Pomological, Nutritional and Phytochemical Properties of Some Plum (Prunus domestica L.) Cultivars and Local Selections Grown in a Collection Orchard Located in South-Western Romania

Abstract

Geometrical and physical properties, dry matter, soluble solids content, titratable acidity, total phenolic content and DPPH radical scavenging activity were investigated in the fresh fruits of six plum (Prunus domestica L.) cultivars (‘Centenar’, ‘Minerva’, ‘Carpatin’, ‘Dobrowica’, ‘Čačanska Lepotica’ and ‘Mirabelle de Nancy’) and two local selections (‘Păscoaia’ and ‘Gogoșele Otăsău’) grown in an experimental plum orchard collection established in 2016 in Orodel, Dolj county (south-western Romania). In addition, phenolic compounds, organic acids and vitamin C contents were determined in fruit flesh and peel by high-performance liquid chromatography. Analysis of the phenolic compounds indicated that chlorogenic acid and catechin hydrate were the predominant phenolic acid and flavonoid, respectively, in the flesh of most of the investigated cultivars. Higher contents of phenolic compounds were found in the peel, where the phenolic profile was dominated by chlorogenic and vanillic acids among the phenolic acids and by rutin among the flavonoids. The highest total phenolic content was measured in the peel of the ‘Centenar’ cultivar (575.64 mg GAE/100 g fw) followed by ‘Čačanska Lepotica’ (536.55 mg GAE/100 g fw), while the flesh of the ‘Mirabelle de Nancy’ (218.36 mg GAE/100 g) and ‘Gogoșele Otăsău’ (152.02 mg GAE/100 g) cultivars were the richest in phenolic compounds and antioxidant activity. In terms of functional characteristics, among the two local selections, ‘Gogoșele Otăsău’ could be considered a valuable plum selection, while Păscoaia’ is more suitable for fresh consumption.

1. Introduction

With a global production of over 12.6 million tons annually in 2019, plums are one of the most popular fruits, with great commercial interest due to the immense variety, wide distribution and high adaptability [1,2]. They represent a large and taxonomically diverse group of stone fruits originating from different climatic and geographical regions and belonging to the subgenus Prunophora of the genus Prunus, in the Pronoideae subfamily of the Rosaceae [3]. The main plum species, both wild and cultivated, are Prunus domestica L., Prunus cerasifera Ehrh., Prunus spinosa L., Prunus institia L. and Prunus salicina Lindley, but the most globally cultivated plum species are the European plum (P. domestica L.) and the Japanese plum (P. salicina Lindl.), on which the plum processing industry is currently based [3,4,5]. Prunus domestica L. varieties are adapted to a wide range of climatic and edaphic factors, being extensively cultivated in Asia, Europe, and North America. Ranking fourth after apple, peach and pear, the European plum varieties are widely cultivated throughout Europe in the temperate fruit-growing zones, Serbia, Romania, Germany, France and Bulgaria being the countries with the largest plum cultivated areas [5,6]. According to FAOSTAT [7], Romania ranks third after China and Serbia as the largest producer of plums. In recent decades, in Romania, the plum varietal assortment has been developed and improved by breeding new autochthonous varieties or by introducing valuable foreign cultivars with higher adaptability and disease tolerance, higher yield and superior nutritional value [8,9]. ‘Centenar’, ‘Silvia’, ‘Minerva’, ‘Carpatin’, ‘Andreea’ and ‘Record’ are some of the new plum cultivars bred and named in Romania.

Plums could constitute a valuable component of our diet as they represent an excellent source of nutrients, such as vitamins, minerals, and carbohydrates (i.e., glucose, fructose, sucrose and sorbitol) [10,11,12]. In addition, fresh fruits contain high levels of fibers (pectin), organic acids (malic and citric acids), tannins, enzymes, and carotenoids, but low levels of proteins (0.7%) and lipids (0.28%) [13]. The minerals in plums, including potassium, phosphorus, calcium, magnesium, iron and zinc, possess alkalizing and mineralizing properties for the human body [14]. The organic acids create complexes with heavy metal ions, thus inhibiting their oxidation–catalyzing action [1]. The high soluble fiber content of plums, such as pectin, as well their sorbitol content, contribute to the well-known laxative and diuretic properties of plums, by drawing water into the colon and maintaining regular bowel movements. Dietary pectin has been linked also to a cholesterol-lowering effect by binding cholesterol molecules and facilitating their excretion from the body [15].

In recent times, the high level of phenolic compounds, including phenolic acids, flavonoids, anthocyanins, and other phenolics in plums, which contribute to their strong antioxidant capacity, have resulted in the increase of consumers’ interest in plums [14,16]. These antioxidant compounds help to scavenge free radicals in the human body, which can contribute to preventing the occurrence of cancer and of other chronic diseases [12,17]. Other health-promoting properties, including anti-inflammatory activity, improving bone health, insulin sensitivity, glucose metabolism, cognition and memory, have been attributed to plums as a result of their high phenolic content and antioxidant capacity, or have been reported from in vitro, animal and clinical studies investigating both plums and related products and extracts [13,16,17,18,19]. Due to these properties, plums are increasingly considered a “health-promoting fruit” or “functional food” [20].

The chemical composition of fresh plums depends on cultivar characteristics, environmental conditions and soil management [13,21]. The sugar content (16–20%), the sugar/acid ratio and the organic acid profile determines to a great extent the plum taste. The phenolic compounds contribute to fruit astringency while anthocyanins are responsible for peel color [1]. The major phenolic compounds found in plums include chlorogenic, neochlorogenic and ferulic acids, quercetin, rutin, proanthocyanidin B1 and kaempferol [13]. Cyanidin 3-xyloside, cyanidin 3-glucoside, cyanidin 3-rutinoside, peonidin 3-glucoside and peonidin 3-rutinoside are the main anthocyanins [22,23], while the major carotenoids detected in plums were lutein and β-carotene in the peel and flesh, respectively, along with zeaxanthin, β-cryptoxanthin and α- and β-carotene [24]. Plums also contain vitamins A, E, K, C, and various B vitamins, including thiamine (B1), riboflavin (B2), niacin (B3), pantothenic acid (B5), pyridoxine (B6), and folate [13,14,25]. The contents of the bioactive compounds in plums vary widely, depending on species, cultivar, rootstock, ripening stage and pre- and post-harvest factors [22,23,26].

Fruits of Prunus domestica L. have great economic importance as, besides being consumed fresh, they provide many processing opportunities. Plums are used in the food industry to produce prunes, compotes, nectars, jams, preserves and alcoholic beverages. The dry matter and sugar content, the color and the maturity at harvest are important parameters of the plums for processing [21,27]. Several studies have been made on the physico-chemical, nutritional and antioxidant properties of various plum cultivars in different ecological conditions [2,4,10,11,22,26]. Currently, one of the priority research directions in plum growing is focused on improving fruit quality and developing new varieties with superior nutritional and functional characteristics under local conditions [27,28]. In this process, autochthonous cultivars and local selections need to be carefully evaluated as they represent a valuable source of germplasm for breeding activities [29,30].

This study seeks to evaluate local Romanian plum selections against established cultivars in terms of nutraceuticals and antioxidants with the hypothesis that local varieties may be more advantageous in regard to nutritional and phytochemical content. Total phenolic content and DPPH radical scavenging activity were spectrophotometrically measured in the fruit flesh and peel of the plum cultivars, while the phenolic and organic acid profiles were assessed using the high-performance liquid chromatography (HPLC)–photodiode array (PDA) analysis.

