Calcium and Potassium Foliar Sprays Affect Fruit Skin Color, Quality Attributes, and Mineral Nutrient Concentrations of ‘Red Delicious’ Apples

ABSTRACT

This study was conducted to evaluate the influences of foliar spray of potassium (K), calcium (Ca), and their combination on the fruit red skin color and quality attributes of ‘Red Delicious’ apple under conditions of south central Iran with warm and dry summer, where low and high temperature differences are low. The trees were sprayed five times using 5 g L^–1 calcium chloride (CaCl₂) at 3-week intervals starting from 3 weeks after full bloom and three times (at 9, 12, and 15 weeks after full bloom) using 2.5 g L^–1 K sources [potassium chloride (KCl), potassium sulfate (K₂SO₄), and potassium nitrate (KNO₃)] during two growing seasons in 2013 and 2014. Anthocyanin, some physicochemical attributes, and fruit mineral concentrations were measured at harvest. Results showed that spraying with K, CaCl₂, and their combinations significantly increased fruit weight, sugar and anthocyanin concentrations, firmness, and K uptake. A combined foliar application of CaCl₂ and each of the K sources was more effective on the improvement of fruit color, firmness, fruit K and Ca uptake, and K/Ca ratio as compared to the case when either K or Ca was applied alone.

KEYWORDS:

• Anthocyanin
• biochemical composition
• fruit firmness
• mineral concentration

Introduction

Red Delicious apple (Malus domestica Borkh.) is one of the most important commercial apple cultivars grown worldwide, including Iran. Solid red color of the skin and fruit size are considered to be two important quality attributes that contribute to consumer preference (Iglesias et al., 2008). Cyanidin-3-galactoside (Idaein) is the main pigment responsible for the red color in apple, which belongs to the anthocyanin group (Saure, 1990). In general, the best skin red color of apples can be observed in the weather conditions with the clear bright days and the cool nights during the pre-harvest period (Iglesias et al., 2002). In contrast, fruits grown in the regions with warm and dry summers usually have poor color and storage life (Saks et al., 1990). Under such conditions, postponing harvest time for better color development would decrease post storage quality attributes, namely, firmness and flavor (Iglesias et al., 2008). Many fruit quality attributes maybe influenced by orchard cultural practices, particularly nutrient management (Fallahi, 1983; Fallahi et al., 1985b). The positive effect of K fertilizer on apple coloration and fruit quality has been previously reported (Nava et al., 2007; Neilsen et al., 1998, 2004). It is important to note that K fertilization has the ability to increase the red color and quality of apple when the leaf K concentration is below 1%, but high K also reduces fruit firmness (Fallahi, 1983; Neilsen et al., 1998). On the other hand, K/Ca ratio below a critical threshold (desirable level) can result in reduced shelf life and storage disorders (Tomala, 1996). According to Dilmaghani et al. (2005) there is an inverse correlation between K/Ca ratios and fruit firmness, indicating that on the calcareous soils, at least eight foliar applications of CaCl₂ solution are necessary for ‘Golden Delicious’ apple. In contrast, Von Bennewitz et al. (2011) revealed that soil application of calcium sulfate (CaSO₄) and K₂SO₄ fertilizer in the acidic soil (pH = 5) do not affect mineral concentrations and quality of fruit in ‘Jonagold’ apple. They also reported that the soluble solids concentration (SSC) was not attributed to the K concentration, and the K/Ca ratio did not affect fruit firmness (Von Bennewitz et al., 2011). The foliar spray is regarded as a credible method to acquire a rapid response to fertilization of fruit trees, especially when soil conditions limit the uptake of elements by the root, or during periods of rapid growth or reproductive growth stages (Swietlik and Faust, 1984). Fars Province is among the areas with rapid extension of apple orchards in the last decade in Iran. This area is located in a lower latitude and has a warmer climatic condition than other apple growing areas in Iran, such as East and West Azerbaijan, Tehran, Khorasan, and Isfahan provinces. Poor coloration, sunscald, and low storage life are among the main issues that need to be considered in the apple orchards of this area. Therefore, our study aimed to evaluate the impact of foliar spray of different K sources (KNO₃, K₂SO₄, and KCl), CaCl₂ and their combination on the skin color (anthocyanin), physicochemical attributes and mineral composition (K, Ca, K/Ca) of fruits, and the relationship between the concentrations of nutrients and quality attributes of ‘Red Delicious’ apple fruits grown in Fars Province under conditions of inadequate K in the leaves.

