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Article

Physiological and Biochemical Analysis of Selenium-Enriched Rice

by
Tianyi Lu
1,
Yanmei Ai
1,
Meng Na
1,
Shangqi Xu
1,
Xiaoping Li
2,
Xianqing Zheng
3,* and
Jihai Zhou
1,*
1
School of Ecology and Environment, Anhui Normal University, Wuhu 241002, China
2
Collaborative Innovation Center of Southern Modern Forestry, Nanjing Forestry University, Nanjing 210037, China
3
Institute of Eco-Environment and Plant Protection, Shanghai Academy of Agricultural Sciences, Shanghai 201403, China
*
Authors to whom correspondence should be addressed.
Agronomy 2024, 14(8), 1715; https://doi.org/10.3390/agronomy14081715
Submission received: 1 July 2024 / Revised: 31 July 2024 / Accepted: 2 August 2024 / Published: 4 August 2024
(This article belongs to the Section Plant-Crop Biology and Biochemistry)
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Versions Notes

Abstract

:
Selenium is an essential trace element in the human body. However, its intake is generally low. Therefore, the production and utilisation of selenium-enriched foods is currently a research hotspot. In this study, the effects of low (0.2 mg·kg−1), medium (1.0 mg·kg−1), and high (5.0 mg·kg−1) concentrations of selenium on the physiological and biochemical characteristics of rice were investigated to develop selenium-enriched rice. High concentrations of selenium have been found to inhibit the growth, physiology, and biochemistry of rice, while low concentrations of selenium promote its growth. The height of mature rice plants exposed to high concentrations of selenium was reduced by 7.20% compared with the height of control rice. Selenium decreased the proline content of rice during the growth period except in mature rice treated with medium and high concentrations of selenium. Excluding high concentrations, selenium treatment increased the soluble sugar content of rice from the tillering to the mature stages. The peroxidase activity of rice at the heading stage treated with medium levels of selenium was significantly higher than that of the control rice, while the superoxide dismutase activity of rice exposed to selenium was significantly enhanced at the mature stage. The malondialdehyde levels of mature rice treated with medium and high levels of selenium were significantly lower than those of the control rice. The selenium content of each plant part was significantly correlated with the soil selenium level. An increase in the soil selenium level facilitated the production of selenium-enriched rice.

1. Introduction

Selenium is an essential micronutrient in humans, with several positive effects on human health [1]. Further, selenium deficiency has been shown to cause alveolar dysplasia in newborns [2]. It enhances the activity of the immune system, prevents oxidative damage, and even exhibits potential antiviral activity in humans [3]. Selenium supplementation in humans is mainly achieved through their diet, and most of the selenium in crops is derived from soil [4]. Selenium promotes photosynthesis, improves plant antioxidant capacity, and inhibits the absorption of potentially toxic elements [5,6,7]. Hasanuzzaman et al. [8] reported that selenium treatment affects plant growth by increasing the levels of chlorophyll a, chlorophyll b, and total chlorophyll in plants. In recent years, the selenium levels in fruits have been enhanced by using selenium-rich soil or spraying leaves with a selenium solution [9]. Groth et al. [10] increased the selenium contents of apples 10- to 14-fold by spraying apple trees with selenium fertiliser. Xu et al. [11] reported that mushrooms are generally enriched with selenium, and these selenium-enriched mushrooms exhibit potential anti-oxidant and anti-cancer effects. Shen et al. [12] reported that rice can be enriched with selenium by applying exogenous selenium to the rice soil. The aforementioned studies provide new ideas for the use of selenium in agricultural production to enhance human health.
Soil selenium is unevenly distributed in China; selenium deficiency is widespread in most areas. However, Hubei Enshi, Shaanxi Ankang, Anhui Shitai, and other regions are enriched in soil selenium (0.23–8.66 mg·kg−1). Accordingly, selenium-rich agricultural products, such as rice, tea, and potatoes, have been widely produced [13]. It has also been found that the enrichment of lettuce plants with Se may be an effective approach to enhancing the Se nutritional status in Se-deficient populations [14]. The selenium content of the edible parts of crops is closely related to the soil selenium content; however, variations in selenium levels do not necessarily have a positive effect on crop growth [9]. Selenium plays a dual role in plant physiology [15]. Low concentrations of selenium promote plant growth, inhibit ageing, and maintain cell composition and activity. High concentrations of selenium can affect the metabolic balance in plants and trigger selenium toxicity [16]. The distribution of selenium in different parts of plants varies across plant species, developmental stages, and physiological conditions [17,18,19]. Selenium levels in the young leaves of plants are often the highest, generally reaching a maximum during seedling growth. Further, the selenium levels in leaves decrease before and during flowering due to their transfer from leaves to seeds [20].
Thus, the rational use of selenium resources is an important factor in human health and the sustainable development of agriculture. In this present study, the selenium-rich soil (0.31–6.59 mg·kg−1) in Dashan Village, Shitai County, Anhui Province was used as a reference [21]. Soils with different selenium concentrations (low, 0.2 mg·kg−1; medium, 1.0 mg·kg−1; and high, 5.0 mg·kg−1) were simulated. We hypothesise that (1) selenium alleviates oxidative senescence in rice and (2) the antioxidants in rice decrease the toxicity of high levels of selenium. The physiological mechanisms of rice enrichment and selenium transport were then investigated. This study will provide a reference for the rational utilisation of soil selenium and the development of selenium-enriched products.