2. Materials and Methods

2.1. Plant Material

Fruits from six plum (Prunus domestica L.) cultivars (‘Centenar’, ‘Minerva’, ‘Carpatin’, ‘Dobrowica’, ‘Čačanska Lepotica’ and ‘Mirabelle de Nancy’) and two local selections (‘Păscoaia’ and ‘Gogoșele Otăsău’) were harvested at the end of July 2024, in the stage of commercial maturity, from an experimental a plum collection orchard established in 2016 in Orodel commune, Cornu village (44°13′ N 23°16′ E), Dolj county (south-western Romania). The area belongs to the temperate climate area, with Mediterranean influences, an annual average temperature of 11 °C (14–21 °C in the summer and −1 °C–−6 °C in the winter) and rainfall of around 600 mm. The orchard was established in 2016, on a soil of reddish preluvosol type, as a result of the activities carried out within the project PN-II-PT-PCCA-2013-2014 (project no. 168/2014, GERMPLUM). Standard growing techniques have been applied since orchard establishment, with winter pruning performed yearly and without fruit thinning. The trees are on their own roots and the planting density was 5 m × 4 m. The fruits were collected from five trees of each cultivar from different sides of the trees and transported to the laboratory for analysis. The ripening period of all evaluated plum cultivars starts at the end of July and continues to the middle of August. About 20 randomly selected fruits from each tree were harvested for analyses. Figure S1, available in the Supplementary Materials section, presents fruit images of the evaluated plum cultivars. The fruits were properly washed with distilled water, drained and their physico-chemical properties were measured. The remaining fruits were stored at −20 °C until analytical analysis. All chemical analyses were performed in three replications.

2.2. Chemicals

Gallic acid, Folin–Ciocalteu’s reagent, 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox), 2,2-diphenyl−1-picrylhydrazyl (DPPH), potassium dihydrogen phosphate and phosphoric acid were purchased from Sigma-Aldrich (Steinheim, Germany), while anhydrous sodium carbonate, sodium hydroxide and malic, citric, tartaric, ascorbic and oxalic acids were purchased from Merck (Darmstadt, Germany). For the chromatographic analysis of the phenolic compounds, standards of the phenolic acids (vanillic, caffeic, chlorogenic, trans-cinnamic, p-coumaric, ferulic, gallic and syringic) and flavonoids (quercetin, rutin, catechin hydrate and epicatechin) were purchased from Sigma (Sigma-Aldrich GmbH, Steinheim, Germany) while methanol (HPLC grade) and acetic acid (analytical grade) were purchased from Merck (Darmstadt, Germany).

2.3. Geometrical and Physical Properties

Fruit length (L), width (W) and thickness (T) of twenty randomly selected fruits were measured by a digital caliper as described by Ertekin et al. [3] and expressed in mm. Based on the results, the geometric mean diameter (D_g) was calculated as follows [3]:

$$D_g={\left(LWT\right)}^{1/3}$$

The sphericity index (S_p) was then calculated as follows:

$$S_p={\displaystyle \frac{D_g}{L}}\times 100$$

The aspect ratio (R_a) and the surface area (S, cm²) of the fruits were calculated by using the following formulas [31]:

$$R_a=W/L$$

$$S=\pi \left(D_g^2\right)$$

Fruit weight and stone weight were measured on twenty randomly selected fruits using an electronic balance with 0.1 mg sensitivity. Based on the results, the pulp ratio (%) was calculated. The fruit volume (V), expressed in cm³, was determined using the liquid displacement method and used to calculate fruit density (g/cm³).

2.4. Dry Matter Content, Soluble Solids Content and Titratable Acidity

The dry matter content (%) was determined by measuring the weight loss of 5 g fresh fruit after drying at 103 °C until reaching a constant weight. The total soluble solids content (SSC, %) was measured in freshly prepared juice by a Hanna digital refractometer (Hanna Instruments, Woonsocket, RI, USA). The results were the mean values ± SD obtained after triplicate analysis. The titratable acidity (TA) was determined in 10 g of homogenate from three fruits, made up to 100 g with distilled water and titrated to pH 8.2 with 0.1 N NaOH solution. The results were expressed as grams of malic acid (MA) per 100 g fresh weight. Two independent extracts were prepared, and each one was titrated in duplicate. The soluble solids content to titratable acid ratio (SSC/TA) was calculated based on the results.

2.5. Extraction Procedure

After removing the stone and separating the peel from the flesh, the peel and flesh samples were shredded and crushed in a mortar with a pestle. One gram of flesh or peel from each fruit samples was mixed with 10 mL methanol in triplicate centrifuge tubes and sonicated for 60 min in an ultrasonic water bath (Bandelin Electronic GmbH, Berlin, Germany). After centrifugation at 2500× g for 5 min, the supernatants were collected, filtered and used in the total phenolic content, total flavonoid content and DPPH radical scavenging activity assays, as well as in the chromatographic analysis of individual phenolic compounds. Extraction was replicated three times for each sample.

2.6. Total Phenolic Content

The total phenolic content was measured in the extracts by the spectrophotometric Folin–Ciocalteu method according to Singleton et al. [32]. Aliquots of appropriately diluted extract (0.1 mL) dispersed in 6 mL of distilled water were combined with 0.5 mL of Folin–Ciocalteu reagent (1:1 with water). After 3 min, 1.5 mL of 20% (w/v) Na₂CO₃ solution was added, and distilled water was added to reach a total volume of 10 mL. After shaking and incubation for 30 min in the dark at 40 °C, the absorbance of the mixture was read at 765 nm on a Varian Cary 50 UV spectrophotometer (Varian Co., Cary, NC, USA). A calibration curve was constructed using gallic acid standard solutions (0–100 mg/L) and the results were expressed as milligrams of gallic acid equivalents (GAE) per 100 g fresh weight (FW).

2.7. DPPH Radical Scavenging Activity

The antioxidant activity of the fruit flesh and peel were tested as the ability to scavenge the DPPH (2,2-diphenyl-1-picrylhydrazyl) free radical following the method described by Brand-Williams et al. [33], with minor modifications. The assay was conducted by reacting 50 μL of adequately diluted flesh or peel extract with 3 mL of 0.004% DPPH solution in methanol. The mixture was homogenized and placed in the dark for 30 min at room temperature, then the absorbance was read at 517 nm against methanol using a Varian Cary 50 UV spectrophotometer (Varian Co., Cary, NC, USA). A control sample, containing 50 μL of methanol in the place of the extract, was used to measure the maximum DPPH absorbance. The DPPH radical scavenging activity was calculated as percentage of inhibition by using the following formula:

DPPH radical inhibition (%) = [1 − A_sample/A_control] × 100

where A_sample is the absorbance of the test sample and A_control is the absorbance of the control. Trolox was used as a standard and the results were expressed as millimoles Trolox per 100 g of fresh weight (FW). The analysis was performed in three independent assays for each sample.

2.8. Chromatographic Analysis of Phenolic Compounds

Individual phenolic compounds were quantified in the methanolic fruit extracts by a RP-HPLC method developed by Nour et al. [34] on a Finningan Surveyor Plus HPLC system (Thermo Electron Corporation, San Jose, CA, USA) equipped with a PDA5P diode array detector (DAD). The separation was carried out on a Hypersil Gold C18 column (5 μm, 250 × 4.6 mm) at 20 °C with a mixture of 1% aqueous acetic acid (eluent A) and methanol (eluent B) as the mobile phase, followed by simultaneous detection at 254, 278 and 300 nm. The following elution conditions were set at a flow rate of 1 mL/min: 0 to 20 min, linear gradient from 90% A to 80% A, 20–27 min, linear gradient from 80% A to 60% A, 27–52 min, 60% A, 52–57 min, linear gradient from 60% A to 80% A and 57–60 min, linear gradient from 80% A to 90% A. A volume of 5 μL methanolic fruit extract was injected after filtration through a nylon syringe filter (0.45 μm). The concentration of phenolic compounds was expressed as mg per 100 g of fresh weight (FW).

2.9. Chromatographic Analysis of Organic Acids

For the extraction of organic acids, fruit flesh and peel homogenates were vortexed for 2 min with 15 mL of distilled water, then centrifuged at 6000 rpm for 10 min. The extraction was performed in triplicate for each sample. The supernatants were filtered (0.45 μm) before injection. Individual organic acids were quantified in the extracts by the HPLC method developed by Nour et al. [35] on a Finningan Surveyor Plus HPLC system (Thermo Electron Corporation, San Jose, CA, USA) equipped with a PDA5P diode array detector (DAD). The separation was carried out under isocratic conditions on a Hypersil Gold aQ column (5 μm, 250 × 4.6 mm) at 10 °C using a 50 mM KH₂PO₄ aqueous solution adjusted to pH 2.8 with ortho-phosphoric acid as the mobile phase. The detector was set at λ = 254 nm for ascorbic acid and λ = 214 nm for the other organic acids. The injection volume was 5 μL and the flow rate of the mobile phase was 0.7 mL/min.

