Hand Thinning Influence on Fruit Quality Attributes of Pomegranate (Punica granatum L. cv. ‘Malase Yazdi’)

ABSTRACT

Pomegranate (Punica granatum L.) is one of the oldest fruit trees that is native to Iran and resistant to hot warm and dry climate. Excessive fruit on the tree reduces fruit size and produces fruit with low quality. Hand thinning of flowers and fruits is a management tool to reduce the crop load and increase the quantitative and qualitative attributes of the fruit before harvest. The main objective of this study was to investigate the impact of different hand thinning levels (control, and removal of 10%, 15%, 20%, and 25% of fruit) on the physic-chemical attributes of pomegranate fruit ‘Malase Yazdi’. These results revealed that removing 20% of fruit gave the highest yield and trees thinned at 15%, 20%, and 25% levels produced heavier fruits at harvest. The highest fruit length and diameter were observed in 20% thinning treatment. Fruits from trees thinned at 20% and 25% had significantly higher volume, juice content, total weight of aril, peel fresh weight and soluble solids concentration than fruits from non-thinned trees. Also, a better red of fruits from trees thinned at 20% and 25% was observed. Thinning had a positive effect on studied quality attributes and thinning at 20% seems to be the optimal treatment for a better yield and fruit quality.

KEYWORDS:

• Crop load
• fruit thinning
• quantitative and qualitative characteristics
• yield

Introduction

Pomegranate (Punica granatum L.) is native to Iran and the Mediterranean areas (Fawole and Opara, 2013). It is believed that over 500 cultivars of pomegranate in the world, of which about 50 cultivars are commercially cultivated (IPGRI, 2001). Pomegranate has high antioxidant activity and is rich in vitamins, polyphenols, minerals, carbohydrates, organic acids, and proteins (Viuda‐Martos et al., 2010). Pomegranate is highly suitable for growing in arid areas due to its resistance to drought, and today it is widely cultivated in the Mediterranean region and California. In addition, pomegranate fruit consumption is increasing worldwide due to its medicinal properties and its many uses in the food industry. Iran is one of the primary and main distribution centers of pomegranate in the world and about 750 pomegranate accessions have been reported from the Yazd pomegranate collection (Shahrbabaki, 1997).

Flowers of pomegranate are generally perfect, containing both male and female parts. However, the pomegranate plant can have three types of flowers including hermaphrodite, male, and intermediate forms. Flowering in the northern hemisphere generally occurs between February to April and in the southern hemisphere between July to August. Depending on cultivar and weather conditions it lasts for up to 3 months or more (Kahramanoglu and Usanmaz, 2016; Newman and Lansky, 2012). The first flowering period of pomegranate begins to appear in mid-April and continues to grow until mid-May under conditions of our study. These flowers are abundant and mostly herbaceous, of which about 25% will set fruit and the rest will senesce and drop. Fruits from the first flowers are large and marketable. The second flowering period of pomegranate grows in limited numbers in late spring and produce small to medium fruits. The third flowering period appears in the middle of summer with a low number and small and incomplete fruit, which have low commercial values; hence, thinning is preferred for such fruit. In most cases, thinning increases the size of the large fruit and decreases the size of moderate ones. Larger fruits have higher market values (Nuncio-Jauregui et al., 2015). Pomegranate naturally produces excessive fruit, resulting in the production of small fruits with a low-quality. Also, the cold resistance is reduced due to the loss of carbohydrate storage in the tree, and because of a high yield, the shoots are weak and for this reason, fruit thinning is necessary. Fruit thinning was first performed by removing extra fruits by hand. Then, mechanical methods were commercially used for the removal of fruits followed by growth regulators on a large scale. In pomegranate, the same as other fruits such as peach, apricot, thinning aimed to remove small and irregular fruits, but mainly to produce large fruits with a high quality that is acceptable to the consumer (Desai et al., 1993; Jafari et al., 2014). Some benefits of fruit thinning include increasing fruit quality and size, improving fruit firmness, reducing fungal diseases, decreasing tree deformation, lowering alternate bearing, and increasing soluble solids concentration (SSC) and titratable acidity (TA) (Jemric et al., 2003).

