Determination and Evaluation of Se-Rich High-Quality Rice Produced by Compound Nutrient Solution

Rice is one of the most important food crops in the world. Its biggest flaw is the low content of protein and essential amino acids, which severely limits its nutritional value. In order to produce high-quality rice with rich Se, we sprayed different concentrations of compound nutrient solution (containing Se (selenium), amino acid compound, zinc and boron) on the rice at different growth stages; and then determined the main nutrient content of their polished rice. The results showed that spraying low concentration compound nutrient solution (Each liter contained 20 mg of Se, 333 mg of complex amino acids, 33 mg of zinc and 33 mg of boron) to rice in the heading stage produced rice with the highest total starch and fat content and lower amylose content. Spraying high concentration compound nutrient solution to rice during the filling stage produced rice with the highest Se content. Multiple sprays of compound nutrient solution produced rice with low protein and low starch. Spraying low concentration compound nutrient solution on rice in milky stage significantly increased the content of protein, total starch, fat, all essential amino acids (Lysine increased by more than 57%), amylopectin and Se in rice; significantly reduced amylose content; significantly improved the nutritional value and taste quality of rice. The conclusion is that spraying low concentration compound nutrient solution on rice in the milky stage can produce rice with the highest content of protein and essential amino acids, higher content of total starch, fat and Se, and the lowest amylose content; significantly improve the nutritional value and taste quality of rice. The technical solution can comprehensively and effectively improve the nutritional value and flavor quality of rice, and has great development and application value.


Introduction
Rice is one of the most important food crops and staple food for more than half of the world's population. However, its biggest drawback is the low content of protein and essential amino acids, especially lysine, which severely limits its nutritional value. To improve the nutritional value of rice, researchers have made unremitting efforts Tian et al., 2004;Xie et al., 2008). Li et al. (2008) analyzed the agrobacterium-mediated high lysine transgenic rice T4 generation. Their results showed that the lysine content of transgenic rice was only about 30% higher than that of the control rice variety. Hasanuzzamana et al. (2020) reported the recent advancement in the beneficial and harmful physiological role of Se (selenium) in plants under abiotic stresses. Recent studies show that Se treatment at a low concentration exerts positive effects on plant growth, development, and yield. Thus, Se may act as a vital element by altering several physiological and biochemical processes. Studies have shown that Se-rich foods have good health effects (Dinh et al., 2008;Santos et al., 2017). Fang et al. (2013) studied the effects of Se solution on the nutritional quality of rice grains. Chen Xue et al. (2017) studied the effects of exogenous Se application on yield and quality of rice and Se distribution in plants. Cong (2011) studied the effects of Se-enriched fertilizer on rice yield and quality. Aboyeji et al. (2020) studied the effects of the zinc sulphate and boron-based foliar fertilizer on growth, yield, minerals, and heavy mtal composition of groundnut. Balawejder et al. (2019) studied the potential effects of foliar fertilizer based on calcined bones, boron and molybdenum on on maize grain production. Jiang et al. (2019) researched the effect of amino acid chelated Se nutrient liquid fertilizer on the yield and Se content of Baixiang 139 rice. Wang et al. (2003) reported the research progress of the biological effects of amino acids on higher plants. However, no major breakthrough has been made in the improvement of rice nutrients, especially essential amino acids such as lysine Tian et al., 2004;Xie et al., 2008;Su et al., 2008;Shu et al., 2010). It is of great scientific significance and application value to improve the nutritional components of rice and increase the content of essential amino acids, especially lysine. To study the technology for producing high-quality rice with rich Se, we sprayed various concentrations of compound nutrient solution on Guanglu Dwarf-4 rice at different growth stages to study the effects of the fertilization period and concentrations of compound nutrient solution on the content of protein, starch, fat, amino acid and Se. We found that spraying low concentration compound nutrient solution on rice in the milky stage can significantly improve the nutritional value and taste quality of rice.

Test Materials and Main Reagents
The rice seed is Guanglu Dwarf-4. Each liter of compound nutrient solution contains 6 g Se, 100 g complex amino acids (Prepared with soybean meal), 10 g zinc, and 10 g boron. In application, the compound nutrient solution is diluted 300 times, 200 times and 100 times to prepare low concentration (C1, Each liter contained 20 mg of Se, 333 mg of complex amino acids, 33 mg of zinc and 33 mg of boron), medium concentration (C2, Each liter contained 30 mg of Se, 500 mg of complex amino acids, 50 mg of zinc and 50 mg of boron) and high concentration (C3, Each liter contained 60 mg of Se, 1000 mg of complex amino acids, 100 mg of zinc and 100 mg of boron) compound nutrient solutions, separately.

Field Experiment
Field experiments were performed by setting 3 parallel samples per group. The design scheme is shown in Appendix A. The area of each test field is 1 m × 4 m, and the line spacing is 200 mm.
The field experiment was conducted in Tuanhu Village, Shuangjiangkou Town, Ningxiang County, Hunan Province from June to September 2018. The test field is paddy soil. The climate was hot and humid during the experiment.

Spray Scheme for Compound Nutrient Solution
The growth period affecting the yield and quality of rice is mainly the heading stage, the filling stage and the milky stage. In the different growth stages of rice, 1-3 times of compound nutrient solution was sprayed on the leaf surface and ear of rice. The amount of spraying is determined by the formation of uniform mist droplets on the surface of rice leaves and ears. The specific spraying scheme is shown in Appendix B.