2.10. Statistical Analysis

The calculation of mean values, standard deviation (SD) and Pearson’s correlation coefficient (r) was conducted using Statgraphics Centurion software (version XVI.I) from StatPoint Technologies, Inc. (The Plains, VA, USA). The statistically significant difference between means was evaluated using the one-way analysis of variance (ANOVA) followed by the Duncan’s multiple range test, and p values less than 0.05 were taken as significant.

3. Results and Discussion

3.1. Geometrical Properties

The results of the geometrical properties of fresh plum fruits are presented in

Table 1. Geometrical properties of the plum cultivars.

	‘Centenar’	‘Minerva’	‘Carpatin’	‘Dobrowica’	‘Čačanska Lepotica’	‘Mirabelle de Nancy’	‘Păscoaia’	‘Gogoșele Otăsău’
Length (mm)	38.68 ± 1.62 bc	37.70 ± 1.33 c	38.40 ± 1.39 bc	40.30 ± 1.92 a	39.07 ± 0.86 ab	32.43 ± 1.62 d	37.53 ± 0.99 c	27.19 ± 1.03 e
Width (mm)	32.46 ± 1.55 d	33.96 ± 1.35 c	33.36 ± 1.27 cd	36.03 ± 1.53 b	34.36 ± 1.42 c	32.16 ± 1.77 d	37.44 ± 1.29 a	24.30 ± 0.52 e
Thickness (mm)	30.65 ± 1.54 d	30.57 ± 1.32 d	30.86 ± 1.06 d	33.48 ± 1.95 b	33.06 ± 1.34 bc	31.82 ± 1.48 cd	35.49 ± 1.59 a	22.53 ± 0.52 e
Geometric mean diameter (Dg, mm)	33.76 ± 1.47 c	33.95 ± 1.26 c	34.06 ± 1.14 c	36.49 ± 1.62 ab	35.40 ± 1.02 b	32.13 ± 1.51 d	36.80 ± 1.20 a	24.60 ± 0.58 e
Sphericity index (Sp, %)	87.28 ± 1.31 d	90.06 ± 1.35 bc	88.72 ± 1.06 cd	90.59 ± 2.77 b	90.62 ± 2.53 b	99.10 ± 1.22 a	98.06 ± 1.89 a	90.52 ± 1.90 b
Aspect ratio (Ra)	0.84 ± 0.02 d	0.90 ± 0.02 b	0.87 ± 0.02 c	0.89 ± 0.04 b	0.88 ± 0.04 bc	0.99 ± 0.01 a	1.00 ± 0.02 a	0.89 ± 0.03 b
Surface area (S, mm2)	35.84 ± 3.10 c	36.24 ± 2.72 c	36.47 ± 2.42 c	41.88 ± 3.79 a	39.37 ± 2.27 b	32.48 ± 3.05 d	42.57 ± 2.77 a	19.01 ± 0.90 e

Different lowercase letters in a row indicate significant differences between cultivars (p < 0.05).

. Significant differences were found between cultivars in terms of sizes and geometrical properties. The results show that the fruits of ‘Dobrowica’ (40.30 mm) and ‘Čačanska Lepotica’ (39.07 mm) had the highest length, followed by ‘Centenar’ (38.68 mm) and ‘Carpatin’ (38.40 mm), while ‘Gogoșele Otăsău’ had the lowest length (27.19 mm). The fruit width varied from 37.44 mm for ‘Păscoaia’ to 24.30 mm for ‘Gogoșele Otăsău’. Ozzengin et al. [4] reported mean width values of 26.26 mm and 28.37 mm for the fruits of ‘Karaca’ and ‘Üryani’ plum cultivars. Manco et al. [28] reported width and length values in the ranges 28.1–54.3 mm and 29.3–60.3 mm, respectively, for the fruits of twenty-nine Prunus domestica varieties from Southern Italy, while Sümbül et al. [27] found between 20.37 mm and 33.11 mm for the width and between 21.42 and 34.44 mm for the length of the fruits of twenty-two plum genotypes growing naturally in the Sivas province in the Central Anatolia region of Türkiye.

The fruit thickness exceeded 30 mm for all cultivars, except the ‘Gogoșele Otăsău’ cultivar (mean thickness = 22.53 mm). The largest fruit thickness was found for ‘Păscoaia’ (35.49 mm) followed by ‘Dobrowica’ (33.48 mm) and ‘Čačanska Lepotica’ (33.06 mm). Ionica et al. [11] reported mean thickness values between 22.84 mm and 51.36 mm for twelve plum cultivars grown in Valcea County, south-western Romania.

The average values of the geometric mean diameter were found to be between 36.80 mm (‘Păscoaia’) and 24.60 mm (‘Gogoșele Otăsău’). However, except ‘Gogoșele Otăsău’, all the other cultivars had the geometric mean diameter higher than 32 mm. Ertekin et al. [3] reported a geometric mean diameter of 36.48 mm for the fruits of the ‘Stanley’ cultivar.

Sphericity is an indicator of the similarity of the fruit’s shape to a sphere of the same volume [3]. The highest sphericity index was found for the fruits of the ‘Mirabelle de Nancy’ (99.10%) and ‘Păscoaia’ (98.06%) cultivars, indicating their tendency toward being spherical in shape. The lowest sphericity index was found for ‘Centenar’ fruits (87.28%), indicating a greater tendency towards an oblong shape. Lower sphericity indexes of 76.3% and 85.8% have been reported previously by Ertekin et al. [3] for the Stanley and Frenze 90 cultivars, respectively. The aspect ratio, given by the ratio of the width to the length, is also an indicative of the fruit tendency toward the spherical vs. oblong shape. The highest mean value was found for the fruits of ‘Păscoaia’ and ‘Mirabelle de Nancy’ cultivars, while the lowest was found for ‘Centenar’.

3.2. Physical Properties

Regarding fruit weight, the cultivars differ significantly. ‘Păscoaia’ was the cultivar with the highest weight (30.09 g), followed by ‘Dobrowica’ (29.48 g), with no significant differences between them. The fruits of the ‘Gogoșele Otăsău’ cultivar had the lowest fruit weight (9.01 g). The same trend was found for fruit volume (

Table 2. Physical properties of the plum cultivars.

	‘Centenar’	‘Minerva’	‘Carpatin’	‘Dobrowica’	‘Čačanska Lepotica’	‘Mirabelle de Nancy’	‘Păscoaia’	‘Gogoșele Otăsău’
Weight (g)	21.90 ± 2.48 cd	23.71 ± 2.63 bc	22.55 ± 2.13 cd	29.48 ± 4.11 a	24.99 ± 1.87 b	21.08 ± 2.94 d	30.09 ± 2.96 a	9.01 ± 0.60 e
Volume (cm3)	21.20 ± 2.40 cd	22.81 ± 2.52 bc	21.59 ± 2.03 cd	27.72 ± 3.85 a	24.58 ± 1.84 b	19.80 ± 2.73 d	29.49 ± 2.96 a	8.22 ± 0.56 e
Stone weight (g)	1.13 ± 0.12 c	0.84 ± 0.07 e	0.96 ± 0.05 d	0.75 ± 0.10 e	1.40 ± 0.14 b	1.66 ± 0.19 a	0.54 ± 0.09 f	0.81 ± 0.06 e
Pulp ratio (%)	94.78 ± 0.98 e	96.43 ± 0.52 c	95.70 ± 0.42 d	97.42 ± 0.51 b	94.34 ± 0.77 e	92.01 ± 1.14 f	98.19 ± 0.48 a	90.98 ± 0.72 g
Density (g/cm3)	1.03 ± 0.00 e	1.04 ± 0.00 d	1.04 ± 0.00 c	1.06 ± 0.00 b	1.02 ± 0.00 f	1.06 ± 0.01 b	1.02 ± 0.01 f	1.10 ± 0.01 a

Different lowercase letters in a row indicate significant differences between cultivars (p < 0.05).

