The Effect of Selenium on Rice Quality Under Different Nitrogen Levels

Abstract

Selenium (Se) is a trace element that is beneficial in enhancing the quality of rice production. However, research on the effects of Se on rice quality under varying nitrogen (N) levels is limited and requires further investigation. This experiment utilized a randomized block design, incorporating an N fertilizer reduction and efficient application mode, with two N levels, CN (225 kg·hm⁻²) and LN (180 kg·hm⁻²), and three Se levels, HSe (0.12 kg·hm⁻²), LSe (0.06 kg·hm⁻²), and 0Se (0.00 kg·hm⁻²). The results indicated that the effects of Se on rice processing quality differ under different N levels. Selenium adversely affected the processing quality under the CN level, whereas it demonstrated some improvement at the LN level. Furthermore, Se application increased the Se content in rice by 46.48–141.82% and enhanced the taste value by 14.88–22.73%. It significantly improved the nutritional and cooking qualities of rice and positively influenced its appearance. Although N levels induced variations, their overall impact remained beneficial. Considering various indicators, applying 0.06 kg·hm⁻² of Na₂SeO₃ under the LN level yielded optimal results. This study provides valuable insights into the effects of Se on rice quality under different N levels. It provides a more scientific basis for the application of selenium fertilizer in rice.

1. Introduction

Rice, one of the three major food crops in the world, is a crucial food source for over half the global population [1]. Rice production is consistently increasing, with a growing emphasis on quality rather than quantity. The global trend in rice cultivation has shifted from solely pursuing high yields to focusing on both high productivity and quality [2]. Cultivating high-quality rice involves selecting superior seeds and implementing appropriate cultivation practices to yield rice with enhanced nutritional value and superior taste.

Selenium (Se), a trace element discovered in 1817 and named after the Greek goddess of the moon, has garnered considerable attention. Initially perceived as toxic, it was later identified by Rotruck in 1973 as a crucial component of the glutathione peroxidase activity center. Further research has revealed its immunomodulatory, anti-aging, anticancer, and cardiovascular protective effects. Selenium deficiency can lead to Kashin–Beck and Keshan diseases. Unlike certain nutrients, Se cannot be synthesized endogenously in the human body [3]. However, inorganic Se intake is associated with a risk of toxicity. Therefore, Se-rich rice has been developed to address this issue. By applying Se in various forms through foliar spraying or soil application, inorganic Se is metabolized to organic forms in rice, ensuring safe human consumption [4,5]. Applying Se elevates Se levels in plants and influences plant morphology, yield, and quality [6]. Studies have revealed antagonistic interactions between Se and various heavy metals, mitigating heavy metal-induced damage to some extent. For example, Se inhibits the translocation of Cd from flag leaves to rice grains [7]. Furthermore, Se enhances the activity of antioxidant enzymes in plants, thereby enhancing their stress tolerance [8]. Moreover, applying Se fertilizers can enhance rice quality. Numerous studies have demonstrated that applying Se fertilizer significantly reduces the chalkiness degree percentage (CP) and chalkiness degree (CD) of rice, thereby enhancing the processing quality of rice and improving its cooking and eating qualities [9,10,11]. Additionally, regarding nutritional quality, Se fertilizer influences protein, amylose, and soluble sugar content [12,13,14]. The most direct effect is an increase in the Se content in the grains [15]. Various forms of Se are predominantly converted into selenomethionine (SeMet) in grains after being transformed by plants [16]. When organic Se fertilizer is applied, the grains also contain selenocysteine and Se-methylselenocysteine (SeMeSeCys) [15]. The Se content of grains is directly influenced by the amount of Se applied. Therefore, the application rate of Se fertilizer should be maintained within a reasonable range to ensure that the Se content in rice does not exceed the standard, thus preventing it from surpassing the appropriate daily intake for humans [17]. Previous studies have indicated that the best results are achieved through foliar application of Se fertilizer at the heading stage [18], with an optimal application rate of 25 g ha⁻¹ [9]. Moreover, Se enhances rice quality and significantly improves the quality of rice products. Processing Se-enriched rice into parboiled rice can enhance its umami and flavor [19]. Adding Se during rice cake production can increase the content of sulfur compounds, alcohols, alkanes, and ketones [20].

In summary, Se has numerous beneficial effects on rice quality and has broad application prospects. A substantial supply of nitrogen (N) is essential during rice growth and development, making applying N fertilizers indispensable for actual production [21]. Moreover, reasonable N fertilizer application measures are key to improving rice quality [22]. As a scientific fertilization approach, a simplified and efficient N fertilizer application model can ensure stable yields while effectively enhancing rice quality [23]. Nitrogen also significantly affects nutrient accumulation in grains and the absorption and utilization of trace elements. Relevant studies have indicated that N directly influences the mineral concentration in rice grains, increasing the concentrations of elements, such as copper (Cu), iron (Fe), magnesium (Mg), and zinc (Zn) [24,25]. Different N supply levels may also influence the effects of Se on rice plants. Limited research has been conducted on the effect of Se on rice quality under varying N supply levels, indicating the need for further studies to address this issue. This experiment is dedicated to exploring whether there are differences in the effects of selenium fertilizer on rice quality under different nitrogen supply levels.

