Fruit Cracking in Pomegranate: Extent, Cause, and Management – A Review

ABSTRACT

Pomegranate (Punica granatum L.) is an economically important fruit crop of tropical and subtropical regions of the world. There has been enormous increase in area, production, and export worldwide over the past decades. But cracking of mature fruit is an important physiological disorder which causes great economic loss to pomegranate. As high losses as 65% have been reported in pomegranate. It is a general problem throughout its growing areas and among all varieties worldwide. Causes associated with fruit cracking may be improper irrigation, environmental factors, and nutritional deficiency, especially boron, calcium, and potash. Besides, it is also reported to be associated with high evapo-transpiration, low humidity, water imbalance, and sharp temperature fluctuation in day and night during fruit growth and development. The cracking is more evident when the fruits are at maturity stage. No single factor can be advocated as efficient enough in controlling fruit cracking. The behavior of fruit cracking in relation to internal fruit composition and quality characteristics, dynamics of water uptake, understanding on how water, gibberellins, abscisic acid, boron, calcium, and the cell wall biosynthesis interacts with fruit cracking, will offer a clearer insights in devising developmental strategies to reduce fruit cracking.

KEYWORDS:

• Causes
• fruit cracking
• physiology
• Punica granatum
• management
• mechanism
• soil-plant-water status

Introduction

Pomegranate (Punica granatum L.) is an economically important commercial fruit plant species belonging to family Punicaceae. The fruit is technically a fleshy berry containing many seeds, each surrounded by a juicy, fleshy aril. The arils develop entirely from outer epidermal cells of the seed, which elongate to a very large extent in a radial direction (Fahan, 1976). From the pattern of CO₂ and ethylene production rates, pomegranate fruits are classified to be nonclimacteric fruits (Elyatem and Kader, 1984; Shulman et al., 1984) and thus, maturation and ripening should take place on the plant before harvest to get quality fruits (Elyatem and Kader, 1984). Pomegranate is native to central Asia, but since the tree is highly adaptive to a wide range of climates and soil conditions, it is grown in many different geographical regions including the Mediterranean basin, Asia, and California (USA) (Holland et al., 2009). In India, owing to its preference for arid and semiarid climate (Banker and Prasad, 1992; Pareek, 1997). About five fold increase in export of this fruit crop has been observed during last decade in value term. Moreover, recent export trends depict a high amenability in supply – demand of Indian pomegranate in international market with higher price. Consequently production is expected to increase by 10-folds and export by nearly seven folds in years to come. However, fruit cracking and sun scald during its growth and development is a serious problem affecting pomegranate fruit industry that causes significant loss of economical yield and commercial value (Panwar et al., 1994; Singh, 1995).

