Effects of Foliar Phosphorus Application at Harvest and Postharvest in Sweet Cherry (Prunus avium L.; cv. Regina) Produced in Southern Chile

Abstract

Southern Chile has become a prominent region for sweet cherry production. However, environmental constraints and low P availability can adversely affect fruit quality and conditions in southern Chile. Therefore, the aim of this study was to evaluate the effects of foliar phosphorus (P) on fruit quality, condition, and antioxidant content at harvest and postharvest storage in sweet cherry (Prunus avium L.) cv. Regina was cultivated under a plastic cover in Southern Chile. For this, sweet cherry trees were subjected to three treatments: control (no P), 1.5 L ha⁻¹, and 2.2 L ha⁻¹ foliar P. In our study, no significant effects were observed on average fruit weight, size, or total soluble solids among P treatments. However, P applications increased the proportion of large fruit (>32 mm), enhanced dark mahogany coloration, and pulp antioxidant content (total phenols and anthocyanins). Interestingly, the 2.2 L ha⁻¹ treatment reduced postharvest disorders, including pitting (70%), dehydration (31%), orange peel (56%), and internal browning (29%) compared to the control trees. These results suggest that foliar P application could be an agronomic tool to improve fruit quality and condition in sweet cherry production under plastic covers cultivated in soils with low P availability.

1. Introduction

Sweet cherry (Prunus avium L.) is an important fruit crop in temperate and Mediterranean climates [1]. The main sweet cherry-producing countries are Turkey, the United States of America, and Chile [1,2]. Interestingly, Chile has increased its sweet cherry production area by 351% in the last 10 years, reaching 76,000 hectares, with more than 625,000 tons of fresh fruit exported during the 2024/2025 season [3]. In this context, southern Chile has emerged as a key production zone for sweet cherry, reaching 3188 ha between La Araucanía and Los Lagos regions (37°35′–40°33′ S) [3,4]. This area is mainly cultivated with late-season cultivars, which are mainly exported to the Chinese market [5], such as Regina. Currently, sweet cherry production is a competitive industry prioritizing higher standards in fruit quality parameters such as firmness, caliber, attractive color, and sweet fruit, and with minimal condition defects, especially in the Chinese market [6,7]. This commercial context has driven producers to optimize preharvest and postharvest practices to ensure marketable fruit quality. Nonetheless, sweet cherry production in southern Chile faces soil and environmental constraints. At the soil level, P available ranges from 7 to 15 mg kg⁻¹ in the Andisols of Southern Chile [8]. However, the appropriate P available for sweet cherry production ranged from 15 to 25 mg kg⁻¹ soil [9]. Therefore, Southern Chile has low P available in the soil for sweet cherry production.

On the other hand, late-spring frosts and rainfall during fruit growth are the main environmental constraints, which significantly compromise sweet cherry fruit quality and condition at harvest and postharvest [10,11]. To mitigate the environmental challenges, several agronomical strategies have been implemented, including the selection of tolerant cultivars (e.g., ‘Regina’), the application of phytohormones, as well as macro- and micronutrients, and the use of protective plastic covers [12,13,14,15]. Although the use of plastic covers has been used as an effective strategy for reducing fruit cracking of sweet cherries, several studies have reported that they may also negatively impact key fruit quality parameters, such as firmness, total soluble solids and titratable acidity [16,17,18]. Thus, new agronomical strategies are needed to improve fruit quality and condition in sweet cherry production under plastic covers growing in soils with low P available.

Phosphorus (P) is an important macronutrient that regulates several physiological and biochemical processes in plants, including membrane integrity, signal transduction, photosynthesis, plant growth, and yield [19,20]. In fruit crops, it has been reported that foliar P applications improve fruit growth, quality, and yield [21,22,23]. However, no information is available on the role of foliar P on fruit quality in sweet cherries cultivated under protective plastic covers in soils with low P available. Thus, we hypothesized that foliar P application improves fruit quality and conditions in sweet cherry (Prunus avium L.) cultivated under plastic cover in Southern Chile. Therefore, the aim of this study was to evaluate the effects of foliar phosphorus (P) on fruit quality, condition, and antioxidant content at harvest and postharvest storage in sweet cherry (Prunus avium L.) cv. Regina was cultivated under a plastic cover in Southern Chile.

