Introduction

The fruit status at harvest is critical to its performance throughout postharvest storage. This comprises an appropriate maturity stage as well as several other factors including nutritional state and tolerance to specific storage diseases (Khan and Ali 2018). A longer shelf life allows for more seasonality, less food waste, and a better possibility of conserving fruit freshness. Weight loss, inferior appearance, softened texture, poor flavor/aroma, and decay are among the important factors limiting the storability and shelf life of apple cultivars (Piestrzeniewicz and Tomala 2000; Ashoori et al. 2013).

Additionally, bruising that occurs along the process of handling can have great economic repercussions, mainly due to the negative changes in appearance, organoleptic attributes, and fruit quality (Van Zeebroeck et al. 2007). To reduce the incidence of fruit damage, knowledge of fruit sensitivity to bruising is of interest to both growers and operators of postharvest handling and marketing facilities (Ericsson and Tahir 1996). Bruising varies among varieties (Gharaghani and Shahkoomahally 2018) and even for the same cultivar, fruit might be more susceptible to bruising under some cultivation practices (i.e., nutrition and irrigation) as well as harvest and postharvest handling conditions (Garcia et al. 1995). Also, bruising may be intensified by some other factors such as fruit water content, temperature, fruit size, and fruit firmness (FF; Jafari and Nassiri 2013; Gharaghani and Shahkoomahally 2018).

Poor skin coloration and low storability of red apples (such as “Red Delicious”) are also two important concerns that prevail in many orchards with warm and dry weather during the summer. In these areas, efforts such as delaying the harvest to improve red color have been unsuccessful because this negatively affects storability and quality attributes such as fruit firmness and flavor (Iglesias et al. 2002). Preharvest spraying with particular compounds/chemicals has been a widely utilized technique across the world to reduce these difficulties and limitations during postharvest storage and handling (Lara 2013). Fruit quality characteristics are substantially influenced by the orchard nutritional practices. Calcium (Ca) and potassium (K) are among the most important nutrient elements that clearly affect the quality of tree fruits (Nava et al. 2008; Ashoori et al. 2013). Moreover, the proper nutrient balance is essential for maintaining fruit quality in apples. An incorrect K/Ca ratio that is below the desirable levels leads to reduced shelf life and storage disorders (Tomala 1996). Ca has an important role in cell membrane integrity and cell wall resistance (Marschner 2012). Ca in particular is a decisive element for apples because they are commercially stored for long periods of time, and other factors generally do not supersede the influence of Ca on storage quality (Von Bennewitz et al. 2011). Foliar application of Ca is often used to control Ca-related disorders such as bitter pits in apples. Increasing the amount and frequency of Ca consumption immediately increases the Ca level of the fruit and can decreases Ca-related disorders and thereby improve apple quality (Kalcsits et al. 2017).

Potassium is the most plentiful mineral component in apple fruit, promoting the transportation and storage of sugar and water in fruits (Marschner 2012). Potassium deficiency results in poor color, in decreased root growth and fruit quality, as well as in burn-up and necrosis in the leaf tips and margins (Qeyami et al. 2020). Appropriate utilization of K fertilizers can motivate a significant yield increase. However, the use of excessively high rates of K can decrease the accessibility of Ca for apple trees (Kuzin et al. 2020). Inconsistent results have been obtained by various researchers when K and Ca fertilizers were used to improve the postharvest quality and storability of apple cultivars (Dilmaghani et al. 2005; Szewczuk et al. 2008; Von Bennewitz et al. 2011). These conflicting results can be due to differences in cultivars, the modes of fertilization, fertilizer source and concentration, water stress, and nutritional status of experimental soil and trees, which could affect nutrient uptake.

Among the modes of fertilization, foliar spray is recognized as a reliable approach for obtaining a quick response to fertilization of fruit trees (Swietlik and Faust 1984), particularly when soil conditions impede element uptake via the root, or during periods of rapid growth or reproductive development stages (Restrepo-Diaz et al. 2008). Solhjoo et al. (2017) evaluated the influence of preharvest foliar spray of various K fertilizers and calcium chloride (CaCl2), and their combination on the fruit quality of “Red Delicious” apple and reported that combined application of CaCl2 and each of the K sources was more effective in the improvement of fruit color, firmness, fruit K and Ca uptake, and K/Ca ratio at harvest time as compared to the case when either K or Ca was applied alone. Moreover, there is little information related to the effectiveness of foliar spray of important K sources (solely or combined with Ca) on postharvest quality and storability of apple cultivars in the literature. Therefore, the objective of this study was to go beyond the harvest time and investigate the effect of foliar application of various K sources (KNO3, K2SO4, and KCl) and CaCl2 and a combined application of some of these fertilizers on fruit color, as well as changes in fruit internal quality attributes and resistance to mechanical damages in the “Red Delicious” apple during storage time.