2. Materials and Methods

2.1. Materials

Rice: Yuzhenxiang, a medium-maturing late indica rice—a variety commonly grown in Dashan Village, Shitai County—was purchased from Mayin Agricultural Materials Management Department, Wuhu City, Anhui Province.
Soil: The soil was collected from farmland in the middle and lower reaches of the Yangtze River. The basic physical and chemical properties of the soil are listed in Table 1.
Se: Sodium selenite (Na2SeO3·5H2O), analytically pure.
Water-soluble fertiliser was purchased from China Stanley Agricultural Group Co., Ltd. (Linyi, China). The primary components of the fertiliser were (N + P2O5 + K2O) ≥ 60%; N:P:K in 1:1:1 ratio; and a 0.2% to 3.0% mixture of Zn, Fe, Mn, and B.

2.2. Experimental Design

Four experimental treatments were designed: CK (control; without selenium addition), LSe (0.2 mg·kg−1 selenium added), MSe (1.0 mg·kg−1 selenium added), and HSe (5.0 mg·kg−1 selenium added). Each experimental treatment was performed in quadruplicate.

2.2.1. Soil Pre-Treatment

The collected soil samples were air-dried and filtered through a 2 mm sieve. The samples were then loaded into pots, each containing 5 kg. The pot dimensions were upper mouth diameter, 20.5 cm; lower mouth diameter, 18.3 cm; and height, 20.6 cm. The pots were sealed tightly to prevent selenium loss during the experiment. Water was then added up to about 2 cm above the soil surface. Based on the selenium concentration of the experimental treatments, the corresponding Na2SeO3·5H2O solution was added. The soil and solution were mixed evenly. The water balance was maintained for one week.

2.2.2. Rice Cultivation

Full rice seeds were selected and rinsed with running tap water. After washing with distilled water three times, the seeds were tiled in a seedling tray with filter paper. The soaked filter paper was incubated at 26 °C for 24 h. The seedlings were then sown in the soil. On day 30 after sowing, healthy seedlings with similar growth levels were selected and transplanted into the pots with the treatments. Each pot carried three holes, with three seedlings per hole. After transplanting, the pots were arranged in completely randomised blocks, and the positions were changed once per week.

2.2.3. Fertilisation and Water Management

Water-soluble fertiliser (25% of 300 kg hm−2 nitrogen fertiliser) was sprayed on days 7 and 21 during the seedling stage. Water was replenished once every 3 days to 2 to 3 cm above the soil surface to promote root growth and stopped two days before transplantation. Nitrogen was applied at a rate of 300 kg hm−2 as a standard fertiliser. A base nitrogen fertiliser was added at a standard rate of 75% before transplanting, and a 25% nitrogen application rate was used as topdressing at the booting stage of rice growth. After transplanting, the water was supplied every day to maintain the water to about 2 cm above the soil surface. Water management was stopped two days before harvest.

2.2.4. Sample Collection

Samples were obtained at the tillering stage (30 days after transplantation), heading stage (63 days after transplantation), and mature stage (106 days after transplantation). Leaves were collected randomly from each pot at the tillering and heading stages. Clean experimental scissors were used to cut the above-ground part of the rice from the soil surface layer and the sample was transferred to a self-sealing bag. The growth and physiological and biochemical indices were measured in the laboratory. All above-ground parts and all roots and soil at a 0–15 cm depth were collected at the mature stage and stored in self-sealing bags. Some fresh rice leaves were stored in an ultra-low temperature refrigerator at −80 °C to analyse the physiological and biochemical characteristics during each growth stage. The mature rice was separated into root, stem, leaf, rice husk, and brown rice. The fresh rice samples were first heated for 2 h at 105 °C and then stored at 80 °C to a constant weight. Large rice samples were then ground while fewer rice samples were crushed in a mortar with a pestle. The crushed rice samples were stored for the measurement of selenium levels in different parts of the rice. The roots were then removed from the soil to reduce the interference of plants, and the air-dried and sieved soil was preserved to determine the soil selenium content.

2.3. Methods

2.3.1. Determination of Plant Height and Biomass

Determination of plant height: The above-ground parts of the rice collected at each growth stage were laid out on a clean experimental operating table. The length of the shear point of the rice to the second-highest leaf tip was measured using a ruler. The measurement value was corrected to two decimal places and recorded as the height of the rice.
Determination of biomass: The collected rice was dried, and the biomass of each pot of rice was measured.

