Application of Plant Growth Regulators During Early Fruit Development Stage Increased Perceived Sweetness of Mango Fruit

Abstract

This study investigated the optimal strategies for improving sugar biosynthesis in mango fruits. Randomized block design was used for experimental treatments. The mango cultivar “Renong-1” was sprayed with five green plant growth regulators, including solutions of SBP (sucrose-based polymers, a new highly efficient and eco-friendly plant growth regulator), SPM (sucrose + potassium dihydrogen phosphate + microelement fertilizer), TPM (taurine + potassium dihydrogen phosphate + microelement fertilize), PFA (potassium fulvic acid), and SOP (seaweed oligosaccharide peptide) at different fruit development stages. Indicators, such as soluble solid content, soluble sugar and starch contents, and activities of 11 enzymes associated with sugar metabolism in physiologically mature and in full ripening fruits were evaluated. The results showed that SBP solution diluted 100-fold exerted the strongest effect on the soluble sugar content and sweetness value of “Renong-1” mango fruits. Based on the linear regression analysis, a significant negative correlation was observed between the activity of acid invertase and the perceived sweetness of physiologically mature fruits, while the activities of other enzymes were significantly negatively correlated with the perceived sweetness of full ripening fruits. According to multiple regression (by lars function in R) and other comprehensive analysis, A1B3 (spraying SBP solution one time in the young fruit stage) was selected as the optimal treatment combination for enhancing “Renong-1” mango perceived sweetness, followed by A1B2 (spraying SBP solution for the first time in the young fruit stage and the second time at medium maturity) as the alternative treatment combination.

1. Introduction

As a well-known tropical and subtropical fruit, the mango fruit (Mangifera indica L., Anacardiaceae) is known as the “king of tropical fruits” and is one of the top five tropical fruits in the world (bananas, pineapples, litchi fruits, mangoes, and longans). Its high sugar, acid, vitamin, and dietary fiber contents, coupled with low amounts of protein, fat, and minerals, make it the most preferred fruit among people. The cultivated area of mangoes in 2022 was nearly 381,000 ha, and the production was about 3.8 million tons of the world [1]. China is the second largest country in mango production and is rich in mango germplasm (about 900 accessions) [2], but only a few varieties with a strong, sweet texture are popularized. Most varieties with appealing appearances exhibit limited sweetness, thereby reducing their values to be used as the primary commercial varieties. To a certain extent, the selection and breeding of new mango varieties with the excellent germplasm resources has also been restricted.

Sugar, the most significant source of chemical energy as well as the end product of photosynthesis, is a key indicator of fruit quality, and the perceived sweetness largely depends on the type and proportion of sugars in a fruit [3]. Much research has given attention to sugar metabolism in fruits, such as peaches [4], strawberries [5], apples [6], tomatoes [7], and other horticultural crops [8]. In the case of mango, the content and proportion of soluble sugars—such as sucrose (S), fructose (F), and glucose (G)—in mango fruits considerably contribute to sweetness, flavor, quality, and commercial value [9,10].

Among the methods for improving sugar accumulation, using various nontoxic plant growth regulators to explore potential technologies to improve mango sugar biosynthesis and thus increase the sweetness value is an effective and green strategy [11]. The common fruit sweeteners available in markets include citrus sweeteners, sweetening liquids, and phytoalexins among others [12,13,14,15], which contain phosphorus, calcium, and various trace elements. However, such sweeteners function by nourishing the tree or as plant stimulants, and their safety in fruits and the ecological environment is difficult to investigate. Furthermore, the effect of the sweeteners on different fruits varies, and no study has identified effective sweeteners for mangoes.

The nontoxic plant growth regulators used in this study as sweeteners, including sucrose-based polymers (SBP), potassium fulvic acid, taurine, and seaweed oligosaccharide peptide [16], are well-known green and nontoxic compounds. Taurine is a green biogenic compound that has been patented for its sweetening effect on fruits [17]. Researchers have conducted several studies to increase the sweetening effect of SBP on fruits. For instance, aqueous foliar sprays of SBP during the fruit-bearing stage can effectively increase sucrose synthase (SS) and sucrose phosphate synthase (SPS) activities in longan (Dimocarpus longan Lour.) leaves, in turn promoting sugar accumulation during the fruit-ripening stage [18,19,20,21]. In addition, the sweetening effect of SBP has been observed in Sanyuehong litchi (Litchi chinensis Sonn.), netted melon (Cucumis melo L.), sweet orange (Citrus sinensis L. Osbeck), pawpaw (Carica papaya L.), and cherry tomato (Solanum lycopersicum (L.) var. cerasiforme Mill.) [22,23]. Potassium fulvic acid is a highly effective macromolecular organic compound characterized by its low molecular weight and easy absorption and utilization by crops to promote crop growth and to increase chlorophyll, vitamin C, and sugar contents. Moreover, the acid enhances resistance to cold, drought, and diseases in plants [24,25], making it the most active organic compound among soil humic acids [26,27,28].

The mango cultivar “Renong-1” has high quality and yield; however, it has a low sugar content and low sweetness. Thus, this study aimed to screen a highly efficient and nontoxic plant growth regulator formulation and establish an effective supporting technology system for improving sugar biosynthesis and sweetness values of mango fruits. The results of this study could provide crucial insights for improving the quality of mangoes through the application of eco-friendly plant growth regulators and technological support systems. Furthermore, it could lay the groundwork for developing superior mango varieties that satisfy consumer preferences, utilizing the rich mango germplasm resources in China.

2. Materials and Methods

2.1. Chemicals and Reagents

All chemicals and reagents for soluble sugar extraction and high-performance liquid chromatography (HPLC) analysis were of the highest available purity. Chromatographically pure acetonitrile and ethyl alcohol were respectively from Thermo Fisher Scientific Company (Waltham, MA, USA) and Shanghai Anpel Experimental Technology Co., Ltd., Shanghai, China. For reference standards, sucrose (57-50-1), glucose (50-99-7), and fructose (57-48-7) were purchased from Shanghai Anpel Experimental Technology Co., Ltd., Shanghai, China). Also used were potassium dihydrogen phosphate (7778-77-0, Shanghai Anpel Experimental Technology Co., Ltd., Shanghai, China), microelement fertilizer (Anyang City Xi Mandi fertilizer Co., Ltd., Anyang, China), Taurine (107-35-7, Shanghai Anpel Experimental Technology Co., Ltd., Shanghai, China), and potassium fulvic acid (Anyang City Xi Mandi fertilizer Co., Ltd., Anyang, China). Water was purified using a Milli-Q deionization unit (Millipore, Bedford, MA, USA).

