Control efficacy of Ca-containing foliar fertilizers on bitter pit in ‘Fuji’ apple and effects on the Ca and N contents of apple fruits and leaves

发布日期：2019-08-15     浏览人数：1048

YU Xian-Mei, WANG Jin-Zheng*, NIE Pei-Xian, XUE Xaio-Min, WANG Gui-Ping, AN Miao

Shandong Institute of Pomology, Tai’an, Shandong 271000, China

Abstract

BACKGROUND: The preharvest application of Ca-containing foliar fertilizers can reduce BP incidence in apples and improve fruit quality by increasing the Ca content and decreasing both the N content and the N/Ca ratio in fruits. In this study, we aimed to investigate the control efficacy of Ca-containing foliar fertilizers on BP in apples and the effects on the Ca and N contents in ‘Fuji’ apple by spraying the foliar fertilizer containing calcium chloride (CaCl₂), calcium nitrate [Ca(NO₃)₂] or calcium formate [Ca(HCOO)₂] at the early stage [5 days after full bloom (DAFB) + 40 DAFB] and at the late stage (80 DAFB + 125 DAFB).

Results: The Ca content of apple fruits was increased and the N content and N/Ca ratio of apple fruits were decreased, however the Ca content, N content and N/Ca ratio of apple leaves were differentially influenced. The BP incidences were reduced significantly with the reduce percentage of 43.2~73.0%, and the efficacy of spraying at the early stage was significantly higher than that of spraying at the late stage.

Conclusion: The foliar fertilizer containing Ca(HCOO)₂, CaCl₂ or Ca(NO₃)₂ could be used at preharvest for the control of BP in apples and quality improvement of apple fruits.

Key words: Bitter pit in apple, Ca-containing foliar fertilizer, Ca content, N content, N/Ca ratio

Introduction

Bitter pit (BP) is a Ca²⁺ deficiency physiological disorder that affects apple fruit production worldwide, especially for bagged apples, and often causes significant economic losses.^1,2 In addition to Ca²⁺, other nutrients, including magnesium (Mg²⁺), potassium (K⁺), boron (B) and nitrogen (N), may play an important role in BP development in apple fruits, and commercial apple growers frequently rely on the [K⁺ + Mg²⁺]/Ca²⁺ and N/Ca²⁺ ratios as guides both for predicting fruit susceptibility to BP and for deciding the application time of Ca fertilizers.^3,4,5 In general, high Ca and B contents are related to a low incidence of BP, especially for moderate and severe BP, whereas high incidence of BP is positively associated with N, K and Mg contents.^3,6,7 In addition, compared with lower fruit Ca²⁺ levels alone, the combination of higher N levels and lower Ca²⁺ levels in fruit tissue is usually more strongly correlated with fruit susceptibility to BP.^3,4,8 Despite abundant research, the physiological causes of BP remain poorly understood; consequently, management techniques to prevent this disorder remain inadequate.

Ca sprays and Ca soil applications throughout the growing season as well as Ca solution dips at postharvest are practiced to supply Ca and reduce BP in apples.⁷ However, applications of foliar Ca products to aboveground parts such as leaves and fruits during the growing season to increase fruit Ca content, improve fruit quality and reduce BP incidence in apples are most widely used and are effective. Lötze et al. ⁹ applied calcium nitrate [Ca(NO₃)₂], calcium carbonate (CaCO₃) and calcium acetate [Ca(COOCH₃)₂] beginning at three different developmental stages (early, middle and late) of fruit growth and reported that late Ca(NO₃)₂ [80 days after full bloom (DAFB)] applications increased the Ca content of fruits at harvest more than early (6 DAFB) and middle (40 DAFB) applications did. BP tended to decrease from the early-to-late applications of Ca(NO₃)₂ and CaCO₃, whereas Ca(COOCH₃)₂ applications did not show any effects in fruit Ca content or BP incidence. Val et al. ¹⁰ showed that in-season CaCl₂ sprays lead to increased Ca concentrations in the skins of the fruits.

