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Article

Deficit Irrigation to Enhance Fruit Quality of the ‘African Rose’ Plum under the Egyptian Semi-Arid Conditions

by
Islam F. Hassan
1,*,
Maybelle S. Gaballah
1,
Hanan M. El-Hoseiny
2,
Mohamed E. El-Sharnouby
3 and
Shamel M. Alam-Eldein
4,*
1
Water Relations and Field Irrigation Department, Agricultural and Biological Research Division, National Research Centre, Giza 12622, Egypt
2
Horticulture Department, Faculty of Desert and Environmental Agriculture, Matrouh University, Marsa Matrouh 51511, Egypt
3
Department of Biotechnology, College of Science, Taif University, P.O. Box 11099, Taif 21944, Saudi Arabia
4
Department of Horticulture, Faculty of Agriculture, Tanta University, Tanta 31527, Egypt
*
Authors to whom correspondence should be addressed.
Agronomy 2021, 11(7), 1405; https://doi.org/10.3390/agronomy11071405
Submission received: 31 May 2021 / Revised: 2 July 2021 / Accepted: 9 July 2021 / Published: 13 July 2021
(This article belongs to the Topic Physiological and Molecular Characterization of Crop Tolerance to Abiotic Stresses)
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Abstract

:
Evolved in South Africa and released to market in 2009, the ‘African Rose’ plum has been introduced and grown under the Egyptian semi-arid conditions since 2010. Within that time, this cultivar has faced significant fruit quality issues, mainly poor color and low total soluble solids (TSS). Several trials using foliarly applied growth regulators have been conducted, but with little conspicuous results on fruit yield and quality. There is very limited information about the relationship between irrigation regime and fruit quality for this cultivar. Therefore, a field experiment was conducted to study the effect of deficit irrigation on the quality of the ‘African Rose’ plum during the 2019 and 2020 seasons. Five-year-old hedge growing trees were subjected to three deficit irrigation regimes: 100% (control), 80%, and 60% of the crop evapotranspiration (ETc) after the pit hardening stage until the end of the harvest season (May to June period) were evaluated. Results indicated that deficit irrigation positively enhanced the levels of abscisic acid (ABA), total phenols, and anthocyanins with improved fruit TSS and maturity index, although fruit yield, acidity, size, and firmness were decreased. Deficit irrigation could be suggested as a sustainable novel solution to improve the fruit quality of the ‘African Rose’ plum grown under the semi-arid conditions of Egypt. Although the total yield and some quality characteristics were not improved, the early harvested fruit with enhanced color and taste could be a good start for additional research to solve other quality-related issues under such conditions.