In this study, we investigated the effects of Se fertilizer on rice quality at varying N levels by examining the processing, appearance, nutritional, cooking, and eating qualities of rice under a simplified and efficient N fertilizer application mode. The aim was to provide a scientific basis for high-quality rice cultivation techniques.

2. Materials and Methods

2.1. Test Materials

The test material was a high-quality Japanese rice variety, Akita Komachi, characterized by 15 leaves on the main stem, a plant height of approximately 100 cm, a total growth period of approximately 150 days, a curved panicle type, strong tillering ability, and classified as premium rice.

The fertilizers used were urea (N content of 46%), potassium chloride (K₂O content of 60%), and superphosphate (P₂O₅ content of 12%). All are produced by China Agricultural Production Materials Group Co., Ltd. (Beijing, China), sodium selenite (Na₂SeO₃ content of 99.99%, Sinopharm Chemical Reagent Co., Ltd., Beijing, China). All fertilizers are provided by the Rice Research Institute of Shenyang Agricultural University.

2.2. Study Sites

The field experiment was conducted at the experimental base of the Rice Research Institute of Shenyang Agricultural University (41°49′ N, 123°34′ E, altitude 41.60 m) in 2021 and 2022. This region is characterized by a temperate continental monsoon climate and features brown soil with medium fertility. The soil fertility conditions are listed in

Table 1. Soil chemical properties for 2021–2022.

Year	pH	Organic Matter (g·kg−1)	Total N (%)	Available K (mg·kg−1)	Olsen-P (mg·kg−1)	Selenium Content (mg·kg−1)	Water-Soluble Selenium (mg·kg−3)	Exchangeable Selenium (mg·kg−3)	Fulvic Acid-Bound Selenium (mg·kg−3)
2021	7.05	25.10	0.11	143.20	35.30	0.23	0.45	1.24	9.39
2022	6.50	39.30	0.16	184.30	87.20	0.59	0.44	6.71	40.46

. The physical and chemical properties of the soil were determined by the Analysis and Testing Center of Shenyang Agricultural University, which also provided the determination methods. Detailed determination methods can be found in the Supplementary Materials.

2.3. Experimental Design

The experiment utilized a randomized block design featuring two N levels, CN (225 kg·hm⁻²) and LN (180 kg·hm⁻²), alongside three Se fertilizer levels, HSe (0.12 kg·hm⁻²), LSe (0.06 kg·hm⁻²), and 0Se (0.00 kg·hm⁻²), resulting in six treatments. Nitrogen fertilizer was applied using a simplified and efficient method with a base fertilizer:tiller fertilizer:panicle fertilizer ratio of 3:3:4. Tiller fertilizer was applied when the leaf age process reached 60% (at the 9-leaf stage), whereas panicle fertilizer was applied when the leaf age process reached 80% (at the 12-leaf stage). Selenium fertilizer was applied using a combination of basal soil application and foliar spraying. Base fertilizer is applied before transplanting, and foliar fertilizer is sprayed at the full heading stage. The specific application schemes are listed in

Table 2. Amount and period of fertilization for each treatment.

Treatment		Basal Fertilizer (kg·hm−2)				Tillering Fertilizer (kg·hm−2)	Panicle Fertilizer (kg·hm−2)
N	Se	N	P	K	Se	N	N	K	Se
CN	Hse	67.50	112.50	56.25	0.09	67.50	90.00	56.25	0.03
	Lse	67.50	112.50	56.25	0.03	67.50	90.00	56.25	0.03
	0Se	67.50	112.50	56.25	-	67.50	90.00	56.25	0.03
LN	Hse	54.00	112.50	56.25	0.09	54.00	72.00	56.25	0.03
	Lse	54.00	112.50	56.25	0.03	54.00	72.00	56.25	0.03
	0Se	54.00	112.50	56.25	-	54.00	72.00	56.25	0.03

Foliar selenium fertilizer was applied during the full heading stage. Base fertilizer was administered one day prior to transplanting. Tillering fertilizer was applied at the ninth leaf stage, while panicle fertilizer was applied at the twelfth leaf stage. The fertilizer is uniformly mixed with sandy soil and applied evenly by the method of broadcasting.

. The same fertilization method was implemented in both years to ensure experimental accuracy. The area of each plot is 12 square meters (3 m × 4 m), with three replicates. Each plot was separated using black PVC partitions that were driven into the soil to a depth of 20 cm to guarantee the isolation of water and fertilizer. Plant spacing was set at 30 cm × 13.3 cm, with two seedlings per hole. Each plot was equipped with an independent irrigation and drainage system.

2.4. Measurement Items and Methods

2.4.1. Measurement of Rice Processing Quality

When the rice is mature, harvest 3 m² of rice for each treatment. Store the harvested paddy in a dark, dry, and ventilated place for 3 months. Once the moisture content dropped to approximately 14.00%, 300 g of rice grains were weighed and processed into brown rice using an FC-2K experimental husker (Yamamoto, Tendo-shi, Japan). Brown rice yield (BRR) was calculated by weighing the product. Brown rice was processed using a VP-32 rice mill (Yamamoto, Tendo-shi, Japan) with a flow rate of 5 and a purity of 2. The processed white rice was weighed, and the milled rice rate (MRR) was calculated.

