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A field experiment was conducted over three growing seasons (2012–14) to study the effect of the foliar application of different potassium (K) fertilizers [potassium phosphate monobasic (KH2PO4), potassium nitrate (KNO3), and humic acid potassium (HAK)] on the fruit growth rate, yield, and quality of ‘Kousui’ japanese pear (Pyrus pyrifola) trees. Except the first year of study, foliar application of K fertilizers generally led to an increase in the concentration of fruit total soluble sugar, titratable acidity (TA) and sweetness, along with an elevated K accumulation in leaf and fruit at maturity. In 2013 and 2014, compared with the control, KNO3 treatment led to an average 16% higher yield, and HAK led to an average 15% higher soluble solid content (SSC). Furthermore, HAK resulted in 26% higher yield in 2014. KNO3 treatment showed 19% higher leaf K concentration, 38% leaf K accumulation, and 43% fruit K accumulation in maturity than the control in 2014. Different effects were found on the concentration of specific types of sugar and organic acid, of which fructose and malate were consistently increased by the K application. With regard to the amino acids, KNO3 and HAK treatments led to a significant increase in the concentration of aspartic acid, which was 12% and 22% higher than the control, respectively. In conclusion, foliar application of KNO3 is an efficient way to increase ‘Kousui’ japanese pear fruit yield, whereas spraying HAK is an effective way to improve the fruit quality.
Japanese pear is one of the leading cultivated fruit trees in temperate regions. After apple (Malus ×domestica) and citrus (Citrus sp.), it is the third largest fruit species in China, both in planting areas and fruit yield. As the top producing country, China grows more than 60% of the world pear (Pyrus sp.) production (Boyer et al., 1943; Wu et al., 2013). K is highly mobile in plants and constitutes up to 10% of plant dry weight (Adams and Shin, 2014; Shin, 2014; Walker et al., 1996). Regarding the total amount of mineral nutrients required by plants, potassium is required in the largest amount after nitrogen (N) (Zörb et al., 2014); moreover, it is the largest nutrient required by the fruit (Lester et al., 2006; Mpelasoka et al., 2003). K activates numerous enzymes, which are critical for various metabolic processes, such as biosynthesis, transport, and transformation of sugar and starch (Baraldi et al., 1991; Karley and White, 2009; Lester et al., 2010a; Niu et al., 2013; Römheld and Kirkby, 2010). Furthermore, K is an essential nutrient involved in the phloem translocation of assimilates, including sucrose movement from shoot to root and to sink tissues such as fruit (Lebaudy et al., 2007). It is generally considered as a quality element, which could increase fruit development with higher quality and longer shelf life by enhancing synthesis and translocation of carbohydrates in plants (Niu et al., 2008). For example, the fruit of ‘Kinnow’ mandarin (Citrus deliciosa × Citrus nobilis) became larger and harder with increasing K supply. In contrast, the number of fruit cells, fruit size, and the SSC were significantly reduced by K deficiency (Ashraf et al., 2010).
