Enhancing Sweet Cherry Quality Through Calcium and Ascophyllum nodosum Foliar Applications

Abstract

Climate change significantly impacts fruit production and yield, affecting its commercial value. Foliar fertilization emerges as a fast and targeted strategy to address crop nutrient deficiencies and enhance fruit quality. Sweet cherry is among the most highly valued and widely appreciated fruit crops globally. This study was conducted over two consecutive years on the sweet cherry cv. Sweetheart. Calcium (300 g hL⁻¹ and 150 g hL⁻¹) and a seaweed-based biostimulant (150 mL hL⁻¹ and 75 mL hL⁻¹), as well as a combination of both nutrients (300 g hL⁻¹ calcium and 150 mL hL⁻¹ seaweed), in addition to a control treatment (water), were applied at the foliar level to improve sweet cherry quality. To assess cherry quality, including biometric, chromatic, texture, and biochemical parameters, as well as the sensory analysis, fruits from each treatment were harvested at the commercial maturity stage. Calcium treatments improved fruit size, total soluble solids, and firmness, while also delaying fruit ripening by increasing titratable acidity. The seaweed-based biostimulant enhanced fruit size, promoted color development, and accelerated ripening. Together, these findings highlight the crucial role of calcium in improving sweet cherry quality and underscore seaweed-based biostimulants as a promising and sustainable strategy for enhancing fruit quality. Although cherry quality is highly affected by environmental conditions, this study demonstrated that calcium fertilization, either alone or in combination with seaweed, enhances sweet cherry quality attributes, making it a suitable strategy for application in commercial orchards and for the global improvement of sweet cherry production.

1. Introduction

Portugal is one of the key producers of flesh fruit crops in Southern Europe, including sweet cherries [1,2]. The country’s Mediterranean climate, characterized by cold winters and dry and hot summers, provides optimal conditions for cherry cultivation, ensuring high fruit quality and yield [3,4].

Sweet cherries (Prunus avium L.) are among the most valued temperate fruits worldwide, prized for their vibrant color, rich flavor, and nutritional benefits [5,6]. Their high consumer demand is driven by their unique combination of sweetness and acidity and their bioactive compounds, which contribute to human health [7,8]. The fruit’s quality attributes, including size, firmness, sweetness, and color, play a crucial role in consumer preferences and purchasing decisions [9,10,11]. Fruit color is one of the most important visual indicators of ripeness and quality, directly influencing marketability. Consumers often associate deeper red hues with higher sugar content, better flavor, and greater freshness, making color a key determinant of cherry acceptance [12]. However, environmental factors, agricultural practices, and postharvest handling can significantly affect consumer satisfaction and market value [13,14].

In fruit trees, climate conditions highly impact the production of edible fruits, playing a fundamental role in determining the quality of sweet cherries [15,16]. Fruit growth, development, ripening, and postharvest quality are influenced by several climatic factors such as temperature, sunlight exposure, wind, humidity, and rainfall [17]. Although adequate winter chilling is essential for proper bud break and productivity in temperate fruit trees [18], extreme temperatures and temperature fluctuations compromise crops, leading to a reduced fruit quality and yield [15,19]. On the other hand, rainfall patterns also play a critical role, as high precipitation near harvest significantly increases fruits cracking susceptibility, a major factor in economic losses by reducing fruits’ marketability [17]. Thus, understanding the relationship between climatic variables and cherry production is crucial for developing adaptive strategies to mitigate environmental stress and ensure high-quality fruit.

