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Article

The Effect of N and KH2PO4 on Skin Color, Sugars, and Organic Acids of “Flame Seedless” Grape

by
Linnan Wu
*,
Fangxia Wang
,
Riye Sha
,
Xujiao Li
,
Kun Yu
and
Jianrong Feng
*
The Key Laboratory of Characteristics of Fruit and Vegetable Cultivation and Utilization of Germolasm Resources of the Xinjiang Production and Construction Crops, Department of Horticulture, College of Agriculture, Shihezi University, Shihezi 832003, China
*
Authors to whom correspondence should be addressed.
Agronomy 2023, 13(3), 902; https://doi.org/10.3390/agronomy13030902
Submission received: 8 January 2023 / Revised: 15 March 2023 / Accepted: 16 March 2023 / Published: 18 March 2023
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Abstract

:
Anthocyanins, soluble sugars, and organic acids play a vital role in the color and flavor of grape berries. N and KH2PO4 are essential nutrients for grape growth and development. However, the research on the effects of foliar spraying of KH2PO4 on the skin color and flavor of grapes under different N levels were not systematic. In this study, “Flame seedless” grapes were used as the test material. There were six treatments in this experiment, including low nitrogen (LN), low nitrogen + KH2PO4 (LNK), moderate nitrogen (MN), moderate nitrogen + KH2PO4 (MNK), high nitrogen (HN), and high nitrogen + KH2PO4 (HNK). Foliar spraying of KH2PO4 on grapes significantly increased total K, anthocyanin contents, and the color index of red grapes (CIRG) in LN, MN, and HN. In the N and KH2PO4 treatments, foliar spraying of KH2PO4 significantly increased the content of methylated, acetylated, and coumarylated anthocyanins under MN treatment. The glucose and fructose contents of MNK were the highest compared to other treatments. The sole use of N showed the highest glucose and fructose contents with MN application. Anthocyanin had a significant positive correlation with soluble sugars; and showed a significant negative correlation with organic acids. Overall, foliar spraying of 0.5% KH2PO4 improved the color and flavor of “Flame seedless” grapes under all N levels, with the most significant effect at MN.

1. Introduction

Grapes (Vitis vinifera L.) are one of the four most widely cultivated fruits in the world. Since 2011, China has become the largest producer of table grapes [1]. In 2022, the table grape production was 12.60 million tons, accounting for 45.61% of the world’s total table grape production(https://www.chinairn.com/scfx/20230111/152009184.shtm accessed on 14 March 2023). Grapes a-re among the most popular fruit crops consumed worldwide, attributable to their unique flavor and appearance. Flavor and appearance are crucial in influencing consumer acceptability of fresh, ripe grapes [2]. The ripening of grapes is a complex process involving the accumulation of soluble sugars, flavonoids, and the catabolism of organic acids [3]. The skin color of grapes is a key quality attribute required by the consumers. In warm climate regions, red table grapes often present color deficits that decrease their commercial value. Grape color can be affected by many different factors, among them, temperature, light intensity, nutrition, and irrigation [4]. Fruit coloration is due to various pigments, such as chlorophylls, carotenoids, and anthocyanins in the skin [5]. Anthocyanins play a decisive role in the red color of grapes [6]. Grapes is primarily due to glucosides of five anthocyanidins: malvidin, petunidin, peonidin, delphinidin, and cyanidin. In grapes, monoglucosides exist as 3-O-glucosides, and acylated anthocyanins are formed by combining with acetic, coumaric, or caffeic acids [7]. Sugars are the primary contributor to the flavor of grapes. In most fleshy fruits, glucose, fructose, and sucrose constitute the major proportion of sugar contents. The high sugar concentration in grapes is a vital component of eating quality that defines maturity and time of harvest [8]. Organic acids are an essential components of fruit taste. Tartaric, citric, and malic acids are the most abundant organic acids found in grapes. High acid contents often reduce the fruit’s quality, but moderate concentrations can improve berry palatability [9].
Nitrogen (N) is a key element that has several roles in the life cycle of plants. A high N status in grapevines may lead to vegetative overgrowth and grape quality reduction [10]. Grapevines with a high N state can have an extended veraison period with delayed grape ripening. The rational application of N fertilization during the key phenological periods could potentially increase the sugar contents and decrease the acidity of grapes [11]. High nitrogen in fruits can lead to poor coloring, which affects apple quality and reduces the value of the commodity [12]. High N increases the chlorophyll and carotenoids content and reduces the anthocyanins content of the colored surface of the fruit, while the application of K increases the anthocyanins content in the peel and promotes fruit redness [13]. Potassium (K) application to table grape clusters after veraison increases the total soluble solids (TSS) [14]. The foliar application of KH2PO4 increased TSS content, juice pH, total anthocyanin, and the color index of red grapes (CIRG) [15]. Numerous studies have reported the impact of N fertilization on grape quality [16,17], flavor components [10], and anthocyanin biosynthesis [18]. Research on the foliar spraying of KH2PO4 on grapes mainly involved berry coloration [15,19]. The effects of foliar spraying of KH2PO4 on the content of soluble sugars, organic acids, and anthocyanins during grape ripening under various N levels remains unclear, especially regarding the impact of K on monomeric anthocyanins and anthocyanin derivatives.
“Flame seedless” grapes were used as the test material in this study. The effect of the N and KH2PO4 fertilizers on grape quality was investigated by analyzing the foliar spraying of KH2PO4 on the content of anthocyanins, soluble sugars, and organic acids under three soil-applied N levels. This study aims to investigate the efficiency of KH2PO4 application in reducing the negative effects of N, especially on the soluble sugar, organic acid, and anthocyanin contents to provide a theoretical basis for the production of high-quality grapes in the Xinjiang region of China.