In our previous research, we found that fruit K concentration decreased sharply with the increase of fruit size during expansion stage to maturation, which suggested that strong K supply was demanded by fruit. Balanced fertilization is an efficient measure to increase the yield and quality of ‘Kousui’ japanese pear. However, compared with the amount of N and phosphorus (P) input to the ‘Kousui’ japanese pear orchard, the supply of K was found to be seriously insufficient. Foliar fertilization was proved to be an efficient way to supplement the nutrients that the plant needed. It is a well-established measure for timely supplement of K to increase yields, fruit size, fruit volume, SSC, and sugar/acid ratio of ‘Gala’ apple (Reuveni et al., 1998a), ‘Valencia’ sweet orange [Citrus sinensis (Calvert and Smith, 1972)], and ‘Williams’ european pear [Pyrus communis (Hudina and Stampar, 2002)]. KH2PO4 (Reuveni et al., 1998b), potassium sulphate [K2SO4 (Sing and McNeil, 1992)], KNO3 (Mukadam and Haldankar, 2012), potassium chloride [KCl (Gill et al., 2005)], and potassium-complex humic acid (Shahryari et al., 2009) have been commercially used in various crops, of which KH2PO4 is routinely used as a foliar fertilizer since it contains both P and K. KCl is the major source of K and usually cheaper than other K sources. However, since it involves chloride ion (Cl−), it is generally not recommended on fruit trees and Cl−-sensitive crop. KNO3 is full of ambiguity that, on one hand, it could supply K and nitrate (NO3−) simultaneously to plants which benefits the fruit development, and avoid NO3− leaching from soil compared with soil application of N (Dong et al., 2005); on the other hand, the foliar application of NO3− easily led to negative effects on fruit quality and even postharvest problems for some fruit, such as muskmelon (Cucumis melo), whose SSC and firmness were decreased by foliar application of KNO3 (Jifon and Lester, 2009). However, when applied to mango (Mangifera indica) trees, higher SSC, less TA, and less firmness was found (Rebolledo–Martínez et al., 2008). These inconsistent results might be related to the spraying time, application rate, and fruit species. Fructose, glucose, sorbitol, and sucrose are the main sugar types, and malate was the main organic acid produced in ‘Huanguan’ chinese white pear fruit [Pyrus bretschneideri (Song et al., 2012)]. Less attention was paid to the effects of K concentration and accumulation in ‘Kousui’ japanese pear leaf and fruit by the foliar application of different K sources, as well as the quality characteristics of ‘Kousui’ japanese pear fruit, such as various sugars, organic acids, and amino acids. The aim of this study was to evaluate the effect of different foliar application strategies of K fertilizers so as to provide a recommendation to growers on how to efficiently increase yield and improve fruit quality.
The trials were conducted over three successive growing seasons on 10-year-old scaffolding ‘Kousui’ japanese pear trees in an orchard from 2012 to 2014. The orchard was located in the town of Ersheng in the city of Jurong city, Jiangsu Province, China. Trees were on callery pear (Pyrus calleryana) seedling rootstock planted at 4 × 4 m. Soil samples were taken from the soil surface 0–30 cm in Fall 2011 and were determined with the following chemical characteristics (Wells, 2009): pH 5.76, 1.16% organic matter, 69.37 mg·kg−1 available N, 11.16 mg·kg−1 available P, and 151.93 mg·kg−1 available K. The soil fertility was comparatively low. Each tree around the drip line in the orchard was supplied by 40 kg commercial organic fertilizer (45% organic matter and 2N–0.4P–1.7K), 2 kg cooked soybean (Glycine max), 2 kg calcium magnesium phosphate, and 0.75 kg K2SO4 in the base fertilization, 0.25 kg K2SO4 after anthesis, and 0.25 kg urea after fruit harvest.
Four treatments were applied to the same set of trees for 3 years. Three uniform trees were selected as a replicate, for a total of three replicates per treatment. Three foliar K sources [KH2PO4, KNO3, and HAK (Foliwell®K; Omex, London, UK)], which contained 0.08% K were administered as follows: 0.3% KH2PO4, 0.22% KNO3, 0.27% HAK, and water as control. HAK was a new-style liquid K fertilizer made by special organic compounds chelating K and no hormone was included. The foliar fertilizers were applied three times (27 Apr., 19 June, and 11 July) on a sunny day between 1500 and 1700 hr. Each tree was sprayed 2 L liquid fertilizer with 0.1% surfactant with Tween® 20 (Sigma-Aldrich, St. Louis, MO). The phenology of ‘Kousui’ japanese pear in 2013 and 2014 were both as follows: full of blossom (3 Apr.), young fruit stage (27 Apr.), expansion phase I (3 June), expansion phase II (2 July), maturity (6 Aug.), 1 month after harvest (8 Sept.), 2 months after harvest (10 Oct.), and leaf abscission (4 Nov.). The phenology of ‘Kousui’ japanese pear in 2012 was 1 week later than that in 2013 and 2014.