In recent years, the agricultural sector has faced challenges related to climate variability, pest management, and market fluctuations, highlighting the need for innovative agricultural practices to enhance productivity and sustainability [20,21,22]. Fertilization plays a crucial role in optimizing the yield, quality, and health of sweet cherry trees [2]. Calcium and seaweed-based biostimulants are increasingly recognized for their benefits among various fertilization strategies. Calcium plays a crucial role in determining the quality and postharvest longevity of sweet cherries, influencing key attributes such as firmness, color, and resistance to physiological disorders [11,23,24,25]. As an essential nutrient, calcium contributes to cell wall stability and membrane integrity, reducing fruit softening and improving overall texture [23,26,27,28]. Adequate calcium levels have been associated with reduced susceptibility to cracking, a significant issue affecting cherry marketability, as well as improved fruit quality and enhanced resistance to postharvest decay [29,30]. On the other hand, seaweed-based biostimulants are considered a new generation of sustainable agro-products that address the demands of the global population, protect the environment, and mitigate climate change [31]. These organic compounds have demonstrated significant potential in enhancing plant growth, increasing crop yields, and improving the quality of final products. Additionally, seaweed extracts are recognized as a promising soil conditioner, aiding plants in coping with abiotic and biotic stresses while boosting their resistance to pests and diseases [32,33,34]. Seaweed extracts act as natural biostimulants in fruit crops by supplying phytohormones, minerals, and bioactive compounds that improve photosynthesis, nutrient uptake, and stress tolerance [35,36]. Their application in sweet cherries, grapes, and blueberries enhances root growth, flowering, fruit set, and size, while also boosting antioxidant activity and osmotic regulation [28,37,38]. These combined effects lead to higher yields and improved fruit quality, positioning seaweed extracts as a sustainable strategy for enhancing crop physiology and productivity [39]. In fruit crops, foliar fertilization enables the rapid absorption of nutrients. As a targeted and eco-friendly approach, it has effectively satisfied plant nutrient requirements, enhancing fruit quality and crop yield [40].

Understanding consumer preferences and the factors influencing cherry quality is essential for optimizing production strategies and improving market competitiveness. Therefore, this study assesses the effects of pre-harvest foliar applications of calcium and a seaweed-based biostimulant (Ascophyllum nodosum), both individually and in combination, on several quality attributes of sweet cherries. These attributes include biometric and colorimetric measurements, fruit firmness, standard quality indicators, and the sensory profile of the cherries.

2. Materials and Methods

2.1. Experimental Trial

Experiments took place in an orchard located in Santa Eulália, São Martinho de Mouros, Resende, Portugal (41°04′55.3″ N 7°53′35.2″ W, altitude 615 m) in 2021 and 2022. A weather station located near the study site registered the weather parameters during the experiments (Figure 1). The minimum mean temperatures were registered in January (5.5 °C in 2021 and 6.59 °C in 2022), while the maximum mean temperatures were recorded in August for 2021 (20.27 °C) and in July for 2022 (24.25 °C). Regarding precipitation in 2021, the rainiest months were January (214.8 mm) and February (213.4 mm), while August was the driest month (5.6 mm). Contrastingly, in 2022, January registered the lowest precipitation level (0.8 mm), whereas October and November were the rainiest months (86.0 mm and 85.6 mm, respectively). Overall, 2022 recorded higher temperatures and lower precipitation levels compared to 2021.

The soil used in the experimental assay was analyzed and classified as a cambissol [41], characterized by a sandy loam texture. Its properties include a pH (H₂O) of 5.9 (lower acid), organic matter content of 22.0 g kg⁻¹ (medium), extractable Egner–Riehm phosphorus (P₂O₅) and potassium (K₂O) of 91.4 mg kg⁻¹ (high) and 270.3 mg kg⁻¹ (very high), respectively, exchangeable Ca²⁺ at 5.4 cmolc kg⁻¹ (medium), exchangeable Mg²⁺ at 3.0 cmolc kg⁻¹ (high), exchangeable K⁺ at 0.6 cmolc kg⁻¹ (high), and a cation exchange capacity (CEC) of 9.3 cmolc kg⁻¹ (medium), determined using ammonium acetate. Based on these results, soil amendments were applied to correct the organic matter and phosphorus levels by incorporating 10 tons per hectare of commercial compost and 90 kg per hectare of phosphorus in the form of single superphosphate (18% P₂O₅), respectively. These corrections were implemented during the crop’s vegetative dormancy period, between September and November of the first year of the trial.

Cherry quality was evaluated through foliar applications of calcium and a seaweed-based biostimulant (Ascophyllum nodosum) on cv. Sweetheart sweet cherry trees. The orchard was planted at a spacing of 4 m × 4 m (625 trees per hectare), with 12 trees selected for each treatment. Treatments included two calcium concentrations (Kit Plant Ca), 300 g hL⁻¹ (Ca300) and 150 g hL⁻¹ (Ca150), two seaweed biostimulant concentrations (Foralg), 150 mL hL⁻¹ (AN150) and 75 mL hL⁻¹ (AN75), a combination of 300 g hL⁻¹ calcium and 150 mL hL⁻¹ seaweed (Mix), and a control where only water was applied. The treatments were applied in accordance with the manufacturers’ recommendations, our previous studies in sweet cherry orchards, and standard agronomic practices routinely adopted in commercial production. Foliar applications were performed during the fruit enlargement phase, with a total of three sprays applied at 8-day intervals. Fruits from the 12 treated trees were harvested at the commercial ripening stage, on 8 July 2021 and 13 July 2022, to analyze sweet cherry quality characteristics.