Figure 1. Schematic representation of factor responsible for fruit cracking

Table 1. Summary of management approaches of fruit cracking in different region

Treatments	Best treatment	Reduction in cracking (%)	Location	Reference
Spraying salicylic acid 100 ppm, MgSO4 0.5%; chelated – Zn at 0.05%, boric acid at 0.05% and CaCl2 at 2% either singly or in combinations.	Four Spray with a mixture containing salicylic acid 100 ppm + magnesium sulfate 0.5% + chelated – Zn 0.05% + boric acid 0.05% + CaCl2 2%.	4.2% compared to control 16.2%	Assiut region, Egypt	Ahmed et al. (2014).
Irrigation at different intervals, mulching, and spraying of boric acid, KNO3 MgSO4	Irrigation at 2 weeks intervals along with mulching with dried grasses and spraying with, 0.005 boric acid, 1% KNO3 or 1% MgSO4	Only 14% fruits cracked	Himachal Pradesh, India	Singh et al. (1990)
Spraying of boron (0.1%, 0.2%, 0.3%, and 0.4%) and GA3 (10, 20, 50, and 100 ppm)	Boron 0.2%	-	Rajasthan, India	Singh et al. (2003)
Borax (0.4%), ZnSO4 (0.5%), and kaolin (4.0%) alone or in combinations with or without mulching with black polythene	Borax 0.4% + ZnSO4 0.5% and mulching of tree basin	18.6% compared to control 41.6%	Rajasthan, India	Singh et al. (2017)
Foliar spray of boric acid (0.2%, 0.4%), FeSO4 (0.5%, 1%), CaCl2 (0.5%, 1%)	Boric acid 0.2%,	3.51% compared to control 31.43%	Karnataka India	Sheikh and Manjula (2006)
Spraying of proline and tryptophan (50, 75, and 100 ppm) full bloom and four weeks later	Tryptophan 100 ppm	7.42% compared to control 29.05%	North Sinai Governorate, Egypt	Omima et al., (2014)
Spraying of borax (0.75% and 1.25%) with irrigation at 9, 14, and 21 days intervals	Irrigation at 14 days + 0.75% borax	2.01% compared to control 20.35%	West Bengal, India	Ahmed, (2009)
Fruit bagging and spraying of kaolin(2.5%, 5%), CaCl2 (2% and 4% and GA3 (50 and 100 ppm)	Fruit bagging followed by CaCl2 and kaolin 5%	Manfaloty 16.0%–1.7% Wonderful 5.5%–0.4%	El-Khatatba region, Egypt	Hegazi et al. (2014)
Two sprays of nano-Ca fertilizer (0.25 and0.50 g Ca/L) and calcium chloride (1% and 2%) at full bloom and 1 month later	Low (0.50 g Ca/L) Ca concentration in the form of nano-Ca formulation	6%–7%	Iran	Sohrab Davarpanah et al. (2018)
Controlled irrigation, bagging, spraying with ZnSo4 1% and kaolin 6%	Controlled irrigation combined with ZnSO4 1% spray	27.57 compared to control 50.76	Middle Sinai – Egypt	El-Rhman (2010)
Foliar application of 2 and 5 ml/l humic acid, kaolin 6%, calcium 3%, and boron 1% separately or in combination	Kaolin 6% and humic acid (2 or 5 ml/l)	5.32% compared to control 32.7%	Darjazin, Iran	Elmira Ghanbarpour et al. (2018)
Application of biostimulants Vipul (5, 10, and 15 ml/l), Spic cytozyme (1, 2, and 4 ml/l), Homobrassinolides (0.5, 1, and 1.5 ml/l), and Biozyme crop plus (1, 2, and 3 ml/l) alone or combinations	Spic cytozyme (4 ml/l) and biozyme crop plus (3 ml/l)	11.3% compared to control 20.0 to	Himachal Pradesh India	Aziz et. al., (2013)
Paclobutrazol 300 ppm, GA3 80 ppm, NAA 40 ppm, CaCl2 3%, boric acid 0.3%, and ZnSO4 0.3% after 2 and 8 weeks from full bloom	Pacloputrazol 300 ppm	15.2% compared to control 49.5%	El-hamam region, near Alexandria City, Egypt	Hoda and Hoda (2013)

Fruit splitting or cracking is a major physiological disorder that adversely affects the production and quality of pomegranate. Cracking and splitting have been described by many ways but the word cracking is evidently popular in most fruits as pomegranate, litchi, apple, sweet cherry, grape, plum, persimmon, avocado, citrus, and tomato. Once the mature fruit gets cracked, it can be invaded by certain fungi or bacteria.

The fruit then loses its market value and often becomes unfit for human consumption (Panwar et al., 1994; Singh, 1995). Cracking of pomegranate fruit is a general problem throughout its growing areas and among all varieties worldwide, but, the magnitude of the problem is depending upon weather, heredity, variety, fruit growth, and cultural practices (Saad et al., 1988). The factors associated with fruit cracking might be high evapo-transpiration, low relative air humidity (RH), water imbalance and sharp temperature fluctuation in day and night during fruit growth and development (Abd and Rahman, 2010). It is also reported to be associated with improper irrigation, environmental factors and nutritional deficiency, especially boron, calcium, zinc, and potash (Gharesheikhbayat, 2006; Saei et al., 2014). The cracking is more evident when the fruits are at maturity stage (Hoda and Hoda, 2013; Shulman et al., 1984; Yazici and Kaynak, 2006). However, for management of fruit cracking, several horticultural practices like spraying of growth promoters, micronutrients, antitranspirants, and regular irrigation with drip and mulching have been recommended (Waskar, 2006). Both macro and micronutrients are needed for proper growth, development and productivity of pomegranate, when grown as a commercial crop. Moreover, Zn, Fe, B, and Mn have been reported to be more limiting micronutrients in pomegranate (Hamouda et al., 2015). Studies on the physiology and mechanism of cracking have further yielded valuable information about the underlying causes of cracking and suggested methods to reduce the problem (Table 1).

This paper reviews the information and work done on fruit cracking an important physiological disorder of pomegranate with respect to extent of losses, mechanism, physiology, factors associated, and management see figure 1.

