Two BGW breeding lines were used for this study. Xihei 88 (BGW-1) and Heidali (BGW-2). These materials were bred and/or preserved in our laboratory (College of Agronomy, Northwest A&F University, Shaanxi, China).

Field experiments were conducted for 2 seasons (2018–2019 and 2019–2020) at the research farm of Northwest A&F University, Yangling, Shaanxi, China (34°17’38”N, 108°4’23” W; elevation 525 m). The soil is classified as Loess loam.

The experiment was set up in a split–split plot design with three replicates. The main plot factors were the control with no Se application (Se₀) and application of 2160 g·ha⁻¹Se ore powder (Se₂₁₆₀) to the soil. Subplot factors were three water regimes: no irrigation throughout the entire growing period (W₀); irrigation at the wintering stage (Feekes 3.0) (W₁); and irrigation at both at the wintering (Feekes 3.0) and green-turning stages (Feekes 4.0) (W₂). Our previous work with 51.3 mg·kg⁻¹ Se ore powder with applied Se concentrations (pure Se) of 1080–4320 g·ha⁻¹ showed that 2160 g·ha⁻¹ Se application had the greatest effect on grain yield and Fe concentration and increased the organic Se concentration in grains to 0.31–0.35 mg·kg⁻¹ (Liu et al., 2021b). Therefore, 2160 g·ha⁻¹ Se ore powder was applied before plowing, and the Se concentration in the soil (0–20 cm) was measured to be 0.65 ± 0.05 mg·kg⁻¹. Before sowing, 600 kg·ha⁻¹ of compound fertilizer (20-20-6, Summit Fertilizer [Qingdao] Co., Ltd., China) containing 20% N, 20% P₂O₅, and 5% K₂O was applied to each plot (9.0×1.2 m²).

The rainfall and daily air temperature were recorded throughout the growth period in the two seasons and are shown in Figure 1 . The rainfall was 169.4 mm and 192.5 mm for the 2 growing seasons. Irrigation was provided using tap water at the wintering stage (60 mm, December 30, 2018 and 2019) and in the greening stages (60 mm, March 15, 2018 and 2019). Herbicides, fungicides, and insecticides were applied whenever necessary. Seeds were sown on October 5, 2018 and October 7, 2019, and plants were harvested on June 2, 2019, and June 5, 2020.

According to the Soil Physicochemical Analysis Handbook (Bao, 2000), before the soil was turned, 20 cm of soil was obtained from a soil sample. After sifting with a 1-mm sieve, 0.2 g was collected, and 6 mL of concentrated sulfuric acid was added. The mixture was then digested with a microwave digester (Mutiwave PRO, Anton Paar, Austria). The deboiling tube was placed on the automatic Kay-type nitrogen analyzer for distillation and titration, and the total nitrogen content of the soil was determined. The boiled liquid was diluted to 50 mL and filtered with a 0.2-um filter membrane, and the Se content was determined using an inductively coupled plasma mass spectrometer (iCAP RQ, Thermo Fisher, USA).

A 0.5-g of soil sample was passed through a 0.15-mm sieve, and 5 mL of 0.800 mol·L⁻¹ of 1/6 K²Cr²O⁷ solution and 5 mL of concentrated H²SO⁴ solution were added and boiled at 180°C for 5 min. The liquid was transferred, and the volume was adjusted to 70 mL, two drops of 1,10-phenanthroline were added, and the solution was titrated with 0.2 mol·L⁻¹ of FeSO⁴ solution until a brick red color appeared. The titration volume was recorded, and a control experiment was performed with powdered SiO² instead of soil. The following formula was used to calculate the soil organic matter content(Os, g·kg^-1):

where V 0 is the volume of FeSO₄ used in the titration (mL); V is the volume of FeSO₄ used in the titration of the soil sample (mL); 1.1 is the oxidation correction factor; and 1.724 is the conversion factor

Then, 2.5-g soil samples were screened using a screen with an aperture of 1 mm, and 0.50 mol·L⁻¹ NaHCO³ and 1 g of phosphorus-free activated carbon were added. The samples were oscillated at 180 r·min⁻¹ for 30 min at 25°C and filtered with phosphorus-free filter paper. A 5-mL volume of a molybdenum-antimony inhibitor was added to 10 mL of filtrate and incubated in a water bath at 40°C for 30 min. The absorbance was measured at wavelength 700 nm using colorimetry, and a standard curve was generated. The soil available phosphorus content (Pa, mg·kg^-1) was calculated as follows:

where ρ is the mass concentration of phosphorus on the standard curve (μg·mL⁻¹) and H is the moisture content of the soil sample (%).