2.2. Sweet Cherry Quality Parameters

2.2.1. Biometric Parameters

A total of 30 fruits were randomly selected from each treatment to assess biometric parameters. Fruit weight (g) was measured using an electronic balance (model EW2200-2NM, Kern, Germany). Fruit dimensions (mm), namely the larger diameter, smaller diameter, and height, were determined using a digital caliper (Mitutoyo, Hampshire, UK). The results were reported as the thirty fruits’ mean value and their standard deviation (SD).

2.2.2. Chromatic Parameters

Fruit color was measured on two opposite sides of the same 30 fruits from each treatment with a colorimeter (CM-2300D, Konica Minolta, Japan), which was calibrated using a standard white plate. Measurements were conducted according to the 1976 CIE (Commission Internationale de l’Éclairage) system, based on a three-dimensional space (CIELAB). The colorimetric coordinates L*, a*, and b* were recorded, where L* indicates lightness, a* reflects the spectrum from green (−a) to red (+a), and b* represents the range from blue (−b) to yellow (+b). From these values, chroma (C*) was calculated using the formula C* = (a*² + b*²)^1/2, while the hue angle (h°) was determined as h° = arctangent (b*/a*) [42]. The results were expressed as the mean of sixty readings (two measurements per fruit for 30 fruits) along with their SD.

2.2.3. Texture Parameters

Texture parameters, including epidermis rupture force (ERF) and fruit firmness (FF), were assessed using the same 30 fruits from each treatment. These measurements were carried out with a Texture Analyser (TA.XT.plus, Stable Micro Systems, Godalming, UK). The maximum force required to penetrate and compress the pulp by 5 mm was recorded at a speed of 1 mm/s, utilizing a 30 N load cell and a 2.0 mm diameter cylindrical probe. Results were reported as the mean value of the 30 fruits along with their SD.

2.2.4. Total Soluble Solids, pH, Titratable Acidity, and Maturity Index

The 30 fruits used for previous measurements were divided into three groups of ten fruits each. After removing the pits and peduncles, fruit juice was extracted from the pulps in each group. Total soluble solids (TSS, in °Brix) were accessed using a digital refractometer (PAL-1, Atago, Tokyo, Japan), and pH was measured using a pH meter (3505, Jenway, UK). To assess titratable acidity (TA, expressed as a percentage of malic acid), 10 mL of juice was diluted with 10 mL of distilled water and titrated with 0.1 mol L⁻¹ sodium hydroxide (NaOH) until the solution reached a pH of 8.2. The maturity index (MI) was then calculated as the ratio of TSS to TA. Results were presented as the mean of three replicates with their SD.

2.3. Sensorial Analysis

The sensory profile of sweet cherries was evaluated by a trained panel from the University of Trás-os-Montes and Alto Douro (UTAD) in a sensory laboratory designed for this type of analysis, following the normative ISO 13299:2016 (Sensory analysis—Methodology—General guidance for establishing a sensory profile). A quantitative descriptive test was selected because the QDA analysis not only provides a detailed profile of the cherry samples but also allows for comparisons between them. Five cherries randomly selected per treatment, at room temperature, were placed in white dishes and randomly given to each panelist to evaluate a set of fifteen cherry attributes, namely appearance, epidermis softness, color intensity, color uniformity, peduncle color, odor intensity, sweet taste, sour taste, bitter taste, astringency, strange taste, cherry flavor, strange flavor, firmness, and succulence, adapted from Chauvin et al. [43]. Each attribute was rated using a scale from 1 (lowest intensity) to 5 (highest intensity) by comparing all the provided samples. Results were reported as the average score for each attribute. Water was provided during the sensory test to avoid fatigue and a decline in the panelists’ sensitivity to the various attributes, and panelists should clean their palates whenever they need to.

