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Abstract [Objectives] This study was conducted to explore the treatment methods of amino acid selenium fertilizer suitable for increasing the selenium content of mangoes and improving the quality of mangoes， so as to provide a theoretical basis for the production of selenium-enriched mangoes.
[Methods] With Tainong mango as a test material， the amino acid selenium foliar fertilizer was applied by eight treatment methods to investigate the changes of selenium， soluble solid， vitamin C， and titratable acid contents in mango flesh and peel.
[Results] Spraying amino acid selenium foliar fertilizer increased the selenium content of mango flesh. The selenium contents of the treatment groups T2， T3， T4 and T5 reached the selenium-enriched standard， and the T2 treatment group had the highest selenium content （0.020 mg/kg）. The selenium contents in the peel of all treatment groups were greater than the corresponding selenium content in the flesh. Except for T1， the vitamin C contents of other treatment groups sprayed with amino acid selenium fertilizer increased compared with the control， and that of the T2 treatment group increased significantly （P<0.05）. Compared with the control， the soluble solids of the treatment groups sprayed with amino acid selenium fertilizer once also increased. The treatment method of spraying amino acid selenium fertilizer with a dosage of 12 000 ml/hm2 once （T2） achieved the best effect of increasing the selenium content and improving the quality of mangoes.
[Conclusions] This study provides a scientific basis for increasing the selenium content of mangoes and improving the current situation of insufficient selenium intake.
Key words Selenium; Mango; Amino acid selenium foliar fertilizer
Received： July 21， 2020 Accepted： September 25， 2020
Supported by Guangxi Science and Technology Major Project （GK AA17202037-2， GK AA17202026）; Guangxi Mango Innovation Team Cultivation Post Project of National Modern Agriculture Industrial Technology System （nycytxgxcxtd-06-02）; Special Fund of Central Government for Local Science and Technology Development （GK ZY19183007）; Special Action of the Science and Technology Vanguard of Guangxi Academy of Agricultural Sciences （GNK JZ202016）; Guangxi Key R&D Program （GK AB16380164）; Guangxi Selenium-enriched Crop Experiment Station （G TS2016011）; Fundamental Scientific Research Fund of Guangxi Academy of Agricultural Sciences （GNK 2020YM109）. Mengling NONG （1980-）， female， P. R. China， senior experimentalist， devoted to research about soil fertilizer and plant nutrition.
*Corresponding author. E-mail： liuyx27@163.com.
Selenium （Se） is one of the essential trace elements for humans and animals to maintain their health. However， soil Se resources are extremely scarce in the world， which is a serious threat to the development of human health， and the daily Se intake of about 1 billion people worldwide is insufficient[1-2]. Inorganic Se is not suitable for direct consumption due to its strong toxicity and low human absorption rate. Inorganic Se is not suitable for direct consumption due to its strong toxicity and low human absorption rate. China’s practice in the prevention and treatment of Keshan disease and Kashin-Beck disease in areas with severe Se deficiency such as Keshan in Heilongjiang and Qinghai-Tibet Plateau has proved that the most economical and effective way to supplement Se is to increase the intake of organic Se[3]. Plants are the main "production workshop" for organic selenium， and most of the Se in humans and animals is obtained from plants. Therefore， increasing the Se content of crops is the key to solving the problem of human Se deficiency. However， factors such as lack of soil Se， low availability， and weak Se accumulation capacity of crops severely restrict the production of selenium-rich crops. The development of crop Se biofortification has become an important measure to increase the Se content of crops.
