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Abstract On the basis of referring plenty of literatures， we summarized the research advance in effects of nitrogen on the internal quality of peach fruit. Most studies have shown that proper nitrogen application can improve internal quality of fruit， and excessive nitrogen application can reduce soluble solid and sugar contents of fruit， increase organic acid content， reduce fruit aroma， increase protein and amino acid contents， and increase or reduce vitamin C content. Relevant issues were discussed.
Key words Peach tree; Nitrogen; Fruit; Soluble solid content; Organic acid; Vitamin C; Protein; Amino acid
With the continuous expansion of Chinas peach industry， the yield of peach is getting higher and higher[1]. Through fertilization， bagging and irrigation， the appearance quality of peaches has been greatly improved. However， the internal quality of peaches is uneven， and most of the peaches are manifested as poor quality， having heavy sour or light flavor， no aroma， and sometimes odor. There are many factors that affect the internal quality of fruit， such as fertilization， irrigation， light and yield. Among them， nitrogen has a greater effect on fruit quality formation. Based on review of a large number of literatures， we summarized the effects of nitrogen on the internal quality of peach fruit， with an attempt to provide a reference for improving the internal quality of peach fruit.
Main Components of Peach Internal Quality
The internal quality of peach fruit is mainly composed of sugars， organic acids， vitamin C， and aroma substances.
Sugar and enzymes related to sugar metabolism
The sweet and sour flavors of peach fruit are mainly determined by the content and ratio of sugars and organic acids. The soluble sugars in peach fruit mainly include sucrose， fructose， glucose and sorbitol[2]. Sucrose is both the main transportation form and the main storage form of sugar， and it can be converted into other sugars. It has been proven that sucrose and its degradation products （fructose， glucose and sorbitol） are important metabolic substances. Therefore， the sucrose metabolism pathway occupies a dominant position in the peach sugar metabolism network. Moriguchi et al.[3]and Vizzotto et al.[4]tested on peaches， proving that acid invertase （AI）， neutral invertase （NI）， sucrose synthase （SS） and sucrose phosphate synthase （SPS） are closely related to sugar metabolism and sugar accumulation， which play an important role in sugar transportation， metabolism and accumulation. Organic acids
Organic acids mainly include malic acid， citric acid and quinic acid[2]. The main ones are malic acid and citric acid. Malic acid accounts for 60% of the total acid. Therefore， peach is a fruit of the malic acid type.
Aroma
More than 100 aroma substances have been identified from peach fruit， including aldehydes， alcohols， esters， lactones， terpenes， and ketones[5-6]. Among these aroma substances， only a few characteristic ones have an important effect on its aroma quality. These substances mainly include C6 aldehydes， alcohols， esters and lactones[7]. C6 aldehydes and alcohols are faint scent type aroma substances， while esters and lactones are fruity type aromasubstances. The C6 aldehydes and alcohols in peach fruit are mainly trans2hexenol， cis3hexenai and trans2hexenal[8]. Terpenes and ketones are the most important floral type aroma substances in peach fruit， of which terpenes， mainly linalool， are more abundant in nectarines with a lower threshold， and have a greater impact on the aroma of nectarines.
Changes in Internal Quality in Different Stages
Sugar change
In the early stage of peach fruit development， the soluble sugars are mainly reducing sugars. In the middle and late stages， reducing sugars remain at relatively low levels， and nonreducing sugars accumulate in large quantities. Throughout the development process， sucrose continues to accumulate， especially during the maturation stage， while fructose and glucose contents show a downward trend in the early stages of fruit development， and remain at a relatively low level in the middle and late stages[9].
Activity of enzymes related to glycometabolism
The changes in the activity of glycometabolismrelated enzymes show that with fruit maturation， the acid invertase activity decreases， and the activity of enzymes in the direction of sucrose synthase decomposition gradually increases. The activity of enzymes in the direction of sucrose synthase synthesis is similar to the change in sucrose phosphate synthase activity. The enzyme activity gradually decreases in the early stage of fruit development， increases gradually in the middle and late stages， and increases rapidly when the fruit is near maturity， at which sucrose accumulates rapidly[10]. During fruit development， fructose and glucose contents are significantly negatively correlated with the activity of sucrose synthase （decomposition direction） ， the correlation coefficients were -0.831 2 and -0.844 5， respectively， and are not significantly related to other enzymes related to sucrose metabolism. The content of sucrose is extremely significantly correlated with the activity of sucrose synthetase （decomposition direction） （r=0.910 6）， and is also significantly related to the activity of sucrose synthase （synthesis direction） and sucrose phosphate synthase （the correlation coefficients were 0.903 7 and 0.971 4， respectively） in the middle and late stages of fruit development， and there is no significant correlation between sucrose content and invertase activity[10]. Organic acids
The organic acid content increases first and then decreases. The variation trend of malic acid content is similar to that of organic acids. The citric acid content increases gradually until the fruit matures and then decreases， and there is little difference between malic acid and citric acid contents during the maturation stage. The activities of organic acidrelated metabolic enzymes are as follows： there is little correlation between changes in citrate synthase （CS） activity and organic acid content; the activity of malic enzyme （ME） gradually increases in the later stages of fruit development， and the malic acid content shows a downward trend; and the activity of malic dehydrogenase （MDH） first increases and then decreases， which is similar to the trend of change in malic acid content[10].
Proteins and amino acids
Both proteins and amino acids are products of nitrogen metabolism. The changes of protein and amino acid contents during fruit development are generally similar （the correlation coefficient is 0.752）， both of which are gradually decreasing， and the decline is larger from 21 to 49 d after peak flowering， during which the protein content decreases from 1.25 to 0.95 mg/g， decreasing by 24.07%， and the amino acid content decreases from 1.35 to 0.377 mg/g， a decrease of 72.00%. From 49 d after peak flowering to fruit maturity， the protein and amino acid contents in the fruit are always lower， and the changes are relatively stable[10].