2.4. Statistical Analysis

The statistical analysis was performed using SPSS software version 27 (SPSS-IBM, Corp., Armonk, NY, USA). Statistical differences were assessed using one-way and two-way analysis of variance (ANOVA), followed by Tukey’s post hoc multiple range test (p < 0.05). One-way ANOVA was used to evaluate the effects of different treatments within each year and to assess the effect of the year within each treatment. Two-way ANOVA (considering two factors) was employed to analyze the combined impact of treatment and year on cherry quality parameters over the two-year trial period.

3. Results

3.1. Effect of Calcium and Seaweed-Based Biostimulant on Sweet Cherry Quality Parameters

3.1.1. Biometric Parameters

All biometric parameters (Figure 2) were influenced by both the year and the interaction between treatment and year (p < 0.001). Treatment also had a significant effect on smaller diameter (p < 0.05) and height (p < 0.01). In 2021, significant differences were observed for all biometric parameters (p < 0.001). Compared with the control, a small improvement in the biometric parameters was observed only in the AN150 treated cherries. Similarly, in 2022, the treatment influenced all biometric parameters (p < 0.001). Most treatments (except AN150) promoted an increase in fruit weight and dimensions, with the greatest effects observed in the mix treatment and in Ca300-treated cherries. In the mix treatment, fruit weight, larger diameter, smaller diameter, and height increased by 19.81%, 7.61%, 6.95%, and 2.83%, respectively, while in Ca300 they increased by 19.95%, 8.63%, 6.82%, and 4.02%. The influence of the year within each treatment also revealed significant differences across all biometric parameters (p < 0.001). Overall, in 2022, all treatments generated smaller cherries, with lower weight and dimensions.

3.1.2. Chromatic Parameters

All chromatic parameters (Figure 3) were impacted by the treatment (p < 0.001), by the year (p < 0.001), and by the interaction between treatment and year (p < 0.001). In 2021, the treatment significantly affected all chromatic parameters (p < 0.001). AN75 treated cherries led to a decrease in L* (−2.04%), a* (−0.56%), b* (−1.69%), C* (−0.62%), and h° (−1.77%), resulting in redder and darker cherries. Conversely, mix treatment increased L* (5.46%), a* (16.46%), b* (27.04%), C* (17.57%), and h° (7.92%), producing lighter cherries. In 2022, the treatment affected all chromatic parameters (p < 0.001). All treatments presented lower values than the control for all chromatic parameters, especially in cherries treated with Ca300 (−6.16% for L*, −45.90% for a*, −56.91% for b*, −46.68% for C*, and −18.36% for h°), meaning redder and darker cherries. Significant differences were also found between years within each treatment for all chromatic parameters (p < 0.001). All chromatic parameters presented lower values in 2022, indicating redder and darker cherries.

3.1.3. Texture Parameters

Epidermis rupture force (ERF) and fruit firmness (FF) (Figure 4) were significantly impacted by the treatment (p < 0.05 for FF, and p < 0.01 for ERF) and by the year (p < 0.001 for both). In 2022, significant differences were observed between treatments. Ca150 treated cherries presented an increase in ERF (25.95%) and FF (20.59%). The effect of the year within each treatment significantly changed both ERF and FF. Overall, a decrease in ERF and FF was observed in 2022.

3.1.4. Total Soluble Solids, pH, Titratable Acidity, and Maturity Index

Within the routine parameters (Figure 5), total soluble solids (TSS), titratable acidity (TA), and maturity index (MI) were influenced by the treatment (p < 0.01 for TSS, and p < 0.001 for TA and MI) and by the year (p < 0.05 for TA, and p < 0.001 for TSS and MI). pH was influenced by the year (p < 0.001) and by the interaction between treatment and year (p < 0.01). In 2021, treatment significantly influenced the pH (p < 0.01) and MI (p < 0.05). Cherries treated with Ca300 led to an increase of 7.69% in pH, while AN150-treated cherries presented an enhancement of 28.62% in MI. In 2022, the treatment influenced TSS, TA, and MI (p < 0.01 for TA, and p < 0.05 for TSS and MI). All treatments produced sweeter cherries, increasing TSS, particularly in cherries treated with Ca150 (16.07%) and Ca300 (14.93%). Simultaneously, Ca300 increased the TA (25.86%) and delayed the MI (−8.71%). Conversely, cherries treated with AN150 evidenced a decrease in TA (−5.17%) and an improvement of MI (15.48%). The effect of the year within each treatment significantly impacted TSS in all treatments; pH in control, mix, Ca300, Ca150, and AN75; TA in control and Ca150; and MI in control, Ca300, Ca150, and AN150. All treatments presented higher TSS, pH, and MI in 2022, while TA was lower or similar in 2022 than those observed in 2021.