Rice Sample Preparation
When Guanglu Dwarf-4 rice is ripe, we harvested their seeds. Two kilograms of rice were obtained from each test group, and the rice was made into polished rice for nutrient composition determination according to Chinese GB/T 21719-2008.

Determination of Se Content in Rice
The pre-treatment of rice Se element is carried out according to Chinese GB 5009.93-2017 regulations. The Se content of rice was determined by PF7 hydride atomic fluorescence spectrophotometer (Beijing Puxi general Instrument Co., Ltd. in China) and SP-722E visible spectrophotometer (Shanghai spectrometer Co., Ltd. in China). Each sample was measured 3 times and averaged. The parameter settings for hydride generation-atomic fluorescence spectrometry are shown in Table 1.

Determination of Main Nutrients in Rice
Total starch was determined by Chinese GB 5009.9-2016 (acid hydrolysis). Amylose was determined by Chinese GB/T 15683-2008. Crude fat was determined by Chinese GB 5009.6-2016；Crude protein was determined by Chinese GB5009.5-2016. Each sample was measured 3 times and averaged.

Determination of Amino Acid Content of Rice Protein
Hunan Food Testing and Analysis Center was entrusted to determine the amino acids content of rice protein according to the requirements of GB 5009.124-2016 using Hitachi L-8900 amino acid automatic analyzer (Shanghai baihe instrument technology Co., Ltd. in China).

Data Processing
The data is processed with Microsoft Excel and SPSS 17.0 (International Business Machines Corporation (IBM), Armonk, New York, U.S.).

Results
The content of Se, amylose, total starch, protein and fat in rice samples was determined. The results are shown in Table 2. The amino acid contents of control rice and three high-protein rice with rich Se were determined. The results are shown in Table 3. jas.ccsenet. C1 compound nutrient solution in the filling stage can significantly increase the content of essential amino acids and conditionally essential amino acids. The synthesis process of essential amino acids, especially lysine, is complicated and the synthesis speed is slow. Spraying the compound nutrient solution (such as C1) of suitable concentration in the milk maturity period after the filling period can increase the synthesis speed of essential amino acids and prolong their synthesis time, because Se, zinc and boron are components (active center) or activator of many metabolic enzymes (about 300 kinds), which can improve the activity of metabolic enzymes; and complex amino acids can provide nitrogen for the synthesis of essential amino acids (Hasanuzzamana et al., 2020;Fang et al., 2013;Balawejder et al., 2019;Niu et al., 2009;Aboyeji et al., 2020). The increase of essential amino acid content in rice, especially lysine content, can significantly improve the nutritional value of rice. Studies have shown that the increase of important nutrients in food can significantly promote the health of the body (Shen & Wu, 2017;Han et al., 2017;Wu et al., 2017). It can be seen that spraying a suitable concentration of the compound nutrient solution during a suitable growth period of rice is a good way to increase the essential amino acid content of rice and improve its nutritional value.

Mechanism of Compound Nutrient Solution Increasing the Content of Nutrients in Rice
This compound nutrient solution contains Se, zinc, boron and various amino acids. Se, zinc and boron are components or activators of many enzymes. They are related to the activities of many enzymes and have many biological functions (Fang et al., 2013;Balawejder et al., 2019;Niu et al., 2009;Aboyeji et al., 2020). Amino acids can be directly absorbed and utilized by rice, promote the metabolism of rice, and also have many biological functions (Jiang et al., 2019;Zhang et al., 2019;Wang et al., 2003). After spraying a low concentration (C1) of compound nutrient solution to rice ears and leaves during milk maturity, Se, zinc, boron and amino acids can be quickly absorbed by the rice. Through synergy, they can significantly increase the activity of many metabolic enzymes, promote photosynthesis in rice, increase the synthesis of essential amino acids, proteins, starch and fat, accelerate the conversion of amylose to amylopectin. In addition, fertilizing rice leaves and ears during the milky stage may extend the synthesis period of various nutrients in rice, especially essential amino acids (worth further study). Therefore, spraying this compound nutrient solution of appropriate concentration on rice during the milky stage can significantly increase the content of protein, starch, fat and essential amino acids, and significantly reduce amylose content, significantly improve the nutritional value and flavor of rice. Because a high concentration of Se has certain toxicity to animals and plants, spraying a high concentration of the compound nutrient solution or multiple sprays of the low concentration compound nutrient solution on rice can inhibit the synthesis of protein and starch and reduce their content (Hasanuzzamana et al., 2020).
In 2019, we used this technical solutions to plant 2,413 acres of rice in 21 villages in Ningxiang and Jingzhou counties in Hunan Province. The harvested selenium-rich rice is fragrant and delicious, and is generally welcomed by consumers, and its sales price has increased by more than 30%. The Se-rich high-quality rice produced by this technical scheme has high nutritional value and unique flavor. Its economic benefits have been significantly improved. This technical scheme surpasses the existing advanced technologies that produce rice, e.g., high lysine transgenic technology, and has great promotion and application value Xie et al., 2008;Fang et al., 2013;Jiang et al., 2019). We have achieved the great goals that agricultural scientists have not achieved for many years.

Conclusion
Spraying C1 compound nutrient solution on rice in the milky stage can produce rice (K3C1) with the highest content of protein, essential amino acids (especially lysine), higher content of total starch, fat and Se, and the lowest amylose content. The technical solution can comprehensively and effectively improve the nutritional value and flavor of rice, and has great development and application value.