1. Introduction

Domesticated in China more than 200 years ago, the plum, a member of the Rosaceae family, is one of the most widely distributed deciduous fruit tree in the world [1]. The European plum, Prunus domestica L., and Japanese plum, Prunus salicina L., can be successfully cultivated in all irrigated regions around the world, including Egypt. The total cultivated area of plums (i.e., mostly the European cultivars) in Egypt is about 1115 hectares with a total annual production of 13,725 tons [2]. The most cultivated cultivars in Egypt are ‘Beauty‘, ‘Hollywood‘, and ‘Santa Rosa‘; however, new cultivars like the ‘African Rose’ cv ARC PR-4 (PR00-01), which is classified as a Japanese plum evolved at ARC Infruitec-Nietvoorbij in South Africa, was introduced to Egypt in 2010 due to its good production at early tree age with no need for pollinators (i.e., honeybees) like other local cultivars, since this cultivar is self-compatible [3]. Under the South African conditions, trees are generally moderate in vigor, semi spreading, bear on both spurs and one-year shoots, and have low-to-moderate chilling requirements (200 to 400 infruitec chilling units); therefore, this cultivar is recommended for cultivation in early maturing areas like deserts. Flowering intensity is high with excellent fruit set; therefore, fruit thinning is recommended for good quality. Fruit yield is good (20 kg/tree) with an average weight of 75–80 g/fruit, and fruit are round and medium in size with bright red skin, yellow flesh, medium firm texture, cling stone, and a minimum total soluble solids (TSS) of 14 °Brix at harvest. For optimum storage (28 days at 0 °C and 90%–95% RH) and good eating quality, harvest should be at TSS of 14 °Brix and firmness of 0.588–0.785 N/mm2 (11 mm probe) [4]. Personal observations with plum growers indicated that under the Egyptian conditions, fruit have shown poor peel color, low sugar, and high acidity contents. This might be due to the hot and dry growing season, which requires sufficient rates of irrigation during fruit ripening to avoid heat stress and preharvest fruit abscission [5]. However, previous reports have indicated that providing excess water after the pit hardening stage until harvest in olive and peach crops has delayed the ripening process, and specific primary and secondary metabolites have been affected leading to a reduced quality, in terms of low TSS and anthocyanin concentrations, as well as preharvest fruit drop [6,7,8,9,10,11]. Likewise, most growers of ‘Crimson Seedless’ grapes in Egypt irrigate their vineyards well to ensure adequate water supply during the ripening stage due to hot summer; meanwhile, they experience poor berry color and low TSS, but deficit irrigation has improved berry maturation with good color and TSS [12].
Fruit quality is the outcome of the improvement in fruit physiochemical characteristics in relation to various environmental and cultural factors during growth and development. Under the Egyptian conditions, most early-harvested plum cultivars, including ‘African Rose’, are harvested during June based on fruit color and TSS [13,14]. In this context, many attempts have been implemented to improve the quality of ‘African Rose’ fruit growing under the Egyptian conditions. For example, the common application of foliarly sprayed Ethephon that was used with other plum varieties did not show much improvement with the ‘African Rose’; instead, it resulted in defoliation, less-colored fruit, and low yield in the following season [15]. Late harvest after June slightly improved fruit color and TSS, but mostly led to low-quality fruit (i.e., increased weight loss, softening, decay) due to the increased levels of temperature and infestation with the Mediterranean fruit fly Ceratitis capitata during July [16], in addition to a low remunerative value due to competition with other fruit types available at the market by this time [17]. These unfavorable results could be due to the difference in plant growth and development in relation to the environmental conditions, primarily since this cultivar came from South Africa, which differs from the Egyptian climatic conditions. To date, very limited information is available on this cultivar in terms of the environmental and cultural requirements.
The plant–water interaction has been of particular consideration in current years; it is usually known that moderate water stress definitely improves plum fruit quality [18]. Deficit irrigation is known as a method that increases a plant’s water use efficiency and improves fruit quality [18,19]. Fruit growth in stone fruit follows a typical double-sigmoid pattern, with rapid exponential growth during cell division (stage 1), followed by a relatively short period of slow growth due to pit hardening and the embryo development phase (stage 2), and finally a period of rapid growth due to cell enlargement prior to harvest (stage 3). The duration of each stage varies with cultivar and location [20]. The response of stone fruit trees to deficit irrigation relies on the stage of fruit growth at which it was applied [21]. Therefore, deficit irrigation applied to plum trees during the period after pit hardening until the end of harvest (stage 3) resulted in a reduction in fruit size and total yield, but improved fruit composition, color, and quality [22,23,24]. Fruit sensitivity to water stress during stage 2 is slight because of the slow growth rate during this period [25]. Water deficit can have a negative effect on peach fruit size, only if the water deficit persists at the onset of stage 3, which is the most sensitive stage to irrigation restriction from a yield point of view [26], although water deficit has shown an improvement in fruit quality in terms of increased TSS [27,28]. The application of deficit irrigation during this period may increase the accumulation of anthocyanins and TSS via reinforcing the internal abscisic acid (ABA) content [29,30,31,32]. Previous findings showed a positive effect of deficit irrigation improving fruit TSS with reduced total acidity (TA) [23,33,34,35]. Therefore, the main aim of this work was to study the effect of different deficit irrigation regimes (100%, 80%, and 60% ETc), applied after the pit hardening stage until the end of harvest season, on the total yield and fruit ripening of ‘African Rose’ plum trees grown under the semi-arid conditions of Egypt.

2. Materials and Methods

2.1. Experiment and Site Climatic Conditions

This research was carried out on five-year-old ‘African Rose’ plum trees, Prunus salicina L., during the 2019 and 2020 seasons. The experimental site was a private orchard located at El-Khatatba, Menoufia Governorate (30°21′60″ N, 30°49′60″ E, 20–50 m height above sea level), Egypt. The climatic conditions of the experimental site are semi-arid without summer rains, as shown in Table 1 [36]. The calculated reference crop evapotranspiration (ETo) of the plum was collected daily during May and June of the 2019 and 2020 seasons from the nearest weather station located 10 km away from the experimental site. Both weather data and ETo were calculated as an average of two seasons using the Food and Agriculture Organization (FAO) Penman–Moteith equation (CROPWAT software, version 8) [37], and are displayed in Table 1. Soil and water analyses were carried out according to Chapman and Pratt [38] and are displayed in Table 2.
Grafted on ‘Nemaguard’ rootstock, the five-year-old ‘African Rose’ trees, planted in sandy loam soil at 2 × 3 m spacing, were carefully selected for similar vigor and size. The training system used in this orchard was a wire-shape and trees were grown on a 6-wire hedge, as shown in Figure 1. Apart from the irrigation, all other agricultural practices, such as fertilization, pruning, spraying Dormex (1%, for uniform flowering), and fruit thinning were conducted according to the standard agricultural practices of plums released by the Egyptian Ministry of Agriculture and Land Reclamation [39]. The experiment layout was set in a randomized complete block design (RCBD) of 3 irrigation treatments with 10 replicates. Each replicate was represented by 10 trees (n = 10) for a total of 300 trees, and the average fruit number was estimated per replicate.