2. Materials and Methods

2.1. Plant Materials and Treatments

The experiment was conducted in a grape standard experimental grape orchard (45°19′ N, 86°03′ E) at the Shihezi Agricultural Science Research Institute in Xinjiang during the 2020 and 2021 seasons. The experimental zone is classified as temperate and arid, with a continental climate. The site’s annual precipitation, annual evaporation, mean annual temperature, and mean humidity are 0.23 m, 1.34 m, 7.8 °C, and 67%, respectively. “Flame seedless” table grapes (Vitis vinifera L.) were used as the test material. “Flame seedless” grape seedlings were planted in 2015 at 3.0 m × 1.0 m space. The plants were trained to a “Y” form. Drip irrigation was used for top dressing and irrigation. Each experimental vine was treated with 400 g P (P2O5) and 480 g K (K2O) throughout the year. The chemical fertilizer was applied with a water–fertilizer integrated device with a flow rate of 2.6 L·h−1 and a working pressure of 0.05–0.1 MPa. A total of 12 irrigations were carried out during the whole growth period, and the irrigation interval was 8–10 days. Three shoots were trained on each vine. Two bunches per shoot were left. The experimental grapes budded on 14 April 2020, flowered on 10 May 2020, and reached full bloom on 16 May 2020. In the second year of the experiment, the grapes budded on 29 March 2021, flowered on 22 May 2021, and reached full bloom on 28 May 2021. The soil was sandy loam with the basic properties: pH, 8.04; organic matter, 26.31 g·kg−1; total N, 0.93 g·kg−1; alkaline hydrolyzable nitrogen, 40.53 mg·kg−1; total phosphorus, 0.78 g·kg−1; available phosphorus, 29.78 mg·kg−1; available potassium, 121.05 mg·kg−1; and electrical conductivity, 2.01 ms·m−1.
In April 2020, 90 grapevines with similar growths were selected for six treatments. The experiment was conducted in a randomized complete block design (RCBD) with three replicates for each treatment. The treatments were: low nitrogen (LN): 200 g N per vine; moderate nitrogen (MN): 400 g N per vine; and high nitrogen (HN): 600 g N per vine. The soil-applied N source was urea (46% N); the remaining three treatments were the combination of N and KH2PO4 treatments, fifteen vines were randomly selected from the LN, MN, and HN treatments, respectively, and sprayed with 0.5% KH2PO4 at 40 and 50 days after the full bloom. The treatments were recorded as LNK, MNK, and HNK treatments, respectively.
Nitrogen Fertilizer was applied at four time points: during the bud break stage (30% of the total amount); during the inflorescence separation stage (30% of the total amount); during the first fruit expansion stage (20% of the total amount); and during the second fruit expansion stage (20% of the total amount). The fertilization method was digging a ring-shaped fertilization area with a diameter of 0.75 m and a depth of 0.05 m with the vines as the center. After the fertilizer was dissolved in 5 L of water, it was evenly poured into the fertilization area and covered with soil.
Sampling started 56 days after full bloom (the beginning of the grape color transformation stage), and repeated every 14, 70 (the commercial mature stage), and 84 (the full mature stage) days after full bloom. The commercial maturity of grape was determined according to the minimum edible standard (15% ≤ total soluble sugar content ≤ 18.00%). Thirty clusters were randomly selected per replication. We collected the clusters from both sides and levels of the grapevines. A total of six berries were picked from each cluster. Two berries were randomly picked from the upper, middle, and lower parts. A total of 540 berries were picked for one treatment, immediately placed into ice boxes, and transported to the laboratory. Fifty berries were randomly selected from each replicate. The grape skin was separated from the flesh. The skin and pulp of the grapes were rapidly frozen in liquid nitrogen and stored at −80 °C for further processing.

2.2. Determination of Mineral Contents

The total N content was determined by the Kjeldahl method [20]. P and K were assayed according to Xia et al. [1]. Approximately 0.2 g of each dried sample was digested with nitric acid (HNO3, Analytical reagent, 98%) and then analyzed using inductively coupled plasma–atomic emission spectrometry ICP-AES ICP6300 (Agilent Inc., Santa Clara, CA, USA).

2.3. Determination of Grape Quality Parameters

The fresh weight of each berry was measured by an electronic scale (JH2102, Amami Seiko, Shanghai, China). When the fruit was cut transversely, the center firmness of the flesh was determined by a GY-4 fruit firmness tester. Twenty separate well-grown fruits were using at 56, 70, and 80 days after full bloom [6].

2.4. Berry Skin Color Evaluation

Thirty berries (i.e., the top, middle, and bottom of 10 bunches) were randomly collected from each replicate and determined at the equator with a CR-200 colorimeter (Minolta, Osaka, Japan). Two bunches per vine were in each replicate. Color development was analyzed by CIRG = [(180 − h°)/(L* + C*)] [21]. The chroma values (C*) = [(a*)2 + (b*)2]0.5, and the hue angle h° (0–360°) = arctangent b*/a*.

2.5. Determination of Fruit Pigment Content

Flavonoid contents were determined according to Cheng et al. [22], and expressed as rutin content. Total anthocyanins in grape berry skins were determined according to the pH differential method [23]. The contents of chlorophyll and carotenoid were assayed using a spectrophotometric method after tissue extraction with 95% ethanol [24]. The absorbance at 470, 649, and 665 nm was determined by a UV-2600 (UV-2600, Shimadzu, Kyoto, Japan) ultraviolet spectrophotometer, with 95% ethanol as the blank control. The calculation formula is as follows:
Ca = 13.95 × OD665 − 6.88 × OD649
Cb = 24.96 × OD649 − 7.32 × OD665
Cchlorophyll = Ca + Cb
Ccarotenoid = (1000 × OD470 − 2.05 × Ca − 114.8 × Cb)/245

2.6. Determination of Anthocyanin Composition

Grape skins 70 days after full bloom in 2021 were used to determine the monomeric anthocyanins according to Liang et al. [25].
The 0.50 g of lyophilized grape skins powder, after grinding and mixing, was weighed and placed into a 10 mL methanol solution containing 2% acetic acid and extracted by ultrasonic shock for 10 min. It was then shaken at 150 rpm on a shaker (Incu-Shaker, BenchmarK Electronics, Inc., Clut, TX, USA) at 25 °C for 30 min and centrifuged at 8000 r·min−1 at 4 °C for 10 min (5427R, Eppendorf, Hamburg, GER). After that, the supernatant was collected. The extraction was repeated five times, and the supernatant was combined. The supernatant was removed by rotary evaporation at 40 °C, and the residue was rescaled to 10 mL with a sample buffer (A:B mobile phase 9:1, A phase 6% acetonitrile solution containing 2% formic acid; B phase 54% acetonitrile solution containing 2% formic acid). Each sample was extracted three times in duplicate. The final sample was filtered with 0.45 µm cellulose acetate membrane and stored at −40 °C for subsequent use.
Ultra-performance liquid chromatography (UPLC) conditions; the column temperature was 40 °C, flow rate was 0.3 mL·min−1, injection volume was 2.0 μL, mobile phase A was acetonitrile, mobile phase B was 3% formic acid solution, gradient elution, liquid A: 5% (0 min) →10% (1 min) →25% (16 min) →40% (18 min) →100% (19 min), return to the initial state at 20 min, and balance for 10 min. The detector wavelength was 520 nm.
The conditions of mass spectrometry were as follows: electrospray ion (ESI) source, MRM (multiple reaction monitoring) mode, ion source temperature 150 °C, dissolvent temperature 400 °C, dissolvent gas flow rate 800 L·h−1, cone-hole gas flow rate 50 L·h−1, and collision gas (high-purity argon) flow rate 0.14 mL·min−1.
The anthocyanin characterization was based on the HPLC-ESI-MS/MS fingerprint library of anthocyanins in grapes and wine established by the Wine Research Center, China Agricultural University. The mass spectral information and retention times in the HPLC-ESI-MS/MS profiles of the samples were analyzed against the spectral library to determine the species and structure of each anthocyanin. The following anthocyanins were detected: (1) cyanidin-3-O-glucoside; (2) cyanidin-3-O-(6-O-p-coumaryl)-glucoside; (3) Cyanidin-3-O-(6-O-acetyl)-glucoside; (4) Delphinidin-3-O-glucoside; (5) Delphinidin-3-O-(6-O-p-coumaryl)-glucoside; (6) malvidin-3-O-glucoside; (7) Malvidin-3-O-(6-O-acetyl)-glucoside; (8) Malvidin-3-O-(t-6-O-p-coumaryl)-glucoside; (9) Malvidin-3-O-(c-6-O-p-couma-ryl)-glucoside; (10) Peonidin-3-O-glucoside; (11) Peonidin-3-O-(t-6-O-p-coumaryl)-glucoside; (12) Peonidin-3-O-(6-O-acetyl)-glucoside; (13) Peonidin-3-O-(c-6-O-p-coumaryl)-glucoside; (14) Peonidin-3-O-(6-O-acetyl)-glucoside; (15) Petunidin-3-O-glucoside; (16) Petunidin-3-O-(6-O-acetyl)-glucoside; (17) Petunidin-3-O-(6-O-p-coumaryl)-glucoside.