). Usenik et al. [22] reported the highest fruit weight in the ‘Valor’ (28.5 g) and ‘Jojo’ (33.2 g) cultivars and significantly lower fruit weight in the ‘Čačanska najbolja’ (25.7 g) and ‘Čačanska rodna’ (19.7 g) cultivars. Manco et al. [28] found fruit weights between 17 g and 100.2 g after analyzing twenty-nine P. domestica varieties grown in Southern Italy, while Milatović et al. [36] reported fruit weights between 22.9 g and 67.7 g in a study on twenty-six plum cultivars grown in the Belgrade area during an eight-year period (2012–2019).

The stone weight is of interest, especially in fruit processing, as it influences the pulp ratio. In our study, the highest stone weight was found in the ‘Mirabelle de Nancy’ (1.66 g) cultivar, followed by ‘Čačanska Lepotica’ (1.40 g) and ‘Centenar’ (1.13 g), and the lowest was found in ‘Păscoaia’ (0.54 g). Dimkova et al. [21] found the highest stone mass in ‘Stanley’ (1.86 g) followed by ‘Jojo’ (1.69 g), while the lowest was found in the fruits of the ‘Čačanska rodna’ cultivar (0.86 g). For the fruits of the ‘Stanley’ cultivar, Ertekin et al. [3] reported a fruit and stone mass of 27.97 g and 2.64 g, respectively, while Sümbül et al. [27] reported fruit and stone weights in the ranges 6.63–25.92 g and 0.37–1.05 g in twenty-two plum genotypes growing naturally in the Sivas province in the Central Anatolia region of Türkiye. The highest pulp ratio was found for the fruits of the ‘Păscoaia’ cultivar (98.19%), followed by the ‘Dobrowica’ (97.42%) and ‘Minerva’ (96.43%) cultivars, and the lowest for ‘Gogoșele Otăsău’ (90.98%). Dimkova et al. [21] reported the lowest pulp ratio in the ‘Stanley’ (93.68%) and ‘Hanita’ (93.15%) cultivars and the highest in the ‘Malvazinka’ (97.67%) cultivar. The bulk densities were found to be between 1.02 g/cm³ and 1.10 g/cm³, in agreement with the fruit density previously reported by Ertekin et al. [3] for the Stanley (1.050 g/cm³) and Frenze 90 (1.029 g/cm³) plum cultivars.

3.3. Dry Matter, Soluble Solids Content and Titratable Acidity

As presented in

Table 3. Dry matter, soluble solids content and titratable acidity of the plum cultivars.

	‘Centenar’	‘Minerva’	‘Carpatin’	‘Dobrowica’	‘Čačanska Lepotica’	‘Mirabelle de Nancy’	‘Păscoaia’	‘Gogoșele Otăsău’
Dry matter (%)	23.35 ± 0.39 b	22.32 ± 0.13 b	24.73 ± 0.13 a	17.43 ± 0.46 d	25.71 ± 0.66 a	20.42 ± 0.18 c	18.16 ± 0.24 d	22.37 ± 0.66 b
Soluble solids content (%)	19.49 ± 0.82 b	18.17 ± 0.94 bcd	21.16 ± 0.47 a	15.88 ± 0.78 e	18.82 ± 0.39 bc	17.14 ± 0.48 d	17.48 ± 0.68 cd	18.12 ± 0.52 bcd
Titratable acidity (g MA/100 g)	0.70 ± 0.05 g	0.64 ± 0.03 g	0.94 ± 0.05 e	1.21 ± 0.04 d	0.82 ± 0.06 f	2.19 ± 0.08 b	1.37 ± 0.03 c	2.34 ± 0.03 a
SSC/TA	27.87 ± 0.56 a	28.53 ± 1.11 a	22.45 ± 0.64 b	12.78 ± 0.67 c	23.09 ± 1.07 b	7.84 ± 0.12 d	12.78 ± 0.67 c	7.74 ± 0.28 d

Different lowercase letters in a row indicate significant differences between cultivars (p < 0.05); SSC/TA—soluble solids content to titratable acid ratio.

, the dry matter of the tested cultivars was between 17.43% and 25.71%. The highest dry matter content was found in the ‘Čačanska lepotica’ (25.71%) and ‘Carpatin’ (24.73%) cultivars and the lowest in the ‘Păscoaia’ (18.16%) and ‘Dobrowica’ (17.43%). A high dry matter content is determinant for obtaining a higher yield, both in plum drying or in other types of processing. From the present results, it can be assumed that ‘Păscoaia’ will not be suitable for the production of dried plums.

The soluble solids content was found to be between 15.88% (‘Dobrowica’) and 21.16% (‘Carpatin’). Lin et al. [37] found SSC values from 11.67 to 20.23 °Brix, with a mean of 15.53 °Brix in the fruits of five plum cultivars from Sichuan. Higher soluble solids contents, between 18.9% (‘Čačanska Najbolja’) and 24.3% (‘Tegera’), have been reported by Dimkova et al. [21] in a study on eight plum cultivars grown in Dryanovo (Gabrovo region, Bulgaria). Lower soluble solids contents have been reported by Ertekin et al. [3] in ‘Frenze 90′ (12.80%) and ‘Stanley’ (14.80%) plums grown in Antalya (Korkuteli), Türkiye and by Usenik et al. [22] (between 13.5 and 15.6%) in four plum cultivars (‘Jojo’, ‘Valor’, ‘Čačanska rodna’ and ‘Čačanska Najbolja’) grown in Slovenia. Manco et al. [28] reported soluble solids contents between 12.7% and 28.9% in twenty-nine P. domestica varieties grown in Southern Italy.

The lowest titratable acidity in plum fruits was found for ‘Minerva’, followed by ‘Centenar’ (0.64% and 0.70% in malic acid, respectively) while the highest was found for ‘Mirabelle de Nancy’ (2.19%) and ‘Gogoșele Otăsău’ (2.34%). Previously, Dimkova et al. [21] reported organic acids content between 0.81% (‘Stanley’) and 1.63% (the ‘Hanita’ cultivar), while Ertekin et al. [3] found titratable acidity from 0.86% to 0.94% for the ‘Frenze 90′ and from 0.37% to 0.40% for ‘Stanley’ cultivars. Lower titratable acidity values, ranging between 0.34% and 0.83%, have been reported by Ionica et al. [11] in twelve plum cultivars grown in Vâlcea County, Romania. Ozzengin et al. [4] reported titratable acidity of 0.46, 0.55 and 1.41 g malic acid/L in the fruits of the ‘Üryani’, ‘Karaca’ and ‘Blackthorn’ plum cultivars, respectively. In Japanese plums, Lin et al. [37] reported TA values ranging from 0.82% to 1.38%, with a mean value of 1.0%. Many studies agreed that both soluble solid content and titratable acidity are strongly influenced by environmental conditions, maturity degree and genotype [3,4].

Soluble solid content plays a significant role in consumer acceptance of most plums, while ripe titratable acidity becomes determinant for consumer decision. In addition, total soluble solids/titratable acidity ratio is an indicator for the determination of the fruit ripening degree [4]. Crisosto [38] showed that plums with a soluble solids concentration between 10 and 11.9% combined with low ripe titratable acidity (≤0.60%) were disliked by 18% of consumers, while plums with soluble solids content ≥ 12.0% had around 75% consumer acceptance, regardless of ripe titratable acidity. However, the decrease in titratable acidity and increase in the SSC/TA ratio during ripening may increase the acceptability of plums. Ripening protocols have been developed and promoted in order to increase the acceptability of plums by decreasing TA and increasing the SSC/TA ratio [38]. Values between 7.74 and 28.53 were found for the SSC/TA ratio in the present study. Dimkova et al. [21] reported between 6.23 and 17.01 for the same ratio, while Usenik et al. [22] reported values between 6.4 and 48.7 and noticed that higher values of this ratio mean sweeter and less acidic fruits. Tomić et al. [30] emphasized the important role of sugars in the formation of fruit flavor, while Colarič et al. [39] showed that SSC/TA ratio and the levels of organic acids have significant impact on the perception of sweetness. Sweet fruits generally contain low levels of organic acids rather than having a high sugar content [40].