2.2. Mango Variety and Treatment

“Renong-1” mango fruits were collected from a mango orchard at the South Subtropical Crop Research Institute, Chinese Academy of Tropical Agricultural Sciences, Zhanjiang city, Guangdong province, China (20°18′ N, 101°18′ E). “Renong-1” is a conventional excellent high-yielding variety independently cultivated by the mango research group at the institute, although it has the disadvantages of low sugar content and low sweet taste.

Field trials were conducted from April to July 2022, where the natural conditions were generally 20–35 °C and 75–100% relativity humidity. Five solutions—SBP, sucrose-potassium dihydrogen phosphate-microelement fertilizer mixture solution, taurine-potassium dihydrogen phosphate-microelement fertilizer mixture solution, potassium fulvic acid solution, and a seaweed oligosaccharide peptide solution—were applied at different fruit growth stages. A total of 63 well-grown “Renong-1” mango trees with consistent growth at the blooming stage were selected and tagged. The trees were divided into 21 groups of 3; 20 groups were sprayed by the 5 solutions at different fruit development stages using a spray gun, and 1 group was blank processed as a control (CK) (

Table 1. The 20 types of spraying treatments used in this study.

No.	Treatment	Plant Growth Regulator	Spraying Period
1	Black (CK)	none	none
2	A1B1	SBP	Young fruit stage—medium maturity—pre-harvest
3	A1B2	SBP	Young fruit stage—medium maturity
4	A1B3	SBP	Young fruit stage
5	A1B4	SBP	Pre-harvest
6	A2B1	SPM	Young fruit stage—medium maturity—pre-harvest
7	A2B2	SPM	Young fruit stage—medium maturity
8	A2B3	SPM	Young fruit stage
9	A2B4	SPM	Pre-harvest
10	A3B1	TPM	Young fruit stage—medium maturity—pre-harvest
11	A3B2	TPM	Young fruit stage-medium maturity
12	A3B3	TPM	Young fruit stage
13	A3B4	TPM	Pre-harvest
14	A4B1	PFA	Young fruit stage—medium maturity—pre-harvest
15	A4B2	PFA	Young fruit stage—medium maturity
16	A4B3	PFA	Young fruit stage
17	A4B4	PFA	Pre-harvest
18	A5B1	SOP	Young fruit stage—medium maturity—pre-harvest
19	A5B2	SOP	Young fruit stage—medium maturity
20	A5B3	SOP	Young fruit stage
21	A5B4	SOP	Pre-harvest

SBP: sucrose-based polymers, a new highly efficient and eco-friendly plant growth regulator; SPM: sucrose + potassium dihydrogen phosphate + microelement fertilizer; TPM: taurine + potassium dihydrogen phosphate + microelement fertilize; PFA: potassium fulvic acid; SOP: seaweed oligosaccharide peptide. “Young fruit stage” means 30 days after flowering, “Medium maturity” means 95 days after flowering, and “Pre-harvest” means 115 days after flowering. A1, A2, A3, A4, and A5 represent SBP solution, SPM solution, TPM solution, PFA solution, and SOP solution, respectively. B1, B2, B3, and B4 represent stages of young fruit—medium maturity—pre-harvest, young fruit—medium maturity, young fruit, and pre-harvest, respectively.

).

SBP was supplied by the School of Environmental and Life Sciences, Nanning Normal of University, Nanning, China and is a new environmentally friendly efficient plant growth regulator. Sucrose crystals were pre-irradiated by γ-rays of 60 Co and turned into an activated form. Then, the activated sucrose was polymerized directly with another monomer (e.g., acrylic acid) to yield SBP.

Based on our prior experiments, the SBP solution (A1) was prepared by diluting the original SBP liquid 100-fold with water. SPM solution (A2) was prepared by mixing 5000 ppm S, 0.4% potassium dihydrogen phosphate, and 500 ppm microelement fertilizer (i.e., 50 g S, 40 g potassium dihydrogen phosphate, 5 g microelement fertilizer, and 10 L water). TPM solution (A3) was prepared by mixing 500 ppm taurine, 0.4% potassium dihydrogen phosphate, and 500 ppm microelement fertilizer (i.e., 5 g taurine, 40 g potassium dihydrogen phosphate, 5 g microelement fertilizer, and 10 L water). PFA solution (A4) was prepared by mixing 20 g potassium fulvic acid and 10 L water. SOP solution (A5) was prepared by mixing 8.3 mL seaweed oligosaccharide peptide solution and 10 L water.

Sampling method: At the physiological maturity stage (about 125 days after flowering), five fruits which were well-shaped, disease-free, and of uniform size and maturity were picked from Renong-1 mango trees from each of five directions (east, south, west, north, and middle). More than half of the harvested fruits were subjected to post-ripening treatment at room temperature (25 °C) until they became edible (i.e., reached the full ripening stage). For both stages, 10 fruits were ground, weighed, extracted, and used to quantify their contents of S, G, and F as well as enzyme activities. The tests were repeated three times, and the average value was calculated.

2.3. Determination of Total Soluble Solid Content

The total soluble solid content of fruit pulps was determined using a digital portable refractometer (PAL-1; Atago Co., Ltd., Tokyo, Japan).

2.4. Determination of Soluble Sugar Contents

The contents of S, F, and G were determined using HPLC coupled with mass spectrometry (HPLC-MS) equipped with an auto sampler, an ultraviolet detector (LC-20A, Shimadzu Inc., Kyoto, Japan), and integration software (v1.5.2). The column was 250 mm × 4.6 mm, i.d., 5 μm ZORBAX SB-C18 (PerkinElmer Inc., Waltham, MA USA, the same below). Briefly, 1 g of fruit pulp was extracted with 85% alcohol and centrifuged, and the supernatant was evaporated in a water bath at 85 °C and then dissolved in 4 mL of water. Afterward, 1 mL of the resulting solution was filtered through a 0.4 μm membrane filter for subsequent liquid-phase analysis under the following chromatographic conditions: mobile phase consisted of acetonitrile and water (72:28, v/v), chromatographic column was an amino column, flow rate was 1.0 mL/min, column temperature was set to 27 °C, injection volume was 10 μL, and the run time was 15 min. The soluble sugar content was calculated with reference to the peak areas of the samples and the corresponding standard curves of the sugars.

2.5. Determination of Total Sugar Content and Calculation of Perceived Sweetness

Total sugar (TS) content was calculated by summing the contents of F, G, and S.

Perceived sweetness was calculated according to the method of Yao et al. [29] as follows: S = 1.00, F = 1.75, and G = 0.75. Therefore, SV = S content × 1.00 + F content × 1.75 + G content × 0.75.