Blanco et al.¹¹ assessed the efficacy of Ca spray formulations containing either calcium chloride (CaCl₂) or calcium propionate as active ingredients (120 or 250 mM Ca) on BP in apple, and found that the addition of appropriate adjuvants to Ca sprays can improve the allocation of Ca to the apple fruits and help reduce the incidence of Ca-related disorders during the postharvest storage period. Wilsdorf et al.¹² evaluated the contributions of Ca root applications and sprays to increases in the Ca content of fruits and reported that Ca sprays were more effective than soil Ca applications were at increasing the Ca content in the fruits, but the efficacy on BP could not be evaluated because no incidence of the disorder occurred. Torres et al.⁷ assessed the effects of combining preharvest Ca applications using soil and/or sprays on ‘Golden Delicious’ and found that the sprays mitigated BP more than Ca soil applications did, and compared with Ca sprays alone, combinations of sprays and soil applications did not improve the results.

The aim of this study was to investigate the control efficacy of Ca-containing foliar fertilizers on BP in ‘Fuji’ apple and the effects on the Ca and N contents of apple fruits and leaves by spraying CaCl₂-, Ca(NO₃)₂- or calcium formate [Ca(HCOO)₂]-containing foliar fertilizer at the early stage (5 DAFB + 40 DAFB) and at the late stage (80 DAFB + 125 DAFB) of fruit growth, after which the incidence of BP at harvest would be estimated and the Ca content, N content and N/Ca ratio of the fruits and leaves at 0 (first sample prior to the 1st spray application), 12, 47, 87, 132 (7 days after each date of spray application) and 165 DAFB (at harvest) would be analyzed. The results of this research would provide a feasible solution for the control of BP in apples and lay the foundation for the quality improvement of bagged fruits.

Experimental

Orchard used for the experiments

The experiments were performed in an orchard of 18-year-old ‘Fuji’ apple (Malus domestica Borkh.) trees. The orchard is located in Mengyin County, Shandong Province, China. This orchard was mature, and its trees were spaced normally (approximately 4 × 1.2 m), grafted onto M9 rootstock, and fully irrigated. All of the trees for spraying experiment were located in the same field, and were subjected to the standard cultural practices of pruning, fertilization, irrigation and crop management.

Ca-containing foliar fertilizers

Fertilizer A: 100 L of foliar fertilizer contains 147.00 g of CaCl₂·2H₂O (1 M), 120.00 g of H₃BO₃, 20.00 g of ZnCl₂·H₂O and 30.00 g of MgCl₂·6H₂O.

Fertilizer B: 100 L of foliar fertilizer contains 236.15 g of Ca(NO₃)₂·4H₂O (1 M), 120.00 g of H₃BO₃, 20.00 g of ZnCl₂·H₂O and 30.00 g of MgCl₂·6H₂O.

Fertilizer C: 100 L of foliar fertilizer contains 130.00 g of Ca(HCOO)₂ (1 M), 120.00 g of H₃BO₃, 20.00 g of ZnCl₂·H₂O and 30.00 g of MgCl₂·6H₂O.

Spray treatments

The Ca-containing foliar fertilizers were sprayed either at the early stage (5 DAFB and 40 DAFB) or at the late stage (80 DAFB and 125 DAFB). Combined with the 3 Ca-containing foliar fertilizers, 6 spraying treatments were used, as shown in Table 1. Spraying one tree served as one treatment, spraying water at the same time served as the control, and each treatment was repeated three times.

BP incidence

Samples consisting of 100 fruits that were randomly collected from each tree at harvest were used to evaluate BP incidence. All of the apples contained in each sample were individually examined for external signs of superficial BP symptoms. The incidence of BP for each sample was calculated as the percentage of fruits that exhibited BP symptoms. Based on the BP incidence, the control efficacy of Ca-containing foliar fertilizers on BP in apple was calculated as follows: Control efficacy (%) = (The BP incidence of control – The BP incidence of treatment) / The BP incidence of control ′ 100%.

Ca and N analyses of fruits and leaves

Fruit and leaf samples

For Ca and N analyses, 15-fruit samples and 100-leaf samples of each tree were collected randomly from locations throughout the tree canopies of the selected ‘Fuji’ orchard at 0 (first sample prior to the 1st spray application), 12, 47, 87, 132 (7 days after each date of spray application) and 165 DAFB (at harvest).

