Phenotypic diversity of nutritional quality attributes and chilling injury symptoms in four early peach [Prunus persica (L.) Batsch] cultivars grown in west central Tunisia

Abstract

The present study aimed to characterize the phenotypic diversity of agronomical and biochemical fruit quality traits in four early peach cultivars. The sensibility to chilling injury symptoms (CI) was studied after two cold storage periods (2 and 4 weeks) at 5 ºC and 95% relative humidity (RH) followed by 2 days at room temperature. Agronomical attributes such as fruit weight, firmness, soluble solids content (SSC), pH, titratable acidity (TA) and color parameters were recorded. Antioxidant compounds such as anthocyanins, flavonoids, total phenolics, vitamin C and relative antioxidant capacity (RAC) were evaluated. Chilling injury symptoms such as mealiness, graininess, flesh browning, flesh bleeding, leatheriness and off-flavor were analyzed. Results revealed high antioxidant compounds in peel regarding to flesh fruit. The antioxidant compounds content in both peel and pulp decreased during cold storage except anthocyanins which exhibited different pattern. After 2 weeks of storage, fruits presented high SSC and low score of chilling injury symptoms. At the end of the trial, the studied cultivars were unacceptable for consumption due to the severity of CI. PCA analysis showed that the cultivars Plagold 5 and Plagold 10 had less sensibility to chilling injury.

Graphical abstract

Introduction

Peaches have a worldwide production of 24.5 million tons in a cultivated area of around 1.5 million ha in 2020 (FAOSTAT 2022). In Tunisia, peach is an emerging fruit crop with a production of 150 thousand tons in a cultivated area of 14.8 thousand ha (FAOSTAT 2022).

Peach fruit contains carbohydrates, organic acids and bioactive compounds such as vitamin C, carotenoids, flavonoids and phenols, increasing the fruit acceptability by consumers (Sajid et al. 2020). According to this potential, there is an increased interest to develop new peach cultivars with improved nutritional quality (Abidi et al. 2015; Saidani et al. 2017). Ceccarelli et al. (2016) reported that peel fruit contains a much higher amount of phenolics and the consumption of unpeeled peaches can be seen as a viable nutritional source for human health benefits. Higher phenolic concentrations, along with higher ascorbic acid values have been also reported in peach peel compared to pulp (Saidani et al. 2017). However, the levels of antioxidants in fruits are known to be influenced by genetic background, maturity stage, environmental factors, culture practices and storage conditions (Manach et al. 2004). Aubert et al. (2014) reported that the most significant changes during cold storage period are reductions of firmness, titratable acidity and organic acids whereas SSC, sugars and phenolic compounds remained constant.

Peach is a climacteric fruit which undergo a rapid softening after harvest. The cold storage is used to maintain the fruit quality traits and to delay fruit ripening. When peach fruits are removed from the cold room and kept at room temperature, the fruit manifest chilling injury symptoms (Abidi et al. 2015). Chilling injury signs are predominantly weight loss, lack of juiciness (mealiness), internal browning, reddish flesh discoloration (bleeding), leatheriness, off-flavor and increased incidence of decay (Lurie and Crisosto 2005).

Early peach cultivars received a short or prolonged storage depending on the shipment period. Studies reporting variability of antioxidants in peel and flesh fruits during cold storage are relatively scarce. The main objective of this study was to evaluate the antioxidant compounds content in peel and flesh fruit and to quantify the chilling injury symptoms of four early peach cultivars during two cold storage periods (2 and 4 weeks at 5 °C and 95%RH). This study was realized with a special focus on selecting cultivars with enhanced antioxidant compounds content and low sensibility to chilling injury.

Material and methods

Plant material and study area

Fruits from 15 years old peach cultivars (Plawhite 5, Plawhite 10, Plagold 5 and Plagold 10) grafted on ‘Garnem’ rootstock were studied (Table 1). Trees were grown in a private orchard located in the region of Regueb (34° 47′45.1’’N; 9°47′54.1’’E; 150 m above sea level), Sidi Bouzid, Tunisia. The cultivars were trained to the standard open vase system, planted at a spacing of 6 m × 3 m and grown under standard conditions of irrigation, fertilization and disease control. Hand thinning was carried out to reduce fruit load to 400 fruits per tree. Fruits were handpicked at commercial maturity and assessed by peel color and flesh firmness. Fruits were considered ripe in the tree when their growth had stopped, exhibited orange-red ground color and were easily detached. The soil of the experimental site was sandy-loam in texture (60% sand, 30% silt, and 10% clay) with about 0.1% organic matter. The study area is characterized by Mediterranean climate with a seasonally irregular restricted rainfall. During the experimental period, the daily mean reference evapotranspiration (ETo) values ranged from 2.0 mm d⁻¹ during the winter to 6.0 mm d⁻¹ during the spring months and the annual ETo was about 1200 mm y⁻¹ whereas the annual precipitation is of 200 mm. In January, the coolest month, the indicated daily temperature is 10 °C, during the month of April the daily temperature is about 20 °C. All cultivars have red skin color and round fruit shape. Plagold 5, Plagold 10 are yellow-flesh peach cultivars whereas Plawhite 5 and Plawhite 10 are white-fleshed peach cultivars. Maturity dates were 20 April, 25 April, 25 April and 05 May for Plawhite 5, Plawhite 10, Plagold 5 and Plagold 10, respectively. The fruit development period from full bloom to the harvest were recorded and expressed as Julian days (Table 1).