2.6. Determination of Starch Content

Starch content was determined using a micro method test kit (Suzhou Comin Biotechnology Co., Ltd., Suzhou, China), where the experimental procedures were performed according to the manufacturer’s instructions.

2.7. Determination of Activities of Enzymes Associated with Sugar Metabolism

The activities of enzymes associated with sugar metabolism were determined using assay kits. The activities of ADP-glucose pyrophosphorylase (AGP), sucrose synthase (SS), protein kinase (PK), phosphoglucomutase (PGM), α-amylase (α-amy), β-amylase (β-amy), debranching enzyme (DBE), isoamylase (ISA), invertase (INV), acid invertase (AI), and sucrose phosphate synthase (SPS) were measured independently using an AGP assay kit, SS assay kit, PK assay kit, PGM assay kit, α-amylase assay kit, β-amylase test kit, starch DBE test kit, ISA test kit, INV test kit, AI test kit, and SPS assay kit (Enzyme-linked Biotechnology Co., Ltd., Shanghai, China), respectively, according to the manufacturer’s instructions.

2.8. Statistical Analysis

The analysis of soluble sugar contents and processing of data obtained via HPLC-MS were performed using the built-in Analyst 1.5.2 (AB SCIEX, Foster City, CA, USA).

Statistical analysis of the data was performed using MS Excel 2010 (Microsoft Corp., Redmond, WA, USA) and IBM SPSS Statistics 22.0 (IBM Corp., Armonk, NY, USA). Multiple regression analyses were conducted using the LASSO (least absolute shrinkage and selection operator) model, implemented in R (v3.6.1) with the Lars package, and the results were further visually presented with the Igraph package.

3. Results

3.1. Sugar Contents Under Different Treatments of ’Renong-1’ Mango Fruits at the Physiologically Maturity Stage

The contents of TSS, S, G, F, TS, and SV in “Renong-1” mango fruits at physiologically maturity stage were illustrated in Figure 1, and the significance test of difference is indicated in

Table 2. The contents of sucrose (S), glucose (G), fructose (F) and starch in physiologically mature “Renong-1” mango fruits under different treatments.

Treatment	S (mg/100 g FW)	G (mg/100 g FW)	F (mg/100 g FW)	Starch (mg/g FW)
CK	1567.69 ± 313.31 c	1363.39 ± 118.73 bc	2219.16 ± 191.93 bc	36.98 ± 1.47 ab
A1B1	207.31 ± 16.74 ij	712.29 ± 30.74 j	1564.54 ± 57.24 g	35.22 ± 7.23 ab
A1B2	1990.52 ± 177.15 b	1461.10 ± 125.00 b	2315.66 ± 157.76 b	40.89 ± 1.95 ab
A1B3	293.10 ± 2.85 hij	784.32 ± 32.37 ij	1663.03 ± 59.83 fg	33.11 ± 2.81 b
A1B4	467.01 ± 16.19 ghi	838.30 ± 60.52 ghij	1983.72 ± 78.77 de	41.54 ± 0.36 ab
A2B1	496.62 ± 50.40 ghi	786.19 ± 55.16 ij	1657.88 ± 62.95 fg	44.95 ± 1.24 a
A2B2	1675.90 ± 138.55 c	1615.07 ± 36.38 a	2580.65 ± 40.96 a	36.52 ± 8.51 ab
A2B3	363.01 ± 40.14 hij	758.55 ± 22.50 ij	1803.20 ± 56.14 ef	38.76 ± 2.18 ab
A2B4	452.27 ± 2.64 ghi	766.97 ± 51.56 ij	1630.09 ± 78.84 fg	33.94 ± 5.24 ab
A3B1	2412.11 ± 254.03 a	865.34 ± 19.54 fghij	1732.01 ± 49.20 fg	34.42 ± 0.42 ab
A3B2	347.70 ± 13.48 hij	719.18 ± 25.13 j	1599.32 ± 46.65 fg	39.92 ± 6.16 ab
A3B3	904.89 ± 49.47 e	1010.75 ± 34.45 ef	2138.45 ± 56.27 bcd	32.89 ± 4.55 b
A3B4	580.43 ± 82.92 fgh	1103.45 ± 45.71 de	1783.30 ± 90.40 efg	32.65 ± 3.83 b
A4B1	2124.00 ± 256.89 b	952.04 ± 120.81 ef	1763.37 ± 103.24 fg	34.36 ± 2.37 ab
A4B2	828.02 ± 170.66 ef	1232.73 ± 73.39 cd	2041.04 ± 75.73 cd	34.77 ± 3.51 ab
A4B3	545.50 ± 95.38 gh	789.30 ± 66.49 hij	1690.72 ± 93.13 fg	30.69 ± 2.99 b
A4B4	671.17 ± 41.22 efg	845.39 ± 81.03 ghij	1817.40 ± 76.85 ef	34.81 ± 7.14 ab
A5B1	1249.98 ± 71.10 d	1175.67 ± 76.93 d	2095.28 ± 124.60 cd	34.29 ± 1.43 ab
A5B2	308.11 ± 8.37 hij	971.45 ± 140.38 ef	1741.10 ± 184.11 fg	36.69 ± 6.18 ab
A5B3	138.63 ± 10.56 j	761.58 ± 18.22 ij	1659.96 ± 19.16 fg	32.23 ± 4.79 b
A5B4	437.91 ± 59.49 ghi	897.23 ± 59.56 fghi	1648.91 ± 56.56 fg	37.25 ± 6.73 ab

A1, A2, A3, A4, and A5 represent SBP solution, SPM solution, TPM solution, PFA solution, and SOP solution, respectively. B1, B2, B3, and B4 represent the fruit stages of young (30 days after flowering)—medium maturity (95 days after flowering)—pre-harvest (115 days after flowering), respectively. Lowercase letters in the same column mean p < 0.05.

and

Table 3. The contents of total soluble solid (TSS) and total sugar (TS), and perceived sweetness (SV) of physiologically maturity “Renong-1” mango fruits under different treatments.