Ca and N analyses

The fruits for Ca and N analyses were carefully washed sequentially with SDS solution, water and deionized water, after which two longitudinal slices were cut from the opposite sides of each fruit, excluding the core and seeds. The complete samples of each group were dried at 105°C for 15 min, dried at 75°C to constant weight and then ground, after which a subsample was nitrified with HNO₃-HClO₄ (3:1).

To investigate the influence of BP on the Ca content and N content in different parts of pitted fruits, the fruits were first washed sequentially with SDS solution, water and deionized water. Afterward, samples of healthy fruits, healthy tissue surrounding the pitted regions and the pitted tissue of fruits were collected, dried at 105°C for 15 min, dried at 75°C to constant weight and then ground, after which a subsample was nitrified with HNO₃-HClO₄ (3:1). Each treatment was repeated three times.

For leaf samples, after the petioles were removed, the leaf tissue was first carefully washed sequentially with SDS solution, water and deionized water. Afterward, the samples were dried at 105°C for 15 min, dried at 75°C to constant weight and then ground, after which a subsample was nitrified with HNO₃-HClO₄ (3:1).

The Ca concentration was determined using inductively coupled plasma atomic emission spectrometry.¹³ The N concentration was determined via the Kjeldahl analysis method.¹⁴ Statistical analysis was performed using SPSS22.0 statistical software package. The mean values of three replicate samples were used for calculating the N/Ca ratio.

Results

Effects of BP disease on the Ca and N contents in different parts of fruits

The Ca contents of both pitted tissue and healthy tissue surrounding the pitted regions were lower than those of healthy fruits, and the N content and N/Ca ratio were higher than those of healthy fruits, although the results did not significantly differ (Fig. 1). Compared with those of healthy fruits, the Ca contents of both pitted tissue and healthy tissue surrounding the pitted regions decreased by 19.37% and 25.32%, respectively, and the N content increased by 7.93% and 13.57%, respectively; consequently, the N/Ca ratio increased by 33.86% and 52.09%, respectively. In addition, the Ca content of the pitted tissue was 7.98% higher than that of the healthy tissue surrounding the pitted regions, whereas the N content and N/Ca ratio of the pitted tissue were 4.97% and 11.99% lower, respectively, than those of the healthy tissue surrounding the pitted regions.

Effects of Ca-containing foliar fertilizers on BP incidence

BP incidence differed among the 7 treatments (including the control), as shown in Table 2. The tested Ca-containing foliar fertilizers significantly reduced the BP incidence in apples. The BP incidence ranged from 3.3-7.0% for the 6 spraying treatments. The control efficacy values were also distinct, as the reduction percentages ranged from 43.2-73.0%. We also found that the effect of spraying at the early stage (5 DAFB + 40 DAFB; treatments I, III and V) of plant growth was significantly greater than that of spraying at the late stage (80 DAFB + 125 DAFB; treatments II, IV and VI); the control efficacy of the 6 spraying treatments was as follows: treatment V (spraying Ca(HCOO)₂-containing fertilizer at 5 DAFB + 40 DAFB) ˃ treatment I (spraying CaCl₂-containing fertilizer at 5 DAFB + 40 DAFB) ˃ treatment III (spraying Ca(NO₃)₂-containing fertilizer at 5 DAFB + 40 DAFB) ˃ treatment II (spraying CaCl₂-containing fertilizer at 80 DAFB + 125 DAFB) ˃ treatment IV (spraying Ca(NO₃)₂-containing fertilizer at 80 DAFB + 125 DAFB) ˃ treatment VI (spraying Ca(HCOO)₂-containing fertilizer at 80 DAFB + 125 DAFB).

Effects of Ca-containing foliar fertilizers on fruit Ca and N contents

The 3 Ca-containing foliar fertilizers clearly increased the Ca content of apple fruits to levels higher than those of the control, but the N contents varied (Fig. 2). When spraying the Ca-containing foliar fertilizers at the early stage, the increase in the Ca content in the fruits varied little among the fruits sprayed with the three fertilizers. The fluctuations were consistent for fertilizer A and fertilizer C, with the exception of the Ca content at harvest. For fertilizer B, the Ca content at 12 DAFB was much higher than that for fertilizer A and fertilizer C, whereas the Ca contents at 47 DAFB and 165 DAFB were less than those for fertilizer A and fertilizer C. On the other hand, spraying fertilizer B reduced the N content of the fruits, but no clear patterns were observed regarding fertilizer A and fertilizer C.