2.2. Irrigation Treatments

As an early-season cultivar, the full bloom of the ‘African Rose’ plum under the Egyptian conditions occurred by early March during both seasons. Trees were subjected to deficit irrigation during the May–June period (end of pit hardening until end of harvest) according to differences in crop evapotranspiration (ETc). The source of drip irrigation was a deep groundwater well, and three irrigation treatments—100% (control), 80%, and 60% ETc—were used. Percentage of ETc was calculated based on the reference crop evapotranspiration (ETo) and crop coefficient factor (Kc) of plums (Table 3), as proposed by Allen et al. [40], using the following equation [37]:
ETc = ETo × Kc
where ETc is the evapotranspiration, ETo is the reference evapotranspiration, and Kc is the FAO coefficient factor.
Total water requirement (WR) of the trees was calculated using the theoretical irrigation rate (m3/ha/season) during the May–June period and ETc values (Table 3), according to the monthly weather data (Table 1). The number of irrigation times varied among the three treatments with a frequency of 1–5 irrigation times per week, based on the soil water content of the three treatments that was monitored weekly using soil tensiometer Model 64xx series (Spectrum Technologies Inc., Aurora, IL, USA). Two lateral lines of irrigation pipes (one on each side of the trees row) with 10 drippers per tree (8 L/h/dripper) were used for the control treatment (100% ETc), whereas 8 and 6 drippers were used for the 80% and 60% ETc treatments, respectively.

2.3. Yield and Fruit Physiochemical Characteristics

By late June in both seasons, fruit were harvested at the commercial maturity, when they reached the minimum standards for harvest with acceptable red skin, yellow pulp, TSS of 12, and TA of 0.8 [5,41]. Yield was represented by total fruit number and weight (kg) per tree. Fruit drop percentage was calculated using the following equation:
[(Initial fruit number − Final fruit number)/Initial fruit number] × 100
A sample of 20 uniform fruit per tree was randomly selected to evaluate the fruit physical and chemical characteristics. Fruit samples were quickly transferred to the quality control laboratory, and fruit weight was measured using a bench-top digital scale Model 6000e (VisionTechShop, Hackensack, NJ, USA). Fruit diameter was measured using digital caliper Model CD-15CPX (Mitutoyo Corp., Kawasaki, Kanagawa, Japan). Fruit firmness (N/mm2) was measured using Effigi penetrometer Model FT327 fitted with an 11 mm diameter plunger tip (QA Supplies LLC, Norfolk, VA, USA). Fruit TSS were determined using a hand-held refractometer Model Palette PR-101 (Atago Co., Tokyo, Japan), and values were indicated as °Brix. Total acidity (TA) was determined by titrating 1 mL of fruit juice against NaOH (0.1 mol/L), and values were expressed as a percentage of malic acid [42]. The values of TSS and TA were used to express the fruit maturity index using the following equation [43];
Maturity index = (TSS/TA) × 10
where TSS is measured in °Brix, TA is measured in percentage of malic acid, and 10 is a conversion factor between both units.
Total phenols were extracted and determined in mg/100 g fresh weight (FW) according to Kim and Lee [44]. Anthocyanins were extracted and determined using a spectrophotometer Model UV 120-02 (Shimadzu Corp., Kyoto, Japan) at wavelength of 535 nm, and values were expressed as mg/100 g FW [45]. Abscisic acid (ABA) was also extracted and expressed as ng/g FW [46].

2.4. Economic Implications

The economic cost of irrigation treatments was compared with Ethephon spray treatment used per hectare (ha). Irrigation cost was based on the price of the cubic meter of water, according to the 2019 and 2020 annual report of the Egyptian Ministry of Water Resources and Irrigation, with an annual price increase from 2019 to 2020 of about 0.15 Egyptian pound (LE) [47]. Additionally, the price of the double application of Ethephon (200 ppm) was calculated according to the suggested prices by the Egyptian Ministry of Agriculture and Land Reclamation in the 2019 and 2020 seasons [39] with an annual increase of about 2.3% [48].

2.5. Statistical Analysis

Data were analyzed using CoStat—Statistics Software (version 4.20) [49]. Data were first run for numerical normality and homogeneity of variance using the Shapiro–Wilk’s and Levene’s tests, respectively, and then a one-way analysis of variance was performed, and means were compared using Duncan’s multiple range tests (DMRT) at p ≤ 0.05. Standard error bars were also added for mean comparisons in the figures [50,51].