2.3.2. Determination of Physiological and Biochemical Indices

The chlorophyll content was determined via 95% ethanol extraction [22]. The following Formulas (1)–(3) were used to calculate the concentrations of total chlorophyll, chlorophyll a, and chlorophyll b:
C a = 13.95 A 665 − 6.88 A 649
C b = 24.96 A 649 − 7.32 A 665
C T = C a + C b = 6.63 A 665 + 18.08 A 649
In the formulae above, Ca represents the content of chlorophyll a; Cb is the content of chlorophyll b; and CT denotes the total chlorophyll content. A665 and A649 represent the absorbance values at wavelengths of 665 and 649 nm, respectively.
The content of malondialdehyde (MDA) was determined using thiobarbituric acid colourimetric methods. An amount of 0.500 g of fresh samples of rice leaves was weighed and homogenised with 10% trichloroacetic acid using a tissue grinder. After centrifugation, the mixed solution (including 2 mL supernatant and 2 mL 0.67% thiobarbituric acid solution) was bathed in boiling water for 10 min to obtain the test solution. The absorbance of the test solution was measured using a microplate reader (wavelength: 450 nm, 532 nm, and 600 nm) [23].
The content of free proline (Pro) was determined using the acid ninhydrin colourimetric method. An amount of 0.500 g of fresh rice leaves was weighed and extracted with 3% sulfosalicylic acid solution in a water bath. Then, the mixture containing 2 mL of extract, glacial acetic acid, and acidic ninhydrin solution was placed in boiling water for 30 min, and the solution turned red. After cooling, toluene was added to transfer all the pigments to the toluene solution. The absorbance (wavelength: 520 nm) of 270 μL upper proline red toluene solution was measured using a microplate reader [24].
The soluble sugar (SS) content was determined via anthrone colourimetry. A total of 0.2 g of fresh rice leaves was weighed and extracted with ultrapure water in boiling water. The extract was repeatedly extracted once after filtration. Then, the mixture (containing 0.5 mL of extract, 1.5 mL of ultrapure water, 0.5 mL of anthrone ethyl acetate, and 5 mL of concentrated sulfuric acid) was fully shaken and immediately placed in boiling water for accurate heat preservation for 1 min. After cooling, 270 μL of the test solution was taken and the absorbance value (wavelength: 630 nm) was measured by a microplate reader [25].
The activity of superoxide dismutase (SOD) was determined via nitro-blue tetrazolium (NBT) colourimetry. An amount of 0.500 g of fresh rice leaf samples was weighed and homogenised with phosphate buffer solution (pH 7.82) using a tissue grinding machine. After centrifugation, 0.05 mL of the supernatant was collected and added to the mixed reaction solution (including the phosphate buffer, methionine, azablue tetrazole, ethylenediamine tetraacetic acid sodium, and riboflavin solution). Instead of enzyme solution, distilled water was used as a control for the dark reaction, and other tubes were subjected to sunlight reaction. The absorbance at 560 nm was measured by a microplate reader with 270 μL of the test solution [26].
The activity of peroxidase (POD) was determined via the guaiacol colourimetric method. An amount of 0.200 g of fresh rice leaves was weighed and added to ultrapure water to grind and extract the enzyme solution using a tissue grinder. The test solution contained 1 mL of 0.1% guaiacol solution, 1 mL of 0.18% hydrogen peroxide solution, and 1 mL of enzyme solution. The absorbance of 270 μL of the solution was measured using a microplate reader (wavelength: 470 nm) [26].
The catalase activity (CAT) was determined via potassium permanganate titration. An amount of 0.200 g of fresh rice leaves was weighed and added to ultrapure water to grind and extract the enzyme solution with a tissue grinder. The test solution contained 1 mL of 0.1% guaiacol solution, 1 mL of 0.18% hydrogen peroxide solution, and 1 mL of enzyme solution. The absorbance of 270 μL of the solution was measured using a microplate reader (wavelength: 470 nm) [27].

2.3.3. Determination of Selenium Content

First, 0.5 g of dried plants or soil was accurately weighed and added to a 50 mL-high borosilicate glass digestion tube, followed by the addition of an acid mixture (plant: volume ratio HNO3:HClO4 = 4:1, 13 mL; soil: volume ratio of HNO3:HClO4 = 3:2, 12 mL). The tube was covered with a small funnel and incubated overnight (for at least 18 h). Next, the solution was digested using an automatic graphite digestion instrument (100 °C (1 h) → 120 °C (2 h) → 180 °C (1 h) → 210 °C) until the solution became transparent. The remaining 2 mL was reduced with 5 mL of 12 mol L−1 (for plant samples) or 10 mL of 6 mol L−1 (for soil samples) hydrochloric acid, and adjusted to a constant volume of 25 mL. The solution was filtered using double-layered filter paper and stored in a centrifuge tube. The selenium content was determined using an AFS-9700 atomic fluorescence spectrophotometer (Beijing Haiguang Instrument Co., Ltd., Beijing, China) [28].

2.3.4. Calculation of the Bioconcentration Factor (BCF) and Translocation Factor (TF)

The BCF was calculated by dividing the selenium content (mg·kg−1) of each part of the rice (root, stem, leaf, rice husk, and brown rice) by the soil selenium content (mg·kg−1)
The TF was calculated from the root to stem, stem to leaf, leaf to rice husk, and rice husk to brown rice. The root-to-stem transport coefficient (TFRoot-Stem) was calculated as follows: TFRoot-Stem = Stem selenium content (mg·kg−1)/Root selenium content (mg·kg−1) [29].

2.4. Data Analysis

Microsoft Excel 2019 and Origin 2021 were used for data processing and mapping, respectively. One-way ANOVA was performed using the SPSS 24 software to test for significant treatment differences. Duncan’s method was used for multiple comparisons (p = 0.05). Pearson correlation coefficients were used for correlation analysis.

3. Results

3.1. Plants Biomass

The height of each rice plant during each period exhibited a decreasing trend with increases in the selenium concentration (Figure 1A). The plant height at the tillering stage was in the following order: CK > LSe > MSe > HSe. There was no significant difference between the treatments and CK at the tillering stage and heading stage. At the mature stage, the plant height of HSe was significantly less than that of CK, by 7.20%. The effects of different concentrations of selenium on the biomass of rice at the mature stage were as follows: the above-ground biomass tended to decrease, and the underground biomass tended to increase, without significant differences (Figure 1B).