Fruit is an important part of people’s daily diet. In order to satisfy the intake of fruit nutrients and supplement selenium， a large number of fruit Se biofortification studies have been carried out， such as strawberries， grapes， pears， apples， plums， etc. Spraying apples and pears with 600 times solution of amino acid Se foliar fertilizer can make the Se content of apple， Qiubai pear and Huagai pear reach 0.014 5， 0.012 and 0.028 mg/kg， respectively， and achieve good results[4-7]. Mango （Mangifera indica L.） is a plant belonging to the genus Mangifera in the Anacardiaceae family. The fruit has a unique flavor and high nutritional value. It enjoys the reputation of "Tropical Fruit King" and is loved by consumers. The mango industry has become one of the main means of income for farmers in China’s hot zone. However， under normal cultivation conditions， the Se content of mangoes is generally low， which does not meet the minimum requirements for selenium-enriched fresh fruits in various provinces and cities （Guangxi， Jiangxi， Hubei， Ankang and other local standards require the lowest value of Se content in selenium-enriched fresh fruits to be 0.01 mg/kg）， which severely restricts the production and consumption of selenium-enriched mangoes. In order to better improve the nutritional value and economic benefits of mango and understand its accumulation of Se， through field trials， mangoes were treated with different Se bio-enhancement measures， and the change characteristics of mango Se content and mango quality were explored， providing scientific basis for increasing mango Se content and improving the current situation of people’s insufficient Se intake. Materials and Methods
Experimental materials
The mangoes tested were Tainong. The test fertilizer was amino acid Se foliar fertilizer， which was developed by the Institute of Agricultural Resources and Environment， Guangxi Academy of Agricultural Sciences， with a Se content of 0.26% and a free amino acid of 0.46%.
Experimental location
The experiment was carried out in the demonstration base of Xingrun Fruit and Vegetable Agricultural Professional Cooperative in Tiandong County， Baise City， Guangxi Zhuang Autonomous Region. The location of the experimental area is in the south subtropical climate， with sufficient annual rainfall， annual average temperature greater than 22 ℃， and annual average sunlight between 1 800 and 1 940 h. It is very suitable for growing mangoes and was once awarded the title of "Hometown of Mango" by the state. Fields that were relatively flat in the orchard， away from roads and other disturbed areas were selected for the experiment. The fruit trees in the test area were all planted in the same batch， and the plants were in good health and growing basically the same. The basic physical and chemical properties of the soil in the field were： total Se 0.24 mg/kg， pH 5.8， organic matter 12.6 g/kg， available phosphorus 2.76 mg/kg， available potassium 16.4 mg/kg， and alkali-hydrolyzale nitrogen 76.9 mg/kg.
Experimental design
The experiment adopted the randomized block design. The spraying of Se fertilizer in the experiment was divided into following ways： blank （CK）， spraying at 9 000 ml/hm2 once （T1）， spraying at 12 000 ml/hm2 once （T2）， spraying at 15 000 ml/hm2 once （T3）， spraying at 18 000 ml/hm2 once （T4）， spraying at 9 000 ml/hm2 twice （T5）， spraying at 12 000ml/hm2 twice （T6）， spraying at 15 000ml/hm2 twice （T7）， and spraying at 18 000 ml/hm2 twice （T8）， each with 3 plots and 4 trees per plot. The Se fertilizer nutrient solution was sprayed for the first time on May 1， 2018， and the treatment group sprayed twice was sprayed for the second time on May 8. Harvesting was performed on July 13.
Determination indicators and methods
Determination of total Se content： After the fruit samples were taken to the laboratory， the flesh and peel were separated， and the fresh samples after separation were tested for total selenium， in accordance with the first method （hydride atomic fluorescence spectrometry） in GB5009.93-2017 "National Food Safety Standard： Determination of Selenium in Foods". The soluble solids were measured by PAL-1 refractometer （Japan）; the titratable acid content was measured by acid-base titration; and vitamin C was measured by 2，6 dichloroindophenol titration method. Statistical analysis
One-way analysis of variance was performed by SPSS22.0 software to compare the Se soluble solid， titratable acid and vitamin C contents of mango flesh and peel in each treatment group for the significance of difference.