Extent of Losses

The loss in yield due to this undesirable phenomenon some time reached to more than 65%, in addition, the cracked fruits are sweeter, loss keeping quality, unfit for shipment, and liable to rot (Malhotra et al., 1983). It is varied among variety to variety but the problem is more severe in harsh climatic conditions than and as high as 65%–75% losses have been reported in pomegranate in arid region (Prasad et al., 2003, Chandra et al., 2011). Singh et. al., (2014) also observed a wide range of variability (18.3%–62.6%) in fruit cracking in pomegranate cultivars in arid region. Pant (1976) observed that fruit cracking amounted to 63% in the spring crop (January–June), 34% in the winter crop (October–March) and only 9.5% in the rainy season crop (July–December) however other physico-chemical properties viz. rind thickness, fruit length and breadth, aril size, juice content, and total soluble solids also varied. The economic loss due to fruit cracking ranges from 10% to 40%, sometimes going up to 70% (Pal et al., 2017). Omima et al., (2014) stated that cracking or splitting is a serious problem in pomegranate orchard as it causes about 50% reduction in fruit marketing value.

Physiology of Fruit Cracking

It has been observed that during drought period, strengthened tissues (skin) lose their ability to divide and enlarge. If after a dry spell, water supply is greatly increased, the meristematic tissues quickly resume growth but not the strengthened tissues (Holland et al., 2009). Fully developed pomegranates crack due to moisture imbalances, as they are very sensitive to variation in soil moisture, relative humidity, rate of transpiration, and rind pliability (Mir et al., 2012). Because of moisture fluctuation, the meristematic tissues quickly resume growth but not the strengthened tissues relatively, owing to differential growth rates, harder tissue rupture (Ram Chandra et al., 2011). Singh et al. (2006) also reported similarly as sudden change in soil moisture causes the moisture stress, which affects the fruit development adversely and leads to fruit cracking. The rapid absorption of water when irrigation is resumed to severely stressed fruit leads to cracking of the skin as water is diverted to the aril and greater stress is placed on the water-deficient skin (Galindo et al., 2014; Lichter et al., 2002). It has also been suggested that asymmetrical stretching of the skin occurs as the aril fills with water. This leads to cracking on the same side of the swelling aril (Galindo et al., 2014). Different management practices reported for controlling fruit cracking has direct or indirect effect on physiology of trees. Hepaksoy et al. (2000) observed significant relationships between fruit cracking and transpiration rate and water use efficiency (WUE). The varieties selected as crack resistant had the highest WUE, whereas, the lowest in susceptible. Aksoy and Akyuz (1993) revealed significant effects of plant nutrients and transpiration rates on fruit splitting. Reduction in fruit cracking in treatment contained borax and kaoline might be due to fact that micronutrients especially boron regulate the proper fertilization and synchronized fruit development as well as antitranspirant reduced the stomatal opening and increase the leaf resistance to water vapor diffusion without affecting the carbon dioxide uptake thus reduce transpiration and regulate elasticity thus fruit skin bears the internal pressure more. This is believed to be the result of vital role of borax associated with enzymes, necessary for the meristematic activities and also for the normal development of the conducting tissue (Singh et al., 2014; Sharma and Belsare, 2009; Yılmaz and Ozguven, 2009). Physiological characters such as calcium content and pectin value have significant effects on the mechanical properties of cell membrane (Cybulska et al., 2011). Cell walls have been recognized as the location where calcium plays a key role in cross-linking with acidic pectin residue (Hepler, 2005). Low-methylated pectin molecules cross-linked with calcium ions make cell walls stiffer, and consequently increase tissue firmness (Brummell et al., 2004; Brummell and Harpster, 2001; Willats et al., 2006). Similarly Yagoden (1990) reported that beneficial effects of zinc on controlling water absorption and nutrient uptake as well as enhancing the biosynthesis of the natural hormone namely IAA surely reflected on reducing fruit cracking per cent.

Associated Factors for Fruit Cracking

Fruit cracking or splitting has been reported for almost every horticultural crop of economic importance. The common occurrence of this disorder has been observed in apricot (Benson, 1994), sweet cherry (Cristian et al., 2013), grape (Considine and Kriedmann 1972), pomegranate (Hoda and Khalil 2013), prune (Milad and Shackel, 1992), nectarine fruit (Caroline et al., 2007), and tomato (Peet, 1992). Certain causative factors viz. cultural, genetic and environmental, plays a determining role in fruit cracking. Deficiencies in nutrients such as calcium and boron in young fruit, drastic undulations in day and night temperatures, irregular watering regimes during fruit ripening, and long dry periods followed by heavy rains or irrigation are the primary contributors to cracking in pomegranate fruit (Galindo et al., 2014; Gharesheikhbayat, 2006; Khalil and Aly, 2013). Fruit cracking rates can also be influenced by elements such as the environment, inherent morphology and physiology, and genetic factors (Khadivi-Khub, 2014).