2. Materials and Methods

2.1. Experimental Site and Meteorological Measurements

The field experiment was conducted at the San José commercial sweet cherry orchard located in La Unión, Los Ríos Region, Chile (40°17′06″ S; 72°57′38″ W), during the 2023/2024 season (Figure 1). Sweet cherry cv. Regina/Gisela 6 rootstock plants were established at 4.5 × 2.0 m (1111 trees ha⁻¹) planting design (North-South orientation) during the 2014 season. Agronomic practices of the sweet cherry orchard, such as pruning, irrigation, pest control, and fertilization, were performed by Ranco Cherries following the technical recommendations of the fruit export industry. The climate of the site is a Mediterranean template with warm summers [24]. The soil texture analysis showed a silt loam surface, while nutrient analysis showed an organic matter content of 8.2%, a pH of 6.59, N of 19.2 mg kg⁻¹, P of 10 mg kg⁻¹, and K of 386.1 mg kg⁻¹. The sweet cherry orchard was covered with a high-density polyethylene (HDPE) plastic (4 m width, a density of 160 g m⁻², 88–90% of total light transmission, 60–65% of diffuse light transmission, and >95% reflectance). The plastic cover was deployed from September 2023 to February 2024, during the fruit-growing period. Weather data were obtained from La Unión Automatic Weather Station (AWS) (https://agrometeorologia.cl, accessed on 15 June 2025). The weather data are shown in Figure 2. During the experiment, the maximum temperature ranged between 32 and 35 °C, mainly during February (Figure 2). Meanwhile, the average minimum temperature was 6.3 °C, and the lowest was 0 °C between September and December. On the other hand, the seasonal rainfall reached 289 mm (Figure 2).

2.2. Treatments

Sweet cherry (P. avium) cv. Regina trees grown under field conditions were subjected to three foliar P treatments: (1) no foliar P application (control), (2) two foliar P applications of 1.5 L ha⁻¹, and (3) three foliar P applications of 2.2 L ha⁻¹ in a water volume of 1000 L ha⁻¹. Control (no foliar P) plants were sprayed only with water. The commercial product was Peloton^® (44% P₂O₅) from FMC^®. The evaluated P doses are recommended by the commercial product. Foliar P applications were performed from the fruit color change on 27 December 2023, 3 January 2024, and 8 January 2024. The application was carried out early in the morning using a backpack spray pump (Cifarelli^®, Model L3EDA, Voghera, Italy) with an output of 5 L min⁻¹ (Figure 3). When ripe, fruit was harvested per tree, cold-stored in a portable refrigerator, and transferred to the Fruit Crops Physiology and Quality Laboratory of Universidad de La Frontera (Temuco, Chile) for fruit quality and antioxidant-related determinations. For antioxidant-related determinations, fruit were dipped in liquid nitrogen and stored at −80 °C. A total of 300 fruit per tree were harvested, considering 150 fruit from the upper zone (h > 1.2 m) and 150 fruit from the lower zone (h < 1.2 m) of the canopy.

2.3. Fruit Quality and Condition Analysis

For fruit quality parameters, a total of 35 fruits per sample was used to determine fruit fresh weight, caliber, total soluble solids, titratable acidity, and maturity index (MI). Fruit fresh weight was determined using a precision balance (Model BA2204B, Biobase Meihua Trading, Jinan, China). Caliber and firmness were determined using a texture meter (FirmPro, Happyvolt, Santiago, Chile) [16]. Fruit color was determined using an export scale as a reference (ASOEX, Las Condes, Chile) [25]. For total soluble solids (TSS) and titratable acidity (TA), a thermos-compensated digital refractometer (ATAGO, Mod. PAL-BX I ACID F5, Saitama, Japan) and an automatic titrator (HANNA Mod. HI-84532, Woonsocket, RI, USA), respectively, were used. The maturity index (MI) was determined by the TSS and TA ratio. Fruit condition was visually determined by a trainer panel, considering fruit cracking, pitting, russet, and browning pedicels.