Compared with the control, the three fertilizers (when sprayed at the late stage) significantly increased the Ca contents of the fruits at each stage, and the order of the effects was fertilizer B ˃ fertilizer A ˃ fertilizer C. However, compared with the control, the three fertilizers had little influence on the N content of the fruits.

Effects of spraying time on fruit Ca and N contents

As shown in Fig. 3, spraying the 3 Ca-containing foliar fertilizers at the early stage (5 DAFB + 40 DAFB) and at the late stage (80 DAFB + 125 DAFB) both increased the Ca content of fruits and decreased the N content of fruits, and the influence of spraying at the early stage on the Ca content and N content was much greater than that of spraying at the late stage; samples at different growth stages differed little. In particular, the Ca content of fruits that were sampled at 12 DAFB and that were from trees sprayed by the tested fertilizers at the early stage was significantly higher than that of the fruits of trees sprayed at the late stage and the control.

For fertilizer A, the Ca content of the fruits of trees sprayed at the early stage was higher than that of the fruits of trees sprayed at the late stage, with the exception of that at 165 DAFB. For fertilizer B, the Ca content of the fruits of trees sprayed at the early stage was higher than that of the fruits of trees sprayed at the late stage, with the exception of that at 47 DAFB. For fertilizer C, the Ca content of the fruits of trees sprayed at the early stage was much higher than that of the fruits of trees sprayed at the late stage. Spraying fertilizer A and fertilizer B at the early stage resulted in less N content in the fruits than did spraying at the late stage, whereas spraying fertilizer C resulted in slightly higher N content than did spraying at the late stage, with the exception of that at 12 DAFB.

Effects of Ca-containing foliar fertilizers on leaf Ca and N contents

As shown in Fig. 4, the leaf N contents in response to the 6 spraying treatments were generally higher than those of the control, whereas the effects of spraying Ca-containing foliar fertilizers on the leaf Ca contents markedly differed. Compared with the control, the 3 fertilizers increased the leaf N content, fertilizer A and fertilizer B decreased the leaf Ca content, and fertilizer C increased the leaf Ca content when each was sprayed at the early stage. When sprayed at the late stage, these 3 fertilizers did not clearly influence the leaf Ca content and N content; and changes in the leaf Ca content and N content varied at different growth stages.

Effects of spraying time on leaf Ca and N contents

As shown in Fig. 5, spraying Ca-containing foliar fertilizers increased the N content of apple leaves, especially for the leaves at harvest, and increased or decreased the Ca content of apple leaves at different stages. For fertilizer A, spraying at the early stage decreased the Ca content, but spraying at the late stage increased the Ca content of apple leaves. For fertilizer B, spraying at the early stage decreased the Ca content of apple leaves, and the Ca content of the apple leaves was similar to that of the control; the Ca content in the leaves increased at 47 DAFB but decreased at 132 DAFB. For fertilizer C, spraying at the early stage increased the Ca content of apple leaves, with the exceptions of decreases in the leaves at 47 DAFB and 165 DAFB, and compared with the control, fertilizer spraying at the late stage decreased the Ca content of apple leaves, with the except of that at 47 DAFB.

Effects of spraying treatments on the N/Ca ratio of apple fruits and leaves

As shown in Fig. 6, compared with the control, the 6 treatments distinctly decreased the N/Ca ratio of the apple fruits. These decreases were sharper in response to spraying at the early stage (treatments I, III and V) than in response to spraying at the late stage (treatments II, IV and VI); the effects of the 6 spraying treatments on the N/Ca ratio of the apple leaves varied. Compared with the control, the 6 treatments, when sprayed at the early stage, increased the N/Ca ratio of the apple leaves, with the exception of treatment V, which reduced the N/Ca ratio of the apple leaves at 12 DAFB and exhibited similar N/Ca ratios at 47 DAFB + 87 DAFB. The N/Ca ratio in response to the 6 spraying treatments sprayed at the late stage was slightly higher than that of the control.