Environmental Factors

Environmental conditions especially temperature and humidity (Khub 2014), temperature difference between day and night (Abd and Rahman, 2010), instant decrease of the temperature (Plamenac, 1972) have been shown to influence fruit cracking. Weather viz. air temperature, wind velocity, air relative humidity, canopy temperature, and fruit surface temperature have also been considered as possible causes of cracking (Frazier and Bowers, 1947). Singh et al., (2014) clearly indicated that relative water content and water potential of leaf as indicative of water stress directly affects the magnitude of cracking in pomegranate fruits. Both these physiological parameters were influenced by high temperature stress as measured by canopy air temperature difference (CATD) and fruit air temperature difference (FATD). Both CATD and FATD were significantly correlated with leaf water potential and relative water content. Stepwise regression analysis indicated that temperature mediated leaf water potential had direct impact on incidence of fruit cracking in pomegranate. Yazici and Kaynak, (2009) also reported higher fruit surface temperature in pomegranate than air temperature but FATD cannot be predicted from the air temperature alone. This might be the reason that under same climatic conditions different varieties shown variable difference in CATD and FATD thus had impact on fruit cracking.

Water Status of Soil and Plant

Studies supported the fact that fruit cracking in pomegranate mainly have occurred due to imbalance of soil moisture and plant water status especially during latter stages of fruit growth and development as pomegranate is very sensitive to these variations (El-Rhman, 2010; Galindo et al., 2014; Hepaksoi et al., 2000; Lichter et al., 2002; Singh et al., 2006). As high demand of water during fruit growth and development is to be met from leaf following source–sink relationship (Hepaksoy et al., 2000), plant water gradient induces the process of water loss from plant either through fruit surface or canopy surface to atmosphere in soil-plant-atmosphere continuum and thus greatly influences water relations and hence lead to incidence of cracking in fruits. It has been shown that in addition to excessive water absorption by the roots, cracking in many kinds of fruit such as apples, peaches, and cherries is also caused by the osmotic absorption of water through the skin of the fruit (Bohlmann, 1962). Under conditions of high availability of water and low evaporative demand, the cracking or splitting of fruits by unseasonal rainfall has been attributed to the development of a high hydrostatic pressure in the fruit (turgor pressure) in excess of the tensile strength of the cell walls (Considine and Brown, 1981). According to Gourley and Howlett (1941) the cracking of fruit in apples and sweet cherries occurs due to excessive cell enlargement of the fruits following a marked increase in the soil moisture. Hartman and Bullis (1929) also reported the role of excessive water absorption by the fruit in cracking of cherries, either directly through the skin in wet weather or by way of the root system and vessels. It was also reported by Meshram et al. (2010); Mars, 2000) that irregular and/or over irrigation during the ripening period and, rains during harvest period causes cracking in pomegranates. In general, heavy rainfall during the period of rapid fruit growth was correlated as a causative factor of the phenomenon in apricot (Gulsen et al., 1995), grape (Clarke et al., 2010), and cherry (Belmans and Keulemans, 1996). In addition, high volume irrigation after a period of water stress has been shown to result in the development of cracks in prune (Milad and Shackel, 1992), tomato (Peet, 1992), and nectarine (Gibert et al., 2007).

Role of soil and plant water status was clearly observed by Akath et al. (2014), as they recorded less fruit cracking in pomegranate trees mulched with black polythene sheet and sprayed with borax + zinc sulfate + kaoline and these treatments were associated with lower fluctuation of soil moisture, plant water potential, and rate of transpiration and also had higher photosynthetic rate, relative water content, stomatal conductance, and reduction of leaf temperature. El-Rhman (2010) also stated that pomegranate fruit is very sensitive to variations in soil moisture with respect to quality and fruit cracking incidence. As water demand during fruit growth and development is to be met from leaf following source-sink gradient relationship (Hepaksoy et al., 2000), plant water gradient induces the process of water loss either through canopy surface or fruit surface to atmosphere in soil-plant-atmosphere continuum and thus greatly influences water relations and hence lead to incidence of cracking in fruits. Several factors are associated with water balance that may lead to cracking. The water potential of the fruit produces the cracking force, and the cell wall and other structures must resist this pressure (Lichter et al., 2002).

