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Abstract ［Objectives］ This study was conducted to investigate the effects of different types of potassium fertilizers on the flavor and quality of pear fruit.
［Methods］ With ‘Xinliqihao’ as a research object, such three treatments as potassium sulfate (T1), potassium nitrate (T2) and potassium chloride (T3) were set up, with no application of potassium fertilizer as control (CK). All the fertilizers were applied into soil in the growing season.
［Results］ The total sugar content of ‘Xinliqihao’ was significantly higher in the potassium fertilizer treatments than that of the control. The total sugar content of T2 (the potassium sulfate treatment) was the highest, which was 34.55% higher than that of the control, sequentially followed by T3 (the potassium nitrate treatment) and T1 (the potassium chloride treatment). The contents of fructose, glucose and sucrose in the potassium sulfate treatment were all the highest, which were 15.96%, 27.32% and 52.81% higher than those of the potassium chloride treatment, respectively, and 11.36%, 27.75% and 20.45% higher than the potassium nitrate treatment, respectively. The total acid content of ‘Xinliqihao’ fruit treated with different types of potassium fertilizers was higher than that of the control. The total acid content of the potassium nitrate treatment was the highest, which was 103.68% higher than that of the control, sequentially followed by the potassium sulfate treatment and the potassium chloride treatment. Both the control and potassium treatments of ‘Xinliqihao’ had the highest malic acid content, and except the potassium chloride treatment with the lowest fumaric acid content, other treatments all had the lowest tartaric acid content. Different types of potassium fertilizer treatments had different effects on the organic acid content of ‘Xinliqihao’ fruit. The types and contents of volatile aroma substances in the fruit of ‘Xinliqihao’ treated with potassium fertilizers were significantly higher than the control. The aroma substances in ‘Xinliqihao’ differed among different potassium fertilizer treatments. Specifically, 78 kinds of aroma substances were detected in the pot assium chloride treatment, and the total content of aroma components was the highest at 746.609 ng/g in the fruit of the potassium sulfate treatment.
［Conclusions］ The potassium fertilizer treatments improved the flavor and quality of ‘Xinliqihao’ pear fruit, and the potassium sulfate treatment exhibited the best effects. Key words Potassium; Pear; Fruit; Sugar acid; Aroma
Potassium is one of the most important nutrient elements in plant metabolism［1］, which plays an important role in plant growth and development. Potassium is required for all stages of pear growth and development. The effect of potassium on fruit trees is mainly reflected in affecting fruit quality and photosynthetic capacity, so it is called "quality element". He et al.［2］have shown that potassium can improve the 100-leaf weight and chlorophyll content of ‘Dangshan pear’ and improve fruit quality. Sun et al.［3］found that the application of potassium fertilizer in kiwifruit can increase leaf chlorophyll content and improve photosynthetic capacity. The results obtained by Qu et al.［4］on apple show that the application of potassium fertilizer can improve the water use efficiency of apple trees by increasing photosynthetic efficiency and reducing transpiration efficiency. Yu et al.［5］suggested that rational application of potassium fertilizer can increase the plant height of glorious orange saplings and the potassium content of various tissues. Wang et al.［6］showed that at a suitable concentration, potassium can promote the growth of pear trees and improve the photosynthetic efficiency of leaves. Wu et al.［7］reported that potassium application promoted the absorption of potassium by ‘Huangguan pear’ trees, and the fruit yield and quality increased with the increase of potassium application to different degrees.
The above results indicate that potassium plays an important role in leaf growth and fruit development, but the effect of potassium fertilizer on the flavor quality of pear fruit has not been reported. In view of this, in this study, with ‘Xinliqihao’ pear fruit as an experimental material, the effects of different types of potassium fertilizers on the flavor and quality of pear fruit such as sugar and aroma were investigated, aiming at providing a theoretical basis for improving pear fruit quality through rational application of potassium fertilizers in pear production.
Materials and Methods
Test materials
The experiment was carried out in the pear orchard of Jinniushan Experimental Base, Shandong Institute of Pomology from 2017 to 2018. The pear orchard was conventionally managed, and the basic nutrient status of the pear orchard soil is shown in Table 1.‘Xinliqihao’ pear trees in the orchard were grafted on Pyrus betulifolia Bunge rootstocks. The trees were 5 years old and had plant spacing of 1 m×4 m. The management level was medium, and the trees were moderate in tree potential, and had a per unit yield of about 30 000 kg/hm2 and a yield per plant of about 12 kg. Experiments were carried out by selecting trees with similar tree phases and loads, and 5 trees were treated for each treatment in three replicates. The treatments of different types of potassium fertilizers referred to the methods of Wang et al.［6］and Wu et al.［7］. The experiment was designed with three treatments: potassium sulfate (T1), potassium nitrate (T2) and potassium chloride (T3), with no application of potassium fertilizer as control (CK). The amount of potassium fertilizer applied was 300 kg/hm2 according to Wu et al.［7］. Treatments T1 and T3 were supplemented with the same amount of nitrogen fertilizer according to the T2 treatment. The potassium fertilizers were applied before germination and during the expansion stage at a ratio of 60%∶40%.