2.7. Determination of Soluble Sugar and Organic Acid Contents

The soluble sugar (i.e., glucose, fructose, and sucrose) contents were determined using the method of Wu et al. [26]. Organic acids were analyzed using high-performance liquid chromatography (HPLC), according to Yang et al. [9].

2.8. Statistical Analysis

The data were analyzed via the analysis of variance (ANOVA), where a significant variance was accepted at p < 0.05 using the IBM SPSS statistical software package 21. Graphs were depicted using the software Origin 2019. The data were presented as mean ± SD values of three biological replicates. The significance was tested at the 5% level using Duncan’s multiple range test.

3. Results

3.1. Effect of N and KH2PO4 on Mineral Nutrient Contents in Grape Berries

The contents of total N, P, and K gradually increased in the six treatments across the maturation time (Table 1). In the two-year experiment, the total N, P, and K contents of the N and KH2PO4 treatments (LNK, MNK, and HNK) were higher than that of the corresponding N treatments (LN, MN and HN) in the three sampling periods, but not significant in all cases. The total N, P, and K of grape berries in the six treatments in 2020 were lower than that in the corresponding six treatments in 2021. Among the three N treatments, the total N, P, and K contents of the HN treatment were the highest in two experimental years, followed by the MN treatment. Among the N and KH2PO4 treatments, the total N, P, and K contents of the HNK treatment were the highest at 56, 70, and 84 days after full bloom in 2020 and 2021 (Table 1).

3.2. Effect of N and KH2PO4 on the Physicochemical Indices in Grape Berries

With the gradual ripening of the grapes in 2020 and 2021, the average berry weight, TSS, and pH gradually increased. Berry firmness gradually decreased for the six treatments (Figure 1). At 70 and 84 days after full bloom in 2021, the single fruit weights of the HNK treatment were significantly higher than that of the MNK and LNK treatment (Figure 1A). At 56 and 84 days after full bloom in 2020, among the three N treatments, the TSS contents of the MN treatment were the highest. Among the N and KH2PO4 treatments in 2020, the MNK treatment had the highest TSS content in the three sampling periods, which were 19.10%, 22.54%, and 25.33%, respectively (Figure 1B). Among the three N treatments, the descending order of pH of grape juice from was: MN > LN > HN in the two experimental years. In the N and KH2PO4 treatments, the pH of the grape juice in the MNK treatment was the highest at 70 and 80 days after full bloom in 2021 (Figure 1C). At 56, 70, and 84 days after full bloom in 2020, among the three N treatments, the firmness of grape berries in descending order was: LN > MN > HN. At 70 and 84 days after full bloom in 2021, among the three N treatments, the firmness of the grape berries in the LN treatment was the highest, 6.55 and 4.71 N, respectively. In the N and KH2PO4 treatments, the firmness of the grape berries in the LNK and MNK treatments was higher than that in the HNK treatment at 70 days after full bloom in 2021 (Figure 1D).

3.3. Effect of N and KH2PO4 on CIRG of Grape Berries

The CIRG gradually increased in the six treatments across the maturation time. In the two-year experiment, the CIRG of the N and KH2PO4 treatments (LNK, MNK, and HNK) was higher than that of the corresponding N treatments (LN, MN and HN) in the three sampling periods, except for HNK and HN treatments at 84 days after full bloom in 2020 (Figure 2A,B and Table 2). The CIRG of the LNK treatment was significantly higher than that of the LN treatment at 70 days after full bloom in 2021. At 84 days after full bloom in 2021, the CIRG of the LNK and MNK treatment was also significantly higher than that of LN and MN, increasing by 12.81% and 11.17%, respectively. At 56 and 70 days after full bloom in 2020 and 2021, among the three N treatments, the MN treatment had the largest CIRG. At 56 and 70 days after full bloom in 2021, among the N and KH2PO4 treatments, the CIRG of the MNK treatment was significantly higher than that of the LNK and HNK treatments. At 84 days after full bloom in 2020 and 2021, among the three N treatments, the descending order of the CIRG of grapes was LN > MN > HN. The same trend was observed in the N and KH2PO4 treatments: LNK > MNK > HNK (Table 2).