Nutritional and Hormonal

In terms of mineral nutrient deficiency, cracks develop due to boron (B) deficiency in apricots (Benson, 1994) and calcium (Ca) deficiency in cherry (Meheriuk et al., 1991). Low calcium concentration in pericarp cells is correlated with fruit cracking in tomato (Astuti, 2002) and litchi (Huang et al., 2005). In litchi cultivar “Bedana,” the greater strength may be attributed to its high calcium content (Leyla and Husnu, 2004). A relationship between fruit cracking and endogenous hormones or mineral nutrition (Ca, Mg, and B) was reported by Qiu et al. (1999) in litchi cv. Nuomici. Both macro and micro-nutrients are needed for proper growth, development and productivity of pomegranate, when grown as a commercial crop. Moreover, Zn, Fe, B, and Mn have been reported to be more limiting micronutrients in pomegranate (Mirdehghan and Rahemi, 2007). Nutritional deficiency, especially boron, calcium, zinc, and potash (Gharesheikhbayat, 2006; Saei et al., 2014) are directly associated with fruit cracking in pomegranate. Nutrients like potassium, calcium, zinc, copper, molybdenum, and manganese are involved in some physiological processes during the fruit growth period, and their deficiency results in fruit cracking (Sheikh, M., and N. Manjula, 2006). Micronutrients especially boron regulate the proper fertilization and fruit development (El-Khawaga, 2007). Pal et al. (2017) reported that increased N, and imbalance between K and Ca also lead to fruit cracking. The reducing effect on fruit cracking % in response to application of boron was mainly attributed to its important role in the extension of plant cell walls through building of pectins as well as enhancing indole-3-acetic acid (IAA) and water uptake (Yagoden, 1990). This is believed to be the result of vital role of borax associated with enzymes, necessary for the meristematic activities and also for the normal development of the conducting tissue (Yilmaz and Ozguven 2009; Sharma and Belsare, 2009). Bohlmann (1962) also opined that application of boron in boron deficient area also check the cracking, the physiological role of boron is due to synthesis of pectic substance in plants. Several studies showed that calcium played a vital role in occurrence of fruit cracking and application of calcium chloride reduced fruit cracking (Khalil and Aly, 2013; Sheikh and Manjula, 2006; Singh et al., 2003). Calcium is an essential element for proper plant growth and development as it has metabolic functions in nutrient uptake and is involved in abiotic and biotic stress resistance. Calcium, an important components of cell wall, contributes to the cohesiveness between cells and the strength of cell walls (Brett and Waldron, 1990) and its concentration was higher in normal fruit compared to cracked fruits (Huang et al., 1999; Li and Huang, 1995; Sanyal et al., 1990). Calcium likely contributes to the greater elasticity, strength, and thickness of epidermal cell walls. Calcium also assists with the deposition of pectin so that fruit are better able to resist cracking under the higher rates of turgor pressure exhibited during water stress (Choi et al., 2010). Ca and Mg are responsible for strengthening the bonds between epidermal and other fruit cells resulting in better strength and low cracking (Poovaiah, 1986). Moreover, the role of Ca in stopping the formation of abscission zone between fruit pedicles and bearing branches as well as regulating the activity of enzymes and photosynthesis (Tony and John, 1994; Mighani et al. 1995; Jackman and Stanley, 1995) could result in controlling fruit splitting percent. Foliar spray of zinc as sulfate reduced percent cracking besides improving fruit yield probably due to its effect on water uptake and transport besides influencing activities of enzymes involved in protein, carbohydrate and nucleic acid metabolism as also reported by Sadeghzadeh (2013). Zinc also plays an important role in regulating absorption of water by plant roots. Simultaneously antitranspirant has been found to reduce fruit cracking. Antitranspirants has general role in reducing the stomatal opening and increase the leaf resistance to water vapor diffusion without affecting the carbon dioxide uptake thus reduce transpiration and regulate elasticity of fruit skin. It also reduces the absorption of radiant energy and thereby reduce leaf temperatures and transpiration rates; emulsions of wax on fruit form thin transparent films which hinder the escape of water vapor from the fruit surface and prevent stomata from opening fully, thus decreasing the loss of water vapor from the leaf reduce transpiration (Yazici and Kaynak, 2006); these may be possible reason of reducing the incidence of fruit cracking in pomegranate. An imbalance between auxins, gibberellins, and cytokinins has also been reported to cause fruit cracking in different crops (Rai et al., 2002). Normal fruits contains higher level of gibberellins and lower level of ABA (Sharma and Dhillon, 2002; Yilmaz and Ozguven, 2003) and an imbalance between the two in fruit pericarp leads to fruit cracking (Li et al., 2014). Yilmaz and Ozguven (2009) found that, the ABA content of the peel was generally higher in cracked fruits than in the peel of healthy (noncracked) fruit.