Table 1.

Pomological and agronomic traits of the studied peach cultivars during two growing seasons (2018/2019)

Traits	Plawhite 5	Plawhite 10	Plagold 5	Plagold 10
Flesh color	White	White	Yellow	Yellow
Flesh texture	Melting	Melting	Melting	Nonmelting
Stone adherence	Clingstone	Clingstone	Clingstone	Clingstone
H (mm)	54.41 ± 3a	49.50 ± 5b	51.30 ± 3a	50.86 ± 2a
SD (mm)	55.43 ± 2a	55.05 ± 4a	55.43 ± 3a	59.27 ± 5a
H/SD (mm)	0.98 ± 0.2a	0.90 ± 0.2a	0.93 ± 0.2a	0.86 ± 0.2a
Flower type	Showy	Showy	Showy	Non-showy
Petiol gland shape	Reniform	Reniform	Circular	Reniform
Fruit development (JD)	75	82	78	88
Annual yield (Kg)	18.00 ± 5a	22.50 ± 6a	20.50 ± 4a	23.50 ± 7a
TCSA (cm2)	34.80 ± 3a	36.60 ± 5a	35.20 ± 4a	36.20 ± 2a
Yield efficiency (kg/cm2)	0.51 ± 0.1a	0.61 ± 0.2a	0.58 ± 0.2a	0.64 ± 0.2a

Values are means of six measurements ± SE. Mean separation within columns by Duncan´s test (P ≤ 0.05). In each column, values with the same letter are not significantly different. Abbreviations: H: height; SD: suture diameter; JD: Julian day; TCSA: trunk cross sectional area

Agronomical traits

Annual yield, cumulative yield, trunk cross sectional area (TCSA), yield efficiency and harvest date were recorded. For all agronomical, biochemical and chilling injury analysis, a representative fruit sample (60 fruits) was taken from ten trees per cultivar over the two growing seasons (2018–2019). Agronomical fruits quality traits in peel and pulp of the fruits were performed at harvest and after the two cold storage periods. Flesh firmness was measured with a penetrometer equipped with an 8 mm diameter flat tip probe and expressed in Newton (N). The juice yield and the pulp/pit ratio were also determined. Soluble solids content (SSC, °Brix) was measured in the juice using a digital hand-held refractometer (Atago, Tokyo, Japan). The initial pH and titratable acidity (TA, g malic acid per 100 g fresh weight sample) were measured in juice by titration using NaOH 0.1 N. The ripening index (RI) was determined as the ratio between soluble solids and titratable acidity (SSC/TA). Data represent the average values of six measurements during the two growing seasons. Peel color of both sides of ten fruits was measured using a CR-200 Minolta Chromameter (Chuo-Ku, Osaka, Japan) and average values of L* (lightness), a* (−a* = greenness, + a* = redness), b* (−b*= blueness, + b* = yellowness), chroma (C*) and Hue (h°) were recorded. The color parameters were measured using Eq. (1).

$${\mathrm{C}}^{\ast }={\left({\mathrm{a}}^{\ast 2},+,{\mathrm{b}}^{\ast 2}\right)}^{1/2}$$ 1

where: C*: chroma is the grade of quantitative difference of Hue parameter with reference to grey color.

$${\mathrm{h}}^{\circ }=\left({\tan }^{-1},{\mathrm{b}}^{\ast }/{\mathrm{a}}^{\ast }\right)$$ 2

where ºh: angle is the qualitative attribute of color.

Biochemical extraction

Fruit samples were washed, towel dried, peeled using a sharp knife and cut into small pieces. Then, 5 g of both peel and flesh were weighted, frozen in liquid nitrogen and stored at (− 20 ºC) until analysis. For phenolic compounds, samples were homogenized in a polytron (T25D Turrax; IKA Works, Inc.; Wilmington, NC) with 10 mL of 0.5 N HCl in methanol/distilled water (80% v/v). The mixture was then centrifuged (SIGMA Laboratory centrifuges 3K18, UK) at 4.000 rpm for 15 min at 4 °C. The extract was used for anthocyanins, flavonoids, total phenolics and antioxidant capacity determinations as described in Abidi et al. (2015). For vitamin C analysis, samples were homogenized with a polytron in 10 mL 5% metaphosphoric acid, centrifuged at 4000 rpm for 15 min at 4 °C and the supernatant was used for vitamin C analysis (Abidi et al. 2015).