Treatment	TSS	TS (mg/100 g FW)	SV (mg/100 g FW)
CK	11.70 ± 0.29 a	5150.23 ± 501.79 b	6473.75 ± 737.99 b
A1B1	7.60 ± 0.54 cdef	2484.14 ± 98.99 i	3479.47 ± 139.07 h
A1B2	9.83 ± 2.49 b	5767.28 ± 151.66 a	7138.74 ± 255.93 a
A1B3	7.23 ± 0.34 cdef	2740.45 ± 115.64 ghi	3791.64 ± 160.64 gh
A1B4	6.17 ± 0.45 f	3289.03 ± 164.59 ef	4567.24 ± 219.39 e
A2B1	7.73 ± 1.34 bcdef	2940.68 ± 104.82 fgh	3987.54 ± 133.90 fgh
A2B2	9.43 ± 1.82 bc	5871.61 ± 100.24 a	7403.33 ± 90.62 a
A2B3	7.70 ± 0.65 bcde	2924.75 ± 132.71 fgh	4087.52 ± 178.30 efg
A2B4	7.40 ± 0.22 bcdef	2849.34 ± 146.54 ghi	3880.16 ± 203.53 gh
A3B1	8.48 ± 2.33 bcde	5009.47 ± 246.62 b	6092.14 ± 219.50 bc
A3B2	7.65 ± 0.09 bcdef	2666.21 ± 101.96 ghi	3685.90 ± 136.94 gh
A3B3	7.28 ± 0.61 cdef	4054.10 ± 118.41 d	5405.25 ± 168.82 d
A3B4	7.75 ± 2.17 bcdef	3467.19 ± 233.28 e	4528.80 ± 294.33 ef
A4B1	8.43 ± 0.94 bcde	4839.41 ± 213.37 bc	5923.93 ± 237.77 c
A4B2	9.23 ± 1.50 bcd	4101.79 ± 161.71 d	5324.39 ± 213.67 d
A4B3	6.93 ± 0.57 ef	3025.53 ± 308.95 fg	4096.25 ± 374.54 efg
A4B4	8.30 ± 0.54 bcdef	3333.96 ± 224.73 ef	4485.66 ± 271.50 ef
A5B1	7.75 ± 0.99 bcdef	4520.93 ± 312.72 c	5798.48 ± 402.95 cd
A5B2	7.10 ± 0.76 def	3020.66 ± 399.03 fg	4083.62 ± 524.83 efg
A5B3	6.13 ± 0.46 f	2560.17 ± 27.09 hi	3614.75 ± 33.66 gh
A5B4	8.00 ± 1.11 bcdef	2984.04 ± 114.73 fgh	3996.42 ± 145.75 fgh

A1, A2, A3, A4, and A5 represent SBP solution, SPM solution, TPM solution, PFA solution, and SOP solution, respectively. B1, B2, B3, and B4 represent the fruit stages of young (30 days after flowering)—medium maturity (95 days after flowering)—pre-harvest (115 days after flowering), respectively. Lowercase letters in the same column mean p < 0.05.

.

The results revealed that for the physiologically maturity mango, treatment groups with SBP enhanced the accumulation of soluble sugars, TS and SV. It is noteworthy that A1B2 (SBP sprayed in the young fruit and medium maturity stages) brought the remarkable-highest TS and SV. A3B1 (TPM sprayed in the young fruit, medium maturity, and pre-harvest stages) obtained the highest S contents of that was significantly higher (p < 0.05) than the rest treatment. A1B2 and A4B3 (PFA sprayed at young fruit stage) treatment groups had the second highest S content and were significantly higher (p < 0.05) than those of the other 18 treatment groups. A2B2 (SPM sprayed at young fruit and medium maturity stages) significantly increased the G, F, and TS contents and SV values. Unexpectedly, the starch content and TSS of the control group did not differ significantly from those of the 20 treated groups.

3.2. Sugar Contents of Full Ripening “Renong-1” Mango Fruits Under Different Treatments

The contents of TSS, S, G, F, TS, and SV in “Renong-1” mango fruits at the post-ripening stage were illustrated in Figure 2, and the significance test of difference is indicated in the

Table 4. The contents of sucrose (S), glucose (G), fructose (F), and starch in fully ripened “Renong-1” mango fruits under different treatments.

Treatment	S (mg/100 g FW)	G (mg/100 g FW)	F (mg/100 g FW)	Starch (mg/g FW)
CK	4931.88 ± 192.29 d	1257.83 ± 62.78 cd	2127.81 ± 58.54 abcde	31.87 ± 3.17 abcd
A1B1	6341.98 ± 205.35 a	607.66 ± 60.65 f	1528.43 ± 106.45 f	32.79 ± 3.76 abc
A1B2	5267.30 ± 244.45 cd	4720.94 ± 523.24 b	1210.97 ± 68.90 f	33.60 ± 3.53 abc
A1B3	4154.87 ± 278.60 ef	12,291.50 ± 239.61 a	2261.68 ± 89.49 abcde	35.78 ± 2.91 abc
A1B4	5201.77 ± 423.54 cd	810.75 ± 511.79 def	2360.27 ± 119.36 ab	37.51 ± 2.93 ab
A2B1	3947.11 ± 138.74 f	1388.53 ± 14.76 c	2353.42 ± 40.79 abc	38.80 ± 0.33 ab
A2B2	3996.12 ± 106.82 f	1512.86 ± 43.80 c	2408.23 ± 31.87 a	38.60 ± 4.71 ab
A2B3	5178.64 ± 59.36 cd	1324.45 ± 45.98 c	2322.93 ± 41.50 abcd	33.58 ± 3.83 abc
A2B4	5713.60 ± 83.99 abc	1152.09 ± 70.90 cde	2057.68 ± 70.06 abcde	35.82 ± 5.12 abc
A3B1	2868.73 ± 191.93 g	1562.80 ± 27.91 c	2348.24 ± 42.33 abc	35.28 ± 0.73 abc
A3B2	4274.14 ± 108.61 ef	1065.57 ± 85.63 cdef	1932.02 ± 36.89 de	39.84 ± 1.53 a
A3B3	4218.23 ± 104.01 ef	1144.48 ± 25.94 cde	2143.03 ± 46.89 abcde	16.19 ± 4.68 f
A3B4	3643.95 ± 98.24 f	1152.97 ± 12.23 cde	1890.75 ± 19.30 e	22.47 ± 2.64 ef
A4B1	4069.15 ± 228.24 ef	1292.52 ± 63.61 cd	2149.81 ± 31.32 abcde	32.05 ± 1.20 abcd
A4B2	4674.29 ± 148.83 de	1254.96 ± 92.06 cd	1946.32 ± 148.31 cde	30.30 ± 8.72 bcde
A4B3	4892.37 ± 133.03 d	1310.99 ± 68.71 cd	2313.53 ± 54.21 abcd	27.77 ± 5.29 cde
A4B4	4181.97 ± 75.52 ef	1276.67 ± 76.61 cd	1954.66 ± 115.03 bcde	30.88 ± 0.62 abcde
A5B1	5669.19 ± 24.66 bc	1123.53 ± 32.36 cde	2016.08 ± 115.90 abcde	30.47 ± 0.83 bcde
A5B2	5762.55 ± 1051.20 abc	688.28 ± 538.59 ef	1448.34 ± 696.39 f	23.83 ± 5.21 def
A5B3	4182.91 ± 73.47 ef	1362.35 ± 63.86 c	2323.43 ± 80.65 abcd	33.19 ± 1.71 abc
A5B4	5941.79 ± 195.67 ab	746.98 ± 79.31 ef	1437.58 ± 61.09 f	28.00 ± 2.95 cde