Genetic Factors

The fruit cracking is a quite common physiological disorder occurred more or less in all the varieties as reported by Malhotra et al. (1983). Prasad and Bankar (2000), reported that out of 9 cultivars studied, Jalore Seedless had the lowest incidence of fruit cracking (36.60%) while as maximum was recorded in Jodhpur Red (78.20%). Significant variations was reported in fruit cracking among different cultivars and found maximum cracking in Jodhpur Red (72.40%) and least in Malta (5.20%) as reported by Singh (2004). Mir et al. (2010) recorded high range of variability among the 10 different genotypes with respect to fruit cracking under Karewa belt of Kashmir. Similarly fruit cracking in different pomegranate varieties ranged from 18.3% in P23 to 62.6% in Jodhpur Red under arid conditions of western Rajasthan (Akath et al., 2014). Trapaidze and Abuladze (1989) evaluated 15 cultivars of pomegranate under subtropical zone of eastern Georgia and found that six cultivars namely Shirvan, Burachni, Apsheronskii-Kranji, Sulu-nar, Kyrmyz Kabukh and Francis were resistant to cracking. They suggested that at least some aspects of fruit splitting in pomegranates are genetically controlled independently from environmental conditions. Hepaksoy et al. (2000) also corroborated this assumption during the studies in variation among the cultivars with respect to fruit cracking in Bornova-Izmir, Turkey and observed that out of six cultivars, two cultivars classified as sensitive to cracking (Koyeegiz and Siyah), two resistant (Kadi and Lefon) and two intermediate (Cekirdeksize Seedless and Feyiz). Tabatabaei and Sarkhosh (2006) reported that most of Iranian pomegranate cultivars tend to split in much earlier stages of fruit development. Samadi (2011) evaluated 48 cultivars including sweet, medium sweet and sour in Afghanistan and observed that cracking is more or less common in all the varieties and pomegranate growing areas of Afghanistan. Sepahvand et al. (2011) also reported similarly trend while evaluating 10 pomegranate cultivars at Karaj condition of Iran. Recently Khaddivi et al. (2018) carried out characterization of 100 pomegranate genotypes in Markazi province of Iran and observed that 90 genotypes did not showed any fruit cracking while six were low in cracking, three were medium and only one genotype was susceptible to cracking. Yuan et al. (2012) reported that fruit-cracking is a severe issue in pomegranate production in Shandong, China which is a prime region of pomegranate cultivation and observed a significant variation (1.5%–45.9%) in major cultivars studied. Similarly Stover and Mercure (2007) also observed a wide range (10%–35%) of fruit cracking in different cultivars under semiarid conditions of Mehdi Rezaei plateau of Iran.

Many morphological characteristics, such as thickness and physical properties of the cuticle, number of hypodermal layers (Bargel and Neinhuis, 2005), fruit shape, and fruit size (Emmonsand Scott, 1998; Koske et al., 1980; Sekse, 1987) affect sensitivity to cracking. However, these properties could be affected by calcium content and pectin value, cell wall structure and component (Cybulska et al., 2011), quantity, and volume of intercellular spaces (Chanliaud et al., 2002). The biomechanical properties of skin have important role in sustaining inner pressure and resistance to fruit cracking. Lane et al. (2000) suggested the skin’s role in cracked resistance is related to the calcium content of the epidermis cells, which increase the cell’s integrity. Role of fruit skin in pomegranate hinder cracking depends on its mechanical properties. Cracking in the wild-type pomegranates with thicker skin and smaller fruits than domestic varieties (Bist et al., 1994) showed that there were some other important factors involved in cracking. Cybulska et al. (2011) suggested that the capability of fruit skin to maintain fruits against cracking depends on the physiological properties such as calcium content rather than skin thickness. Further it is quite likely that rind thickness influence elasticity of fruit skin which in turns checks cracking (Ramezanian et al., 2009). Physiological characters such as calcium content and pectin value have significant effects on the mechanical properties of cell membrane (Cybulska et al., 2011). Many physical characters contribute to resistance to fruit cracking. The sensitive cultivar had thinner skin, greater ratio of seeds weight to skin weight and significantly smaller size. Skin thickness against fruit volume, indicates relation between skin thickness and fruit volume with fruit cracking. In contrast there were reports that skin thickness was significantly different between the two cultivars, but no relation was observed between skin thickness and fruit cracking (Saei et al., 2014). Some authors have indicated that under water stress peel mechanical properties change, the peel extensibility tending to decline (Aloni et al., 1999; Matthews et al., 1987) and becoming thicker and stiffer (Lawlor and Leach, 1985). These changes could explain the susceptibility of pomegranate fruits to cracking. Pal et al. (2017) reported that the rind and aril development phases are critical for cracking and any adverse condition may cause cracking, sooner or later. The recent result indicated that arils were the principal sources of pressure to the fruit skin. The increase in aril pressure would put pressure on the peel and make it susceptible to cracking (Galindo et al., 2014). Saei et al. (2014) reported that large fruits are likely to reflect more cracking by varying biomechanical forces on the growing fruits making them susceptible to cracking. He further observed that that fruit shape had an influential effect on fruit cracking. Fruits with a ratio (length/diameter) < 1 were susceptible to fruit cracking. Considine and Brown (1981) demonstrated that in spheroid fruits, pressure on the containing membrane increases when the shape deviates from a perfect spheroid toward oblate or prolate. This shape deviation implied the inner fruit pressure this made more sensitive to cracking. There are studies on the relationship between fruit cracking and expression of the expansin gene. During growth, plant cells secrete a protein called expansin, which unlocks the network of cell-wall polysaccharides, permitting turgor-driven cell enlargement. Expansin weakens the noncovalent binding between wall polysaccharides which results in cracking.