3.4. Effect of N and KH2PO4 on the Pigment Content of Grape Skin

With the gradual ripening of the grapes, the total chlorophyll and carotenoid contents in the grape skins continuously decreased (Figure 3A,B), but the flavonoid and anthocyanin contents continued to increase (Figure 3C,D). The flavonoid contents of the HNK treatment were significantly higher than that of the HN treatment at 56 and 84 days after full bloom in 2020 and at 84 days after full bloom in 2021. The flavonoid contents of LNK at 70 days after full bloom in 2020 and 56 days after full bloom in 2021 was significantly higher than that of the LN treatment (Figure 3C). The anthocyanin contents of the LNK treatment were significantly higher than that of the LN treatment, which increased by 30.26% at 70 days after full bloom in 2020. The anthocyanin contents of MNK and HNK were also significantly higher than that of MN and HN at 84 days after full bloom in 2020 (Figure 3D).
Among the three N treatments, the HN treatment had the highest total chlorophyll content at 70 and 84 days after full bloom in both seasons. Among the N and KH2PO4 treatments, the HNK treatment had the highest total chlorophyll content at 70 and 84 days after full bloom in 2020 and 2021 (Figure 3A). The carotenoid content of the HNK was significantly higher than that of HN at 56 and 70 days after full bloom in 2021, with an increase of 41.07% and 20.51%, respectively. At 56 and 84 days after full bloom in 2021, the carotenoid contents of the LN treatment were the highest. Among the N and KH2PO4 treatments, the carotenoid contents were the highest in the LNK treatment at 70 and 84 days after full bloom in 2020. The carotenoid content of the MNK treatment was significantly higher than that of the LNK and HNK treatments at 70 d after full bloom in 2021 (Figure 3B).
Among the three N treatments, the flavonoid contents were highest in the LN treatments at 70 and 84 d after full bloom in 2020. The MN treatments contained more flavonoids than the LN and HN treatments at 56 d after full bloom in 2020. At 56, 70, and 84 days after full bloom in 2021, the flavonoid contents of the LN treatment were the highest, which were 2.66, 4.24, and 4.66 mg·g−1 FW, respectively. Among the N and KH2PO4 treatments, the flavonoid contents of the LNK treatment were the highest at 56 and 70 days after full bloom in 2020 and 2021 (Figure 3C). Among the three N treatments, at 56, 70, and 84 days after full bloom in 2020, the anthocyanin contents of the MN treatment were the highest, which were 5.51, 21.71, and 49.76 mg·g−1 FW, respectively. At 56 and 70 days after full bloom in 2021, the anthocyanin contents of the MN treatment were the highest, 8.52 and 23.55 mg·g−1 FW, respectively. Among the N and KH2PO4 treatments, the anthocyanin contents of the MNK treatment were significantly higher than that of the LNK and HNK treatments at 56 and 84 days after full bloom in 2020. The anthocyanin contents of the MNK treatment were the highest at 56, 70, and 84 days after full bloom in 2021 (Figure 3D).

3.5. Effect of N and KH2PO4 on the Anthocyanin Composition of ‘Flane Seedless’ Grape Skin

The total anthocyanin contents in the berry skin of the N and KH2PO4 treatments (LNK, MNK, and HNK) were higher than their corresponding N treatments (LN, MN, and HN). Among the three N treatments, the total anthocyanin contents of LN and MN treatments were significantly higher than that of the HN treatment, in descending order of LN > MN > HN (Table 3). Among the N and KH2PO4 treatments, the total anthocyanin content of the MNK treatment was 317.31 mg·kg−1 DW, significantly higher than that of the LNK and HNK treatments. A total of 17 monomeric anthocyanins was detected in the LN and MNK treatments, including 5 acetylation, 7 coumarylation, and 5 basic anthocyanins. Peonidin-3-O-glucoside was the primary component of LN, accounting for 38.04% of the total anthocyanins. Cyanidin-3-O-glucoside was the primary component of MNK, accounting for 44.43% of the total anthocyanins. A total of 15 monomeric anthocyanins was detected in the MN, LNK, and HNK treatments, and malvidin-3-O-(c-6-O-p-coumaryl)-glucoside and peonidin-3-O-(6-O-acetyl)-glucoside were not detected compared to the LN and MNK treatments. Cyanidin-3-O-glucoside and Peonidin-3-O-glucoside were the primary components of anthocyanins treated with MN and LNK, which accounted for 79.18% and 78.15% of the total anthocyanins, respectively. A total of 14 monomeric anthocyanins was detected in the HN treatment. Delphinidin-3-O-(6-O-p-coumaryl)-glucoside was not detected in the HN treatment, and the most predominant anthocyanin component in the HN treatment was Peonidin-3-O-glucoside (Table 3).
The percentage of coumaroylation and acetylation modification of N and KH2PO4 treatments (LNK, MNK, and HNK) was higher than their corresponding N treatments (LN, MN, and HN). Among the six treatments, the total anthocyanins modification percentage was the highest in LN (68.86%) and the lowest in MNK (50.12%) (Table 4). The six treatments were dominated by methylation modifications, followed by coumarylation modifications, and acetylation modifications accounting for the smallest proportion. The methylation percentages of LN and HN were the highest, amounting to 66.51% and 63.16%, respectively.

3.6. Effect of N and KH2PO4 on the Soluble Sugar Contents in Grape Berries

The contents of glucose, fructose, and sucrose gradually increased in the six treatments across the maturation time (Figure 4A,B). In the two-year experiment, the soluble sugar content of the N and KH2PO4 treatments (LNK, MNK, and HNK) was higher than that of the corresponding N treatments (LN, MN, and HN) in three sampling periods. Among the three N treatments, the glucose content of the MN treatment was the highest at 70 and 84 days after full bloom in 2020 and 2021. Among the N and KH2PO4 treatments, the glucose content of the MNK treatment was the highest in two experimental years. Among the three N treatments, the fructose content in the MN treatment was the highest at 56 and 70 days after full bloom in the two experimental years. Among the N and KH2PO4 treatments, the fructose content of the MNK treatment was the highest at 56, 70, and 84 days after full bloom in 2020, which were 76.40, 97.08, and 103.43 mg·g−1 FW, respectively. The glucose content of the MNK treatment was the highest at 56 and 70 days after full bloom in 2021, which were 61.37 and 99.00 mg·g−1 FW, respectively, followed by the LNK treatment. Among the three N treatments, the LN treatment had the highest sucrose content, while the HN treatment had the lowest in two experimental years. Among the N and KH2PO4 treatments, the LNK treatment had the highest sucrose content, while the sucrose content in the HNK treatment was the lowest at 56, 70, and 84 days after full bloom in 2020 and 2021.

3.7. Effect of N and KH2PO4 on the Organic Acid Contents in Grape Berries

The contents of tartaric acid, malic acid, citric acid, and shikimic acid in the grape berry demonstrated a decreasing trend during the ripening and development (Figure 5A–C). In the two-year experiment, the organic acids content of the N and KH2PO4 treatments (LNK, MNK, and HNK) was lower than the corresponding N treatments (LN, MN, and HN) in the three sampling periods, but the difference was insignificant. At 56 and 84 days after full bloom in 2020, the tartaric acid content in the HN treatment was the highest, and the MN treatment was the lowest among the three N treatments. At 70 and 84 days after full bloom in 2021, the tartaric acid content of the HN treatment was the highest, and the LN treatment was the lowest. Among the N and KH2PO4 treatments, the tartaric acid content of the MNK treatment was the lowest at 56, 70, and 84 days after full bloom in 2020, which was 9.02, 8.99, and 7.51 mg·g−1 FW, respectively. The tartaric acid content of the HNK treatment was the highest at 70 and 84 days after full bloom in 2021. Among the three N treatments, the malic acid content in the HN treatment was significantly higher than that in the LN and MN treatments in two experimental years. Among the N and KH2PO4 treatments, the malic acid content of the LNK treatment was significantly higher than that of the HNK and MNK treatments in 2020. The malic acid content in the HNK treatment was significantly higher than that in the LNK and MNK treatments during the three sampling periods in 2021. Among the three N treatments, at 70 days after full bloom in 2020, the citric acid content in the LN treatment was significantly higher than that in the MN and HN treatments, increasing by 26.44% and 29.37%, respectively. The citric acid content in the HN treatment was the largest during the three sampling periods in 2021. Among the N and KH2PO4 treatments, the citric acid content in the LNK treatment was significantly higher than that in the MNK and HNK treatments at 56 and 70 days after full bloom in 2020. The citric acid content in the HNK treatment was the highest during the three sampling periods in 2021. At 56 and 70 days after full bloom in 2020, among the three N treatments, the shikimic acid content in the LN treatment was the highest (Figure 5C). At 56 and 70 days after full bloom in 2021, the shikimic acid content of the HN treatment was the highest. Among the N and KH2PO4 treatments, the shikimic acid content of the LNK treatment was the highest at 56 and 70 days after full bloom in 2020. The shikimic acid content of the LNK treatment was significantly higher than that of the MNK and HNK treatments at 70 days after full bloom in 2021.