Materials and methods

Plant material and experimental treatments

The experiment was conducted in a commercial orchard located at Kudian (29° 50′ N, 52° 11′ E, 7213 ft), with a total area of 90 hectares in Fars province, in south central Iran. This site displays a temperate and semi-arid climate. According to statistics of Fars meteorological bureau (40 years), the average high and low temperatures and rainfall in the vegetation period (March to September) are 32.4 °C, 15.4 °C, and 54 mm, respectively. The mean annual temperature is 18 °C. The average annual precipitation is 324 mm, which is distributed throughout the year, but it primarily occurs between November and February.

This study was conducted during two consecutive growing seasons (2013 and 2014) on 14-year-old apple trees (Malus × domestica Borkh. cv. Red Delicious) on MM 106 rootstock at 4 × 2.5 m spacing and a modified leader training system. A total of 32 uniform and healthy trees were selected and tagged as experimental plant materials in a block of this commercial orchard. The orchard floor was managed using mechanical cultivation between rows and chemically controlled weed free strip (2 m width) within the rows. Very light pruning (including removal of dead, infected, broken, as well as thinning of crowded branches and shoots) and fruitlet thinning were done manually in all of the experimental trees. Trees were irrigated using a drip irrigation system with 2-day intervals and the amount of irrigation water was adjusted based on the trees’ requirement during the growing season. The regular nutrition program of the orchard consisted of an application of 20 t ha^–1 of decomposed dairy cattle manure, 250 kg ha^–1 potassium sulfate, and 100 kg ha^–1 of triple super phosphate every other year to the soil; 100 kg of urea annually divided in two applications of equal amounts in autumn (late October) and spring (mid-May) using a fertigation system; plus one foliar spray of a mix of trace elements in early June. The common plant protection plan for the orchard was based on two sprays for codling month (Diazinon in late May and Phosalone in early July), one spray of TOPAZ^® fungicide for powdery mildew (early June), and one spray of Omite for spider mite (early August).

Soil samples from the 0–30-cm and 31–60-cm depths and leaf samples were collected and analyzed according to standard methods as described by Horneck et al. (1989) and Fallahi and Simons (1993), respectively. Preliminary evaluation of soil and plant mineral status in the experimental site showed that the soil type is calcareous with loamy texture, and the amount of K is about marginal in the soil while the leaves are faced with a shortage of K and several other nutrients (Tables 1 and 2). Basic physicochemical properties of the soil and leaf analysis, which were measured in the year before the start of the experiment, are given in Tables 1 and 2. The arable soil layer (0–30 cm) had low concentrations of iron (Fe), manganese (Mn), and copper (Cu); borderline levels of K; normal level of zinc (Zn) and organic carbon; but a high concentration of phosphorous (P). The sub-arable soil layer (31–60 cm) showed the borderline amounts of K; standard level of Mn, Fe, Zn, and Cu; low concentrations of organic carbon; but high concentration of P (Westwood, 1993). Leaf analysis showed low concentrations of K, Cu, and Zn; standard levels of Fe, Mn, and P; a high concentration of boron (B); and borderline nitrogen (N) as shown in Westwood (1993) and Ferree and Warrington (2003). In this orchard, the amount of K was insufficient in the leaves.

Table 1. Soil physical and chemical analyses at the experimental orchard.

	Soil texture
	Sand	Silt	Clay	Cu	Mn	Zn	Fe	K	P	N^z	OC^y	TNV^x	SP^w
Soil depth (cm)	%			(ppm)						(%)				pH	EC (ds/m)
0–30	50	32	18	0.3	0.8	2.1	0.9	250	45	0.2	2.14	40	29	7.39	2.58
30–60	50	30	20	0.6	7	1.3	5.3	261	26	0.12	1.36	45	30	7.46	1.09

Note. Soil samples were taken at 0–30 and 30–60 cm depth.

^zTotal nitrogen; ^yOrganic carbon; ^xTotal neutralizing value; ^wSaturation percentage.

