Ferric Oxide Nanoparticles Foliar Application Effectively Enhanced Iron Bioavailability and Rice Quality in Rice (Oryza sativa L.) Grains

Abstract

Synergizing iron nutrition and rice quality is essential for the development of integrated high-quality rice. In this study, a two-year field experiment was conducted to investigate the influence of ferric oxide nanoparticles (Fe₂O₃ NPs) foliar spraying on rice yield, quality, and iron bioavailability, with spraying water as the control (CK). Our results demonstrate that Fe₂O₃ NPs foliar application increase grain yield by 1.22–3.97% for the improved filled grain rate and 1000-grain weight, essentially attributed to improved net photosynthetic rate and SPAD value after heading. In addition, Fe₂O₃ NPs application achieved a higher rate of brown rice, polished rice, and head rice, and decreased chalkiness grain rate and chalkiness degree. Rice taste value treated with Fe₂O₃ NPs application was notably increased by 2.75–9.43% compared to CK, respectively, which is also reflected in the superior breakdown value (5.85–15.18%) and inferior setback value (12.38–28.19%). Meanwhile, foliar spraying Fe₂O₃ NPs significantly increased the iron content (16.97–58.74% and 26.48–94.01%) and proportion (2.90–5.35% and 13.10–26.44%), while they decreased the molar ratio of phytate to Fe (19.70–33.67% and 31.55–45.77%) in brown rice and polished rice, increasing iron bioavailability. Our findings indicate that Fe₂O₃ NPs can be effectively applied as a foliar fertilizer to enhance rice yield, quality, and iron nutrition.

1. Introduction

Iron (Fe) is an indispensable micronutrient for humans, underpinning critical physiological processes, including oxygen transport and storage, enzymatic function, DNA synthesis, and mitochondrial energy production [1]. Fe deficiency is causally linked to iron-deficiency anemia, cardiovascular disorders, adverse pregnancy outcomes, impaired pediatric neurodevelopment, cutaneous lesions, and an elevated cancer risk [2]. Current estimates indicate that more than 25% of the global population is affected by Fe deficiency [3,4]. Since the human body is incapable of origin Fe biosynthesis and relies exclusively on dietary acquisition [5], biofortification of staple crops to elevate Fe density represents a pivotal strategy for mitigating this worldwide nutritional shortfall [6].

Among various staple crops, rice supplies the primary source of energy and nutrition to more than 70% of the world’s population [7], and its intrinsic quality profoundly shapes rice consumers’ daily life [8]. Nevertheless, rice products typically contain insufficient Fe to satisfy human daily requirements [9,10], underscoring its exceptional potential as a target for Fe biofortification [11]. Among the available strategies, Fe fertilizer application is regarded as the most rapid, operationally simple, and economically viable route to elevate grain Fe concentrations in rice [12,13]. In addition to its human health implications, Fe is indispensable to plant metabolism, serving as an essential cofactor in chlorophyll biosynthesis, photosynthetic electron transport, respiratory redox reactions, and enzymatic antioxidant defense [14]. Adequate Fe nutrition therefore not only stimulates vegetative growth and photosynthetic efficiency, but also enhances grain yield and promotes further Fe enrichment in the grain [15,16,17,18]. Contemporary consumer expectations, however, have evolved beyond single-nutrient enrichment. Driven by rising living standards and increasingly sophisticated markets, demand has shifted toward rice genotypes and production systems that deliver integrated quality attributes—namely, superior milling performance, appealing appearance, excellent palatability, and elevated nutritional value [8]. Despite the agronomic benefits of Fe fertilization, its impact on this multifaceted quality profile remains inadequately characterized and warrants systematic investigation.

In field practice, foliar application of Fe fertilization is preferred over soil application because it minimizes Fe immobilization by the complex soil matrix and circumvents the inefficient long-distance translocation from root to grain, thereby achieving greater grain Fe enrichment [19]. Beyond its application method, the chemical form of Fe fertilizer strongly determines its bioavailability [16,20]. The products most frequently used are inorganic salts, organic complexes, and synthetic chelates [21,22]. Inorganic formulations generally display superior foliar penetration and faster leaf greening, whereas chelated Fe exhibits higher phloem mobility and more efficient internal transport [13,16,23]. Nevertheless, conventional Fe fertilizers are highly vulnerable to environmental conditions, exhibit low use efficiency, and may pose phytotoxicity and environmental contamination risks [16,18,24,25,26]. Advances in nanotechnology have introduced Nano-Fe fertilizers that feature high surface energy, uniform particle dispersion, excellent stability, enhanced bioavailability, and markedly reduced toxicity [18,27,28], which have already proven effective in green bean [29], cucumber [30], and wheat [31]. In rice, most Nano-Fe studies have concentrated on growth promotion, stress resistance, and Fe accumulation [18,32,33], whereas fundamental grain-quality responses remain largely unexplored.