3.4. Total Phenolic Content and DPPH Radical Scavenging Activity

The results on total phenolic content presented a high variability, both in the flesh and the peel (Figure 1). In the flesh, the highest total phenolic content was found in ‘Mirabelle de Nancy’ (218.36 mg GAE/100 g), ‘Gogoșele Otăsău’ (152.02 mg GAE/100 g) and ‘Čačanska Lepotica’ (125.64.64 mg GAE/100 g) and the lowest was found in ‘Dobrowica’ (68.36 mg GAE/100 g) and ‘Păscoaia’ (70.18 mg GAE/100 g). In the peel, the highest levels of phenolics were found in ‘Centenar’ (575.64 mg GAE/100 g), ‘Čačanska Lepotica’ (536.55 mg GAE/100 g) and ‘Minerva’ (466.55 mg GAE/100 g), while the lowest was found in ‘Păscoaia’ (166.55 mg GAE/100 g). It turns out that in terms of total phenolic content, the most valuable local selection seems to be ‘Gogoșele Otăsău’. Khallouki et al. [41] detected 45.6 mg/100 g and 70.1 mg/100 g total phenolics in the peels and flesh of the Prunus domestica fruits, while Navarro et al. [42] reported TPC results ranging from 109 to 179 mg GAE/100 g FW. Other determinations of total phenolic content in previous studies revealed a large variability, with values ranging between 18.4 and 495 mg GAE/100 g fresh weight (FW) [43,44,45]. Ayub et al. [14] reported between 125.0 and 373.6 mg GAE/100 g in ten plum cultivars, while Rop et al. [10] found total phenolic content ranging from 227 to 495 mg of gallic acid/100 g of fresh mass in some plum cultivars grown in orchards from the south-western part of the White Carpathians. In good agreement with the present results, Ozzengin et al. [4] reported TPC values between 83.33 and 440.67 mg GAE/100 g in some wild plum cultivars, while Lin et al. [37] noted highly significant differences in TPC among five Japanese plum cultivars (p < 0.05), with values varying from 108.54 to 271.74 mg of GAE/100 g. In a systematic review on the health effects of plums, Igwe and Charlton [16] noticed the dramatically increased plum-based research since the 1990s and attributed this interest to the high levels of phenolic compounds observed in plums, including flavonoids, and particularly the subclass of anthocyanins. Some of the health promoting properties of plums, including antioxidant, antimicrobial, anti-inflammatory, antidiabetic and antimutagenic activities, have been attributed primordially to their high phenolic content and antioxidant capacity [13,16,45,46,47,48,49].

In agreement with previous studies [4,30,42,50,51], significant and strong positive correlations (p < 0.05) were found for both flesh and peel samples between TPC values and DPPH radical scavenging activity, with correlation coefficients of 0.9868 and 0.9942 for flesh and peel, respectively. Moreover, all previous studies agreed on the superiority of the peel as compared with the flesh in terms of total phenolic content and antioxidant activity [10]. Usenik et al. [26] reported that TPC in P. domestica peel was at least ten times higher than the TPC of the flesh. They reported that, although the fruit peel represented on average 9.8% of the edible part of the fruit, it contained 53.6% to 63.9% of the TPC of the edible part. Celik et al. [1] attributed the more intense occurrence of phenolic compounds in the peel to the light, based on light requirement in the synthesis of flavonol glycosides in plants. Numerous studies have shown that synthesis of phenolic compounds is affected by complex interactions of environmental and developmental factors, such as light, temperature, level of sugar and plant hormones, and they accumulate first in the skin then in flesh [51,52]. In our study the peel/flesh total phenolic (TP) ratio varied from 1.44 to 3.98 times, in agreement with the findings of Drogoudi and Pandelidis [53], who reported peel/flesh ratio between 2.2 and 14.6 times in 43 local and foreign European and Japanese plum cultivars.

In the flesh, the highest DPPH antioxidant activity values were found in the fruits of the ‘Mirabelle de Nancy’ (1.19 mmol Trolox/100 g), ‘Gogoșele Otăsău’ (0.81 mmol Trolox/100 g) and ‘Minerva’ (0.75 mmol Trolox/100 g) cultivars, while, in the peel, the ‘Centenar’ (2.45 mmol Trolox/100 g) and ‘Čačanska Lepotica’ (2.34 mmol Trolox/100 g) cultivars stood out. Generally, the local selection ‘Gogoșele Otăsău’ showed good results in total phenolic content and antioxidant activity both in the flesh and the peel and could be of interest in new commercial orchards in Romania and wider or for potential breeding purposes.

3.5. Phenolic Compounds

Of the individual phenolic acids tested, chlorogenic acid was dominant in the flesh, followed by the ferulic and vanillic acids, while among flavonoids, catechin hydrate predominated, followed by epicatechin (

Table 4. Content of phenolic compounds (mg/100 g) in the fruit flesh of the plum cultivars.

	‘Centenar’	‘Minerva’	‘Carpatin’	‘Dobrowica’	‘Čačanska Lepotica’	‘Mirabelle de Nancy’	‘Păscoaia’	‘Gogoșele Otăsău’
Vanillic acid	1.79 ± 0.07 b	0.54 ± 0.03 e	1.04 ± 0.05 d	0.10 ± 0.01 fg	1.42 ± 0.06 c	7.61 ± 0.24 a	0.22 ± 0.02 f	nd
Rutin	nd	nd	nd	0.23±0.01 b	nd	nd	nd	0.51±0.02 a
Quercetin	nd	nd	nd	nd	nd	nd	nd	0.07 ± 0.01
Gallic acid	0.01 ± 0.01 bc	0.01 ± 0.01 c	0.03 ± 0.01 b	nd	0.03 ± 0.01 b	0.41 ± 0.02 a	0.01 ± 0.01 c	0.01 ± 0.01 bc
Catechin hydrate	2.47 ± 0.09 e	2.01 ± 0.08 f	1.97 ± 0.08 f	7.54 ± 0.23 c	1.76 ± 0.07 f	13.94 ± 0.34 a	4.16 ± 0.14 d	10.73 ± 0.28 b
Syringic acid	nd	0.05 ± 0.01 b	nd	nd	nd	nd	nd	0.07 ± 0.01 a
Epicatechin	0.46 ± 0.02 e	1.44 ± 0.05 c	0.22 ± 0.01 f	2.31 ± 0.07 b	0.36 ± 0.02 e	6.98 ± 0.20 a	1.16 ± 0.04 d	0.09 ± 0.01 f
Trans cinnamic acid	nd	nd	nd	nd	nd	nd	0.06 ± 0.01	nd
Chlorogenic acid	6.19 ± 0.23 b	1.70 ± 0.06 e	1.98 ± 0.08 d	nd	7.48 ± 0.31 a	2.39 ± 0.06 c	0.38 ± 0.01 f	nd
Caffeic acid	nd	nd	nd	nd	nd	nd	nd	nd
Coumaric acid	0.54 ± 0.02 b	0.25 ± 0.01 e	nd	0.29 ± 0.01 d	0.59 ± 0.02 a	0.35 ± 0.02 c	0.36 ± 0.02 c	0.62 ± 0.03 a
Ferulic acid	2.53 ± 0.10 a	1.06 ± 0.06 d	1.59 ± 0.07 c	1.52 ± 0.07 c	0.74 ± 0.03 e	0.51 ± 0.02 f	1.13 ± 0.06 d	2.22 ± 0.09 b

Different lowercase letters in a row indicate significant differences between cultivars (p < 0.05); nd—not detected.