Antioxidant determinations

Antioxidant compounds in both peel and pulp of the fruits were measured using a spectrophotometer (Jenway 6300, UK). Calibration curves of the analysed antioxidant compounds were prepared daily using the correspond standard.

The anthocyanin content was calculated with 535 and 700 nm absorbance using the molar extinction absorptivity coefficient ε = 25,965/cm M and expressed in mg of cyanidin 3-glucoside equivalents (C3GE) per kg of FW.

Flavonoid content was determined as reported in Zhishen et al. (1999). The extract (1 ml) was diluted with 2 ml of distilled water, mixed with 0.3 ml of NaNO₂ for 5 min, incubated with 0.3 ml of AlCl₃ for 2 min and 2 ml of NaOH was added and shaken vigorously. The absorbance was measured at 510 nm and the results were expressed in mg of catechin equivalents (CE) per 100 g of FW.

Total phenolic content was determined as reported in Saidani et al. (2017). The method consisted of mixing 0.5 ml of the extract in 8 ml of distilled water with 0.5 ml of Folin-Ciocalteu reagent. The solution was mixed, incubated for 3 min, then 1 ml of sodium carbonate (Na₂CO₃) was added and the samples were incubated at 25 °C in darkness for one hour. The absorbance was measured at 725 nm and results were expressed in mg of gallic acid equivalents (GAE) per 100 g of FW.

For vitamin C determination one ml of the extract was mixed with equal amounts of trichloroacetic acid (TCA), 0.8 ml of phosphoric acid, 0.8 ml of 2,2‘-bipyridyl and 0.4 ml FeCl₃. The solution was mixed and incubated at 37 °C for 60 min. Then the absorbance was measured at 525 nm and results were expressed as mg of ascorbic acid (AsA) per 100 g of FW (Okamura 1980).

Free radical scavenging activity of peel and pulp was assessed by the 1,1-diphenyl2-picrylhydrazyl (DPPH) assay and the Ferric Reducing Antioxidant Power (FRAP) assay. The DPPH assay was performed using the method adapted from Brand-Williams et al. (1995). Fruit extracts (100 μl) were mixed with 2.9 ml DPPH and reaction was allowed to stand 10 min in darkness at room temperature afterward the absorbance was measured at 515 nm. The FRAP assay was performed using a TPTZ (2,4,6-tripyridylS-triazine) solution according to Fu et al. (2020). In fact, 4.9 mL of FRAP solution (2.5 ml of 0.1 M acetate buffer [pH = 3.6], 250 µl of 10 mM TPTZ, and 250 µl of 20 mM FeCl3) was mixed with 0.1 mL of sample extract. Thereafter, the obtained mixture was shaken vigorously and incubated in the dark for 10 min at room temperature. Finally, the absorbance was measured at 593 nm. The antioxidant capacity was calculated using a Trolox standard curve and expressed as µg of Trolox equivalents (TE) per g of FW.

Chilling injury symptoms

The chilling injury symptoms were analyzed after 2 and 4 weeks of cold storage at 5 ºC and 95% RH (relative humidity) followed by 2 days at room temperature according to Crisosto et al. (1999). Fruits were evaluated for CI symptoms such as flesh mealiness, greaniness, leatheriness, flesh browning, internal reddening and off-flavour. Fruits with dry appearance were considered mealy. Mealiness, graininess and off flavor was scored as the proportion of fruit affected with these symptoms in the sample. Internal browning was visually scored on a scale of 1 (no browning) to 6 (severe browning). Bleeding was visually scored on a scale of 1 (no bleeding) to 3 (more than 50% of the flesh with bleeding). Eventually, the degree of CI (CI index) was visually assessed according to the global fruit appearance of each cultivar, from healthy fruit with no symptoms (1) to severe CI symptoms (6) when the fruit was extremely affected with CI symptoms.

Statistical analysis

All traits were measured for each cultivar separately over the two growing seasons. The presented values of agronomical and biochemical fruit quality traits are means of six measurements. All statistical analyses were performed using SPSS 20.0 (SPSS Int. J. Mol. Sci. 2010, 11 6932 Inc., Chicago, IL). When analysis of variance (ANOVA) showed statistical differences (P ≤ 0.05), means were separated by Duncan’s multiple range test. Principal component analysis (PCA) of agronomic and biochemical traits and CI symptoms was carried out using SPSS 20.0. The component matrix was evaluated and orthogonal factors were rotated using variance maximizing (varimax).