Other management measures of the test trees were consistent. The fruit was harvested around July 25, and fruit samples were collected from the east, south, west, and north of each tree. From each treatment, fruit with appropriate maturity, no pests and diseases and no mechanical damage, having typical characteristics of the variety, was selected and mixed, and 100 individuals per treatment were transported back to the laboratory for immediate treatment. The fruit was chopped after removal of the fruit core, and divided into two parts, one of which was used for the determination of fruit sugar and acid contents, and the other was used for fruit aroma determination.
Determination indexes and methods
The determination of fruit sugar, acid and aroma referred to the method of Wei et al.［8］.
Extraction and determination of fruit sugar and acid components: A certain amount of fruit flesh (5 g) was accurately weighed and ground with the addition of 15 ml of 80% ethanol. The mixture was heated in a water bath at 75 ℃ for 30 min and centrifuged at 4 000 g/min for 5 min. The supernatant was transferred to a 25 ml volumetric flask, and the remaining precipitate was added with 10 ml of 80% ethanol, followed by heating in a water bath at 75 ℃ for 30 min and centrifuging at 4 000 g/min for 5 min, obtaining the supernatant which was transferred to the 25 ml volumetric flask. The supernatant was diluted to constant volume, and the extract was then evaporated to dry at 60 ℃. The residue was dissolved with 5 ml of redistilled water, for later determination by 510 Waters High Performance Liquid Chromatograph. The column used for the analysis of the sugar components was an amino column Kromasil 250 mm×4.6 mm, and the HPLC determination was carried out with the mobile phase of acetonitrile: water (80∶20) at a flow rate of 1 ml/min under an injection volume of 15 μl, using a RID 10- A differential refractive index detector. The column for organic acid analysis was C18 Kromasil 250 mm× 4.6 mm, and the HPLC was performed with 10 mmol/L ammonium dihydrogen phosphate (having pH adjusted to 2.8 with phosphoric acid)∶methanol (97∶3) as the mobile phase at a flow rate of 0.9 ml/min under an injection volume of 10 μl, using a Waters 2487 dual-wavelength UV detector with a detection wavelength of 214 nm. The contents of the sugar and acid components were calculated using an N2000 chromatography workstation (Ver. 3.30). Determination of volatile aroma substances in fruit: The fruit core was removed by quadruple method, and the remaining part was then cut into pieces (length, width and height of about 0.2 cm) and mixed. A certain amount of the sample (5 g) was accurately weighed into a sample bottle and added with 10 μg of 3-nonanone (0.4 mg/ml) as the internal standard at the bottom of the 10 ml sample bottle which was then sealed with PTFE butyl rubber spacer. The extraction and determination of the volatile components of the fruit were carried out by static headspace GC-MS using a Perkin Elmer Turbo Matrix 40 Trap headspace sampler and a Shimadzu GCMS-QP2010 gas chromatography-mass spectrometer. Qualitative method for volatile components: The mass spectra of unknown compounds were searched by computer and matched with NIST05 mass spectrometry library, to confirm various volatile components combined with atlas analysis and data analysis. Quantitative method: According to the peak area normalization method, the mass percentage of each compound was determined, and 3-nonanone was selected as an internal standard for accurate quantification.
The test data were processed using Microsoft Excel 2010. The variance and correlation analysis were performed using DPS data processing software, and the significant test of the average was performed by t test.
Results and Analysis
Effects of different types of potassium fertilizers on sugar and acid components in ‘Xinliqihao’ Fruit
The results showed that the total sugar content of ‘Xinliqihao’ was significantly higher in the potassium fertilizer treatments than that of the control. The total sugar content of T2 (the potassium sulfate treatment) was the highest, which was 34.55% higher than that of the control, sequentially followed by T3 (the potassium nitrate treatment) which increased by 15.67% compared with the control and T1 (the potassium chloride treatment) which increased by 12.16% compared with the control. The total acid content of ‘Xinliqihao’ fruit treated with different types of potassium fertilizers was higher than that of the control. The total acid content of the potassium nitrate treatment was the highest, which was 103.68% higher than that of the control, sequentially followed by the potassium sulfate treatment which increased by 53.16% compared with the control and the potassium chloride treatment which increased by 38.96% compared with the control.
