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Article

Selenium Biofortification Improves Grain Quality and Reduces Arsenic Accumulation in Rice Under Alternate Wetting and Drying Irrigation

by
María J. Poblaciones
1,
Luis Vicente
1,
Damián Fernández-Rodríguez
1,
Ángel Albarrán
1,
David Peña
2 and
Antonio López-Piñeiro
3,*
1
Área de Producción Vegetal, Escuela de Ingenierías Agrarias—Instituto Universitario de Investigación del Agua, Cambio Climático y Sostenibilidad, Universidad de Extremadura, Avda. Adolfo Suárez s/n, 06007 Badajoz, Spain
2
Área de Edafología y Química Agrícola, Escuela de Ingenierías Agrarias—Instituto Universitario de Investigación del Agua, Cambio Climático y Sostenibilidad, Universidad de Extremadura, Avda. Adolfo Suárez s/n, 06007 Badajoz, Spain
3
Área de Edafología y Química Agrícola, Facultad de Ciencias—Instituto Universitario de Investigación del Agua, Cambio Climático y Sostenibilidad, Universidad de Extremadura, Avda. de Elvas s/n, 06071 Badajoz, Spain
*
Author to whom correspondence should be addressed.
Agronomy 2026, 16(13), 1220; https://doi.org/10.3390/agronomy16131220
Submission received: 22 May 2026 / Revised: 17 June 2026 / Accepted: 23 June 2026 / Published: 24 June 2026
(This article belongs to the Special Issue Agroecological Strategies for Improving Nutritional Quality of Cereal Crops)
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Abstract

Rice production is under increasing threat from adverse climatic trends that exacerbate water scarcity and compromise food safety. The need to transition toward water-saving irrigation is urgent, as is the requirement of addressing the dual burden of selenium (Se) deficiency and arsenic (As) toxicity. This 3-year field study (2020–2022) is the first to evaluate the effects of integrated water-saving irrigation. Permanent flood irrigation (Flood) or alternate wetting and drying was used, in which fields were reflooded when the soil matric potential reached −20 kPa (Reflood-20) and −70 kPa (Reflood-70); the effects of foliar Se biofortification at 15 g Se ha−1 with sodium selenate (15-Se) or no Se (No-Se) on rice production and Se and As accumulation were also investigated. The results identified the Reflood-20 regime as the optimal strategy, achieving 36% water savings without significant grain yield penalties while enhancing grain quality. Foliar Se application successfully increased the dehulled grain Se content by 10.7-fold, effectively meeting human dietary requirements. The As contents were decreased by 27.6% due to water restriction, and an additional 10% loss was observed because of Se supplementation. Analysis of the straw also showed a 23.5% decrease in As and a 5.7-fold increase in Se. Consequently, the synergy between moderate deficit irrigation and Se biofortification provides a robust, cost-effective framework for the large-scale production of safer, nutrient-dense rice, reconciling resource efficiency with food security.