Material and Methods

Plant Materials and Treatments

This study was conducted through two consecutive growing seasons in a commercial apple orchard located in Kudian village, Shiraz County, Fars province, southern Iran. The experimental site exhibits dry and hot summers but cold and humid winters receiving 275 mm precipitation annually. Experimental plants were 14-year-old “Red Delicious” apple trees grafted on MM 106 rootstocks and planted at 4 × 2.5 m apart from each other. They were grown in loamy soil under a drip irrigation system and trained in a modified leader and received the usual care of local commercial apple orchards, such as pruning, pollination, nutrition, as well as weed and pest control. Before starting the research, the nutrient contents in the soil and leaves were determined (Table 1). According to Neilsen and Neilsen (2003), the arable and sub-arable soil layers in the experimental site showed the standard levels of K, but in the leaves, it was near the deficit level or below the normal range (1.5–2.5%; Table 2).

Table 1 Physical and chemical properties of soil in the experimental orchard
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Table 2 Leaf nutrient content of apple cultivar “Red Delicious” in the experimental orchard before starting the treatments
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The study was performed according to a completely randomized block design with six treatments (chemicals) and four replications, each including a whole tree, and in total consisting of 24 trees (6 treatments × 4 replications). Foliar spray treatments include sole application of three sources of K (KCl [60% K2O, 47% Cl], K2SO4 [50% K2O, 45% SO3], and KNO3 [44% K2O, 14% N]), at a concentrations of 2.5 g × l−1 for each of their oxides (K2O), sole application of CaCl2 at a concentration of 5 g × l−1, and combined application of KNO3 + CaCl2 at the aforementioned concentrations and the control. CaCl2 was sprayed five times (starting from 3 weeks after full bloom with 3‑week interval) and K sources were sprayed three times (at 9, 12, and 15 weeks after full bloom), in each growing season. Control trees were sprayed with water. The period of 145 days after full bloom (25 and 29 April, respectively, in the first and second season) was considered for the date of commercial harvest by performing the starch-iodine test. Fruit samples consisting of 60 fruits were randomly taken at harvest time from each replicate (each apple tree was considered as one replication) and divided into three sets of 20 fruits for three times of measurements (at harvest and stored at 0 °C [RH = 85%] for 2 and 4 months).

Fruit Color Measurement

Following harvest, the peel of five randomly selected apples from each replicate was marked at two points, 180° apart on each fruit, and color on each side was measured using a colorimeter (CR-400/410, Konica Minolta, Japan). Fruit chromaticity was recorded according to the Commission Internationale d’Eclairage (CIE) with parameters L* (light to dark), a* (green to red), and b*(blue to yellow) color space coordinates. Chroma C* is calculated as (a* + b*)1/2. The hue angle (h°) is calculated from the arc tangent of b*/a* and expressed in degrees from 0 to 360, where 0° = red, 90° = yellow, 180° = green, and 360° = blue (McGuire 1992).

Total Soluble Solids, Titratable Acidity, Firmness, and Dry Matter

Sub-samples of five unpeeled apple fruits were used as a blended composite of wedges to determine total soluble solids (TSS) and total acidity (TA). The TSS was determined using a refractometer (ATAGO ATC-IE model, Japan) and expressed in Brix, while TA was determined using NaOH (0.3 N) according to A.O.AC. (1995). The TSS/TA ratio was also calculated. Fruit firmness (FF) was determined using a Force Gauge (FG-5005, Lutron Electronic Enterprise co. Ltd, Taiwan) on opposite sides of the five randomly selected fruits in each replicate and was expressed in Newton (N). Dry matter (DM) was determined using two plugs of 10 mm diameter excised from five randomly selected fruits in each replicate. Fruit tissue was sampled perpendicular to the fruit surface. Plugs were weighed on a preweighed foil tray. The samples were then oven-dried at 70 °C for 20 h until a constant weight was reached and were then re-weighed once more. Dry matter was expressed as the percentage of the dry weight of the initial fresh sample.