3. Results

3.1. Fruit Drop, Yield, and Physical Characteristics

3.1.1. Fruit Drop (%)

Results indicated that percentage of fruit drop was insignificantly different between the two levels of water deficit (80% and 60% ETc) during both seasons, whereas the difference was significant in comparison to 100% ETc (the control) (Figure 2).

3.1.2. Number of Fruits and Fruit Weight (kg) Per Tree

Both the number of fruits and fruit weight (kg) per tree decreased with the continuous reduction in ETc percentage, even though these two parameters were noticeably higher in 2020 (Table 4). The most conspicuous reduction was noticed with the 60% ETc treatment, which was also significantly different from the application of 80% ETc, except for the number of fruits per tree during the second season. It could also be noticed that water stress affected the percentage of increase in the number of fruits per tree from one season to another by 40.7% with 60% ETc, which was higher than that of 80% ETc and the control (9.4% and 10.4%, respectively); however, the changes in fruit weight from one season to another were almost the same.

3.1.3. Fruit Diameter (mm) and Firmness (N/mm2)

Like the number of fruits, fruit diameter and firmness were also negatively affected with reduced ETc during both seasons (Table 5). The largest and the firmest fruit were recorded with the control treatment (100% ETc). Fruit diameter were significantly different among all treatments in both seasons. Fruit firmness was insignificantly different between 80% ETc and the control, as well as between the two levels of water deficit during the first and the second seasons, respectively.

3.2. Fruit Chemical Characteristics

Under the deficit irrigation conditions of 60% ETc, the fruit TSS was the greatest, while fruit from the control trees recorded the lowest TSS in both seasons (Table 6). In contrast, TA content significantly decreased with more water deficit compared to the control during both seasons. Therefore, the highest fruit maturity index was recorded from trees that received 60% ETc (Figure 3). The 60% ETc treatment also had fruit with the greatest total phenols and anthocyanins, followed by fruit from trees that received 80% ETc, and then fruit from the control trees, which had the lowest contents (Table 7). Similarly, ABA content significantly increased with more water deficit in both seasons (Figure 4).

3.3. Economic Implications

The variable cost and economic indices for all deficit irrigation treatments and the control against spraying Ethephon during the period of experiment are displayed in Table 8. Data showed that reducing the water amount remarkably decreased the total cost, which was then smaller than that of spraying Ethephon in both seasons.

