Co-application of mineral and organic fertilizers under deficit irrigation improves the fruit quality of the Wonderful pomegranate

Introduction

Pomegranate (Punica granatum L.), a popular fruit in tropical and subtropical regions and an important crop for newly reclaimed soils in Egypt, has attracted attention for its health benefits (Lansky & Newman, 2007). The acceptability of pomegranate fruit to processors and consumers depends on a combination of several quality characteristics related to its mechanical and physicochemical properties, e.g., the attractiveness, smoothness, and color of the skin; the amount of cracking; the presence of small seeds in the aril; the incidence of fruit sunburn (which causes dark-brown to black discoloration of the affected skin area); and the content of sugars, anthocyanin, and vitamin C (Al-Said, Opara & Al-Yahyai, 2009; Hmid et al., 2018).

Although pomegranate is fairly drought resistant, it requires regular watering to produce a high yield and heavy fruit (Holland, Hatib & Bar-Yaakov, 2009; Taha, 2018). Nevertheless, some studies have shown that reduced irrigation regimes in combination with organic/mineral fertilization during the fruit growth period can not only reduce water use but also positively affect fruit quality characteristics such as taste (by an increase of total soluble solids (TSS)) and color (Crisosto et al., 1994; Torrecillas et al., 2000; Mpelasoka, Behboudian & Mills, 2001; Pérez-Pastor et al., 2007; Parvizi & Sepaskhah, 2015). In contrast, extreme watering may adversely affect fruit quality indicators by increasing vegetative growth, which promotes nutritional imbalance (Herrero & Guardia, 1992).

In general, water availability is the most critical and limiting factor for expanding cultivation and plant production in arid and semiarid regions. Therefore, worldwide research attention has been directed toward identifying efficient strategies for reducing water use during crop irrigation (Quda, 2016). Deficit irrigation is one such strategy for improving water productivity (i.e., the crop yield per m³ of applied water). Under regulated deficit irrigation, irrigation is applied below the maximum crop evapotranspiration rate (ETc) during certain noncritical stages of the crop cycle, whereas complete irrigation is used during the dormant period of the growing season; under sustained deficit irrigation, watering is reduced to a fraction of the ETc uniformly throughout the growing season (Ruiz-Sanchez, Domingo & Castel, 2010). Compared with full irrigation, scarcity irrigation during the flowering and fruit set stages has previously been shown to increase the quantity of pomegranate fruit per tree without having adverse effects on the fresh weight or yield (Intrigliolo et al., 2013), whereas sustained deficit irrigation increases the number of fruit (Intrigliolo et al., 2013), fruit peel redness and firmness, and the soluble solids content (Peña, Artés-Hernández & Aguayo, 2013). Pomegranate trees can activate physiological mechanisms to regulate their water status under scarcity conditions (Galindo et al., 2014); consequently, several studies have investigated pomegranate fruit properties under deficit irrigation (Mellisho et al., 2012; Rodríguez et al., 2012; Intrigliolo et al., 2013; Mena et al., 2013; Galindo et al., 2014; Parvizi, Sepaskhah & Ahmadi, 2014; Centofanti et al., 2017) both at harvest time and in storage (Laribi et al., 2013; Peña, Artés-Hernández & Aguayo, 2013).

Organic cultivation, which is considered a benign and sustainable alternative production method, can also improve pomegranate fruit production. For example, soil organic matter plays a vital role in improving the physicochemical attributes of fruits (Marathe et al., 2009); it has been shown to affect nutrient fluxes (Marathe et al., 2012) in sweet orange (Citrus sinensis L.) and increase microbial biomass in pomegranate (Marathe et al., 2010; Marathe et al., 2011; Marathe et al., 2017; Mir, Sharma & Kumar, 2015).

Therefore, for pomegranate with its the long maturation time, a more rational approach to organic cultivation, including the exploitation of various locally available organic materials such as farmyard manure, poultry manure, vermin compost, and green manure, could be practically implemented to help rejuvenate depleted soil fertility and enrich the pool of nutrients available to the plants, which could benefit the medicinal properties of the fruit and other plant parts. In addition, the co-application of modest amounts of mineral fertilizers with organic manure can provide nutrient requirements (Baghdadi et al., 2018).