3.8. Correlation Analysis of Sugar and Acid Contents and Color Index

Flavonoids, anthocyanins, and CIRG had a significant positive correlation with glucose, fructose, sucrose, and total sugar contents and showed a significant negative correlation with tartaric acid, malic acid, citric acid, shikimic acid, and total acid contents. Total chlorophyll and carotenoids showed a significant negative correlation with glucose, fructose, sucrose, and total sugars. The total chlorophyll and carotenoids were not significantly correlated with tartaric acid (Figure 6).

4. Discussion

4.1. Effect of N and KH2PO4 on Color and Anthocyanin Composition

The final expression of red fruit skin color is the result of the combined effect of chlorophyll, carotenoids, anthocyanins, and other pigments in the fruit skin; it is especially the content of anthocyanins that plays a decisive role in fruit color. Chlorophyll content has a certain interference on red development [27]. When the anthocyanin content is relatively stable, the fruits with high chlorophyll content are dark red, while those with low content are bright red [28]. The results by Okba et al. [29]. suggest that K fertilizer, applied foliarly, improved the “Canino” apricot’s color. It also increased anthocyanins as compared to the control in apples [30]. In this study, foliar spraying of KH2PO4 reduced the total chlorophyll and increased the anthocyanin contents of the LN- and MN-treated grapes. This result indicated that foliar spraying of KH2PO4 reduced the interference of chlorophyll with the chromogenic background and promoted the accumulation of anthocyanins in the grape skin.
Malvidin 3-O-glucoside was the primary anthocyanin component detected in “Summer Black” [1], “Moldova” [31], and “Merlot” [32] grapes. The anthocyanins in red-pink grape skins were mostly Peonidin 3-O-glucoside and Cyanidin 3-O-glucoside; but Malvidin 3-O-glucoside was mostly found in the skins of black-blue grapes [33]. In the current study, Peonidin 3-O-glucoside and Cyanidin 3-O-glucoside were the primary anthocyanin component detected in “Flame seedless” grapes. Anthocyanin derivatives were more color stable than monomeric anthocyanins in the grapes [34]. The ratio of the composition of each monomeric anthocyanin also affected the color and stability of the grapes and wines, respectively. Prior studies demonstrated that the red hue of wine was enhanced with increasing methylation [35]. However, the red color of “Frontenac” grapes with a high anthocyanin content was higher than that of “Cabernet Sauvignon” grapes with a high methylation degree [35]. It was concluded that the content of anthocyanins had a greater impact on the color of grapes. Methylation and acylation modifications generally take place in most grape cultivars for anthocyanins and increase their structural stability. The stability of anthocyanins is greatly influenced by their structure, which is highly stable with high degree of methylation [36]. Nonacylated anthocyanins are particularly susceptible to degradation at high temperatures. Higher proportions of acylated anthocyanins, which are more chemically stable than non-acylated anthocyanins, were observed in the berries of Cabernet Sauvignon and Merlot [37]. The present study indicated that the highest degree of total anthocyanin modification was observed in the LN treatment, which predicted a higher antioxidant activity and a more stable structure (Table 4). Vines treated with MNK had the highest anthocyanin contents, indicating a higher redness value (Table 3). Foliar spraying of KH2PO4 significantly increased the content of methylated, acetylated, coumarylated, and total modified anthocyanins in MN-treated grape berries, indicating that this improved the color and stability of MN-treated grapes.

4.2. Effect of N and KH2PO4 on Sugar and Organic Acid Contents

Fruit quality was improved due to lower acid and increased soluble sugar contents [38]. A low N promoted the rapid growth of roots, increased the transport of sucrose to roots, and led to rapid sugar metabolism [39]. Conversely, a high N reduced the contents of soluble sugars and total flavonoid contents, resulting in poor fruit quality. A high N inhibited the accumulation of sucrose and glucose in apples [40]. The TSS of yellow melon increased as a function of the increased levels of K fertilizer [41]. The application of aqueous K solutions to grapevine clusters increased the TSS content in “Flame seedless” grape berries [42]. With the application of K fertilizer, fructose, glucose, and sucrose accumulation rates significantly increased during apple fruit development [43]. The current study showed that foliar spraying of KH2PO4 increased TSS, glucose, fructose, and sucrose contents in the LN, MN, and HN treatments. The reason may be that K increase photosynthesis and sugar production to enter the target organs [44]. Foliar spraying of potassium sulfate effectively arrested organic acid content in grapes of Crimson Seedless cultivar [45]. Grape acidity results from the ratio between free organic acids (i.e., malic and tartaric acids) and organic acids neutralized by K+ [46]. The application of K “Flame seedless” grape clusters after veraison increased K content compared to the control berries [42]. This study found that foliar spraying of KH2PO4 significantly increased total K content in LN, MN, and HN in two experimental years. Foliar spraying of KH2PO4 decreased tartaric acid, malic acid, citric acid, and shikimic acid contents in the LN, MN, and HN treatments. The possible reason is that K+ neutralizes the organic acids, and reduced their contents in the grapes [46].

4.3. Correlation Analysis

Fructose was positively correlated with total flavonoids and anthocyanins [47]. The anthocyanin concentrations were significantly positively correlated with glucose and fructose contents during the ripening of the blood orange, with correlation coefficients of 0.810 and 0.799, respectively [48]. The total anthocyanins increased with an increase in grape berry sugar [34]. TSS was positively correlated with anthocyanin synthesis during fruit development [49]. The anthocyanin contents were significantly positively correlated with TSS, sucrose, and glucose contents in “Summer black” grape berries [1]; this is consistent with previous results on sugar-induced anthocyanin accumulation in “Cabernet Sauvignon” grape berries [50]. Correlation analysis in this study showed that anthocyanins were positively correlated with glucose, fructose, sucrose, and total sugar contents. The possible reasons were that sugar was the basis of secondary metabolites, where glycolysis provides the raw material for anthocyanin synthesis, and sugar promotes anthocyanin synthesis through a signaling mechanism [51]. Anthocyanins were significantly negatively correlated with malic acid (correlation coefficient was −0.798) and was negatively correlated with citric acid and total acid content [52]. This study showed that “Flame seedless” grape anthocyanins were negatively correlated with malic acid, shikimic acid, and total acid contents. Organic acid in fruit may reduce the content of anthocyanin by replacing the aglycones molecule of the anthocyanins. Therefore, it was speculated that a decrease in organic acid contents may also contribute to an increase anthocyanin contents [23]. However, this mechanism was equivocal and requires further demonstration.