4. Discussion

Under the semi-arid conditions of Egypt, ‘African Rose’ plum fruit manifested poor peel color, low sugar, and high acidity contents, which means low-grade quality that may lead to poor marketing (personal communication). Due to the limited reports on the environmental and cultural requirements of this cultivar, the objective of this research was to determine whether deficit irrigation could be used as a sustainable novel tool to improve fruit quality, instead of commonly used methods such as foliar spray of growth regulators, which did not show much improvement in fruit quality, particularly those used as harvest indices like fruit color and TSS [4].
Results suggested that reducing the amount of water available for the trees directly after the pit hardening stage until the end of the harvest season negatively affected the total yield due to the reduction in total harvested fruit number, weight and diameter (Table 4 and Table 5). The reduction in the number of harvested fruit per tree was the result of increased fruit drop percentage under deficit irrigation conditions, as shown in Figure 2. In this regard, it is worth mentioning that at the tree level, the difference in fruit drop percentage was almost the same for all 10 trees per replicates, and same observation was noticed for all three replicates, since the selected trees were similar in vigor and size; however, the higher fruit drop percentage was recorded for trees located close to the end of the tree lines, and this could be justified because these trees are more exposed to sun (data not shown). It has been reported that stone fruit trees subjected to deficit irrigation during stage 3 of fruit growth have shown early ripening, associated with yield reduction because of the smaller fruit size at harvest due to the reduced rates of mesocarp cell expansion [52,53,54,55,56,57]. It has also been reported that under such environmental conditions, water flow from the mother plant to the fruit was insufficient to balance fruit transpiration rate, which caused the reduction in fruit water status and growth [58]. In addition, the increased levels of ABA under drought conditions may trigger ethylene production, which promotes the activity of hydrolytic enzymes, such as endo-beta-glucan, cellulase, and polygalacturonase, at the abscission zone of the fruit petiole inducing preharvest fruit abscission [59,60,61]. The present study showed a reduction in both fruit size and firmness (Table 5). Plum fruit has been reported to be sensitive to water shortage, which affects cell turgidity and negatively impacts fruit size and firmness [8,62,63,64], which was similar to the results of the current study. Water shortage stimulates glycosidases causing changes in pectin polymers of the cell wall leading to the deterioration of the cell wall and loss of firmness [65]. In contrast, a previous report on three other Japanese plum cultivars, grown under the semi-arid conditions of Tunisia, showed a positive impact of moderate water deficit (50% crop water requirements) on fruit firmness [23]. This suggests that fruit firmness in response to water deficit may be a cultivar-related parameter. Previous reports have shown that reduced water content has resulted in improved TSS and reduced TA, resulting in an improved maturity index (TSS: TA ratio) [66,67,68], which is similar to the results of the current study, as indicated in Table 6 and Figure 3. Similarly, it was also reported that deficit irrigation increased preharvest fruit drop, and the resultant low yield significantly improved the TSS of plum fruit at harvest [69,70].
Reduced irrigation has been shown to trigger ethylene biosynthesis leading to the accumulation of glucose, fructose, and sucrose through the inhibition of pectinase and polygalacturonase [71,72]. Glucose and fructose have also been reported as the compounds largely responsible for the active osmoregulation in moderately stressed trees [73], and total sugars were reported higher in water-stressed plants than the non-stressed ones [74]. This might also explain the slight reduction in the number of fruits with 60% ETc compared to the control in the 2020 season, in comparison to the 2019 season (Table 4), because of the accumulated sugars that might led to a higher carbon: nitrogen ratio, which positively affected flowering and fruit set in the 2020 season, as previously reported [75]. However, fruit TSS and TA were almost the same in both seasons (Table 6), and this could be related to increased fruit competition in the second season, as previously reported [76]. Research has previously reported an increase in sugar accumulation as a result of cell dehydration under stressful conditions [7,77,78]. Sugar accumulation under drought conditions could also be from the tree reserve by active osmoregulation to maintain cell turgidity and minimize damages caused by water stress [64]. Moreover, sugars could also be produced as a result of partial photosynthesis due to non-uniform stomatal closure in response to increased ABA levels under drought conditions [79]. These sugars were not completely utilized for fruit volume growth even after irrigation was resumed [80]. An increase in fruit sugar content is usually associated with improved TSS [78]. The reduction in fruit TA under deficit irrigation conditions (Table 6) may have resulted from a reduction in malic acid content, which is associated with reduced water content, as previously reported [81,82,83,84]. In addition, the reduction in stomatal conductance and the photosynthesis rate of stressed plants could be two other reasons for the low availability of metabolites that are used to build up acids [85,86]. Moreover, the reduction in acids under water stress conditions is mainly related to the increase in the respiration rate [87], since acids are being used as substrates in the second stage of cell respiration (tricarboxylic acid cycle) [88].
Results of the current study showed that under deficit irrigation conditions, there was an increase in fruit total phenols, anthocyanins (Table 7), and ABA (Figure 4), which were also observed following two successive seasons of water deficit (2019 and 2020) (Table 7 and Figure 4). Previous research has reported an increase in the biosynthesis of anthocyanins and an increase in ABA signaling, associated with improved fruit color under drought conditions [89,90,91]. Similarly, an increase in phenylalanine ammonia-lyase has been shown to be responsible for the biosynthesis of phenols [92] from the increase in ABA levels [93].
Phenols are one of the most important groups of secondary metabolites in plants that act as antioxidants that protect cell structure and improve plant tolerance to stressful conditions [94,95]. In addition, phenolic compounds have been shown to be the major influence on the sensory quality of the fruit (e.g., color, flavor, and taste) [96]. Anthocyanins are the prominent phenolic compounds responsible for plum fruit color, which accumulate first in the peel and then in the pulp [97]. Anthocyanins also contribute to the plant antioxidant capacity more than other phenolic compounds [98]. The biosynthesis of ABA usually occurs in the roots under drought conditions, and can increase 50 times, which is considered the highest change of any phytohormones under such conditions [99]. The ABA concentration has been shown to promote root growth and adjust shoot growth [100] via the regulation of transpiration and photosynthesis [101]. It has also been shown to induce stomatal closure and reduce transpiration [102]. An increase in ABA concentration has also been reported to increase the catechin and malvidin synthesis in moderately water-stressed plum trees, which may improve fruit color and sugar contents with a reduction in total acidity [103]. The role of ABA on sugar contents came through the accumulation of assimilates from the phloem into the fruit by strengthening sink capacity [104,105]. An increase in ABA has been reported as an indicator of the beginning of fruit senescence [106], which may shorten the storability of fruit. This is also could explain the reduction in fruit firmness of the ‘African Rose’ plum fruit from trees subjected to deficit irrigation in the present study (Table 5). It has also been reported that the accumulation of sugars in combination with low nitrogen content under drought conditions has induced senescence-like symptoms [107]. The reduction in total yield and fruit diameter (Table 4 and Table 5) may have resulted from an increase in ABA content (Figure 4), since ABA content has been negatively correlated to fruit weight, as reported in lemons [108]. Fruit weight reduction in water-stressed peaches was a result of simultaneous declines in fruit dimensions and pulp weight [109]. Fruit ABA has also been shown to be involved in ethylene production, which in turn affects cellulase and polygalacturonase activity, leading to fruit softening [60]. This might be the reason of low fruit firmness in the current study (Table 5).
In general, deficit irrigation during stage 3 of fruit growth was shown to improve plum fruit quality through the enhancement of peel-to-pulp color ratio and the biosynthesis of quality-related components via the upregulation of related genes leading to an improved fruit maturation rate. Improved fruit color and TSS could also be related to reduced shoot growth and canopy size, which improved the light regime around the fruit [110,111,112,113,114]. This could also lead to poor fruit color, since improved water and nitrogen uptake resulted in enhanced vegetative growth, which in turn reduced light penetration into the canopy and around the fruit [115].
Overall, despite the negative effect of water deficit conditions on ‘African Rose’ fruit yield, size, and firmness, the results of the present study indicate a positive role of water deficit on other fruit ripening aspects such as color and taste as a result of early fruit maturity, compared to the well-irrigated trees. This led to early harvested fruit, avoiding a marked increase in fruit drop due to hot weather, and the overrun of fruit flies during July. Moreover, the economic value of using deficit irrigation is less costly in comparison to spraying other growth regulators like Ethephon. It could be mentioned that irrigation cost was lower as a whole (and was not eliminated due to early maturity or increased fruit drop) when compared to Ethephon treatment, because Ethephon treatment also led to improved maturity and/or increased fruit drop percentage (data not shown), the same as the water deficit conditions. In addition to that, the high remunerative value of the fruit due to low competition with the few other fruit types available at the market at this time could compensate for the reduction in yield of trees subjected to deficit irrigation compared to the well-irrigated ones.

