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Abstract [Objectives] This study was carried out to explore the selenium??enriching effect of different crops， and provide certain references for production of selenium??enriched agricultural products. [Methods] Four kinds of crops （tomato， leaf mustard， peanut， and carrot） were exogenous selenium fertilizer treated， to explore the selenium??enriching effects on these crops. [Results] Exogenous application of microbial nano??selenium can significantly increase the selenium content of the crops， of which the effects on big tomato （Weiyanuo） were the best. After spraying， the selenium content increased by 1.39 times， and at the same time the sugar content of tomato was also increased. [Conclusions] This study is expected to provide references for increase of the selenium content of agricultural products.
Key words Microbial nano??selenium； Tomato； Leaf mustard； Peanut； Carrot； Selenium??enriching effects
As a trace element， selenium （Se） was discovered and named by Swedish scientist Berlius in 1817. The Chinese Nutrition Society listed selenium as one of the essential nutrients for the human body in 1988. Studies have shown that selenium has functions of anti??cancer and anti??oxidation， immunity improvement， prevention and treatment of cardiovascular diseases， liver protection and detoxification[1]. Besides， selenium also can regulate plant growth， increase glutathione peroxidase activity， improve stress tolerance， and improve quality and yield[2]. The Chinese Nutrition Society recommends the daily intake of selenium is 50 ??g， while international organizations recommend the daily intake of 60 ??g of selenium. Selenium cannot be synthesized in the human body. It has to be absorbed from foods to realize rapid supplementation. Crops can be transformed into safe and effective organic selenium and then be easily absorbed by human body. Taking selenium??enriched agricultural products is an important method for improving selenium shortage of human body. Most regions in China are selenium??deficient areas， only a few regions are natural selenium??enriched areas （such as Ziyang in Shaanxi and Enshi in Hubei） that can engage in the production of natural selenium??enriched agricultural products. For selenium??deficient areas， it is necessary to increase the selenium content of agricultural products through exogenous selenium addition by technical means， such as application of selenium in soil， spraying selenium on leaves， soaking seeds with selenium， and water culture of selenium fertilizer[3-4]. In this experiment， through treatment of exogenous selenium fertilizer on four kinds of crops （tomato， leaf mustard， peanut， and carrot）， we explored the selenium??enriching effects of different crops， to provide references for production of selenium??enriched agricultural products. Materials and Methods
Experimental materials
Four kinds of crops （tomato， leaf mustard， peanut， and carrot） were selenium fertilizer treated. In the experiment， microbial nano??selenium fertilizer was developed and produced by Sichuan Zhongnong Herun Technology Co.， Ltd， with the red nano??selenium content not less than 1 500 mg/kg. The tomato experiment and the leaf mustard experiment were carried out in the garden of Foshan Institute of Agricultural Sciences. The peanut experiment and the carrot experiment were carried out in Lihuoli Agricultural and Sideline Products Plantations in Datang Town of Sanshui District of Foshan City.
Experiment design
Tomato selenium??enriching experiment：
Big tomato variety was Weiyanuo， and small tomato variety was Hongguanyin.
Selenium fertilizer application scheme： in the treatment area， foliar spraying， root spraying， and root spraying were performed with 150 times， 100 times， and 100 times diluted selenium fertilizer solution on November 8， November 18， and November 30， 2017 separately. For the control area， the selenium fertilizer solution was not sprayed. In this experiment， the selenium was the only variable， and all the others adopted the same cultivation management measures.
Leaf mustard selenium??enriching experiment：
Selenium fertilizer application scheme： continuously sprayed three times with 150 times diluted selenium fertilizer solution every 7-10 days， and stopped spraying 15 days before harvesting. The specific application was as follows： in the treatment area， foliar spraying was performed with 150 times diluted selenium fertilizer solution on November 8， November 18， and November 30， 2017 separately； in the control area， no selenium fertilizer solution was sprayed. In this experiment， the selenium was the only variable， and all the others adopted the same cultivation management measures.
Peanut selenium??enriching experiment：
Selenium fertilizer application scheme： continuously sprayed three times with 200 times diluted selenium fertilizer solution every 10 days after peanut flowering.