1. Introduction

With 2030 approaching, progress in reducing hunger and food insecurity is still too slow, which is putting the achievement of Sustainable Development Goal (SDG) Target 2.1 at serious risk. In addition, global nutrition indicators show that the world is not on track to achieve SDG Target 2.2. As a result, billions of people still lack consistent access to nutritious, safe, and sufficient food [1], a situation that is of growing concern given ongoing population growth and shifting climatic patterns [2]. In this regard, previous studies have shown that changes in precipitation patterns and increased evapotranspiration resulting from rising temperatures lead to significant declines in yields of major cereals, such as wheat, maize, and rice [3,4,5]. In this context, one of the main global challenges is meeting the growing demand for food in a sustainable manner, both from an environmental and an economic perspective [2].
Rice (Oryza sativa L.) is the most important staple grain, as it feeds nearly two-thirds of the world’s population and is a significant source of fiber, protein, vitamins, antioxidants, and other biomolecules that are beneficial for health [6]. In addition, rice is widely recognized as a popular gluten-free grain, making it especially suitable for individuals with celiac disease [7]. However, the traditional rice irrigation system involves continuous flooding throughout the entire growing season, resulting in high water consumption. Notably, Zhang et al. [8] indicated that more than 60% of agricultural water use is currently devoted to rice cultivation. Consequently, water scarcity in rice-growing regions has become a major obstacle to improving the yield and sustainability of rice cultivation [9]. This situation is further exacerbated in rice-producing regions with a Mediterranean climate, where droughts are becoming increasingly frequent and severe [10], making it necessary to provide extraordinary direct aid to ensure the economic viability of the sector. Therefore, there is an urgent need to develop strategies to improve the resilience of rice, especially in the producing regions of vulnerable countries such as Spain and Italy, the largest rice producers in Europe, where the sector faces important challenges due to climate change [10,11].
One of the most promising and widely used water-saving techniques is alternate wetting and drying (AWD). This technique could be readily implemented using existing infrastructure in rice fields, alternating between drying phases and re-irrigation when soil moisture declines to a predetermined threshold [12]. Thus, the implementation of AWD leads to significant improvements in water-use efficiency [13]. However, one of the main challenges in implementing it is concerns about its impact on crop yields. Specifically, compared to permanent flooding, AWD irrigation has been reported to reduce rice yield [13] by up to 33% [14]. In this regard, Vicente et al. [10] noted that the implementation of AWD irrigation, especially under severe conditions, could reduce the effectiveness of herbicides, suggesting that weed control must be taken into account under AWD irrigation systems. Weeds are a major biotic constraint in rice production, as they compete with rice plants for essential resources and can cause significant yield reductions [15]. However, other studies have observed no significant effect [16] or even an increase in rice yield under AWD irrigation [17]. Rice field drying can enhance root system development, leading to more productive tiller growth [18]. Consequently, increasing the number of effective tillers is a key determinant of panicle and spikelet production, which ultimately contributes to higher grain yield [19]. These controversial findings may be due to the wide variability in AWD management, which is related to factors such as the frequency of drying and flooding cycles, soil types, agricultural practices, and climate [20,21]. This situation underscores the importance of conducting further studies to better understand the effects of AWD on rice yield.
Selenium (Se) is an essential micronutrient for both humans and animals, primarily due to its role in enhancing antioxidant activity. Adequate Se intake has been associated with several health benefits, including cancer prevention, cell protection, improved immune system function, and defense against aging-related processes and cardiovascular diseases [22,23]. However, Tolu et al. [24] have recently estimated that up to one billion people worldwide have insufficient Se intake—the adequate intake of which for adults is 70 μg day−1 [25]—largely due to the low concentrations found in staple food crops. Therefore, consuming Se-fortified food products is considered the most effective and safest strategy for supplementing Se intake [26]. In this context, rice is one of several dietary components that significantly contribute to Se intake [22]. As awareness of the importance of dietary Se supplementation has increased, Se-fortified rice has gained growing popularity among consumers [27]. Furthermore, recent studies have highlighted the positive effects of Se on plant growth and development. In this regard, several studies have shown that foliar Se application can enhance the photosynthetic rate of rice, promoting dry matter accumulation and consequently increasing rice yield [28,29]. These effects could be particularly beneficial under AWD management, potentially improving both crop yield and grain quality, aspects that have remained largely unexplored until now. Furthermore, Se is often found in association with other metals and metalloids, such as cadmium, lead, and arsenic [30]. Specifically, Se and arsenic (As) share important similarities in their geochemical behavior and often occur together in soils [31]. In this sense, compared to other crops, rice accumulates As more efficiently, making it one of the main contributors of As to the human food chain [32]. Consequently, rice may be a significant source of human exposure to As, which could lead to serious health problems [33]. A recent study demonstrated that Se biofortification can improve rice yield while potentially reducing As accumulation [34]. Similarly, Pokhrel et al. [35] indicated that selenate is especially effective at decreasing As bioaccumulation in rice grains. However, these studies were conducted under pot experimental conditions with permanent flooding irrigation. Therefore, field studies are needed to evaluate the combined effects of Se application and AWD strategies on rice grain quality and yield. Such research would support informed decision-making by rice growers and contribute to the transition towards sustainable and high-nutritional-value rice agroecosystems.
Thus, the present field study represents the first evaluation of the impacts of two intensities of AWD combined with Se application on grain quality and productivity. To achieve this, the effects of sodium selenate application under moderate and severe AWD were assessed on rice yield components and As and Se bioaccumulation in straw and whole and dehulled rice grains over three consecutive growing seasons.

2. Materials and Methods

2.1. Study Site

A field experiment was carried out during three consecutive rice-growing seasons (2020–2022) in southern Spain (38°92′ N, 6°96′ W). Based on the climatic classification proposed by Papadakis [36], the study area is located in the semi-arid Mediterranean region. During the experimental period, a mean annual temperature of 16.6 °C and an average annual rainfall of 394 mm were recorded at the site (Figure 1). The field had been cultivated with rice under conventional permanently flooded irrigation conditions for approximately 15 years prior to the study. Following harvest, which typically occurred between September and October, the soil remained fallow until the next sowing period in April or May.
According to the soil classification system of the Food and Agriculture Organization [37], the soil was identified as a Hydragic Anthrosol, the initial physicochemical properties (0–20 cm) of which at the beginning of the study are presented in Table 1.