The pH was determined using 1:2.5 (w/v) soil:water extracts. Alkali-hydrolysable N was determined by the method used by Lu (1999). Available phosphorus was measured using extracts of hydrochloric acid–ammonium fluoride [HCl–NH4F (Horta and Torrent, 2007)]. Available potassium was determined by extracting the soil with 1 mol·L−1 ammonium acetate (CH3COONH4), and then measured by flame photometry (Allen et al., 1974). All extractions were performed in triplicate. The analysis values from the triplicate extractions were averaged before statistical analysis was performed.
In 2012–14, 30 ‘Kousui’ japanese pear fruit were collected per tree at maturity stage and weighed. Fruit firmness was assessed with a penetrometer (FT327; Effegi, Alfonsine, Italy) with an 11.3-mm probe. SSC was determined using a refractometer (PAL-1; Atago, Tokyo, Japan). Total soluble sugar was measured using the anthrone–sulphuric acid colorimetric method (Alexander and Edwards, 2003). TA was determined using standard acid–base titration (Sánchez, 2015).
In 2013 growing season, to evaluate the foliar K fertilizers on fruit growth rate and K concentration in fruit and leaves during the fruit development, eight fruits were randomly picked from four orientations on 27 Apr., 3 June, 2 July, and 6 Aug. At the same time, eight mature leaves from the midportion of the bearing branch of every tree were collected. In addition, leaves were also collected on 6 Sept., 6 Oct., and 4 Nov., respectively. After weighing the fruit, one quarter of these fruit were randomly selected, homogenized, and thoroughly dried. Leaves were rinsed twice with tap water and twice with deionized water, wiped with a paper towel, and thoroughly dried. Fruit and leaf samples were digested with a sulfuric acid–hydrogen peroxide (H2SO4–H2O2) assimilating method in a digestion furnace (280 °C, 2 kW, 1 h), and K concentration was determined by a flame photometer (AP1200; Aopu Analytical Instruments, Shanghai, China).
In 2013 growing season, a 2-g portion of the frozen fruit sample was ground with 20 mL of ultrapure water and then centrifuged at 20,000 gn for 15 min at 4 °C. The supernatant was recovered and immediately filtered through a 0.45-mm filter (SepPak; Waters, Milford, MA) to eliminate large particles. The extraction was stored at −80 °C in a sealed tube for the high-performance liquid chromatography (1200; Agilent, Santa Clara, CA) determination of individual sugar and organic acid concentration. Sweetness value = fructose × 1.75 + glucose × 0.70 + sorbitol × 0.40 + sucrose × 1.00 (Song et al., 2012).
The individual sugar concentration was determined using the following specific conditions as described by Colaric et al. (2007) with some modifications: 4.6 × 250–mm, 5-μm column (CapCell Pak NH2; Shiseido, Tokyo, Japan), constant temperature of 50 °C, and mobile phase of acetonitrile-water (80/20; v/v). An evaporative light scattering detector (Alltech 3300 ELSD; Grace, Deerfield, IL) was used with a flow rate of 1.0 mL·min−1, drift tube temperature of 80 °C, and nitrogen flow rate of 2.0 mL·min−1. The individual organic acid concentration was determined using the following specific conditions as described by Xu et al. (2012) with slight revisions as follows: 4.6 × 250-mm, 5-μm column (C18, Waters) with 1 mL·L−1 phosphoric acid aqueous solution, flow rate of 0.8 mL·min−1, column temperature of 45 °C, injection volume of 20 μL, and detection at 210 nm using an ultraviolet spectrophotometer.