Studies have shown that the control and potassium treatments of ‘Xinliqihao’ fruit both showed the highest fructose content, sequentially followed by glucose and sucrose. The contents of fructose, glucose and sucrose in the potassium sulfate treatment were all the highest, which were 15.96%, 27.32% and 52.81% higher than those of the potassium chloride treatment, respectively, and 11.36%, 27.75% and 20.45% higher than the potassium nitrate treatment, respectively. Both the control and potassium treatments of ‘Xinliqihao’ had the highest malic acid content, and except the potassium chloride treatment with the lowest fumaric acid content, other treatments all had the lowest tartaric acid content. Different types of potassium fertilizer treatments had different effects on the organic acid content of ‘Xinliqihao’ fruit. The fruit in the potassium nitrate treatment had the highest malic acid and citric acid contents; the potassium chloride treatment showed the highest oxalic acid and tartaric acid contents; and the potassium sulfate treatment exhibited the highest quinic acid content.
Effects of different types of potassium fertilizers on the aroma of ‘Xinliqihao’ fruit
Studies have shown that the types and contents of volatile aroma substances in the fruit of ‘Xinliqihao’ treated with potassium fertilizers were significantly higher than the control. The aroma substances in ‘Xinliqihao’ differed among different potassium fertilizer treatments. Specifically, 78 kinds of aroma substances were detected in the potassium chloride treatment, followed by 73 kinds in the potassium nitrate treatment and 72 kinds in the potassium sulfate treatment, and the control only contained 33 kinds. The differences in aroma substances among the different potassium fertilizer treatments of ‘Xinliqihao’ fruit were mainly from other substances (including olefins, acids, etc.). The most such substances were detected in the potassium chloride treatment, 32 kinds in total, followed by 31 kinds in the potassium nitrate treatment, and 25 kinds which are the least in the potassium sulfate treatment. In the fruit of ‘Xinliqihao’ treated with different potassium fertilizers, the most common types of esters, 15 kinds, were found in the potassium sulfate treatment, followed by 14 kinds in the potassium chloride and potassium nitrate treatments, and only 7 kinds were detected in the control. Sixteen kinds of alcohol substances, which were the most, were detected in the potassium sulfate treatment, followed by 12 kinds in the potassium chloride and potassium nitrate treatments, and the control only contained 7 kinds. Eleven kinds of aldehydes, i.e., the most aldehydes, were detected in the potassium chloride treatment, followed by 9 kinds in the potassium sulfate and potassium nitrate treatments, and there were only 5 kinds in the control.
There were also differences in the aroma components of ‘Xinliqihao’ between different potassium fertilizer treatments. The total content of aroma components in fruit was 746.609 ng/g with potassium sulfate, followed by 713.268 ng/g with potassium chloride, and 700.707 ng/g with potassium nitrate, and these values were 104.44%, 95.31% and 91.87% higher than the control, respectively. In the ‘Xinliqihao’ fruit treated with different potassium fertilizers, the highest content of esters was 64.404 ng/g for potassium sulfate, followed by 21.525 and 21.909 ng/g for potassium chloride and potassium nitrate, respectively, and these values were 15.55, 5.197, 5.29 times of the control, respectively. The highest content of alcohols was 222.184 ng/g for potassium sulfate, followed by 192.815 ng/g for potassium nitrate and 96.437 ng/g for potassium chloride, and these values were 172.16%, 149.41%, and 74.73% of the control, respectively. The potassium chloride treatment showed the highest content of aldehydes at 556.6 ng/g, followed by the potassium sulfonate treatment at 416.395 ng/g, and the potassium nitrate treatment at 406.07 ng/g, and these values were 2.613, 1.95, and 1.91 times of the control, respectively. The potassium nitrate treatment exhibited the highest content of ketones, sequentially followed by the potassium sulfate treatment and the potassium chloride treatment, and these values were 16.48, 6.13 and 4.70 times of the control, respectively. As to the content of other substance, it was the highest in the potassium chloride treatment, followed by the potassium sulfate treatment, and the lowest in the potassium nitrate treatment, and the three were 140.60%, 138.15%, 125.28% of the control, respectively. Shuwei WEI et al. The Effect of Different Potassium Fertilizers on Fruit Flavor Quality of ‘Xinliqihao’ Pear
Conclusions and Discussion
Potassium is one of the main nutrients that are indispensable in plants. The nutritional status of potassium is closely related to the growth and fruit quality of fruit trees［1］. Potassium ion as one of the most abundant cations in plants is involved in the physiological processes of growth and development of many plants, such as water metabolism, photosynthesis, assimilation transport, and enzyme activity. Potassium is an activator of various enzymes, which can promote the binding of proteases and their coenzymes［9］, thereby accelerating the activation of enzymes. It can also bind to the activation site of the protease to promote protein allostery, which in turn leads to an increase in the activation sites of the protease and an increase in the rate of protein-catalyzed reaction.