3.2. Sensorial Analysis

Within the sensory attributes (Figure 6), appearance, color uniformity, peduncle color, sweet taste, sour taste, strange taste, and firmness were affected by the year (p < 0.05), while the treatment influenced color intensity and color uniformity (p < 0.05). Color intensity was also impacted by the interaction between treatment and year (p < 0.01). In both years, 2021 and 2022, color intensity was the only sensory attribute affected by the treatment (p < 0.01 for 2021, and p < 0.001 for 2022), reaching the highest scores in Ca300 and Ca150 (4.8 in 2021 and 4.9 in 2022).

4. Discussion

According to our results, in the first year, foliar fertilization with Ascophyllum nodosum, specifically AN150 treated cherries, increased fruit weight and dimensions (Figure 2). These findings align with our previous study, where we observed similar fruit size and dimensions improvements when AN150 was applied to sweet cherry trees cv. Sweetheart. However, our results also revealed a significant decrease in total production, with yields 49.71% lower than the control, when seaweed was applied in the first year of study [44]. Nevertheless, an improvement in fruit size has been reported following the application of seaweed to sweet cherry trees, whose application on cvs. Sweetheart and Skeena led to an increase in fruit dimensions, including both weight and diameter [45]. Similar findings were reported by Gonçalves et al. [46], who observed that the use of Ascophyllum nodosum on cv. Staccato produced larger fruits than control cherries. Similarly, in cvs. Kordia and Regina, applying a plant extract biostimulant also increased fruit diameter [47]. Conversely, mix treatment- and Ca300-treated cherries in the second year resulted in more prominent cherries (Figure 2). The increased fruit size and weight in both treatments suggest that calcium and nutrient mix applications may enhance the structural integrity of the fruit, allowing it to grow larger and heavier. These findings align with the results previously reported by Correia et al. [25] and Correia et al. [28], who observed an improvement in fruit size when calcium and seaweed (particularly in calcium fertilization) were applied to cvs. Sweetheart and Skeena. According to the same authors, the enhancement in sweet cherry size was most pronounced when calcium and seaweed were combined in a single treatment, similar to what was achieved in our mix treatment. Moreover, in cv. Lapins, the later foliar application of CaCl₂ (62 DAFB) promoted an increase in fruit weight and diameter [23]. In addition to improved fruit size, we have reported a significant enhancement in orchard profitability with Ca300 treatment, with a 135.80% increase in total production compared to the control and achieving the highest productivity of 26.86 t/ha [44], highlighting the important role of calcium in sweet cherry quality. Our results showed strong positive correlations between all biometric parameters (Supplementary Table S1), suggesting uniform and synchronous fruit growth. This underscores the effectiveness of calcium and seaweed as valuable compounds for the development and growth of cherries.

Fruit color is a fundamental attribute of fruit quality, playing a crucial role in shaping consumer preferences and significantly influencing the fruit’s market value [48]. In our study, all chromatic parameters were significantly influenced by the treatment, year, and their interaction, reinforcing that fruit coloration is a dynamic trait shaped by multiple interacting factors (Figure 3). In 2021, the AN75 treatment produced redder and darker cherries, as evidenced by decreases in all chromatic coordinates. Similar results were reported by Gonçalves et al. [46], who observed lower values for the chromatic coordinates L*, C*, and h°, suggesting that biostimulants may enhance fruit ripening. In fact, in sweet cherry, biostimulant treatments, such as seaweed extracts, have been linked to an increase in anthocyanin content, which can contribute to color development and darker fruit skins [47,49]. Conversely, the mix treatment increased L*, a*, b*, C*, and h° values, resulting in lighter cherries. In cv. Sweetheart, the same cultivar used in this study, higher C* values were reported in cherries treated with a combined fertilization of calcium and Ascophyllum nodosum, leading to lighter-colored cherries [25]. In 2022, all treatments resulted in lower chromatic values than the control, particularly for cherries treated with Ca300. The significant reductions in L*, a*, b*, C*, and h° indicate the production of redder and darker cherries and a higher level of maturity [2]. These findings contrast those reported in the literature, as calcium is generally associated with delaying color development [24]. In cvs. 0900 Ziraat and Merton Late, fertilization with CaCl₂ resulted in higher L*, C*, and h° values than the control, indicating less color development and lighter cherries [26]. However, the same authors reported similar L*, C*, and h° values in CaCl₂-treated and untreated cherries for cv. Sweetheart. Nevertheless, positive correlations were found between all chromatic coordinates and all biometric parameters (Supplementary Table S1), suggesting that larger fruits are likely still in the growing stage and developing their color.