A1, A2, A3, A4, and A5 represent SBP solution, SPM solution, TPM solution, PFA solution, and SOP solution, respectively. B1, B2, B3, and B4 represent the fruit stages of young (30 days after flowering)—medium maturity (95 days after flowering)—pre-harvest (115 days after flowering), respectively. Lowercase letters in the same column mean p < 0.05.

and

Table 5. Total soluble solid (TSS) and total sugar (TS) contents and perceived sweetness (SV) of fully ripened “Renong-1” mango fruits under different treatments.

Treatment	TSS	TS (mg/100 g FW)	SV (mg/100 g FW)
CK	14.40 ± 0.62 ab	8317.53 ± 310.30 cdef	9598.92 ± 412.88 def
A1B1	13.50 ± 0.62 ab	8492.07 ± 336.18 cde	9472.48 ± 398.37 defg
A1B2	13.37 ± 0.55 ab	11,199.22 ± 866.98 b	10,927.22 ± 776.66 b
A1B3	13.93 ± 1.29 ab	18,708.05 ± 503.73 a	17,331.44 ± 363.19 a
A1B4	14.37 ± 0.85 ab	8386.94 ± 676.67 cde	9940.31 ± 589.46 cde
A2B1	14.07 ± 1.17 ab	7689.06 ± 115.68 fghi	9106.99 ± 100.27 fgh
A2B2	13.83 ± 1.89 ab	7931.25 ± 219.55 defgh	9345.16 ± 233.70 efg
A2B3	13.67 ± 1.27 ab	8840.42 ± 74.11 c	10,237.10 ± 98.07 c
A2B4	14.27 ± 0.74 ab	8938.10 ± 139.00 c	10,178.61 ± 177.97 c
A3B1	9.90 ± 2.95 c	6790.22 ± 173.19 jk	8150.25 ± 167.77 ij
A3B2	13.13 ± 2.64 b	7284.96 ± 176.32 ij	8454.35 ± 198.76 hi
A3B3	13.63 ± 0.95 ab	7519.46 ± 149.98 ghi	8826.89 ± 180.16 gh
A3B4	12.77 ± 0.90 b	6700.72 ± 153.15 k	7817.48 ± 166.21 j
A4B1	14.83 ± 0.40 ab	7525.99 ± 211.02 ghi	8800.71 ± 235.20 gh
A4B2	14.03 ± 1.05 ab	7889.67 ± 368.21 efghi	9021.57 ± 465.97 fgh
A4B3	15.07 ± 1.11 ab	8532.16 ± 220.01 cd	9924.28 ± 234.67 cde
A4B4	13.93 ± 0.47 ab	7413.31 ± 192.90 hi	8560.13 ± 286.24 hi
A5B1	15.73 ± 0.50 a	8824.19 ± 165.12 c	10,039.97 ± 261.16 cd
A5B2	12.90 ± 0.80 b	7911.97 ± 238.51 defghi	8813.35 ± 706.62 gh
A5B3	13.67 ± 1.27 ab	7882.72 ± 265.53 efghi	9270.68 ± 319.66 efg
A5B4	14.30 ± 1.41 ab	8140.44 ± 109.37 defg	9017.78 ± 109.36 fgh

A1, A2, A3, A4, and A5 represent SBP solution, SPM solution, TPM solution, PFA solution, and SOP solution, respectively. B1, B2, B3, and B4 represent the fruit stages of young (30 days after flowering)—medium maturity (95 days after flowering)—pre-harvest (115 days after flowering), respectively. Lowercase letters in the same column mean p < 0.05.

.

For the fully ripened mango, A1B3 (SBP sprayed in the young fruit stage) affected the improvement of the G content; meanwhile, TS content and SV were the brightest. The TS content of 100 g of fresh mango flesh in the A1B3 treatment group was 18,708.05 mg; and the SV was 17.3% in weight, which was 7.7% higher than that of the control group (9.6%) (Table 5). Furthermore, the TS content and SV of the A1B2 (SBP sprayed in the young fruit and medium maturity stages) were considerably higher than those of the other treatment groups but lower than those of the A1B3 treatment group (Table 5). In terms of S content, A1B1 (SBP sprayed in the young fruit, medium maturity, and pre-harvest stages) was observably more than any others, which reached to 6341.98 mg per 100 g of fresh mango flesh after spraying. Similarities to the physiologically maturity mango were found in the following points: F content was the highest in the A2B2 treatment group, although the effect of the treatment was insignificant; overall, most treatments with SBP solutions exerted strong effects on S, G, F, and TS contents, SV; the starch content and TSS of full ripening fruits differed considerably, but almost no significant differences were observed among all groups.

3.3. The Activities of Enzymes Associated with Sugar Metabolism in Physiologically Maturity and Full Ripening Fruits Under Different Treatments

The activities of enzymes associated with sugar metabolism (AGP, SS, PK, PGM, α-amy, β-amy, DBE, ISA, INV, AI, and SPS) in physiologically mature and fully ripened “Renong-1” mangoes under different treatments are presented in

Table 6. Activities of enzymes associated with sugar metabolism in physiologically mature “Renong-1” mango fruits under different treatments.