Previous studies have shown that potassium is closely related to soluble sugar in fruits, and the application of appropriate potassium can significantly promote the synthesis of sucrose in apple［10］and strawberry［11］. The results of this study are basically consistent with previous studies. Potassium promotes the synthesis of sucrose, glucose and fructose in the fruit of ‘Xinliqihao’. It is speculated that the reason might be that potassium promotes the accumulation of fructose, sucrose and glucose in fruits by increasing the activity of sucrose phosphate synthase［11］, and can also regulate sucrose phosphate synthase, starch synthase and α-amylase, thereby affecting sugar metabolism. Meanwhile, this study also found that among the different types of potassium elements, potassium sulphate had a most significant effect in increasing soluble sugar in fruit, while the mechanism needs further study.
There are different opinions on the effect of potassium on the organic acids in fruits. Some studies suggest that potassium can promote the accumulation of organic acids in fruits［12］, while some studies have suggested that potassium reduces the content of organic acids in fruits［13］. There are also studies deeming that the effect of potassium on the organic acid content in fruits is not significant［14］. The results of this study indicated that potassium treatment improved the organic acid content of ‘Xinliqihao’ fruit, but different organic acids differed in the degree of response to potassium treatment, and different types of potassium fertilizers differed in the effect on the organic acid content of ‘Xinliqihao’ fruit. The impact is also different. The mechanism of potassium affecting the organic acid content of pear fruit needs further study. Studies have shown that foliar application of potassium fertilizer (potassium dihydrogen phosphate) can increase the aroma components and content in apple fruit［15］, and within a certain concentration range, with the increase of the spraying concentration, the contents and types of volatile substances increase gradually. The results of this study also confirmed that the application of potassium fertilizers significantly increased the types and contents of the volatile aroma components of ‘Xinliqihao’ fruit. It is speculated that the production of fruit aroma substances is based on nutritive substances such as fatty acids (such as linoleic acid, linolenic acid), amino acids, monosaccharides and glycosides, which are synthesized under the direct and indirect synthesis of enzymes. The potassium fertilizer treatments increased the content of monosaccharides in the aroma substrate of pear fruit, and promoted the synthesis of volatile aroma substances, while the specific mechanism needs further study.
References
［1］ZHANG SL. Pear science［M］. Beijing: China Agriculture Press, 2013: 453-455. (in Chinese).
［2］HE ZJ, TONG YA, ZHANG GW, et al. Effect of potash application on the output and quality of Dangshan Suli pear variety in loess area［J］. Journal of Fruit Science, 2002, 19(1): 8-11. (in Chinese).
［3］SUN Q, YANG J, ZHANG SY, et al. Potassium nutrition on photosynthesis and chlorophyll fluorescence in Actinidia chinensis Planch leaves［J］. Journal of Anhui Agricultural University, 2007, 34(2): 256-261. (in Chinese).
［4］QU GM, SHU HR, WANG HX. Effect of potassium on water use efficiency and relevant parameters of apple trees［J］. Acta Pedologica Sinica, 2000, 37(2): 257-262. (in Chinese).
［5］YU QQ, DENG L, HE SL, et al. Effect of K fertilization on growth potassium uptake and utilization of young Jincheng sweet orange tree［J］. South China Fruits, 2015, 44(5): 1-4. (in Chinese).
［6］WANG YZ, ZHANG HP, HUANG XS, et al. Effect of potassium supply on plant potassium distribution and growth and leaf photosynthetic capacity of Pyrus pyrifolia［J］. Journal of Nanjing Agricultural University, 2017, 40(1): 60－67. (in Chinese)
［7］WU X, SHEN CW, DING YF, et al. Potassium accumulation in ‘Huangguan’ pear fruits and leaves and their response to different potassium application［J］Journal of Plant Nutrition and Fertilizer, 2016, 22(5): 1425-1432. (in Chinese)
［8］WEI SW, ZHANG Y, WANG HW, et al. Effects of organic fertilizer and bagging on taste quality of Pyrus bretschneideri［J］. Plant Nutrition and Fertilizer Science, 2012, 18(5): 1269-1276. ［9］CERA ED. A structural perspective on enzymes activated by monovalent canons［J］. J Biol Chem, 2006, 281: 1305-1308.