Soil samples weighing 5 g were screened using a screen with a 1-mm aperture, and 50 mL of 1mol·L⁻¹ ammonium acetate solution was added. The samples were oscillated at 180 r·min⁻¹ for 30 min at 25°C and filtered with filter paper, and the content of available potassium content was determined with a flame photometer (M410, Sherwood, UK).

Soil samples weighing 10 g were screened using a screen with a 1-mm aperture, and 25 mL of distilled water was added to make a soil suspension. The pH value of the soil solution was measured with a pH meter.

The soil contained 9.7 g·kg⁻¹organic matter content, 1.5 g·kg⁻¹ total N, 10.5 mg·kg⁻¹ available P, 250.1 mg·kg⁻¹ available K, and 0.24 mg·kg⁻¹ total Se and had a pH of 8.2.

Five flag leaves of wheat were labeled and sampled. Net photosynthetic rate (Pn) was measured in the field between 9:00 am and 11:00 am during stages of anthesis (Feekes 10.5.3–5.4), early grain-filling (GF-1, Feekes 11.1), and mid grain-filling (GF-2, Feekes 11.1) (Miller, 1999) using a portable photosynthesis system (LI-6400, LI-COR Inc. Lincoln, NE, USA). Measurements were taken in a 6-cm² leaf chamber under ambient CO₂ concentration of 380–400 μmol CO₂·mol^-1, a radiation of 1400 μmol·m⁻² s⁻¹, and a leaf temperature of 28°C.

Plants were sampled in a 1-m² plot with four rows for yield components. Spike numbers were measured from the samples and then transformed to spike number per m². Kernel number (spike⁻¹) was calculated as the average of ten spikes randomly selected from the samples. Plots were harvested at the maturity stage (Feekes 11.3–11.4) by small-plot combines (4LXNK-1.0, Weihui Xinnongke Machinery Factory, China). Grain yield (t·ha⁻¹) was determined from the harvested weight adjusted to a 12% moisture content. Thousand kernel weight (g) was measured using 1,000 grains from the harvested samples in each plot.

The N content of grains was measured according to the Kjeldahl nitrogen determination method (AACC, 2000) with an automatic nitrogen determination analyzer (Kjeltec 8400, FOSS). Grain protein content (%) was calculated by multiplying the N content by a coefficient of 5.7.

Amylose and amylopectin fractions from wheat starch were isolated and obtained using the butanol precipitation method as reported previously (Zhu et al., 2008). The amylose and amylopectin contents were determined by dual-wavelength iodine binding colorimetry (Zhu et al., 2008). Total starch content was the sum value of amylose and amylopectin content.

A 0.2-g sample of dried grain powder was extracted using 6.0 ml of 80% (v/v) ethanol in a water bath at 80°C for 30 min and centrifuged at 5000×g for 10 min. This extraction procedure was performed three times, and the supernatants were collected, mixed, and then diluted with 80% ethanol to 25.0 mL for the measurement of soluble sugar and sucrose contents according to the protocol as described by Buysse and Merckx (1993).

After harvest, the plants were washed with tap water and then with deionized water to remove dust and soil. Subsequently, the washed plants were separated into the various parts (grain, spike-stalk + glume, leaves, stem + leaf sheath, and root) and then dried in a forced–air oven at 75°C to a constant weight. The oven–dried samples were ground into fine powder using a freezing mixer mill (MM400, Retsch, Haan, Germany) after passing through a 100-mesh sieve and then stored in a sealed plastic bag.