Table 2. Leaf chemical composition of the experimental orchard.^z

(ppm)					(% dry weight)
B	Cu	Mn	Zn	Fe	K	P	N
65	5	37	10	73	1.13	0.12	1.63

Note. ^zSamples of 50 leaves were collected in mid-August in the year before start of the experiment from mid-terminal and new year extension growth around the trees.

The trees were sprayed with three sources of K (KCl, K₂SO₄, and KNO₃; 3.5 L per tree) at rates of 2.5 g L^–1 as well as 5 g L^–1 CaCl₂ for each of their oxides (K₂O) or (CaO) as follows:

• T1: Control trees (standard practice: split application of 250 g NH₂CONH₂ and 30 g EDDHA per tree based on soil test and leaf analysis recommendations);

• T2: T1+CaCl₂ (commercial flake, 78% CaO) was sprayed five times (starting from 3 weeks after full bloom (WAFB) with intervals of 3 weeks) in each growing season;

• T3: T1+KNO₃ (44% K₂O, 14% N) was sprayed three times: 9, 12, and 15 WAFB in each growing season;

• T4: T1+K₂SO₄ (50% K₂O, 45% SO₃) was sprayed three times: 9, 12, and 15 WAFB in each growing season;

• T5: T1+KCl (60% K₂O, 47% Cl) was sprayed three times: 9, 12, and 15 WAFB in each growing season;

• T6: T2+T3 (CaCl₂+KNO₃);

• T7: T2+T4 (CaCl₂+K₂SO₄);

• T8: T2+T5 (CaCl₂+KCl).

A randomized complete block design (RCBD) was used with eight treatments and four single-tree replicates (each apple tree was considered as one replication and a total of 32 trees were selected). Fruits harvested 145 days after full bloom based on Cornell starch-iodine starch staining pattern (starch index = 3.5; Blanpied and Silsby, 1992). Fruit samples consisting of 30 fruits were randomly taken at harvest from each replicate for determination of both physical and chemical properties.

Fruit physical attributes

Immediately after harvest the weight (g), length (cm), diameter (cm), and fruit shape index (L/D) of 30 fruits per single-tree replicate (four replicates) were recorded using proper digital balance and caliper, respectively. Flesh firmness was determined using a Force Gauge (FG-5005, Lutron, Taiwan) on the two opposite sides of the fruit and expressed as Newton.

For bruise test on fruits, a round steel ball (110 g) was hurled from a height of 30 cm into a vertical hollow and then through perforated PVC pipe, thereby hitting opposite sides of the fruit (10 fruits per tree or replicate, four replicates) along its equatorial axis. After testing, fruits were placed in the laboratory for about 24 h to allow bruise development. Bruise sizes were measured using a slice of tissue obtained by cutting the fruit at a vertical axis through the center of the bruise, down to the fruit core. Bruise diameter (d) and depth (h) were measured using a digital caliper. Bruise volume (BV, cm³, Formula 1) was calculated based on Mohsenin (1986) as follows:

(1)

Fruit chemical attributes

For measuring anthocyanin concentration, peel tissues (10 fruits per replicate, four replicates) were separated with a peeler from the two opposite sides (on both the colored and opposite sides of fruit) and immediately frozen in liquid N₂. One gram of frozen–crushed tissues was homogenized using methanol-HCl (99:1) solution in the dark and centrifuged at 4 °C and 12,000 rpm for 20 min. Then, anthocyanin concentration was measured based on the pH differential method by Lee et al. (2005) Absorbance of each extraction was measured using a spectrophotometer (T60U, PG Instruments, Leicestershire, UK) at 510 and 700 nm in buffers of pH 1.0 and 4.5. Anthocyanin concentration (cyanidin-3-galactoside) was calculated according to the following formula:

where A = [(A520 nm – A 700 nm) pH 1.0 – (A520 nm – A700 nm) pH 4.5]; MW (molecular weight) = 445.2 (g.mol^–1) for cyanidin-3-galactoside; DF = dilution factor; l = path length (1 cm); ε = 30,200 (L.mol^–1.cm^–1) molar extinction coefficient. Finally, data were expressed as mg cyanidin-3-galactoside per g of fresh weight.