3.2. Selenium Accumulation in Rice

3.2.1. Changes in the Selenium Levels of Different Parts of Rice

The selenium levels of different parts of rice during the mature stage increased with increasing selenium concentration (Figure 2). In comparison to CK, the selenium content in rice roots in the HSe treatment significantly increased, by 6847.75% (Figure 2A). The selenium levels of rice leaves treated with MSe and HSe increased significantly, by 1715.89% and 2195.58%, respectively, in comparison to CK (Figure 2C). No selenium enrichment was found in the rice stems (Figure 2B), rice husks (Figure 2D), and brown rice (Figure 2E) of CK, whereas the selenium enrichment increased significantly with increases in the selenium concentration. The selenium levels of rice stems treated with LSe, MSe, and HSe were 0.21 mg kg−1, 2.29 mg kg−1, and 26.20 mg kg−1, respectively. The selenium levels of rice husks treated with LSe, MSe, and HSe were 0.33 mg kg−1, 3.89 mg kg−1, and 32.32 mg kg−1, respectively. The selenium levels of brown rice treated with LSe, MSe, and HSe were 0.54 mg kg−1, 13.86 mg kg−1, and 18.70 mg kg−1, respectively.
The distribution of selenium in the different parts of rice was greatly affected by the soil selenium concentration (Figure 3). The selenium was mainly distributed in the roots and leaves of the CK plants. The proportion of selenium in different parts of rice under the LSe treatment was in the following order: leaf > root > brown rice > rice husk > stem. In the MSe treatment, these proportions were leaf > brown rice > root > rice husk > stem, while in the HSe treatment, they were root > brown rice > rice husk > leaf > stem. The distribution of selenium in brown rice increased first and then decreased with increases in the soil selenium concentration. The distribution of selenium in the stems and rice husks increased with increases in the soil selenium concentration. The distribution of selenium in the leaves decreased with increases in the soil selenium concentration. The distribution of selenium in the roots first decreased and then increased with increases in the soil selenium concentration.

3.2.2. Changes in Selenium Enrichment and Transport of Rice

The enrichment coefficient and transport coefficient of selenium in the different parts of rice varied as a function of the selenium concentration. The changes in the enrichment coefficient were consistent with the changes in the selenium content of the different parts of the rice (Figure 4). Overall, the selenium enrichment ability of the different parts of the rice exhibited the following trend: BCFleaf > BCFroot > BCFbrown rice > BCFrice husk > BCFstem. The selenium transport capacity of the different parts of the rice exhibited the following trend: TFstem-leaf > TFrice husk-brown rice > TFroot-stem > TFleaf-rice husk.

3.3. Determination of Chlorophyll Content

Selenium enhanced the content of chlorophyll a in rice at the tillering and mature stages but decreased the levels at the heading stage (Figure 5A). The chlorophyll a level at the tillering stage under the LSe treatment was significantly reduced, by 18.28%, compared with CK. The chlorophyll a levels of LSe and MSe at the heading stage were significantly reduced, by 15.86% and 11.13%, respectively, compared with CK. The chlorophyll a content of HSe at the maturity stage was significantly increased, by 7.62%, compared with CK.
Selenium reduced the chlorophyll b levels of rice at the heading and mature stages (Figure 5B). The chlorophyll b concentration in the tillering stage under LSe treatment was significantly reduced, by 21.02%, compared with CK. The chlorophyll b levels of LSe, MSe, and HSe in the heading stage were significantly reduced by 18.58%, 11.91%, and 10.77%, respectively, compared with CK. The chlorophyll b levels of LSe, MSe, and HSe at the mature stage were significantly reduced by 55.4%, 19.31%, and 42.23% compared with CK, respectively.
Selenium also decreased the total chlorophyll content of rice at the heading and mature stages (Figure 5C). The total chlorophyll content in the tillering stage under LSe treatment was significantly reduced, by 18.97%, compared with CK. The total chlorophyll levels of LSe, MSe, and HSe at the heading stage were significantly reduced, by 16.58%, 11.34%, and 8.02%, respectively, compared with CK. The total chlorophyll levels of LSe, MSe, and HSe at the mature stage were significantly reduced by 27.67%, 9.2%, and 19.34% compared with CK, respectively.
The ratio of chlorophyll a to chlorophyll b in rice leaves decreased gradually during the growth period, but selenium significantly increased the ratio of chlorophyll a to chlorophyll b in mature rice (Figure 5D). In comparison to CK, the chlorophyll a to chlorophyll b ratios in the mature stage under the LSe, MSe, and HSe treatments were significantly increased by 135.38%, 27.20%, and 86.27%, respectively. These findings indicate that selenium improved the light energy utilisation efficiency of rice.

3.4. Levels of Osmotic Adjustment in Rice

Different concentrations of selenium reduced the production of Pro in the early and middle stages of rice growth. High concentrations of selenium (MSe, HSe) increased the content of Pro in the late growth stage (Figure 6A). The Pro content of rice leaves at the tillering stage decreased gradually with increases in the selenium concentration, with the LSe, MSe, and HSe treatments showing significantly reduced Pro levels of 11.17%, 28.75%, and 34.94%, respectively, when compared to CK. The Pro levels of LSe, MSe, and HSe at the heading stage were significantly lower than those of CK by 43.55%, 59.18%, and 39.08%, respectively. The Pro content decreased first and then increased with increases in the selenium concentration. The Pro content in the HSe treatment at the mature stage was significantly increased by 92.39% compared with CK.
The tillering stage exhibited no significant differences in the levels of soluble sugars (SS) between treatments under different treatments (Figure 6B). The SS contents of LSe and HSe at the heading stage increased significantly, by 25.11% and 133.20%, respectively, compared with CK. No significant differences were found in the SS levels of the different treatments in the mature stage.