Few studies have assessed the effects of irrigation regimes on the nutritional aspects of pomegranate fruit, the physiological and production responses of pomegranate trees to water stress, or the effects of coapplying organic and mineral fertilizers under scarcity irrigation (Holland, Hatib & Bar-Yaakov, 2009; Abdel-Sattar et al., 2021). Previous field studies in southeastern Spain have examined the effects of different deficit irrigation applications on yield performance (Intrigliolo et al., 2013; Abdel-Sattar et al., 2021) and some fruit composition characteristics during fruit growth (Mellisho et al., 2012). Recently, it was also shown that deficit irrigation in combination with organic manure/mineral fertilizer or organic manure alone could improve the marketable fruit of Wonderful pomegranate trees, which may be attributable to the organic manure reducing the water stress of the growing plants (Abdel-Sattar et al., 2021).

Building on these previous studies, the objective of the present study was to examine the separate and interactive effects of various irrigation regimes and fertilizer treatments on the quality of the Wonderful cultivar of pomegranate.

Materials & Methods

Results

Effects of irrigation regimes on the chemical properties of fruit

The main effects of the irrigation treatments on the chemical properties of Wonderful pomegranate fruit in the 2018 and 2019 seasons are shown in Fig. 1. All of the chemical attributes were significantly affected by the irrigation treatments in 2018 (P <0.05), with the application of 80% ETc (I₂) having a positive effect on all chemical properties except acidity, TSS/acidity, and reducing sugars when compared with the effects obtained with the 100% ETc (I₁) and 60% ETc (I₃) applications. Contrastingly, the application of 60% ETc significantly increased acidity and decreased all other chemical properties in the fruit. Similar results were obtained in 2019, i.e., 80% ETc (I₂₎ significantly increased the values of all chemical properties except for the acidity and reducing sugar content of the fruit (Fig. 1). Across the two seasons, the 80% ETc (I₂) application had a higher positive effect on all chemical properties of pomegranate fruit except acidity, TSS/acidity, and reducing sugars than did the 100% ETc (I₁) and 60% ETc (I₃) applications.

Interaction effects of irrigation and fertilizer treatments in relation to the physical properties of fruit

The two-way interactions between the irrigation regimes and fertilizer treatments on the physical properties of Wonderful pomegranate fruit grown in the 2018 and 2019 seasons are shown in Figs. 2 and 3. In 2018, the highest fruit weight was recorded with I₂+T₃ (333.75 g), followed by I₁+T₂ (327.75 g), and then I₁+T₅(325.50 g). In addition, the % fruit peel was significantly affected by the interaction between the irrigation regimes and fertilizer treatments, with the lowest percentages (and therefore the highest quality) being observed with the I₂+T₃ and I₂+T₄ treatments. These findings indicate that the use of organic fertilizer alone or in combination with mineral fertilizer enhanced the water content of the soil under deficit irrigation, which allowed the water supply to be reduced to 80% of the total requirement of the plants. The aril %, length, diameter, and volume of the fruit were also significantly affected by the interaction between the irrigation regimes and fertilizer treatments (Figs. 2 and 3), with higher values and therefore higher fruit quality being observed with the I₂+T₃ and I₂+T₄ treatments. The findings for 2018 were in agreement with the results from the 2019 season (Figs. 2 and 3).

The interaction effects of the irrigation regimes and fertilizer treatments on the skin color parameters of Wonderful pomegranate fruit grown in 2018 and 2019 are presented in Table 7. In 2018, the highest skin color (except b*) parameter values were observed with the I₂+T₃treatment, followed by the I₂+T₄ and I₁+T₃ treatments.

Table 7:

Interaction effects of three irrigation regimes and five fertilizer treatments on the skin color parameters of Wonderful pomegranate fruit in the 2018 and 2019 growing seasons.