2.2. Experimental Design and Field Management

The experiment was designed as a randomized complete block with a split-plot arrangement and three repetitions, resulting in a field trial with 18 basic plots. Main plots were exposed to permanent flood irrigation (Flood), where water depth was maintained at 10 cm from the soil surface from 7 to 12 days after sowing until 10 to 15 days before rice harvest, which represented the traditional flooding practice. In addition, reflooding was performed whenever the soil matric potential reached −20 kPa (at 15–20 cm depth) (Reflood-20) or −70 kPa (at 15–20 cm depth) (Reflood-70). Therefore, during the growing season, the Reflood-20 treatment typically involved 8–9 drying–flooding cycles, whereas 5–6 such cycles were generally implemented under the Reflood-70 treatment. The soil matric potential was then monitored on a daily basis with Watermark 200SS sensors (Irrometer, Riverside, CA, USA). The mean volumes of water applied under the different treatments were 21,786 m3 ha−1 (Flood), 13,932 m3 ha−1 (Reflood–20), and 10,643 m3 ha−1 (Reflood-70), as measured using flow meters (Hidroconta, Murcia, Spain). Subplots were used to measure foliar application of 0 (distilled-water spray) and 15 g Se ha−1 using sodium selenate (Na2SeO4), referred to as 15-Se and No-Se, respectively. Spraying was performed using a hand-held sprayer at the anthesis stage (>80% plants flowering) using an aqueous solution at a rate of 800 L ha−1. To avoid cross-contamination, Se was applied under wind speeds below 10 km h−1 and at a constant pressure of 2 bar. Furthermore, a conical hood attached to a hand-held sprayer was used to ensure accurate application and effectively prevent spray drift.
The rice variety used throughout the three years of the study was Sirio, obtained from Copsemar (Madrid, Spain). In all treatments, sowing was carried out each year during the first half of May using the Sola Neumansem 799 planter (Sola, Barcelona, Spain) at a seeding rate of 160 kg ha−1, while harvesting took place in early October. Nitrogen fertilization was applied annually at a total rate of 115 kg N ha−1, divided between basal and top-dressing applications. Herbicides (such as Pendimethalin, Imazamox, Bentazone, and MCPA) were also applied for weed control. All other agronomic practices were conducted according to local management recommendations.

2.3. Rice Performance

Rice plants were sampled at physiological maturity using two randomly selected quadrats of 1 m2 per basic plot, which were then combined (2 m2), corresponding to a total sampled area of 6 m2 per treatment, each of which has an area of 270 m2. Within these areas, grain yield was calculated after weighing the filled grains collected from panicles and adjusted to a standard moisture content of 0.14 g H2O g−1 fresh weight. Aboveground biomass production was also measured. In addition, the 1000-grain weight was determined using a Swantech-SC2 (Sadkiewicz Instruments, Bydgoszcz, Poland).

2.4. Soil and Plant Analysis for Selenium and Arsenic

Each October (in 2020–2022), after the rice harvest, four to five subsamples of soil at 0–20 cm depth were collected from each plot and combined to form a composite sample. These soil samples were air-dried at room temperature (20–25 °C), ground, and sieved to <2 mm to determine available Se and As contents. For this purpose, extracts were obtained using KH2PO4 (0.016 mmol, pH 4.8) (ratio 10 g dry weight soil: 30 mL KH2PO4 w/v), and the contents were determined using an Inductively Coupled Plasma Mass Spectrometer (ICP-MS) (Agilent 7500ce, Agilent Technologies, Palo Alto, CA, USA) operating in the hydrogen gas mode.
For mineral analysis (Se, As), samples of straw, grain, and dehulled grain were ground to a particle size of less than 0.45 mm, and 1 g was digested in a mixture of ultra-pure nitric acid and 30% hydrogen peroxide using a microwave digestion system (Mars X, CEM Corp, Matthews, NC, USA). The solution was diluted and analyzed by ICP-MS, with quality control ensured by the use of blanks and standard reference materials (tomato leaf, NIST 1573a), resulting in a 95% recovery rate for the minerals [38].

2.5. Statistical Analysis

All data were initially evaluated for normality and homoscedasticity using the Shapiro–Wilk and Levene tests, respectively. Split-split-plot analysis of variance (ANOVA) within mixed-design models was employed to analyze the effects of the growing season (2020–2022), irrigation system (Flood, Reflod-20, Reflod-70), Se foliar treatments (No-Se, 15-Se), and their interactions on key crop parameters. The parameters examined included grain yield, biomass production, 1000-grain weight, and Se and As concentrations in whole and dehulled grains and in rice straw. When the results of the ANOVAs indicated significant differences, mean comparisons were performed using Fisher’s protected least significant difference (LSD) test, with a significance level of p ≤ 0.05. All analyses were performed using the Statistix v. 8.10 (Analytical Software, Tallahassee, FL, USA) package.