). Chlorogenic acid has been proven to protect human LDL (low-density lipoprotein) and to act as a scavenger against reactive oxygen and nitrogen species [54]. Chlorogenic acid intake can reduce the risks of chronic metabolic diseases, neurodegenerative and age-related disorders, including atherosclerosis, hypertension, heart failure, and other cardiovascular risks, such as obesity and type 2 diabetes [55]. In the flesh, the highest content of chlorogenic acid was found in ‘Čačanska Lepotica’ (7.48 mg/100 g), followed by ‘Centenar’ (6.19 mg/100 g). Worthy of note is the high contents of vanillic acid (7.61 mg/100 g), catechin hydrate (13.94 mg/100 g) and epicatechin (6.98 mg/100 g) in the flesh of the ‘Mirabelle de Nancy’ cultivar.

The major phenolic acids identified in the fruit peel were chlorogenic and vanillic acids, depending on the cultivar, while among flavonoids, rutin was dominant, followed by catechin hydrate (

Table 5. Content of phenolic compounds (mg/100 g) in the peel of the plum cultivars.

	‘Centenar’	‘Minerva’	‘Carpatin’	‘Dobrowica’	‘Čačanska Lepotica’	‘Mirabelle de Nancy’	‘Păscoaia’	‘Gogoșele Otăsău’
Vanillic acid	25.12 ± 0.58 a	12.53 ± 0.26 e	10.03 ± 0.21 f	3.09 ± 0.08 g	13.45 ± 0.31 d	24.31 ± 0.45 b	2.12 ± 0.06 h	15.64 ± 0.36 c
Rutin	39.51 ± 0.77 c	52.75 ± 1.23 b	20.04 ± 0.51 e	62.36 ± 2.05 a	26.67 ± 0.88 d	13.89 ± 0.33 f	52.50 ± 1.88 b	24.66 ± 0.38 d
Quercetin	nd	nd	nd	0.11 ± 0.01	nd	nd	nd	nd
Gallic acid	0.20 ± 0.01 e	0.23 ± 0.02 de	0.78 ± 0.03 b	0.27 ± 0.02 d	0.43 ± 0.03 c	1.49 ± 0.06 a	0.06 ± 0.01 f	0.73 ± 0.03 b
Catechin hydrate	4.33 ± 0.16 d	0.58 ± 0.03 g	2.31 ± 0.11 f	0.41 ± 0.02 g	6.57 ± 0.26 b	5.18 ± 0.16 c	3.83 ± 0.12 e	9.09 ± 0.28 a
Syringic acid	nd	nd	nd	nd	nd	nd	nd	nd
Epicatechin	2.00 ± 0.08 c	1.73 ± 0.07 d	1.22 ± 0.05 e	nd	nd	6.54 ± 0.27 a	1.85 ± 0.05 cd	3.53 ± 0.09 b
Trans cinnamic acid	nd	nd	nd	nd	nd	nd	nd	nd
Chlorogenic acid	21.86 ± 0.88 a	5.13 ± 0.21 d	4.02 ± 0.14 e	0.56 ± 0.02 g	9.88 ± 0.33 c	4.62 ± 0.19 de	2.42 ± 0.07 f	12.55 ± 0.44 b
Caffeic acid	0.24 ± 0.02 a	nd	nd	nd	nd	0.05 ± 0.01 c	nd	0.14 ± 0.01 b
Coumaric acid	nd	nd	nd	0.44 ± 0.02 b	nd	0.06 ± 0.00 c	0.04 ± 0.00 c	1.64 ± 0.06 a
Ferulic acid	0.97 ± 0.04 e	1.23 ± 0.07 d	1.48 ± 0.05 c	2.27 ± 0.07 b	0.23 ± 0.01 f	0.26 ± 0.02 f	0.08 ± 0.01 g	2.78 ± 0.11 a

Different lowercase letters in a row indicate significant differences between cultivars (p < 0.05); nd—not detected.

). Rutin is also acknowledged as a potent antioxidant with many pharmacological activities, including anti-inflammatory, anticarcinogenic, neuroprotective, antidiabetic, antimicrobial, anti-viral, antiallergic and antithrombotic activities [56,57].

The highest rutin content was found in ‘Dobrowica’ (62.36 mg/100 g), followed by ‘Minerva’ (52.75 mg/100 g) and ‘Păscoaia’ (52.50 mg/100 g). In a study on the chemical composition and antioxidant capacity of the fruits of European plum cultivar ’Čačanska Lepotica’, influenced by different rootstocks, Trendafilova et al. [23] found rutin as the major flavonoid, with average values of 1.98 mg/100 g FW. Khallouki et al. [41] also reported rutin as the major flavonoid in the peel of the fruits of Prunus × domestica L. (18.6 mg/100 g) from the Lorraine region of Europe (‘Mirabelle‘). They found also 9.4 mg/100 g chlorogenic acid in the flesh extract of Prunus domestica L. as determined by reverse-phase analytical HPLC–ESI-MS. The results of Celik et al. [1] indicated that chlorogenic acid was the predominant phenolic compound in P. domestica L. fruits grown in Türkiye (115.65 mg/100 g). They also quantified higher levels of p-coumaric (18.63 mg/100 g) and ferulic (15.23 mg/100 g) acids, but lower levels of rutin (1.92 mg/100 g) and vanillic acid (3.59 mg/100 g). Lombardi-Boccıa et al. [58] also identified phenolic compounds of organically grown plum (P. domestica L.) fruits and reported 3.75 mg chlorogenic acid/100 g, 2.26 mg caffeic acid/100 g and 0.93 mg ferulic acid/100 g.

In good agreement with our results, Treutter et al. [59] measured the content of rutin between 5.5 and 119.4 mg/100 g, accounting for ~50% of the total flavonols. In addition, they reported chlorogenic and p-coumaric acids in the range 0.3–171.6 mg/100 g and 0.2–14.6 mg/100 g, respectively, in the peels of twenty-eight analyzed plum varieties and clones. Lower chlorogenic acid contents have been reported by Tomić et al. [30] (0.14–2.29 mg/100 g FW), who found neochlorogenic acid as the predominating hydroxycinnamic acid in the studied cultivars (0.26–23.26 mg/100 g FW). However, among the flavonols, they also found in the first place rutin (quercetin-3-rutinoside), with the highest content in the ’Piskavac’ cultivar (1.32 mg/100 g FW). Usenik et al. [22], in a study on the quality changes during ripening of four plum cultivars, found also neochlorogenic acid as the major hydroxycinnamic acid derivative (19.3–120 mg/100 g FW), followed by chlorogenic acid (12.3–54.7 mg/100 g FW), rutin (3.8–12.4 mg/100 g FW) and p-coumaroylquinic acid (0.0–7.6 mg/100 g FW). Rutin was also found as the predominant flavonol in this study. Liaudanskas et al. [60] found also that chlorogenic acid predominated among the phenolic acids (21.98–312.67 mg/100 g) and rutin predominated in the flavonol group (12.16–47.88 mg/100 g) in plum fruits grown in Lithuania.

Recently, in a study on chemical composition of a large number of European plum cultivars grown in Norway, Fotirić Akšic et al. [61] reported that the most dominant phenolic compounds were 5-O-caffeoylquinic (chlorogenic) acid (0–16.18 mg/100 g) and rutin (0.05–12.23 mg/100 g), with a share of around 30% of the total sum of phenols. Significant differences between cultivars were found in the content of individual phenolic compounds in all studies. Genetical factors and environmental conditions (especially temperature and light) are considered the main factors responsible for the differences in the levels of phenolic compounds of various cultivars [1]. In the past few years, researchers have focused on gene expression in synthesizing phenolic compounds. Phenolic substances are biosynthesized by various enzymes in the phenylpropanoid pathway, of which phenylalanine ammonia-lyase, chalcone synthase, hydroxycinnamic acid transferase, and 4-count rate CoA ligase are the key ones [62,63]. In addition, the variations in TPC are also attributed to the differences in fruit grown in different regions and to the adaptation to the cultivar conditions [64], mainly as a result of the complex role of phenolic compounds in the growth, development and protection of plants [65].