Treatment	2018			2019
	L*	a*	b*	L*	a*	b*
I1+T1	52.47	46.02	25.22	52.76	46.58	25.44
I1+T2	54.77	43.53	23.57	55.65	43.50	23.78
I1+T3	56.71	48.00	22.82	57.33	48.80	23.31
I1+T4	56.03	46.09	23.64	56.49	46.42	24.25
I1+T5	54.18	44.68	23.78	54.42	45.36	24.47
I2+T1	52.17	48.94	24.75	52.07	49.70	24.83
I2+T2	60.03	46.13	22.61	59.54	45.78	23.05
I2+T3	63.46	54.70	22.23	63.52	54.69	21.76
I2+T4	62.06	51.32	23.03	61.78	51.38	22.77
I2+T5	57.53	49.70	24.59	56.67	48.30	24.22
I3+T1	45.23	38.76	26.12	44.89	37.75	26.12
I3+T2	45.76	39.65	20.76	45.18	39.58	20.80
I3+T3	52.86	46.36	23.14	52.32	46.66	22.75
I3+T4	50.29	45.85	23.41	49.92	45.98	23.41
I3+T5	48.51	42.70	25.99	48.24	41.71	25.91
LSD0.05	0.48	0.76	1.01	0.98	1.43	1.35

DOI: 10.7717/peerj.11328/table-7

Notes:

I₁: 100% ETc; I₂: 80% ETc; I₃: 60% ETc; T₁: 100% mineral fertilizer; T₂: 100% organic manure; T₃: 75% mineral fertilizer + 25% organic manure; T₄: 50% mineral fertilizer + 50% organic manure; T₅: 25% mineral fertilizer + 75% organic manure.

Discussion

Our results showed that 100% and 80% ETc applications significantly increased all physical properties of Wonderful pomegranate in the two tested seasons, whereas 60% ETc significantly reduced all physical properties. Thus, the water deficit is a limiting factor for Wonderful pomegranate fruit quality. Abdel-Sattar & Mohamed (2017) reported similar results, i.e., that deficit irrigation management was generally the main reason for reduced fruit volume and weight as well as shape index in Manfalouty pomegranate fruit. In addition, our results were in agreement with the findings of Rodríguez et al. (2012), Mellisho et al. (2012), Intrigliolo et al. (2013), Mena et al. (2013), Galindo et al. (2014), Parvizi, Sepaskhah & Ahmadi (2014), and Centofanti et al. (2017).

In the present study, all pomegranate skin color parameter values were significantly reduced with the 60% ETc application, indicating that a water deficit also affected the skin color of the fruit. Furthermore, deficit irrigation reduced the values of a* and b*, which is in agreement with the findings of Laribi et al. (2013), who observed redder and darker fruit at harvest under deficit irrigation. These skin color characteristics predominantly arise from an increase in anthocyanins and sugars and from reduced vegetative growth (i.e., nutritional status) (Intrigliolo et al., 2013; Laribi et al., 2013; Martínez-Nicolás et al., 2019). Our results are also in agreement with those of Gelly et al. (2003) for peach, Prunus persica (L.) Batsch, and Martínez-Nicolás et al. (2019) for the Wonderful and Mollar de Elche pomegranate cultivars.

Similarly, all of the skin color parameters in Wonderful pomegranate fruit grown in the 2018 and 2019 seasons were significantly affected by fertilizer treatment. Application of 100% organic fertilizer significantly increased all of the parameters except a* and b*, which were decreased, relative to the application of 100% mineral fertilizer. The co-application of mineral and organic fertilizers also had a significant effect on all skin color parameters, with the highest values being obtained with the 75% mineral + 25% organic fertilizer treatment. This may have been due to the organic fertilizer improving the physiochemical properties of the soil, e.g., aeration, water movement, water-holding capacity, cation exchange capacity, and availability of nutrients (Mahdy, 2012), which in turn resulted in improved skin color parameters. These findings agree with those reported in several other studies (e.g., Crisosto et al., 1994; Mpelasoka, Behboudian & Mills, 2001; Intrigliolo et al., 2013; Laribi et al., 2013; Martínez-Nicolás et al., 2019).