5. Conclusions

Most plum cultivars in Egypt, including the ‘African Rose’ cultivar, are harvested based on fruit color, TSS, and TA, but the ‘African Rose’ has shown low-quality fruit under the Egyptian conditions, even with late harvest after June. The findings of the current study reflect that deficit irrigation, applied to ‘African Rose’ plum trees by the end of the pit hardening stage until the end of the harvest season, enhanced fruit ripening and led to early harvest in comparison to well-irrigated trees. This deficit irrigation technique could be considered as a sustainable novel solution to improve the fruit quality of the ‘African Rose’ plum grown under the semi-arid conditions of Egypt. Although the total yield and some quality characteristics were not improved under such conditions, the improvement in plant tolerance to water shortage and early harvested fruit with good sensory parameters like color and taste, which are the two main factors that affect customer’s decisions, may be a good start to solve the quality problems of this cultivar with good remunerative value for growers due to less market competition with other fruits during June. However, future research on the effect of deficit irrigation on enhancing fruit ripening is needed, taking into account total yield and other quality parameters like fruit size, firmness, and flavor components, as well as fruit storability. This may require additional research work at the molecular level of drought-tolerance and quality-related genes.

Author Contributions

Conceptualization, I.F.H., M.S.G., H.M.E.-H., and S.M.A.-E.; investigation, I.F.H., M.S.G., and H.M.E.-H.; methodology, I.F.H., M.S.G., and S.M.A.-E.; project administration, I.F.H., M.S.G., M.E.E.-S., and S.M.A.-E.; resources, I.F.H. and M.E.E.-S.; writing—original draft, I.F.H. and S.M.A.-E.; writing—review and editing, S.M.A.-E. All authors have read and agreed to the published version of the manuscript.