Bruise Measurement

For the bruise test, a set of five fruits was selected randomly in each replicate, then a spherical steel ball (110 g) was thrown from 30 cm into a vertical hollow porous cylinder constructed of PVC, striking the fruit on opposing sides along its equatorial axis (Banks and Joseph 1991). Fruits were left in the laboratory for roughly 24 h after testing to allow for bruise formation. Bruise sizes were determined by cutting a slice of tissue from the fruit along a vertical axis through the bruise center and down to the fruit core. A digital caliper was used to measure the diameter (d; or BD) of the bruise as well as the depth (h; or BH). Bruise volume (BV; in mm3; Eq. 1) and general bruise susceptibility (GBS; Eq. 2; Eq. 3) were calculated based on Mohsenin (1986) as follows:

$$BV=(\pi h/24)(3d2+4h2)$$
(1)

General bruise susceptibility (GBS; mm3J−1) is defined as the ratio of bruise volume to the impact energy, IE (J):

$$GBS=BV/I$$
(2)
$$IE=\textit{mighD}$$
(3)

where mi is the mass of the impacting (or falling) object (kg), g is the acceleration due to gravity (ms−2), and hD is the drop height (m).

Weight Loss

Weight loss (WL) was recorded during storage, using five fruits per replicate, calculated as follows:

$$\textit{Weight}\,loss=((Wi-Wf)/Wi)\times 100$$
(4)

Wi being the initial sample weight and Wf the final sample weight. Results are reported as percentage of weight loss.

Analysis of Fruit Mineral Concentration

In order to analyze the fruit mineral composition, two longitudinal slices were obtained from each fruit (10 fruits per replicate), including the peel. Samples were dried in an oven at 75 °C for 72 h. Then ash was produced at 500 °C for 8 h, dissolved in 5 ml of 2N HCl and filtered using a filter paper. Finally, the ashes were diluted to a volume of 50 ml with deionized water. The concentration of K was measured using flame photometry (Jenway-PFP7, England) at 766.5 nm (Chapman and Pratt 1961) and Ca was estimated using an atomic absorption spectrophotometer (Shimadzu-AA-670, Japan) at 422.7 nm (Waling et al. 1989) and. The results are expressed as mg × kg−1 dry weight.

Statistical Analysis

Statistical analysis was performed using SAS software (version 9.1) according to a randomized complete block design (RCBD) within repeated measurement design having four replications. Primarily, the data of two growing seasons were compared to evaluate the significance of the year effect. Results showed that the effect of year was significant for almost all measured traits at p ≤ 5%, except for TSS, TA, and WL. Thus, the data of 2 years were averaged in the case of a nonsignificant year effect, while the 2‑year data are presented for others. Data were analyzed using the GLM procedure and mean values were compared by the LSD test at a significance level of 0.05. To illustrate the association among fruit quality attributes as well as between nutrient concentrations and fruit quality characteristics, the correlations between all measured traits of “Red Delicious” apple fruits (generated based on the average data of two growing seasons) were calculated based on Pearson’s correlation coefficient.

Results and Discussion

Fruit Color

As shown in Table 3, the sole application of K sources in addition to a combined spray with KNO3 + CaCl2 significantly enhanced the rate of skin color development as compared to the control fruits; however, the sole application of CaCl2 had a little effect on fruit color. There were no significant differences in fruit color properties between trees that received different types of K fertilizers. Treatments with K sources significantly decreased the h° and increased a* value and C*, indicating that K sources increased the fruit peel red color and decreased the yellow color of “Red Delicious” apple. The b* value and h° of the control trees were higher than those of treated trees, indicating the yellow color of the fruits in these treatments. L* was not clearly affected significantly by the treatments; however, it was lower in control trees. Results showed a positive correlation (R = 0.98) between a* value and C* value and also between b* value and h° (R = 0.64). Moreover, K concentration was negatively correlated with a* value (R = −0.51) (Table 4). Chroma is the intensity or purity of the h° and it shows the brightness of fruits. Therefore, K treatments increased the color brightness compared to the control. Nava et al. (2008) have reported that K fertilization increased the a* value and reduced the b* value of “Fuji” apples, which is in agreement with the results of this research. Kadir (2005) also reported lower hue angle in Ca-treated fruits of “Jonathan” apple. Abdel-Hafeez et al. (2010) found that KCl and CaCl2 treatments had the least significant values of hue angle on plum, but KNO3 and other Ca fertilizers had no effect on fruit color properties. Qeyami et al. (2020) demonstrated that varying concentrations of CaCl2 and K2SO4 improved the color of apples. Optimum levels of K can enhance phloem loading, transporting, and unloading of sucrose (Marschner 2012). Furthermore, K is involved in a variety of enzymatic activities and is a key component in the formation of fruit color (Gill et al. 2012; Tripathi and Uniyal 2018). Therefore, it becomes apparent that sugar translocation into the fruit is aided by K, which is necessary for the production of polyphenols like anthocyanin (Solhjoo et al. 2017).