The application of 100% and 80% ETc significantly decreased the chemical properties of the Wonderful pomegranate, whereas 60% ETc significantly increased acidity and decreased all other chemical properties. In 2019, the 80% ETc treatment significantly increased all chemical properties of fruit except acidity and reducing sugar content. In general, these results agree with those of Abdel-Sattar & Mohamed (2017), who reported that fruit quality indexes such as TSS/acidity ratio, TSS, and acidity, as well as anthocyanin and vitamin C content, increased under deficit irrigation in Manfalouty pomegranate fruit, whereas the acidity of the fruit juice decreased. Our results also support those reported by Laribi et al. (2013), Intrigliolo et al. (2013), and Martínez-Nicolás et al. (2019) for other pomegranate cultivars.

When assessing the subplot effects of fertilizer treatments, we found that all of the physical properties were significantly affected by the fertilizer treatment in both seasons, with co-applications of mineral and organic fertilizers producing higher values for some parameters than were obtained with 100% mineral fertilizer alone. However, 100% mineral fertilizer produced higher fruit aril, volume, diameter, and length values with identical shape index values. Among the co-applications of mineral and organic fertilizers, a combination of 75% mineral + 25% organic fertilizers provided the highest value for every property. The improved physical and chemical characteristics achieved through co-application of mineral and organic fertilizers may be associated with the beneficial changes in soil P, K, and N, and in soil organic matter and organic matter-dependent soil properties, as well as the availability of nutrients (Mahdy, 2012; Baghdadi et al., 2018). Similarly, Marathe et al. (2017) reported that the application of organic fertilizers, such as farmyard manure and poultry manure, significantly enhanced pomegranate fruit yield and quality compared with the application of inorganic fertilizers due to the excess in nutrient availability. These results are also in line with those reported by Mellisho et al. (2012), Rodríguez et al. (2012), Intrigliolo et al. (2013), Mena et al. (2013), Galindo et al. (2014), Parvizi, Sepaskhah & Ahmadi (2014), and Centofanti et al. (2017), who associated the high-quality attributes of fruit with application of organic fertilizers.

Our results also showed that all of the measured chemical properties of Wonderful pomegranate fruit grown in 2018 and 2019 were significantly influenced by fertilizer treatment. Specifically, 100% organic fertilizer or the co-application of organic and mineral fertilizers significantly increased all chemical properties except acidity, which significantly decreased, whereas 100% mineral fertilizer generated lower values for all chemical properties except acidity. Among the co-application treatments, the highest values for all parameters except acidity and TSS/acidity ratio were obtained with the 75% mineral + 25% organic fertilizer treatment. The organic fertilizer therefore seems to have directly and indirectly enhanced the physiochemical properties of the soil and the availability of nutrients (Mahdy, 2012; Baghdadi et al., 2018; Abdel-Sattar et al., 2021). Similarly, Ray et al. (2014) reported that the fruit of pomegranate plants treated with 300-g N + 1-kg neem cake per plant showed high TSS (12.29°Bx), total sugar (10.74%), reducing sugar (9.78%), non-reducing sugar (1.09%), and ascorbic acid (21.93 mg 100 mL⁻¹ juice) contents as well as low acidity (0.39%). Amin, Ali & El-Moneim (2017) also showed that a mixture of organic and biofertilizers (organic N and PK raw mineral rocky materials at 2,000 g plus NPK biofertilizer at 300 mL plant⁻¹) provided the greatest improvement in all of the studied growth parameters for pomegranate, with the promotion of mineral composition and total chlorophyll noted in young trees treated with mixed organic materials (NPK at 2,000 g plant⁻¹) and biofertilizers (NPK at 300 mL plant⁻¹). Furthermore, Aly & Zagzog (2019) found that the application of biofertilizer and organic fertilizer enhanced all vegetative growth parameters of Zaghloul date palm (Phoenix dactylifera L.), with the replacement of mineral fertilizer in this scenario also reducing pollution. Moreover, OlyaieTorshiz et al. (2017) suggested that the use of biofertilizer in combination with organic fertilizers, particularly granular humic acid, could be used in pomegranate orchard management to improve pomegranate yield and prevent crop losses resulting from cracking, nutrient deficiency, and Ectomyelois ceratoniae Zeller infestation. The findings of the present study were also in agreement with those reported by Intrigliolo et al. (2013), Mena et al. (2013), Galindo et al. (2014), Parvizi, Sepaskhah & Ahmadi (2014), and Centofanti et al. (2017) in relation to the enhancement of soil porosity and infiltration rate as well as water retention after organic fertilizer application.