Treatment	AGP	SS	PK	DBE	PGM	α-Amy	β-Amy	ISA	INV	AI	SPS
A1B1	3.53 ± 0.23	0.74 ± 0.07	3.43 ± 0.31	0.82 ± 0.12	937.21 ± 97.59	1.05 ± 0.10	3.75 ± 0.10	4.17 ± 0.45	0.60 ± 0.05	1.05 ± 0.08	0.81 ± 0.05
A1B2	3.69 ± 0.29	0.68 ± 0.02	3.39 ± 0.22	0.75 ± 0.02	1172.30 ± 180.71	1.28 ± 0.15	3.39 ± 0.45	5.13 ± 0.22	0.62 ± 0.03	0.90 ± 0.08	0.96 ± 0.08
A1B3	2.84 ± 0.21	0.72 ± 0.07	3.36 ± 0.23	0.67 ± 0.05	1351.26 ± 173.55	0.93 ± 0.12	2.57 ± 0.18	4.95 ± 0.64	0.68 ± 0.05	0.79 ± 0.01	0.78 ± 0.01
A1B4	3.07 ± 0.20	0.72 ± 0.09	2.84 ± 0.26	0.68 ± 0.04	1300.22 ± 8.90	0.74 ± 0.11	2.73 ± 0.26	3.6 ± 0.11	0.53 ± 0.05	0.77 ± 0.05	0.74 ± 0.06
A2B1	4.43 ± 0.12	0.64 ± 0.06	3.75 ± 0.22	0.88 ± 0.06	1711.53 ± 120.80	1.31 ± 0.13	3.93 ± 0.30	5.55 ± 0.88	0.94 ± 0.05	1.07 ± 0.06	1.19 ± 0.09
A2B2	3.97 ± 0.33	0.62 ± 0.07	2.52 ± 0.23	0.96 ± 0.10	1761.93 ± 48.13	0.99 ± 0.09	3.17 ± 0.42	4.37 ± 0.37	0.76 ± 0.09	0.84 ± 0.04	0.92 ± 0.03
A2B3	2.63 ± 0.07	0.68 ± 0.01	2.56 ± 0.11	0.77 ± 0.08	1714.28 ± 104.27	0.81 ± 0.10	3.33 ± 0.20	3.84 ± 0.24	0.70 ± 0.07	0.77 ± 0.05	0.90 ± 0.08
A2B4	3.78 ± 0.31	0.71 ± 0.04	2.93 ± 0.36	0.91 ± 0.09	1062.59 ± 104.09	0.90 ± 0.05	2.64 ± 0.31	4.59 ± 0.29	0.59 ± 0.07	0.71 ± 0.03	0.81 ± 0.14
A3B1	4.45 ± 0.73	0.95 ± 0.05	3.43 ± 0.28	1.07 ± 0.11	1471.14 ± 165.51	1.48 ± 0.18	4.68 ± 0.37	4.62 ± 0.52	0.68 ± 0.05	0.80 ± 0.09	0.99 ± 0.05
A3B2	4.69 ± 0.67	0.83 ± 0.03	3.87 ± 0.27	0.91 ± 0.08	1697.12 ± 117.55	1.38 ± 0.09	4.07 ± 0.05	5.53 ± 0.78	0.72 ± 0.07	1.23 ± 0.10	1.12 ± 0.08
A3B3	3.25 ± 0.11	0.61 ± 0.03	3.31 ± 0.18	0.95 ± 0.07	1566.45 ± 42.92	1.13 ± 0.04	3.16 ± 0.27	5.19 ± 0.41	0.80 ± 0.11	0.87 ± 0.03	0.91 ± 0.06
A3B4	3.02 ± 0.39	0.72 ± 0.02	3.52 ± 0.14	0.98 ± 0.15	1544.42 ± 116.81	1.25 ± 0.15	3.19 ± 0.32	3.45 ± 0.18	0.56 ± 0.04	0.65 ± 0.05	0.78 ± 0.08
A4B1	3.76 ± 0.36	0.75 ± 0.11	3.00 ± 0.03	0.91 ± 0.07	1533.83 ± 144.87	0.93 ± 0.06	3.57 ± 0.28	4.45 ± 0.42	0.76 ± 0.06	0.98 ± 0.16	0.84 ± 0.03
A4B2	3.15 ± 0.29	0.72 ± 0.12	2.72 ± 0.12	0.77 ± 0.09	1008.37 ± 112.35	1.05 ± 0.08	3.41 ± 0.03	4.02 ± 0.40	0.62 ± 0.01	0.81 ± 0.05	0.93 ± 0.05
A4B3	3.19 ± 0.26	0.57 ± 0.07	3.62 ± 0.31	0.61 ± 0.06	1075.51 ± 94.81	0.95 ± 0.09	2.77 ± 0.23	3.72 ± 0.31	0.61 ± 0.02	0.86 ± 0.03	0.61 ± 0.07
A4B4	3.16 ± 0.32	0.57 ± 0.08	3.64 ± 0.21	0.76 ± 0.07	1361.01 ± 40.31	0.79 ± 0.10	3.54 ± 0.32	4.62 ± 0.44	0.62 ± 0.05	0.82 ± 0.04	0.73 ± 0.07
A5B1	4.12 ± 0.35	0.89 ± 0.08	2.46 ± 0.19	1.02 ± 0.16	1351.26 ± 5.17	1.34 ± 0.14	3.73 ± 0.12	5.57 ± 0.82	0.87 ± 0.05	1.09 ± 0.09	1.19 ± 0.09
A5B2	3.32 ± 0.23	0.63 ± 0.02	3.56 ± 0.25	0.80 ± 0.13	917.93 ± 134.49	1.15 ± 0.15	3.63 ± 0.29	4.71 ± 0.15	0.58 ± 0.01	0.85 ± 0.11	0.97 ± 0.09
A5B3	3.49 ± 0.20	0.74 ± 0.06	2.40 ± 0.30	0.79 ± 0.06	1401.88 ± 17.48	1.03 ± 0.14	3.23 ± 0.09	2.94 ± 0.33	0.51 ± 0.06	0.85 ± 0.07	0.83 ± 0.11
A5B4	3.11 ± 0.24	0.58 ± 0.05	3.66 ± 0.13	0.82 ± 0.10	1910.40 ± 100.00	1.18 ± 0.07	3.41 ± 0.17	4.00 ± 0.38	0.53 ± 0.02	0.61 ± 0.08	0.81 ± 0.03

A1, A2, A3, A4, and A5 represent SBP solution, SPM solution, TPM solution, PFA solution, and SOP solution, respectively. B1, B2, B3, and B4 represent the fruit stages of young (30 days after flowering)—medium maturity (95 days after flowering)—pre-harvest (115 days after flowering), respectively. The unit is U/g.

and

Table 7. Activities of enzymes associated with sugar metabolism in fully ripened “Renong-1” mango fruits under different treatments.