Effect of Nitrogen Application on the Internal Quality of Peach Fruit
Appropriate nitrogen application increases the content of soluble solids or sugar
If nitrogen is insufficient in soil， nitrogen application can increase the content of soluble solids or sugars to a certain extent， thereby improving the internal quality. Li et al.[11]showed that insufficient or excessive nitrogen supplication significantly reduced the content of soluble solids in Bayuecui peach fruit. When the amount of nitrogen increased from 0 to 600 kg/hm2， the content of soluble solids increased. When the amount of nitrogen was 150 kg/hm2， the content of soluble solids was the highest， reaching 12.6%， which was 1.2 percentage points higher than that with no nitrogen application. When the nitrogen application rate was the highest （600 kg/hm2）， the content of soluble solids was the lowest， as low as 10.9%， which was 0.5 percentage point lower than without nitrogen application. After that， as the amount of nitrogen increased， the content of soluble solids gradually decreased. The experiments carried out by Kong et al.[12]showed that when the medium amount of nitrogen was applied to the peach orchard， the soluble solid content of peach fruit was 10.2%， which was higher than the peach orchards applied with nitrogen at the low， high， and ultrahigh levels. Zhao et al.[10]showed that in the late stages of peach fruit development， the fructose content of nitrogentreated fruit was higher than that of the control， and the mediumnitrogen treatment exhibited the highest fructose content， followed by the highnitrogen treatment， and the difference between the mediumnitrogen treatment and the highnitrogen treatment was significantly different from the control， and that between the lownitrogen treatment and the control was not significant. The glucose contents of the nitrogen treatments were all lower than the control. The difference between the highnitrogen treatment and the control was significant， while those between other treatments were not significant. As to the effects on sucrose content， the mediumnitrogen treatment exhibited the highest sucrose content， followed by the lownitrogen treatment， and the control was the lowest. The differences between the various treatments were significant.
With the increase of nitrogen， the soluble solid content of Shuguang nectarine continued to increase， but began to decrease when it reached a certain level. The soluble solid content was 8.2%-8.7%， 10.0%-12.9%， 10.3%-10.8% and 9.8%-10.2% in N1， N2， N3 and N4， respectively. It could be seen that N2 showed the highest value， and the soluble sugar content exhibited a similar law， which was also the highest in N2. And the soluble sugar content was the lowest in N1， while N3 and N4 showed close values[13]. Wang et al.[14]also reported similar conclusions. When the nitrogen application rate was 150-300 kg/hm2， the solublesolid content was higher than no application of nitrogen fertilizer， and at this time， it was the highest， accompanied by the sugaracid ratio at a high level[11]. With the increase of the amount of nitrogen， the soluble solid content of the fiveyearold Shuguang nectarine increased first and then decreased[15]. Foreign studies have found that as the amount of nitrogen increased from 33 to 147 kg/hm2，the content of soluble solids also increased accordingly[16].
Effects of excessive nitrogen application on soluble solid content or sugar content
Highest soluble solid or sugar content without nitrogen application For soils with higher nitrogen contents， the application of nitrogen fertilizer reduces the soluble solid content or sugar content in fruit. The content of soluble solids without nitrogen fertilizer application is higher than that with nitrogen fertilizer application. In other words， the control has higher content of soluble solids than nitrogen fertilizer treatments.
Wang et al.[17]showed that when nitrogen fertilizer was not applied，the soluble sugar of nectarine fruit was 9.63%， and whenthe amount of nitrogen was 677 kg/hm2， it decreased to 4.8%. The application of 1.5 kgof urea to a 8yearold Zaoshengshuimitaopeach plant reduced the soluble solid content of fruit from 11.56% to 10.56%， and the total sugar content decreased from 9.2% to 7.55%[18].
When the nitrogen application rate was increased from 0 to 675 kg/hm2， the soluble sugar content of Zhongyou 4 nectarine fruit decreased from 9.6% to 4.8%. When the amount of nitrogen was 300 kg/hm2， the soluble sugar content was 7.0% [17]. There are still many researches reporting that the contents of soluble solids in nitrogen fertilizer treatments were lower than those of fruit with the controls （without any nitrogen fertilizer）[15-16，19-20]， which indicates that the nitrogen contents in soils of peach orchards in China are relatively sufficient， and it is inappropriate to apply nitrogen fertilizer in a large quantity.