Table 3 Effect of preharvest foliar spray of potassium sources and calcium chloride on skin color of “Red Delicious” apple fruits, harvested in two growing seasons
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Table 4 Correlations among all measured traits of “Red Delicious” apple fruits treated with preharvest foliar spray of potassium sources and calcium chloride (generated based on average data of two growing seasons)
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Fruit Firmness

As expected, the FF decreased significantly (p < 0.05) through the storage period (Fig. 1a,b). In both years, trees treated with CaCl2 and KNO3 + CaCl2 produced firmer fruits compared to the control during the storage period, which is in agreement with results reported by Dilmaghani et al. (2005), Ajender and Chawla (2019), and Ranjbar et al. (2020). Significant positive correlations (R = 0.73) were found between the FF and TA in this study, while the FF had significant negative correlations with the TSS, TSS/TA, DM, and WL (R = −0.5, −0.8, −0.76, −0.77, respectively) (Table 4). This finding is in agreement with the results of other researchers who reported a correlation between citrus fruit weight loss and FF (Khorram et al. 2017). Shirzadeh et al. (2011) also showed that Ca regulated the peroxidase and catalase enzymes activity in apple fruits, thus reducing physiological disorders by delaying cell breakdown and preserving better firmness and lower weight loss during storage. Also, FF decreases due to the breakdown of a major component of the cell wall (insoluble protopectins) into water-soluble pectin composites during storage, which eventually affects the cell wall stability or skin softening (Tripathi et al. 2019). Calcium acts as an intermolecular binder material to fix the protein–pectin complexes of the middle lamella. In the cell wall, Ca interacts with pectic acid to form Ca pectate, which maintains the structure of the cell wall (Ranjbar et al. 2020). These results are in line with the findings of Qeyami et al. (2020), who reported that foliar spray of Ca enhances the firmness of apples. Also, the beneficial effect of Ca dips (2% w/v) (solely or combined with hot water) on the firmness of kiwifruit during storage has been reported by Shahkoohmahali and Ramezanian (2015). Other researchers have observed the positive effect of Ca on FF in different apple cultivars (Fallahi et al. 1988; Benavides et al. 2002; Neilsen et al. 2005). By contrast, Von Bennewitz et al. (2011) showed that apple firmness was not affected by soil application of CaSO4 and K2SO4 (each being used alone or in combination) during storage. Our results in both seasons revealed that the separate application of KNO3 or K2SO4 caused no significant differences in FF for up to 2 months after harvest, compared to the control, but at the end of the storage period (4 months) the control samples showed higher FF compared to those treated with KNO3 and KCl, respectively, in the first and second seasons, (Fig. 1a,b). Also, KCl-treated trees produced firmer fruits only at harvest time. Szewczuk et al. (2008) reported that soil application of different K fertilizer forms did not cause any significant changes in FF of “Golden Delicious” apple after storage. However, the positive effect of K on apricot firmness has been reported in the study by Moradinezhad and Dorostkar (2021). Potassium preserves water content in the tissues and thus leads to FF and strength (EL-Seginy 2006). Fruit resistance to penetration occurs in the presence of K, which may be due to an increase in the thickness of the sclerenchyma cell wall (Fallah et al. 2021).