5. Conclusions

Two consecutive years of experiments indicated that foliar spraying of 0.5% KH2PO4 significantly increased total K, CIRG, carotenoid, flavonoid, and anthocyanin contents in LN-, MN-, and HN-treatments, but it slightly decreased organic acid and total chlorophyll contents. HN increased organic acid contents and decreased the sucrose, total sugar, anthocyanin, CIRG, and firmness of the grapes. The MN treatment increased the TSS and pH of grape berries. The application of LN treatment decreased the average berry weight of grape berries. Flavonoids, anthocyanins, and CIRG were significantly positively correlated with soluble sugar and negatively correlated with organic acid. The combined application (HNK) had the highest total N, P, and K during berry ripening in all six treatments. Foliar spraying of 0.5% KH2PO4 significantly increased methylated, acylated, and coumarylated anthocyanins in the MN treatment at 70 days after full bloom in 2021. Overall, foliar spraying of KH2PO4 improved the flavor and color of grapes. Nitrogen fertilization (400 g/vine) significantly increased soluble sugar and anthocyanin contents.

Author Contributions

Conceptualization, L.W. and K.Y.; methodology, K.Y.; software, F.W.; formal analysis, X.L.; investigation, R.S.; data curation, X.L. and R.S.; writing—original draft preparation, L.W.; writing—review and editing, L.W. and J.F.; visualization, F.W.; project administration, J.F.; funding acquisition, K.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the National Natural Science Foundation of China (Grant No. 31760550) and National Key Research and Development Project and Fundamental (Grant No. 2018YFD1000200).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding authors.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Effects of foliar spraying with KH2PO4 on berry fresh weight (g) (A), total soluble solid content (TSS) (B), juice pH (C), and berry firmness (D) of grapes under three soil-applied N application levels. Different lowercase letters indicate significant differences between the three N treatments in the same period and year; Different uppercase letters indicate significant differences between N and KH2PO4 treatments in the same period and year, using DMRT at p < 0.05.
Figure 1. Effects of foliar spraying with KH2PO4 on berry fresh weight (g) (A), total soluble solid content (TSS) (B), juice pH (C), and berry firmness (D) of grapes under three soil-applied N application levels. Different lowercase letters indicate significant differences between the three N treatments in the same period and year; Different uppercase letters indicate significant differences between N and KH2PO4 treatments in the same period and year, using DMRT at p < 0.05.
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Figure 2. Effects of foliar spraying KH2PO4 on the general appearance of “Flame seedless” grape clusters in 2020 (A) and 2021 (B) under three soil-applied N levels.
Figure 2. Effects of foliar spraying KH2PO4 on the general appearance of “Flame seedless” grape clusters in 2020 (A) and 2021 (B) under three soil-applied N levels.
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Figure 3. Effects of foliar spraying with KH2PO4 on total chlorophyll (A), carotenoid (B), flavonoid (C), and anthocyanin (D) contents of grape skins under three N application levels. Different lowercase letters within the same period and year indicate significant differences between the three N treatments; Different uppercase letters on data within the same period and year indicate significant difference between N and KH2PO4 treatments r, using DMRT at p < 0.05. * represents the significant differences between HNK and HN, MNK, and MN or LNK and LN within the same period.
Figure 3. Effects of foliar spraying with KH2PO4 on total chlorophyll (A), carotenoid (B), flavonoid (C), and anthocyanin (D) contents of grape skins under three N application levels. Different lowercase letters within the same period and year indicate significant differences between the three N treatments; Different uppercase letters on data within the same period and year indicate significant difference between N and KH2PO4 treatments r, using DMRT at p < 0.05. * represents the significant differences between HNK and HN, MNK, and MN or LNK and LN within the same period.
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Figure 4. Effects of foliar spraying of KH2PO4 on soluble sugars of grape berry under three soil-applied N levels. Soluble sugar of grape berries in 2020 (A) and 2021 (B). Different lowercase letters within the same period and year indicate significant differences between the three N treatments; Different uppercase letters on data within the same period and year indicate significant difference between N and KH2PO4 treatments r, using DMRT at p < 0.05.
Figure 4. Effects of foliar spraying of KH2PO4 on soluble sugars of grape berry under three soil-applied N levels. Soluble sugar of grape berries in 2020 (A) and 2021 (B). Different lowercase letters within the same period and year indicate significant differences between the three N treatments; Different uppercase letters on data within the same period and year indicate significant difference between N and KH2PO4 treatments r, using DMRT at p < 0.05.