Effects of excessive nitrogen application on soluble solid content or sugar content
If the nitrogen content in soil is high， the excessive application of nitrogen at this time will cause the peach tree growing too vigorously， which will further affect the absorption of other elements by the tree body and reduce the nutritional quality of the fruit[21-23]. Studies at abroad have shown that excessive nitrogen application reduces the soluble solid content of fruits[16]. Zhu et al.[16]found on the nectarine that the soluble solid content showed a downward trend with the increase of nitrogen， and as the amount of nitrogen increased from 300 to 600 and 1 200 g/plan， the soluble solid content decreased regularly， which were 8.50%， 8.06% and 7.85% at the three application rates， respectively. When the nitrogen applied to Yuhualu and Zhonghuashoutao peach plants increased from 100 to 125 g/plant， the soluble solid content decreased from 14.14% to 10.83%[24]. Based on the application of organic fertilizer and biogas residue to Shenzhoumitao peach trees， the application of 150 kg/hm2 urea reduced the soluble solidcontent of fruit from 13.56% to 10.94% and from 13.41% to 10.47%， respectively[25]. It shows that when sufficient organic fertilizer is applied， nitrogen fertilizer should not be applied again， because organic fertilizer already has higher nitrogen content. The application of highnitrogen content fertilizers such as human feces and urine will also reduce the soluble solid content of peach fruits. Experiments have shown that the application of 20 kg/plant of feces and urine to Chunlei peach reduced the soluble solid content of fruit by 1.25percentage points compared with the control[26]. In addition， the nitrogen content in soil also has a certain effect on the internal quality. Fan[27]investigated the nitrogen content in different soils of Shenzhoumitao peach orchards， and the results showed that the total nitrogen and alkalihydrolyzale nitrogen contents were negatively correlated with the soluble solid， total sugar and reducing sugar contents in fruit. To the contrary， Zhong et al.[28]investigated nine representative plots in Nyingchi area， Tibetan where Prunus mira Koehne is concentrated to study the relationship between nine soil factors and the quality of Tibetan P. mira. The results showed that there was a strong positive correlation between total sugar content in P. mira fruit and rapidly availablenitrogen in soil. It shows that due to years of planting in the production area of Shenzhoumitao peach， the nitrogen fertilizer application rate is large， while Nyingchi， P. mira production area in Tibet is in a wild state without or very less application of nitrogen fertilizer， exhibiting nitrogen deficiency， so the application of appropriate amount of nitrogen fertilizer is helpful to improve the internal quality. However， some studies have suggested that nitrogen has no significant effect on the content of soluble solids[29]. Agricultural Biotechnology 2020
Effect of nitrogen application on organic acids
Numerous studies have shown that nitrogen application in soil increases organic acid content in fruits. As the nitrogen application rate increased from 300 to 600 and 1 200 g/plant， the acid content of 4yearold Shuguang nectarine fruit increased regularly， from 0.45% to 0.52% and 0.63%， and the no nitrogen application treatment showed the lowest content of 0.38%[16]. The content of titratable acid in Zhonghuashoutao peach increased significantly with the increase of fertilizing amount. When the nitrogen application rates were 0.75， 1.5， and 2.25 kg/plant， the titratable acid contents were 0.43%， 0.47% and 0.49%， respectively， while the value was 0.29% without the application of nitrogen[14]. When the nitrogen application rate was increased from 0 to 675 kg/hm2， the total acid content of Zhongyou 4 nectarine fruit increased from 0.5% to 0.6%， and the total acid content in fruit with the highest nitrogen application （677 kg/hm2） was the highest[17]. Jia et al.[30]believed that the highest contents of titratable acid， citric acid and malic acid were found in the fruits with highnitrogen treatment. When the soil nitrogen content was high， on the basis of applying organic fertilizer and biogas residue， applying urea at 150 kg/hm2 improved the acid content of Shenzhoumitao peach compared with the control[25]. Liu et al.[31]obtained the same results on Feicheng peach. The application of human feces and urine with high nitrogen content also had an effect on the acid content in fruit. After applying 20 kg/plant of human feces and urine to Chunlei peach plant， the acid content increased comparing with the control[26]. If the soil has a high nitrogen content， the application of nitrogen will increase the acid content in fruit. The nitrogen content in the soil is related to the acid content in fruit. Fan[27]reported that when the total nitrogen and alkalihydrolyzale nitrogen contents in soil of Shenzhoumitao peach orchards were high， the acid content in fruit increased correspondingly， and the sugaracid ratio decreased. Other studies have shown that there is no relationship between nitrogen application and acid content[13，15]. Before sprouting， the application of urea to Zaohongbaoshi nectarine in greenhouse did not change the titrated acid content compared with no nitrogen application[32].
Effect of nitrogen application on vitamin C
The effects of nitrogen on the vitamin C content of peach fruits are different. Some studies have shown that nitrogen application reduces the vitamin C content in fruits. When the nitrogen application rate on 4yearold Shuguang nectarine was 300， 600， and 1 200 g/plant， respectively， the vitamin C content continued to decrease with the increase of nitrogen application， and a maximum of 0.297 μg/g was found without nitrogen application[16]. The 8yearoldZaoshengshuimitao peach plant applied with 1.5 kg of urea decreased the vitamin C content in fruit from 13.64 to 9.24 mg/100 g compared with the control[18]. Based on the application of organic fertilizer and biogas residue， Shenzhoumitao peach applied with urea at 150 kg/hm2 showed a decrease in vitamin C[25]. Nitrogen application also reduced the vitamin C content of Zhongyou 4 nectarine[17]. Some studies have shown that in the nitrogen application treatments， as the amount of nitrogen applied increased， the vitamin C in fruit first decreased and then increased[17]or first increased and then decreased[13， 15]， but Liu et al.[26]showed that the content of vitamin C in nitrogentreated fruits did not change compared with the control. Effect of nitrogen application on protein and amino acid contents
There have been few studies on the effects of nitrogen fertilizer on the protein and amino acid contents in fruit. Existing research results show that the application of nitrogen fertilizer increases the contents of protein and amino acids in fruit. With the increase of nitrogen application， the protein and amino acid contents in fruit increased[10]. Jia et al.[21]reported that the contents of free amino acids such as arginine in fruit treated with high nitrogen were too high. Based on the application of organic fertilizer and biogas residue to Shenzhoumitao peach， after the application of 150 kg/hm2 of urea， the results showed that the protein content in fruit increased[25]. In addition， when the soil nitrogen content is high， the protein content in fruit increases accordingly. The higher the total soil nitrogen and alkalihydrolyzable nitrogen contents in Shenzhoumitao peach orchards， the higher the protein content in fruit[28].