To evaluate fruit soluble sugar (glucose) and starch concentration, the fruit samples (from 10 fruits per replicate, four replicates) were dried at 75 °C for 72 h and grounded. Then, 0.1 g of fruit powder was dissolved in 13 ml of ethanol (80%). Sugar concentration was estimated using a spectrophotometer at 490 nm according to Buysee and Merckx’s (1993) method and starch measurement was taken at 630 nm by Cready et al.’s (1950) method. The results were expressed on a dry weight (DW) basis. Sub-samples of 10 unpeeled apple fruits were used for total acidity (TA %) determination. Percentage of total acidity in fruit juice was determined by titrating the homogenate with 0.3 M NaOH and was calculated as malic acid according to Fallahi et al. (1985c) Flavor index (Sugar/TA ratio) was also calculated.

Analysis of fruit mineral concentration

In order to assess the fruit mineral composition, two longitudinal slices were obtained from each fruit (10 fruits per replicate, four replicates), including the peel. The fruit samples were dried in an oven at 75 °C for 72 h. Ash was produced at 500 °C for 8 h, dissolved in 5 ml of 2N HCl and filtered through a filter paper, then diluted to a volume of 50 ml with deionized water. Concentration of Ca was determined using an atomic absorption spectrophotometer (Shimadzu-AA-670, Shimadzu, Kyoto, Japan) at 422.7 nm (Waling et al., 1989) and K was estimated using flame photometry (Jenway-PFP7, Jenway, Staffordshire, UK) at 766.5 nm (Chapman and Pratt, 1961). The results were expressed on a DW basis.

Statistical analysis

Statistical analysis was performed by SAS software (version 9) according to a RCBD with four replications. Data were analyzed using general linear model (GLM) and means were compared by least significant differences (LSD) test at a significance level of P < 0.05.

Results and discussion

Fruit physical attributes

In both seasons, fruit weight was increased by application of all treatments. It is noteworthy that fruit weight usually has an inverse relationship with yield (Fallahi et al., 1985a). Although the yield was not measured in this experiment, fruit size from 2013 was significantly larger than those of 2014, due to the possibly lighter crop load in 2013. Almost all treatments significantly increased the fruit weight in both cropping years, but in 2013, [K+CaCl₂] treatments resulted in higher mean fruit weight compared with the single application (Table 3). This may be because K⁺, Ca²⁺, and Cl^– ions are needed for cell division and cell expansion to ultimately control the growth (Marschner, 2012). In addition, based on leaf analysis the concentration of K and other mineral compounds was inadequate (Table 2), so it was clear that foliar application of these mineral nutrients should have a positive effect on the fruit growth. Compared to the control treatment (T₁), the fruit length was only affected by KNO₃ (T₆) and [K+CaCl₂] treatments (T₇, T₈) in both years (Table 3). However, fruit diameter and shape index (L/D) did not differ significantly among the studied treatments. Cytokinin (CK) and Gibberellin (GA) are two families of phytohormones, which are mostly responsible for L/D ratio and possibly in warmer conditions (such as our experimental site in Fars Province), K and other treatments are not significantly affecting endogenous CK or GA levels (Westwood, 1993).

Table 3. Effect of foliar application of CaCl₂, K sources, and their combinations on fruit physical properties of ‘Red Delicious’ apple over two cropping years (2013 and 2014).

	Fruit length (cm)		Fruit diameter (cm)		Length to diameter ratio		Fruit weight (g)		Firmness (N)		Bruise volume (cm^3)
Treatments	2013	2014	2013	2014	2013	2014	2013	2014	2013	2014	2013	2014
T1:Control	6.44b	6.34b	6.90b	7.36ab	0.93a	0.86a	148.65c	145.20b	39.98d	44.30c	3.77bc	3.25b
T2:CaCl2	6.68a	6.32b	7.06b	7.20ab	0.95a	0.89a	160.32b	148.37a	44.50a	46.70a	3.60c	3.07b
T3:KNO3	6.67a	6.60a	7.05b	7.53ab	0.94a	0.87a	159.30b	148.56a	39.92cd	45.20abc	4.03ab	3.39b
T4:K2SO4	6.62ab	6.33b	7.03b	7.09b	0.94a	0.89a	158.70b	148.14a	40.65bcd	44.40bc	3.63c	3.24b
T5:KCl	6.63ab	6.29b	7.01b	7.20ab	0.95a	0.87a	160.07b	148.15a	42.18bc	46.30ab	4.13a	4.01a
T6:KNO3+CaCl2	6.67a	6.68a	6.92b	7.54ab	0.96a	0.89a	166.15a	149.57a	43.50ab	47.10 a	4.06ab	2.88bc
T7:K2SO4+CaCl2	6.72a	6.61a	7.07ab	7.40ab	0.95a	0.89a	164.55a	149.60a	44.63a	46.51a	3.68c	3.14b
T8:KCl+CaCl2	6.84a	6.82a	7.31a	7.70a	0.94a	0.89a	166.17a	149.16a	43.54ab	46.54a	3.72c	3.18b

Note. Values within each column followed by the same letter are not significantly different according to LSD test (p ≤ 0.05). Data are means of four replicates (n = 4).