2.4. Determination of Antioxidant-Related Parameters in Fruit

As antioxidant-related parameters, antioxidant activity (AA), total phenols (TP), and total anthocyanins (TA) were determined in pulp and skin of sweet cherry fruit. For these determinations, a spectrophotometer (Genesys 10S UV-VIS) was used. For AA and TP, 0.15 g of each tissue sample was macerated with ethanol (80% v/v) and centrifuged at 13,000 rpm for 10 min at 4 °C. The AA was determined using the protocol described by Chinnici et al. [26], where the stable free radical 2,2-diphenyl-1-picryl-hydrazyl (DPPH) is used. The absorbance was determined at 515 nm. The Folin–Ciocalteau method [27] was used to determine total phenols. The absorbance was measured at 765 nm, using gallic acid as a standard. The AA and TP results were expressed as mg of Trolox equivalents per gram of fresh weight (mg TE g⁻¹ FW) and as mg of gallic acid equivalents per gram of fresh weight (mg GAE g⁻¹ FW), respectively. For total anthocyanins, the pH differential method was used according to Strack and Wray [28]. For this, pulp and skin (× g) were separately macerated with acidified ethanol (pH 1.0) and shaken overnight at 4 °C. The absorbance was determined at 530 and 675 nm spectrophotometrically. The results are expressed as mg of cyaniding 3-O-glycoside equivalents per gram of fresh weight (mg C3G g⁻¹ FW).

2.5. Postharvest Storage Conditions and Fruit Quality Assessment

To evaluate fruit quality and condition during postharvest storage, sweet cherry fruit were pre-cooled using a hydrocooler until pulp temperature reached approximately 3–4 °C and were then kept in a cold room for 24 h. Subsequently, the fruit were immersed in a chlorinated solution and manually packed in transparent plastic clamshell containers with a capacity of 1 kg. The clamshells were subjected to forced-air cooling (Californian-type tunnel) to reduce fruit temperature to 0 °C. Finally, fruits were stored for 35 days at 0 °C and 90% relative humidity, controlled by a humidification system in the cold room. Before conducting the measurements, the fruit were acclimatized for 16 h at 20 °C. Fruit weight, firmness, fruit diameter, total soluble solids (TSS), titratable acidity (TA), and maturity index (MI) were determined to assess fruit quality. In addition, the incidence of pedicel browning, pitting, orange peel, and shriveled fruit was visually estimated as indicators of fruit condition. The appearance and severity of internal browning (IB) were assessed following the methodology described by Youryon et al. [29]. A panel of four trained evaluators performed a hedonic evaluation using a five-point scale to score the fruit tissue as follows: 0 = no symptoms; 1 = small translucent spots turning brown (≤5% of the surface area); 2 = approximately 10% of the surface area showing IB symptoms; 3 = approximately 20%; 4 = approximately 30%; and 5 = more than 30% of the pulp surface area exhibiting IB symptoms. The Browning Index (BI) was calculated using Formula (1):

BI = Σ (browning score × percentage of fruit within each class).

(1)

2.6. Experimental Design and Statistical Analysis

The experiment was performed in a randomized block design with three treatments and four repetitions per treatment, each corresponding to a row of four consecutive trees, where the two central trees were evaluated. The Shapiro–Wilk and Levene tests were used to test the normality and homoscedasticity of the data. Then, an analysis of variance (ANOVA) was used to analyze the data, followed by Fisher’s LSD multiple comparison tests (p ≤ 0.05). Infostat^® Software Version 2017 was used to perform statistical analyses. The dataset was subjected to principal component analysis (PCA), preserving as much statistical information as possible. The analysis was carried out using R software version R 4.3.1 (R Core Team, Statistical computing, Vienna, Austria, 2023).

3. Results

3.1. Fruit Physical Quality

Our results revealed no significant differences in fruit quality parameters among treatments in upper and lower canopy zones, with the exception of fruit firmness (

Table 1. Fruit quality parameters at harvest of sweet cherry (P. avium) cv. Regina subjected to foliar P treatments.