A 0.5-g powder sample was placed into the polytetrafluoroethylene digestion tube with 10.0 mL HNO₃ and 2.0 mL H₂O₂ and then was digested in a closed microwave digestion system (MARS6, CEM, Matthews, NC, USA). After cooling to room temperature, the digested solution was transferred to a volumetric flask and diluted with ultrapure water to 25 mL for the determination of Fe, Zn, Mn, and Cu concentrations using atomic absorption spectrometry (PinAAcle 900F, PerkinElmer Enterprise Management Co. Ltd. Waltham, MA, USA) according to the Standard Method GB/T 5009.14-2017 developed by the Ministry of Health of China. About 5.0 mL 6.0 mol·L⁻¹ HCl was added to the obtained digested solution, and then the mixed solution was diluted with ultrapure water to 25 mL. The 10.0 mL mixed solution was transferred to a reaction vessel, with 2.5 mL 100 g·L⁻¹ K₃Fe (CN)₃ being added for total Se concentration in grains determination using a liquid-phase atomic fluorescence spectrometer (LC-AFS9780, Beijing Haiguang Instrument Co. Ltd. Beijing, China) according to the Standard Method GB/T 5009.93-2017 developed by the Ministry of Health of China.

Se distribution and accumulation in each part were calculated as follows:

where D S e is the proportion of Se distributed in the grain, spike-stalk + glume, leaves, stem + leaf sheath and root (%); C i is the Se concentration in the above-mentioned part (mg·kg⁻¹); D w is the dry weight of above-mentioned parts (kg·plant⁻¹); and A w is the Se concentration in the whole wheat plant (mg·plant⁻¹).

The translocation factor (TF) of Se from the root to grain ( T F r o o t   − g r a i n ) was calculated as follows:

where C g r a i n is the Se concentration in the above-mentioned part (mg·kg⁻¹); D r o o t is the Se concentration in the above-mentioned part (mg·kg⁻¹).

Soil samples were collected at depths of 0–20 cm and 20–40 cm from the experimental site and then air-dried at room temperature. After removing plant residues, the soil samples were homogenized and sieved through a 100-mesh for measuring the soil bioavailable Se concentration according to the method described by Li et al. (2016).

A 1.0-g amount of soil was added into a 100-mL centrifuge tube and extracted with 10.0 mL 0.25 mol·L⁻¹ KCl (soil/liquid = 1:10) by shaking at 200×g for 1 h at 25°C. Afterward, the mixture was centrifuged at 3000×g for 10 min and filtered through a 0.45-μm filter. The supernatant was collected for soluble Se determination. The residue in the above tube was continuously extracted with 10.0 mL 0.7 mol·L⁻¹ KH₂PO₄ (pH 5.0) and shaken at 200×g and 25°C for 4 h. Then, the mixed solution was centrifuged at 3000×g for 10 min and filtered. The collected supernatant was used for exchangeable Se determination. The soil available Se concentration value was the sum of soluble Se and exchangeable Se concentration.

The data were recorded as mean values ± standard deviations (SD). The statistical analysis was carried out by analysis of variance (ANOVA) procedures using JMP V12.0 statistical software from SAS (version 9, SAS Institute, Inc. Cary, NC, USA). Significant differences among water regimes as well as between Se₀ (without Se added) and Se₂₁₆₀ (2160 g·ha⁻¹ Se ore powder added) were detected using Fisher’s protected LSD at α = 0.01 and α = 0.05, respectively.

As shown in Figure 2 , the net photosynthetic rate (Pn) of BGW (BGW-1 and BGW-2) initially significantly increased initially and then decreased from anthesis to the GF-2 stage, and its highest value was observed at the GF-1 stage in both seasons. After soil application of Se₀ and Se₂₁₆₀, irrigation significantly increased the Pn of BGW at the anthesis, GF-1, and GF-2 stages (α = 0.01). There were no significant differences in Pn between W₁ and W₂ treatments. Within the same water regime (W₀, W₁, or W₂), the Se₂₁₆₀ fortification did not result in any significant change in Pn at each stage compared to control samples with no Se fortification.

Grain yield and its components increased with increasing irrigation amounts in both seasons ( Table 1 ). Regardless of the Se treatment, BGW had a significantly higher grain yield, spike number, and kernel number under W₂ treatment than under W₀ treatment (α = 0.01), but no significant differences were observed between W₁ and W₀ treatments. In addition, there were no significant differences in the 1000-kernel weight among the 3 water regimes. Se₂₁₆₀ application significantly increased the grain yield, kernel number, and thousand kernel weight, compared to Se₀ application (α = 0.05), but no significant differences were observed for spike number under the three water regimes (W₀, W₁, and W₂).