According to Coimbra et al. (2011), each fresh fruit sample (2.5 g) was placed in a coated polytetrafluoroethylene test tube with a screw cap. To each sample, 10 mL of 6 m HCl was added. The tube was filled with nitrogen over a 15-min period and sealed. The hydrolysis reaction was performed for 24 h at 110 °C using a heating block. The tube was allowed to cool to room temperature, and the solution was filtered through wet filter paper and collected in a 50-mL volumetric flask. The residue was mixed with 20 mL of citrate buffer (pH 2.2) and the amino acid profile was determined using an automatic amino acid analyzer (Biochrom 30; Biochrom, Cambridge, UK). The 18 L-amino acid standards [i.e., glycine (Gly), alanine (Ala), serine (Ser), proline (Pro), valine (Val), threonine (Thr), cysteine (Cys), leucine (Leu), isoleucine (Ile), aspartic acid (Asp), glutamic acid (Glu), methionine (Met), histidine (His), lysine (Lys), phenylalanine (Phe), arginine (Arg), tyrosine (Tyr), and tryptophan (Trp)] were purchased from Sigma-Aldrich.
The fruit number per tree was counted during the fruit expansion stage. The biomass of the fruit was calculated by single fruit weight and fruit number. According to the method of leaf-to-fruit ratio (LFR) reported in apple trees by Sabbatini and Flore (2006), we corrected the LFR as 37:1, by which we can calculate the fruit biomass at the base of leaf biomass. The total K accumulation in leaves and fruit were calculated according to the following equations:
Data were analyzed by analysis of variance using SAS (version 9.3; SAS Institute, Cary, NC). Means were compared for treatment effects using a Fisher’s protected least significant difference at P < 0.05.
On the whole, in the three growing seasons, foliar K fertilization resulted in an increasing trend in fruit weight (Fig. 1B). In the first growing season (2012), there were large differences in the number of fruit that the trees bore. The number of fruit in the KH2PO4 and HAK treatments was almost half of that in the KNO3 treatment (Fig. 1A), which led to great difference in yield (Fig. 1C). It was suggested that the fruit number was very essential and during the subsequent two seasons, the fruit number was hand controlled around 100. In 2013 and 2014, three foliar K sources led to an increasing trend in fruit yield, firmness (Fig. 1D), SSC (Fig. 1F), and TA (Fig. 1G). The application of KNO3 in these two seasons led to an average 16% higher yield than the control. The yield of HAK treatment was significantly lower than that of the KNO3 treatment in 2013, whereas HAK resulted in 26% higher yield than the control in 2014. No significant differences were found in the yield of KH2PO4 treatment either in 2013 or 2014. From 2013 to 2014, comparing with the control, SSC (Fig. 1E), total soluble sugar, and TA were increased by an average of 15%, 25%, and 21%, respectively, in HAK treatment.
Citation: HortTechnology hortte 26, 3; 10.21273/HORTTECH.26.3.270
As shown in Fig. 2A and 2B, the single fruit weight and daily weight gains of fruit in the control gradually increased with fruit growth. In maturity, the fruit weight of KNO3 treatment was significantly higher than that of the control and HAK. The daily weight gain from the expansion phase II to maturity (i.e., 2 July to 6 Aug.) was on average 10 times higher than that from the young fruit stage to the expansion phase I (i.e., 27 Apr. to 3 June), and was on average 0.7 times higher than that from expansion phase I to expansion phase II (3 June to 2 July). Upon foliar application of KH2PO4 and KNO3, the single fruit weights were 16% and 17% higher than the control at the expansion phase II (2 July) and maturity (6 Aug.), respectively. During expansion phase II (2 July) and maturity (6 Aug.), daily weight gains were 21% and 18% higher in plants treated with KNO3 than in the control.
Citation: HortTechnology hortte 26, 3; 10.21273/HORTTECH.26.3.270
The leaf K concentration in the control plants was among the lowest during all developmental stages (Fig. 3A). From young fruit to maturity, K concentration was significantly increased by foliar KNO3 and HAK. From fruit harvest to leaf fall, leaf K concentration and accumulation was increased first and then decreased (Fig. 3A and 3C). During fruit development, the fruit K concentration decreased from 2.5% to 0.15%. There was no significant difference in the accumulation of K in leaves between plants treated with KNO3 and HAK at maturity (i.e., 6 Aug.; Fig. 3C). Furthermore, K accumulation was 38% and 14% higher in the leaves of samples sprayed with HAK (KNO3) and KH2PO4 than in those of the control, respectively (Fig. 3C). However, there was no significant difference between the fruit K accumulation in KNO3 and KH2PO4 (Fig. 3D), which was an average of 22% and 43% higher than that in HAK and the control, respectively. From expansion stage II to maturity, K accumulation in leaves and fruit accounted for 61% and 91% of the total K accumulation. Therefore, it could be concluded that expansion stage II is the critical stage of ‘Kousui’ japanese pear tree for K demand.