［10］MOSA WFAE, EL-MEGEED NAA, PASZT LS. The effect of the foliar application of potassium, calcium, boron and humic acid on vegetative growth, fruit set, leaf mineral, yield and fruit quality of ‘Anna’ apple trees［J］. Amer JExp Agr, 2015, 8: 224-234.
［11］AHMAD H, SAJID M, ULLAH R, et al. Dose optimization of potassium (K) chandler for yield and quality increment of strawberry (Fragaria×ananassa Duch)［J］. American J Exp Agr, 2014, 4: 1526-1535.
［12］ZHAN C, TONG YZ, LU YL, et al. Effects of different potassium fertilizers on production, quality and storability of Fuji apple［J］. Journal of Plant Nutrition and Fertilizer, 2016, 22(1): 216-224. (in Chinese)
［13］GUO W. Effects of irrigation with potassium on photosynthetic characteristics, mineral nutrition and fruit quality of Red Fuji apple leaves［D］. Yangling: Northwest A&F University, 2009. (in Chinese)
［14］LIU YN, MA HY, XIAN AM, et al. Effect of potassium application on yield and fruit quality of pineapple［J］. China Fruits, 2015, 5: 55-58. (in Chinese)
［15］TANG Y, SONG LQ, SUN YX, et al. Effects of foliage application of potassium dihydrogen phosphate on blade, fruit quality and aroma components of red general Fuji Apple［J］. Shandong Agricultural Sciences, 2017, 49(5): 82-85. (in Chinese)
［Methods］ With ‘Xinliqihao’ as a research object, such three treatments as potassium sulfate (T1), potassium nitrate (T2) and potassium chloride (T3) were set up, with no application of potassium fertilizer as control (CK). All the fertilizers were applied into soil in the growing season.
［Results］ The total sugar content of ‘Xinliqihao’ was significantly higher in the potassium fertilizer treatments than that of the control. The total sugar content of T2 (the potassium sulfate treatment) was the highest, which was 34.55% higher than that of the control, sequentially followed by T3 (the potassium nitrate treatment) and T1 (the potassium chloride treatment). The contents of fructose, glucose and sucrose in the potassium sulfate treatment were all the highest, which were 15.96%, 27.32% and 52.81% higher than those of the potassium chloride treatment, respectively, and 11.36%, 27.75% and 20.45% higher than the potassium nitrate treatment, respectively. The total acid content of ‘Xinliqihao’ fruit treated with different types of potassium fertilizers was higher than that of the control. The total acid content of the potassium nitrate treatment was the highest, which was 103.68% higher than that of the control, sequentially followed by the potassium sulfate treatment and the potassium chloride treatment. Both the control and potassium treatments of ‘Xinliqihao’ had the highest malic acid content, and except the potassium chloride treatment with the lowest fumaric acid content, other treatments all had the lowest tartaric acid content. Different types of potassium fertilizer treatments had different effects on the organic acid content of ‘Xinliqihao’ fruit. The types and contents of volatile aroma substances in the fruit of ‘Xinliqihao’ treated with potassium fertilizers were significantly higher than the control. The aroma substances in ‘Xinliqihao’ differed among different potassium fertilizer treatments. Specifically, 78 kinds of aroma substances were detected in the pot assium chloride treatment, and the total content of aroma components was the highest at 746.609 ng/g in the fruit of the potassium sulfate treatment.
［Conclusions］ The potassium fertilizer treatments improved the flavor and quality of ‘Xinliqihao’ pear fruit, and the potassium sulfate treatment exhibited the best effects. Key words Potassium; Pear; Fruit; Sugar acid; Aroma
Potassium is one of the most important nutrient elements in plant metabolism［1］, which plays an important role in plant growth and development. Potassium is required for all stages of pear growth and development. The effect of potassium on fruit trees is mainly reflected in affecting fruit quality and photosynthetic capacity, so it is called "quality element". He et al.［2］have shown that potassium can improve the 100-leaf weight and chlorophyll content of ‘Dangshan pear’ and improve fruit quality. Sun et al.［3］found that the application of potassium fertilizer in kiwifruit can increase leaf chlorophyll content and improve photosynthetic capacity. The results obtained by Qu et al.［4］on apple show that the application of potassium fertilizer can improve the water use efficiency of apple trees by increasing photosynthetic efficiency and reducing transpiration efficiency. Yu et al.［5］suggested that rational application of potassium fertilizer can increase the plant height of glorious orange saplings and the potassium content of various tissues. Wang et al.［6］showed that at a suitable concentration, potassium can promote the growth of pear trees and improve the photosynthetic efficiency of leaves. Wu et al.［7］reported that potassium application promoted the absorption of potassium by ‘Huangguan pear’ trees, and the fruit yield and quality increased with the increase of potassium application to different degrees.