3. Results and Discussion

3.1. Effects on Grain and Biomass Production

As shown in the analysis of variance (Table 2), grain yield, biomass production, and 1000-grain weight were significantly influenced by the year of the study, the irrigation system, and Se application (the latter did not influence grain yield). However, none of the interactions in the model had a significant effect on any of the production parameters studied.
The effect of the growing year varied across the parameters evaluated. While the grain yield in 2022 was 22.4% higher than the average of the preceding two years, biomass production peaked in 2021—specifically, it was 39.3% and 11.2% higher than in 2020 and 2022, respectively (Table 3). The highest grain yield in 2022 may be attributed to the higher minimum temperatures (15.5 °C) recorded from June to mid-July, encompassing critical vegetative and reproductive stages, compared with 13.9 °C in 2020 and 13.6 °C in 2021, (Figure 1). Consistently, Shi et al. [39] demonstrated that low-temperature stress during rice reproductive stages induces yield losses via reduced spikelet fertility. Notably, the mean grain yield obtained in this trial, 8983 kg ha−1, substantially exceeded the regional average for the Guadiana River area (7400 kg ha−1) [40]. Conversely, the 1000-grain weight was significantly higher in 2020, being 6.7% higher than in 2021 and 10.6% higher than in 2022 (Table 3). This pattern likely reflects a fundamental physiological source–sink trade-off in which limited resources are partitioned between competing growth and reproductive processes [41]. In fact, Li et al. [42] reported a negative correlation between the 1000-grain weight and grain yield, although this relationship could depend on the rice variety.
The yields declined as water scarcity intensified. The intermediate treatment, Reflood-20, did not significantly reduce grain yield, despite minor decreases in biomass (6.8%) and 1000-grain weight (by 2.2%). Conversely, the severe treatment significantly reduced grain yield by 14.1% compared to Reflood-20 (Table 3). Notably, Reflood-20 achieved 36% water savings compared to traditional flooding, offering a cost-effective strategy for enhancing regional crop sustainability without any extra financial investment. These findings align with those of Zhang et al. [21], though the yield reduction under severe stress (14.2%) was less pronounced than their reported 18% for severities greater than −40 kPa. Considering that drought events are becoming increasingly recurrent in this area, any reduction in water consumption is critical.
No significant increase in grain yield was observed with agronomic biofortification with Se, although biomass and 1000-grain weight improved by 10.2% and 2.2%, respectively (Table 3). The lack of a significant impact on yield corroborates the findings of Ramos et al. [43] in their field trial in Brazil that spanned four years and involved 13 cultivars, in which they used a rate of 60 g ha−1. However, Luo et al. [44] found a 6.3% increase in fragrant rice. The positive effect observed on the 1000-grain weight and biomass was consistent with the findings of Boldrin et al. [45], despite conflicting findings in other studies [46,47,48].

3.2. Effects on Available Se and as in the Soil

Available concentrations of As and Se levels were not significantly influenced by study year, irrigation system, or Se application (Table 2). Available Se levels ranged from 1.35 to 1.70 µg kg−1, while As levels fluctuated between 58.4 and 87.5 µg kg−1. These values are consistent with regional findings by Rodrigo et al. [49], who, for 19 locations in the same region, reported that bioavailable Se and As represent approximately 2.5% and 3% of their total concentration, respectively, with a range between 1.7 and 6.3 µg kg−1 for Se and between 60 and 460 µg kg−1 for As. According to the Hawkesford and Zhao [50] soil classification system, this soil can be considered marginal for Se and non-toxic regarding As [51].