2.4.2. Measurement of Rice Appearance Quality

An ES-1000 rice appearance analyzer (Shizuoka, Fukuroi, Japan) was used to randomly select and measure more than 1000 grains of milled rice. This instrument assesses the head rice rate (HRR), chalkiness degree (CD), and chalkiness percentage (CP) of milled rice.

2.4.3. Cooking and Eating Quality of Rice

Next, 30 g of the polished rice was weighed and washed until the water was clear. The rice was soaked for 30 min, after which 42 g of pure water was added. The rice was covered with a paper lid and steamed for 30 min. After steaming, the heat was turned off and the sample was allowed to sit for 10 min. Next, the paper lid was removed, and the rice was stirred thoroughly and covered again with the paper lid. The mixture was then allowed to cool in a fume hood for 20 min. The paper lid was then replaced with a metal lid and allowed to cool to room temperature for 90 min. Finally, 8 g of the sample was pressed into cakes, and the balance, appearance, hardness, viscosity, and taste value were evaluated using an STA-1A rice taste analyzer (SATAKE, Saitama, Japan).

2.4.4. Nutritional Quality of Rice

Selenium Content in Rice

Take a sufficient amount of polished rice samples and grind them in a ball mill. After grinding, sieve them through a 100-mesh sieve and retain the polished rice powder for subsequent determination. Subsequently, 0.50 g of the polished rice powder was weighed into a glass test tube, and 1 mL of hydrogen peroxide and 6.00 mL of nitric acid were added, followed by microwave digestion. After digestion, the mixture was allowed to cool, the acid was removed, and the solution was diluted to the desired volume. The Se content in the polished rice was determined using an Agilent 7500 inductively coupled plasma mass spectrometer (China). The determination was carried out by the Analysis and Testing Center of Shenyang Agricultural University, which also provided the methodology.

Determination of Rice Protein Components

A portion (0.10 g) of the polished rice powder was weighed. Albumin, globulin, prolamin, and glutenin were extracted sequentially and quantified using Bradford’s BCA protein quantification method [26]. A BCA protein quantification kit (Suzhou Keming Biotechnology Co., Ltd., Suzhou, China) was used to determine the protein content of each component.

Determination of Amino Acid Content in Rice

A portion (0.02 g) of the polished rice powder was measured into a hydrolysis tube, and 15 mL of 6 mol mL⁻¹ hydrochloric acid and one drop of n-octanol defoamer were added. After evacuating for 10 min, the hydrolysis tube was sealed and placed in a drying oven set to 110 °C for 24 h for hydrolysis. After cooling, the volume of the solution was adjusted, the acid removal process was repeated thrice, and the sample was transferred to a sample tube. Samples were analyzed using a Hitachi L8800 automatic amino acid analyzer (Hitachi, Tokyo, Japan).

2.5. Data Processing

When measuring the samples used, three replicates are included to ensure analysis of variance (ANOVA) can be performed. The data were recorded and organized using Microsoft Excel 2021, processed and statistically analyzed using SPSS 22.0 (Softonic International, Barcelona, Spain), and graphs were drawn using Origin 2024 (OriginLab, Northampton, MA, USA). The correlation heat map was drawn using ChiPlot (https://www.chiplot.online/, accessed on 3 May 2025). Correlation analysis was performed using the pearsonr—mantel test. Principal component analysis was conducted using Origin 2024.

3. Results

3.1. Effect of Se on Rice Processing and Appearance Quality Under Different Nitrogen Levels

The effects of Se on the processing and appearance of rice are shown in Figure 1. Under the CN level, using Se fertilizer significantly improved the appearance quality, specifically CD and CP, while having a minimal impact on processing quality, which includes BRR, MRR, and HRR. For CP, the HSe and LSe treatments reduced the rate by 50.52% and 44.33%, respectively, compared to that under the 0Se treatment in 2021, whereas reductions of 23.72% and 35.90% were observed in 2022. Regarding CD, reductions were 56.67% and 50.00% in 2021, and 48.00% and 64.00% in 2022. Under the LN level, Se fertilizer significantly decreased the CD, although other aspects of appearance and processing quality did not reach significance. In 2021, the HSe and LSe treatments reduced CD by 20.00% and 28.00%, respectively, compared to the treatment without Se fertilizer. Additionally, rice processing quality was influenced by N supply levels, with LN levels outperforming CN application levels. Applying Se fertilizer significantly reduced the CD and CP of rice over the two years, with a minor impact on processing quality. Among the different N supply levels, LSe treatment yielded the best results.

3.2. Effects of Se on Rice Nutritional Quality Under Different Nitrogen Levels

3.2.1. Effects of Se on Rice Se Content Under Different Nitrogen Levels

The effects of N, Se, and their interactions were significant or highly significant in both years (Figure 2). Under the CN levels, N application significantly increased the Se content in rice, with the overall performance being ranked as HSe > LSe > 0Se. In 2021, the Se and LSe treatments resulted in an increase in the Se content of 141.82% and 91.82%, respectively, compared to that under the 0Se treatment. In 2022, these increases are 94.31% and 87.80%, respectively. Under the LN level, Se application significantly enhanced Se content in rice. In contrast to the CN level, the overall performance was ranked LSe > HSe > 0Se. In 2021, the Se and LSe treatments increased the Se content by 72.26% and 130.66%, respectively, compared to that under the 0Se treatment, whereas the increases were 46.48% and 106.34%, respectively, in 2022. Overall, the Se content in rice was influenced by N supply levels, exhibiting variations under different N supply conditions.