3.6. Organic Acids

Acids are important components characterized by a high stability during processing and storage, involved in the formation of plum flavor and in the gelation of pectin [30]. The results on organic acids content in the fruit flesh and peel (

Table 6. Content of organic acids (mg/100 g) in the fruit flesh of the plum cultivars.

	‘Centenar’	‘Minerva’	‘Carpatin’	‘Dobrowica’	‘Čačanska Lepotica’	‘Mirabelle de Nancy’	‘Păscoaia’	‘Gogoșele Otăsău’
Malic acid	527.39 ± 15.66 g	497.47 ± 13.12 g	585.07 ± 17.22 f	1035.05 ± 26.98 d	656.83 ± 12.35 e	1758.04 ± 38.33 b	1181.90 ± 25.99 c	1863.26 ± 36.66 a
Tartaric acid	101.58 ± 3.12 de	104.64 ± 2.66 de	300.14 ± 7.22 c	107.76 ± 2.45 d	97.09 ± 1.77 ef	328.57 ± 8.66 b	89.12 ± 1.44 f	375.74 ± 10.11 a
Citric acid	16.57 ± 0.35 h	20.20 ± 0.43 g	41.63 ± 0.65 c	51.18 ± 0.88 b	30.86 ± 0.50 e	68.67 ± 1.66 a	39.61 ± 1.11 d	24.04 ± 0.68 f
Oxalic acid	35.13 ± 1.11 a	14.67 ± 0.52 c	8.48 ± 0.26 e	11.90 ± 0.44 d	28.65 ± 0.86 b	9.05 ± 0.25 e	6.04 ± 0.18 g	7.37 ± 0.22 f
Ascorbic acid	3.63 ±0.12 b	1.32 ± 0.08 e	2.14 ± 0.09 d	1.03 ± 0.05 e	6.06 ± 0.23 a	2.74 ± 0.12 c	2.14 ± 0.11 d	2.73 ± 0.12 c

Different lowercase letters in a row indicate significant differences between cultivars (p < 0.05).

and

Table 7. Content of organic acids (mg/100 g) in the peel of the plum cultivars.

	‘Centenar’	‘Minerva’	‘Carpatin’	‘Dobrowica’	‘Čačanska Lepotica’	‘Mirabelle de Nancy’	‘Păscoaia’	‘Gogoșele Otăsău’
Malic acid	790.19 ± 18.88 g	901.82 ± 23.77 e	814.66 ± 14.67 fg	1423.56 ± 38.88 d	860.05 ± 17.55 ef	2138.07 ± 46.66 b	1544.34 ± 28.77 c	2291.41 ± 43.87 a
Tartaric acid	140.29 ± 2.23 f	155.48 ± 2.65 e	333.17 ± 5.51 b	141.25 ± 3.37 f	99.30 ± 1.98 g	288.66 ± 9.97 c	187.84 ± 4.66 d	365.00 ± 12.28 a
Citric acid	28.31 ± 0.68 e	22.65 ± 0.45 g	100.01 ± 1.77 a	90.45 ± 1.03 c	25.55 ± 0.51 f	94.69 ± 1.34 b	84.21 ± 0.79 d	16.27 ± 0.35 h
Oxalic acid	13.23 ± 0.26 e	14.19 ± 0.31 d	51.65 ± 1.11 a	21.18 ± 0.54 b	11.89 ± 0.20 f	15.08 ± 0.45 c	12.50 ± 0.36 ef	8.66 ± 0.18 g
Ascorbic acid	3.70 ± 0.15 bc	2.75 ± 0.12 d	2.51 ± 0.11 d	1.73 ± 0.08 e	4.08 ± 0.16 b	7.11 ± 0.23 a	3.42 ± 0.12 c	2.85 ± 0.13 d

Different lowercase letters in a row indicate significant differences between cultivars (p < 0.05).

, respectively) showed that malic acid was the predominant organic acid, both in the flesh and the peel, of the examined plum cultivars. Other previous studies also reported malic acid as the most common acid in plums, along with other acids such as citric, quinic, shikimic, tartaric and fumaric in significantly lower proportions [22,26,30,66,67,68]. Higher malic acid content were found in the peels as compared with the flesh of all the plum cultivars. Statistically significant differences (p < 0.05) in the contents of the organic acids were found between the studied plum cultivars. The malic acid content ranged between 497.47 mg/100 g (‘Minerva’) and 1863.26 mg/100 g (‘Gogoșele Otăsău’) in the fruit flesh and between 790.19 mg/100 g and 2291.41 mg/100 g (‘Gogoșele Otăsău’) in the fruit peel. Usenik et al. [22] reported malic acid contents between 900 and 2180 mg/100 g throughout the maturity period of some plum cultivars, Celik et al. [1] reported 601 mg malic acid/100 g in native wild plum species and Lombardi-Boccia et al. [58] found 1980 mg/100 g malic acid in yellow plums (Prunus domestica L.) from conventional and organic productions. Ionica et al. [11] found also malic acid as the predominant organic acid in plums, with levels between 177.53 mg/100 g in the ‘Renclod Althan’ cultivar and 780.07 mg/100 g in the ‘Tuleu Gras cl. 14′ cultivar, while Tomić et al. [30] reported between 670 mg/100 g and 1670 mg/100 g of malic acid in various plum (Prunus domestica L.) cultivars grown in Serbia. Recently, in a study on organic acid composition of the flesh and peel of plum fruits, Xiao et al. [67] found malic acid as the most abundant compound, accounting for 177.58 to 1568.43 mg/100 g FW and 327.68 to 2187.46 mg/100 g FW in the fruit flesh and peel, respectively.

The levels of citric acids in plums ranged between 16.27 mg/100 g in the peel of ‘Gogoșele Otăsău’ and 100.01 mg/100 g in the peel of the ‘Carpatin’ cultivar. In good agreement with our results, Lombardi-Boccia et al. [58] reported 25.7 mg citric acid/100 g in yellow plums from conventional and organic productions, Ionica et al. [11] found citric acid content ranging from 10.49 mg/100 g in ‘Alina’ to 83.80 mg/100 g in the ‘Oltenal’ plum cultivar, while Tomić et al. [30] found between 30 and 180 mg citric acid per 100 g. Oxalic acid content ranged from 6.04 to 35.13 mg/100 g in the fruit flesh and from 8.66 to 41.65 mg/100 g in the peel. Previously, Fotirić Akšić et al. [61] found mean values of oxalic acid content between 12 and 153 mg/100 g dry weight after analyzing sixty-eight Norwegian plum cultivars, while Lin et al. [37] found oxalic acid accounting for 0.94% of the total acids in five Japanese plum cultivars (between 12 and 43 mg/100 g). Xiao et al. [67] detected slight amounts of tartaric (0.70–288.40 and 1.09–419.47 mg/100 g FW), citric (2.70–71.96 and 9.03–79.14 mg/100 g FW) and oxalic acids (2.66–51.69 and 2.15–45.63 mg/100 g FW) in the flesh and peel of plum fruits, respectively. In good agreement with our results on malic acid content in the flesh (656.83 mg/100 g) and peel (860.05 mg/100 g) of the ’Čačanska Lepotica’ plums, Trendafilova et al. [23], in a study on the influence of different rootstocks on the chemical composition of the fruits of European plum cultivar ’Čačanska Lepotica’, found malic acid contents between 730 and 890 mg/100 g in the flesh and between 650 and 940 mg/100 g in the peel.

The highest vitamin C content was measured in the peel of the ‘Mirabelle de Nancy’ cultivar (7.11 mg/100 g) and in the flesh of the ‘Čačanska Lepotica’ cultivar (6.06 mg/100 g). Higher ascorbic acid contents were reported by Nergiz and Yıldız [69] in the Victoria and Stanley plum varieties (19.26 mg/100 g and 23.43 mg/100 g, respectively), while Lombardi-Boccia et al. [58] reported only 1.60 mg/100 g ascorbic acid content in yellow plums. In addition to the cultivar characteristics, the large variability in the organic acid content of plums could be attributed to environmental factors and post-harvest fruit physiology [1].