3.3. Effects on Se and As Contents on Whole and Dehulled Grain

Selenium concentrations in the whole and dehulled grains were significantly influenced by all the studied factors and their interactions in the model (Table 2). Under flooded conditions without external Se application, the Se content ranged from 19.8 to 27.5 µg kg−1. These levels were low, but slightly higher than those found by Rodrigo et al. [49] in this region. Higher values (67 μg kg−1) were found by Matos-Reyes et al. [52] in south-east Spain, and by Ventura et al. [53] in Portugal (40 μg kg−1) for white rice grain. As observed by Lidon et al. [54], climatic conditions significantly influence the patterns of Se uptake, translocation, and final accumulation in rice grains. In the present study, whole grains accumulated more Se in 2021, exceeding 2020 and 2022 levels by 60.7% and 23.6%, respectively. However, upon husk removal, the highest Se content was recorded in 2020 (156.2 µg kg−1), followed by 2021 (126.3 µg kg−1) and 2022 (102.3 µg kg−1). These findings indicate that Se accumulation within the husk varied annually, with significantly higher levels observed in 2021 compared to the other two years (Table 3).
In the Reflood-20 irrigation system, Se accumulation significantly improved in both paddy and dehusked rice. This enhancement is likely due to the aerobic conditions inherent in this management system, which favor the presence of selenate—a highly water-soluble Se species [55].
The application of 15 g Se ha−1 as sodium selenate led to a 9.7-fold increase in Se in the whole grains, rising from 34 to 330 µg kg−1. The efficiency of foliar Se application in increasing the grain Se content is well documented, though the magnitude varies with genetics, application timing, and environmental factors. Lidon et al. [54] quantitatively reported increases ranging from a modest 5-fold enrichment in cultivars like Ariete and Ceres at doses up to 100 g Se ha−1 to over 800-fold in genotypes such as Albatros and OP1105 when doses reach 300 g Se ha−1. More recently, Ramos et al. [43], through evaluating 13 irrigated rice cultivars, further highlighted this genetic variability, showing that it varied by up to 73 times in hybrids compared to control levels, under identical field conditions. Regarding timing, Deng et al. [56] found that Se concentrations in brown rice can increase by up to 2-fold when applied at full heading compared to at the late tillering stage, as nutrients are prioritized for grain filling during later stages.
The accumulation was even higher when the husk was removed, with a 10.7-fold increase, rising from 21.9 to 234.7 µg kg−1, indicating that Se was more effectively partitioned into the endosperm rather than the husk. Lidon et al. [54], in the Lisbon area (Portugal), found lower increases in whole grains of between 1.57 and 2.04 following the application of 20 g Se ha−1, with an average Se loss of 28.3% during dehusking.
When examining the three-way interaction (year × irrigation × Se application), 2020 stands out for both grains. Under the Reflood-20 regime with Se application, the concentration reached 596 µg kg−1 in whole grains and 460 µg kg−1 in dehulled grains (Figure 2A,B). This higher accumulation did not occur in subsequent years, when accumulation did not follow a clear pattern with respect to irrigation (Figure 2B,C). As the maximum tolerable level of Se intake is 400 μg per day [22], even at the highest level of 460 μg kg−1 observed here, the consumption of 300 g of husked rice would result in an intake of 138 μg of Se. This level is enough to cover the European Recommended Dietary Allowance (RDA) of Se for humans (55–70 μg Se day−1 for women and men, respectively) [57], but remains below the 200 µg day−1 threshold suggested by some studies for specific health benefits, such as cancer risk reduction [58].
Regarding As, its content ranged from 92.7 to 895.5 μg kg−1 in whole grains and from 91.0 to 524.1 μg kg−1 in the dehulled grains, showing an average reduction of 17% with hull removal. Notably, As accumulation in 2022 was significantly higher, by 2.6-fold on average, than in the other study years. These levels align with findings from S.W. Spain (70–820 μg kg−1) [49] and those reported by Moreno-Jimenez et al. [32]. To minimize the dietary intake of As, the FAO/WHO Codex Alimentarius has established a maximum threshold of 0.2 mg kg−1 for inorganic As in polished/white rice, which is often stricter depending on the product, regional standards, and consumer EFSA guidelines. Reducing As levels in grain is therefore critical to minimize intake of this Class 1, non-threshold carcinogen.
Arsenic accumulation, both in whole and dehulled grains, was higher in the final year studied (Table 3). Rodrigo et al. [49] found that rice monoculture—a standard practice in Spanish rice cultivation utilized in this trial—contributes to elevated As levels. This could be due to, under flooded conditions, inorganic As being reduced from As(V) to mobile As(III) [59], which is more efficiently assimilated and translocated into the grain [60]. Consequently, water restriction strategies proved highly effective at reducing As accumulation, leading to reductions of 26.1% and 49.6% under moderate and severe restrictions, respectively, in whole grains, and 27.6 and 47.6%, respectively, in dehulled grains (Table 3). Ma et al. [61] also found a 55% higher As concentration with the use of continuous flooding compared to intermittent flooding. Moreno-Jiménez et al. [32] found that, as successive sprinkler irrigation over 7 years decreased, the total As in dehulled grain decreased to one-sixth of its initial concentration in the flooded system (from 550 to 90 µg As kg−1). Using a pot experiment, Jahan et al. [62] found that the use of intermittent flooding reduced As uptake by between 16% and 64% compared to that seen with continuous flooding, without necessarily compromising crop yield. This demonstrated that reducing the soil redox potential through alternative irrigation minimizes the predominance of arsenite—a more toxic and mobile As species [63]—thereby limiting its concentration in the soil solution and subsequent plant uptake [64].
Selenium biofortification significantly reduced the As content by 9.7% and 10% in whole and dehulled grains, respectively (Table 3). Previous assessments have revealed that Se biofortification can mitigate As accumulation in rice while alleviating the harmful effect of As on grain yield [65]. Zheng et al. [27] found that As–Se co-exposure in germinated rice could reduce the As level in polished rice by up to 73.8%. Previous research has also indicated that Se alleviates heavy metal and metalloid toxicity in plants through various mechanisms, including the inhibition of membrane lipid peroxidation, reducing element uptake and translocation, and alleviating chloroplast damage [66,67,68,69].
A recent study by Huang et al. [70] indicated that foliar selenate effectively decreases cadmium As accumulation by promoting As(V) reduction for efflux, enhancing cell wall sequestration, and boosting antioxidant defenses. These mechanisms are driven by the down-regulation of key transport genes, highlighting Se application as a promising strategy for managing As contamination in rice systems. However, Wang et al. [71] found that while Se application substantially increased grain Se concentrations, it had no significant effects on As concentrations in brown or milled rice. Therefore, it is necessary to continue investigating the effect of applying small doses of selenium on arsenic levels in primary crops.
Upon examining the interaction of the main factors—year of study, irrigation system, and Se application—across all years, increasing water restriction consistently led to lower As accumulation. The minimum As concentration in dehulled grain was recorded under the Reflood-70 regime with Se application in 2020 and 2021. In 2022, however, the cumulative effects of crop management and incorporation of As-rich straw residues likely led to elevated As accumulation across all fractions (Figure 3B). Given the variability reported in the literature, the average 10% As reduction achieved in this three-year field trial represents a noteworthy outcome.