3.2.2. Effects of Se on Rice Protein Components Under Different Nitrogen Levels

Nitrogen significantly affected rice protein components, except for globulin in 2021 and glutenin in 2022. Additionally, the effect of Se on the rice protein components reached a significant level, except for prolamin in 2021 (Figure 3). Furthermore, the interaction effects of N and Se significantly influenced each protein component in 2021, except for globulin. Overall, variations were observed between the years. Under the CN level, applying Se fertilizer in 2021 significantly reduced the content of albumin and globulin, while significantly increasing the glutenin content. In 2022, applying Se fertilizer enhanced the content of each rice component, achieving a significant level of albumin. Conversely, under LN conditions, applying Se fertilizer in 2021 significantly decreased the content of various protein components in rice other than glutelin. However, applying Se fertilizer significantly increased the content of various protein components in rice, excluding globulin, In 2022. Notably, under the same Se level, the protein content in the CN treatment was generally higher than that in the LN treatment, indicating that N also influences the protein content in rice.

3.2.3. Effects of Se on Rice Amino Acid Content Under Different Nitrogen Levels

Applying Se fertilizer enhanced the content of essential amino acids in rice (

Table 3. Presents the effect of selenium on the content of essential amino acids in rice under varying nitrogen levels.

Year	Treatment	Val	Met	Ile	Leu	Tyr	Phe	Lys
2021	CNHSe	4.08 ± 0.08 a	0.54 ± 0.02 bc	2.10 ± 0.02 bc	6.46 ± 0.08 a	2.51 ± 0.12 a	5.60 ± 0.05 a	2.24 ± 0.03 b
	CNLSe	3.79 ± 0.03 b	0.52 ± 0.02 bc	2.10 ± 0.10 bc	6.17 ± 0.16 b	2.41 ± 0.12 ab	5.22 ± 0.04 b	2.10 ± 0.03 c
	CN0Se	3.30 ± 0.06 d	0.59 ± 0.02 ab	1.89 ± 0.04 d	5.72 ± 0.12 c	2.25 ± 0.05 bc	5.33 ± 0.14 b	1.92 ± 0.04 d
	LNHSe	3.60 ± 0.06 c	0.54 ± 0.02 bc	2.15 ± 0.05 b	5.78 ± 0.03 c	1.94 ± 0.09 d	4.10 ± 0.04 d	2.44 ± 0.04 a
	LNLSe	4.23 ± 0.08 a	0.47 ± 0.04 c	2.40 ± 0.02 a	6.72 ± 0.04 a	2.08 ± 0.10 cd	4.54 ± 0.06 c	1.83 ± 0.03 e
	LN0Se	3.47 ± 0.02 cd	0.68 ± 0.06 a	1.96 ± 0.04 cd	5.50 ± 0.12 c	2.00 ± 0.05 d	4.38 ± 0.11 c	1.76 ± 0.04 f
	F-value
	N	0.88 ns	0.28 ns	9.25 *	2.56 ns	55.18 **	419.07 **	48.62 **
	Se	59.56 **	9.52 **	16.87 **	43.97 **	2.15 ns	0.11 ns	742.39 **
	N*Se	32.69 **	2.14 ns	3.16 ns	24.25 **	3.43 ns	23.19 **	162.30 **
2022	CNHSe	4.05 ± 0.08 c	0.74 ± 0.08 b	2.82 ± 0.06 c	6.71 ± 0.15 c	3.87 ± 0.09 d	4.26 ± 0.11 d	2.85 ± 0.06 b
	CNLSe	4.01 ± 0.08 cd	0.62 ± 0.03 c	2.74 ± 0.06 d	6.44 ± 0.14 e	3.69 ± 0.09 f	4.05 ± 0.11 e	2.78 ± 0.06 c
	CN0Se	3.59 ± 0.07 e	0.77 ± 0.03 b	2.73 ± 0.06 d	6.61 ± 0.14 d	3.83 ± 0.09 e	4.49 ± 0.12 b	2.60 ± 0.06 f
	LNHSe	4.22 ± 0.08 b	0.90 ± 0.04 a	2.95 ± 0.07 b	7.08 ± 0.15 b	4.32 ± 0.10 a	4.56 ± 0.12 a	3.03 ± 0.06 a
	LNLSe	4.34 ± 0.08 a	0.69 ± 0.06 bc	3.01 ± 0.07 a	7.10 ± 0.15 a	4.07 ± 0.10 b	4.48 ± 0.12 b	2.74 ± 0.06 d
	LN0Se	3.90 ± 0.02 d	0.93 ± 0.08 a	2.67 ± 0.06 e	6.40 ± 0.14 f	3.94 ± 0.10 c	4.32 ± 0.11 c	2.63 ± 0.06 e
	F-value
	N	76.13 **	24.53 **	2089.43 **	2544.76 **	5218.00 **	2130.55 **	385.06 **
	Se	77.43 **	20.89 **	2285.40 **	1763.16 **	1109.44 **	518.45 **	4719.20 **
	N*Se	3.04 ns	1.40 ns	1482.39 **	2193.30 **	570.67 **	2117.17 **	576.56 **

Different letters denote significant differences between treatments, with p-values less than 0.05. Note: * and ** indicate significant differences at the 0.05 and 0.01 levels. Valine (Val); methionine (Met); isoleucine (Ile); leucine (Leu); tyrosine (Tyr); phenylalanine (Phe); lysine (Lys).