Results and Analysis
Effect of amino acid Se foliar fertilizer on Se content of mango
The results showed that different amino acid Se foliar fertilizer treatments had no significant effect on the Se content of mango flesh （P>0.05）. The Se contents of some treatment groups increased， and the Se contents of the flesh were in the range of 0.006-0.020 mg/kg. The Se contents in the flesh of T6， T7， T8 sprayed twice with Se fertilizer were less than or equal to the control. The spraying of amino acid Se foliar fertilizer had a significant effect on the Se content of mango peel （P<0.05） （Table 1）. The Se content in the peel of the T2 treatment group was the highest， reaching 0.039 mg/kg， which was 2.6 times that of the blank control （Table 1）. In the treatment groups sprayed with Se fertilizer once， the Se content of the peel increased first and then decreased with the increase of the spray concentration. There were no significant differences between the treatment groups sprayed with Se fertilizer twice （P>0.05）. Including the control， the Se contents in the peel of all treatment groups were greater than the corresponding Se content in the flesh （Table 1）.
Effects of amino acid Se leaf fertilizer on mango quality
The titratable acid contents of mangoes in all treatment groups ranged from 0.13% to 0.18%. The titratable acid content of the T7 treatment group was significantly higher than those of other treatment groups （P<0.05）， reaching 0.18%， which was 1.29 times that of the control （Table 1）. Except for T7， there were no significant differences between all other amino acid Se foliar fertilizer treatment groups and the control group （P>0.05） （Table 1）. Spraying amino acid Se fertilizer could effectively increase the vitamin C content of mangoes. Except for T1， the vitamin C contents of other treatment groups sprayed with amino acids all increased compared with the control， and the value of the T2 treatment group significantly increased compared with the control （P<0.05）， reaching 0.039 mg/100 g （Table 1）. Different amino acid Se fertilizer treatment groups had different effects on mango soluble solids. The treatment groups sprayed once showed an increase compared with the control， and the T4 treatment group had a significant increase （P<0.05）. The soluble solid contents of the treatment groups sprayed twice with amino acid Se foliar fertilizer decreased compared with the control， and the values of the T6 and T8 treatment groups significantly decreased （P<0.05） （Table 1）. Conclusions and Discussion
Crop Se biofortification aims to increase the amount of Se in crops that can be absorbed and utilized by the human body through agricultural measures and genetic breeding methods， and ultimately improve the current situation of insufficient human Se intake. The targets of crop Se biofortification are mainly human and animal edible plants. There are various ways of biofortification， which can be roughly summarized as the following： applying exogenous Se to the soil， spraying exogenous Se to the leaves， activating soil Se through microorganisms， breeding high-efficiency Se-enriched varieties through traditional breeding methods， and enhancing the Se-enrichment capacity of crops through transgenic technology[8-9]. In this study， the method of spraying exogenous Se on the leaves was used to carry out the research. The results showed that spraying amino acid Se foliar fertilizer increased the Se content of mango flesh， and the Se contents of the flesh of the treatment groups T2， T3， T4 and T5 reached the Se-enriched standard （the local standards of Guangxi， Jiangxi， Hubei and Ankang require the lowest value of Se content in Se-enriched fresh fruits to be 0.01 mg/kg）， and the T2 treatment group had the highest Se content. It can be seen that mangoes can absorb and transform the exogenous Se sprayed on the leaves to a certain extent， but the Se element transported to the flesh is limited， because even if the concentration and frequency of Se fertilizer spraying are increased， the Se content in the flesh does not increase significantly. The Se contents of the T6， T7 and T8 treatment groups sprayed with the Se fertilizer twice were all less than or equal to the control. From this study， the Se storage capacity of the peel was greater than that of the flesh. Including the control， the Se contents of the peel were greater than the corresponding Se content of the flesh. It can be seen that in the practice of crop Se biofortification， in addition to providing sufficient bioavailable Se to crops， the distribution and transportation of Se in various organs of crops is still the key to the restriction in production of Se-rich crops.