Results and discussion

Agronomical traits

Mean annual yield varied from 18.0 kg/tree in Plawhite 5 to 23.5 kg in Plagold 10 (Table 1). The yield efficiency varied from 0.51 kg/ cm² in Plawhite 5 to 0.64 kg/cm² in Plagold 10. The studied cultivars were conducted under standard conditions of irrigation and fertilization. The same field conditions resulted in similar yield and yield efficiency values. These findings are in accordance with the study of Milatovic et al. (2010) reporting that yield depends on the genetic background of the cultivar and on agronomical and environmental factors. The yield efficiency values showed in our study are in the interval [0.35 to 0.76 kg/cm²] reported by Loreti and Massai (1998) in peach cultivar “Springcrest” grafted on different rootstocks.

Fruit quality evaluation

Fruit weight, weight loss, firmness, SSC, TA and RI of the studied cultivars are shown in Fig. (1a-f). The fruit weight showed a gradual decrease during storage and statistically significant difference (P < 0.05) were observed among the storage periods (Fig. 1a). The studied peach cultivars showed similar behavior regarding fruit weight loss (Fig. 1b). After 30 days of cold storage, the highest weight loss was observed in Plawhite 5 (14.22%) while the lower weight loss value (6.6%) was observed in Plagold 10. Our results are in accordance with the study of Khan et al. (2018) reporting that peach weight loss increased as fruit ripening and storage period progressed.

Fig. 1.

Influence of storage period on fruit weight (a), weight loss (b), flesh firmness (c), soluble solids content (SSC) (d), titratable acidity TA (e) and ripening index (RI) (f) in peach cultivars at harvest and after 2 or 4 weeks of cold storage (5ºC and 95% Relative Humidity) during 2 years of study (2018–2019). Values are means of six measurements (n = 6) ± SE. Different letters a, b, c, indicate significant difference among storage periods in each cultivar (ns, not significant, *P < 0.05, **P < 0.01) according to Duncan’s multiple range test

The fruit firmness values ranged from 55 N in the cv. Plawhite 10 to 61 N in the cv. Plagold 10. At the end of the storage period, the lowest fruit firmness was observed in the cv. Plawhite 5 (13.92 N). Results showed that flesh firmness differed significantly (P < 0.05) between the cold storage periods (Fig. 1c). Our results showed a gradual decrease of flesh firmness for all the studied cultivars. Hence, the cultivars Plawhite 5 and Plawhite 10 showed a rapid decrease in firmness as compared to Plagold 5 and Plagold 10. Our results are in line with the study of Liu et al. (2019) reporting that fruit firmness decreased gradually over the storage time. Lurie and Crisosto, (2005) reported that flesh firmness dropped during ripening within 4 days confirming the melting-flesh type of the cultivar.

At harvest, the SSC ranged from 9.0°Brix in the cv. Plagold 10 to 9.7°Brix in the cv. Plagold 5 (Fig. 1d) and increased during the two cold storage periods showing statistically significant difference (P < 0.05) with the values obtained at harvest. The obtained SSC at harvest are greater than the minimum (8°Brix) established by the European Union to market peaches and nectarines (R-CE no. 1861/2004), although most previous findings considered that a mean value of SSC over 10°Brix is the minimum value for consumer acceptance (Aubert et al., 2014). The increase in SSC during storage might be due to the breakdown of complex organic metabolites into sugars (Mahajan and Goswami 2004) or as a consequence of fruit water loss (Infante et al. 2008).

Results showed remarkable differences among cultivars in their juice acidity levels (Fig. 1e). The TA showed a gradual decrease in all the studied cultivars which might be due to the use of organic acids in respiration mechanism as reported by Xi et al. (2016). Infante et al. (2008) reported that cold storage significantly reduced titratable acidity and increased the ripening index ratio but did not affect SSC which remained constant. In this line, Crisosto and Valero (2008) reported that in most peach cultivars, acidity decreased about 30% during ripening. Aubert et al. (2014) studying the effects of storage on physicochemical nectarine fruit quality traits, reported that the most significant changes are the decrease of firmness and TA whereas SSC and phenolic compounds remained constant during the same period.

The RI ranged from 9.0 in cv. Plagold 10 to 15.6 in cv. Plagold 5 (Fig. 1f) showing high variability between cultivars as a result of the observed differences in juice SSC and TA. The obtained values of RI, are in accordance with the study of Ceccarelli et al. (2016) reporting that RI increased in the studied cultivars after four weeks of cold storage.