Treatment		Control	1.5 L ha−1	2.2 L ha−1	p-Value
Fresh weight (g)	Upper	14.8 ± 0.6 a	13.6 ± 0.3 a	14.2 ± 0.2 a	0.23
	Lower	14.0 ± 0.7 a	13.1 ± 0.3 a	13.8 ± 0.4 a	0.52
Caliber (mm)	Upper	32.0 ± 0.1 a	31.7 ± 0.1 a	31.4 ± 0.0 a	0.15
	Lower	31.6 ± 0.2 a	30.7 ± 0.1 a	31.2 ± 0.1 a	0.24
Firmness (g mm−1)	Upper	323.8 ± 9.3 a	321.6 ± 8.1 a	306.0 ± 8.0 a	0.18
	Lower	340.0 ± 4.7 a	331.2 ± 6.0 a	293.2 ± 9.1 b	0.01

Different lowercase letters indicate significant differences among the P treatments for the same canopy zone. The value represents the mean ± SE (n = 100). Upper and lower indicate the canopy zone of each tree, calculated per treatment.

). In our study, fruit weight ranged from 13.6 to 14.8 g in the upper canopy zone and between 13.1 and 14.0 g in the lower canopy zone. Fruit caliber remained consistent across treatments, with a range between 31.4 and 32.0 mm in the upper canopy zone, while in the lower canopy it ranged from 30.7 to 31.6 mm, with no significant differences among treatments. However, a decreasing fruit caliber trend with an increasing P dose was observed in both canopy zones, particularly in the lower canopy zone. We observed that 2.2 L ha⁻¹ treatment slightly decreased fruit firmness in upper (5.5%) and lower (13.7%) canopy zones compared to the control trees (Table 1).

Regarding fruit size distribution, we observed a higher proportion of large fruit (>32 mm) with the 2.2 L ha⁻¹ foliar P treatment, followed by the control and the 1.5 L ha⁻¹ treatment in the upper canopy zone (

Table 2. Fruit size distribution at harvest of sweet cherry (P. avium) cv. Regina subjected to three P treatments.

Canopy	Foliar P	Fruit Size Distribution (%)
Zone	Treatment	28 mm	30 mm	32 mm	>32 mm
Upper	Control	9.28 ± 0.56 b	21.78 ± 1.02 a	23.22 ± 0.92 b	43.40 ± 2.0 b
	1.5 L ha−1	12.86 ± 0.70 a	23.92 ± 1.02 a	37.86 ± 0.72 a	37.86 ± 1.84 b
	2.2 L ha−1	8.40 ± 0.48 b	20.18 ± 0.96 a	16.60 ± 0.94 c	48.90 ± 2.04 a
	p-value	0.0029	0.18	0.0018	0.0016
Lower	Control	5.54 ± 0.64 b	23.76 ± 0.92 b	22.50 ± 0.90 a	46.62 ± 1.74 a
	1.5 L ha−1	11.60 ± 0.72 a	24.10 ± 1.04 b	16.26 ± 0.72 b	43.40 ± 2.44 b
	2.2 L ha−1	13.22 ± 0.66 a	31.08 ± 1.18 a	15.72 ± 0.54 b	31.96 ± 1.70 c
	p-value	0.011	0.023	0.014	0.028

Different lowercase letters indicate significant differences among the P treatments for the same canopy zone. The value represents the mean ± SE (n = 100). Upper and lower indicate the canopy zone of each tree, calculated per treatment.

). Otherwise, our results revealed that foliar P treatment at 1.5 L ha⁻¹ significantly increased 38% the 32 mm fruit compared to the control. For 28 mm sizes, 1.5 L ha⁻¹ treatment showed an increase of 38.5% compared to control trees (Table 2). Meanwhile, no differences were found in 30 mm among treatments. In the lower canopy, the 2.2 L ha⁻¹ treatment showed the highest percentages of 28 mm and 30 mm fruit, but a reduced percentage of fruit >32 mm. By contrast, control treatment had the highest proportion of >32 mm fruit (46.62%), followed by the 1.5 L ha⁻¹ treatment (43.40%).