Ferric fertilizers are typically applied at heading [13,34], and the post-heading grain-filling phase is the critical stage not only for both Fe enrichment, but also the determination of yield and quality traits. Owing to their rapid foliar penetration and superior crop uptake [24], ferric oxide nanoparticles (Fe₂O₃ NPs) may influence the entire quality-formation process, yet their impact on final rice quality remains unknown. In this study, three dosages of Fe₂O₃ NPs were foliar-sprayed during the gestation stage, with distilled water serving as the control, in order to evaluate effects on grain yield, iron nutrition, and rice quality. The overarching goal was to assess the practical feasibility of Fe₂O₃ NPs for concurrently enhancing grain yield, iron bioavailability, and rice quality.

2. Materials and Methods

2.1. Experimental Site

Field trials were run consecutively in 2020 and 2021 on the research farm of Yangzhou University, Jiangsu, China (32°23′24″ N, 119°25′12″ E). The station lies in a rice–wheat rotation belt within the mid-lower Yangtze Basin and experiences a warm, humid subtropical climate (Figure 1). Topsoil (0–20 cm) was sandy loam with pH 6.51 and contained 24.4 g kg⁻¹ organic matter, 1.3 g kg⁻¹ total nitrogen (N), 104.2 mg kg⁻¹ available N, 35.4 mg kg⁻¹ available phosphorus (P), and 72.5 mg kg⁻¹ available potassium (K).

2.2. Agronomic Management Systems

Nanjing 9108, a regional japonica cultivar (~150–155 d growth cycle), was used. Seeds were sown in plastic trays, kept in darkness for 48 h, and then transferred to a moist nursery. Twenty-five-day-old seedlings were manually transplanted at 1.33 × 10⁶ hills ha⁻¹ on 15 June 2020 and 16 June 2021; the harvest dates were 30 October 2020 and 31 October 2021, respectively.

The trial followed a randomized complete block layout with three replicates. Twelve plots (10 m × 6 m = 60 m² each) received one of four foliar sprays: deionized water (CK) or ferric oxide nanoparticles (Fe₂O₃ NPs) at 0.5 (NF_0.5), 1 (NF₁) or 2 (NF₂) mmol L⁻¹. Fe₂O₃ NPs (≥99.99% purity; 30–50 nm spherical, reddish powder) were supplied by Shanghai Chaowei Nanotechnology Co., Ltd., Shanghai, China.

To prepare the treatment solutions, 320, 640, and 1280 mg of Fe₂O₃ NPs were completely dispersed in 2 L ultrapure water by 30 min ultrasonication to yield 0.5, 1, and 2 mmol L⁻¹ suspensions, respectively. Each suspension was uniformly sprayed at the booting stage (as the rice spike begins to emerge from the leaf sheath) under calm, clear weather. All chemicals were analytical grade and used as received.

Urea (46% N) was split-applied at 270 kg ha⁻¹: 30% at transplanting, 30% seven days later (tillering), and 40% at panicle initiation. Calcium superphosphate (135 kg P ha⁻¹) and potassium chloride (270 kg K ha⁻¹) were incorporated basally. Irrigation, pest, disease, and weed management followed standard regional recommendations.

2.3. Sampling and Data Collection

2.3.1. Yield and Yield Components

At maturity, plants within a 1 m² area per plot were hand-cut, threshed, and sun-dried to 14% moisture for yield determination. From each plot, 20 representative hills were selected to record panicle number; a subsample of 12 hills was used to quantify spikelets per panicle, seed-setting rate, and 1000-grain weight.

2.3.2. Net Photosynthetic Rate and SPAD

Flag-leaf net photosynthetic rate (Pn) at heading and grain filling was recorded with a Li-6400 portable system (Li-6400, LI-COR, Lincoln, NE, USA). Conditions were set to 1000 μmol m⁻² s⁻¹ light intensity, 400 μmol mol⁻¹ CO₂, 30–35 °C, and 500 μmol s⁻¹ flow. The central portion of each leaf was clamped for 2 min between 09:00 and 12:00 under full sun. Three plants per replicate were assessed.

Flag-leaf SPAD readings were obtained with a SPAD-502 m (Minolta Camera Co., Ltd., Tokyo, Japan). Three positions—the lower quarter, midpoint, and upper quarter of the leaf—were averaged per leaf to obtain the representative SPAD value for each treatment.