Citation: HortTechnology hortte 26, 3; 10.21273/HORTTECH.26.3.270
Foliar application of K fertilizers resulted in a significant increase in the concentration of fructose, which was on average 22% higher than in the control (Fig. 4A). The sorbitol concentration of the KH2PO4 and HAK treatment groups were an average of 35% higher than in the control (Fig. 4B). The glucose concentration in the fruit of KH2PO4 treatment was the highest of all, followed by that of the KNO3 treatment (Fig. 4C). HAK and KNO3 treatment resulted in a significant increase in sucrose concentration compared with the control and KH2PO4 treatment (Fig. 4D). Total sugar and sweetness of fruit of each treatment were consistent with the fructose concentration, which were both 28% higher than the control (Fig. 4E and 4F).
Citation: HortTechnology hortte 26, 3; 10.21273/HORTTECH.26.3.270
Total acid concentration in fruit was significantly increased by the foliar application of K fertilizers (Fig. 5G). KH2PO4 treatment led to a 105%, 61%, 112%, and 59% increase in malate, succinic acid, shikimic acid, and citric acid, respectively (Fig. 5A–C and 5F). However, the concentration of malate and shikimic acid in HAK treatment was 43% and 142% higher than that of the control. Compared with the control, KNO3 treatment had no significant difference of acid concentration except for malate. Concerning oxaloacetic acid and tartaric acid of the fruit (Fig. 5D and 5E), it was observed that there were no significant differences with different treatments.
Citation: HortTechnology hortte 26, 3; 10.21273/HORTTECH.26.3.270
Except for the essential amino acid Trp, the concentration of the 17 amino acids was examined in the ‘Kousui’ japanese pear fruit (Table 1). A significant increase was found in the concentration of some nonessential amino acids, such as Asp, Ser, Glu, and Gly. Asp is in the highest level in nonessential amino acid group, accounting for 67% to 72% of the total nonessential amino acids, and 49% to 55% of total amino acids. Foliar application of KNO3 and HAK treatments led to 12% and 22% higher Asp, respectively, than in the control, whereas KNO3 treatment resulted in 25%, 49%, and 23% higher Ser, Glu, and Gly than in the control and in the KH2PO4 treatment. However, of all the seven essential amino acids presented in ‘Kousui’ japanese pear fruit, Val and Thr were the most abundant amino acids, representing 22% and 20% of the total essential amino acid concentration, respectively. Hardly any effect was observed on the essential amino acid concentration by the foliar application of the three K fertilizers, except for the concentration of Thr in HAK treatment, which was 13% higher than that in the control. With regard to the total amino acid concentration, foliar application of KNO3 and HAK led to 17% and 15% higher amounts than the control, respectively.
In the present study, two successive growing seasons (2013 and 2014) of foliar application of KNO3 fertilizer led to a significant increase in fruit weight and daily fruit weight gain, whereas spraying with KH2PO4 had no significant effect. Spraying with KNO3 was previously reported to increase fruit size in ‘Patharnakh’ japanese pear (Gill et al., 2012), and the increase was greatest at a dose of 1.5% KNO3. A study of plum trees [Prunus salicina (Southwick et al., 1996)] revealed that there was a correlation among the dry weight, size of fruit, and the K concentration of leaves, showing that a moderate fruit size required a higher K concentration in the leaves. This was consistent with our results of KNO3 treatment, which could efficiently increase K levels both in the leaves and the fruit (Fig. 3). From the expansion phase I to leaf abscission (i.e., 3 June to 4 Nov.), the K concentration of leaves significantly decreased in the control, but this reduction could be alleviated by foliar application of K fertilizers. Sufficient K level in the leaves during the expansion phase is critical for fruit development, since a large amount of photosynthetic production is required for fruit enlargement. It has been shown that leaf K concentration had a nutrient backflowing process after fruit harvest, which might be related to the nutrient reflux of K (Fig. 3A). Compared with the control, the foliar application of K fertilizers significantly increased K backflow efficiency in leaves. Proe et al. (2000) found that increasing the nutrient supply increased the amount of K remobilized from 49% to 57% of initial concentration in scots pine. This might reflect the contrasting physiological roles of N and K and the greater proportion of K available for remobilization. In the study, the leaf K concentration in the control was 22% lower than those treated with KNO3 at the expansion phase I, which was suggested that there might be some risks of K insufficiency if no additional K fertilizer was supplied at the start of expansion phase I.