In our study, the P application increased the fruit in the Dark Mahogany category in the upper canopy zone, where the 2.2 L ha⁻¹ P treatment exhibited 20% higher levels compared to the control treatment, while the 1.5 L ha⁻¹ treatment was 12% greater than the control treatment (

Table 3. Fruit color distribution at harvest of sweet cherry (P. avium) cv. Regina subjected to three P treatments.

Canopy	Foliar P	Fruit Color Distribution (%)
Zone	Treatment	Red	Mahogany Red	Dark Mahogany
Upper	Control	20.18 ± 0.82 a	43.92 ± 1.48 a	35.90 ± 1.30 b
	1.5 L ha−1	18.40 ± 0.66 a	41.26 ± 1.24 a	40.36 ± 1.34 a
	2.2 L ha−1	18.04 ± 0.64 a	38.76 ± 1.42 a	43.22 ± 1.62 a
	p-value	0.104	0.254	0.0012
Lower	Control	24.82 ± 0.88 a	47.50 ± 1.56 a	27.68 ± 1.06 c
	1.5 L ha−1	16.26 ± 0.60 b	37.32 ± 1.26 b	46.42 ± 1.78 a
	2.2 L ha−1	22.86 ± 0.58 a	41.78 ± 1.32 a	35.54 ± 1.18 b
	p-value	0.002	0.001	0.0029

Different lowercase letters indicate significant differences among the P treatments for the same canopy zone. The value represents the mean ± SE (n = 100). Upper and lower indicate the canopy zone of each tree, calculated per treatment.

). Meanwhile, no differences were observed in the Red and Mahogany Red categories among treatments in the upper canopy zone. In the lower canopy zone, the 1.5 L ha⁻¹ treatment exhibited the highest percentage of fruit in the Dark Mahogany category (46.42%), followed by the 2.2 L ha⁻¹ treatment and control trees (Table 3).

3.2. Fruit Chemical Quality and Condition in P. avium Fruit

Our results revealed no significant differences in total soluble solids (TSS), titratable acidity, and maturity index among treatments in both canopy zones (Figure S1). In our study, cracking and russet defects were slightly reduced in the 1.5 L ha⁻¹ treatment (1.0% and 1.0%, respectively) compared to the control and 2.2 L ha⁻¹ treatment in the upper canopy zone (Figure 4). Meanwhile, pitting was the most prevalent defect in all treatments in the upper canopy, reaching its highest value in the 2.2 L ha⁻¹ treatment (15.5%) and its lowest level in the control (11.14%). Browning pedicels remained relatively stable among treatments in the upper canopy. In the lower canopy zone, pitting was also the predominant defect and increased with P doses, from 10.00% in the control to 14.88% in the 2.2 L ha⁻¹ treatment. Likewise, 2.2 L ha⁻¹ treatment resulted in the highest incidence of browning pedicels (Figure 4).

3.3. Antioxidant-Related Parameters in Sweet Cherry Fruit

Our results revealed that total phenols (TP) increased progressively in pulp with foliar P dose applications (Figure 5A). Thus, 2.2 L ha⁻¹ treatment exhibited the highest TP values among treatments in the upper canopy, which were 2-fold greater compared to control trees. Likewise, TP increased 2.1-fold in 2.2 L ha⁻¹ treatment compared to control trees in the lower canopy. A similar pattern was observed for total anthocyanins (TA) in the pulp, increasing 33.2% in the upper and 36% in the lower canopy zones compared to control plants (Figure 5E). Meanwhile, antioxidant activity (AA) in the pulp remained stable among treatments for both canopy zones, with values between 2.62 and 2.73 µM TE g⁻¹ FW (Figure 5C). By contrast, skin tissue displayed the opposite trend in TP in both canopy zones. Thus, TP decreased 42.9% in the upper canopy and 42.2% in the lower canopy compared to control trees (Figure 5B). On the other hand, TA showed a tendency to decrease with no significant differences among foliar P treatments (Figure 5F). Otherwise, AA showed no significant differences among treatments in both canopy zones (Figure 5D).