As compared to non-treated fruits (T₁), the fruit firmness was significantly increased by spraying of CaCl₂ (solely and along with K sources) and KCl (T₅); however, a single application of K₂SO₄ (T₄) and KNO₃ (T₃) had no significant impact on this parameter in both years (Table 3). KCl resulted in overall improvement of metabolism including mineral uptake, particularly Ca that contributed to higher fruit firmness (Table 4). Also, fruit concentration of Ca was positively correlated with fruit firmness in both years (r² = 0.83** and r² = 0.75* in 2013 and 2014, respectively) as previously reported by Fallahi et al. (1985b). Fruit firmness is an integral part of quality and is of increasing importance in today’s marketplace. Larger fruits have less firmness than the smaller, because the dilution effects of fruit Ca are more apparent than that on other elements in large fruit (Fallahi et al., 1985a). The results of this research showed that, in spite of larger fruits of trees, which were treated using CaCl₂ (alone or in combination with K sources), fruits were markedly firmer than others due to the increased fruit Ca (Table 4). This result confirms that addition of Ca would lead to increased cell wall resistance and firmness (Fallahi et al., 1997). Our findings also showed a negative correlation between fruit firmness and the K/Ca ratio in the fruits (Table 5) as was reported by Dilmaghani et al. (2005). A positive correlation was observed between fruit starch concentration and firmness (r² = 0.48 and r² = 0.57* in 2013 and 2014, respectively; data not shown). Similarly, Fallahi et al. (2013) have shown that fruit starch degradation pattern (SDP) at harvest had strong negative correlations with fruit firmness at harvest and after storage; thus, firmness could be predicted using a simple SDP test in apples. It seems that the ripening process can be delayed by foliar application of Ca through a reduction in respiration and ethylene production, preventing softness and starch molecule breakdown (Fallahi et al., 1985a).

Table 4. Concentrations of K and Ca, and K/Ca ratio in ‘Red Delicious’ apple fruits harvested in 2013 and 2014.

	Potassium (mg.Kg^–1 DW)		Calcium (mg.Kg^–1 DW)		K/Ca ratio
Treatments	2013	2014	2013	2014	2013	2014
T1:Control	7451.5c	7686.5c	276.48c	321.50c	27ab	24ab
T2:CaCl2	7671.0b	7971.0b	374.05ab	408.55b	21c	20c
T3:KNO3	7793.0ab	8147.0ab	271.98c	313.98c	29a	26a
T4:K2SO4	7978.8a	8120.3ab	290.33c	305.33c	28ab	28a
T5:KCl	7870.3ab	8278.8a	371.43ab	412.43b	22bc	21bc
T6:KNO3+CaCl2	7774.5ab	8074.5b	366.40b	556.40a	21bc	15d
T7:K2SO4+CaCl2	7767.5ab	7977.5b	382.63ab	398.83b	20c	20bc
T8:KCl+CaCl2	7809.0ab	8139.0ab	405.43a	444.93b	19c	19cd

Note. Values within each column followed by the same letter are not significantly different according to LSD test (p ≤ 0.05). Data are means of four replicates (n = 4). DW: dry weight.

In both seasons, sole application of KCl markedly increased bruise volume compared to non-treated fruits, whereas no differences were detected between control and other treatments (Table 3). KCl-treated trees also showed significantly higher sugar concentration, which increases their bruise susceptibility because of higher cell turgidity as previously reported by Tahir et al. (2007). Stresses in the tissues are higher in turgid fruit, so they are more susceptible to bruising, and turgidity changes seem to be the cause of the lower susceptibility of fruits to damage after storage (Garcı́a et al., 1995). Although this effect was only observed at harvest time and not after 2 and 4 months of storage (data not shown), to make sure, bruising should be examined for more years. No significant correlation was found between fruit minerals (K, Ca, K/Ca) and bruise volume in either year (Table 5). Also, in this study as previously reported by Mowatt (1997) there was no relationship between fruit firmness and bruise.