Effect of nitrogen application on fruit aroma
Jia et al.[33]studied the effects of different fertilization treatments on peach fruit flavor. The results showed that different nitrogen fertilizer levels had an effect on the formation of peach fruit flavor substances. With the treatment of 80 mg/L nitrogen fertilizer， the contents of γcaprolactone， γcaprylolactone and γdecalactone in fruit were higher than other treatments， while the contents of trans2hexenal and iso3hexenol were lower. Jia et al.[23]reported that the aromatic substance content in fruit treated with high nitrogen decreased. The content of decanolactone was the highest in fruit at the medium nitrogen level， while it was the lowest in the highnitrogen treatment[31].
Optimal nitrogen application period and amount
Murata Ryuichi[34]carried out an experiment of spraying 0.5%urea at different time on Sunagowase peach （N1May 8， N2May 30， N3June 23）. The results showed that the N1 treatment had a high sugar content of fruit， there was no difference between N2 and untreated plants， and the N3 treatment had a low sugar content. And they concluded that in order to obtain highquality peach fruit， the trees must absorb sufficient nitrogen before sprouting to increase the nitrogen content at the beginning of growth， and the nitrogen fertilizer should be applied as early as possible. Zheng et al.[35]deem that it is better to apply nitrogen before sprouting. The recommended amount of nitrogen in foreign peach trees was generally pure N 100-200 kg/hm2 per year[36-37]. The nitrogen was recommended to be 10-12 kg per 0.1 hm2 peach orchard in Japan[38]. Li et al.[11]showed that the suitable nitrogen rate for Bayuecui peach was 150-300 kg/hm2， and the fruit quality was the best at this rate. The results of multisite research experiments in Pinggu District， Beijing showed that the recommended annual nitrogen of young and adult trees were 100-150 kg/hm2 and 150-200kg/hm2， respectively[39].
Conclusions and Discussion
Most studies have shown that proper nitrogen amount can improve the internal quality. Excessive nitrogen application reduces soluble solid and sugar contents in fruit， increases organic acid content， reduces fruit aroma， increases protein and amino acid contents， and increases or decreases vitamin C content.
The effect of nitrogen on sugar content is the result of the coordinated action of nitrogen on nitrogen and sugar metabolism enzymes. The mechanism is not clear on peach trees. The study of Zhao et al.[40]on sugar beet showed that when nitrogen was applied excessively， the enzyme activity in the direction of sucrose synthase synthesis decreased， but the enzyme activity in the direction of decomposition increased as the ammonium nitrogen increased. The sucrose phosphate synthase activity has a quadratic parabola relationship with the amount of nitrogen. When excessive nitrogen is applied， the sucrose phosphate synthase activity in roots and leaves decreases. It can be inferred that peach trees can increase the sucrose synthase （synthesis direction） and sucrose phosphate synthase activity and increase sugar content when nitrogen is applied properly. When there is more nitrogen， the activity of sucrose synthase （synthesis direction） and sucrose phosphate synthase become lower， the sucrose synthase （decomposition direction） activity is strong， and the invertase activity is enhanced.
In the regulation of carbon and nitrogen metabolism， there are three vital enzymes， namely， NR， PEPCase and SPS[41]， the activity of which can be regulated by nitrogen nutrition signals. When NO-3 concentration increases， the activity of protein kinase is improved， making PEPC and SPS enzyme proteins phosphorylated， while phosphorylated PEPC has high activity， is conducive to the reaction of PEP to oxaloacetate， and promotes amino acid synthesis. Meanwhile， phosphorylated SPS activity is inactivated， limiting sucrose synthesis[41-42]. Zhou[43]showed that when sugar beet absorbs too much nitrogen， plants must change the inorganic nitrogen absorbed into organic nitrogen including amino acids and proteins. Otherwise， the plants will be poisoned due to excessive ammonia accumulation. When sugar beet absorbs large amounts of NO-3 and NH+4， it absorbs the same amounts of cations and anions to achieve the balance between anions and cations; and in the case of increased nitrogen in the body， the contents of amino acids and organic acids will also follow the increase. As these solutes are increasing， the osmotic pressure of the cells increased， resulting in an increase in the water content in sugar beet and a decrease in the sugar content.
Proteins and amino acids are products of nitrogen metabolism in fruit. During the fruit development， the changes of protein and amino acid contents are similar， showing a gradual downward trend. The protein and amino acid contents in the early fruit development period are relatively high， which indicates that the nitrogen metabolism is relatively vigorous， and the higher protein and amino acid contents can satisfy the demand for nitrogen. At the young fruit stage， with the fruit development， the protein and amino acid contents in fruit decrease from the highest to the higher level， and from the late development stages to fruit maturity， the protein and amino acid contents further decrease， and decrease to the lowest when the fruits are mature[10].
For a very important process of synthesizing amino acids and proteins with photosynthetic products and nitrogen， humans have different requirements for different crops. For cereal and legume crops， higher protein content is expected in their grains. However， for sugar crops， it is desirable to have a higher sugar content and the lowest possible amount of harmful nitrogen based on a certain yield. Therefore， sugar beet can only accumulate more sugar under proper nitrogen supply[43]. The main purpose of producing fruit is to have sweet or sweet and sour fruit with aroma and high Vc content. The protein and amino acid contents should not be used as an indicator of the good quality of peach fruit.