Fig. 1
figure 1

Changes in fruit firmness of “Red Delicious” apple during storage time treated with preharvest foliar spray of potassium sources and calcium chloride. a,b First and second season data, respectively

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Total Soluble Solids

The TSS of fruit was increased significantly (p < 0.05) by prolonging the storage period (Fig. 2). The TSS levels significantly increased by KNO3 + CaCl2 treatment only at harvest time and then stabilized during the storage period. Treatment with CaCl2 did not affect TSS levels during storage. In general, fruits in the control treatment yielded the lowest TSS value at harvest and during storage, while the sole application of K sources showed the highest TSS during storage. The positive correlation was observed between TSS with TSS/TA ratio and DM (R = 0.69 and R = 0.57, respectively). Changes in TSS during the storage period are a result of two aspects: respiration rates and water loss by evaporation. An increase in fruit TSS with soil application of K has previously been reported by other researchers (Dilmaghani et al. 2005; Von Bennewitz et al. 2011). These findings are not in line with the results of Ranjbar et al. (2020), who reported that Ca treatments in “Red Delicious” apple resulted in low TSS compared to the control. The role of K in increasing fruit TSS might be attributed to its participation in sugar translocation from leaves to fruits (Prasad et al. 2015), resulting in higher-quality fruits with increased TSS (Qeyami et al. 2020). Inconsistent observations regarding the “Golden Delicious” apple suggest that the different forms of K fertilizers in the soil do not result in any significant effects in TSS after storage (Szewczuk et al. 2008).

Fig. 2
figure 2

Changes in TSS of “Red Delicious” apple during storage time treated with preharvest foliar spray of potassium sources and calcium chloride (as the effect of the year was not significant for this trait, data represent the average of two growing seasons)

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Total Acidity

As shown in Fig. 3, data indicated that TA decreased with the progress in storage period up to the 4 months of storage in all treatments. The same results have been reported by Ranjbar et al. (2020) on “Red Delicious” apple. The TA increased with all K sources (~ 0.61%) as compared to the control (0.52%) just at harvest, but the KNO3 + CaCl2 treatment significantly rendered more organic acids in the fruit than those of the control treatment until the end of storage.

Fig. 3
figure 3

Changes in titratable acidity of “Red Delicious” apple during storage time treated with preharvest foliar spray of potassium sources and calcium chloride (as the effect of year was not significant for this trait, data represent the average of two growing seasons)

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The application of Ca delays fruit ripening (Mosa et al. 2015) and reduces ethylene production and the respiration rate, thereby reducing the rate of TA changes (Ranjbar et al. 2018, 2020). Decreased acidity may be related to postharvest ripening, because organic acids are involved in respiratory metabolism (Valero and Serrano 2010). These results are in agreement with results of Abdel-Hafeez et al. (2010), who found that spraying KNO3 on plum can lead to higher percentages of TA throughout the storage period. Some reports showed that the preharvest application of Ca has no substantial effect on the content of organic acids in grape (Ciccarese et al. 2013) and blueberry (Angeletti et al. 2010) during storage. In another study, Prasad et al. (2015) observed increased TA content in mango by foliar application of CaCl2 and KNO3. Therefore, it can be concluded that preharvest spray of K‑containing treatments can maintain the sensory quality of apple fruit.

Dry Matter

The DM percentage showed a significant increase as a result of prolonged storage periods in both seasons (Fig. 4), but this increase was slower for the control treatment during the first and second seasons. During storage, fruit DM was significantly increased by all K sources and KNO3 + CaCl2 treatments compared to the control in both seasons. Also, during storage, control treatment exhibited the lowest DM in both seasons. During the second season, the CaCl2 treatment showed a significantly higher percentage of DM as compared to the control. This result could be related to the increase in TSS content, which was affected by the K treatments. The high level of sugar and organic acids brings about an increase in the amount of the fruit DM, which acts as one of the significant factors in ameliorating the quality of fruits. Furthermore, the increased DM that is associated with improved cell resources results in longer postharvest life. It has been reported that DM affects fruit quality during storage (Buxton 2005) and “Hayward” kiwifruits with low DM showed more imminent collapse, softening, and decay during storage (Ferguson et al. 2003).