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Figure 5. Effects of foliar spraying of KH2PO4 on organic acids of grape berries under three soil-applied N levels. Organic acid of grape berries in 2020 (A) and 2021 (B); Shikimic acid of grape berries (C). Different lowercase letters within the same period and year indicate significant differences between the three N treatments; Different uppercase letters on data within the same period and year indicate significant difference between N and KH2PO4 treatments r, using DMRT at p < 0.05.
Figure 5. Effects of foliar spraying of KH2PO4 on organic acids of grape berries under three soil-applied N levels. Organic acid of grape berries in 2020 (A) and 2021 (B); Shikimic acid of grape berries (C). Different lowercase letters within the same period and year indicate significant differences between the three N treatments; Different uppercase letters on data within the same period and year indicate significant difference between N and KH2PO4 treatments r, using DMRT at p < 0.05.
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Figure 6. Correlation between sugar and acid contents and coloring index of “Flame seedless” grape berries. * and ** is p < 0.05 and 0.01, respectively.
Figure 6. Correlation between sugar and acid contents and coloring index of “Flame seedless” grape berries. * and ** is p < 0.05 and 0.01, respectively.
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Table
Table 1. Effect of foliar spraying with KH2PO4 on total N, P, K contents (mg·g−1 DW) of “Flame seedless” grape under three soil-applied N levels.
Table 1. Effect of foliar spraying with KH2PO4 on total N, P, K contents (mg·g−1 DW) of “Flame seedless” grape under three soil-applied N levels.
TreatmentTotal NTotal PTotal K
202020212020202120202021
LN5.20 ± 0.35b5.23 ± 0.31b3.52 ± 0.36c3.62 ± 0.31c7.37 ± 0.65c *7.44 ± 0.56b *
LNK5.38 ± 0.45C5.58 ± 0.41B4.14 ± 0.34C4.34 ± 0.49C11.10 ± 0.78C11.54 ± 0.96B
56dMN5.45 ± 0.45ab5.59 ± 0.34a4.57 ± 0.49b4.61 ± 0.34b10.56 ± 0.65b *10.68 ± 0.61b *
MNK5.58 ± 0.49B5.62 ± 0.41B4.82 ± 0.46B4.87 ± 0.44B12.54 ± 0.96B13.10 ± 0.78A
HN5.55 ± 0.51a5.62 ± 0.53a5.37 ± 0.52a5.43 ± 0.42a11.75 ± 0.91a *11.96 ± 0.52a *
HNK5.75 ± 0.59A6.22 ± 0.59A5.44 ± 0.48A5.48 ± 0.57A13.62 ± 0.98A13.88 ± 0.56A
LN5.74 ± 0.62c5.77 ± 0.60b3.92 ± 0.21b3.96 ± 0.13b7.76 ± 0.80b *7.79 ± 0.68b *
LNK5.95 ± 0.13C5.95 ± 0.34C4.29 ± 0.32C4.43 ± 0.31B11.29 ± 0.79C12.18 ± 0.75B
70dMN5.78 ± 0.45b5.81 ± 0.45a4.52 ± 0.38a4.55 ± 0.43a10.95 ± 0.88ab *11.03 ± 0.44a *
MNK6.09 ± 0.56B6.13 ± 0.16B5.24 ± 0.49B5.29 ± 0.37B12.62 ± 0.76B13.29 ± 0.79B
HN5.81 ± 0.24a5.85 ± 0.12a4.55 ± 0.34a *4.59 ± 0.41a *12.73 ± 0.59a *12.98 ± 0.51a *
HNK6.22 ± 0.54A6.28 ± 0.55A5.71 ± 0.45A5.76 ± 0.43A13.89 ± 0.79AB14.10 ± 0.76A
LN6.19 ± 0.57b6.23 ± 0.34b4.30 ± 0.23b *4.37 ± 0.35b *10.36 ± 0.75b *10.41 ± 0.57b *
LNK6.74 ± 0.61B6.78 ± 0.45B5.19 ± 0.51B5.21 ± 0.15B11.72 ± 0.88B12.62 ± 0.76B
84dMN6.49 ± 0.32b6.53 ± 0.43b4.35 ± 0.44b *4.38 ± 0.54b *12.98 ± 0.79a *13.21 ± 0.47a *
MNK7.52 ± 0.61A7.58 ± 0.34A5.49 ± 0.51AB5.52 ± 0.25AB15.56 ± 0.87A15.88 ± 0.98A
HN7.53 ± 0.29a7.62 ± 0.12a5.02 ± 0.46a *5.18 ± 0.43a *13.67 ± 0.89a *13.96 ± 0.58a *
HNK7.69 ± 0.48A7.76 ± 0.44A5.95 ± 0.45A6.05 ± 0.41A15.88 ± 0.98A15.99 ± 0.87A
Different lowercase letters after data within the same column and period indicate a significant difference between the three N treatments; Different uppercase letters after data within the same column and period indicate a significant difference between the combined N and KH2PO4 treatments using Duncan’s multiple range test (DMRT) at p < 0.05. * represents significant differences between HNK and HN, MNK, and MN or LNK and LN within the same column and period. All values are expressed as the means ± SD of three replicates (n = 3).
Table
Table 2. Effect of foliar spraying with KH2PO4 on berry CIRG of “Flame seedless” grapes under three soil-applied N levels.
Table 2. Effect of foliar spraying with KH2PO4 on berry CIRG of “Flame seedless” grapes under three soil-applied N levels.
CIRG 56 d70 d84 d
LN2.34 ± 0.33c3.82 ± 0.11b4.67 ± 0.26a
LNK2.43 ± 0.36B4.50 ± 0.35A4.70 ± 0.19A
2020MN2.75 ± 0.18a4.07 ± 0.36a4.50 ± 0.11a
MNK2.89 ± 0.26A4.18 ± 0.41B4.64 ± 0.27A
HN2.54 ± 0.26b3.53 ± 0.24b3.96 ± 0.18b
HNK2.54 ± 0.19B3.69 ± 0.37C3.93 ± 0.23B
LN2.63 ± 0.36ab3.00 ± 0.22b *3.98 ± 0.35a *
LNK2.62 ± 0.41B3.29 ± 0.16B4.49 ± 0.31A
2021MN2.85 ± 0.22a3.31 ± 0.18a3.76 ± 0.23a *
MNK2.99 ± 0.18A3.42 ± 0.20A4.18 ± 0.28B
HN2.15 ± 0.41b2.90 ± 0.21b3.63 ± 0.17a
HNK2.54 ± 0.39B3.24 ± 0.11B3.76 ± 0.23C
Different lowercase letters after data within the same column and period indicate a significant difference between the three N treatments; Different uppercase letters after data within the same column and period indicate a significant difference between the combined N and KH2PO4 treatments using Duncan’s multiple range test (DMRT) at p < 0.05. * represents the significant differences between HNK and HN, MNK, and MN or LNK and LN within the same column and period. All values are expressed as the means ± SD of three replicates (n = 3).
Table
Table 3. Effect of foliar spraying of KH2PO4 on anthocyanin composition contents in grape skin under three soil-applied N levels.
Table 3. Effect of foliar spraying of KH2PO4 on anthocyanin composition contents in grape skin under three soil-applied N levels.