The color parameters (a*, b*, L*, C* and h°) of the peach peel color are presented in the Fig. 2 (a-e). The degree of red blush (a* value) was maintained during the cold storage periods (Fig. 2a). Results showed that the brightness (L* value) of peach peel increased as the storage period increased (Fig. 2c). This change in fruit peel color may be associated with the ripening process of the peach fruit as it was reported in Nuzzi et al. (2015).

Fig. 2.

Variation in fruit peel color from the peach cultivars during harvest and after two cold storage periods (2 and 4 weeks). L* means the brightness of the sample while a* and b* represent the color directions, C* = chroma and h° = Hue angle. Values are means of six measurements (n = 6) ± SE. Different letters a, b, c, indicate significant difference among storage periods in each cultivar (ns, not significant, *P < 0.05, **P < 0.01) according to Duncan’s multiple range test

Biochemical traits

Anthocyanin content

The analysis of anthocyanins in peel and pulp of peach fruits at harvest and after two cold storage periods are shown in Table 2. The analysis of anthocyanins in fruit tissue showed high content in peel than in pulp of the fruit. At harvest, anthocyanins values ranged from 2.18 to 5.38 mg CE/100 g of FW in peel fruit and from 1.2 to 2.43 mg CE/100 g of FW in flesh fruit. These findings are in accordance with the study of Saidani et al. (2017) reporting that peel fruits showed significantly higher contents of antioxidants than pulp. In this line, Crisosto and Valero (2008) suggested that it is recommended to eat peaches with the peel to ensure intake of most of the antioxidant compounds. The evolution of anthocyanins over the storage periods, showed an increase during the first cold storage period (2 weeks) in both peel and pulp and then decreased. Our findings are in accordance with the study of Tsaltani et al. (2010) reporting that anthocyanin increases in 4 week-stored peach fruits resulted in an extended and intense bleeding by day 5 of storage.

Table 2.

Influence of storage period on anthocyanins, flavonoids, total phenolics, vitamin C and relative antioxidant capacity among four peach cultivars during harvest, 2 weeks and 4 weeks cold storage periods at 5 °C and 95% RH

Cultivars	Peel						Pulp
	TP	TF	TAC	Vitamin C	RAC	FRAP	TP	TF	TAC	Vitamin C	RAC	FRAP
Harvest
Plawhite 5	80.2 ± 7a	9.56 ± 6b	2.18 ± 0.4c	4.80 ± 0.4ª	425.0 ± 52a	380.0 ± 12a	61.8 ± 10a	4.02 ± 0.5b	1.90 ± 0.2a	2.10 ± 0.5b	318.9 ± 5a	303.5 ± 10a
Plawhite 10	61.0 ± 5b	16.03 ± 4a	3.63 ± 0.5b	3.70 ± 0.5ª	418.10 ± 36a	373.10 ± 5a	48.70 ± 12b	7.30 ± 0.4a	1.20 ± 0.4b	2.35 ± 0.3b	367.2 ± 8a	312.20 ± 8a
Plagold 5	73.1 ± 8ª	10.63 ± 3b	5.38 ± 0.6ª	3.00 ± 0.7ª	325.8 ± 34b	280.80 ± 5b	42.3 ± 16b	3.12 ± 0.6b	2.43 ± 0.3a	3.50 ± 0.2a	286.1 ± 9b	241.10 ± 5b
Plagold 10	48.6 ± 8c	12.52 ± 2b	4.25 ± 0.7b	2.70 ± 0.5b	329.0 ± 20b	286.0 ± 10b	41.50 ± 25b	4.60 ± 0.6b	1.71 ± 0.2a	2.60 ± 0.3b	224.2 ± 7b	218.50 ± 5b
2 weeks cold storage
Plawhite 5	46.4 ± 8ª	8.77 ± 2b	9.53 ± 0.8a	2.75 ± 0.9ª	242.2 ± 15a	297.5 ± 5a	40.4 ± 10a	2.93 ± 0.4c	2.40 ± 0.5b	2.02 ± 0.5b	248.3 ± 3b	203.50 ± 3b
Plawhite 10	50.4 ± 9ª	14.50 ± 2ª	8.98 ± 0.1ª	1.23 ± 0.8b	240.4 ± 13b	195.5 ± 3b	35.2 ± 15b	6.69 ± 0.3a	3.08 ± 0.4a	1.36 ± 0.7b	293.0 ± 3a	248.00 ± 2a
Plagold 5	52.7 ± 7ª	10.98 ± 1b	7.82 ± 0.5b	2.10 ± 0.7ª	143.4 ± 10b	190.5 ± 5b	43.3 ± 13a	4.36 ± 0.5b	2.99 ± 0.2a	3.00 ± 0.4a	249.1 ± 5b	204.10 ± 5b
Plagold 10	42.4 ± 6b	14.61 ± 1ª	4.51 ± 0.6c	1.32 ± 0.4b	241.1 ± 21ª	200.1 ± 10ª	41.2 ± 18a	3.11 ± 0.8c	2.69 ± 0.4b	1.93 ± 0.5b	256.8 ± 8b	211.50 ± 4b
4 weeks cold storage
Plawhite 5	43.4 ± 7a	8.40 ± 1ª	7.20 ± 0.4ª	1.48 ± 0.3b	202.6 ± 24ª	157.2 ± 4ª	46.5 ± 16a	1.2 ± 0.5c	4.31 ± 0.4a	1.60 ± 0.3b	125.5 ± 7a	110.50 ± 2a
Plawhite 10	39.2 ± 5b	4.80 ± 2b	5.50 ± 0.4b	2.24 ± 0.5ª	165.8 ± 25b	120.5 ± 5b	41.0 ± 17a	2.4 ± 0.9b	2.75 ± 0.6c	1.19 ± 0.2b	132.9 ± 3a	117.50 ± 5a
Plagold 5	39.5 ± 4b	6.00 ± 1b	4.4 ± 0.8b	2.11 ± 0.4ª	123.1 ± 20a	150.5 ± 10a	27.8 ± 16c	3.7 ± 1.0a	3.46 ± 0.7b	2.79 ± 0.4a	119.6 ± 5b	101.50 ± 5b
Plagold 10	41.5 ± 3a	10.80 ± 2ª	2.33 ± 0.1c	1.18 ± 0.2b	137.5 ± 13c	92.3 ± 5c	31.8 ± 13b	1.2 ± 0.3c	2.47 ± 0.1c	1.59 ± 0.8b	135.0 ± 3a	120.20 ± 5a
ANOVA
C	*	*	*	*	*	*	*	*	*	*	*	*
T	*	*	*	*	*	*	*	*	*	*	*	*
C*T	*	*	ns	ns	*	*	*	*	ns	ns	*	*