Fig. 4
figure 4

Changes in dry matter of “Red Delicious” apple during storage time treated with preharvest foliar spray of potassium sources and calcium chloride. a,b First and second season data, respectively

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Weight Loss

Weight loss increased significantly with the prolongation of the storage period (Fig. 5). This increase was more rapid in the control group than other treatments. All treatments significantly reduced WL as compared to the control treatment at the end of storage, and the control and the KNO3 + CaCl2 treatments showed the highest and lowest WL at the end of storage, respectively. These findings were comparable to those of Ranjbar et al. (2018) and Ajender and Chawla (2019), who reported that an increased storage period leads to higher weight loss in apple. The positive role of Ca in reducing WL has been reported in many studies (Mahajan and Dhatt 2004; Ashour 2000; Shirzadeh et al. 2011). Normally, WL occurs during storage due to the fruit respiration process, the transference of humidity, and a mild process of oxidation (Ayranci and Tunc 2003). Calcium ions increase the stability of cell walls by binding non-esterified pectin (Angeletti et al. 2010) and maintain integrity and functionality of the membrane (Saure 2005; Shirzadeh et al. 2011). Despite the fact that plant cell walls are permeable to water, slower breakdown of the cell-wall structure might lead to increased resistance to water flux (Angeletti et al. 2010) and, as is common knowledge, K prevents water loss and WL by increasing the fruit sugar concentration or accordingly may result in osmoregulation effects in the tissue. Shirzadeh et al. (2011) reported that application of Ca delays cells breakdown, maintaining the higher firmness and reducing WL during storage by influencing the activity of catalase and peroxidase enzyme in the apple.

Fig. 5
figure 5

Changes in weight loss of “Red Delicious” apple during storage time treated with preharvest foliar spray of potassium sources and calcium chloride (as the effect of year was not significant for this trait, data represent the average of two growing seasons)

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Bruise Susceptibility

In both growing seasons, BD, BH, BV, and GBS of treated and nontreated fruits significantly decreased during the first 2 months of storage and then remained almost constant until the end of the storage period (Figs. 6, and 7 and 8 and 9). In fact, fruits were more susceptible to bruise damage at harvest time, but this tended to decrease as the time of storage increased. This necessitates caution in the handling process to protect this apple variety from impact damage at harvest and during initial storage (Gharaghani and Shahkoomahally 2018). Contrary to our findings, Abi Tarabay et al., (2018) reported that bruising percent increased with prolonged storage time. As shown in Figs. 6 and 7 and 8 and 9, nonsignificant differences were observed between treatments in both growing seasons except at harvest time in the second growing season when fruits treated with KCl showed the highest bruise susceptibility. The values of BH, BV, and GBS were significantly higher for the KCl-treated fruits than the control only at harvest time (Figs. 7b, and 8b, and 9b). In general, the bruise was not influenced effectively by K and Ca treatments. Although Ca had no effect on bruise reduction in this study, a decrease in the BS in Ca (NO3)2-treated apples was reported in the study by Jafarian et al. (2021). They also observed the highest BS in untreated trees on the first day. Similarly, Tahir and Ericsson (2001) observed a positive effect of Ca application on BS reduction.

Fig. 6
figure 6

Changes in bruise diameter of “Red Delicious” apple during storage time treated with preharvest foliar spray of potassium sources and calcium chloride. a,b First and second season data, respectively

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Fig. 7
figure 7

Changes in bruise depth of “Red Delicious” apple during storage time treated with preharvest foliar spray of potassium sources and calcium chloride. a,b First and second season data, respectively

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Fig. 8
figure 8

Changes in bruise volume (BV) of “Red Delicious” apple during storage time treated with preharvest foliar spray of potassium sources and calcium chloride. a,b First and second season data, respectively

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Fig. 9
figure 9

Changes in general bruise susceptibility (GBS) of “Red Delicious” apple during storage time treated with preharvest foliar spray of potassium sources and calcium chloride. a,b First and second season data, respectively

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Correlation analysis showed that there is a strong positive correlation (R = 0.90) between BH and BV. In addition, the BH and BV in the fruit had significantly positive correlation (R = 0.78 and R = 0.8, respectively) with the BD. A positive correlation was recorded for BV (R = 0.56) with b* (Table 4). The tendency of lower susceptibility to bruise damage in the latter stages of storage can be explained by the differences in turgidity and the increased fruit skin resistance as well as the change in the fruit texture during storage (Van Zeebroeck et al. 2007). The same patterns have been noted by Yurtlu and Erdogan (2005) whereby BV and BS of “Starkspur Golden Delicious” apple and “Williams” pear decrease with increased storage time.