3.5. Antioxidant Enzyme Activity

The SOD activities in the tillering stage of LSe and HSe were significantly reduced, by 11.09% and 8.78%, respectively, compared with CK. The SOD activity of LSe at the heading stage was significantly increased by 3.99% compared with CK. The SOD activities of the LSe, MSe, and HSe treatments increased significantly at the mature stage, by 19.25%, 34.83% and 49.15%, respectively, compared with CK, with statistically significant differences between the treatments (Figure 7A). The effects of different concentrations of selenium on the POD activity of rice at the tillering and heading stages were similar: low concentrations of selenium (LSe) decreased POD activity, while medium (MSe) and high (HSe) concentrations of selenium increased POD activity. In the mature stage, POD activity first increased and then decreased with increasing selenium concentration (Figure 7B). No significant differences were found in the POD activity of different treatments at the tillering stage. At the heading stage, the HSe treatment significantly increased POD activity by 114.08% compared with CK. At maturity, no significant difference in POD activity was found between the LSe, HSe, and CK treatments, while the POD activity of the MSe treatment was significantly increased by 137.99% compared with CK.
Different concentrations of selenium primarily reduced the activity of CAT in rice leaves during the early and middle stages of growth; in the later stage, the LSe treatment exhibited a decrease in the activity of CAT, while the MSe and HSe treatments exhibited an increase (Figure 7C). At the tillering and heading stages, no significant differences were detected in CAT activity among the different treatments. Compared with CK, the CAT activities of MSe and HSe at the maturity stage were significantly increased by 92.31% and 166.42%, respectively.
The MDA content of rice leaves increased in the HSe treatment at the heading stage but decreased in the presence of different concentrations of selenium during the other stages of rice growth. In particular, during the maturity stage, the MSe and HSe treatments had the greatest effects on the MDA content (Figure 7D). No significant differences were found in the MDA levels of the treatments during the tillering and heading stages. The MDA levels of the MSe and HSe treatments at the mature stage were significantly lower than those in CK by 46.19% and 49.96%, respectively. These findings indicate that the addition of selenium decreased the production of MDA.

3.6. Correlations between the Selenium Content and Physiological Characteristics of Rice Growth

A correlation analysis was performed to further explore the relationships between soil selenium levels, the selenium levels of various parts of the rice, and the physiological characteristics of rice growth (Figure 8). The results revealed that the selenium levels of the root, stem, leaf, rice husk, and brown rice were significantly positively correlated with the Pro content, SOD activity, CAT activity, and soil selenium content (p < 0.01). Significant (p < 0.05) or extremely significant negative correlations were found between the height of rice, chlorophyll a content, MDA, and selenium content in the soil and the various parts of the rice. The results of multivariate analysis showed that stage and treatment and their interaction have significant impact on SOD, CAT, MDA, SS in our study (Table 2).

4. Discussion

4.1. Effects of Se Fertilization on Plant Biomass

This study found that high concentrations of selenium significantly inhibited the height of rice at the mature stage but had little effect on the above-ground and underground biomasses (Figure 1). The plant height was significantly negatively correlated with the soil and rice selenium levels (Figure 8), which is consistent with the findings of previous studies. Lanza et al. [30] reported that plants exhibit dose-dependent effects of selenium. High concentrations of selenium may have toxic effects on plants, whereas appropriate concentrations of selenium have a positive effect on plant growth. Liang et al. [31] also reported that the effects of selenium on the height of rice were dose-dependent. When the selenium concentration increased to a certain level, the plant height reached its maximum and was then inhibited. Chen et al. [24] found that the yield of wheat first increased and then decreased with increases in the selenium fertiliser concentration from zero to 1200 kg·ha−1. The high concentration of selenium may partially inhibit the absorption of other nutrients by plants [32].

4.2. Se Accumulation in Rice

Selenium in plants can self-metabolise by competing with sulphur metabolic pathways. For example, selenite is metabolised to other forms of selenium after absorption by roots and is transported to the above-ground part of the plant. Selenite and its metabolites are mainly enriched in the roots in forms that are difficult for plants to absorb [33]. Di et al. [34] found that the basal application and foliar spraying of sodium selenite significantly affected selenium accumulation in various parts of wheat and improved the selenium utilisation rate in wheat. This present study found that the selenium content of each part (root, stem, leaf, rice husk, and brown rice) of mature rice increased with increases in the soil selenium content (Figure 2). A significant positive correlation was found between the levels of selenium in rice and the soil (Figure 8). Increases in the soil selenium content improve the bioavailability of selenium, leading to greater absorption and enrichment. This present study did not detect selenium accumulation in the stems, rice husk, and brown rice of rice grown in soil without selenium, while the selenium content of brown rice treated with a low concentration of selenium (0.2 mg·kg−1) reached 0.54 mg·kg−1. The standard range of the selenium enrichment of rice in China is 0.04–0.30 mg·kg−1 [35]. Accordingly, the brown rice in this study was excessively enriched in selenium, indicating its strong ability to enrich selenium in the soil. Sharma et al. [36] analysed the soil selenium enrichment ability of two different rice varieties and found that the selenium accumulation of rice grains and leaves in selenium-enriched soil was significantly higher than in the control treatment. Consistent with the study conducted by Sharma et al. [37], this present study also found that the selenium enrichment ability of the different parts of rice decreases in the following order: BCFleaf > BCFroot > BCFbrown rice > BCFrice husk > BCFstem, while the transport ability of selenium in rice was as follows: TFstem-leaf > TFrice husk-brown rice > TFroot-stem > TFleaf-rice husk (Figure 4). These findings demonstrate that it is feasible to produce selenium-enriched rice using selenium-rich soil.
Our experimental rice exceeded the content standard of selenium-enriched rice, primarily owing to the differences in rice varieties. In the future, multiple varieties of rice should be researched.

4.3. Effects of Se Fertilization on Chlorophyll Content

In this study, the chlorophyll levels of rice during the different growth periods were dynamically monitored. The results revealed that the low concentration of selenium reduced the contents of chlorophyll a, b, and total chlorophyll at the tillering and heading stages in comparison to the control. At the mature stage, the chlorophyll a level was mainly increased, while the levels of chlorophyll b and total chlorophyll were decreased (Figure 5). These findings are basically consistent with the results of other studies. For example, Iqbal et al. [38] showed that the application of low-concentration selenium at the heading stage increased the chlorophyll a and total chlorophyll levels of wheat and regulated the growth of wheat. Li et al. [39] also reported that the chlorophyll content of mustard first increased and then decreased when the concentration of sodium selenite was 0–40 mg·L−1. This may be due to changes in the chlorophyll content of rice during the different growth and development stages, or due to the fact that different methods of exogenous selenium application and different genotypes of rice varieties altered the physiological response of rice [31,34].