Nitrogen fertilizer will increase the content of bitter amino acids （arginine， isoleucine， phenylalanine， methionine and histidine） in watermelon， and reduce the contents of sweetness （glycine， alanine， serine and threonine） and umami amino acids （aspartic acid and glutamic acid）[44]. The study by Lu et al.[45]showed that excessive application of nitrogen fertilizer increased protein content. The total amino acid content of strawberry fruit increased with the increase of nitrogen application， and it was the highest when the nitrogen was applied at a higher amount[46]. Zhao et al.[47]reported that the nitrogen content in cucumber fruit gradually increased with the increase of nitrogen application amount. When the nitrogen application amount was 375 kg/hm2， the contents of free amino acids， soluble proteins and crude proteins reached their maximum values. Qu et al.[48]showed that the amount of nitrogen application was positively correlated with the contents of total nitrogen， harmful nitrogen， amino acids and amides in sugar beet. References
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Key words Peach tree; Nitrogen; Fruit; Soluble solid content; Organic acid; Vitamin C; Protein; Amino acid
With the continuous expansion of Chinas peach industry， the yield of peach is getting higher and higher[1]. Through fertilization， bagging and irrigation， the appearance quality of peaches has been greatly improved. However， the internal quality of peaches is uneven， and most of the peaches are manifested as poor quality， having heavy sour or light flavor， no aroma， and sometimes odor. There are many factors that affect the internal quality of fruit， such as fertilization， irrigation， light and yield. Among them， nitrogen has a greater effect on fruit quality formation. Based on review of a large number of literatures， we summarized the effects of nitrogen on the internal quality of peach fruit， with an attempt to provide a reference for improving the internal quality of peach fruit.
Main Components of Peach Internal Quality
The internal quality of peach fruit is mainly composed of sugars， organic acids， vitamin C， and aroma substances.
Sugar and enzymes related to sugar metabolism
The sweet and sour flavors of peach fruit are mainly determined by the content and ratio of sugars and organic acids. The soluble sugars in peach fruit mainly include sucrose， fructose， glucose and sorbitol[2]. Sucrose is both the main transportation form and the main storage form of sugar， and it can be converted into other sugars. It has been proven that sucrose and its degradation products （fructose， glucose and sorbitol） are important metabolic substances. Therefore， the sucrose metabolism pathway occupies a dominant position in the peach sugar metabolism network. Moriguchi et al.[3]and Vizzotto et al.[4]tested on peaches， proving that acid invertase （AI）， neutral invertase （NI）， sucrose synthase （SS） and sucrose phosphate synthase （SPS） are closely related to sugar metabolism and sugar accumulation， which play an important role in sugar transportation， metabolism and accumulation. Organic acids
Organic acids mainly include malic acid， citric acid and quinic acid[2]. The main ones are malic acid and citric acid. Malic acid accounts for 60% of the total acid. Therefore， peach is a fruit of the malic acid type.
Aroma
More than 100 aroma substances have been identified from peach fruit， including aldehydes， alcohols， esters， lactones， terpenes， and ketones[5-6]. Among these aroma substances， only a few characteristic ones have an important effect on its aroma quality. These substances mainly include C6 aldehydes， alcohols， esters and lactones[7]. C6 aldehydes and alcohols are faint scent type aroma substances， while esters and lactones are fruity type aromasubstances. The C6 aldehydes and alcohols in peach fruit are mainly trans2hexenol， cis3hexenai and trans2hexenal[8]. Terpenes and ketones are the most important floral type aroma substances in peach fruit， of which terpenes， mainly linalool， are more abundant in nectarines with a lower threshold， and have a greater impact on the aroma of nectarines.
Changes in Internal Quality in Different Stages
Sugar change
In the early stage of peach fruit development， the soluble sugars are mainly reducing sugars. In the middle and late stages， reducing sugars remain at relatively low levels， and nonreducing sugars accumulate in large quantities. Throughout the development process， sucrose continues to accumulate， especially during the maturation stage， while fructose and glucose contents show a downward trend in the early stages of fruit development， and remain at a relatively low level in the middle and late stages[9].
Activity of enzymes related to glycometabolism
The changes in the activity of glycometabolismrelated enzymes show that with fruit maturation， the acid invertase activity decreases， and the activity of enzymes in the direction of sucrose synthase decomposition gradually increases. The activity of enzymes in the direction of sucrose synthase synthesis is similar to the change in sucrose phosphate synthase activity. The enzyme activity gradually decreases in the early stage of fruit development， increases gradually in the middle and late stages， and increases rapidly when the fruit is near maturity， at which sucrose accumulates rapidly[10]. During fruit development， fructose and glucose contents are significantly negatively correlated with the activity of sucrose synthase （decomposition direction） ， the correlation coefficients were -0.831 2 and -0.844 5， respectively， and are not significantly related to other enzymes related to sucrose metabolism. The content of sucrose is extremely significantly correlated with the activity of sucrose synthetase （decomposition direction） （r=0.910 6）， and is also significantly related to the activity of sucrose synthase （synthesis direction） and sucrose phosphate synthase （the correlation coefficients were 0.903 7 and 0.971 4， respectively） in the middle and late stages of fruit development， and there is no significant correlation between sucrose content and invertase activity[10]. Organic acids
The organic acid content increases first and then decreases. The variation trend of malic acid content is similar to that of organic acids. The citric acid content increases gradually until the fruit matures and then decreases， and there is little difference between malic acid and citric acid contents during the maturation stage. The activities of organic acidrelated metabolic enzymes are as follows： there is little correlation between changes in citrate synthase （CS） activity and organic acid content; the activity of malic enzyme （ME） gradually increases in the later stages of fruit development， and the malic acid content shows a downward trend; and the activity of malic dehydrogenase （MDH） first increases and then decreases， which is similar to the trend of change in malic acid content[10].
Proteins and amino acids
Both proteins and amino acids are products of nitrogen metabolism. The changes of protein and amino acid contents during fruit development are generally similar （the correlation coefficient is 0.752）， both of which are gradually decreasing， and the decline is larger from 21 to 49 d after peak flowering， during which the protein content decreases from 1.25 to 0.95 mg/g， decreasing by 24.07%， and the amino acid content decreases from 1.35 to 0.377 mg/g， a decrease of 72.00%. From 49 d after peak flowering to fruit maturity， the protein and amino acid contents in the fruit are always lower， and the changes are relatively stable[10].