Despite pomegranate’s huge health benefits and economic impacts, little attention has been given to the impact of pomegranate fruit hand thinning (Jafari et al., 2014; Mohsen and Osman, 2015; Samara and Shalan, 2013), chemical thinning (Sharma and Singh, 2000), and flower thinning (Desai et al., 1993). Jafari et al. (2014) reported that hand thinning of ‘Malase Torshe Saveh’ pomegranate fruits improved fruit size and fruit quality attributes. In that study, hand thinning of fruits was applied when fruitlet diameter was about 30 mm, and they found that fruit weight, length, diameter, and volume increased with increasing thinning levels. Fruit from trees thinned at 20% and 30% had significantly higher SSC than those from unthinned trees. Samara and Shalan (2013) showed that fruit thinning and fruit bagging significantly reduced the number of fruits, but fruit weight and size showed significant increases compared to the control. Mohsen and Osman (2015) studied the physical and chemical properties of pomegranate fruits (‘Wonderful’) in an experiment with four levels of hand thinning. The results showed that thinning significantly increased the fruit weight, weight of 1000 seeds, SSC, anthocyanin and ascorbic acid contents. Also, hand thinning increased total phenolic compounds and antioxidant activity, but it decreased the yield and total acidity.

Given the importance of fruit thinning practice in pomegranate growing for improving fruit quality, our study aims to investigate the effects of different levels of fruit thinning on the quantitative and qualitative characteristics of ‘Malase Yazdi’ pomegranate.

Materials and Methods

This study was conducted on a commercial pomegranate orchard in Ardakan, Iran (32°20^′N, 53°46^′E), during the 2017–2018 growing seasons. The twenty experimental trees were uniform, 20 years old and planted at 2 × 3 m spacing. Other than thinning treatments, all cultural practices such as irrigation, pruning, and fertilization in these trees were similar to those in commercial pomegranate orchards in the region.

The treatments consisted of five levels of thinning [control (no thinning), and removal of 10%, 15%, 20%, and 25% of the total fruit crop load]. The research started when most of the flowers converted to fruits and fruits reached about a walnut-size depending on the regional weather from June to July (Jafari et al., 2014). Small and weak fruit were removed at different directions. Fruits become ready for harvesting about 5 to 7 months after the appearance of blossoms and they are harvested with special secateurs such as outer appearance especially the peel color (red) and inner color. Five ripe fruits from different sites of each tree were harvested in November and immediately transported to the laboratory to measure both quantitative and qualitative traits. Fruit weight (g), fruit volume (cm³), fruit length and diameter (mm), fresh weight of peel (g), total weight of arils (g), and juice percentage (%) were determined. The pH of juice was measured using a digital pH meter (PL 500, Taiwan). SSC (degree of brix) was determined by a manual refractometer (Atago, NSG Precision Cells, Inc., Hicksville, USA). Titratable acidity was quantified by titration to pH 8.3 with 0.1 N sodium hydroxide (NaOH) solution and expressed as % citric acid. Ascorbic acid (mg of ascorbic acid per 100 g sample) was determined by titration of fruit juice with 2,6- dichlorophenol indophenol (Ting and Rouseff, 1981). Juice color was assessed according to the grading scale (Ayala-Zavala et al., 2007). Total phenolic compounds were evaluated by Folin–Ciocalteau method. Absorption was measured at 760 nm using a spectrophotometer and the results were expressed as mg of Gallic acid per 100 g of fruit juice (Waterhouse, 2002).

Data analysis

This experiment design was a randomized complete block with five treatments and four one-tree replications. Data were analyzed using SAS v.9.1 software, and mean values were compared based on Duncan’s test at 5% probability level.

Results

Fruit thinning was effective on the weight and volume of fruits, total weight of arils, volume of juice, and fresh weight of peel, but did not affect the fresh and dry weights of the seeds. The fresh weight of pomegranate fruit increased with increasing thinning levels. A 20% thinning increased the fruit weight of about 26% compared to the control treatment. Also, fruits from trees thinned at 20% presented the highest length and diameter (82.78 mm and 78.2 mm, respectively), while the lowest values were observed in control. Two higher levels of thinning significantly increased the aril weight by 18% in 20% thinning compared to the control and also significantly influenced fruit volume by 22% in 20% thinning compared to the control (Table 1). Four levels of thinning treatment increased the amount of fruit juice in comparison to untreated fruits, and the highest amount of juice was obtained in 20% and 25% thinning, with a 25% increase in 20% thinning. According to the results, three high levels of thinning produced the highest fresh weight of peel (Table 1).