Units and abbreviations: TP = Total phenolics (mg GAE/100 g of FW); TF = Flavonoids (mg CE/100 g of FW); TAC = anthocyanin (mg C3Geq kg⁻¹ of FW); Vitamin C (mg AsA/100 g of FW); RAC = Relative Antioxidant Capacity (μg TE/g of FW). AsA = Ascorbic acid; C3GE = Cyanidin-3-glucoside equivalents; CE = Catechin equivalents; GAE = Gallic acid equivalents; TE = Trolox equivalent; Ferric reducing antioxidant power (FRAP). Values are the means of six measurements (n = 6) ± SE. Different letters (a, b, c) within each column and for each storage period indicate significant differences between cultivars (n.s., not significant, *P < 0.05) according to Duncan's test

Flavonoid content

The flavonoids behavior in peel and pulp of the fruit is reported in Table 2. The analysis revealed that flavonoid content was higher in the peel of the fruit. At harvest, fruits showed high flavonoid content and then decreased during the two cold storage periods. The cultivar Plawhite 10 showed high flavonoid content at harvest (16.03 and 7.30 mg CE/100 g of FW in peel and pulp, respectively) and then decreased within storage periods. Our results are in accordance with the study of Stojanovic et al. (2016) in peach and nectarine cultivars, presenting values in the range of [21.9—94.9 mg CE/100 g FW] and [5.0 58.9 mg CE/100 g FW] in peel and pulp extracts, respectively.

Total phenolic content

Total phenolics varied significantly between fruit peel and pulp and among storage periods as shown in Table 2. At harvest, the amount of total phenolics ranged from 48.6 to 80.2 mg GAE/100 g FW for peel extracts and from 41.5 to 61.8 mg GAE/100 g FW for pulp extracts. Our results are in accordance with Chang et al. (2000) showing values in the range of [41.5–76.5 mg GAE/100 g FW] and [87.7–189.6 mg GAE/100 g of FW] in pulp and peel of peach fruits, respectively. Throughout the analysis we observed that the total phenolic content in peel of the fruit was significantly higher than the pulp. Saidani et al. (2017) reported that phenolic compounds varied between tissues of the same fruit and are mostly concentrated in the epidermal part. The peel and pulp of the fruit showed a gradual decrease of the total phenolic content during storage. Our findings are in accordance with the study of Tsantili et al. (2010) reporting significant changes in total phenolics in peach fruit stored at 5 °C for 2 or 4 weeks. Hence, total phenolics content decreased greatly, as observed in long-stored fruit immediately after store removal.