After applying Se₂₁₆₀ to the soil, irrigation slightly decreased the grain protein and amylose contents and increased the amylopectin, total starch, and soluble sugar contents, but no significant differences were found among the three water regimes in either season ( Table 2 ). Following Se₀ application, the sucrose content in grains was significantly higher under W₂ treatment than under W₀ treatment (α = 0.01), but no significant differences were observed between W₁ and W₀ treatments. After applying Se₂₁₆₀, the sucrose content significantly increased with irrigation and with an increasing irrigation amount (α = 0.01).

Significantly higher grain protein, amylose, amylopectin, total starch, soluble sugar, and sucrose contents were observed for BGW grown in Se₂₁₆₀-treated soils under the three water regimes than for BGW grown in the control soils in both seasons (α = 0.05).

Fe, Zn, and Mn concentrations in grains of BGW grown in control soils were slightly increased by irrigation in both seasons but were significantly increased with Se₂₁₆₀ addition (α = 0.01) ( Table 3 ). Regardless of the Se treatment, the Cu concentration in grain was not significantly affected by irrigation. When Se₂₁₆₀ was added to the soil, the Fe concentration in grain was significantly increased with increasing irrigation amount (α = 0.01), but no significant differences were observed for the Zn or Mn concentration under W₁ or W₂ treatment.

Se₂₁₆₀ application significantly increased the Fe, Zn, and Mn concentrations in grains under the three water regimes (W₀, W₁, and W₂) in two seasons compared to Se₀ application in both seasons (α = 0.05) but did not significantly affect the Cu concentration in grain.

In control soils, Se uptake of Se in grains, leaves, stem + leaf sheath, and roots of BGW was not significantly affected by irrigation in either season ( Table 4 ) but was significantly increased when Se₂₁₆₀ was applied to the soil (α = 0.01). Regardless of the Se treatment, irrigation had no significant effect on the Se concentration in the spike-stalk + glume. After applying Se₂₁₆₀, the Se concentration in the grains, leaves, and roots of BGW were significantly increased with an increasing irrigation amount (α = 0.01), but no significant differences were found for the stem + leaf sheath between W₁ and W₂ treatments. Regardless of the water regime, BGW grown in soils with the addition of Se₂₁₆₀ had a significantly higher Se concentration in the 5 plant parts than BGW grown in control soils (α = 0.05). The TF_root-grain of BGW grown in Se₂₁₆₀-treated soil was increased by irrigation in both seasons, but no significant differences were found among the three water regimes ( Table 4 ). Se₂₁₆₀ application did not significantly affect TF_root-grain under any of the water regimes in either season.

As shown in Figure 3 , a large proportion of the total Se (27.8–38.9%) was distributed in roots under all the treatments in both seasons, whereas a small amount of Se (9.3–12.8%) was observed in grains. Irrigation had no significant effect on the Se distribution in each part of BGW grown in control soils, but it significantly decreased and increased the Se distribution in spike-stalk + glume and leaves, respectively, when applying Se₂₁₆₀ to the soil (α = 0.01). Moreover, following Se₂₁₆₀ application, the Se distribution decreased in roots (BGW-1, W₁: 31.8% vs. 30.2%, W₂: 30.8% vs. 29.8%; BGW-2, W₁: 37.3% vs. 35.7%, W₂: 37.3% vs. 35.1%) and increased in grains (BGW-1, W₁: 10.8% vs. 12.8%, W₂: 11.6% vs. 12.5%; BGW-2, W₁: 11.4% vs. 12.7%, W₂: 10.8% vs. 12.0%) with increasing irrigation amounts in both seasons. After Se₂₁₆₀ application, there were no significant differences in Se distribution in the stem + sheath among the three water regimes in either season, but the lowest and highest Se distributions in roots and grains, respectively, were observed under W₂ treatments (α = 0.01).

A significantly higher Se distribution in the roots of BGW-1 (2019–2020) and BGW-2 (both seasons) and a significantly lower Se distribution in the stem + sheath of BGW were observed in Se₂₁₆₀-treated soils under the three water regimes than in control soils (P = 0.05). Moreover, BGW grown in Se₂₁₆₀-treated soils showed a significantly higher Se distribution in grains than BGW grown in control soils under the W₂ treatment (α = 0.05).