). Under the CN level, Se fertilizer significantly increased the contents of Val, Ile, and Lys (although the increase in Ile did not reach significant levels in 2022), specifically manifested as HSe > LSe > 0Se. Conversely, it reduced the Met and Phe contents, with the reduction in Phe reaching significant levels in 2022, represented as 0Se > HSe > LSe. Under the LN level, applying Se fertilizer increased the contents of Val, Ile, Leu, and Lys, achieving significant levels in 2022, specifically shown as LSe > HSe > 0Se (except Lys). While Se fertilizer reduced Met, this effect did not reach a significant level, represented as 0Se > HSe > LSe. Overall, applying Se fertilizer increased the content of Val, Ile, and Lys, which are essential amino acids for humans, while decreasing the Met content. Additionally, the effect of Se on rice amino acids was influenced by the N supply level, demonstrating certain variations.

Regarding non-essential amino acids, applying Se fertilizer generally increased the content of each amino acid (

Table 4. The effect of selenium on the content of non-essential amino acids in rice under varying nitrogen levels.

Year	Treatment	Asp	Thr	Ser	Glu	Gly	Ala	Cys	His	Arg	Pro
2021	CNHSe	6.74 ± 0.06 a	1.68 ± 0.06 bc	2.56 ± 0.10 a	11.55 ± 0.22 a	3.12 ± 0.02 a	3.40 ± 0.10 a	2.26 ± 0.01 b	1.36 ± 0.01 a	7.53 ± 0.09 a	2.78 ± 0.05 a
	CNLSe	6.32 ± 0.16 ab	1.60 ± 0.02 bc	2.50 ± 0.09 a	11.19 ± 0.08 a	2.84 ± 0.01 b	3.22 ± 0.09 ab	2.11 ± 0.01 c	1.27 ± 0.03 bc	6.96 ± 0.09 b	2.54 ± 0.04 b
	CN0Se	6.29 ± 0.16 ab	1.57 ± 0.02 bc	2.47 ± 0.05 a	11.19 ± 0.23 a	2.86 ± 0.02 b	3.08 ± 0.07 bc	1.82 ± 0.08 d	1.21 ± 0.03 c	6.40 ± 0.20 c	2.48 ± 0.06 b
	LNHSe	5.93 ± 0.09 bc	2.12 ± 0.08 a	2.40 ± 0.09 a	8.16 ± 0.09 c	1.73 ± 0.01 d	3.16 ± 0.06 b	2.05 ± 0.01 c	1.33 ± 0.02 ab	6.51 ± 0.17 c	2.09 ± 0.06 c
	LNLSe	6.45 ± 0.12 a	1.55 ± 0.02 c	2.19 ± 0.08 b	9.80 ± 0.22 b	2.22 ± 0.01 c	2.89 ± 0.01 c	2.39 ± 0.01 a	1.35 ± 0.01 a	6.33 ± 0.11 c	2.25 ± 0.04 c
	LN0Se	5.65 ± 0.14 c	1.69 ± 0.03 b	2.42 ± 0.05 a	9.36 ± 0.19 b	2.25 ± 0.02 c	3.05 ± 0.07 bc	1.77 ± 0.08 d	1.20 ± 0.03 c	6.29 ± 0.06 c	2.12 ± 0.06 c
	F-value
	N	15.69 **	23.40 **	10.10 *	223.88 **	8215.31 **	15.46 **	0.01 ns	0.97 ns	31.98 **	108.86 **
	Se	5.65 *	28.73 **	2.41 ns	6.50 *	65.41 **	8.32 *	77.46 **	21.08 **	14.24 **	3.79 ns
	N*Se	6.82 *	15.14 **	1.86 ns	17.22 ns	712.88 **	3.19 ns	21.41 **	3.37 ns	6.38 *	8.36 **
2022	CNHSe	7.52 ± 0.19 d	2.41 ± 0.06 d	3.64 ± 0.08 e	14.90 ± 0.30 e	2.70 ± 0.02 e	4.37 ± 0.10 d	0.25 ± 0.01 a	1.19 ± 0.03 c	8.21 ± 0.26 b	2.98 ± 0.08 c
	CNLSe	7.31 ± 0.18 e	2.26 ± 0.06 f	3.50 ± 0.07 f	14.47 ± 0.30 f	2.60 ± 0.02 f	4.22 ± 0.09 f	0.23 ± 0.01 c	1.13 ± 0.03 d	7.98 ± 0.25 c	2.82 ± 0.07 f
	CN0Se	7.60 ± 0.19 c	2.39 ± 0.06 e	3.74 ± 0.08 d	15.47 ± 0.32 d	2.86 ± 0.02 c	4.34 ± 0.10 e	0.21 ± 0.01 d	1.13 ± 0.03 d	7.80 ± 0.24 d	2.91 ± 0.08 d
	LNHSe	8.08 ± 0.20 a	2.60 ± 0.07 a	3.98 ± 0.08 b	15.95 ± 0.33 a	2.90 ± 0.03 b	4.65 ± 0.10 a	0.24 ± 0.01 b	1.27 ± 0.04 a	8.76 ± 0.27 a	3.16 ± 0.08 a
	LNLSe	7.99 ± 0.20 b	2.49 ± 0.07 c	3.84 ± 0.08 c	15.58 ± 0.32 b	2.72 ± 0.02 d	4.54 ± 0.10 c	0.24 ± 0.01 b	1.20 ± 0.03 b	8.78 ± 0.28 a	3.11 ± 0.08 b
	LN0Se	7.25 ± 0.18 f	2.58 ± 0.07 b	4.15 ± 0.09 a	15.54 ± 0.32 c	3.02 ± 0.03 a	4.62 ± 0.10 b	0.20 ± 0.01 e	1.09 ± 0.03 e	8.25 ± 0.26 b	2.86 ± 0.07 e
	F-value
	N	1813.95 **	5305.32 **	8025.21 **	6911.03 **	21,063.09 **	8841.53 **	0.60 ns	561.30 **	3402.90 **	2291.22 **
	Se	958.59 **	829.81 **	1601.91 **	1091.92 **	22,233.48 **	614.65 **	1147.90 **	2084.00 **	745.54 **	1383.92 **
	N*Se	2127.98 **	27.09 **	33.93 **	1412.05 **	357.59 **	20.38 **	101.96 **	668.89 **	104.06 **	1199.10 **