The above results indicate that potassium plays an important role in leaf growth and fruit development, but the effect of potassium fertilizer on the flavor quality of pear fruit has not been reported. In view of this, in this study, with ‘Xinliqihao’ pear fruit as an experimental material, the effects of different types of potassium fertilizers on the flavor and quality of pear fruit such as sugar and aroma were investigated, aiming at providing a theoretical basis for improving pear fruit quality through rational application of potassium fertilizers in pear production.
Materials and Methods
Test materials
The experiment was carried out in the pear orchard of Jinniushan Experimental Base, Shandong Institute of Pomology from 2017 to 2018. The pear orchard was conventionally managed, and the basic nutrient status of the pear orchard soil is shown in Table 1.‘Xinliqihao’ pear trees in the orchard were grafted on Pyrus betulifolia Bunge rootstocks. The trees were 5 years old and had plant spacing of 1 m×4 m. The management level was medium, and the trees were moderate in tree potential, and had a per unit yield of about 30 000 kg/hm2 and a yield per plant of about 12 kg. Experiments were carried out by selecting trees with similar tree phases and loads, and 5 trees were treated for each treatment in three replicates. The treatments of different types of potassium fertilizers referred to the methods of Wang et al.［6］and Wu et al.［7］. The experiment was designed with three treatments: potassium sulfate (T1), potassium nitrate (T2) and potassium chloride (T3), with no application of potassium fertilizer as control (CK). The amount of potassium fertilizer applied was 300 kg/hm2 according to Wu et al.［7］. Treatments T1 and T3 were supplemented with the same amount of nitrogen fertilizer according to the T2 treatment. The potassium fertilizers were applied before germination and during the expansion stage at a ratio of 60%∶40%.
Other management measures of the test trees were consistent. The fruit was harvested around July 25, and fruit samples were collected from the east, south, west, and north of each tree. From each treatment, fruit with appropriate maturity, no pests and diseases and no mechanical damage, having typical characteristics of the variety, was selected and mixed, and 100 individuals per treatment were transported back to the laboratory for immediate treatment. The fruit was chopped after removal of the fruit core, and divided into two parts, one of which was used for the determination of fruit sugar and acid contents, and the other was used for fruit aroma determination.
Determination indexes and methods
The determination of fruit sugar, acid and aroma referred to the method of Wei et al.［8］.
Extraction and determination of fruit sugar and acid components: A certain amount of fruit flesh (5 g) was accurately weighed and ground with the addition of 15 ml of 80% ethanol. The mixture was heated in a water bath at 75 ℃ for 30 min and centrifuged at 4 000 g/min for 5 min. The supernatant was transferred to a 25 ml volumetric flask, and the remaining precipitate was added with 10 ml of 80% ethanol, followed by heating in a water bath at 75 ℃ for 30 min and centrifuging at 4 000 g/min for 5 min, obtaining the supernatant which was transferred to the 25 ml volumetric flask. The supernatant was diluted to constant volume, and the extract was then evaporated to dry at 60 ℃. The residue was dissolved with 5 ml of redistilled water, for later determination by 510 Waters High Performance Liquid Chromatograph. The column used for the analysis of the sugar components was an amino column Kromasil 250 mm×4.6 mm, and the HPLC determination was carried out with the mobile phase of acetonitrile: water (80∶20) at a flow rate of 1 ml/min under an injection volume of 15 μl, using a RID 10- A differential refractive index detector. The column for organic acid analysis was C18 Kromasil 250 mm× 4.6 mm, and the HPLC was performed with 10 mmol/L ammonium dihydrogen phosphate (having pH adjusted to 2.8 with phosphoric acid)∶methanol (97∶3) as the mobile phase at a flow rate of 0.9 ml/min under an injection volume of 10 μl, using a Waters 2487 dual-wavelength UV detector with a detection wavelength of 214 nm. The contents of the sugar and acid components were calculated using an N2000 chromatography workstation (Ver. 3.30). Determination of volatile aroma substances in fruit: The fruit core was removed by quadruple method, and the remaining part was then cut into pieces (length, width and height of about 0.2 cm) and mixed. A certain amount of the sample (5 g) was accurately weighed into a sample bottle and added with 10 μg of 3-nonanone (0.4 mg/ml) as the internal standard at the bottom of the 10 ml sample bottle which was then sealed with PTFE butyl rubber spacer. The extraction and determination of the volatile components of the fruit were carried out by static headspace GC-MS using a Perkin Elmer Turbo Matrix 40 Trap headspace sampler and a Shimadzu GCMS-QP2010 gas chromatography-mass spectrometer. Qualitative method for volatile components: The mass spectra of unknown compounds were searched by computer and matched with NIST05 mass spectrometry library, to confirm various volatile components combined with atlas analysis and data analysis. Quantitative method: According to the peak area normalization method, the mass percentage of each compound was determined, and 3-nonanone was selected as an internal standard for accurate quantification.