In addition to increasing the Se content of crops， Se biofortification will also have a certain impact on crop quality. Generally speaking， there have been many studies on Se biofortification promoting the improvement of crop quality. For example， spraying amino acid Se foliar fertilizer can not only increase strawberry fruit firmness， soluble solids and vitamin content， but also reduce its titratable acid content， as well as increasing the single-grape weight and soluble solid content and improving the quality traits of Qiubai pear and Huagai pear such as total sugar content， vitamin C content and single-fruit weight[10-12]. In this study， spraying amino acid Se fertilizer effectively increased the vitamin C content of mangoes. Except for T1， the vitamin C contents of other treatment groups sprayed with amino acid Se fertilizer increased compared with the control， and the value of the T2 treatment group increased significantly compared with the control （P<0.05）， reaching 0.039 mg/100 g. Meanwhile， the soluble solids of the treatment groups sprayed with amino acid Se fertilizer once also increased compared with the control. In summary， spraying amino acid Se foliar fertilizer increased the Se content of mango flesh. The Se contents of the treatment groups T2， T3， T4 and T5 reached the Se-enriched standard， and the T2 treatment group had the highest Se content. The Se contents of the peel of all treatment groups were greater than the corresponding Se content of the flesh， and the storage capacity of the peel was greater than that of the flesh. Spraying amino acid Se fertilizer increased the vitamin C content of mangoes， and the soluble solid contents of the treatment groups sprayed once also increased. The vitamin C content of the T2 treatment group increased significantly. The treatment method of spraying amino acid Se fertilizer with a dosage of 12 000 ml/hm2 once （T2） achieved the best effect of increasing the Se content and improving the quality of mangoes.
References
[1] WILLIAMS PN， LOMBI E， SUN GX， et al. Selenium characterization in the global rice supply chain[J]. Environmental science & technology， 2009， 43（15）： 6024-6030.
[2] COMBS GF. Selenium in global food systems[J]. British journal of nutrition， 2001， 85（5）： 517-547.
[3] STRANGES S， MARSHALL JR， NATARAJAN R， et al. Effects of long-term selenium supplementation on the incidence of type 2 diabetes： A randomized trial[J]. Annals of Internal Medicine， 2007， 147（4）： 217-223.
[4] LI EM， XU K， AN XH， et al. Study on absorption， translocation and accumulation characteristics of amino acid selenium foliar fertilizer on apple[J]. South China Fruits， 2014， 43（6）： 105-107. （in Chinese）
[5] SHI XB， LIU FZ， WANG XD， et al. Effect of amino acid selenium foliar fertilizer on selenium content and quality of pear fruit[J]. South China Fruits， 2016， 45（5）： 105-107. （in Chinese）
[6] DENG XF， LIU XW， YE ZJ， et al. Effects of spraying different selenium sources in different periods on the absorption and distribution of selenium in grapes[J]. South China Fruits， 2018， 47（5）： 82-86. （in Chinese）
[7] WU DD， WU YY， HUANG YC， et al. Effects of different types of selenium fertilizers on selenium content and quality of Lingfeng grapes[J]. South China Fruits， 2018， 47（4）： 132-134. （in Chinese）
[8] NORTON GJ， DEACON C， XIONG L， et al. Genetic mapping of the rice ionome in leaves and grain： identification of QTLs for 17 elements including arsenic， cadmium， iron and selenium[J]. Plant and Soil， 2010， 329（1）： 139-153.
[9] WHITE PJ. Selenium accumulation by plants[J]. Annals of Botany， 2015， 117（2）： 217-235.
[10] WANG XD， LIU FZ， WANG S， et al. Effect of spraying amino acid selenium foliar fertilizer on leaf and fruit quality of strawberry in protected cultivation[J]. China Fruits， 2014（4）： 37-39. （in Chinese）
[11] WANG HB， WANG XD， YAO XY， et al. Application effect of amino acid-Se foliar fertilizer on Muscat Hamburg[J]. Sino-Overseas Grapevine & Wine， 2011（5）： 47-49. （in Chinese）
[12] SHI XB， LIU FZ， WANG XD， et al. Effect of amino acid selenium foliar fertilizer on selenium content and quality of pear fruit[J]. South China Fruits， 2016， 45（5）： 105-107， 112. （in Chinese）
Editor： Yingzhi GUANG Proofreader： Xinxiu ZHU
[Methods] With Tainong mango as a test material， the amino acid selenium foliar fertilizer was applied by eight treatment methods to investigate the changes of selenium， soluble solid， vitamin C， and titratable acid contents in mango flesh and peel.