3.4. Effects on Straw Se and as Contents

Because straw incorporation is a standard agronomic practice that significantly influences Se and As accumulation, its composition is essential for understanding system behavior. The selenium content was significantly influenced by the study year, Se application, and their interaction, as well as the three-way year × irrigation system × Se application interaction. Regarding the As content, the irrigation system, Se application, and the year x irrigation interaction were the variables significantly affecting it (Table 2).
In the control treatments, the straw Se ranged from 19.8 to 77.6 µg kg−1, levels that approach the recommended minimum for animal feed (50–100 µg kg−1). The significantly higher Se levels in 2021—which were 108% and 57.0% higher than those in 2020 and 2022, respectively—may be attributed to lower mean maximum temperatures (30.9 °C) during critical reproductive stages in July, compared to the higher temperatures recorded in the other years (Table 3). In control plots, i.e., in flood conditions without Se application, Se varied from 19.8 µg kg−1 in 2020 to 77.6 µg kg−1 in 2021, without significant differences. Levels between 49 and 711 µg Se kg−1 were found by Guan et al. [72] in Zhejiang Province (China), showing that straw, a typical form of agricultural waste, exhibited substantial bioaccumulation of Se from soil. This finding is also of interest for the use of rice in animal feed, as this value is close to the recommended minimum Se intake for maintaining normal animal health, established at 50–100 µg kg−1 of dry matter (DM). Regarding the years of the study, Se content in the straw was much higher in 2021, 57.0% higher than in 2022, and 108% higher than in 2020 (Table 3). The highest Se contents, obtained not only in straw but also in whole and dehulled grain in 2021, could be attributed to the lower mean maximum temperatures (30.9 °C) recorded in July that year, an important period in the crop’s reproductive cycle, compared to those in 2020 and 2022: 36.8 °C and 37.7 °C, respectively (Figure 1). In fact, abiotic stress factors, particularly high temperatures, can cause stomatal closure in rice plants [73], thereby limiting one of the main pathways for Se uptake into plant tissues and reducing the efficiency of foliar Se absorption [74]. Furthermore, minimizing Se volatilization is a key strategy for increasing its accumulation in rice grains and improving the effectiveness of biofortification [75], especially under high-temperature conditions, which promote selenium methylation and subsequent volatilization [76].
On average, the irrigation system did not significantly influence Se accumulation in the straw. However, when examining the three-way interaction (year × irrigation × Se application), the highest level of Se accumulation was recorded in 2020 under the Reflood-20 regime with Se application, resulting in a concentration of 528.6 µg kg−1. This trend was not consistently observed in subsequent years, when accumulation lacked a clear association with irrigation (Figure 2A). Selenium application led to an average 5.7-fold increase from 62.4 µg kg−1 up to 357.9 µg kg−1. Although research on rice straw biofortification is limited, these increases notably exceed the 3.1-fold improvements reported for wheat [36] or triticale [20]; the increases were higher than the 3.1-fold increase obtained in these materials. Importantly, even the peak values remained well below the 4–5 mg Se kg−1 toxicity threshold [77], ensuring safety for agricultural use.
It is estimated that the concentration of As in straw is, on average, 22 times higher than that in grain [62]. In this study, the straw As level was, on average, 24.6-fold higher, ranging from 1707 µg kg−1 to 14,295 µg kg−1. In control plots (i.e., in the flood irrigation system without Se application), the concentration ranged between 11,900 µg kg−1 in 2022 and 14,295 µg kg−1 in 2021, aligning with the upper limits reported by Khan et al. [78] under wetland conditions in Bangladesh, with a variation between 2640 and 12,520 µg kg−1. Given that straw incorporation is a typical agronomic practice that can significantly increase As levels, the reduction in the As content is critical. Ma et al. [61] quantified the amount of As added by the straw, at 3400 µg kg−1 of total As in approximately 20 g ha−1, which is not significant compared to the soil’s endogenous As pool. On average, significantly less As accumulated in 2020 than in 2022, although there was no significant difference compared with 2021. Examining Figure 3A, which shows the year x irrigation system interaction, there was less accumulation with the traditional irrigation method but higher accumulation with both reflooding systems. The water management system is the key factor contributing to As biogeochemistry in rice systems, drastically affecting its bioavailability and subsequent accumulation in plant tissues. The reduction in the As content in the grain under limited water availability was also evident in the straw, where the concentration decreased by 41.0% under moderate restriction (Reflood-20) and by 63.6% under severe restriction (Reflood-70), reaching contents of 6573–9065 µg kg−1 and 3279–6640 µg kg−1, respectively, compared with that achieved using traditional flooding, at 11,000–13,000 µg kg−1. These results exceed the 28% reduction reported by Jahan et al. [62] in pot experiments using alternate wetting and drying.
Furthermore, Se application significantly reduced As accumulation in straw from an average of 9263 µg kg−1 to 7086 µg kg−1, representing a 23.5% reduction (Table 3). This trend mirrored the pattern observed in dehulled grains (Figure 3A), although it was not significant across any of the irrigation systems studied. The greatest reductions occurred during the first two years, with significant differences between the Flood and Reflood-20 treatments (averaging 50.6%) and between the Reflood-20 and Reflood-70 treatments (46.1%). In 2022, these reductions reached 17.6% and 26.7%, respectively. These findings align with the 28% reduction reported by Singh et al. [79] in their results of a hydroponic pot experiment.