4.4. Effects of Se Fertilization on Osmotic Adjustment

Plants respond to abiotic stresses such as water deficits by accumulating various osmotic protective agents. Pro plays an important role in the osmotic regulation of plant stress resistance [40]. This study showed that the growth of rice led to an initial decrease in the free Pro content of leaves, followed by an increase (Figure 6A). This may be due to the need to increase osmotic agents during the tillering and mature stages to maintain a steady state of plant growth and development. The Pro content of rice leaves at the tillering and heading stages decreased significantly with different concentrations of selenium, while the Pro content increased significantly under medium and high concentrations of selenium during the mature stage. A significant positive correlation existed between the Pro content during the mature stage and the selenium content of soil and rice (Figure 8). Sharma et al. [36] analysed the selenium accumulation and antioxidant properties of rice in selenium-free and selenium-containing soils, finding that the Pro content of rice was significantly increased in selenium-containing soils. Khalofa et al. [41] showed that high concentrations of selenium significantly increased the Pro content of quinoa. These findings are similar to those observed during the mature stage in this study. This may be because a high concentration of selenium adversely affects the homeostasis of the internal environment of rice during the mature stage, and the plant increases its Pro content to maintain osmotic regulation. In the early stages of rice growth (tillering and heading), the growth of rice is vigorous, with a relatively weak effect of selenium on the internal environment of the plant.
Soluble sugars (SS) are crucial for cell growth and the maintenance of osmotic homeostasis [42]. In this study, rice leaves were found to produce higher levels of SS during the maturity stage, whereas medium and high concentrations of selenium reduced the levels of SS; low and high concentrations of selenium during the heading stage significantly increased the SS content (Figure 6B). These findings are similar to the results reported by Sharma et al. [37]. The addition of selenium can significantly increase the SS content in the early stages of wheat growth, but the SS content gradually decreases after the heading stage. In addition, the level of SS in mustard was found to decrease following the addition of high levels of selenium, which may be related to the toxicity of high concentrations of selenium [34]. Yu et al. [43] also found that when the concentration of selenium in hydroponics was 0–1.6 mmol·L−1, the SS content of cabbage first increased and then decreased with increases in the selenium concentration. This may be because selenium can regulate the activity of sucrose metabolic enzymes, which, in turn, affects the levels of SS [44].

4.5. Effects of Se Fertilization on Antioxidant Enzyme Activity

SOD, POD, CAT, glutathione peroxidase (GSH-Px), and glutathione-S-transferases (GST) are enzymatic antioxidants, whereas, ascorbic acid (AsA), GSH, tocopherols, and phenolic compounds are non-enzymatic antioxidants. Altogether, these constitute the antioxidant system of plants [45]. In a study of the effect of selenium on the antioxidant properties of rice, Sharma et al. [36] found that the growth and development of rice in selenium-containing soil were inhibited. Further, the level of peroxidation was enhanced. The flowering period was delayed by 10 days. The POD activity was significantly increased, while the CAT activity was decreased. This present study revealed that the activities of SOD and CAT in rice did not change significantly from the tillering to the heading stage and did not change under different concentrations of selenium. However, the activities of these two enzymes were significantly enhanced during the mature stage, especially in the HSe treatment. Nevertheless, the POD activity during the tillering stage was significantly higher than that in the heading and mature stages and the effect of selenium on the POD activity at different stages of plant growth increased with increasing selenium concentration (Figure 7). A significant positive correlation existed between the SOD and CAT activity during the mature stage and the selenium levels of soil and rice (Figure 8). This can be attributed to the excessive levels of selenium damaging plant growth, resulting in an increase in the POD activity. Selenium-induced toxicity is mainly related to the non-specific substitution of sulphur in proteins, resulting in intracellular enzyme and protein metabolic dysfunction. In addition, excessive selenium can lead to the formation of reactive oxygen species (ROS) and subsequent membrane damage caused by lipid peroxidation, resulting in phytotoxicity [46,47]. Iqbal et al. [38] demonstrated that selenium significantly enhanced the antioxidant defence system in wheat under environmental stress and increased the CAT activity under environmental stress. Studies have also shown that high concentrations of selenium can lead to oxidative stress in plants. In order to protect against oxidative stress, plants synthesise reducing agents, including SOD, POD, and CAT. These antioxidants directly and indirectly react with ROS [48]. Another study reported contrasting effects of selenium on SOD and MDA. The SOD activity first increased and then decreased, while the MDA content first decreased and then increased with increases in the selenium concentration, which was consistent with the results of this present study, indicating that the increase in SOD activity reduced the degree of membrane lipid peroxidation, thereby decreasing the production of MDA [31]. At the same time, our conjecture is verified: the antioxidants in rice decrease the toxicity of high levels of selenium.
Hartikainen et al. [49] reported that selenium has dual effects on the body: a low concentration of selenium acts as an antioxidant to inhibit lipid peroxidation, while a high concentration of selenium is a pro-oxidant, which promotes the production and accumulation of membrane lipid peroxidation products. The accumulation of MDA also indicates that plants are damaged by the peroxidation of ROS [50], and the application of selenium alleviates the harmful effects of leaf senescence in rice [51]. In this study, the MDA level increased with the growth of rice, especially from the heading to the maturity stages. Different concentrations of selenium decreased the production of MDA (Figure 7D). The correlation analysis also demonstrated that the MDA level decreased with increases in the soil selenium content, with a very significant negative correlation (Figure 8). Dai et al. [52] found that soil selenium had no significant effect on the MDA content of the plant cell membrane in the range of 0.5–1.25 mg·kg−1, while the MDA content decreased significantly when the soil selenium content was 2.5–20 mg·kg−1. This is corroborated by the results of this present study. Iqbal et al. [38] also reported that selenium significantly enhanced the antioxidant defence system of wheat under environmental stress and reduced the MDA content, possibly due to increased accumulation of MDA in rice. MDA is a product of membrane lipid peroxidation in response to physiological changes during plant growth and ageing. Selenium is a micronutrient required for plant growth and plays a role in regulating plant resistance and maintaining cell structure integrity. This proves our first hypothesis.