[Results] Spraying amino acid selenium foliar fertilizer increased the selenium content of mango flesh. The selenium contents of the treatment groups T2， T3， T4 and T5 reached the selenium-enriched standard， and the T2 treatment group had the highest selenium content （0.020 mg/kg）. The selenium contents in the peel of all treatment groups were greater than the corresponding selenium content in the flesh. Except for T1， the vitamin C contents of other treatment groups sprayed with amino acid selenium fertilizer increased compared with the control， and that of the T2 treatment group increased significantly （P<0.05）. Compared with the control， the soluble solids of the treatment groups sprayed with amino acid selenium fertilizer once also increased. The treatment method of spraying amino acid selenium fertilizer with a dosage of 12 000 ml/hm2 once （T2） achieved the best effect of increasing the selenium content and improving the quality of mangoes.
[Conclusions] This study provides a scientific basis for increasing the selenium content of mangoes and improving the current situation of insufficient selenium intake.
Key words Selenium; Mango; Amino acid selenium foliar fertilizer
Received： July 21， 2020 Accepted： September 25， 2020
Supported by Guangxi Science and Technology Major Project （GK AA17202037-2， GK AA17202026）; Guangxi Mango Innovation Team Cultivation Post Project of National Modern Agriculture Industrial Technology System （nycytxgxcxtd-06-02）; Special Fund of Central Government for Local Science and Technology Development （GK ZY19183007）; Special Action of the Science and Technology Vanguard of Guangxi Academy of Agricultural Sciences （GNK JZ202016）; Guangxi Key R&D Program （GK AB16380164）; Guangxi Selenium-enriched Crop Experiment Station （G TS2016011）; Fundamental Scientific Research Fund of Guangxi Academy of Agricultural Sciences （GNK 2020YM109）. Mengling NONG （1980-）， female， P. R. China， senior experimentalist， devoted to research about soil fertilizer and plant nutrition.
*Corresponding author. E-mail： liuyx27@163.com.
Selenium （Se） is one of the essential trace elements for humans and animals to maintain their health. However， soil Se resources are extremely scarce in the world， which is a serious threat to the development of human health， and the daily Se intake of about 1 billion people worldwide is insufficient[1-2]. Inorganic Se is not suitable for direct consumption due to its strong toxicity and low human absorption rate. Inorganic Se is not suitable for direct consumption due to its strong toxicity and low human absorption rate. China’s practice in the prevention and treatment of Keshan disease and Kashin-Beck disease in areas with severe Se deficiency such as Keshan in Heilongjiang and Qinghai-Tibet Plateau has proved that the most economical and effective way to supplement Se is to increase the intake of organic Se[3]. Plants are the main "production workshop" for organic selenium， and most of the Se in humans and animals is obtained from plants. Therefore， increasing the Se content of crops is the key to solving the problem of human Se deficiency. However， factors such as lack of soil Se， low availability， and weak Se accumulation capacity of crops severely restrict the production of selenium-rich crops. The development of crop Se biofortification has become an important measure to increase the Se content of crops.