Discussion

Influence of BP on the Ca and N contents in different parts of fruits

BP is a major problem in apple-growing industry worldwide and can result in severe economic losses, as the affected fruits cannot be sold in fresh markets.^1,2 BP occurs mainly during the period of cold storage but can occur before harvest in severe cases. BP is characterized by brown corky spots just beneath the skin, and these spots dehydrate over time and form depressions in the skin of the fruits.^15,16

The universally acknowledged hypothesis associated with BP formation in fruits during production is a deficiency in Ca²⁺ due to the improper uptake of nutrients by plants; however, the mechanisms involved in this process have not been explained.^5,17 At the physiological level, Ca²⁺ is transported from the root cortex through the xylem and is distributed to vegetative and reproductive tissues.¹⁸ The rate of water supply and nutrients to fruits is lower than that to the rest of the plant, and Ca²⁺ delivery to organs depends on the concentration of Ca²⁺ in the xylem sap, transpiration and growth rate.¹⁹ 

Ca reductions can disrupt cell membranes; this disruption can lead to cell death and tissue collapse, resulting in BP.^4,20 BP is also related to high N content and N/Ca ratios in fruits.^4,7 However, Chamel and Bossy²¹ reported that the concentrations of Ca²⁺ in pitted tissue are higher than those in healthy tissue surrounding the pitted regions. de Freitas et al.⁴ reported similar concentrations of N in both pitted and healthy fruits and reported that a higher N/Ca²⁺ ratio in pitted fruit solely results from lower tissue Ca²⁺. Miqueloto et al.²² reported that fruits that exhibit BP symptoms have higher values of N/Ca ratios in the peel and pulp.

In our experiment, we analyzed the Ca content and N content and then calculated the N/Ca ratio in the different parts of pitted fruits. We found that the Ca contents of pitted tissue and healthy tissue surrounding the pitted regions were lower than those of healthy fruits and that the N content and N/Ca ratio were higher; at the same time, the Ca content of the pitted tissue was higher than that of the healthy tissue surrounding the pitted regions, whereas the N content and N/Ca ratio of the pitted tissue were lower (Fig. 1). Our results were consistent with those of previous reports ^4,5,7,21 and confirmed that BP is related to low Ca contents and high N contents of fruits. The Ca content in the pitted tissue was also higher than that in the healthy tissue surrounding the pitted regions, whereas the N content and N/Ca ratio in the pitted tissue were lower.

Efficacy of Ca sprays

Preharvest foliar Ca applications to increase the Ca content of apples and therefore reduce BP incidence have become standard practice; however, inconsistent results are often obtained after applying treatments because of the varying responses of apple cultivars to foliar Ca sprays.^10,23 Results have ranged from positive effects ^9,11,24,25 to little effect ^26,27 on fruit Ca content and BP control.

Lötze et al. ⁹ applied Ca(NO₃)₂, CaCO₃ and Ca(COOCH₃)₂ at three different developmental stages (early, middle and late) of fruit growth. Those authors reported that early-to-late applications of Ca(NO₃)₂ and CaCO₃ decrease the incidence of BP, whereas Ca(COOCH₃)₂ applications did not influence the fruit Ca content or BP incidence. Val et al. ¹⁰ observed that in-season CaCl₂ sprays leads to increased Ca concentrations in the skin. Blanco et al. ¹¹ assessed the efficacy of Ca spray formulations containing either CaCl₂ or calcium propionate via laboratory and field experiments and reported that the addition of appropriate adjuvants to Ca sprays improves the distribution of Ca and helps reduce the BP incidence during the postharvest storage period.

Ortiz et al. ²⁸ reported that dipping ‘Golden Reinders’ fruits in 2% (w/v) CaCl₂ solution prior to storage notably increases the Ca content and enhances the production of aromatic volatile compounds after mid-term storage of the fruits. Guerra et al.²⁹ reported that CaCO₃ is useful in organic production for decreasing BP incidence in ‘Reinette’ apple cultivars during medium-term storage. Wilsdorf et al.¹² reported that Ca sprays can effectively increase fruit Ca contents, but the treatments could not be evaluated with respect to BP because no incidence of the disorder occurred.