Fruit chemical properties

Results showed that all K sources treatments (single or in combination with CaCl₂) significantly increased anthocyanin concentration compared to the non-treated plants, but the highest anthocyanin values were achieved by sole application of KCl treatment in both experimental years. Although single application of CaCl₂ significantly increased anthocyanin concentration compared to the untreated plants, K sources treatments resulted in higher anthocyanin concentration (Table 6). In addition, there was a positive correlation between fruit K concentration and anthocyanin in both years (r² = 0.82** and r² = 0.55* in 2013 and 2014, respectively; Table 5). Potassium foliar application (1.5 and 3 g L^–1 Potassium Metalosate) was reported to influence anthocyanin concentration in pomegranate (Tehranifar and Mahmoodi Taber, 2009). It seems that K is an important element in the anthocyanin pathway and could be a cofactor in the activation of some specific enzymes, similar to the UDPGalactose:flavanoide-3-o-glicosiltransferase (Nava et al., 2007). Although, the apple yield or crop load was not measured in this experiment, this experiment’s results show a positive correlation between total sugar concentration and anthocyanin concentration of the fruit in both seasons (r² = 0.54* and r² = 0.50* in 2013 and 2014, respectively; figure not shown) which is in agreement with a previous study by Fallahi et al. (2013). On the other hand, the fruit sugar concentration was positively correlated with the fruit K concentration (r² = 0.76** and r² = 0.86** in 2013 and 2014, respectively; Table 5), suggesting that color improvement could be due to the role of K in enhancing sugar translocation into the fruit, which is important for synthesis of polyphenols such as anthocyanins (Nava et al., 2007).

Table 5. Pearson correlation between fruit mineral concentrations and fruit quality attributes of ‘Red Delicious’ apples harvest in 2013 and 2014.

	‘r’ value (2013)			‘r’ value (2014)
Fruit properties	K	Ca	K/Ca	K	Ca	K/Ca
Anthocyanin (mg.g^–1 FW)	0.82**	ns	ns	0.55*	ns	ns
Firmness (N)	ns	0.83**	–0.85**	ns	0.75**	–0.80**
Total sugar (mg.g^–1 DW)	0.76**	0.19^ns	ns	0.86*	ns	ns
TA%	0.50*	ns	ns	0.56*	ns	ns
Sugar/TA ratio	0.55*	0.36^ns	–0.24^ns	0.70**	ns	ns
Starch (mg.g^–1 DW)	ns	0.50*	–0.53*	ns	0.64*	–0.64*
Bruise volume (cm^3)	ns	ns	ns	0.22^ns	–0.10^ns	0.11^ns

Note. ns = non-significant. *Significant at 5%; **Significant at 1%. Data are means of four replicates (n = 4).

In both years, CaCl₂ treatments (solely or combined with K sources) significantly enhanced starch accumulation in the fruit, whereas no significant changes were observed using a single application of K sources than the untreated trees, albeit the lowest starch concentration was obtained in KNO₃ and K₂SO₄ treatments as 35.50 and 26.50 mg g^–1 DW in 2013 and 2014, respectively (Table 6). There was a negative correlation between K/Ca ratio and apple starch concentration (r² = –0.50* and r² = –0.64*, in 2013 and 2014, respectively; Table 5). Furthermore, a positive correlation was observed between fruit Ca and starch concentration (r² = 0.50* and r² = 0.64* in the first and second year, respectively; Table 5), and between fruit firmness and starch concentration (not shown). It has been established that calcium delays ripening by reducing fruit respiration and Ethylene emissions and thereby slightly retarding the climacteric rise and reducing the climacteric maximum and preventing softness and starch molecule break down (Fallahi et al., 1985a; Marcelle et al., 1989; Recasens et al., 2004).

Table 6. Effect of foliar application of CaCl₂, K sources, and their combinations on fruit chemical properties of ‘Red Delicious’ apple over two cropping years (2013 and 2014).