5. Conclusions

Selenium can attenuate the membrane lipid peroxidation caused by rice senescence during the mature stage and decrease the production of MDA. Further, it can affect the osmotic and oxidative balance of rice. The toxic effects of high concentrations of selenium in rice are inhibited by increasing the Pro content and antioxidant enzyme activity (mainly SOD and CAT). Selenium supplementation contributes to the enrichment and transport of selenium in the various parts of rice, with strong enrichment by brown rice. Selenium-enriched rice can be produced by applying low concentrations of selenium. This study offers theoretical guidance for the production of selenium-enriched rice.

Author Contributions

Conceptualization, T.L., J.Z. and XZ.; methodology, T.L., J.Z. and X.Z.; software, T.L. and Y.A.; validation, T.L. and Y.A.; formal analysis, X.Z.; investigation, X.L., M.N. and S.X.; data curation, T.L., J.Z. and Y.A.; writing—original draft preparation, T.L., J.Z. and Y.A.; writing—review and editing, T.L., J.Z. and Y.A.; visualisation, X.L., M.N. and S.X.; supervision, T.L., J.Z. and X.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the National Natural Science Foundation of China (41660615), National Agricultural Experimental Station for Agricultural Environment, Feng xian (grant number: NAES035AE03), the Natural Science Foundation of Anhui Province (2108085MD128), and the Key Research and Development Project of Wuhu City (2022yf56).