Fruit firmness is a key indicator of sweet cherries’ storage potential and overall quality, influencing consumers’ preference [26,50]. In our study, the treatments applied significantly impacted both ERF and FF (Figure 4), highlighting the potential of specific agricultural interventions to influence fruit characteristics. In 2022, calcium treatments, particularly the Ca150 treatment, proved especially effective, resulting in increased ERF and FF. Calcium treatments are well documented as enhancing fruit firmness by strengthening cell walls [51,52,53]. The observed improvements in fruit firmness with the Ca150 treatment are consistent with findings from previous studies that have verified that calcium applications can enhance the sweet cherry texture. Several authors have demonstrated that calcium sprays, especially foliar CaCl₂ applications, have significantly increased sweet cherry firmness [23,25,26,53,54,55]. Similarly, calcium treatments increased firmness in Chinese cherry (Cerasus pseudocerasus) [56]. A positive correlation between ERF and FF (R = 0.861, p = 0.000), indicating a strong relationship between texture parameters. Therefore, our results underscore the benefits of calcium in promoting cell wall integrity and improving cherry firmness and quality. Additionally, positive correlations were found between texture parameters and all biometric parameters and between texture parameters and all chromatic coordinate (Supplementary Table S1), meaning that larger fruits are firmer and lighter. This reinforces the idea that fruits are likely still in the growth phase, undergoing size expansion and gradually developing their characteristic color as they progress toward ripening.

The results of this study demonstrate that both treatment and year significantly influenced key physiological parameters of cherries, such as TSS, TA, pH, and MI. These parameters are critical indicators of fruit quality and ripening, with TSS being associated with sweetness, TA reflecting sourness, and MI serving as an overall indicator of ripening progression [2,57]. In our study, the treatments applied to cherries significantly impacted the routine parameters measured, particularly TSS, TA, and MI (Figure 5). All treatments produced sweeter cherries, as evidenced by increased TSS, particularly in cherries treated with calcium. These results are consistent with previous studies that reported enhanced sugar content in cherries following calcium treatments [58], likely due to its role in regulating the accumulation of soluble sugars in fruits [59]. The increase in TSS in treated cherries suggests that calcium fertilization may have promoted the accumulation of sugars, contributing to a sweeter taste, which is an important attribute for marketability. At the same time, calcium treatments, particularly Ca300, resulted in higher TA and a delayed MI, suggesting that calcium likely slows down the fruit ripening process. This aligns with a previous study by Erbaş and Koyuncu [26], who stated higher TA content in cvs. 0900 Ziraat, Sweetheart, and Merton Late compared to the control group. Similarly, Correia et al. [25] observed a delay in fruit’s maturity in cv. Sweetheart when calcium was applied in sweet cherry trees. Likewise, in Chinese cherry (Cerasus pseudocerasus), calcium treatments significantly increased total soluble solids and titratable acidity [56]. In horticultural crops, calcium is essential for fruit ripening as it affects physical and biochemical processes. Calcium helps delay fruit softening and maintain post-harvest quality by slowing down the ripening process, ultimately extending the fruit’s shelf life [60]. In sweet cherries, calcium fertilization has been associated with suppressed fruit ripening [26]. Conversely, AN150-treated cherries exhibited a decrease in TA and an improvement in MI, indicating that this treatment may have promoted a faster ripening process while lowering the fruit’s acidity, which could benefit flavor development. These findings are in line with those reported by Gonçalves et al. [46], who observed reduced acidity and enhanced fruit ripening in cv. Staccato when treated with biostimulants such as Ascophyllum nodosum. Within routine parameters, positive correlations were found between TSS and pH (R = 0.692, p = 0.000), and pH and MI (R = 0.653, p = 0.000). In contrast, negative correlations were found between pH and TA (R = −0.359, p = 0.032). This highlights a strong relationship among routine quality parameters, suggesting that riper cherries were sweeter and less acidic. Nevertheless, routine parameters, namely TSS, pH, and MI, were negatively correlated with all biometric, chromatic, and texture parameters, while TA was positively correlated with texture parameters (Supplementary Table S1). These findings suggest that bigger cherries were also firmer, sourer, less sweet, and less mature, supporting the idea that these fruits are still developing.