Treatment	AGP	SS	PK	DBE	PGM	α-Amy	β-Amy	ISA	INV	AI	SPS
A1B1	3.95 ± 0.37	0.71 ± 0.01	3.13 ± 0.05	0.65 ± 0.04	1381.76 ± 159.66	0.97 ± 0.10	2.86 ± 0.29	4.36 ± 0.58	0.70 ± 0.07	1.01 ± 0.09	0.86 ± 0.09
A1B2	4.01 ± 0.47	0.66 ± 0.02	2.73 ± 0.21	0.67 ± 0.07	1038.66 ± 104.97	0.97 ± 0.12	2.73 ± 0.45	4.28 ± 0.37	0.57 ± 0.03	1.04 ± 0.11	1.15 ± 0.09
A1B3	3.85 ± 0.28	0.51 ± 0.05	2.26 ± 0.14	0.62 ± 0.07	1332.42 ± 140.72	0.78 ± 0.12	2.81 ± 0.29	3.66 ± 0.42	0.60 ± 0.01	0.70 ± 0.15	0.92 ± 0.05
A1B4	3.49 ± 0.54	0.57 ± 0.05	3.27 ± 0.36	0.64 ± 0.08	1442.55 ± 70.72	0.86 ± 0.07	3.32 ± 0.30	2.89 ± 0.26	0.60 ± 0.05	0.88 ± 0.11	1.14 ± 0.1
A2B1	3.53 ± 0.16	0.30 ± 0.01	1.31 ± 0.23	1.87 ± 0.02	820.28 ± 156.43	0.95 ± 0.05	2.07 ± 0.26	6.76 ± 0.31	0.53 ± 0.03	1.07 ± 0.02	0.96 ± 0.01
A2B2	3.54 ± 0.26	0.60 ± 0.02	2.41 ± 0.33	0.87 ± 0.09	1520.28 ± 156.47	1.15 ± 0.15	2.97 ± 0.26	4.76 ± 0.41	0.71 ± 0.09	1.01 ± 0.04	0.86 ± 0.07
A2B3	3.36 ± 0.19	0.59 ± 0.07	2.92 ± 0.16	0.53 ± 0.06	762.35 ± 88.60	0.93 ± 0.06	2.83 ± 0.14	2.91 ± 0.24	0.59 ± 0.04	0.76 ± 0.07	0.79 ± 0.06
A2B4	3.69 ± 0.33	0.57 ± 0.04	3.43 ± 0.27	0.58 ± 0.04	671.89 ± 168.63	0.95 ± 0.07	3.43 ± 0.08	5.22 ± 0.11	0.61 ± 0.02	0.90 ± 0.09	0.93 ± 0.14
A3B1	4.25 ± 0.29	0.72 ± 0.05	3.51 ± 0.32	0.91 ± 0.05	1640.3 ± 263.86	1.29 ± 0.08	3.40 ± 0.17	4.47 ± 0.75	0.80 ± 0.09	0.89 ± 0.11	1.18 ± 0.16
A3B2	3.84 ± 0.23	0.63 ± 0.05	2.71 ± 0.09	0.79 ± 0.08	1539.69 ± 204.05	1.06 ± 0.11	3.60 ± 0.27	3.54 ± 0.36	0.96 ± 0.15	1.14 ± 0.06	1.21 ± 0.14
A3B3	4.33 ± 0.33	0.60 ± 0.03	3.93 ± 0.39	0.85 ± 0.04	1000.65 ± 134.39	1.18 ± 0.11	3.27 ± 0.28	4.80 ± 0.42	0.58 ± 0.05	0.89 ± 0.14	0.88 ± 0.02
A3B4	3.80 ± 0.19	0.55 ± 0.08	3.56 ± 0.25	0.53 ± 0.03	1180.47 ± 79.16	1.14 ± 0.09	3.65 ± 0.45	3.71 ± 0.27	0.57 ± 0.07	0.82 ± 0.07	1.05 ± 0.02
A4B1	3.51 ± 0.26	0.61 ± 0.06	2.81 ± 0.21	0.85 ± 0.03	1103.47 ± 159.25	1.14 ± 0.08	3.60 ± 0.46	5.07 ± 0.66	0.76 ± 0.06	1.06 ± 0.07	0.90 ± 0.09
A4B2	3.95 ± 0.28	0.61 ± 0.08	2.93 ± 0.10	0.83 ± 0.08	1387.44 ± 221.44	1.04 ± 0.09	3.65 ± 0.29	4.49 ± 0.27	0.59 ± 0.06	0.82 ± 0.04	1.03 ± 0.06
A4B3	3.36 ± 0.24	0.70 ± 0.06	2.74 ± 0.21	0.67 ± 0.02	791.48 ± 82.45	0.93 ± 0.10	3.28 ± 0.24	3.35 ± 0.29	0.74 ± 0.06	0.79 ± 0.06	1.08 ± 0.07
A4B4	2.36 ± 0.21	0.80 ± 0.05	2.72 ± 0.01	0.77 ± 0.01	591.38 ± 80.35	0.63 ± 0.80	2.28 ± 0.21	2.15 ± 0.01	0.84 ± 0.03	0.63 ± 0.02	0.78 ± 0.02
A5B1	4.32 ± 0.15	0.63 ± 0.03	3.91 ± 0.40	0.45 ± 0.08	832.07 ± 59.47	1.27 ± 0.16	3.85 ± 0.19	4.87 ± 0.24	0.87 ± 0.02	1.33 ± 0.11	0.89 ± 0.08
A5B2	3.42 ± 0.33	0.57 ± 0.04	2.72 ± 0.22	0.57 ± 0.09	1651.11 ± 118.14	1.23 ± 0.18	3.03 ± 0.27	4.99 ± 0.49	0.55 ± 0.04	0.67 ± 0.06	1.04 ± 0.05
A5B3	3.66 ± 0.03	0.36 ± 0.04	2.85 ± 0.21	0.72 ± 0.01	1485.19 ± 153.71	0.98 ± 0.15	2.95 ± 0.46	3.16 ± 0.20	0.63 ± 0.05	0.89 ± 0.13	0.91 ± 0.03
A5B4	3.64 ± 0.26	0.55 ± 0.06	3.36 ± 0.48	0.59 ± 0.05	1489.16 ± 115.34	1.01 ± 0.06	3.66 ± 0.20	4.41 ± 0.36	0.54 ± 0.05	0.80 ± 0.03	1.00 ± 0.02

A1, A2, A3, A4, and A5 represent SBP solution, SPM solution, TPM solution, PFA solution, and SOP solution, respectively. B1, B2, B3, and B4 represent the fruit stages of young (30 days after flowering)—medium maturity (95 days after flowering)—pre-harvest (115 days after flowering), respectively. The unit is U/g.

.

To identify the key enzymes that serve as the basis for selecting optimal treatment combinations, a multiple linear regression analysis was conducted. The effects of 13 variables (TSS, starch, AGP, SS, PK, DBE, PGM, α-amy, β-amy, ISA, INV, AI, and SPS) on SV, F, G, and S were estimated, and the results are described by the following two equations.