Table 1. Effect of thinning treatments on the crop load, yield, fruit weight, length, diameter and volume, aril and peel fresh weight and fruit juice volume of pomegranate fruits ‘Malase Yazdi’

Thinning treatment	Crop load	Yield (kg/tree)	Fruit weight (g)	Fruit length (mm)	Fruit diameter (mm)	Fruit volume (cm^3)	Aril weight (g)	Peel fresh weight (g)	Juice volume (ml)
Control	80.3 a	19.07c	236.68b	75./18b	70.6b	239.58b	157.8c	48.67c	98.25c
Hand thin. 10%	69.7b	17.6e	245.71b	76.30ab	73.93ab	247.8b	170.95bc	52.7bc	116.37b
Hand thin. 15%	66.3c	19.7b	300.75a	79.30b	75.97ab	282.61ab	181.65abc	64.82ab	117.00b
Hand thin. 20%	63.3d	20.3a	319.61a	82.7a	82.02a	307.09a	193.08ab	65.65ab	130.5ab
Hand thin. 25%	58.3e	18.2d	308.76a	80.4ab	75.43ab	314.5a	207.3a	68.8a	136.25a

Means followed by different letters in each column indicate significant difference at p < 0.05 (Duncan test)

Thinning was effective on soluble solids concentration, pH, TA, and juice color of pomegranate fruit, but did not affect the amount of ascorbic acid and phenolic compounds (Table 2). Total titratable acidity increased by fruit thinning and reached the highest levels in the 15% and 20% fruit thinning. Thinning increased SSC and the three high levels of thinning showed the highest amounts with a 14% increase in 20% thinning. Values of pH were lowermost in 15% and 20% thinning treatments compared to untreated fruits. Peel color of pomegranate fruits increased by 33% in 20% and 25% thinning treatments (Table 2).

Table 2. Effect of thinning treatments on the SSC, TA, pH, color, ascorbic acid and total phenolic content of pomegranate ‘Malase Yazdi’

Thinning treatment	SSC (° Brix)	TA	pH	Color	Ascorbic acid (mg/100 g)	Toral phenolic content (mg GAE/100 g)
Control	13.37c	0.75b	3.65a	3b	17.25a	283.65a
Hand thin. 10%	13.75c	0.83ab	3.56ab	3b	18.4a	270.02a
Hand thin. 15%	14.75ab	1.0a	3.48b	4b	23.06a	267.82a
Hand thin. 20%	15.5a	0.97a	3.5b	4.5a	18.4a	263.54a
Hand thin. 25%	15.25ab	0.93ab	3.56ab	4.5a	23.01a	263.65a

Means followed by different letters in each column indicate significant difference at p < 0.05 (Duncan test)

Discussion

Fruit production in pomegranate trees depends on several factors such as tree genotype, environment, relative humidity, sunlight, soil type, irrigation, fertilization, pruning, and crop load (Ahmed Ali et al., 2006). The fruit thinning is regularly applied in pomegranate orchards in order to achieve regular yield and better quality (Vasantha Kumar, 2006).

The effects of chemical flower thinners on fruit quality have been studied on apple and stone fruit trees and showed a significant fruit size increase (Fallahi and Greens, 2010). Also, blossom thinning is currently used in many apple-producing areas to enhance flower initiation for next year (Racskó, 2006). Fruit tissues act like strong sinks for photosynthates, nutrients, and water, and thus increasing the number of these sinks (number of fruit), would adversely affect fruit size and quality due to the increased completion among fruits for carbohydrates, nutrients, and water (Rodrigues et al., 2019). When fruit thinning is practiced, the remaining fruits receive more photosynthetic materials, resulting in fruit with larger cells, weights, and sizes (Mohsen and Osman, 2015). The fruit size is closely related to the thinning intensity, and crop load, age of wood, flower bud quality, competition between clusters, and canopy are important factors that affect thinning response (Link, 2000). A negative correlation was reported previously between vegetative growth and pomegranate fruit production (Hernandez et al., 2012).

Based on the results of the present study, the highest yield was achieved by removing 20% of fruits. The best results for fruit weight and size were obtained in 20% and 25% thinning treatments (Table 1). Hand-thinned fruits in about 4 weeks after full bloom in ‘Wonderful’ pomegranate have been reported to be very effective in increasing average fruit weight and quality (Kahramanoglu et al., 2018). Fruit thinning of pomegranate (‘Malase Torshe Saveh’) at 40% level, produced heavier and larger fruits and the highest yield (Jafari et al., 2014). Mohsen and Osman (2015) found that the fruit length and diameter of pomegranate fruit (‘Wonderful’) increased with increasing hand thinning level. Also, they showed that the tree yield varies with respect to the level of thinning. The highest level of thinning (one fruit per cluster) produced the lowest yield and two fruits per cluster led to the highest yield. Hussein et al. (1994) observed that hand thinning increased the fruit weight. Our results are in agreement with Samara and Shalan (2013) in ‘wonderful’ cultivar.