Selenium distribution in each part represents the proportion in the total Se taken up by the whole plant. W₀, no irrigation throughout the entire growing period; W₁, irrigated at the wintering stage (Feekes 3.0); W₂, irrigated at both the wintering and greening stage (Feekes 4.0); Se₀, no Se fertilizer; Se₂₁₆₀, 2160 g·ha^-1 pure Se; BGW-1, black-grained wheat genotype Xihei 88; BGW-2, black-grained wheat genotype Heidali.

In control soil, the bioavailable Se concentration in the 0–20-cm layer was not significantly affected by irrigation but was significantly reduced in Se₂₁₆₀-treated soils in both seasons (α = 0.01) ( Table 5 ). Moreover, following Se₂₁₆₀ application, the soil available Se concentration significantly decreased with an increasing irrigation amount (α = 0.01). The bioavailable Se concentration in the 0–20-cm layer of Se₂₁₆₀-treated soil was significantly higher than that in control soil under the 3 water regimes (α = 0.05). Irrigation and Se₂₁₆₀ application did not affect the bioavailable Se concentration in the 20–40-cm layer of soil in either season.

Wheat is the second most produced cereal grain worldwide, and it plays an essential role in the human diet. It is also a staple food for nearly half of the Chinese population, where up to 85% of wheat is consumed as flour-derived products. The overall demand for foodstuffs with high nutritional value is increasing due to the increasing prevalence of lifestyle diseases worldwide. In addition to traditional cereal crops, color-grained wheat provides an opportunity for cultivation and processing (Padhy et al., 2022). Feng et al. (2009) reported that irrigation significantly affected grain yield and the 1000- kernel weight of green-grained wheat, and their highest values were found under irrigation treatment (irrigation at fifth leaf emergence, anthesis, and grain-filling stages). The 1000-kernel weight of color-grained wheat significantly differed between irrigation two and three times at 15 and 25 days after anthesis (Yang et al., 2010). Higher antioxidant contents and lower grain yields have been observed for selected pigmented wheat in an organic cropping system and in drier years (Zrckova et al., 2018). Drought stress (drought induced by skipping irrigation at the fourth leaf and anthesis stages) significantly reduced gas exchange parameters and grain yields in wheat (Ejaz et al., 2022). In the present study, the least rainfall was recorded between December and January and between February and March in both years ( Figure 1 ), occurring during the wintering and green-turning stages, consistent with the time of irrigation. In this study, the net photosynthetic rate of BGW was remarkably improved by water application. The highest grain yield, spike number, and kernel number were found in BGW at the wintering and green-turning stages. Non-significant differences in the 1000-kernel weight may be due to the amount and timing of irrigation.

Previous studies have reported that Se fertilizer promotes photosynthetic efficiency regarding pigments contents and gas exchange parameters, thereby markedly boosting plant growth and biomass accumulation (Alves et al., 2020). The different results of the net photosynthetic rate observed in this study may have been due to Se fertilizer and its application method. Alves et al. (2020) applied 500 mL of 10 μM sodium selenite solution daily to rice plants during the entire experiment, whereas, in the present study, 2160 g·ha⁻¹ Se ore powder was applied and mixed thoroughly with the soil before plowing.

Supplemental Se resulted in a much higher grain yield under normal water stress conditions (Nawaz et al., 2015). Almost all yield and yield components of wheat were significantly increased by exogenous Se application (Se fertigation and Se foliar spraying) (Nawaz et al., 2015). Se application can improve plants’ defense systems by enabling them to recruit and boost beneficial microorganisms in rhizosphere soil, providing further protection (Li et al., 2022). The application of 600 kg·ha⁻¹ organic Se fertilizer to soil substantially enhances wheat yield (Chen et al., 2023). In this study, the highest grain yield and its components in BGW were observed with Se₂₁₆₀-treated soil under irrigation treatments compared to rain-fed conditions, consistent with previous studies. This may be explained by the improved photosynthetic traits and increased sugar content (as observed in the present study), which contribute to crop growth (Guo et al., 2022). In addition, soil texture, soil physicochemical characteristics, and the method and timing of Se application influence its relative effectiveness in improving crop yield. Irrigation with soil application of Se ore powder maximizes the effect of soil chemistry and microbiology on Se uptake and accumulation, thus improving grain yield and its components.