The test data were processed using Microsoft Excel 2010. The variance and correlation analysis were performed using DPS data processing software, and the significant test of the average was performed by t test.
Results and Analysis
Effects of different types of potassium fertilizers on sugar and acid components in ‘Xinliqihao’ Fruit
The results showed that the total sugar content of ‘Xinliqihao’ was significantly higher in the potassium fertilizer treatments than that of the control. The total sugar content of T2 (the potassium sulfate treatment) was the highest, which was 34.55% higher than that of the control, sequentially followed by T3 (the potassium nitrate treatment) which increased by 15.67% compared with the control and T1 (the potassium chloride treatment) which increased by 12.16% compared with the control. The total acid content of ‘Xinliqihao’ fruit treated with different types of potassium fertilizers was higher than that of the control. The total acid content of the potassium nitrate treatment was the highest, which was 103.68% higher than that of the control, sequentially followed by the potassium sulfate treatment which increased by 53.16% compared with the control and the potassium chloride treatment which increased by 38.96% compared with the control.
Studies have shown that the control and potassium treatments of ‘Xinliqihao’ fruit both showed the highest fructose content, sequentially followed by glucose and sucrose. The contents of fructose, glucose and sucrose in the potassium sulfate treatment were all the highest, which were 15.96%, 27.32% and 52.81% higher than those of the potassium chloride treatment, respectively, and 11.36%, 27.75% and 20.45% higher than the potassium nitrate treatment, respectively. Both the control and potassium treatments of ‘Xinliqihao’ had the highest malic acid content, and except the potassium chloride treatment with the lowest fumaric acid content, other treatments all had the lowest tartaric acid content. Different types of potassium fertilizer treatments had different effects on the organic acid content of ‘Xinliqihao’ fruit. The fruit in the potassium nitrate treatment had the highest malic acid and citric acid contents; the potassium chloride treatment showed the highest oxalic acid and tartaric acid contents; and the potassium sulfate treatment exhibited the highest quinic acid content.
Effects of different types of potassium fertilizers on the aroma of ‘Xinliqihao’ fruit
Studies have shown that the types and contents of volatile aroma substances in the fruit of ‘Xinliqihao’ treated with potassium fertilizers were significantly higher than the control. The aroma substances in ‘Xinliqihao’ differed among different potassium fertilizer treatments. Specifically, 78 kinds of aroma substances were detected in the potassium chloride treatment, followed by 73 kinds in the potassium nitrate treatment and 72 kinds in the potassium sulfate treatment, and the control only contained 33 kinds. The differences in aroma substances among the different potassium fertilizer treatments of ‘Xinliqihao’ fruit were mainly from other substances (including olefins, acids, etc.). The most such substances were detected in the potassium chloride treatment, 32 kinds in total, followed by 31 kinds in the potassium nitrate treatment, and 25 kinds which are the least in the potassium sulfate treatment. In the fruit of ‘Xinliqihao’ treated with different potassium fertilizers, the most common types of esters, 15 kinds, were found in the potassium sulfate treatment, followed by 14 kinds in the potassium chloride and potassium nitrate treatments, and only 7 kinds were detected in the control. Sixteen kinds of alcohol substances, which were the most, were detected in the potassium sulfate treatment, followed by 12 kinds in the potassium chloride and potassium nitrate treatments, and the control only contained 7 kinds. Eleven kinds of aldehydes, i.e., the most aldehydes, were detected in the potassium chloride treatment, followed by 9 kinds in the potassium sulfate and potassium nitrate treatments, and there were only 5 kinds in the control.