Sweetness = 297.0 × TSS + 0.0 × Starch + 0.0 × AGP + 0.0 × SS − 842.5 × PK + 792.9 × DBE + 0.0 × PGM + 924.3 × α-Amylase + 0.0 × β-Amylase + 334.9 × Isoamylase + 1262.3 × INV − 1064.4 × AI − 821.5 × SPS

(1)

Sweetness = 0.0 × TSS − 75.0 × Starch + 1649.7 × AGP + 3367.2 × SS − 2067.0 × PK +1812.0 × DBE − 0.4 × PGM − 2774.1 × α-Amylase − 2923.6 × β-Amylase − 528.8 × Isoamylase + 2480.1 × INV − 2981.8 × AI − 4621.4 × SPS

(2)

In Equation (1), the model derived from statistical data for fruit samples at the green-ripening stage demonstrated that the degree of influence on SV decreased in the order of INV > AI > α-amy > PK > SPS > DBE > ISA > TSS, with PK, AI, and SPS exerting significant negative effects (Figure 3A).

In Equation (2), the model derived from statistical data for fruit samples at the fully ripe stage revealed that the degree of influence on SV decreased in the order of SPS > SS > AI > β-amylase > α-amylase > INV > PK > DBE > AGP > isoamylase > starch > PGM, with SPS, AI, β-amylase, α-amylase, PK, isoamylase, starch, and PGM exerting significant negative effects (Figure 3B).

It should be noted that at the green-ripening stage, four enzymes (AGP, SS, PGM, and β-Amylase) are determined to have no effect on SV, in contrast to the fully ripe stage, where none are decided. Additionally, the coefficient of each enzyme in Equation (2) is larger than that in Equation (1).

3.4. Selection of the Optimal Treatment Combination

To determine the optimal treatment combination in the view of sweetness, both TS content and SV indicators were evaluated. At the physiologically mature stage, A1B2 and A2B2 treatment groups whose TS and SV values were significantly greater than that of the control group were selected; similarly, at the fully ripened stage, treatment combinations of A1B2 and A1B3 were selected. Considering that A2B2 treatment showed higher values of TS and SV only in the early stage but not in the fully ripened stage, it should be discarded (Table 4 and Table 5). Overall, A1B3 was selected as the optimal treatment combination for its best effects on enhancing mango perceived sweetness, followed by A1B2 as the alternative treatment.

4. Discussion

4.1. Section of Testing Factors Associated with Sweetness

The taste texture of fruit is a mixture trait comprising a series of aspects, including pulp hardness, texture, smell, sweetness, sourness, and so on, with each affected by various factors. In the case of sweetness, it is well known that many compounds, such as soluble sugars, acids, and alcohols, are involved, again with each containing numerous components [30]. Obviously, it is impossible to test so many chemicals and their contents with a few experiments implemented in one study, whereas picking out those most important factors for examination is reasonable. There, we here checked that the contents of sugars (S, F, and G), TSS and starch were in line with previous studies.

On the other hand, soluble sugar accumulation is closely associated with the activities of enzymes involved in sugar metabolism [3]. Moreover, plant growth regulators influence fruit development and ripening, as well as sugar accumulation and metabolism in fruits, by affecting these sugar-associated enzymes [23]. Consequently, the activities of AGP, SS, PK, PGM, α-amylase, β-amylase, DBE, ISA, INV, AI, and SPS were selected for examination, referencing numerous previous studies.

4.2. Effects of Different Plant Growth Regulators on Sweetness

The results of this study indicated that TS content and SV value of “Renong-1” mango fruits were not significantly affected after spraying with SOP or PFA; however, S content was increased substantially by TPM, and G and F contents were increased considerably by SPM. The SBP exerted the strongest effect on the sugar content, with significant increases observed in TS content and SV when compared to the control group, particularly at the full ripening stage (Figure 2 and Figure 3, Table 2, Table 3, Table 4, Table 5 and Table 6).

Seaweed oligosaccharide peptide (SOP) and potassium fulvic acid (PFA) are both nutrients and regulators [16,28]. They did not improve the sugar content in this study, possibly because they functioned merely as nutrients, and the dosage used was too small to nourish a large tree. Similar to SOP and PFA, SPM and TPM consist of a mixture solution containing sucrose/taurine, potassium dihydrogen phosphate, and microelement fertilizers, which are richer in nutrients and can therefore increase certain sugar contents. Unlike the first four mixtures, SBP is limited in nutrients but has significantly higher contents of various sugars, suggesting it functions as a stimulant. During the SBP production process, sucrose crystals were pre-irradiated with γ-rays, thereby making it possible to obtain activators that may affect sugar metabolism. Nevertheless, the efficiency of SBP in the present study is consistent with that of previous studies [18,22].

It should be noted that compared to the CK group, no tested growth regulators were able to enhance starch content or TSS content at either tested stage (Figure 2 and Figure 3, Table 2, Table 3, Table 4, Table 5 and Table 6), indicating that these indices are not relevant for selecting the optimal treatment. Given that TS was enhanced at the full ripening stage, it can be inferred that more sugar-like compounds were present before fruit harvest. Thus, if starch was not affected, it suggests that other polysaccharides, such as fibers, must have increased. Therefore, further studies are recommended to analyze other quality compounds.

4.3. Effects of Activity of Different Enzymes on Sweetness

As mentioned previously, plant growth regulators influence sugar accumulation through enzymes [30]. Consequently, monitoring the relationship between enzyme activity and sweetness can provide insights into the underlying mechanisms [6]. Compared to other mathematical models, the LASSO (least absolute shrinkage and selection operator) is less accurate (p-values are not provided) but simpler and more concise, with the coefficients of insignificant variables being directly expressed as zero, whereas significant variables (either positive or negative) are screened out. Upon utilizing this model, we observe that nearly every enzyme (with the exception of α-amylase) maintains its effect trend (positive or negative) on SV from the fruit’s green-ripening stage to the fully ripened stages, while the absolute values of their coefficients gradually increase. The results indicate that these enzymes are closely related to sugar accumulation during mango fruit development, and therefore, some of them could potentially be used as markers to determine which plant growth regulators are more effective. To document this issue, it is necessary to dynamically monitor enzyme activity and sugar content from the young fruit stage through to harvest and full ripeness in future studies.

5. Conclusions

At the present study, a randomized block design was employed to investigate the effects of various plant growth regulators on sugar biosynthesis during fruit development and ripening stages. Key physiological and biochemical indicators, including enzyme activities related to sugar metabolism, are comprehensively analyzed to identify optimal treatment combinations. In summary, the SBP solution sprayed once in the young fruit stage had the optimal sweetening effect on “Renong-1” mango fruits, with significant effects being observed on increasing sugars, i.e., S, G, and F contents. It is hope that the application of SBP will benefit for the field of fruit physiology study and sustainable agriculture production in the near future.