Management of Fruit Cracking

Several practices like spraying of growth promoters, micronutrients, antitranspirants, and regular irrigation with drip and mulching have been recommended (Waskar, 2006) for managing fruit cracking in pomegranate but effects are variable depending on different conditions. Singh et al. (1990) suggested different cultural practices to minimize premature fruit cracking of pomegranate which include irrigation at weekly intervals, mulching with dried grass or farmyard manure, spraying with 0.005 or 0.002% boric acid, 1% KNO₃ or 1% MgSO₄. Hegazi et al. (2014) obtained lowest percentage of fruit creaking in Manfaloty and Wonderfull pomegranates with fruit bagging followed by kaolin 5% and spraying by CaCl2 and GA3 treatments compared with the control treatment. Singh et al. (2017) reported that foliar application of borax (0.4%), zinc sulfate (0.5%), and kaolin (4%) with mulching of tree basin (black polythene sheet 150 µ) is more effective in minimizing fruit cracking rather than sprays alone. It was observed during various studies correct nutrition with calcium, magnesium, boron, and zinc were very effective in reducing fruit splitting in various pomegranate cultivars (El-Kholy, 1999; Kuldeep et al., 2001; Abdel- Aziz et al., 2001; Hasaballa, 2002; Abdelaal, 2007; Morsy et al., 2008). Singh et al. (2003) obtained least cracking with the application of boron at 0.2%, which in turn produced the highest yield. Soni et al. (2000) reported that soil application of 0.5% borax at the time of fruit setting, and irrigation at 10 days’ intervals decreased the fruit cracking and improved the yield in pomegranate cv. Jalore Seedless. Similarly Ahmad (2009) also reported that soil application of 0.75% borax at the time of fruit setting, and irrigation at nine days interval decreased the fruit cracking and improved the yield in pomegranate cv. Ganesh. Misra and Khan (1981) reported that the role of boron application may probably due to translocation of sugars and synthesis of cell wall material and increase in methyl esterase activity.

Calcium, an important components of cell wall, contributes to the cohesiveness between cells and the strength of cell walls (Brett and Waldron, 1990) and its concentration was higher in normal fruit compared to cracked fruits in litchi (Huang et al., 1999; Li and Huang, 1995; Sanyal et al., 1990). Calcium availability during early stage of fruit ontogeny is important for cracking resistance (Huang et al., 2005). Similarly Sohrab Davarpanah et al. (2018) reported that foliar fertilization with a low (0.50 g Ca L⁻¹) Ca concentration in the form of a nano-Ca formulation resulted in decreases in pomegranate fruit cracking than those obtained with higher doses of CaCl₂ (2.73 and5.45 g Ca L⁻¹) while other physic-chemical quality parameters were not affected by Ca foliar fertilization .

Foliar spray of zinc as sulfate reduced percent cracking besides improving fruit yield (Singh et al., 2017) probably due to its effect on water uptake and transport besides influencing activities of enzymes involved in protein, carbohydrate, and nucleic acid metabolism as reported by Sadeghzadeh (2013). While some workers reported that application of zinc sulfate and paclobatrazol were effective in reducing the percentage of fruit splitting (Prasad et al. (2003) and El- Khawga, (2007). Abd El Rahman (2010) obtained minimum cracked fruits by using the controlled irrigation combined with zinc sulfate (ZnSO₄) spray at 1%.