	Anthocyanin (mg.g^–1 FW)^z		Starch (mg.g^–1 DW)^z		Total sugar (mg.g^–1 DW)		TA%^z		Flavor index
Treatments	2013	2014	2013	2014	2013	2014	2013	2014	2013	2014
T1:Control	27.20d	24.70d	39.23de	28.76cd	8.98f	10.15d	0.48c	0.56b	18.80c	18.18c
T2:CaCl2	28.75cd	27.10cd	42.60cd	33.80b	11.43e	11.70c	0.50bc	0.59ab	22.50bc	19.64bc
T3:KNO3	34.25ab	32.62ab	35.50e	31.36bc	13.65cd	12.83b	0.57ab	0.61a	24.00b	20.89b
T4:K2SO4	35.75a	31.10ab	37.07de	26.50d	14.43bcd	12.80b	0.57ab	0.63a	25.50ab	20.38b
T5:KCl	32.40abc	33.00a	38.70de	30.57bc	16.00a	14.52a	0.61a	0.61a	26.50ab	23.62a
T6:KNO3+CaCl2	32.20abc	30.20ab	50.73a	38.10a	14.80abc	12.69b	0.61a	0.63a	21.40b	20.13b
T7:K2SO4+CaCl2	32.17abc	30.50ab	44.92bc	33.02b	13.08d	12.75b	0.54abc	0.625a	24.40b	20.39b
T8:KCl+CaCl2	31.50bc	29.70bc	47.02ab	40.48a	15.30ab	12.40bc	0.51bc	0.625a	30.11a	19.80bc

Note. Values within each column followed by the same letter are not significantly different according to LSD test (p ≤ 0.05). Data are means of four replicates (n = 4).

^zFW–DW: fresh or dry weight. TA: Titratable acidity.

This study’s results showed that in 2013 apple trees had significantly higher fruit SSC than 2014, possibly because of their larger fruits and the lower yield in 2013. This observation agrees with Fallahi et al. (1985a) who reported an inverse relationship between SSC and fruit yield. This experiment’s results revealed that K application (alone or with CaCl₂) markedly increased the total sugar concentration of fruits compared to the untreated ones (Table 6). In both years, the highest and the least total sugar values were related to the KCl and the control treatments, respectively. Accordingly, there was a significant correlation between the fruit sugar concentration and K concentration in 2013 (r² = 0.76**) and in 2014 (r² = 0.86**; Table 5), which is in agreement with earlier studies (Fallahi, 1983; Nava et al., 2007). The T₂ treatment (CaCl₂) increased the total sugar concentration in comparison with T₁ (control); however, the positive effect of Ca (alone) was significantly lower than the effect of K treatments. This finding is in line with the observation of Lu et al. (2013) who reported that foliar spray of CaCl₂ (10 g L^–1, at fruit set stage) significantly increased the total sugar concentration of ‘Fuji’ apple.

The total acidity (TA) in apple fruit was significantly elevated by K foliar application (solely or combined with CaCl₂) except for T₇ and T₈ in 2013 (Table 6). A low acidity at harvest can lead to a poor organoleptic quality of the apple after storage, hence, to a lower consumer’s acceptance. This finding confirms that K seems to be involved in the biosynthesis of organic acids of apple (Nava et al., 2007). The accumulation of organic acid anions (like malic acid) in plant tissues is often the consequence of K transport without accompanying anions, because in charge compensation, K is the dominant cation for counterbalancing immobile anions in the cytoplasm, chloroplasts, and quite often also for mobile anions in vacuoles, the xylem, and the phloem (Marschner, 2012). Similar results were obtained by Nava et al. (2007) and Neilsen et al. (1998; 2004) reporting that the TA of several apple cultivars (Fuji, Gala, Fiesta, Spartan, and Summerland McIntosh) was positively affected by K fertilization (soil application or fertigation). This results of this study showed that sole application of CaCl₂ had no effect on the fruit acidity in both years (Table 6). This is consistent with Ramezanian et al. (2009) who indicated that CaCl₂ spray at different concentrations had no significant effect on the TA of ‘Malase-Yazdi’ pomegranate. Accordingly, there was a positive correlation between fruit acidity and K concentration in both years (r² = 0.50* and r² = 0.56*; Table 5). Casero et al. (2004) also found a positive correlation between fruit acidity and K concentrations in ‘Golden Smoothee’ apples.