Different letters denote significant differences between treatments, with p-values less than 0.05. Note: * and ** indicate significant differences at the 0.05 and 0.01 levels. Aspartic acid (Asp); threonine (Thr); serine (Ser); glutamic acid (Glu); glycine (Gly); alanine (Ala); cysteine (Cys); histidine (His); arginine (Arg); proline (Pro).

). Under the CN level, Se fertilizer significantly enhanced the contents of Cys, His, and Arg (although His did not reach a significant level), with the specific order being HSe > LSe > 0Se. In 2022, applying Se fertilizer significantly reduced the levels of Asp, Ser, Glu, and Gly. Under the LN level, Se fertilizer significantly increased the content of Cys, His, and Arg (notably, Arg did not reach a significant level in 2021), with the content of Cys following the order LSe > HSe > 0Se, whereas His and Arg exhibited interannual differences. Applying Se fertilizer reduced the Ser and Gly contents, although the overall decrease was not statistically significant. Specifically, the Ser content followed the order 0Se > HSe > LSe, whereas Gly showed interannual variations. Overall, applying Se fertilizer increased the content of Cys, His, and Arg, while decreasing the content of Ser and Gly. Furthermore, the effect of Se on the amino acid content of rice was influenced by N supply levels, with variations observed across different N supply conditions.

3.3. Effect of Se on the Cooking and Eating Quality of Rice Under Different Nitrogen Levels

Nitrogen and Se significantly influence the cooking and eating quality of rice, and their interactions also achieved a notable level of significance in 2022. The observed trends were generally consistent over the two years(Figure 4). Under the CN level, adding Se fertilizer significantly improved the cooking and eating quality of rice, enhancing its appearance, balance, and taste value while reducing its hardness. In 2021, the HSe and LSe treatments resulted in increases of 20.24% and 14.88%, respectively, compared with those under the 0Se treatment, while it showed increases of 16.38% and 16.95%, respectively, in 2022. Under the LN level, applying Se fertilizer further improved appearance, balance, viscosity, and taste value, with significant enhancements in appearance, balance, and taste value. Although the hardness was slightly reduced, this change was not statistically significant. In 2021, the HSe and LSe treatments yielded increases of 20.90% and 20.62%, respectively, compared to that under the 0Se treatment, whereas they increased by 22.73% and 17.17%, respectively, in 2022. Furthermore, at the same Se level, a 20% reduction in N application resulted in better outcomes than conventional N application. The optimal treatment over two years was identified as the LNHSe treatment.

3.4. Correlation and Principal Component Analysis of Se on Rice Quality Under Different Nitrogen Levels

Under the CN level, the Se content in rice exhibited a significant negative correlation with CD in appearance and a significant or highly significant positive correlation with Val, Lys, and Arg in amino acids (Figure 5). Additionally, it demonstrated a significant negative correlation with globulin in protein components; a significant or highly significant positive correlation with appearance, balance, and viscosity in cooking and eating quality; and a highly significant negative correlation with hardness. The correlations of albumin and gliadin were notably similar, presenting significant or highly significant positive correlations with processing (HRR) and appearance quality (CD, and CP). They also display significant or highly significant negative correlations with Cys, Phe, and His in amino acids while showing significant positive correlations or no correlation with other amino acids. Conversely, globulin and prolamins exhibited a significant negative correlation with processing quality (HRR), except for globulin, and a significant negative correlation with CD and CP regarding appearance quality (excluding globulin). Globulin showed highly significant positive correlations with Gly and Phe, whereas glutenin showed highly significant positive correlations with Cys, Phe, and His. The remaining amino acids showed either a significant negative or no correlation with globulin or glutenin. Selenium, globulin, and Val, Leu, and Arg contents were strongly correlated with taste quality.