Carrot selenium??enriching experiment：
Selenium fertilizer application scheme： in the treatment area， foliar spraying was performed with 200 times??diluted selenium fertilizer solution in the seedling stage； when carrot seeds appear， applied the selenium fertilizer through irrigation at the volume of 30 L/hm2， after 10 days， foliar spraying was performed with 150 times diluted selenium fertilizer solution， then continuously sprayed every 7 days till the volume was adequate. For the control area， the selenium fertilizer solution was not sprayed. In this experiment， the selenium was the only variable， and all the others adopted the same cultivation management measures.
Measurement indicators
The selenium content of tomato， leaf mustard， peanut， and carrot was measured using Fluorescence Spectrophotometry （GB 5009.93??2017）， and the chlorophyll content and fruit sugar content were measured for tomato.
Data processing
The experimental data was processed using EXCEL 2007， and variance analysis was carried out using SPSS 20.0 software.
Results and Analysis
At present， there is no unified selenium??enrichment standard in China. References are mainly made to the supply and marketing cooperation industry standard （GH/T1135??2017） implemented in 2017 and national food industry standard （HB001/T??2013） implemented in 2013. In the supply and marketing cooperation industry standard， the total selenium content of vegetable （dry basis）， beans （fresh basis） selenium??enriched agricultural products is 0.10-1.00 mg/kg. In the national food industry standard， the total selenium content of beans （fresh basis） and vegetable （fresh basis） selenium??enriched foods is 0.02-0.28 mg/kg and 0.01-0.09 mg/kg， respectively.
Selenium??enriching effect of different vegetables
As shown in Table 1， the selenium content of tomato， leaf mustard， peanut， and carrot treated by microbial nano??selenium fertilizer was significantly higher than that in the control group. Compared with the control group， the selenium content of big tomato （Weiyanuo） fruit increased by 13.9 times， the selenium content of small tomato （Hongguanyin） fruit increased by 11.5 times， the selenium content of leaf mustard increased by 6.35 times， the selenium content of peanut increased by 3.6 times， and the selenium content of carrot increased by 1.4 times.
In the experiment carried out in Institute of Agricultural Sciences， the selenium content was the highest in the control big tomato （Weiyanuo） fruit， followed by the leaf mustard， and it was the lowest in the small tomato （Hongguanyin）. In the experiment carried out in Lihuoli Agricultural and Sideline Products Plantations， the selenium content of peanut was higher than that of carrot.
In this experiment， the selenium content of tomato and leaf mustard before treatment was all lower than 0.10 mg/kg. After the treatment with microbial nano??selenium， the selenium content was significantly increased and reached the value specified in standard GH/T1135??2017 for selenium??enriched agricultural products. The selenium content of control peanut （fresh basis） was 0.053 mg/kg， and the selenium content of control carrot （fresh basis） was 0.014 mg/kg. According to HB001/T??2013， it has reached the selenium??enrichment standard， because Lihuoli Agricultural and Sideline Products Plantations in Datang Town of Sanshui District of Foshan City is located in the selenium??enriched area. Through using the microbial nano??selenium， it is able to greatly increase the selenium content of agricultural products.
Effects of microbial nano??selenium fertilizer on the sugar degree of tomato
The sugar degree concerns the flavor of tomato. According to Table 2， the selenium fertilizer treatment can increase the sugar degree of tomato. The sugar degree of big tomato （Weiyanuo） fruit increased 0.38， and the sugar degree of small tomato （Hongguanyin） fruit increased 0.57.
Conclusions and Discussions
（1） Through treatment of exogenous selenium fertilizer on four kinds of crops （tomato， leaf mustard， peanut， and carrot）， it indicates that exogenous application of microbial nano??selenium can significantly increase the selenium content of the crops. The study of Han Yawen shows that foliar spraying of selenium fertilizer can effectively increase the selenium content of tomato fruit. Yang Huifang carried out the foliar spraying of selenium fertilizer on tomato， carrot， and garlic， and found that it can significantly increase the total selenium， organic selenium， and inorganic selenium content of plants. These findings are consistent with the results of this experiment. In this experiment， the effect of big tomato （Weiyanuo） is the best， the foliar spraying increased the selenium content by 13.9 times， followed by the small tomato （Hongguanyin） with increase of selenium content up to 11.5 times.
（2） This experiment indicates that the microbial nano??selenium fertilizer treatment can increase the sugar content of tomato， and moderate concentration selenium treatment can effective improve the quality of tomato fruit， increase the soluble sugar content and sugar??acid ratio of tomato， which is similar to the findings of Shao et al[7] and Han et al[5].