Gago et al.⁶ reported that CaCl₂ (1.5%, w/v) alone and CaCl₂ plus 1-methylcyclopropene (1-MCP) reduce BP intensity by reducing moderate and severe BP incidence and that the treated fruits were lighter but firmer fruit than were the control fruits. By using an increasing number of CaCl₂ sprays, Torres et al.⁷ reported a trend toward reduced BP but no clear effect on maximizing fruit Ca contents.

In our study, spraying Ca(HCOO)₂-containing fertilizer at 5 DAFB + 40 DAFB had the greatest effect on BP control among the 6 spray treatments and reduced the percentage of BP incidence by 73.0%; this fertilizer was followed by CaCl₂-containing fertilizer and Ca(NO₃)₂-containing fertilizer, each of which reduced the percentage of BP incidence by 70.3% and 67.6%, respectively (Table 2). Compared with the control, the 3 tested Ca-containing foliar fertilizers clearly increased the Ca content of apple fruits, whereas the N content varied (Fig. 2). With respect to the apple leaves, the N contents in response to the spraying treatments were generally higher than the N contents of the control, but the effects on the Ca contents varied (Fig. 4). Consequently, the 3 tested fertilizers distinctly decreased the N/Ca ratio of apple fruits; the decrease in response to spraying at the early stage was greater than that in response to spraying at the late stage, whereas the effects on the N/Ca ratio of leaves varied (Fig. 6).

Our results showed positive effects of Ca-containing foliar fertilizers on decreasing BP incidence and improving the mineral nutrient contents in apple fruits, as described previously.^{6,7,9,11,12,28,29} Our results also indicated that the tested Ca fertilizers containing CaCl₂, Ca(NO₃)₂ and especially Ca(HCOO)₂ could potentially control BP in apple and improve the mineral nutrient contents in apple fruits.

Influence of spraying time

The efficacy of Ca absorption and effectiveness at decreasing BP in fruits and influencing mineral nutrients can differ if Ca is applied during different developmental stages.^10,30 While no general agreement exists regarding the best time to apply Ca sprays, some studies have found that the best time is immediately after full bloom, when fruits are small.⁷ 

Early applications may be advantageous because a less-developed cuticle favors the penetration of Ca into apples.²⁵ Raese and Drake²⁴ reported a decrease in BP incidence when Ca was applied to ‘Golden Delicious’ apples at preharvest. Fertilization with Ca during the period of fruit enlargement greatly improved fruit Ca concentrations and alleviated the occurrence of BP during storage.³¹ Lötze et al. ⁹ reported very low BP incidences, and significant differences were observed between Ca(NO₃)₂ mid-stage treatments and other treatments as well as between Ca(NO₃)₂ and CaCO₃ early treatments and other treatments; Ca(COOCH₃)₂ applications did not show any trends in fruit BP incidence when applied during the early, middle or late stage. Torres et al.⁷ reported the mid-season (May) CaCl₂ sprays greatly mitigated BP, and a trend toward reduced BP was reported as the number of CaCl₂ sprays increased; however, this trend was not clearly an effect of maximizing fruit Ca content.

In many studies, Ca sprays have been most effective during the second half of the fruit-growing period.^9,32 Wojcik ³³ alleviated the susceptibility of apple fruits to BP by spraying CaCl₂ in the summer and autumn. According to Casero et al.³², starting Ca sprays at 10 DAFB does not increase Ca accumulation in apples, whereas starting at 70 DAFB increases the Ca absorption rate and accumulation in fruits. This effect likely occurs because Ca is provided mainly by root absorption during the first period of fruit growth, whereas during the second period of fruit growth, during which time the Ca absorption is reduced, Ca sprays may be more effective. Lötze et al. ⁹ reported that late (80 DAFB) Ca(NO₃)₂ applications increased the Ca content of fruits at harvest more than early (6 DAFB) and middle (40 DAFB) applications did.