Fruit is an important part of people’s daily diet. In order to satisfy the intake of fruit nutrients and supplement selenium， a large number of fruit Se biofortification studies have been carried out， such as strawberries， grapes， pears， apples， plums， etc. Spraying apples and pears with 600 times solution of amino acid Se foliar fertilizer can make the Se content of apple， Qiubai pear and Huagai pear reach 0.014 5， 0.012 and 0.028 mg/kg， respectively， and achieve good results[4-7]. Mango （Mangifera indica L.） is a plant belonging to the genus Mangifera in the Anacardiaceae family. The fruit has a unique flavor and high nutritional value. It enjoys the reputation of "Tropical Fruit King" and is loved by consumers. The mango industry has become one of the main means of income for farmers in China’s hot zone. However， under normal cultivation conditions， the Se content of mangoes is generally low， which does not meet the minimum requirements for selenium-enriched fresh fruits in various provinces and cities （Guangxi， Jiangxi， Hubei， Ankang and other local standards require the lowest value of Se content in selenium-enriched fresh fruits to be 0.01 mg/kg）， which severely restricts the production and consumption of selenium-enriched mangoes. In order to better improve the nutritional value and economic benefits of mango and understand its accumulation of Se， through field trials， mangoes were treated with different Se bio-enhancement measures， and the change characteristics of mango Se content and mango quality were explored， providing scientific basis for increasing mango Se content and improving the current situation of people’s insufficient Se intake. Materials and Methods
Experimental materials
The mangoes tested were Tainong. The test fertilizer was amino acid Se foliar fertilizer， which was developed by the Institute of Agricultural Resources and Environment， Guangxi Academy of Agricultural Sciences， with a Se content of 0.26% and a free amino acid of 0.46%.
Experimental location
The experiment was carried out in the demonstration base of Xingrun Fruit and Vegetable Agricultural Professional Cooperative in Tiandong County， Baise City， Guangxi Zhuang Autonomous Region. The location of the experimental area is in the south subtropical climate， with sufficient annual rainfall， annual average temperature greater than 22 ℃， and annual average sunlight between 1 800 and 1 940 h. It is very suitable for growing mangoes and was once awarded the title of "Hometown of Mango" by the state. Fields that were relatively flat in the orchard， away from roads and other disturbed areas were selected for the experiment. The fruit trees in the test area were all planted in the same batch， and the plants were in good health and growing basically the same. The basic physical and chemical properties of the soil in the field were： total Se 0.24 mg/kg， pH 5.8， organic matter 12.6 g/kg， available phosphorus 2.76 mg/kg， available potassium 16.4 mg/kg， and alkali-hydrolyzale nitrogen 76.9 mg/kg.
Experimental design
The experiment adopted the randomized block design. The spraying of Se fertilizer in the experiment was divided into following ways： blank （CK）， spraying at 9 000 ml/hm2 once （T1）， spraying at 12 000 ml/hm2 once （T2）， spraying at 15 000 ml/hm2 once （T3）， spraying at 18 000 ml/hm2 once （T4）， spraying at 9 000 ml/hm2 twice （T5）， spraying at 12 000ml/hm2 twice （T6）， spraying at 15 000ml/hm2 twice （T7）， and spraying at 18 000 ml/hm2 twice （T8）， each with 3 plots and 4 trees per plot. The Se fertilizer nutrient solution was sprayed for the first time on May 1， 2018， and the treatment group sprayed twice was sprayed for the second time on May 8. Harvesting was performed on July 13.
Determination indicators and methods
Determination of total Se content： After the fruit samples were taken to the laboratory， the flesh and peel were separated， and the fresh samples after separation were tested for total selenium， in accordance with the first method （hydride atomic fluorescence spectrometry） in GB5009.93-2017 "National Food Safety Standard： Determination of Selenium in Foods". The soluble solids were measured by PAL-1 refractometer （Japan）; the titratable acid content was measured by acid-base titration; and vitamin C was measured by 2，6 dichloroindophenol titration method. Statistical analysis
One-way analysis of variance was performed by SPSS22.0 software to compare the Se soluble solid， titratable acid and vitamin C contents of mango flesh and peel in each treatment group for the significance of difference.