Our findings indicate that the use of organic fertilizer alone or in combination with mineral fertilizer enhances the water content of the soil under deficit irrigation, allowing the amount of water supplied to be reduced to 80% of the total requirement of the plants. Specifically, fruit weight, aril %, length, diameter, and volume were significantly affected by the interaction between the irrigation regimes and fertilizer treatments. These findings were apparent in both growing seasons and were likely due to the organic fertilizer having a beneficial effect on water content and releasing more nutrients into the soil in an available form. Similar trends were observed by Intrigliolo et al. (2013), Parvizi, Sepaskhah & Ahmadi (2014), and Centofanti et al. (2017), who found that deficit irrigation is usually the principal reason for reduced fruit volume, length, diameter, weight, and aril.

The interaction effects of the irrigation regimes and fertilizer treatments also increased the skin color parameters (except b*) in Wonderful pomegranate fruit in both seasons, again because the application of mineral fertilizer in combination with organic fertilizer improved the water content of the soil under deficit irrigation, which released more nutrients into the soil in an available form, allowing for a reduction in amount of water supplied to pomegranate (80% of the total requirement). Similarly, Mena et al. (2013) found that reduced irrigation caused a dramatic reduction in bioactive phenolic compounds, particularly anthocyanin, in pomegranate fruit, which resulted in color changes to the juice (it became more yellowish). The results of Gelly et al. (2003) for peach, Laribi et al. (2013) for Mollar de Elche pomegranate, Abdel-Sattar & Mohamed (2017) for Manfalouty pomegranate, and Martínez-Nicolás et al. (2019) for the Wonderful and Mollar de Elche pomegranate cultivars are also in agreement with our findings.

In both seasons, the chemical properties of Wonderful pomegranate fruit (with the exception of acidity and the TSS/acidity ratio) were improved via the interacting effects of the irrigation regimes and fertilizer treatments. This is likely because the organic fertilizers released more nutrients into the soil in an available form and had a beneficial effect on the water-holding capacity of the soil, again allowing the water supply to be reduced to 80% of the total requirement. The increase in chemical properties such as the vitamin C, anthocyanin, TSS, reducing sugar, and non-reducing sugar contents was observed with all co-application treatments. However, reducing the amount of water to 60% ETc resulted in lower quality fruit. These findings agree with those of Khattab et al. (2011) and Abdel-Sattar & Mohamed (2017), who concluded that deficit irrigation could improve vitamin C, anthocyanin, and TSS content, increase TSS/acidity ratio, and reduce acidity in pomegranate fruit. Similarly, Mena et al. (2013) found that reducing the irrigation levels to 43% and 12% of ET₀ dramatically reduced the amount of bioactive phenolic compounds, particularly anthocyanin, in pomegranate fruit, which accordingly caused the color of the aril juice to become more yellowish.

Organic fertilizer has a micronutrient content that is several fold higher than that of mineral fertilizer, which leads to the mineralization of organically bound forms, the establishment of organic chelates of higher stability, or the production of stable water-soluble complexes, which have a reduced susceptibility to absorption, precipitation, and/or fixation (Mir, Sharma & Kumar, 2015; Shambhavi et al., 2020). Therefore, as previously mentioned the co-application of organic and mineral fertilizers increases the water-holding capacity of the soil under deficit irrigation and allows the water supply to the plants to be reduced (to 80% of the total requirement in the current study) (Küçükyumuk, Yildiz & MerIç, 2020).

Conclusions

We examined the separate and interactive effects of various irrigation regimes and fertilizer treatments on the quality of the Wonderful cultivar of pomegranate. Our results suggest that, under deficit irrigation, the co-application of organic and mineral fertilizers produces better quality pomegranate fruit than does the application of mineral fertilizer alone. Reducing the water supply to 80% ETc improves fruit quality when compared with the fruit produced by 100% and 60% ETc applications. The best combination of mineral and organic fertilizers was 75% mineral + 25% organic fertilizers, which produced the highest values of fruit quality. Using this technique could promote the partitioning of metabolites, which would favor the fruiting of horticultural trees in arid growing regions.

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