Fruit Mineral Concentration

In general, the K concentration of fruits increased by spraying treatments with K sources (solely or along with CaCl2) compared to the control trees. However, the highest values of K concentration of fruit were observed in K2SO4 and KCl in first and second season, respectively (Table 5). In both seasons, the lowest K concentration of fruit was recorded in the control fruits. A noticeable increase of the K concentration was also recorded in single treatment of CaCl2 compared to untreated control trees (Table 3). The K concentration in fruit was correlated with some of the fruit color attributes (Table 4). A positive and significant correlation (R = 0.32) was observed between K concentration and L*, while negative associations were seen between K and a* (R = −0.51) and K and c* (R = −0.48). Kadir (2005), applying a foliar spray of CaCl2 (4 and 8 times, 9 kg ha−1 during the growing season), reported significantly enhanced K concentrations in the peel and flesh of “Jonathan” apples, which is in accordance with the results of current study.

Table 5 Effect of preharvest foliar spray of potassium sources and calcium chloride on concentrations of potassium (K), calcium (Ca), and K/Ca ratio in treated “Red Delicious” apples, harvested in two growing seasons
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The highest fruit Ca concentration was recorded in fruits treated with CaCl2 (solely or KNO3 + CaCl2) as well as KCl. On the other hand, the lowest Ca levels were found in control fruits and other K sources in both growing seasons (Table 4). Spraying solely KCl resulted in an increased fruit Ca concentration compared to other K sources. Consistent with the results of this study, increased apple fruit Ca content as a result of preharvest Ca treatment has been reported in several studies (Benavides et al. 2002; Neilsen and Neilsen 2001; Jafarpour and Poursakhi 2011). Similarly, Asgharzade et al. (2012) stated that the Ca content of apples treated with CaCl2 was increased, which confirms the results of this study.

Chloride (in the form of KCl fertilizer) may act as a neutralizer ion, balancing inorganic cations such as Ca+2 and resulting in improved Ca mobility and transfer to the fruit. An antagonistic effect between K and Ca has been reported in the soil, which means use of K fertilizer can reduce the absorption of Ca by the roots (Neilsen and Neilsen 2003; Dilmaghani et al. 2005). However, it appears that this antagonistic action was formed only in the soil, not within the plant. Foliar treatment of K and Ca revealed no antagonistic interaction in this investigation. Not surprisingly, Ca concentration was positively correlated with FF (R = 0.33) and negatively correlated with GBS (R = −0.33), meaning that the fruit with higher Ca was firmer and less susceptible to mechanical damage (Table 4).

Over the two consecutive cropping seasons, the trend of K/Ca ratios was different. In general, CaCl2 (solely or combined with KNO3) as well as KCl produced a lower K/Ca ratio than the control or other treatments (Table 5). The highest ratio belonged to KNO3 (in the first growing season) and K2SO4 (in both growing seasons) treatments (29 and 28, respectively). The K/Ca ratio of the control trees was approximately 25. Therefore, the sole application of CaCl2 or the combination of KNO3 and CaCl2 led to the more appropriate ratio (approximately 15 ~ 21) than single K sources application (22 ~ 29). Ratios of K/Ca < 28 and K/Ca < 30 have been recommended for apple in Poland and Italy, respectively (Piestrzeniewicz and Tomala 2000; Drahorad and Aichner 2001). Given the K/Ca ratios reported in this study, foliar application of CaCl2 might be recommended for optimizing the ratio when K fertilizers are applied in an apple orchard. Due to the diluting effect, apple trees with a reduced fruit load would yield bigger fruits with lower Ca contents (Fallahi and Simons 2008). However, in the current study, bigger fruit were found in trees that received Ca coupled with KNO3 treatments, as well as greater K and Ca concentrations. As a result, rather than yield impacts, this higher absorption should constitute a real treatment effect. Interestingly, the K/Ca ratios were negatively correlated with FF (R = −0.36) and positively correlated with GBS (R = 0.31), which means fruit with a higher K/Ca ratio was less firm and more prone to mechanical damage (Table 4).

Conclusion

The results obtained in the present study indicate that none of the sole applications of K sources and Ca could effectively improve storability, while the best results were obtained when the combination of KNO3 and CaCl2 was applied. Thus, foliar application of K combined with Ca could positively affect postharvest quality of apple. Considering different sources of K, foliar spray of KCl was more effective in fruit quality only at harvest, but there were no significant differences between K sources after 4 months of storage. Also, results showed that the “Red Delicious” apple is more susceptible to bruise damage at harvest time. This means that the handling of fruits at harvest and during the initial phases of storage require special attention to protect this variety from impact damage.