Data Availability Statement

The original contributions presented in this study are included in the article; further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. Effects of selenium on the height (A) and biomass (B) of rice. CK (control; no selenium added), LSe (0.2 mg·kg−1 selenium added), MSe (1.0 mg·kg−1 selenium added), HSe (5.0 mg·kg−1 selenium added). Different lowercase letters indicate significant differences between different treatments during the same period (p < 0.05).
Figure 1. Effects of selenium on the height (A) and biomass (B) of rice. CK (control; no selenium added), LSe (0.2 mg·kg−1 selenium added), MSe (1.0 mg·kg−1 selenium added), HSe (5.0 mg·kg−1 selenium added). Different lowercase letters indicate significant differences between different treatments during the same period (p < 0.05).
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Figure 2. Selenium content in rice roots (A), stems (B), leaves (C), husks (D), and brown rice (E). CK (control; no selenium added), LSe (0.2 mg·kg−1 selenium added), MSe (1.0 mg·kg−1 selenium added), HSe (5.0 mg·kg−1 selenium added). Different lowercase letters indicate significant differences between the different treatments during the mature stage (p < 0.05).
Figure 2. Selenium content in rice roots (A), stems (B), leaves (C), husks (D), and brown rice (E). CK (control; no selenium added), LSe (0.2 mg·kg−1 selenium added), MSe (1.0 mg·kg−1 selenium added), HSe (5.0 mg·kg−1 selenium added). Different lowercase letters indicate significant differences between the different treatments during the mature stage (p < 0.05).
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Figure 3. Proportion of selenium in different parts of rice. CK (control; no selenium added), LSe (0.2 mg·kg−1 selenium added), MSe (1.0 mg·kg−1 selenium added), HSe (5.0 mg·kg−1 selenium added). BRSe represents the selenium content of brown rice; RHSe denotes the selenium content of rice husks; LeafSe represents the selenium content of leaf, StemSe denotes the selenium content of stem, RootSe denotes the selenium content of root.
Figure 3. Proportion of selenium in different parts of rice. CK (control; no selenium added), LSe (0.2 mg·kg−1 selenium added), MSe (1.0 mg·kg−1 selenium added), HSe (5.0 mg·kg−1 selenium added). BRSe represents the selenium content of brown rice; RHSe denotes the selenium content of rice husks; LeafSe represents the selenium content of leaf, StemSe denotes the selenium content of stem, RootSe denotes the selenium content of root.
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Figure 4. Bioconcentration factors (BCF) and translocation factors (TF) of selenium in rice under different selenium treatments: CK (control; no selenium added), LSe (0.2 mg·kg−1), MSe (1.0 mg·kg−1), and HSe (5.0 mg·kg−1). ‘-’ indicates no detectable selenium or invalid calculation. TFRH-BR: transport coefficient from rice husk to brown rice. TFL-RH: transport coefficient from leaf to rice husk. TFS-L: transport coefficient from stems to leaves. TFR-S transport coefficient from roots to stems. BCFBR: enrichment coefficient in brown rice. BCFRH: enrichment coefficient in rice husk. BCFL: enrichment coefficient in leaves. BCFS: enrichment coefficient in stems. BCFR: enrichment coefficient in roots.
Figure 4. Bioconcentration factors (BCF) and translocation factors (TF) of selenium in rice under different selenium treatments: CK (control; no selenium added), LSe (0.2 mg·kg−1), MSe (1.0 mg·kg−1), and HSe (5.0 mg·kg−1). ‘-’ indicates no detectable selenium or invalid calculation. TFRH-BR: transport coefficient from rice husk to brown rice. TFL-RH: transport coefficient from leaf to rice husk. TFS-L: transport coefficient from stems to leaves. TFR-S transport coefficient from roots to stems. BCFBR: enrichment coefficient in brown rice. BCFRH: enrichment coefficient in rice husk. BCFL: enrichment coefficient in leaves. BCFS: enrichment coefficient in stems. BCFR: enrichment coefficient in roots.
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Figure 5. Effects of selenium on chlorophyll a (A), chlorophyll b (B), total chlorophyll (C) content, and chlorophyll a/b (D) in rice. CK (control; without selenium), LSe (0.2 mg·kg−1 selenium addition), MSe (1.0 mg·kg−1 selenium addition), HSe (5.0 mg·kg−1 selenium addition). Different lowercase letters indicate significant differences between the different treatments during the same period (p < 0.05).
Figure 5. Effects of selenium on chlorophyll a (A), chlorophyll b (B), total chlorophyll (C) content, and chlorophyll a/b (D) in rice. CK (control; without selenium), LSe (0.2 mg·kg−1 selenium addition), MSe (1.0 mg·kg−1 selenium addition), HSe (5.0 mg·kg−1 selenium addition). Different lowercase letters indicate significant differences between the different treatments during the same period (p < 0.05).
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Figure 6. Effects of selenium on proline (A) and soluble sugar (B) levels in rice. CK (control; no selenium added), LSe (0.2 mg·kg−1 selenium added), MSe (1.0 mg·kg−1 selenium added), HSe (5.0 mg·kg−1 selenium added). Different lowercase letters indicate significant differences between the different treatments during the same period (p < 0.05).
Figure 6. Effects of selenium on proline (A) and soluble sugar (B) levels in rice. CK (control; no selenium added), LSe (0.2 mg·kg−1 selenium added), MSe (1.0 mg·kg−1 selenium added), HSe (5.0 mg·kg−1 selenium added). Different lowercase letters indicate significant differences between the different treatments during the same period (p < 0.05).
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Figure 7. Effects of selenium on SOD (A), POD (B), CAT (C) activity, and MDA (D) content in rice. CK (control; no selenium added), LSe (0.2 mg·kg−1 selenium added), MSe (1.0 mg·kg−1 selenium added), HSe (5.0 mg·kg−1 selenium added). Different lowercase letters indicate significant differences between the different treatments during the same period (p < 0.05).
Figure 7. Effects of selenium on SOD (A), POD (B), CAT (C) activity, and MDA (D) content in rice. CK (control; no selenium added), LSe (0.2 mg·kg−1 selenium added), MSe (1.0 mg·kg−1 selenium added), HSe (5.0 mg·kg−1 selenium added). Different lowercase letters indicate significant differences between the different treatments during the same period (p < 0.05).
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Figure 8. Correlations between the Se content and physiological characteristics of rice growth. SOSe denotes the soil selenium content; RSe is the selenium content of roots; STSe represents the selenium content of stems; LSe refers to the leaf selenium content; RHSe is the selenium content of rice husks; BRSe indicates the selenium content of brown rice; S/R-DW is the above-ground and belowground biomass, respectively; Chla is chlorophyll a; Chlb is chlorophyll b; TChl is total chlorophyll. * indicates p < 0.05, ** indicates p < 0.01.
Figure 8. Correlations between the Se content and physiological characteristics of rice growth. SOSe denotes the soil selenium content; RSe is the selenium content of roots; STSe represents the selenium content of stems; LSe refers to the leaf selenium content; RHSe is the selenium content of rice husks; BRSe indicates the selenium content of brown rice; S/R-DW is the above-ground and belowground biomass, respectively; Chla is chlorophyll a; Chlb is chlorophyll b; TChl is total chlorophyll. * indicates p < 0.05, ** indicates p < 0.01.
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Table 1. Basic physical and chemical properties of the test soil.
Table 1. Basic physical and chemical properties of the test soil.
MaterialpHNutrient Content (g·kg−1)Se Content
(mg·kg−1)
Organic
Carbon		Total
Nitrogen		Total
Phosphorus		Total
Potassium
Soil5.517.150.180.531.630.22
Table 2. Analysis of variance of plant height, TChl, SOD, POD, CAT, Pro, MDA, and SS with the degree of freedom (DF), F values, and significance levels.
Table 2. Analysis of variance of plant height, TChl, SOD, POD, CAT, Pro, MDA, and SS with the degree of freedom (DF), F values, and significance levels.
VariablesDFF Value
HeightTChlSODPODCATProMDASS
Stage2107.92 ***0.47 193.5 ***12.66 ***11.9 ***18.77 ***164.59 ***13.29 ***
Treatment32.14 1.49 5.58 **6.07 **6.15 **1.96 9.18 ***8.28 ***
Stage × treatment60.18 1.53 7.51 ***1.10 6.83 ***1.79 10.27 ***19.94 ***
Data based on analysis of variance; *** significance level at p < 0.001; ** significance level at p < 0.01 (LSD test).
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Lu, T.; Ai, Y.; Na, M.; Xu, S.; Li, X.; Zheng, X.; Zhou, J. Physiological and Biochemical Analysis of Selenium-Enriched Rice. Agronomy 2024, 14, 1715. https://doi.org/10.3390/agronomy14081715

AMA Style

Lu T, Ai Y, Na M, Xu S, Li X, Zheng X, Zhou J. Physiological and Biochemical Analysis of Selenium-Enriched Rice. Agronomy. 2024; 14(8):1715. https://doi.org/10.3390/agronomy14081715

Chicago/Turabian Style

Lu, Tianyi, Yanmei Ai, Meng Na, Shangqi Xu, Xiaoping Li, Xianqing Zheng, and Jihai Zhou. 2024. "Physiological and Biochemical Analysis of Selenium-Enriched Rice" Agronomy 14, no. 8: 1715. https://doi.org/10.3390/agronomy14081715

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