2.3.3. Rice Quality

After harvesting, rice grains from each plot were sun-dried and stored at room temperature for a minimum of one month for grain quality analysis. A 100 g sample of rice grains was polished using a dehusker and separated into broken and unbroken grains. Grain quality analysis was conducted in accordance with the national standards of the People’s Republic of China (GB/T 17891–2017). The brown rice rate, milled rice rate, and head rice rate are expressed as percentages of the total (100 g) rice grains. The chalkiness rate and chalkiness degree of rice were assessed using a rice appearance scanner (MRS-9600TFU2L, Shanghai, China).

Protein content in milled rice was determined using the Kjeldahl method with an automatic Kjeldahl apparatus (Kjeltec 8200, Foss, Hillerød, Denmark). Amylose content was measured using a Near-Infrared Grain Analyzer (Infratec 1241, Foss, Denmark).

Rice palatability was quantified using a taste analyzer (Satake Corporation, Higashi-Hiroshima, Japan). In total, 30 g head rice was rinsed three times in 40 g deionized water, soaked 30 min, then steamed 30 min under filter paper. After 10 min warming, samples were fan-cooled by blowing air for 20 min and cooled naturally for 90 min at ambient temperature before analysis.

Rice-flour pasting was analyzed using a rapid viscosity analyzer (RVA, Super3, Newport Scientific, Warriewood, Australia). A 3 g sample (14% moisture basis) was dispersed in 25 g distilled water in an aluminum canister. The RVA was programmed to hold the sample at 50 °C for 1 min, increase to 95 °C at 11.84 °C/min, hold for 2.5 min, and then cool to 50 °C at 11.84 °C/min. Peak viscosity, trough viscosity, final viscosity, breakdown value, setback value, and consistency viscosities were extracted from the resulting curves.

2.3.4. Iron Content and Distribution

Samples (0.5 g) were digested with 5 mL of high-purity nitric acid in a microwave dissolver (CEM-MARS 5, Matthews, NC, USA). The digested solution was diluted and ferric content was analyzed using a plasma emission spectrometry-atomic absorption spectrometer (iCAP 6300, Thermo Fisher, Waltham, MA, USA). Accuracy was verified using certified reference material NIST SRM 1568b (rice flour). The mean recovery of Fe was 96.4 ± 2.1% (n = 6), indicating acceptable accuracy of the digestion-ICP-MS protocol. The proportion of ferric in each grain component relative to total ferric content was calculated using the following formula:

$$P(\%)={\displaystyle \frac{{SC}_c\times P_c}{{SC}_g}}\times 100$$

(1)

where ${SC}_c$ refers to the ferric content in each component of the rice grain, $P_c$ refers to the proportion of each component in the rice grain, and ${SC}_g$ refers to the iron content in the rice grain.

2.3.5. Determination of the Phytic Acid Content and Molar Ratio of Phytic Acid to Fe

Phytic acid content was determined using the method of Wang et al. [8]. Initially, 0.25 g of the sample was mixed with diluted hydrochloric acid and then shaken and centrifuged to obtain the supernatant. This supernatant was subsequently combined with chromogenic agents, namely ferric chloride (FeCl₃) and sulfosalicylic acid, as well as a standard solution prepared from sodium phytate. The absorbance of the solution was measured at 500 nm. The phytic acid content in the rice was calculated based on these measurements. The molar ratio of phytic acid to iron was obtained by dividing the millimoles of phytic acid by the millimoles of iron.

2.3.6. Statistical Analysis

The data are presented as mean ± standard deviation (SD), and statistical analyses were conducted using an LSD test using IBM SPSS Statistics software (Version 26.0, Armonk, NY, USA). One-way analysis of variance (ANOVA) was conducted to identify significant differences among treatment means, with significance set at p < 0.05.

3. Results

3.1. Yield and Its Components

As shown in

Table 1. Effect of foliar spraying Fe₂O₃ NPs concentration on yield and its components.

Year	Treatment	Panicles (×104 hm−2)	Spikelets Per Panicle	Filled Grain Rate (%)	1000-Grain Weight (g)	Grain Yield (t hm−2)
2020	CK	340.64 ± 14.87 a	128.30 ± 5.06 a	90.45 ± 1.24 c	27.37 ± 0.11 c	10.42 ± 0.10 b
	NF0.5	341.29 ± 10.16 a	128.01 ± 3.10 a	91.79 ± 1.14 b	27.54 ± 0.14 b c	10.54 ± 0.13 a b
	NF1	338.69 ± 10.61 a	127.92 ± 8.56 a	92.93 ± 1.18 a	27.69 ± 0.18 b	10.71 ± 0.10 a
	NF2	342.45 ± 11.49 a	129.48 ± 6.12 a	94.89 ± 0.54 a	27.90 ± 0.16 a	10.81 ± 0.14 a
2021	CK	350.98 ± 13.10 a	126.88 ± 5.21 a	92.51 ± 1.95 c	27.47 ± 0.20 c	10.56 ± 0.14 d
	NF0.5	351.03 ± 15.71 a	127.20 ± 4.14 a	94.30 ± 1.38 b	27.70 ± 0.13 b c	10.70 ± 0.16 c
	NF1	349.54 ± 16.51 a	126.24 ± 5.73 a	95.56 ± 1.36 a	27.98 ± 0.21 a b	10.89 ± 0.15 b
	NF2	350.12 ± 10.25 a	126.77 ± 3.11 a	97.59 ± 0.44 a	28.15 ± 0.15 a	11.01 ± 0.17 a

Note: Values within the same column followed by different letters are significantly different at the 0.05 probability level.