Anthocyanin CompositionLNLNKMNMNKHNHNK
Cyanidin derivatives
1Cyanidin-3-O-glucoside42.01 ± 5.37b60.21 ± 0.25B62.85 ± 11.76a140.98 ± 29.83A29.77 ± 2.59c38.00 ± 3.65C
2cyanidin-3-O-(6-O-p-coumaryl)-glucoside2.88 ± 0.09a3.71 ± 0.64B3.31 ± 0.62a6.17 ± 1.33A1.47 ± 0.21b2.22 ± 0.34B
3Cyanidin-3-O-(6-O-acetyl)-glucoside0.36 ± 0.05a0.38 ± 0.01B0.36 ± 0.08a0.65 ± 0.15A0.19 ± 0.01b0.25 ± 0.03B
Subtotal45.25 ± 2.33b64.3 ± 4.01B66.52 ± 3.45a147.8 ± 8.67A31.43 ± 1.32c40.47 ± 3.12B
%28.3036.7642.0646.5835.0736.83
Delphinidin derivatives
4Delphinidin-3-O-glucoside7.80 ± 0.80a3.79 ± 0.64B5.18 ± 0.87b17.02 ± 5.12A1.59 ± 0.10c2.63 ± 0.84B
5Delphinidin-3-O-(6-O-p-coumaryl)-glucoside0.51 ± 0.08a0.16 ± 0.08B0.25 ± 0.04b0.62 ± 0.13A0.00 ± 0.00c0.14 ± 0.02B
Subtotal8.31 ± 0.57a3.95 ± 0.56B5.43 ± 0.33b17.64 ± 0.87A1.59 ± 0.23c2.77 ± 0.11B
%5.202.263.435.561.772.52
Malvidin derivatives
6Malvidin-3-O-glucoside30.49 ± 0.17a18.69 ± 2.02B13.92 ± 2.85b33.25 ± 4.54A7.87 ± 0.66c9.97 ± 0.47C
7Malvidin-3-O-(6-O-acetyl)-glucoside0.81 ± 0.07a1.23 ± 0.13B0.50 ± 0.09b2.18 ± 0.08A0.42 ± 0.08b0.68 ± 0.11C
8Malvidin-3-O-(t-6-O-p-coumaryl)-glucoside1.19 ± 0.26a0.63 ± 0.20B0.50 ± 0.14b1.97 ± 0.27A0.28 ± 0.03c0.42 ± 0.10B
9Malvidin-3-O-(c-6-O-p-coumaryl)-glucoside0.22 ± 0.04a0.00 ± 0.00B0.00 ± 0.00b0.21 ± 0.02A0.00 ± 0.00b0.00 ± 0.00B
Subtotal32.71 ± 1.55a20.55 ± 0.98B14.92 ± 0.63b37.61 ± 1.01A8.57 ± 0.67c11.07 ± 0.69C
%20.4511.759.4311.859.5610.08
Peonidin derivatives
10Peonidin-3-O-glucoside60.83 ± 7.49a76.49 ± 2.26A62.40 ± 11.96a87.96 ± 26.73A43.57 ± 4.35b49.28 ± 6.67B
11Peonidin-3-O-(t-6-O-p-coumaryl)-glucoside2.36 ± 0.07a3.01 ± 0.60B2.30 ± 0.52a8.19 ± 1.63A1.34 ± 0.21b1.72 ± 0.38B
12Peonidin-3-O-(6-O-acetyl)-glucoside0.64 ± 0.07a0.98 ± 0.14B0.61 ± 0.18a1.75 ± 0.31A0.43 ± 0.09b0.67 ± 0.15B
13Peonidin-3-O-(c-6-O-p-coumaryl)-glucoside0.37 ± 0.01a0.35 ± 0.08B0.31 ± 0.08a0.83 ± 0.14A0.21 ± 0.02b0.28 ± 0.05B
14Peonidin-3-O-(6-O-acetyl)-glucoside0.19 ± 0.01a0.00 ± 0.00B0.00 ± 0.00b0.32 ± 0.06A0.00 ± 0.00B0.00 ± 0.00b
Subtotal64.39 ± 6.20a80.83 ± 5.23A65.62 ± 4.55a99.05 ± 6.23A45.55 ± 3.32b51.95 ± 2.12B
%40.2646.2141.4831.2250.8347.28
Petunidin derivatives
15Petunidin-3-O-glucoside8.90 ± 0.38a5.10 ± 0.57B5.51 ± 1.17b14.60 ± 3.11A2.39 ± 0.26c3.45 ± 1.02B
16Petunidin-3-O-(6-O-acetyl)-glucoside0.69 ± 0.02a1.13 ± 0.17A0.65 ± 0.05a1.23 ± 0.06A0.41 ± 0.06b0.66 ± 0.14B
17Petunidin-3-O-(6-O-p-coumaryl)-glucoside0.37 ± 0.07a0.19 ± 0.04B0.18 ± 0.02b0.61 ± 0.12A0.09 ± 0.00c0.16 ± 0.06B
Subtotal9.27 ± 0.23a5.29 ± 0.23B5.69 ± 0.15b15.21 ± 0.67A2.48 ± 0.11c3.61 ± 0.21B
%5.793.023.604.792.773.29
Total159.93 ± 11.09a174.92 ± 13.64B158.18 ± 30.42a317.31 ± 23.64A89.62 ± 8.64b109.87 ± 6.11C
Different lowercase letters after data within the same line indicate a significant difference between the three N treatments; Different uppercase letters after data within the same line indicate a significant difference between the combined N and KH2PO4 treatments at p < 0.05. All values are expressed as the means ± SD of three replicates (n = 3).
Table
Table 4. Effects of foliar spraying of KH2PO4 on the content and percentage of anthocyanin modification in grape skins under three soil-applied N levels in 2021.
Table 4. Effects of foliar spraying of KH2PO4 on the content and percentage of anthocyanin modification in grape skins under three soil-applied N levels in 2021.
TreatmentMethylationAcetylationCoumarylationTotal Anthocyanins Modification
mg·kg−1%mg·kg−1%mg·kg−1%mg·kg−1%
LN106.37 ± 6.54a66.512.69 ± 0.56a1.687.90 ± 0.89a4.94110.12 ± 4.12a68.86
LNK106.67 ± 4.55B60.983.72 ± 0.12B2.138.05 ± 0.42B4.60110.92 ± 2.03B63.41
MN86.23 ± 3.54b54.512.12 ± 0.32a1.346.85 ± 0.37a4.3390.15 ± 1.57b57.00
MNK151.87 ± 7.32A *47.866.13 ± 0.46A *1.9318.60 ± 1.01A *5.86159.31 ± 3.59A *50.21
HN56.60 ± 2.34c63.161.45 ± 0.37b1.623.39 ± 0.12b3.7858.26 ± 1.08c65.01
HNK66.63 ± 2.13C60.652.26 ± 0.28C2.064.94 ± 0.36C4.5069.24 ± 1.05C63.02
Different lowercase letters after data within the same column and period indicate a significant difference between the three N treatments; Different uppercase letters after data within the same column and period indicate a significant difference between the combined N and KH2PO4 treatments using Duncan’s multiple range test (DMRT) at p < 0.05. * represent the significant differences between HNK and HN, MNK, and MN or LNK and LN within the same column. All values are expressed as the means ± SD of three replicates (n = 3).
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MDPI and ACS Style

Wu, L.; Wang, F.; Sha, R.; Li, X.; Yu, K.; Feng, J. The Effect of N and KH2PO4 on Skin Color, Sugars, and Organic Acids of “Flame Seedless” Grape. Agronomy 2023, 13, 902. https://doi.org/10.3390/agronomy13030902

AMA Style

Wu L, Wang F, Sha R, Li X, Yu K, Feng J. The Effect of N and KH2PO4 on Skin Color, Sugars, and Organic Acids of “Flame Seedless” Grape. Agronomy. 2023; 13(3):902. https://doi.org/10.3390/agronomy13030902

Chicago/Turabian Style

Wu, Linnan, Fangxia Wang, Riye Sha, Xujiao Li, Kun Yu, and Jianrong Feng. 2023. "The Effect of N and KH2PO4 on Skin Color, Sugars, and Organic Acids of “Flame Seedless” Grape" Agronomy 13, no. 3: 902. https://doi.org/10.3390/agronomy13030902

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