Results and Analysis
Effect of amino acid Se foliar fertilizer on Se content of mango
The results showed that different amino acid Se foliar fertilizer treatments had no significant effect on the Se content of mango flesh （P>0.05）. The Se contents of some treatment groups increased， and the Se contents of the flesh were in the range of 0.006-0.020 mg/kg. The Se contents in the flesh of T6， T7， T8 sprayed twice with Se fertilizer were less than or equal to the control. The spraying of amino acid Se foliar fertilizer had a significant effect on the Se content of mango peel （P<0.05） （Table 1）. The Se content in the peel of the T2 treatment group was the highest， reaching 0.039 mg/kg， which was 2.6 times that of the blank control （Table 1）. In the treatment groups sprayed with Se fertilizer once， the Se content of the peel increased first and then decreased with the increase of the spray concentration. There were no significant differences between the treatment groups sprayed with Se fertilizer twice （P>0.05）. Including the control， the Se contents in the peel of all treatment groups were greater than the corresponding Se content in the flesh （Table 1）.
Effects of amino acid Se leaf fertilizer on mango quality
The titratable acid contents of mangoes in all treatment groups ranged from 0.13% to 0.18%. The titratable acid content of the T7 treatment group was significantly higher than those of other treatment groups （P<0.05）， reaching 0.18%， which was 1.29 times that of the control （Table 1）. Except for T7， there were no significant differences between all other amino acid Se foliar fertilizer treatment groups and the control group （P>0.05） （Table 1）. Spraying amino acid Se fertilizer could effectively increase the vitamin C content of mangoes. Except for T1， the vitamin C contents of other treatment groups sprayed with amino acids all increased compared with the control， and the value of the T2 treatment group significantly increased compared with the control （P<0.05）， reaching 0.039 mg/100 g （Table 1）. Different amino acid Se fertilizer treatment groups had different effects on mango soluble solids. The treatment groups sprayed once showed an increase compared with the control， and the T4 treatment group had a significant increase （P<0.05）. The soluble solid contents of the treatment groups sprayed twice with amino acid Se foliar fertilizer decreased compared with the control， and the values of the T6 and T8 treatment groups significantly decreased （P<0.05） （Table 1）. Conclusions and Discussion
Crop Se biofortification aims to increase the amount of Se in crops that can be absorbed and utilized by the human body through agricultural measures and genetic breeding methods， and ultimately improve the current situation of insufficient human Se intake. The targets of crop Se biofortification are mainly human and animal edible plants. There are various ways of biofortification， which can be roughly summarized as the following： applying exogenous Se to the soil， spraying exogenous Se to the leaves， activating soil Se through microorganisms， breeding high-efficiency Se-enriched varieties through traditional breeding methods， and enhancing the Se-enrichment capacity of crops through transgenic technology[8-9]. In this study， the method of spraying exogenous Se on the leaves was used to carry out the research. The results showed that spraying amino acid Se foliar fertilizer increased the Se content of mango flesh， and the Se contents of the flesh of the treatment groups T2， T3， T4 and T5 reached the Se-enriched standard （the local standards of Guangxi， Jiangxi， Hubei and Ankang require the lowest value of Se content in Se-enriched fresh fruits to be 0.01 mg/kg）， and the T2 treatment group had the highest Se content. It can be seen that mangoes can absorb and transform the exogenous Se sprayed on the leaves to a certain extent， but the Se element transported to the flesh is limited， because even if the concentration and frequency of Se fertilizer spraying are increased， the Se content in the flesh does not increase significantly. The Se contents of the T6， T7 and T8 treatment groups sprayed with the Se fertilizer twice were all less than or equal to the control. From this study， the Se storage capacity of the peel was greater than that of the flesh. Including the control， the Se contents of the peel were greater than the corresponding Se content of the flesh. It can be seen that in the practice of crop Se biofortification， in addition to providing sufficient bioavailable Se to crops， the distribution and transportation of Se in various organs of crops is still the key to the restriction in production of Se-rich crops.