Effect of Nitrogen Application on the Internal Quality of Peach Fruit
Appropriate nitrogen application increases the content of soluble solids or sugar
If nitrogen is insufficient in soil， nitrogen application can increase the content of soluble solids or sugars to a certain extent， thereby improving the internal quality. Li et al.[11]showed that insufficient or excessive nitrogen supplication significantly reduced the content of soluble solids in Bayuecui peach fruit. When the amount of nitrogen increased from 0 to 600 kg/hm2， the content of soluble solids increased. When the amount of nitrogen was 150 kg/hm2， the content of soluble solids was the highest， reaching 12.6%， which was 1.2 percentage points higher than that with no nitrogen application. When the nitrogen application rate was the highest （600 kg/hm2）， the content of soluble solids was the lowest， as low as 10.9%， which was 0.5 percentage point lower than without nitrogen application. After that， as the amount of nitrogen increased， the content of soluble solids gradually decreased. The experiments carried out by Kong et al.[12]showed that when the medium amount of nitrogen was applied to the peach orchard， the soluble solid content of peach fruit was 10.2%， which was higher than the peach orchards applied with nitrogen at the low， high， and ultrahigh levels. Zhao et al.[10]showed that in the late stages of peach fruit development， the fructose content of nitrogentreated fruit was higher than that of the control， and the mediumnitrogen treatment exhibited the highest fructose content， followed by the highnitrogen treatment， and the difference between the mediumnitrogen treatment and the highnitrogen treatment was significantly different from the control， and that between the lownitrogen treatment and the control was not significant. The glucose contents of the nitrogen treatments were all lower than the control. The difference between the highnitrogen treatment and the control was significant， while those between other treatments were not significant. As to the effects on sucrose content， the mediumnitrogen treatment exhibited the highest sucrose content， followed by the lownitrogen treatment， and the control was the lowest. The differences between the various treatments were significant.
With the increase of nitrogen， the soluble solid content of Shuguang nectarine continued to increase， but began to decrease when it reached a certain level. The soluble solid content was 8.2%-8.7%， 10.0%-12.9%， 10.3%-10.8% and 9.8%-10.2% in N1， N2， N3 and N4， respectively. It could be seen that N2 showed the highest value， and the soluble sugar content exhibited a similar law， which was also the highest in N2. And the soluble sugar content was the lowest in N1， while N3 and N4 showed close values[13]. Wang et al.[14]also reported similar conclusions. When the nitrogen application rate was 150-300 kg/hm2， the solublesolid content was higher than no application of nitrogen fertilizer， and at this time， it was the highest， accompanied by the sugaracid ratio at a high level[11]. With the increase of the amount of nitrogen， the soluble solid content of the fiveyearold Shuguang nectarine increased first and then decreased[15]. Foreign studies have found that as the amount of nitrogen increased from 33 to 147 kg/hm2，the content of soluble solids also increased accordingly[16].
Effects of excessive nitrogen application on soluble solid content or sugar content
Highest soluble solid or sugar content without nitrogen application For soils with higher nitrogen contents， the application of nitrogen fertilizer reduces the soluble solid content or sugar content in fruit. The content of soluble solids without nitrogen fertilizer application is higher than that with nitrogen fertilizer application. In other words， the control has higher content of soluble solids than nitrogen fertilizer treatments.
Wang et al.[17]showed that when nitrogen fertilizer was not applied，the soluble sugar of nectarine fruit was 9.63%， and whenthe amount of nitrogen was 677 kg/hm2， it decreased to 4.8%. The application of 1.5 kgof urea to a 8yearold Zaoshengshuimitaopeach plant reduced the soluble solid content of fruit from 11.56% to 10.56%， and the total sugar content decreased from 9.2% to 7.55%[18].
When the nitrogen application rate was increased from 0 to 675 kg/hm2， the soluble sugar content of Zhongyou 4 nectarine fruit decreased from 9.6% to 4.8%. When the amount of nitrogen was 300 kg/hm2， the soluble sugar content was 7.0% [17]. There are still many researches reporting that the contents of soluble solids in nitrogen fertilizer treatments were lower than those of fruit with the controls （without any nitrogen fertilizer）[15-16，19-20]， which indicates that the nitrogen contents in soils of peach orchards in China are relatively sufficient， and it is inappropriate to apply nitrogen fertilizer in a large quantity.