Under the LN level, Se content in rice exhibited significant and highly significant positive correlations with amino acids (Cys, Val, Leu, and His), a significant negative correlation with Met, and significant negative correlations with protein components (albumin and globulin). In addition, a highly significant positive correlation was observed with the glutelin levels. Furthermore, the Se content showed significant positive correlations with the appearance and balance of cooking and eating quality in rice. Among the protein components, albumin and prolamin displayed highly significant positive correlations with processing quality and significant negative correlations with Cys and His among the amino acids. The remaining amino acids demonstrated a significant positive correlation or no correlation. Both globulin and glutelin exhibited highly significant positive correlations with the processing quality (HRR), with glutelin showing highly significant positive correlations with Cys and His. In contrast, the other amino acids displayed significant negative correlations or no correlations with globulin and glutelin. Selenium content, HRR, globulin, and amino acids (Asp, Thr, Glu, Ala, Val, Ile, Leu, Thr, Lys, Arg, and Pro) were strongly correlated.

Considering both N levels, Se content, HRR, globulin, and amino acids (Asp, Thr, Ala, Val, Ile, Leu, Thr, Lys, Arg, and Pro) were strongly correlated (Figure S1).

Principal component analysis was conducted by selecting rice processing quality (HRR), appearance quality (CD and CP), nutritional quality (content of seven essential amino acids and Se content), and cooking and eating quality (taste value). The results (Figure 6) showed that under conventional N application levels, there was a notable overlap between the HSe and LSe treatments, indicating similarity. In contrast, they were highly separated from the 0Se treatment, suggesting that Se has a significant effect on rice quality under conventional N application levels. Under the CN level, there was a notable overlap between the HSe and LSe treatments and between the HSe and 0Se treatments. However, the separation between LSe and 0Se was relatively high, indicating that Se significantly affected rice quality under LN conditions. At the same Se level, the overlap among treatments was relatively high, indicating a strong similarity between these treatments. Based on a comprehensive analysis of various indicators, the LN+LSe treatment performed the best and was considered the optimal treatment for this experiment.

4. Discussion

4.1. Effects of Se on Rice Processing and Appearance Quality Under Different Nitrogen Levels

Selenium application significantly improved various rice quality indicators, although differences were observed under varying N supply levels. Processing quality is a crucial aspect of rice quality and an essential measure for determining rice grades. Previous studies have demonstrated that applying Se fertilizer can increase the chlorophyll content and transpiration rates in leaves, thereby enhancing the growth conditions of seedlings [27,28]. Furthermore, when combined with silicon (Si) or Zn, Se can enhance both the net photosynthetic rate and photosynthetic efficiency [14,29,30], subsequently improving the synthesis and accumulation of starch and enhancing the processing quality of rice [16]. The findings of this experiment differ from those of previous studies. In this study, applying Se fertilizer reduced the BRR, MRR, and HRR of rice, adversely affecting the processing quality. This effect was particularly pronounced in 2021, whereas the reduction was somewhat alleviated in 2022, and there was even a slight improvement under the LN level. This may be attributed to the increased Se content in the soil in 2022, particularly the increase in soluble Se content, with more Se present in the soil in the form of organic Se, which mitigates the adverse effects of Se on rice processing quality [31]. Furthermore, this experiment utilized sodium selenite as the Se source, whereas previous studies predominantly used selenomethionine. This difference also supported the improved processing quality observed in 2022. This indicated that organic Se is more conducive to enhancing the processing quality of rice than inorganic Se. Additionally, the processing quality of rice is influenced by N, and an appropriate level of N can enhance processing quality [22]. The effect of Se on processing quality varies under different N supply levels, and the specific reasons and mechanisms warrant further research.

Appearance quality, a crucial component of the rice evaluation system, plays a significant role in quality assessment. Previous studies have demonstrated that Se fertilizer is beneficial for enhancing the appearance of rice, significantly reducing both CD and CP [14]. Application of various concentrations of chelated Se notably decreased the CD and CP of fragrant rice. Furthermore, EDTA-Se substantially improves the quality of fragrant rice [13]. The results of this experiment align closely with previous findings. In this study, applying Se fertilizer reduced both CD and CP, thereby enhancing the appearance quality of rice. Notably, the reduction in CP was particularly pronounced under the CN level but was not significant under the LN level. This suggested that while Se significantly influences the reduction of CP, its effectiveness is also affected by N supply [32]. Applying Se fertilizer significantly reduced CD across both N levels, indicating that Se has a marked effect on decreasing CD, which is less susceptible to variations in N supply. Overall, Se fertilizer improved the processing and appearance quality of rice to some extent. However, its effectiveness is influenced by the N supply level, potentially related to the absorption of Se and expression of the root NRT1.1B gene [32].