Vitamin C content

The analysis of vitamin C in the peel and pulp extracts among the two cold storage periods are shown in Table 2. The vitamin C at harvest ranged from 2.70 to 4.80 mg AsA/100 g of FW in peel extract and from 2.1 to 3.5 mg AsA/100 g of FW in pulp extract. Our results are in the same range previously described by several studies (Abidi et al. 2015; Saidani et al. 2017). The content of ascorbic acid dropped notably during the cold storage in both peel and pulp in all the studied cultivars as this antioxidant compound is highly oxidizable. Our findings are in line with the study of Khan et al. (2018) reporting that the level of ascorbic acid in the peach cultivars showed decreasing trend as ripening period progressed. In this line, Zhao et al. (2018) reported that the ascorbic acid content in peach, nectarine and apricot fruits decreased during the cold storage periods.

Relative antioxidant capacity (RAC)

The antioxidant capacity of the peel and pulp of peaches fruits was assessed by DPPH and FRAP assays to validate the results (Table2). The RAC values were higher in peel extracts than in pulp and showed a gradual decrease during the two cold storage periods. The observed RAC values are in accordance with the study of Stojanovic et al. (2016) showing that peel extracts from all peach cultivars exhibited appreciably higher scavenging activities compared to the pulp extracts. Rossato et al. (2009) reported that the phenolic content and antioxidant activity varied among cultivars and among different fruits within the same cultivar depending on the stage of maturity, agricultural practices and environmental conditions. Changes in the antioxidant capacity in peel and pulp of peach fruits were consistent with changes in flavonoid, total phenolics and vitamin C content. The observed values of RAC under the DPPH and FRAP assays showed that the DPPH scavenging ability was stronger than FRAP in both peel and pulp of the fruit which may be related to the specific antioxidant compounds in peach fruit.

Chilling injury symptoms

Fruit phenotypes following the chilling injury symptoms are presented in Fig. 3. Fruits showed continuous decrease of the appearance with progression of storage period. The analysis and quantification of chilling injury symptoms observed in the studied cultivars are showed in Fig. 4. After 2 weeks of cold storage, CI symptoms were observed with minor proportion. Hence, flesh mealiness varied from 10% in Plagold 5 to 25% in Plawhite 10 (Fig. 4a) whereas the graininess varied from 5% in Plagold 5 to 15% in Plawhite 10 (Fig. 4b). The bleeding rating was observed in the cv. Plagold 10 with proportion lower than 50% of flesh fruit (Fig. 4c). Lurie and Crisosto (2005) reported that the red flesh color observed could be due to the characteristic pigmentation of fruit flesh or a result of fruit senescence process and not a chilling injury symptom. Tsaltani et al. (2010) reported that bleeding seemed to be a result of the combination of low temperature storage prior to ripening time at elevated temperature. The browning rating was observed mainly in the cultivars Plawhite 5 and Plawhite 10 (Fig. 4 d). Abidi et al. (2015) reported that the internal browning is generally related to the increased activities of polyphenoloxidade and peroxidase that provoke oxidation of phenolic compounds. The duration of cold storage (2 or 4 weeks) increased the severity of mealiness, bleeding and browning in the flesh fruit. Hence, after 4 weeks of cold storage, the analysis of the chilling injury symptoms revealed that the cultivars Plawhite 5 and Plawhite 10 showed high proportion of mealiness (50 and 55%, respectively) and high browning rate (5 and 4, respectively). The fruit external and internal qualities were negatively affected by the CI symptoms. The Plawhite 10 was greatly affected by the chilling injury symptoms (CI = 5) followed by the cv. Plawhite 5 (CI = 4). The two cultivars Plagold 5 and Plagold 10 showed significantly (P < 0.05) low mealiness and browning. Bustamante et al. (2016) reported that one of the principal symptoms of CI in peach is flesh mealiness, which is the consequence of altered cell wall metabolism. In the same line, Brummell et al. (2004) reported that the degree of mealiness in melting flesh varieties increased substantially with increased cold storage time. Our results are in accordance with D’Aquino et al. (2019) reporting that the peach appearance changed slightly during 2 weeks of storage at 5 °C, while worsened at a faster rate after 4 weeks of storage.

Fig. 3.

Appearance of chilling injury symptoms in the studied cultivars during the two cold storage periods (2 and 4 weeks). (a): cv.Plawhite 5; (b): cv.Plawhite 10; (c): cv.Plagold 5; (d): cv.Plagold 10; (e): Plawhite 5 after 2 weeks of cold storage; (f): Plawhite 10 after 2 weeks of cold storage; (g): Plagold 5 after 2 weeks of cold storage; (h): Plagold 10 after 2 weeks of cold storage; (i): Plawhite 5 after 4 weeks of cold storage; (j): Plawhite 10 after 4 weeks of cold storage; (k): Plagold 5 after 4 weeks of cold storage; (l): Plagold 10 after 2 weeks of cold storage

Fig. 4.