Funding

Research Supporting Project number (TURSP-2020/139), Taif University, Taif, Saudi Arabia.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors thank Taif University, Taif, Saudi Arabia for the financial support of this research under the project number: TURSP-2020/139. The authors also gratefully thank the owner and the technical workers of the plum orchard, Menoufia, Egypt, as well as the staff of the Echo Physiology Laboratory at the National Research Institute, Giza, Egypt for their excellent technical assistance. Authors’ appreciation also extends to Harlene Hatterman-Valenti, North Dakota State University, USA for her valuable comments to improve the readability of the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Flowering and fruit development of 5-year-old ‘African Rose’ plum in a wire training system on a 6-wire hedge under the semi-arid conditions of El-Khatatba, Menoufia, Egypt.
Figure 1. Flowering and fruit development of 5-year-old ‘African Rose’ plum in a wire training system on a 6-wire hedge under the semi-arid conditions of El-Khatatba, Menoufia, Egypt.
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Figure 2. Effect of deficit irrigation (as a percentage of crop evapotranspiration, ETc) on the ‘African Rose’ plum fruit drop percentage during the 2019 and 2020 seasons. Standard error bars and Duncan’s multiple range test (DMRT) were used for mean comparisons within the same year (n = 10). Capital and small letters represent DMRT for 2019 and 2020, respectively.
Figure 2. Effect of deficit irrigation (as a percentage of crop evapotranspiration, ETc) on the ‘African Rose’ plum fruit drop percentage during the 2019 and 2020 seasons. Standard error bars and Duncan’s multiple range test (DMRT) were used for mean comparisons within the same year (n = 10). Capital and small letters represent DMRT for 2019 and 2020, respectively.
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Figure 3. Effect of deficit irrigation (as a percentage of crop evapotranspiration, ETc) on maturity index of ‘African Rose’ plum fruit during the 2019 and 2020 seasons. Standard error bars and DMRT were used for mean comparisons within the same year (n = 10). Capital and small letters represent DMRT for 2019 and 2020, respectively.
Figure 3. Effect of deficit irrigation (as a percentage of crop evapotranspiration, ETc) on maturity index of ‘African Rose’ plum fruit during the 2019 and 2020 seasons. Standard error bars and DMRT were used for mean comparisons within the same year (n = 10). Capital and small letters represent DMRT for 2019 and 2020, respectively.
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Figure 4. Effect of deficit irrigation (as a percentage of crop evapotranspiration, ETc) on ABA content of ‘African Rose’ plum fruit during the 2019 and 2020 seasons. Standard error bars and DMRT were used for mean comparisons within the same year (n = 10). Capital and small letters represent DMRT for 2019 and 2020, respectively.
Figure 4. Effect of deficit irrigation (as a percentage of crop evapotranspiration, ETc) on ABA content of ‘African Rose’ plum fruit during the 2019 and 2020 seasons. Standard error bars and DMRT were used for mean comparisons within the same year (n = 10). Capital and small letters represent DMRT for 2019 and 2020, respectively.
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Table
Table 1. Weather data of El-Khatatba, Menoufia, Egypt and reference crop evapotranspiration (ETo) of the plum as the average of the 2019 and 2020 seasons.
Table 1. Weather data of El-Khatatba, Menoufia, Egypt and reference crop evapotranspiration (ETo) of the plum as the average of the 2019 and 2020 seasons.
MonthMin Temp
(°C)		Max Temp
(°C)		Humidity
(%)		Rainfall
(mm)		Wind Speed
(km/Day)		Radiation
(MJ/m2/Day)		ETo
(mm/Day)
May14.131.8540.021624.56.05
June17.534.6560.019026.36.23
Table
Table 2. Soil and water analysis of the experimental site.
Table 2. Soil and water analysis of the experimental site.
Soil Analysis (0–40 cm)Water Analysis
pH7.4pH7.3
Sand (%)93.4
Silt (%)3.9
Clay (%)2.7
Total dissolved salts (ppm)600Total dissolved salts (ppm)400
CaCO3 (%)5.6
Ca2+ (meq/100 g)4Ca2+ (meq/L)3.1
Mg2+ (meq/100 g)2Mg2+ (meq/L)2.5
Na+ (meq/100 g)7.2Na+ (meq/L)2.7
K+ (meq/100 g)1.3K+ (meq/L)0.2
Cl− (meq/100 g)3.2Cl− (meq/L)2.3
So42− (meq/100 g)9.3So42− (meq/L)4.1
CO3− (meq/100 g)-
HCO3− (meq/100 g)2HCO3− (meq/L)2.1
Table
Table 3. Plum water requirement (WR) based on three different evapotranspiration levels (ETc = 100%, 80%, or 60%), calculated from the reference evapotranspiration (ETo) and coefficient factor (Kc) during the May–June period (data are the average of the 2019 and 2020 seasons).