In the present study, spraying the 3 Ca-containing foliar fertilizers at the early stage (5 DAFB + 40 DAFB) (treatments I, III and V) reduced the BP incidence by 70.3%, 67.6% and 73.0%. These reductions were much greater than those resulted from spraying at the late stage (80 DAFB + 125 DAFB) (treatments II, IV and VI) by 54.1%, 45.9%and 43.2% (Table 2). In addition, we found that treatment V and treatment VI, that is, spraying Ca(HCOO)₂-containing fertilizer at the early stage and at the late stage, reduced the BP incidence by 73.0% (the best result among the 6 treatments) and 43.2% (the lowest percentage reduction of the 6 treatments), respectively, indicating that spraying the same fertilizer at different growth stages results in different reductions in BP incidence; these results are consistent with the results of Lötze et al.⁹ for Ca(NO₃)₂ applications. In general, the present results are consistent with those of Raese and Drake²⁴, Zheng et al.³¹ and Torres et al.⁷ and are partially consistent with those of Lötze et al.⁹, but they disagree with those of Wojcik³³ and Casero et al.³².

We found that spraying the 3 Ca-containing foliar fertilizers at the early stage and late stage both greatly increased the fruit Ca content and decreased the fruit N content, and the influence of spraying at the early stage on the Ca content and N content of fruits was much greater than that of spraying at the late stage; little differences were observed among samples at different growth stages (Fig. 3). We also observed a significantly higher Ca content in fruits at 12 DAFB in response to the 3 tested fertilizers sprayed at the early stage than in both the fruits of trees sprayed at the late stage and the control fruits, confirming the advantages of early Ca applications.^7,25,31

Spraying the 3 Ca-containing foliar fertilizers at the early and late stages increased the N contents of apple leaves, especially in the samples at 165 DAFB. The influences on the Ca contents varied, as samples at different stages either increased or decreased (Fig. 5). The Ca content is generally much higher in leaves, which have greater transpiration rates than do fruits, because Ca is imported into fleshy fruits only in small amounts;^3,5 however, few studies have investigated the effects of Ca application on the mineral nutrients of apple leaves. In the present study, we found that the Ca contents (0.067~0.371%) and N contents (0.278~1.877) of apple fruits were much lower than those of apple leaves (0.699~2.219% and 2.107~2.799%, respectively). Also, compared with the control, the 3 tested Ca-containing foliar fertilizers influenced the fruit Ca content much more than the fruit N content, leaf Ca content and leaf N content (Fig. 3 and Fig. 5).

Conclusion

Spraying foliar fertilizer containing CaCl₂, Ca(NO₃)₂ or Ca(HCOO)₂ at the early stage (5 DAFB + 40 DAFB) and at the late stage (80 DAFB + 125 DAFB) improved the Ca content, N content and the N/Ca ratio of apple fruits and leaves and consequently reduced the incidence of BP in the fruits. The efficacy of spraying at 5 DAFB + 40 DAFB was significantly better than that of spraying at 80 DAFB + 125 DAFB and that of the control, and the order of the efficacies efficacy was as follows: spraying Ca(HCOO)₂-containing fertilizer at 5 DAFB + 40 DAFB ˃ spraying CaCl₂-containing fertilizer at 5 DAFB + 40 DAFB ˃ spraying Ca(NO₃)₂-containing fertilizer at 5 DAFB + 40 DAFB ˃ spraying CaCl₂-containing fertilizer at 80 DAFB + 125 DAFB ˃ spraying Ca(NO₃)₂-containing fertilizer at 80 DAFB + 125 DAFB ˃ spraying Ca(HCOO)₂-containing fertilizer at 80 DAFB + 125 DAFB. The results of this research will provide a feasible solution for the control of BP in apple and lay the foundation for the quality improvement of bagged fruits.

Acknowledgments

This work was supported by the National Specific Funding for Modern Agriculture (apple) Industry Technology System (CARS-28), the Technology Research and Demonstration on Apple Production and Quality Control of Flowers and Fruits (2014BAD16B02-2), and the Key Technology Research on Quality Improvement and Standardization Production for Famous and Special Fruits in Shandong (2014CXZ04). 

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  Figure captions

  Table 1 Spraying treatments of Ca-containing foliar fertilizers

Fig. 1 Ca and N contents in different parts of pitted apple fruits

Table 2 Effects of Ca foliar fertilizers on the incidence of BP in apples

Note: The data with different uppercase letters and different lowercase letters indicate significant differences at the 0.01 level and the 0.05 level, respectively.