The correlation coefficients between the content of phenolic compounds, organic acids, total phenolics (TPC) and DPPH radical scavenging activity (RDA) in the fruit flesh and peel of the examined plum cultivars are presented in

Table 8. Correlation coefficients between phenolic compounds, organic acids, total phenolic contents and DPPH radical scavenging activity (RSA) in the fruit flesh of the plum cultivars.

	Rutin	Gallic Acid	Catechin Hydrate	Epicatechin	Chlorogenic Acid	Caffeic Acid	Coumaric Acid	Ferulic Acid	Malic Acid	Tartaric Acid	Citric Acid	Oxalic Acid	Ascorbic Acid	TPC	RSA
Vanillic acid	−0.35	0.97 **	0.51	0.87 **	0.22	−0.24	−0.01	−0.48	0.37	0.38	0.64	−0.01	0.17	0.78 *	0.77 *
Rutin	1	−0.23	0.50	−0.21	−0.50	−0.20	0.38	0.47	0.61	0.48	−0.13	−0.33	−0.19	0.10	0.08
Gallic acid		1	0.69	0.92 **	0.02	−0.10	−0.05	−0.54	0.52	0.47	0.74 *	−0.21	0.05	0.80 *	0.79 *
Catechin hydrate			1	0.71 *	0.45	−0.33	0.20	−0.14	0.93 **	0.65	0.62	−0.49	−0.21	0.68	0.66
Epicatechin				1	−0.16	−0.22	−0.13	−0.58	0.49	0.27	0.81 *	−0.29	−0.20	0.65	0.68
Chlorogenic acid					1	0.71 *	0.40	−0.05	−0.51	−0.34	−0.31	0.89 *	0.85 **	0.17	0.12
Caffeic acid						1	0.43	−0.39	−0.26	−0.29	−0.13	0.50	0.85 **	0.07	0.05
Coumaric acid							1	0.22	0.31	−0.08	−0.37	0.48	0.62	0.41	0.34
Ferulic acid								1	−0.08	0.09	−0.58	0.25	−0.11	−0.30	−0.36
Malic acid									1	0.66	0.47	−0.58	−0.12	0.58	0.53
Tartaric acid										1	0.32	0.26	−0.09	0.52	0.48
Citric acid											1	−0.51	−0.22	0.27	0.27
Oxalic acid												1	0.65	0.01	−0.02
Ascorbic acid													1	0.28	0.20
TPC														1	0.99 **
RSA															1

* p < 0.05; ** p < 0.01.

and

Table 9. Correlation coefficients between phenolic compounds, organic acids, total phenolic contents and DPPH radical scavenging activity (RSA) in the peel of the plum cultivars.

	Rutin	Gallic Acid	Catechin Hydrate	Epicatechin	Chlorogenic Acid	Caffeic Acid	Coumaric Acid	Ferulic Acid	Malic Acid	Tartaric Acid	Citric Acid	Oxalic Acid	Ascorbic Acid	TPC	RSA
Vanillic acid	−0.60	0.52	0.43	0.61	0.70	0.69	−0.01	−0.15	0.09	0.16	−0.35	−0.23	0.65	0.66	0.70
Rutin	1	−0.81 *	−0.63 *	−0.55	-0.25	−0.18	−0.12	0.13	−0.25	−0.62	−0.03	−0.23	−0.60	−0.15	−0.20
Gallic acid		1	0.35	0.77 *	−0.11	−0.01	0.14	−0.03	0.53	0.66	0.33	0.19	0.69 *	−0.12	−0.04
Catechin hydrate			1	0.39	0.51	0.44	0.57	0.01	0.51	0.38	−0.42	−0.43	0.39	0.36	0.39
Epicatechin				1	0.07	0.29	0.19	−0.14	0.70 *	0.58	0.14	−0.22	0.77 *	−0.03	0.05
Chlorogenic acid					1	0.90 **	0.16	0.05	−0.18	−0.09	−0.68	−0.33	0.11	0.80 *	0.78 *
Caffeic acid						1	0.32	0.22	0.10	0.11	−0.47	−0.32	0.13	0.56	0.56
Coumaric acid							1	0.77 *	0.67	0.54	−0.33	−0.28	−0.26	0.02	0.04
Ferulic acid								1	0.25	0.40	−0.17	0.15	−0.63	−0.06	−0.07
Malic acid									1	0.58	0.14	−0.38	0.37	−0.30	−0.22
Tartaric acid										1	0.24	0.36	0.12	−0.37	−0.35
Citric acid											1	0.59	0.14	−0.87 **	−0.84 **
Oxalic acid												1	−0.28	−0.50	−0.53
Ascorbic acid													1	0.15	0.24
TPC														1	0.99 **
RSA															1

* p < 0.05; ** p < 0.01.

, respectively. In the fruit flesh, the data indicated that vanillic acid had a strong positive correlation with gallic acid and epicatechin, while chlorogenic acid was strongly correlated with caffeic acid content. The ascorbic acid content was significantly correlated with chlorogenic and caffeic acid contents, but poorly correlated with radical scavenging activity, both in the flesh and the peel, thus demonstrating the weaker contribution of ascorbic acid to the antioxidant activity of plums as compared with the phenolic compounds. Celik et al. [1] also reported a significant and positive correlation between vitamin C and chlorogenic and p-coumaric acid; however, a negative correlation between vitamin C and gallic acid was observed. TPC and RDA were positively and strongly correlated with vanillic acid content, which was to be expected considering the high content of vanillic acid in the fruit flesh. Xiao et al. [67] found that the total phenolic content in the peel and flesh showed a high positive correlation with epicatechin and gallic acid. It is worth noting the strong negative correlation between total phenolic content and citric acid content. A negative correlation was noticed between oxalic and malic acid contents, which is in agreement with the findings of Fotirić Akšić et al. [61] in the fruits of some European plum cultivars grown in Norway.

As presented above, a significant and very strong correlation was found between TPC and RDA, both in the fruit flesh and the peel. These findings are in agreement with previous studies reporting a correlation between total polyphenolic contents and antioxidant activity in plums [4,10,30,42,50,51].

In the peel, total phenolic content was positively and strongly correlated with chlorogenic acid content, while it was negatively correlated with citric acid content. A strong negative correlation was found between rutin and gallic acid, as well as between rutin and catechin hydrate. Similar to the fruit flesh, a positive correlation was found between gallic acid and epicatechin content, as well as between chlorogenic and caffeic acid. Although rutin content is high in the peel, no significant correlation (p > 0.05) was found between rutin content and radical scavenging activity, thus demonstrating the cumulative role of all phenolic compounds in determining the antioxidant activity.

4. Conclusions

The study provides a comparative analysis of biometric properties, dry matter, soluble solids content, titratable acidity, total phenolic content, DPPH radical scavenging activity, phenolic and organic acid profiles of six plum cultivars and two local plum selections grown in a collection orchard in south-western Romania. The ‘Păscoaia’ local selection had the largest fruits, while ‘Gogoșele Otăsău’ plums were the smallest. ‘Păscoaia’ had also the highest pulp ratio but a low dry matter content, which makes it unsuitable for the production of dried plums. Both the peel and flesh of plum fruits contain significant amounts of bioactive compounds and antioxidant activity. Chlorogenic acid was the predominant phenolic acid both in the fruit flesh and the peel, while among the flavonoids, rutin was dominant, followed by catechin hydrate. A large variability was found in the organic acid profile depending on the cultivar; however, malic acid was the most abundant both in the flesh and the peel. The local selections ‘Gogoșele Otăsău’ and ‘Păscoaia’ present valuable characteristics and could be used as a source of germplasm to improve the quality properties of plums; first, due to its outstanding bioactive content and antioxidant activity, and second, due to the large size and attractive appearance of the fruits. Future research will need to screen the nutritional and bioactive characteristics of the plum cultivars across multiple growing seasons, as well as their postharvest evolution.