Distribution of chilling injury symptoms in peach cultivars after storage at 5 ºC for 2 and 4 weeks and then ripening for 2 days at room temperature. Mealiness was scored as proportion of mealy fruits in the sample. Bleeding was scored on a scale of 1 (no bleeding) to 3 (more than 50% of the flesh with bleeding). Browning was scored on a scale of 1 (no browning) to 6 (severe browning). Values are the means of 20 peach fruits (n = 20) ± SE. Different letters a, b, c, indicate significant difference among storage periods in each cultivar (ns, not significant, *P < 0.05, **P < 0.01) according to Duncan’s multiple range test

Principal Components Analysis (PCA)

The principal component analysis was conducted to evaluate the separation of cultivars based on agronomical traits, antioxidant compounds and chilling injury symptoms after 4 weeks of cold storage as shown in Fig. 5. The principal components 1 and 2 (PC1 and PC2) accounted for 44.21 and 33.25% of total variance, respectively. The PCA revealed interesting grouping (Fig. 4A) indicating that in this case, the genotype seems to be relevant in modulating the agronomical and biochemical status. PC1 was able to separate the cultivars Plagold 5 and Plagold 10 from Plawhite 5 and Plawhite 10 cultivars. Among the traits that most contribute to PC1 separation were SSC, firmness, fruit size and browning. Meanwhile the most important traits leading to the PC2 separation were flavonoids content in both peel and pulp of the fruit, vitamin C in pulp and bleeding. The two cultivars Plawhite 5 and Plawhite 10 had positive values of PC1 while the Plagold 5 and Plagold 10 cultivars had negative values. The PCA analysis showed that Plawhite 5 and Plawhite 10 had higher values of fruit size and SSC but also higher sensibility to browning than Plagold 5 and Plagold 10. In the other hand, Plawhite 10 had positive values in PC2 due to the highest values of flavonoids both in peel and pulp and lowest vitamin C content. While Plagold 10 showed average values for these fruit quality traits. The PCA analysis showed that Plawhite 5 and Plagold 5 cultivars had lowest values of flavonoids both in peel and pulp and highest concentrations of vitamin C. The grouping of peach cultivars by PCA clearly permits to discriminate the cultivars Plawhite 5 and Plawhite 10 due to the significantly higher mealiness, browning and CI index. The identification of the molecular basis of the response of the studied cultivars to cold storage is a future challenge, since it could aid in defining strategies for the improvement of the organoleptic quality of peach fruits while fruits are stored at low temperatures (Bustamante et al. 2016).

Fig. 5.

Principal component analysis of main agronomical traits, bioactive compounds and chilling injury symptoms in peach cultivars. Chilling injury symptoms analyzed after 4 weeks of cold storage at 5 ºC and 95% HR then ripening at room temperature for 2 days. Abbreviations: FW: fruit weight, RAC: relative antioxidant capacity, SSC: soluble solids content, TA: titratable acidity

Conclusion

This work showed the differential effects of cold storage on agronomical and biochemical fruit quality traits over two cold storage periods. The studied cultivars presented similar behavior regarding the agronomical traits among storage periods and years of study. Hence, the significant changes in the agronomical traits during the cold storage period were a gradual decrease in flesh firmness and titratable acidity whereas the SSC increased. The analysis of the antioxidant compounds in peel and flesh fruits showed higher variability between cultivars and storage periods. Peel extract presented higher phenolic compounds content compared to flesh fruit at harvest and during cold storage periods. Phenolic compounds decreased during cold storage periods except for anthocyanins which exhibited different pattern. After two weeks of cold storage the cultivar Plawhite 5 and Plawhite 10 developed mealiness and flesh browning. After 4 weeks of cold storage the postharvest fruit quality was affected regardless of the genetic capability against chilling injury. The Plagold 5 and Plagold 10 cultivars maintend their firmness and developed less symptoms of chilling injury as compared to the Plawhite cultivars presenting the suitability of these cultivars to transport and handling during fruit shipment.

Abbreviations

AsA

Ascorbic acid; C3GE: Cyanidin-3-glucoside equivalents

CE

Catechin equivalents

FRAP

Ferric reducing antioxidant power

FW

Fresh weight

GAE

Gallic acid equivalents

N

Newton

RAC

Relative antioxidant capacity

RI

Ripening index

SSC

Soluble solids content

TA

Titratable acidity

TE

Trolox equivalent

Authors’ contributions

Walid Abidi conceived the research, performed chilling injury analysis. Rawaa Akrimi performed biochemical analysis. The two authors wrote and approved the manuscript.

Funding

This work was supported by the Regional Agriculture Center of Sidi Bouzid.

Data availability

All data generated or analysed during this study are included in this published article.

Code availability

Not applicable.

Declarations

Conflict of Interest

The authors declare that they have no conflict of interest.