Fruit taste is a key quality attribute of cherries, influenced by their sweetness, flavor, and firmness [61]. Beyond the evident physiological disorders in sweet cherries, consumer preferences are primarily driven by firmness and flavor [62]. In our study, only color intensity evidenced significant differences in both years, with calcium treatments achieving higher scores (Figure 6). However, despite no statistical differences being found, the evaluator panel also indicated calcium treatments that produced sweeter cherries and with more excellent color uniformity.

In our study, foliar fertilization with calcium and a seaweed-based biostimulant resulted in different trends during the two years of trial, highlighting the effect of climate conditions in our results. Overall, in 2022, all treatments generated smaller cherries, with lower weight and dimensions. Likewise, Correia et al. [25] observed smaller cherries during the second year of their trial. Additionally, all chromatic parameters presented lower values in 2022, indicating redder and darker cherries. At the same time, cherries were ripper, sweeter, and less firm in 2022. Despite this, fertilization with the seaweed-based biostimulant improved fruit size and color in the first trial year and the MI in both years. Similar findings were stated by Zhi and Dong [63], who observed an enhancement of sweet cherry quality attributes, such as size and color, following the application of seaweed-based biostimulants. On the other hand, continuous fertilization with calcium improved fruit size and dimensions, TSS, pH, firmness, and color in the second trial year but increased TA and delayed MI. This variability underscores the importance of considering environmental factors when evaluating the efficacy of treatments in agricultural studies. Agriculture and climate change are closely related, as climate change is a significant cause of both biotic and abiotic stresses, which could negatively impact the agricultural productivity [64]. Within climatic factors, temperature is an important factor for the growth and development of fruit crops. Extreme temperatures negatively impact pollination, flowering, and fruit sets in fruit crops [15]. Moreover, temperature variations highly affect fruit quality attributes by changing sugars synthesis, color and firmness, and delaying ripening [65]. Our study’s second trial year, 2022, was extremely hot and dry (Figure 1). These variations in temperature and rainfall across the years could have affected the physiological processes of the trees, including fruit set, growth rates, and nutrient uptake, resulting in smaller and less firm cherries, as well as variations in several quality attributes, such as color, TSS, or ripening. Excessive heat stress negatively impacted fruit firmness and led to accelerated softening, which can shorten shelf life [66].

5. Conclusions

Our study demonstrates that calcium and Ascophyllum nodosum-based biostimulants represent complementary and sustainable strategies to improve sweet cherry quality. Calcium is an essential element for fruit growth and development, and it improves fruit size, TSS, pH, firmness, and color but increases TA and delays ripening. This suggests that calcium applications, at the appropriate concentration, can improve fruit characteristics, making it a potential tool for enhancing the post-harvest quality of cherries. Our work also used an innovative and sustainable strategy to improve cherry quality parameters by applying a seaweed-based biostimulant. Foliar application of Ascophyllum nodosum enhanced fruit size and color development. Simultaneously, seaweed promoted a decrease in TA and accelerated ripening.

This study demonstrates that calcium fertilization, applied alone or in combination with seaweed biostimulants, significantly enhances sweet cherry quality traits. These findings highlight a practical and sustainable strategy for commercial orchards, with strong potential to improve the global competitiveness and resilience of sweet cherry production. Furthermore, the complementary effects of calcium and seaweed biostimulants offer growers flexible management options to target different market demands, from extended storage potential to early harvest opportunities. Considering the increasing challenges posed by climate variability, the integration of calcium nutrition and seaweed biostimulants represents a climate-adaptive strategy to improve cherry quality and sustainable production under variable environmental conditions. However, given the increasing challenges posed by climate variability, future research should focus on implementing adaptive agricultural practices to mitigate environmental stress and enhance cherry quality under changing climatic conditions.