Effects of excessive nitrogen application on soluble solid content or sugar content
If the nitrogen content in soil is high， the excessive application of nitrogen at this time will cause the peach tree growing too vigorously， which will further affect the absorption of other elements by the tree body and reduce the nutritional quality of the fruit[21-23]. Studies at abroad have shown that excessive nitrogen application reduces the soluble solid content of fruits[16]. Zhu et al.[16]found on the nectarine that the soluble solid content showed a downward trend with the increase of nitrogen， and as the amount of nitrogen increased from 300 to 600 and 1 200 g/plan， the soluble solid content decreased regularly， which were 8.50%， 8.06% and 7.85% at the three application rates， respectively. When the nitrogen applied to Yuhualu and Zhonghuashoutao peach plants increased from 100 to 125 g/plant， the soluble solid content decreased from 14.14% to 10.83%[24]. Based on the application of organic fertilizer and biogas residue to Shenzhoumitao peach trees， the application of 150 kg/hm2 urea reduced the soluble solidcontent of fruit from 13.56% to 10.94% and from 13.41% to 10.47%， respectively[25]. It shows that when sufficient organic fertilizer is applied， nitrogen fertilizer should not be applied again， because organic fertilizer already has higher nitrogen content. The application of highnitrogen content fertilizers such as human feces and urine will also reduce the soluble solid content of peach fruits. Experiments have shown that the application of 20 kg/plant of feces and urine to Chunlei peach reduced the soluble solid content of fruit by 1.25percentage points compared with the control[26]. In addition， the nitrogen content in soil also has a certain effect on the internal quality. Fan[27]investigated the nitrogen content in different soils of Shenzhoumitao peach orchards， and the results showed that the total nitrogen and alkalihydrolyzale nitrogen contents were negatively correlated with the soluble solid， total sugar and reducing sugar contents in fruit. To the contrary， Zhong et al.[28]investigated nine representative plots in Nyingchi area， Tibetan where Prunus mira Koehne is concentrated to study the relationship between nine soil factors and the quality of Tibetan P. mira. The results showed that there was a strong positive correlation between total sugar content in P. mira fruit and rapidly availablenitrogen in soil. It shows that due to years of planting in the production area of Shenzhoumitao peach， the nitrogen fertilizer application rate is large， while Nyingchi， P. mira production area in Tibet is in a wild state without or very less application of nitrogen fertilizer， exhibiting nitrogen deficiency， so the application of appropriate amount of nitrogen fertilizer is helpful to improve the internal quality. However， some studies have suggested that nitrogen has no significant effect on the content of soluble solids[29]. Agricultural Biotechnology 2020
Effect of nitrogen application on organic acids
Numerous studies have shown that nitrogen application in soil increases organic acid content in fruits. As the nitrogen application rate increased from 300 to 600 and 1 200 g/plant， the acid content of 4yearold Shuguang nectarine fruit increased regularly， from 0.45% to 0.52% and 0.63%， and the no nitrogen application treatment showed the lowest content of 0.38%[16]. The content of titratable acid in Zhonghuashoutao peach increased significantly with the increase of fertilizing amount. When the nitrogen application rates were 0.75， 1.5， and 2.25 kg/plant， the titratable acid contents were 0.43%， 0.47% and 0.49%， respectively， while the value was 0.29% without the application of nitrogen[14]. When the nitrogen application rate was increased from 0 to 675 kg/hm2， the total acid content of Zhongyou 4 nectarine fruit increased from 0.5% to 0.6%， and the total acid content in fruit with the highest nitrogen application （677 kg/hm2） was the highest[17]. Jia et al.[30]believed that the highest contents of titratable acid， citric acid and malic acid were found in the fruits with highnitrogen treatment. When the soil nitrogen content was high， on the basis of applying organic fertilizer and biogas residue， applying urea at 150 kg/hm2 improved the acid content of Shenzhoumitao peach compared with the control[25]. Liu et al.[31]obtained the same results on Feicheng peach. The application of human feces and urine with high nitrogen content also had an effect on the acid content in fruit. After applying 20 kg/plant of human feces and urine to Chunlei peach plant， the acid content increased comparing with the control[26]. If the soil has a high nitrogen content， the application of nitrogen will increase the acid content in fruit. The nitrogen content in the soil is related to the acid content in fruit. Fan[27]reported that when the total nitrogen and alkalihydrolyzale nitrogen contents in soil of Shenzhoumitao peach orchards were high， the acid content in fruit increased correspondingly， and the sugaracid ratio decreased. Other studies have shown that there is no relationship between nitrogen application and acid content[13，15]. Before sprouting， the application of urea to Zaohongbaoshi nectarine in greenhouse did not change the titrated acid content compared with no nitrogen application[32].
Effect of nitrogen application on vitamin C
The effects of nitrogen on the vitamin C content of peach fruits are different. Some studies have shown that nitrogen application reduces the vitamin C content in fruits. When the nitrogen application rate on 4yearold Shuguang nectarine was 300， 600， and 1 200 g/plant， respectively， the vitamin C content continued to decrease with the increase of nitrogen application， and a maximum of 0.297 μg/g was found without nitrogen application[16]. The 8yearoldZaoshengshuimitao peach plant applied with 1.5 kg of urea decreased the vitamin C content in fruit from 13.64 to 9.24 mg/100 g compared with the control[18]. Based on the application of organic fertilizer and biogas residue， Shenzhoumitao peach applied with urea at 150 kg/hm2 showed a decrease in vitamin C[25]. Nitrogen application also reduced the vitamin C content of Zhongyou 4 nectarine[17]. Some studies have shown that in the nitrogen application treatments， as the amount of nitrogen applied increased， the vitamin C in fruit first decreased and then increased[17]or first increased and then decreased[13， 15]， but Liu et al.[26]showed that the content of vitamin C in nitrogentreated fruits did not change compared with the control. Effect of nitrogen application on protein and amino acid contents
There have been few studies on the effects of nitrogen fertilizer on the protein and amino acid contents in fruit. Existing research results show that the application of nitrogen fertilizer increases the contents of protein and amino acids in fruit. With the increase of nitrogen application， the protein and amino acid contents in fruit increased[10]. Jia et al.[21]reported that the contents of free amino acids such as arginine in fruit treated with high nitrogen were too high. Based on the application of organic fertilizer and biogas residue to Shenzhoumitao peach， after the application of 150 kg/hm2 of urea， the results showed that the protein content in fruit increased[25]. In addition， when the soil nitrogen content is high， the protein content in fruit increases accordingly. The higher the total soil nitrogen and alkalihydrolyzable nitrogen contents in Shenzhoumitao peach orchards， the higher the protein content in fruit[28].
Effect of nitrogen application on fruit aroma
Jia et al.[33]studied the effects of different fertilization treatments on peach fruit flavor. The results showed that different nitrogen fertilizer levels had an effect on the formation of peach fruit flavor substances. With the treatment of 80 mg/L nitrogen fertilizer， the contents of γcaprolactone， γcaprylolactone and γdecalactone in fruit were higher than other treatments， while the contents of trans2hexenal and iso3hexenol were lower. Jia et al.[23]reported that the aromatic substance content in fruit treated with high nitrogen decreased. The content of decanolactone was the highest in fruit at the medium nitrogen level， while it was the lowest in the highnitrogen treatment[31].