In addition to increasing the Se content of crops， Se biofortification will also have a certain impact on crop quality. Generally speaking， there have been many studies on Se biofortification promoting the improvement of crop quality. For example， spraying amino acid Se foliar fertilizer can not only increase strawberry fruit firmness， soluble solids and vitamin content， but also reduce its titratable acid content， as well as increasing the single-grape weight and soluble solid content and improving the quality traits of Qiubai pear and Huagai pear such as total sugar content， vitamin C content and single-fruit weight[10-12]. In this study， spraying amino acid Se fertilizer effectively increased the vitamin C content of mangoes. Except for T1， the vitamin C contents of other treatment groups sprayed with amino acid Se fertilizer increased compared with the control， and the value of the T2 treatment group increased significantly compared with the control （P<0.05）， reaching 0.039 mg/100 g. Meanwhile， the soluble solids of the treatment groups sprayed with amino acid Se fertilizer once also increased compared with the control. In summary， spraying amino acid Se foliar fertilizer increased the Se content of mango flesh. The Se contents of the treatment groups T2， T3， T4 and T5 reached the Se-enriched standard， and the T2 treatment group had the highest Se content. The Se contents of the peel of all treatment groups were greater than the corresponding Se content of the flesh， and the storage capacity of the peel was greater than that of the flesh. Spraying amino acid Se fertilizer increased the vitamin C content of mangoes， and the soluble solid contents of the treatment groups sprayed once also increased. The vitamin C content of the T2 treatment group increased significantly. The treatment method of spraying amino acid Se fertilizer with a dosage of 12 000 ml/hm2 once （T2） achieved the best effect of increasing the Se content and improving the quality of mangoes.
References
[1] WILLIAMS PN， LOMBI E， SUN GX， et al. Selenium characterization in the global rice supply chain[J]. Environmental science & technology， 2009， 43（15）： 6024-6030.
[2] COMBS GF. Selenium in global food systems[J]. British journal of nutrition， 2001， 85（5）： 517-547.
[3] STRANGES S， MARSHALL JR， NATARAJAN R， et al. Effects of long-term selenium supplementation on the incidence of type 2 diabetes： A randomized trial[J]. Annals of Internal Medicine， 2007， 147（4）： 217-223.
[4] LI EM， XU K， AN XH， et al. Study on absorption， translocation and accumulation characteristics of amino acid selenium foliar fertilizer on apple[J]. South China Fruits， 2014， 43（6）： 105-107. （in Chinese）
[5] SHI XB， LIU FZ， WANG XD， et al. Effect of amino acid selenium foliar fertilizer on selenium content and quality of pear fruit[J]. South China Fruits， 2016， 45（5）： 105-107. （in Chinese）
[6] DENG XF， LIU XW， YE ZJ， et al. Effects of spraying different selenium sources in different periods on the absorption and distribution of selenium in grapes[J]. South China Fruits， 2018， 47（5）： 82-86. （in Chinese）
[7] WU DD， WU YY， HUANG YC， et al. Effects of different types of selenium fertilizers on selenium content and quality of Lingfeng grapes[J]. South China Fruits， 2018， 47（4）： 132-134. （in Chinese）
[8] NORTON GJ， DEACON C， XIONG L， et al. Genetic mapping of the rice ionome in leaves and grain： identification of QTLs for 17 elements including arsenic， cadmium， iron and selenium[J]. Plant and Soil， 2010， 329（1）： 139-153.
[9] WHITE PJ. Selenium accumulation by plants[J]. Annals of Botany， 2015， 117（2）： 217-235.
[10] WANG XD， LIU FZ， WANG S， et al. Effect of spraying amino acid selenium foliar fertilizer on leaf and fruit quality of strawberry in protected cultivation[J]. China Fruits， 2014（4）： 37-39. （in Chinese）
[11] WANG HB， WANG XD， YAO XY， et al. Application effect of amino acid-Se foliar fertilizer on Muscat Hamburg[J]. Sino-Overseas Grapevine & Wine， 2011（5）： 47-49. （in Chinese）
[12] SHI XB， LIU FZ， WANG XD， et al. Effect of amino acid selenium foliar fertilizer on selenium content and quality of pear fruit[J]. South China Fruits， 2016， 45（5）： 105-107， 112. （in Chinese）
Editor： Yingzhi GUANG Proofreader： Xinxiu ZHU