3.4. Fruit Quality and Conditions at Postharvest in Sweet Cherry cv. Regina

In our study, no statistically significant differences (p > 0.05) were observed for any of the evaluated fruit quality parameters, such as fresh weight, caliber, firmness, TSS, TA, and MI in sweet cherry at 35 days of post-harvest storage among P treatments (Table S1). However, our results revealed significant differences among P treatments in fruit condition after 35 days of storage (Figure 6). Thus, pitting incidence showed a clear decreasing trend with increasing P doses. The control treatment had the highest pitting value (0.71%), followed by the 1.5 L ha⁻¹ treatment (0.63%), while the 2.2 L ha⁻¹ treatment showed a significant reduction to 0.21%, showing a reduction of 70% compared to control trees. On the other hand, the dehydration condition was also significantly reduced by P application, decreasing 31% in the 2.2 L ha⁻¹ treatment compared to the control plants. Likewise, orange peel disorder was decreased in the 2.2 L ha⁻¹ treatment, exhibiting a 56% reduction compared to the control treatment. Interestingly, internal browning also decreased notably with P treatments, showing a reduction of 51% in 1.5 L ha⁻¹ and 29% in 2.2 L ha⁻¹ treatments in comparison with the control treatment. By contrast, brown pedicel incidence was unaffected by P treatments (Figure 6).

3.5. Principal Component Analysis (PCA) in Response to Foliar Phosphorus Treatment

In our study, PCA allowed for multivariate integration of fruit quality and condition parameters, differentiating samples according to foliar P treatments and position in the canopy (Figure 7). The PC1 explained the largest proportion of variability (24.9%) and was primarily associated with skin color (Red, Mahogany Red, and Dark Mahogany), phenols and anthocyanins in the pulp, and physical quality (size, fresh weight, and firmness). The PC2 explained a smaller proportion (15.4%) and was related to titratable acidity, firmness, and antioxidants in the skin. The biplot showed a clear separation of treatments, where P applications (1.5 and 2.2 L ha⁻¹) were positively associated with larger-sized fruits, darker color, and higher phenolic content in the pulp, while the control group tended to cluster with smaller-sized fruits, a higher proportion of defects, and lower color intensity. Likewise, the position in the canopy partially modulated this response: in the upper zone, the fruits were more strongly associated with intense colors and higher levels of phenols, while in the lower zone, there was greater dispersion and a tendency toward smaller sizes.

4. Discussion

Currently, Southern Chile (37°35′–40°33′ S) has become an important sweet cherry production area, reaching 3188 ha. However, low soil P availability and environmental constraints negatively affect sweet cherry quality and condition in southern Chile [8,30]. Therefore, new strategies are needed to improve fruit quality and condition in sweet cherry production under plastic covers in low soil P availability. Thus, we evaluated the effects of foliar phosphorus (P) applications on fruit quality, condition, and antioxidant-related parameters at harvest and postharvest in sweet cherry (cv. Regina) grown under plastic cover. It has been reported that P plays a key role in improving fruit quality and yield in fruit crop species [31]. However, in our study, no significant changes were observed in key fruit quality traits such as fruit weight, caliber, total soluble solids (TSS), and titratable acidity (TA). Our results agree with Schreiner [32], who showed that three applications of foliar P during fruit growth had no effects on fruit quality and yield in Vitis vinifera plants. Likewise, Khalid et al. [33] showed no significant changes in fruit caliber and fruit weight in Citrus nobilis plants under 250 mL of foliar P application applied once during the season. However, the authors reported that foliar P application increased total soluble solids (TSS). By contrast, Dang et al. [22] reported that fruit growth and TSS were increased in Citrus grandis plants subjected to 0.6 L per tree of foliar P application, which was applied once every two months during fruit growth. According to Nartvaranant et al. [34], P application can contribute to improving photosynthesis performance, increasing carbohydrate accumulation in fruit, resulting in higher TSS, fruit growth, and yield. Therefore, the effects of foliar P on fruit quality appear to be influenced by multiple factors, including the fruit species, application rate, timing, and the specific developmental stage at which it is applied. Although no differences were observed in fruit caliber as average considering all fruit by each treatment, foliar P treatments positively affected fruit size distribution, color, and conditions.