Table 3. Plum water requirement (WR) based on three different evapotranspiration levels (ETc = 100%, 80%, or 60%), calculated from the reference evapotranspiration (ETo) and coefficient factor (Kc) during the May–June period (data are the average of the 2019 and 2020 seasons).
100% ETc80% ETc60% ETc
MayJuneMayJuneMayJune
ETo6.056.234.844.983.633.73
Kc0.900.850.900.850.900.85
WR (mm/m2/day)4.363.973.573.172.612.38
WR (m3/ha/day)62.7858.8841.1237.5722.5521.12
WR (m3/ha/month)1883.41766.41233.61127.1676.5633.6
Table
Table 4. Effect of deficit irrigation (as a percentage of crop evapotranspiration, ETc) on the total yield of ‘African Rose’ plum trees during the 2019 and 2020 seasons (n = 10).
Table 4. Effect of deficit irrigation (as a percentage of crop evapotranspiration, ETc) on the total yield of ‘African Rose’ plum trees during the 2019 and 2020 seasons (n = 10).
TreatmentNumber of Fruits/Tree Fruit Weight/Tree (kg)
2019202020192020
100% ETc (control)327.8 a362.0 a22.5 a26.5 a
80% ETc305.3 b334.0 b20.0 b23.5 b
60% ETc236.6 c332.0 b17.5 c20.7 c
Means followed by the same letter within a column are not significantly different using DMRT at p ≤ 0.05.
Table
Table 5. Effect of deficit irrigation (as a percentage of crop evapotranspiration, ETc) on fruit size and firmness of ‘African Rose’ plum trees during the 2019 and 2020 seasons (n = 10).
Table 5. Effect of deficit irrigation (as a percentage of crop evapotranspiration, ETc) on fruit size and firmness of ‘African Rose’ plum trees during the 2019 and 2020 seasons (n = 10).
TreatmentFruit Diameter
(mm)		Fruit Firmness
(N/mm2)
2019202020192020
100% ETc (control)48.2 a50.5 a0.056 a0.056 a
80% ETc46.0 b48.7 b0.054 a0.042 b
60% ETc44.0 c46.7 c0.047 b0.041 b
Means followed by the same letter within a column are not significantly different using DMRT at p ≤ 0.05.
Table
Table 6. Effect of deficit irrigation (as a percentage of crop evapotranspiration, ETc) on total soluble solids (TSS) and total acidity (TA) of ‘African Rose’ plum fruit during the 2019 and 2020 seasons (n = 10).
Table 6. Effect of deficit irrigation (as a percentage of crop evapotranspiration, ETc) on total soluble solids (TSS) and total acidity (TA) of ‘African Rose’ plum fruit during the 2019 and 2020 seasons (n = 10).
TreatmentTSS (°Brix)TA (%)
2019202020192020
100% ETc (control)12.9 c12.9 c0.94 a0.89 a
80% ETc14.9 b14.9 b0.82 b0.80 b
60% ETc15.8 a15.9 a0.80 c0.78 c
Means followed by the same letter within a column are not significantly different using DMRT at p ≤ 0.05.
Table
Table 7. Effect of deficit irrigation (as a percentage of crop evapotranspiration, ETc) on total phenol and anthocyanin contents of ‘African Rose’ plum fruit during the 2019 and 2020 seasons (n = 10).
Table 7. Effect of deficit irrigation (as a percentage of crop evapotranspiration, ETc) on total phenol and anthocyanin contents of ‘African Rose’ plum fruit during the 2019 and 2020 seasons (n = 10).
TreatmentTotal Phenols (mg/100 g FW)Anthocyanins (mg/100 g FW)
2019202020192020
100% ETc (control)100.5 c105.9 c40.7 c44.5 c
80% ETc115.3 b120.9 b74.5 b75.2 b
60% ETc119.4 a129.9 a80.2 a85.0 a
Means followed by the same letter within a column are not significantly different using DMRT at p ≤ 0.05.
Table
Table 8. Total cost of deficit irrigation treatments and foliar sprays of Ethephon on “African Rose” plum trees during the 2019 and 2020 seasons.
Table 8. Total cost of deficit irrigation treatments and foliar sprays of Ethephon on “African Rose” plum trees during the 2019 and 2020 seasons.
TreatmentCost (LE/ha)
20192020
100% ETc (control)6513.86523.6
80% ETc4213.84220.1
60% ETc2338.32341.8
Ethephon; two sprays (2000 ppm each)5236.05356.4
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Hassan, I.F.; Gaballah, M.S.; El-Hoseiny, H.M.; El-Sharnouby, M.E.; Alam-Eldein, S.M. Deficit Irrigation to Enhance Fruit Quality of the ‘African Rose’ Plum under the Egyptian Semi-Arid Conditions. Agronomy 2021, 11, 1405. https://doi.org/10.3390/agronomy11071405

AMA Style

Hassan IF, Gaballah MS, El-Hoseiny HM, El-Sharnouby ME, Alam-Eldein SM. Deficit Irrigation to Enhance Fruit Quality of the ‘African Rose’ Plum under the Egyptian Semi-Arid Conditions. Agronomy. 2021; 11(7):1405. https://doi.org/10.3390/agronomy11071405

Chicago/Turabian Style

Hassan, Islam F., Maybelle S. Gaballah, Hanan M. El-Hoseiny, Mohamed E. El-Sharnouby, and Shamel M. Alam-Eldein. 2021. "Deficit Irrigation to Enhance Fruit Quality of the ‘African Rose’ Plum under the Egyptian Semi-Arid Conditions" Agronomy 11, no. 7: 1405. https://doi.org/10.3390/agronomy11071405

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