4. Conclusions

This study demonstrates that the integration of the Reflood-20 irrigation regime with foliar Se biofortification at 15 g Se ha−1 is the most effective agronomic strategy for optimizing rice production in water-scarce environments. This management technique enabled water savings of 36% without any significant reduction in grain yield, effectively bridging the gap between resource efficiency and crop productivity. Furthermore, this combined strategy successfully enhanced the dehulled grain Se content by 10.7-fold to meet human dietary requirements, while simultaneously suppressing As accumulation by 27.6% through water restriction and an additional 10% via Se supplementation. Therefore, these results support the recommendation to implement an irrigation system based on alternating wetting and drying, ensuring that the soil water potential does not fall below −20 kPa, together with foliar Se application at anthesis. This combined management strategy would be practical for growers in Mediterranean rice-growing systems and would improve sustainability, nutritional quality, and food security. Nevertheless, further studies are needed to analyze the long-term effects of this technique across different rice varieties.

Author Contributions

Conceptualization, M.J.P. and A.L.-P.; methodology, M.J.P., L.V., D.F.-R. and Á.A.; software, L.V. and Á.A.; validation, M.J.P., D.P. and A.L.-P.; formal analysis, M.J.P., L.V., D.F.-R. and Á.A.; investigation, M.J.P., L.V., D.F.-R., Á.A., D.P. and A.L.-P.; resources, Á.A. and A.L.-P.; data curation, M.J.P., D.P. and A.L.-P.; writing—original draft preparation, M.J.P., L.V., Á.A. and A.L.-P.; writing—review and editing, M.J.P., D.P. and A.L.-P.; visualization, M.J.P. and D.F.-R.; supervision, M.J.P., Á.A. and A.L.-P.; project administration, A.L.-P.; funding acquisition, A.L.-P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Grant TED2021–129790B-I00 funded by MCIN/AEI/10.13039/501100011033 and by European Union NextGeneration EU/PRTR and Grant RTI2018–095461-B-I00, PID2021–123062OB-100 funded by FEDER, UE, and Grant GR21038, GR24018 funded by the Extremadura Regional Government.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request due to the need to ensure that the dataset is used appropriately within the methodological framework of the study.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agronomy 16 01220 g001
Figure 1. Mean rainfall (blue bars), average maximum (red line) and minimum (green line) temperatures, and rice crop evapotranspiration (ETc; cyan line) recorded at the field location during the rice growing season in 2020–2022.
Figure 1. Mean rainfall (blue bars), average maximum (red line) and minimum (green line) temperatures, and rice crop evapotranspiration (ETc; cyan line) recorded at the field location during the rice growing season in 2020–2022.
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Figure 2. Effect of study year, irrigation system, and Se application interactions on whole grain (A), dehulled grain (B), and straw (C) Se contents. Different letters indicate significant differences (p ≤ 0.05) according to the LSD test.
Figure 2. Effect of study year, irrigation system, and Se application interactions on whole grain (A), dehulled grain (B), and straw (C) Se contents. Different letters indicate significant differences (p ≤ 0.05) according to the LSD test.
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Figure 3. Effect of study year, irrigation system, and Se application interactions on dehulled grain (A) and effect of study year and irrigation system interactions on straw As content (B). Different letters indicate significant differences (p ≤ 0.05) according to the LSD test.
Figure 3. Effect of study year, irrigation system, and Se application interactions on dehulled grain (A) and effect of study year and irrigation system interactions on straw As content (B). Different letters indicate significant differences (p ≤ 0.05) according to the LSD test.
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Table 1. Physicochemical soil properties at study outset.
Table 1. Physicochemical soil properties at study outset.
PropertiesValue
Sand (%)62.0
Silt (%)23.4
Clay (%)14.6
TOC (g kg−1)8.64
pH (H2O)5.51
EC (dS m−1)2.81
N (g kg−1)0.76
CEC (cmolc kg−1)10.8
Available As (µg kg−1)70.3
Available Se (µg kg−1)1.32
TOC: Total Organic Carbon; EC: Electrical Conductivity; N: Total Nitrogen; CEC: Cation Exchange Capacity.