4.2. Effects of Se on the Nutritional Quality of Rice Under Different Nitrogen Levels

Selenium significantly enhances the nutritional quality of rice. As a trace element, it can be toxic to both plants and humans when present in excessive amounts, with inorganic Se posing a greater hazard than its organic form [4]. During plant production, most Se is transformed from its inorganic to organic form, resulting in its presence in various parts of the rice plant. In rice grains, Se predominantly exists as SeMet [16]. SeMet is the primary component involved in the absorption, utilization, and translocation of Se in rice plants [32]. Furthermore, when the Se source is an organic fertilizer, it can also be found in the form of methylselenocysteine (SeMeCys), selenocystine (SeCys₂), and SeMeSeCys within the grains [15,33]. Additionally, applying Se fertilizer positively influences the amino acid content of rice [12,34]. This experiment demonstrated that applying Se fertilizer significantly increased the Se content in rice grains, which is consistent with the results of previous studies [31]. Simultaneously, applying Se fertilizer enhanced the levels of Val, Arg, Leu, and Lys, which are essential amino acids for humans; however, the Met content decreased. This decrease may be attributed to the fact that Se is primarily synthesized and transported in plants in the form of SeMet, with Met in grains coexisting with Se as SeMet. Lys, Thr, and Met have been identified as the first, second, and third most limiting amino acids, respectively, in the human body, highlighting their critical importance in human health. Notably, Lys is a vital indicator of nutritional quality [31,32]. The application significantly enhanced the nutritional quality of rice. Furthermore, using Se fertilizer increased the content of non-essential amino acids, including Cys, His, and Arg. However, the overall Cys content in 2022 was relatively low, potentially because of the increased organic Se content in the soil, which enhances the conversion of selenocysteine (SeCys) and results in a greater amount of Cys combining with Se and existing in the form of SeCys [15]. Applying Se markedly increased the content of aspartate (Lys and Ile) and pyruvate (Val and Leu) family amino acids. This enhancement is likely attributable to the involvement of Se in the synthesis and metabolism of aspartate and pyruvate family amino acids, which play crucial roles in these processes. Additionally, Val, Leu, and Ile are branched-chain amino acids characterized by small hydrocarbon side chains that are regulated by the same enzymes, suggesting that applying Se fertilizer may enhance the activity of these enzymes. Although this study revealed the potential promoting effects of Se on amino acid accumulation, the specific pathways and molecular mechanisms involved remain unclear, necessitating further in-depth metabolomic and functional genomic studies.

The main storage proteins in rice are albumin, globulin, prolamins, and glutelin. Albumin and globulin were predominantly distributed in the pericarp, seed coat, and aleurone layers and exhibited high levels of Lys and Thr. However, these proteins are susceptible to loss during processing, significantly reducing their nutritional value. Prolamins and glutelin are located in the endosperm. Although prolamins have an overall low nutritional value regarding amino acid content, glutelin, the primary storage protein in rice, is rich in various amino acids and constitutes a significant component of the nutritional quality of rice [35]. The results of this experiment indicated that Se application significantly increased the glutelin content in rice, although its effects on the other three protein components varied across different years. For prolamin, the trends were inconsistent over the two years studied. Gliadin showed a highly significant positive correlation with processing quality, and the notable increase in gliadin content in 2022 may be a crucial factor contributing to the improvement in processing quality in that year. This was likely due to the enhancement of soil Se and the availability of Se. Glutenin exhibited a highly significant negative correlation with processing quality and was negatively correlated with CD and CP. This may explain why Se is beneficial for improving the appearance quality of rice but adversely affects processing quality. Furthermore, glutenin is influenced by N; as the N levels increase, the glutenin content in rice also increases [36]. However, an excessively high glutenin content can significantly degrade rice quality [37]. The effect of Se on rice protein components varied slightly under different N supply levels; however, the overall effect remained minimal.

The cooking and eating qualities of rice are crucial indicators of its palatability. Studies have demonstrated that applying Se fertilizer significantly enhances the taste of rice [10] and increases the content of 2-acetyl-1-pyrroline (2-AP), a flavor compound in rice [30]. This effect is particularly pronounced in aromatic rice production [13,18,38]. Furthermore, Se has been shown to enhance the flavor profile of rice products, improve the umami and overall taste of parboiled rice [19], and increase the concentration of alcohols and alkanes in rice cakes [20]. The conclusions drawn from this experiment are consistent with those of previous studies, indicating that applying Se fertilizer markedly improves the appearance, viscosity, and balance of cooking and eating quality in rice while reducing its hardness. Consequently, this significantly enhances the taste and overall quality of rice. Regarding amino acid content, Se elevated the levels of Val, Leu, Ile, and Lys. These amino acids exhibited a significant positive correlation with the appearance and balance of cooking quality and a significant negative correlation with hardness. This may represent a key reason for the improvement in the cooking and eating qualities of rice. In addition, branched-chain amino acids (Val, Leu, and Ile) contribute to the enhancement of fruit flavor. Furthermore, Leu and Ile act as precursors of aromatic substances, and an increase in their concentration can promote the synthesis of aromatic compounds, thereby improving flavor. Among the protein components, the content of globulin exhibits a strong negative correlation with taste, demonstrating a highly significant relationship. Additionally, the Se content in rice is highly negatively correlated with globulin, suggesting that applying Se fertilizer can reduce globulin levels in rice, thereby enhancing its cooking and eating quality. Moreover, the cooking and eating qualities of rice are influenced by N, with elevated N levels leading to decreased rice quality. Selenium has a pronounced positive effect on the cooking and eating quality of rice and is less influenced by N levels.

5. Conclusions

The application of Se fertilizer has a notable adverse effect on the processing quality of rice, which was influenced by nitrogen levels, improved processing quality was observed under reduced N conditions. Selenium significantly enhanced the appearance, nutritional value, and cooking and eating qualities of rice, demonstrating notable effects at varying N levels. It increased the essential amino acid content in rice, improved its flavor, and significantly enhanced its cooking and eating qualities. Considering all primary quality indicators, LN+LSe treatment emerged as the optimal choice.