integrity. Foliar application of calcium and potassium during fruit development and the maturation stage improves melon fruit texture through turgor maintenance [77].

• Foliar application of micronutrients, such as calcium, zinc, and boron, is effectively used to control melon craking in C. melo cv. Grand Riado (Sakata) in Spain [78].

• Recent advances in nanotechnology indicate that using nanomaterials in agriculture may aid in increasing productivity. Furthermore, nanofertilizers and nanonutrients can help to reduce fruit cracking. Davarpanah et al. [79] investigated the effects of foliar fertilization with nano-nitrogen and urea fertilizers containing nanoparticles (nN) on pomegranates, finding that fruit quality and yields improved. Furthermore, they also reported that foliar treatment with the nano-calcium fertilizer significantly reduced fruit cracking in pomegranates when compared to the control treatment.

• Grafting of susceptible cultivars with resistant rootstock with a vigor root system reduces the incidence of cracking [80].

5. Conclusion

The problem of fruit cracking in melons is a complex phenomenon that is caused by a variety of factors. Advances in research on fruit cracking in terms of causes, physiology, and effective management clearly demonstrate that the problem cannot be considered in isolation but that both external and internal conditions must be considered holistically. For fruit cracking management, horticultural practices such as spraying growth promoters, micronutrients, antitranspirants, and regular drip irrigation with mulching have been recommended. Each factor can only be advocated as being sufficiently effective in controlling fruit cracking. At best, the role of integrated orchard management, which aims to reduce water, nutrition, and physiological factors that contribute to fruit cracking, should be considered. Several studies have confirmed that nanoparticle-based fertilizers have the potential to increase crop yield and quality under various biotic and abiotic stress conditions. However, the use of nano-based fertilizers to prevent fruit cracking is still in its infancy. Despite the fact that it is a natural occurrence, proper monitoring and development of accurate management practices are required to overcome the severity of this natural threat and avoid massive economic loss in the melon fruit industry.