There were also differences in the aroma components of ‘Xinliqihao’ between different potassium fertilizer treatments. The total content of aroma components in fruit was 746.609 ng/g with potassium sulfate, followed by 713.268 ng/g with potassium chloride, and 700.707 ng/g with potassium nitrate, and these values were 104.44%, 95.31% and 91.87% higher than the control, respectively. In the ‘Xinliqihao’ fruit treated with different potassium fertilizers, the highest content of esters was 64.404 ng/g for potassium sulfate, followed by 21.525 and 21.909 ng/g for potassium chloride and potassium nitrate, respectively, and these values were 15.55, 5.197, 5.29 times of the control, respectively. The highest content of alcohols was 222.184 ng/g for potassium sulfate, followed by 192.815 ng/g for potassium nitrate and 96.437 ng/g for potassium chloride, and these values were 172.16%, 149.41%, and 74.73% of the control, respectively. The potassium chloride treatment showed the highest content of aldehydes at 556.6 ng/g, followed by the potassium sulfonate treatment at 416.395 ng/g, and the potassium nitrate treatment at 406.07 ng/g, and these values were 2.613, 1.95, and 1.91 times of the control, respectively. The potassium nitrate treatment exhibited the highest content of ketones, sequentially followed by the potassium sulfate treatment and the potassium chloride treatment, and these values were 16.48, 6.13 and 4.70 times of the control, respectively. As to the content of other substance, it was the highest in the potassium chloride treatment, followed by the potassium sulfate treatment, and the lowest in the potassium nitrate treatment, and the three were 140.60%, 138.15%, 125.28% of the control, respectively. Shuwei WEI et al. The Effect of Different Potassium Fertilizers on Fruit Flavor Quality of ‘Xinliqihao’ Pear
Conclusions and Discussion
Potassium is one of the main nutrients that are indispensable in plants. The nutritional status of potassium is closely related to the growth and fruit quality of fruit trees［1］. Potassium ion as one of the most abundant cations in plants is involved in the physiological processes of growth and development of many plants, such as water metabolism, photosynthesis, assimilation transport, and enzyme activity. Potassium is an activator of various enzymes, which can promote the binding of proteases and their coenzymes［9］, thereby accelerating the activation of enzymes. It can also bind to the activation site of the protease to promote protein allostery, which in turn leads to an increase in the activation sites of the protease and an increase in the rate of protein-catalyzed reaction.
Previous studies have shown that potassium is closely related to soluble sugar in fruits, and the application of appropriate potassium can significantly promote the synthesis of sucrose in apple［10］and strawberry［11］. The results of this study are basically consistent with previous studies. Potassium promotes the synthesis of sucrose, glucose and fructose in the fruit of ‘Xinliqihao’. It is speculated that the reason might be that potassium promotes the accumulation of fructose, sucrose and glucose in fruits by increasing the activity of sucrose phosphate synthase［11］, and can also regulate sucrose phosphate synthase, starch synthase and α-amylase, thereby affecting sugar metabolism. Meanwhile, this study also found that among the different types of potassium elements, potassium sulphate had a most significant effect in increasing soluble sugar in fruit, while the mechanism needs further study.
There are different opinions on the effect of potassium on the organic acids in fruits. Some studies suggest that potassium can promote the accumulation of organic acids in fruits［12］, while some studies have suggested that potassium reduces the content of organic acids in fruits［13］. There are also studies deeming that the effect of potassium on the organic acid content in fruits is not significant［14］. The results of this study indicated that potassium treatment improved the organic acid content of ‘Xinliqihao’ fruit, but different organic acids differed in the degree of response to potassium treatment, and different types of potassium fertilizers differed in the effect on the organic acid content of ‘Xinliqihao’ fruit. The impact is also different. The mechanism of potassium affecting the organic acid content of pear fruit needs further study. Studies have shown that foliar application of potassium fertilizer (potassium dihydrogen phosphate) can increase the aroma components and content in apple fruit［15］, and within a certain concentration range, with the increase of the spraying concentration, the contents and types of volatile substances increase gradually. The results of this study also confirmed that the application of potassium fertilizers significantly increased the types and contents of the volatile aroma components of ‘Xinliqihao’ fruit. It is speculated that the production of fruit aroma substances is based on nutritive substances such as fatty acids (such as linoleic acid, linolenic acid), amino acids, monosaccharides and glycosides, which are synthesized under the direct and indirect synthesis of enzymes. The potassium fertilizer treatments increased the content of monosaccharides in the aroma substrate of pear fruit, and promoted the synthesis of volatile aroma substances, while the specific mechanism needs further study.
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