K has been widely known as a “quality element” in fruit production, which can significantly improve fruit quality (Zörb et al., 2014). Sugar and acids concentration are regarded as two of the most important parameters of fruit quality. It was shown that in ‘Kousui’ japanese pear fruit, fructose and malate were dominant in the soluble sugars and organic acids, respectively (Figs. 4 and 5), consistent with the report by Chen et al. (2007) in the fruit of ‘Niitaka’ japanese pear. In the present study, fructose concentration was found to account for 28% to 31% of the total sugars (Fig. 4A and 4E), and malate 67% to 85% of the total organic acids concentration (Fig. 5A and 5G). It was found that foliar spraying with K fertilizers could increase fruit firmness and total soluble sugar, in agreement with a previous study (Lester et al., 2010b). HAK treatment strongly increased fruit firmness, SSC, and total soluble sugar in 2013 and 2014 (Fig. 1), which was suggested as an efficient measurement to improve fruit quality. The application of K fertilizers all increased the total concentration of organic acids (Fig. 5G), which agreed with Mukadam and Haldankar (2012) in karonda (Carissa carandas). It was clearly shown that different accompanied ions might result in a specific difference in the metabolism of various kinds of sugars and organic acids in the fruit (Figs. 4 and 5).
In the present study, foliar application of K fertilizers had no effect on the total concentration of fruit essential amino acids. However, the concentration of Asp, Ser, and Glu, which belong to the nonessential amino acids, increased to various degrees (Table 1). Asp had an important role in fruit flavor (Ardö, 2006), and was the most abundant amino acid in ‘Kousui’ japanese pear, accounting for a maximum of 75% of the total amino acid concentration. Asp was significantly increased upon spraying with KNO3 and HAK (Table 1). During NO3− assimilation, NO3− is first assimilated as ammonia in leaves and is then converted into Glu by the glutamine synthetase and glutamate synthase pathways (Rufty et al., 1981). When ammonia is assimilated into glutamate and glutamine, it can be converted into other amino acids by transamination, and this process is promoted by K (Mengel et al., 1981), since K functions as a cofactor that promotes the assimilation of amino acids. It was discovered by Armengaud et al. (2009) that regulation of enzymes at the level of transcripts and proteins was likely to play an important role in Arabidopsis (Arabidopsis thaliana) adaptation to K deficiency by decreasing negative metabolic charge, such as Glu, Asp, nitrate, and malate, whereas K supply could increase negative charge. Lin et al. (2004) reported that the taste, aroma, and Glu and Asp concentration of muskmelon could be improved when K was present at a level of 240 mg·L−1 in the nutrient solution, which was consistent with our present study.
Comparatively speaking, foliar application of KNO3 could significantly increase the K accumulation, the fructose and sucrose concentration, and some amino acids in the fruit, such as Asp, Ser, Glu, and Gly. Further, compared with KNO3, HAK was more helpful to improve the fruit quality by increasing fruit firmness, the total SSC, and sweetness. Farmers should adopt different strategies according to the aim of whether to increase the fruit yield or improve the fruit quality.
Contributor Notes
This research was supported by the Special Fund for Agro-scientific Research in the Public interest (201203013) and China Agriculture Research System (CARS-29-15).