In summary， exogenous application of microbial nano??selenium can significantly increase the selenium content of tomato， leaf mustard， peanut， and carroty， and can increase the sugar degree of tomato. In actual production， it is required to pay attention to the volume of selenium application. Some studies show that in certain range， the selenium content of plants increases with the increase of the volume of selenium application， but if the concentration is too high， the selenium concentration of plants will decline， and it will even be detrimental to plants[7-8]. In addition， different crops have different selenium??enrichment effects， and the industrialization of selenium??enriched agricultural products needs further study[9-10].References [1] PENG YX， CHEN ZF. Physiologic function of selenium and development utilization of rich selenium fruits[J]. Research of Agricultural Modernization， 2007（3）： 381-384. （in Chinese）
[2] LI N， WANG X， WAN K， et al. Research progress of effects of selenium on plant growth[J]. Guangdong Agricultural Sciences， 2010（9）： 38-40. （in Chinese）
[3] DU XF， WAN YX， WANG YH， et al. Research progress on selenium??rich vegetables and mechanism of selenium??enriched[J]. Northern Horticulture， 2015（20）： 186-188. （in Chinese）
[4] SUN XP， WU W， LUO YJ， et al. Research progress in selenium contents improvement in crop[J]. Hunan Agricultural Sciences， 2015（7）： 144-147. （in Chinese）
[5] HAN YW， HAN YY， GUO WX， et al. The effect of application of selenate and selenite to soil on the fruit quality of tomato[J]. Chinese Agricultural Science Bulletin， 2014（13）： 220-224. （in Chinese）
[6] YANG HF， LIANG XA， CHANG JT， et al. Effects of foliar spraying of selenium fertilizer on selenium enrichment of different vegetables and yield， 2014（11）： 158-161. （in Chinese）
[7] SHAO XR， HAN YY， QI CH， et al. Effects of different concentrations of selenium fertilizer on tomato quality[J]. Vegetables， 2017（8）： 25-28. （in Chinese）
[8] ZHAI ZJ， HAO LF， HAN PY， et al. Influence of exogenous selenium on selenium enrichment and yield of pakchoi[J]. Journal of Shanxi Agricultural Sciences， 2014（7）： 701-703. （in Chinese）
[9] LI N. Research status and development of selenium rich products[J]. Qinghai Prataculture， 2016（4）： 46-50. （in Chinese）
[10] XU QG， LIU HM， HUANG F. Discussions on development and popularization of selenium??enriched agricultural products in China[J]. Crop Research， 2013， 27 （5）： 461-464. （in Chinese）
Key words Microbial nano??selenium； Tomato； Leaf mustard； Peanut； Carrot； Selenium??enriching effects
As a trace element， selenium （Se） was discovered and named by Swedish scientist Berlius in 1817. The Chinese Nutrition Society listed selenium as one of the essential nutrients for the human body in 1988. Studies have shown that selenium has functions of anti??cancer and anti??oxidation， immunity improvement， prevention and treatment of cardiovascular diseases， liver protection and detoxification[1]. Besides， selenium also can regulate plant growth， increase glutathione peroxidase activity， improve stress tolerance， and improve quality and yield[2]. The Chinese Nutrition Society recommends the daily intake of selenium is 50 ??g， while international organizations recommend the daily intake of 60 ??g of selenium. Selenium cannot be synthesized in the human body. It has to be absorbed from foods to realize rapid supplementation. Crops can be transformed into safe and effective organic selenium and then be easily absorbed by human body. Taking selenium??enriched agricultural products is an important method for improving selenium shortage of human body. Most regions in China are selenium??deficient areas， only a few regions are natural selenium??enriched areas （such as Ziyang in Shaanxi and Enshi in Hubei） that can engage in the production of natural selenium??enriched agricultural products. For selenium??deficient areas， it is necessary to increase the selenium content of agricultural products through exogenous selenium addition by technical means， such as application of selenium in soil， spraying selenium on leaves， soaking seeds with selenium， and water culture of selenium fertilizer[3-4]. In this experiment， through treatment of exogenous selenium fertilizer on four kinds of crops （tomato， leaf mustard， peanut， and carrot）， we explored the selenium??enriching effects of different crops， to provide references for production of selenium??enriched agricultural products. Materials and Methods
Experimental materials
Four kinds of crops （tomato， leaf mustard， peanut， and carrot） were selenium fertilizer treated. In the experiment， microbial nano??selenium fertilizer was developed and produced by Sichuan Zhongnong Herun Technology Co.， Ltd， with the red nano??selenium content not less than 1 500 mg/kg. The tomato experiment and the leaf mustard experiment were carried out in the garden of Foshan Institute of Agricultural Sciences. The peanut experiment and the carrot experiment were carried out in Lihuoli Agricultural and Sideline Products Plantations in Datang Town of Sanshui District of Foshan City.