Pomegranate fruit juice was uppermost in 25% thinning of fruits (Table 2). Thinning increased the percentage of fruit juice compared to control fruits, which is in agreement with that of Jafari et al. (2014) who achieved the highest value of juice in 40% thinning. Also, Ahmed Amin et al. (2000) found that the application of girdling, fruit thinning, and thinning with girdling increased the percentage of juice by 12.2% compared with the control. The juice percentage of pomegranate increased 80.26% in ‘Ganesh’ and 77.72% in ‘Qandahari’ cultivar by blossom thinning, which showed significant increases compared to control fruits (Sharma and Singh, 2000).

In the present study, the highest average total weight of aril was found at 25% thinning level (Table 1). Similarly, researchers found that the highest levels of thinning resulted in the highest aril weight (Padmavathamma and Hulamani, 1996). The volume of juice, fruit weight, and total aril weight are closely correlated, and each of these characteristics is considered as an indicator of the fruit size. The number of arils per fruit has a strong correlation with the fruit size, and larger fruits are heavier and have a greater total aril weight (Wetzstein et al., 2011). Mohsen and Osman (2015) revealed that all hand thinning treatments increased the weight of 100 arils of pomegranate ‘Manfalouty’, and the highest values were observed when thinning one fruit per cluster. According to Table 2, the fruit thinning at different levels affected pH juice and the lowest values were observed in 15% and 20% treatments. The highest and lowest amounts of total acid content were obtained in untreated and treated fruits with one fruit per cluster (Mohsen and Osman, 2015). These results do not agree with our results. This result could be due to the high amount of nutrients and photosynthetic assimilates in the fruit. In a study by Hussein et al. (1994), hand thinning resulted in increased acidity but Padmavathamma and Hulamani (1996) found that hand thinning did not affect TA.

With increasing levels of thinning, SSC increased significantly compared to control treatment. The highest value of SSC (15.25) was observed in fruits of 20% thinning and the lowest SSC (13.37) belonged to unthinned treatments (Table 2), which was in agreement with Mohsen and Osman (2015). They found that the highest amount of SSC was related to the highest level of thinning (one fruit per cluster), and the lowest was recorded for control trees at the lowest level of thinning (four fruits per cluster).

Due to the importance of soluble solids concentration, ascorbic acid, and anthocyanins, they play an important role in fruit post-harvest quality (Mir et al., 2012). Mohsen and Osman (2015) found that the highest SSC (17.4%) was observed in one fruit per branch treatment and the lowest SSC (14.4%) was obtained in control treatment. Hussein et al. (1994) showed that hand thinning increased acidity compared to the control in pomegranate fruit ‘Manfalouty’. The results of apricot hand thinning showed that this treatment increased the soluble content and other qualitative traits of the fruit (Edson et al., 1993). After fruit thinning, the leaf area ratio was greater in the remaining fruits than in the control. More leaf area and the production of photosynthetic materials further increase soluble solids.

According to Table 2, with increasing thinning intensity, the juice color index increased and reached the highest values at 20% and 25% levels. Pomegranate maturity is usually determined based on skin color assessment, sugar content, fruit acidity, and flavor (Al-Said et al., 2009). The color of many fresh and processed fruits affects their consumer acceptance (Manerab et al., 2012). The increase in red fruit color is due to the rise of anthocyanin content. Such an improvement in color can be due to an increase in the leaf area of the fruit, which is in agreement with the findings of other thinning-related researches (Fallahi et al., 1997; Jafari et al., 2014).

The contents of polyphenols were decreased during the ripening process of pomegranate fruit. This trend can be attributed to variations in the hydrolysis of glycosides, and the oxidation of phenols by polyphenol oxidase (Remorini et al., 2008). In the present study, thinning did not affect TPC (Total phenolic content) and no significant differences were observed among treatments. In agreement with our results, Padmavathamma and Hulamani (1996) revealed that fruit thinning did not affect the total phenolic content in pomegranate juice, which could be due to climatic conditions or cultivar effects. This finding is in contrast with those of other researchers such as Mohsen and Osman (2015). They showed that thinning increased total phenolic compounds and fruits of the highest level of thinning treatment (one fruit per cluster) had the highest phenol content. Also, an increase of TPC was observed in peach fruits from trees receiving a severe thinning (Drougoudi et al., 2009).

Conclusion

Pomegranate fruit quality appeared to be significantly improved by fruit thinning. Both thinning levels (20% and 25%) affect positively the most fruit characteristics with a preference for treatment 20% which resulted in a better yield.