Thus, we observed that the 2.2 L ha⁻¹ foliar P application increased the proportion of large fruit (>32 mm) and the 1.5 L ha⁻¹ treatment increased the proportion of 32 mm fruit compared to the control treatment in the upper canopy zone. Interestingly, our results revealed that 1.5 L ha⁻¹ and 2.2 L ha⁻¹ foliar P treatments enhanced fruit color, increasing the proportion of fruit in Dark Mahogany and Red Mahogany categories in the upper canopy zone. This response could be attributed to an indirect stimulation of total phenols and anthocyanin biosynthesis pathways, which have been associated with secondary metabolite production [35]. In this context, we observed that foliar P treatments improved total phenols and anthocyanins in pulp in upper and lower canopy zones.

Similar enhancements in color development by foliar P have been reported by Stampar et al. [36], who showed that foliar P application improved fruit color in Malus domestica. The authors showed that P treatment increased anthocyanin and flavonoid levels in M. domestica fruit. Thus, Chen et al. [37] reported that flavonoids and anthocyanins are responsible for the color in sweet cherry fruit. According to Li et al. [35], P interacts with phenols and anthocyanin endogenous biosynthesis pathways, increasing phenolic compounds levels. Skin coloration is a key market parameter for consumers, especially for the Asian market, which favors darker and fully pigmented fruit [38].

Our results also revealed that cracking and russet defects were reduced in the 1.5 L ha⁻¹ treatment of P at harvest. Fruit condition parameters are critical indicators of postharvest performance, especially where sweet cherries travel between 20 and 35 days in marine containers [39,40]. Our results showed that foliar P application reduced the incidence of postharvest disorders such as pitting, dehydration, orange peel, and internal browning. The most significant reduction was observed for pitting (70% decrease in 2.2 L ha⁻¹ treatment), followed by orange peel (56%) and internal browning (up to 51%). On the other hand, no significant differences among treatments were observed in fruit weight, caliber, firmness, TSS, and TA. Thus, the lack of significant improvement on these fruit quality parameters might be explained by the fact that foliar P alone may not be sufficient to influence these parameters under plastic covers, which can impose microclimatic conditions that reduce sugar accumulation and firmness [12,16].

Nevertheless, the improvement in postharvest conditions, such as pitting, dehydration, orange peel, and internal browning, represents an important contribution to sweet cherry management, especially in regions facing climatic constraints like Southern Chile [10,11]. Thus, foliar P application could be an interesting tool for improving fruit parameters at harvest and postharvest, such as greater caliber, color, and condition, considering that sweet cherry production is a competitive industry, prioritizing higher standards in fruit quality parameters such as caliber, attractive color, and fruit condition [6,7]. In our study, the PCA showed that P applications increased the proportion of fruit in the Mahogany Red and Dark Mahogany categories. This response is associated with the stimulation of anthocyanin and phenol biosynthetic pathways, as we discussed in M. domestica and P. avium. Our PCA confirms that, although P treatments did not significantly modify fruit quality parameters (physical and chemical parameters), they improved key market attributes: color, antioxidants, and postharvest condition. This reinforces the conclusion that foliar P fertilization is a useful tool for optimizing the commercial quality of cherries under plastic covers and in soils with low available P.

5. Conclusions

Our study showed that foliar P treatments did not significantly affect fruit quality parameters such as fresh weight, caliber, and total soluble solids. However, P treatments positively influenced fruit size distribution by increasing the proportion of larger fruit (>32 mm) and enhancing skin coloration, particularly in the Dark Mahogany category. On the other hand, 2.2 L ha⁻¹ P treatment significantly improved the antioxidant levels of fruit by increasing total phenols and anthocyanin levels in the pulp. At postharvest storage, foliar P treatments reduced the incidence of physiological disorders such as pitting, dehydration, orange peel, and internal browning, indicating better fruit condition. These findings suggest that foliar P treatments could be an important agronomic strategy to enhance commercial sweet cherry under protective plastic covers cultivated in soils with low P availability, like Southern Chile. However, further studies are needed to elucidate the biochemical and molecular mechanisms by which phosphorus influences fruit skin color, antioxidant level, and reduces the incidence of physiological disorders in sweet cherry grown under plastic covers.