Optimal nitrogen application period and amount
Murata Ryuichi[34]carried out an experiment of spraying 0.5%urea at different time on Sunagowase peach （N1May 8， N2May 30， N3June 23）. The results showed that the N1 treatment had a high sugar content of fruit， there was no difference between N2 and untreated plants， and the N3 treatment had a low sugar content. And they concluded that in order to obtain highquality peach fruit， the trees must absorb sufficient nitrogen before sprouting to increase the nitrogen content at the beginning of growth， and the nitrogen fertilizer should be applied as early as possible. Zheng et al.[35]deem that it is better to apply nitrogen before sprouting. The recommended amount of nitrogen in foreign peach trees was generally pure N 100-200 kg/hm2 per year[36-37]. The nitrogen was recommended to be 10-12 kg per 0.1 hm2 peach orchard in Japan[38]. Li et al.[11]showed that the suitable nitrogen rate for Bayuecui peach was 150-300 kg/hm2， and the fruit quality was the best at this rate. The results of multisite research experiments in Pinggu District， Beijing showed that the recommended annual nitrogen of young and adult trees were 100-150 kg/hm2 and 150-200kg/hm2， respectively[39].
Conclusions and Discussion
Most studies have shown that proper nitrogen amount can improve the internal quality. Excessive nitrogen application reduces soluble solid and sugar contents in fruit， increases organic acid content， reduces fruit aroma， increases protein and amino acid contents， and increases or decreases vitamin C content.
The effect of nitrogen on sugar content is the result of the coordinated action of nitrogen on nitrogen and sugar metabolism enzymes. The mechanism is not clear on peach trees. The study of Zhao et al.[40]on sugar beet showed that when nitrogen was applied excessively， the enzyme activity in the direction of sucrose synthase synthesis decreased， but the enzyme activity in the direction of decomposition increased as the ammonium nitrogen increased. The sucrose phosphate synthase activity has a quadratic parabola relationship with the amount of nitrogen. When excessive nitrogen is applied， the sucrose phosphate synthase activity in roots and leaves decreases. It can be inferred that peach trees can increase the sucrose synthase （synthesis direction） and sucrose phosphate synthase activity and increase sugar content when nitrogen is applied properly. When there is more nitrogen， the activity of sucrose synthase （synthesis direction） and sucrose phosphate synthase become lower， the sucrose synthase （decomposition direction） activity is strong， and the invertase activity is enhanced.
In the regulation of carbon and nitrogen metabolism， there are three vital enzymes， namely， NR， PEPCase and SPS[41]， the activity of which can be regulated by nitrogen nutrition signals. When NO-3 concentration increases， the activity of protein kinase is improved， making PEPC and SPS enzyme proteins phosphorylated， while phosphorylated PEPC has high activity， is conducive to the reaction of PEP to oxaloacetate， and promotes amino acid synthesis. Meanwhile， phosphorylated SPS activity is inactivated， limiting sucrose synthesis[41-42]. Zhou[43]showed that when sugar beet absorbs too much nitrogen， plants must change the inorganic nitrogen absorbed into organic nitrogen including amino acids and proteins. Otherwise， the plants will be poisoned due to excessive ammonia accumulation. When sugar beet absorbs large amounts of NO-3 and NH+4， it absorbs the same amounts of cations and anions to achieve the balance between anions and cations; and in the case of increased nitrogen in the body， the contents of amino acids and organic acids will also follow the increase. As these solutes are increasing， the osmotic pressure of the cells increased， resulting in an increase in the water content in sugar beet and a decrease in the sugar content.
Proteins and amino acids are products of nitrogen metabolism in fruit. During the fruit development， the changes of protein and amino acid contents are similar， showing a gradual downward trend. The protein and amino acid contents in the early fruit development period are relatively high， which indicates that the nitrogen metabolism is relatively vigorous， and the higher protein and amino acid contents can satisfy the demand for nitrogen. At the young fruit stage， with the fruit development， the protein and amino acid contents in fruit decrease from the highest to the higher level， and from the late development stages to fruit maturity， the protein and amino acid contents further decrease， and decrease to the lowest when the fruits are mature[10].
For a very important process of synthesizing amino acids and proteins with photosynthetic products and nitrogen， humans have different requirements for different crops. For cereal and legume crops， higher protein content is expected in their grains. However， for sugar crops， it is desirable to have a higher sugar content and the lowest possible amount of harmful nitrogen based on a certain yield. Therefore， sugar beet can only accumulate more sugar under proper nitrogen supply[43]. The main purpose of producing fruit is to have sweet or sweet and sour fruit with aroma and high Vc content. The protein and amino acid contents should not be used as an indicator of the good quality of peach fruit.
Nitrogen fertilizer will increase the content of bitter amino acids （arginine， isoleucine， phenylalanine， methionine and histidine） in watermelon， and reduce the contents of sweetness （glycine， alanine， serine and threonine） and umami amino acids （aspartic acid and glutamic acid）[44]. The study by Lu et al.[45]showed that excessive application of nitrogen fertilizer increased protein content. The total amino acid content of strawberry fruit increased with the increase of nitrogen application， and it was the highest when the nitrogen was applied at a higher amount[46]. Zhao et al.[47]reported that the nitrogen content in cucumber fruit gradually increased with the increase of nitrogen application amount. When the nitrogen application amount was 375 kg/hm2， the contents of free amino acids， soluble proteins and crude proteins reached their maximum values. Qu et al.[48]showed that the amount of nitrogen application was positively correlated with the contents of total nitrogen， harmful nitrogen， amino acids and amides in sugar beet. References
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