Table 2. Summary of the three-way ANOVAs showing, DF (degrees of freedom), F-values and level of significance (* p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001) of the effect of the Study year, Irrigation system, Se application and their interactions on grain yield, biomass production, 1000-grain weight, Se and As contents in soil, grain, dehulled grain and straw of rice.
Table 2. Summary of the three-way ANOVAs showing, DF (degrees of freedom), F-values and level of significance (* p ≤ 0.05, ** p ≤ 0.01, *** p ≤ 0.001) of the effect of the Study year, Irrigation system, Se application and their interactions on grain yield, biomass production, 1000-grain weight, Se and As contents in soil, grain, dehulled grain and straw of rice.
Year (Y)Irrigation (I)Y × ISelenium (Se) Y × Se I × SeY × I × Se
DF2241224
Grain yield21.57 ***10.93 ***1.960.010.672.311.90
Biomass production43.26 ***4.40 *1.0511.47 **1.211.110.40
1000 grain weight43.59 ***6.11 **0.615.23 *1.230.490.28
Se soil0.540.800.144.782.590.870.97
As soil1.381.720.560.380.250.531.09
Se whole grain13.00 ***12.89 ***13.90 ***477.01 ***10.47 ***11.67 ***12.14 ***
As whole grain116.95 ***34.90 ***1.821.510.020.761.32
Se dehulled14.59 ***13.00 ***19.69 ***680.58 ***8.18 ***9.93 ***15.68 ***
As dehulled105.16 ***92.38 ***4.54 **7.23 *11.33 ***1.213.00 *
Se straw63.3 ***0.932.18655.3 ***21.2 ***1.262.52 *
As straw2.5698.07 ***5.92 ***21.27 ***0.410.820.22
Table 3. Effect of Study year, irrigation system, and Se application on grain yield, biomass production, 1000-grain weight, Se and As contents in soil, grain, dehulled grain, and straw of rice (means ± standard error). Different letters (if any) indicate significant differences (p ≤ 0.05) according to the LSD test.
Table 3. Effect of Study year, irrigation system, and Se application on grain yield, biomass production, 1000-grain weight, Se and As contents in soil, grain, dehulled grain, and straw of rice (means ± standard error). Different letters (if any) indicate significant differences (p ≤ 0.05) according to the LSD test.
ParameterYearIrrigation SystemSe Application
202020212022FloodReflood-20Reflood-7015-SeNo-Se
Grain yield (kg ha−1)8309 ± 232 b8409 ± 213 b10,230 ± 411 a9436 ± 609 a9418 ± 389 a8094 ± 312 b8984 ± 2858982 ± 268
Biomass production (kg ha−1)6287 ± 227 c8760 ± 197 a7875 ± 246 b8084 ± 558 a7531 ± 310 b7307 ± 331 b8013 ± 322 a7268 ± 234 b
1000 grain weight (g)23.9 ± 0.2 a22.4 ± 0.2 b21.6 ± 0.2 c23.1 ± 1.3 a 22.6 ± 0.4 b22.2 ± 0.3 b22.9 ± 0.2 a22.4 ± 0.3 b
Se soil (µg kg−1)1.5 ± 0.031.5 ± 0.051.4 ± 0.031.4 ± 0.041.5 ± 0.041.5 ± 0.41.5 ± 0.031.4 ± 0.04
As soil (µg kg−1)65.3 ± 2.474.5 ± 3.671.1 ± 4.475.2 ± 3.066.2 ± 3.168.6 ± 4.368.7 ± 4.271.2 ± 3.7
Se grain (µg kg−1)182 ± 53.8 b225 ± 53.8 a140 ± 81.9 c158 ± 61.0 b231 ± 57.5 a157 ± 36.0 b330 ± 61.5 a34 ± 18.2 b
As grain (µg kg−1)225 ± 28.9 b206 ± 29.1 b564 ± 81.7 a444 ± 62.1 a328 ± 53.8 b224 ± 35.2 c317 ± 63.2346 ± 44.5
Se dehulled (µg kg−1)156 ± 40.3 a126 ± 27.5 b102 ± 22.0 c103 ± 24.7 c154 ± 39.7 a127 ± 26.5 b235 ± 18.7 a22 ± 2.4 b
As dehulled (µg kg−1)195 ± 19.3 b189 ± 13.7 b332 ± 23.8 a319 ± 23.9 a231 ± 21.2 b167 ± 14.5 c226 ± 17.4 b251 ± 22.4 a
Se Straw (µg kg−1)143 ± 31.4 c298 ± 51.3 a190 ± 33.2 b204 ± 46.8221 ± 44.0205 ± 39.0358 ± 6.7 a62 ± 23.8 b
As Straw (µg kg−1)7641 ± 1089 b7979 ± 1191 ab8904 ± 564 a12,553 ± 623 a7403 ± 422 b4566 ± 605 c7086 ± 676 b9263 ± 850 a
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Poblaciones, M.J.; Vicente, L.; Fernández-Rodríguez, D.; Albarrán, Á.; Peña, D.; López-Piñeiro, A. Selenium Biofortification Improves Grain Quality and Reduces Arsenic Accumulation in Rice Under Alternate Wetting and Drying Irrigation. Agronomy 2026, 16, 1220. https://doi.org/10.3390/agronomy16131220

AMA Style

Poblaciones MJ, Vicente L, Fernández-Rodríguez D, Albarrán Á, Peña D, López-Piñeiro A. Selenium Biofortification Improves Grain Quality and Reduces Arsenic Accumulation in Rice Under Alternate Wetting and Drying Irrigation. Agronomy. 2026; 16(13):1220. https://doi.org/10.3390/agronomy16131220

Chicago/Turabian Style

Poblaciones, María J., Luis Vicente, Damián Fernández-Rodríguez, Ángel Albarrán, David Peña, and Antonio López-Piñeiro. 2026. "Selenium Biofortification Improves Grain Quality and Reduces Arsenic Accumulation in Rice Under Alternate Wetting and Drying Irrigation" Agronomy 16, no. 13: 1220. https://doi.org/10.3390/agronomy16131220

APA Style

Poblaciones, M. J., Vicente, L., Fernández-Rodríguez, D., Albarrán, Á., Peña, D., & López-Piñeiro, A. (2026). Selenium Biofortification Improves Grain Quality and Reduces Arsenic Accumulation in Rice Under Alternate Wetting and Drying Irrigation. Agronomy, 16(13), 1220. https://doi.org/10.3390/agronomy16131220

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