Experiment design
Tomato selenium??enriching experiment：
Big tomato variety was Weiyanuo， and small tomato variety was Hongguanyin.
Selenium fertilizer application scheme： in the treatment area， foliar spraying， root spraying， and root spraying were performed with 150 times， 100 times， and 100 times diluted selenium fertilizer solution on November 8， November 18， and November 30， 2017 separately. For the control area， the selenium fertilizer solution was not sprayed. In this experiment， the selenium was the only variable， and all the others adopted the same cultivation management measures.
Leaf mustard selenium??enriching experiment：
Selenium fertilizer application scheme： continuously sprayed three times with 150 times diluted selenium fertilizer solution every 7-10 days， and stopped spraying 15 days before harvesting. The specific application was as follows： in the treatment area， foliar spraying was performed with 150 times diluted selenium fertilizer solution on November 8， November 18， and November 30， 2017 separately； in the control area， no selenium fertilizer solution was sprayed. In this experiment， the selenium was the only variable， and all the others adopted the same cultivation management measures.
Peanut selenium??enriching experiment：
Selenium fertilizer application scheme： continuously sprayed three times with 200 times diluted selenium fertilizer solution every 10 days after peanut flowering.
Carrot selenium??enriching experiment：
Selenium fertilizer application scheme： in the treatment area， foliar spraying was performed with 200 times??diluted selenium fertilizer solution in the seedling stage； when carrot seeds appear， applied the selenium fertilizer through irrigation at the volume of 30 L/hm2， after 10 days， foliar spraying was performed with 150 times diluted selenium fertilizer solution， then continuously sprayed every 7 days till the volume was adequate. For the control area， the selenium fertilizer solution was not sprayed. In this experiment， the selenium was the only variable， and all the others adopted the same cultivation management measures.
Measurement indicators
The selenium content of tomato， leaf mustard， peanut， and carrot was measured using Fluorescence Spectrophotometry （GB 5009.93??2017）， and the chlorophyll content and fruit sugar content were measured for tomato.
Data processing
The experimental data was processed using EXCEL 2007， and variance analysis was carried out using SPSS 20.0 software.
Results and Analysis
At present， there is no unified selenium??enrichment standard in China. References are mainly made to the supply and marketing cooperation industry standard （GH/T1135??2017） implemented in 2017 and national food industry standard （HB001/T??2013） implemented in 2013. In the supply and marketing cooperation industry standard， the total selenium content of vegetable （dry basis）， beans （fresh basis） selenium??enriched agricultural products is 0.10-1.00 mg/kg. In the national food industry standard， the total selenium content of beans （fresh basis） and vegetable （fresh basis） selenium??enriched foods is 0.02-0.28 mg/kg and 0.01-0.09 mg/kg， respectively.
Selenium??enriching effect of different vegetables
As shown in Table 1， the selenium content of tomato， leaf mustard， peanut， and carrot treated by microbial nano??selenium fertilizer was significantly higher than that in the control group. Compared with the control group， the selenium content of big tomato （Weiyanuo） fruit increased by 13.9 times， the selenium content of small tomato （Hongguanyin） fruit increased by 11.5 times， the selenium content of leaf mustard increased by 6.35 times， the selenium content of peanut increased by 3.6 times， and the selenium content of carrot increased by 1.4 times.
In the experiment carried out in Institute of Agricultural Sciences， the selenium content was the highest in the control big tomato （Weiyanuo） fruit， followed by the leaf mustard， and it was the lowest in the small tomato （Hongguanyin）. In the experiment carried out in Lihuoli Agricultural and Sideline Products Plantations， the selenium content of peanut was higher than that of carrot.