Malnutrition, unhealthy diets, and lifestyle changes are the major risk factors for chronic diseases in humans, adversely affecting sustainable development goals. Color-grained wheat and its derived products are key to global nutritional security (Padhy et al., 2022). Biofortification is a strategy employed to produce crop products rich in deficient elements, but the concentrations of other nutrients important for human consumption should not be adversely affected (Sunic and Spanic, 2024). Improved nutrient uptake largely depends on the crop species and its growth environment (Drahoňovský et al., 2016). The total protein content of purple- and green-grained wheat was significantly affected by irrigation (Yang et al., 2010). The differing grain protein contents in the present study may be due to the timing and amount of irrigation. Se supplementation increases mineral uptake and water-soluble protein and sugar contents (Alves et al., 2020). The combination of Se and high nitrogen increased protein concentrations but decreased Fe, Mn, Cu, and Zn concentrations in rice grains (Teixeira et al., 2021). The findings of this study concerning the grain protein content are consistent with those of previous studies, a result that may be due to the increased nitrogen uptake under Se fertilization (Guo et al., 2022). Se fertilizer had a positive effect on carbohydrate accumulation in rice grains (Teixeira et al., 2021), and the mechanism involved carbon fixation, transport, and metabolism. Water-stressed wheat plants fertigated with Se had a higher total soluble sugar content than those under a normal water supply (Nawaz et al., 2015). The soluble sugar content in tomato increased considerably after the application of Se-enriched fertilizer (Huang et al., 2021). Significantly higher soluble sugar and sucrose contents were observed in BGW after Se₂₁₆₀ application, possibly due to increased acid invertase activity, because it plays an important role in sugar accumulation (Zhu et al., 2017). Sugar transporter genes may be upregulated in response to Se application (Ren et al., 2022).

Water stress markedly decreased the grain Fe, Zn, and Mg concentrations after exogenous Se supply (Nawaz et al., 2015). Na₂SeO₄ as a Se fertilizer application enhanced the quality of water-stressed plants (Nawaz et al., 2015). Water stress reduced the micromineral content in wheat grains (Silva et al., 2020). However, Se-enriched irrigation had little effect on the concentrations of macro- or microelements (Cu, Fe, I, K, Mg, Mn, P, and Zn) in the dry weights of green pea and carrot (Ragalyi et al., 2022). The increased Fe concentration in grains by Se fertigation might be attributed to increased osmoprotectant production or to the activities of catalase, peroxidase, and ascorbate peroxidase (Nawaz et al., 2015). Se facilitates the biosynthesis of pigments, such as chlorophyll, by improving nutrient accumulation, thus benefitting the photosynthetic system (Alves et al., 2020). This could explain the increased Fe, Zn, and Mn concentrations observed in the present study. The Fe, Zn, and Mn concentrations significantly increased in BGW grains due to irrigation after applying Se₂₁₆₀ application to the soil, but Cu concentrations in grains showed no positive effects, suggesting that Se ore powder fertigation has synergistic effects on Fe. The concentration of Fe, Zn, and Mn concentrations were increased in BGW grains following irrigation and Se application. This finding was not in accordance with the observations reported by Chen et al. (2023), who observed an increase in the Ca concentration following Se fertilization. This may be attributable to variations in the types of Se fertilizer and growing environments.

Se is crucial for human and animal nutrition because of its function as a co-factor in many enzymes (Gupta et al., 2021). In China, approximately 51% of soils are deficient in Se (Dinh et al., 2018), resulting in people in this region experiencing Se deficiency. Therefore, biofortification technology for increasing the Se concentration in crop plants has become a popular research field in recent years.

Se application to staple crops can enhance the Se concentration in grains, thereby meeting the demand for Se in the human diet (Gupta et al., 2021). A proper dose of Se fertilizer increases Se accumulation in different plant parts (Teixeira et al., 2021). Se application significantly increases the Se concentration in wheat organs, and the leaves have the strongest Se accumulation ability, followed by grains, stems, and glumes (Ma et al., 2022). The Se contribution in different parts of the wheat plant after the application of different organic Se fertilizer levels to the soil has been shown to have the following order: roots > grains> ear stem > glume > leaves > stem (Chen et al., 2023). The findings of this study confirmed that soil application of Se ore powder resulted in a much higher Se accumulation in each part of BGW plants. These results are consistent with the findings of Deng et al. (2018), who observed a greater proportion of Se in rice grains and beans after Se ore powder application. The higher Se accumulation in each plant part by Se fertigation may be attributed to the greater abundance of oxidizing bacteria and the improved soil redox environment created by irrigation conditions, which retain more available Se for plants, reducing residual soil Se and increasing the Se concentration in various plant parts (Zhou et al., 2022).