Positive effects of antitranspirant on reducing fruit cracking is well reported by different workers. Antitranspirant reduce the stomatal opening and increase the leaf resistance to water vapor diffusion without affecting the carbon dioxide uptake thus reduce transpiration and regulate elasticity of fruit skin and minimize cracking. Kaolin application helped reduce pomegranate surface temperatures and fruit cracking by reflecting light away from the fruit thereby preventing sun-scorching of the pomegranate skin (Gindaba and Wand, 2005; Yazici and Kaynak, 2006). Undulations in fruit surface temperatures combined with increased water evaporation from the surface are known factors that induce pomegranate fruit cracking (Galindo et al., 2014). El-Rhman (2010) reported a 9% reduction in pomegranate fruit cracking after kaolin application during water stress. Elmira Ghanbarpour et al. (2018) reported reduction of cracking in Pomegranate fruit after foliar application of humic acid (2 ml L⁻¹), calcium-boron (3% and1%) and Kaolin (6%) during water stress. Glen et al., (2002) also reported similar effect of Kaolin on reducing fruit cracking and improve fruit size and fruit color.

Preharvest application of some growth regulators and mineral nutrients such as pacloputrazol, GA₃, NAA, CaCl₂, boric acid, and ZnSO₄ at the early stages of fruit growth in pomegranate trees had a positive influence in decreasing fruit cracking percentage and improving fruit quality. Spraying with gibberellic acid (GA3) or benzyl adenine (BA) significantly reduced fruit cracking (Mohamed, 2004; Yılmaz and Ozguven, 2009). Hoda and Hoda (2013) studied the effect of foliar application by using some growth regulators and found that the extent of fruit cracking was reduced significantly with application of 300 ppm pacloputrazol, while aril, fruit juice, total soluble solids (TSS) and acidity were increased. Khalil and Aly (2013) also reported fewer occurrences of fruit cracking with application of 300 ppm paclobutrazol. El–Khawaga (2007) who reported that cracking fruits in Manfalouty pomegranate cultivar were reduced by applying of paclobutrazol. El Sayed et al. (2014) reported tryptophan treatment at 100ppm proved to be most efficient treatment in reducing fruit cracking Manfalouty pomegranate variety. Kuldeep Kumar et al. (2017) observed that 2, 4-D and NAA @10 ppm recorded significantly lower incidence of cracking in pomegranate. Sharma et al. (2018) observed that preharvest sprays of Surround WP reduced the fruit cracking in ‘Kandhari’ pomegranates by 46% along with significantly higher cosmetic appeal, red skin color and higher levels of total anthocyanin content. El-Wafa (2014) reported that incidence of fruit cracking in pomegranate cv. Wonderful was found significantly low in all the bags compared to nonbagged fruit while lowest incidence was recorded in fruits bagged with prgmen bag compared to other bags. Wei et al., (2009) while evaluating the different bagging on off-season longan fruit found that white adhesive-bonded fabric bag with about 70% light transmittance, and black adhesive-bonded fabric bag with <10% light transmittance reduced cracking incidence significantly as compared to the control (5.1% and 11.6% vs 32.8%).

Conclusion

The problem of fruit cracking in pomegranate is a complex phenomenon with a large number of factors playing a contributory role. Advances in research on the fruit cracking in relation to causes, its physiology, and effective management clearly elucidate that the problem cannot be considered in isolation but both external and internal conditions need to be taken into account in holistically. Much of the work has been attempted to relate the propensity of pomegranate fruit cracking to the anatomical features of the fruit, role of climate, pericarp thickness, and pattern of fruit growth, endogenous calcium and genetic constitution of the cultivars. The relevance of these parameters in relation to fruit cracking is supported by the differences between cultivars, susceptible, and resistant to fruit cracking. Efforts to control or reduce fruit cracking include cultural measures, and the use of plant regulators and other chemicals which modify the fruit growth process. None single factor can be advocated as efficient enough in controlling fruit cracking. The role of an integrated orchard management which aims to minimize stress of water, nutrition and physiological factors that contribute to fruit cracking should be considered at best. The behavior of fruit cracking in relation to internal fruit composition and quality characteristics, dynamics of water uptake, understanding on how water, gibberellins, abscisic acid, calcium, and the cell wall biosynthesis interacts with fruit cracking, will offer a clearer insights in devising developmental strategies to reduce fruit cracking.