In comparison with untreated trees, the flavor index (sugar/acid ratio) was increased by all K treatments (except for T₈ in 2014; Table 6). In 2014, the sole application of KCl (T₅) resulted in the highest sugar/acid ratio. In contrast, single application of CaCl₂ (T₂) had no significant effect compared to the control in both years. A significant positive correlation was found between the sugar/acid ratio and the fruit’s K concentration (r² = 0.50* and r² = 0.70** in 2013 and 2014, respectively; Table 5). These results are in line with Cronje et al. (2009) who reported that application of KNO₃ (2%) on Litchi cultivar, Maritius, at fruitlet stage increased the fruit K content and the SSC/TA, and resulted in a sweet and sour blended flavor, which could be regarded as more beneficial for consumer acceptance.

Fruit mineral concentration

In general, the K concentration of fruit increased by K sources (solely or along with CaCl₂) treatments compared to non-treated trees. However, the highest values of fruit K concentration were recorded from K₂SO₄ (T₄) and KCl (T₅) applications in 2013 and 2014, respectively (Table 4). In both years, the least fruit K concentration was observed in non-treated fruits (T₁). A significant increase of the K concentration was also observed when using CaCl₂ (T₂) compared with untreated trees (Table 4). Similarly, Kadir (2005) stated that foliar application of CaCl₂ (four and eight times during the growth season, 9 Kg ha^–1) significantly raised the K concentration of the peel and flesh of ‘Jonathan’ apples.

The highest fruit Ca concentration was obtained from the trees treated by CaCl₂ (solely or with K sources) and sole application of KCl (T₅). In contrast, the lowest mean values were observed in control trees (T₁) and in those sprayed with some of the K sources (T₃, T₄) in both years (Table 4). Single application of KCl (T₅) resulted in an increase in the Ca uptake in the fruit compared with the other two K sources. It is possible that chloride (with the use of KCl fertilizer) plays a role as a neutralizer ion for balancing inorganic cations such as Ca⁺² and increases mobility and transportation of Ca to the fruit. It has been reported earlier that there is an antagonistic effect between K and Ca in the soil and the use of K fertilizer helped reduce the absorption of Ca by the roots (Dilmaghani et al., 2005; Neilsen et al., 2004). However, it seems that this antagonistic effect was only established in the soil and not inside the plant. In our study, an antagonistic relationship between K and Ca has not been observed through foliar spraying treatments.

Over the two studied cropping seasons, the K/Ca ratios showed significant differences. In general, CaCl₂ (single or combined with various K sources) and KCl treatments produced a lower ratio than the control or other treatments (Table 4). The greatest ratio was related to T₄ (K₂SO₄) and T₃ (KNO₃) treatments (mean values of 28–29). The K/Ca ratio of the untreated trees (T₁) was approximately 25. K/Ca < 28 and K/Ca < 30 have been recommended in Poland and Italy, respectively (Drahorad and Aichner, 2001; Piestrzeniewicz and Tomala, 2001). Therefore, the combination of each K source with CaCl₂ led to the more appropriate ratio (approximately 19–21) than single K application (26–30). Considering the K/Ca ratios obtained in this study to whenever K fertilizers are used in an apple orchard, foliar application of CaCl₂ could be recommended for adjusting the ratio. Apple trees with a lighter fruit load would produce larger fruits with lower Ca concentrations, due to the dilution effect (Fallahi and Simons, 1993). However, in this study, the larger fruit size in trees receiving Ca combined with K treatments also had higher fruit K and Ca concentrations. Thus, this increased uptake should be a true treatment effect rather than yield effects.

Conclusion

Considering that these two main limitations (warm climate during pre-harvest period and sub-optimal level of effective nutrients, such as K and Ca) for poor red color development and low storage life on red apples in the region (Fars Province), results showed that foliar application of Ca in combination with various K sources has the potential to improve fruit color, firmness, and other quality attributes simultaneously. In this regard, a combined foliar application of CaCl₂ and K of different sources was more effective on the improvement of fruit quality attributes (especially color, firmness, fruit K and Ca uptake, and K/Ca ratio) than separate applications of each of these compounds. The results of this 2-year experiment could have a major impact on the apple industry in the region, because they unveil a realistic situation that exists in the majority of apple orchards where climate is warm and nutrient levels are sub-optimal.