Se primarily accumulates in the roots under selenite fertigation treatments, while Se is largely transported to shoots under selenate fertigation treatments (Zhang et al., 2018). In this study, following soil application of Se₂₁₆₀, the irrigation treatments significantly increased the Se concentration in the roots, leaves, and grains of BGW, suggesting that the combination of Se ore powder application and irrigation enhances the Se uptake capacity. Se likely facilitates the response to the irrigation water supply in the expression of sulfate transporter OsSULTR1.2, phosphate transporter OsPT2, and the silicon transporter OsNIP2.1 in roots, thereby improving the Se uptake capacity in roots and increasing the Se concentration in leaves and grains (Teixeira et al., 2021). BGW grown in soil with 2160 g·ha⁻¹ Se ore powder was a more efficient Se accumulator under the W₂ treatment due to Se being readily taken up by the roots, yielding a significantly higher Se concentration in the grains than that under the W₁ treatment. The potential for Se uptake by wheat in Se-enriched soils is greatly enhanced by irrigation, which is especially important in the presence of elevated water amounts, as this increases Se uptake by BGW. The use of irrigation both during the wintering and greening stages in soil with Se₂₁₆₀ addition did caused Se enrichment in BGW, thus, it is recommended to be used for biofortification. Se-enriched BGW could be useful as a raw material in Se-deficient areas where the population largely depends on cereal foods. The results of this study confirm that irrigation after Se₂₁₆₀ application can not only achieve BGW with high grain yield but also produce higher grain nutritional quality.

The edible parts of crop plants are the main sources of dietary Se, while the Se concentration in crops is determined by Se bioavailability in the soil. Most Se in the soil is not available to plants (Ali et al., 2017). Se⁴⁺, Se⁶⁺, and elemental Se (Se⁰) are the main forms distributed in soil (Guo et al., 2022). Among these, Se⁴⁺ and Se⁶⁺ are the main Se fractions determining the bioavailable Se concentrations in the soil. Se fractions are separated into soluble Se, exchangeable Se, Fe/Mn oxide-bound Se, organic matter-bound Se, and residual Se (Ali et al., 2017). However, the distribution patterns of Se fractions in soil are in a state of dynamic equilibrium regulated by adsorption/desorption, precipitation/dissolution, and oxidation/reduction processes (Guo et al., 2022). The intensity of these transformation processes is controlled by soil properties, such as soil texture, moisture, pH, redox conditions, organic matter, and microbial functions (Guo et al., 2022).

Flooded environments promote residual Se transformation into Fe/Mn oxide-bound Se, and thus increasing the Se activity of Se in the soil (Deng et al., 2018). Flood irrigation reduced the bioavailability of soil Se, and the concentration of water-soluble and ion-exchangeable Se was reduced from 8.0% to 5.0% of the total Se (Song et al., 2020). Se in the soil of riverside flood plains can be transformed into water-soluble and ion-exchangeable Se (VI) (Zhang et al., 2023). Moreover, water retention regulation and organic matter transport affect Se speciation and partitioning in the soil of mollisol lands far from river channels. Se fertigation with nanobubbles reduces soil Se accumulation and increases the Se content in cucumber (Zhou et al., 2022). The results of this study confirmed our hypothesis that irrigation and increasing irrigation amount would decrease soil residual Se in the 0–20-cm layer following Se₂₁₆₀ application, ultimately improving wheat grain yield and quality. This result may be ascribed in part to the transformation of organic-bound Se and infiltrated oxygenated water, which increase water-soluble and ion-exchangeable Se (Zhang et al., 2023). By analyzing the variation in the soil bioavailable Se concentration in the 0–20-cm layer under the same water regimes, we found that Se ore powder application promoted soil bioavailable Se accumulation. Se in ore powder is mainly present as Se⁴⁺, and Se⁰ is formed at a ratio of 4:6 (Deng et al., 2018). Thus, changes among valence states caused by the soil application of Se ore powder can affect Se fractions because of their differences in migration mobility and binding intensity, further affecting Se bioavailability in the soil.