In this experiment， the selenium content of tomato and leaf mustard before treatment was all lower than 0.10 mg/kg. After the treatment with microbial nano??selenium， the selenium content was significantly increased and reached the value specified in standard GH/T1135??2017 for selenium??enriched agricultural products. The selenium content of control peanut （fresh basis） was 0.053 mg/kg， and the selenium content of control carrot （fresh basis） was 0.014 mg/kg. According to HB001/T??2013， it has reached the selenium??enrichment standard， because Lihuoli Agricultural and Sideline Products Plantations in Datang Town of Sanshui District of Foshan City is located in the selenium??enriched area. Through using the microbial nano??selenium， it is able to greatly increase the selenium content of agricultural products.
Effects of microbial nano??selenium fertilizer on the sugar degree of tomato
The sugar degree concerns the flavor of tomato. According to Table 2， the selenium fertilizer treatment can increase the sugar degree of tomato. The sugar degree of big tomato （Weiyanuo） fruit increased 0.38， and the sugar degree of small tomato （Hongguanyin） fruit increased 0.57.
Conclusions and Discussions
（1） Through treatment of exogenous selenium fertilizer on four kinds of crops （tomato， leaf mustard， peanut， and carrot）， it indicates that exogenous application of microbial nano??selenium can significantly increase the selenium content of the crops. The study of Han Yawen shows that foliar spraying of selenium fertilizer can effectively increase the selenium content of tomato fruit. Yang Huifang carried out the foliar spraying of selenium fertilizer on tomato， carrot， and garlic， and found that it can significantly increase the total selenium， organic selenium， and inorganic selenium content of plants. These findings are consistent with the results of this experiment. In this experiment， the effect of big tomato （Weiyanuo） is the best， the foliar spraying increased the selenium content by 13.9 times， followed by the small tomato （Hongguanyin） with increase of selenium content up to 11.5 times.
（2） This experiment indicates that the microbial nano??selenium fertilizer treatment can increase the sugar content of tomato， and moderate concentration selenium treatment can effective improve the quality of tomato fruit， increase the soluble sugar content and sugar??acid ratio of tomato， which is similar to the findings of Shao et al[7] and Han et al[5].
In summary， exogenous application of microbial nano??selenium can significantly increase the selenium content of tomato， leaf mustard， peanut， and carroty， and can increase the sugar degree of tomato. In actual production， it is required to pay attention to the volume of selenium application. Some studies show that in certain range， the selenium content of plants increases with the increase of the volume of selenium application， but if the concentration is too high， the selenium concentration of plants will decline， and it will even be detrimental to plants[7-8]. In addition， different crops have different selenium??enrichment effects， and the industrialization of selenium??enriched agricultural products needs further study[9-10].References [1] PENG YX， CHEN ZF. Physiologic function of selenium and development utilization of rich selenium fruits[J]. Research of Agricultural Modernization， 2007（3）： 381-384. （in Chinese）
[2] LI N， WANG X， WAN K， et al. Research progress of effects of selenium on plant growth[J]. Guangdong Agricultural Sciences， 2010（9）： 38-40. （in Chinese）
[3] DU XF， WAN YX， WANG YH， et al. Research progress on selenium??rich vegetables and mechanism of selenium??enriched[J]. Northern Horticulture， 2015（20）： 186-188. （in Chinese）
[4] SUN XP， WU W， LUO YJ， et al. Research progress in selenium contents improvement in crop[J]. Hunan Agricultural Sciences， 2015（7）： 144-147. （in Chinese）
[5] HAN YW， HAN YY， GUO WX， et al. The effect of application of selenate and selenite to soil on the fruit quality of tomato[J]. Chinese Agricultural Science Bulletin， 2014（13）： 220-224. （in Chinese）
[6] YANG HF， LIANG XA， CHANG JT， et al. Effects of foliar spraying of selenium fertilizer on selenium enrichment of different vegetables and yield， 2014（11）： 158-161. （in Chinese）
[7] SHAO XR， HAN YY， QI CH， et al. Effects of different concentrations of selenium fertilizer on tomato quality[J]. Vegetables， 2017（8）： 25-28. （in Chinese）
[8] ZHAI ZJ， HAO LF， HAN PY， et al. Influence of exogenous selenium on selenium enrichment and yield of pakchoi[J]. Journal of Shanxi Agricultural Sciences， 2014（7）： 701-703. （in Chinese）
[9] LI N. Research status and development of selenium rich products[J]. Qinghai Prataculture， 2016（4）： 46-50. （in Chinese）
[10] XU QG， LIU HM， HUANG F. Discussions on development and popularization of selenium??enriched agricultural products in China[J]. Crop Research， 2013， 27 （5）： 461-464. （in Chinese）