, data from both years consistently demonstrates that foliar application of Fe₂O₃ NPs exerts a positive effect on grain yield. Compared to the control (CK), grain yield increased by 1.15–3.74% in 2020 and 1.33–4.26% in 2021, respectively. ANOVA further revealed that the yield increments reached statistical significance under the NF₁ and NF₂ treatments in both years. A detailed analysis of yield components indicated that the observed yield increase was primarily attributed to improvements in the filled grain rate and 1000-grain weight. Over the two-year study, all Fe₂O₃ NPs treatments significantly increased the filled grain rate by 1.48–4.91% in 2020 and 1.93–5.49% in 2021, respectively. Additionally, under the NF₁ and NF₂ treatments, the 1000-grain weight exhibited significant increases of 1.17–1.94% in 2020 and 1.86–2.48% in 2021, separately. In contrast, no significant differences were detected in panicle number or spikelets per panicle between the Fe₂O₃ NPs treatments and CK across both years.

3.2. Net Photosynthetic Rate and SPAD Value

After heading, both the net photosynthetic rate and SPAD values showed a progressive decline; however, foliar application of Fe₂O₃ NPs effectively mitigated this downward trend (

Table 2. Effects of foliar spraying Fe₂O₃ NPs concentration on net photosynthetic rate and SPAD value after heading.

Year	Treatment	Net Photosynthetic Rate (μmol m−2 s−1)				SPAD Value
		Heading	20 Days After Heading	40 Days After Heading	Maturity	Heading	20 Days After Heading	40 Days After Heading	Maturity
2020	CK	27.42 ± 0.44 d	19.22 ± 0.25 c	12.36 ± 0.27 c	5.45 ± 0.14 a	44.77 ± 0.68 c	32.07 ± 0.35 c	20.17 ± 0.31 c	9.03 ± 0.15 a
	NF0.5	27.83 ± 0.35 c	19.61 ± 0.21 b c	12.77 ± 0.44 b	5.48 ± 0.09 a	45.47 ± 0.75 b c	33.03 ± 0.50 b	20.90 ± 0.26 b	9.10 ± 0.10 a
	NF1	28.22 ± 0.24 b	20.03 ± 0.19 ab	13.04 ± 0.36 a b	5.51 ± 0.12 a	46.10 ± 0.80 a b	33.73 ± 0.36 a b	21.47 ± 0.21 a	9.13 ± 0.06 a
	NF2	28.57 ± 0.28 a	20.38 ± 0.23 a	13.34 ± 0.47 a	5.55 ± 0.16 a	46.57 ± 0.60 a	34.23 ± 0.31 a	21.87 ± 0.15 a	9.17 ± 0.08 a
2021	CK	27.61 ± 0.38 d	19.65 ± 0.29 c	12.50 ± 0.30 c	5.20 ± 0.14 a	45.27 ± 0.61 c	32.73 ± 0.31 c	20.53 ± 0.15 d	9.10 ± 0.10 a
	NF0.5	27.98 ± 0.49 c	20.06 ± 0.27 b c	12.85 ± 0.54 b	5.24 ± 0.12 a	46.03 ± 0.79 b c	33.43 ± 0.54 b	21.33 ± 0.21 c	9.17 ± 0.15 a
	NF1	28.36 ± 0.28 b	20.50 ± 0.22 a b	13.19 ± 0.20 a	5.26 ± 0.10 a	46.50 ± 0.56 a b	33.97 ± 0.43 a b	21.90 ± 0.14 b	9.20 ± 0.12 a
	NF2	28.79 ± 0.64 a	20.81 ± 0.20 a	13.47 ± 0.29 a	5.29 ± 0.18 a	47.03 ± 0.88 a	34.30 ± 0.50 a	22.27 ± 0.15 a	9.27 ± 0.07 a

Note: Values within the same column followed by different letters are significantly different at the 0.05 probability level.

). At the heading stage, the net photosynthetic rate under Fe₂O₃ NPs treatments were 1.50–4.19% higher in 2020 and 1.34–4.27% higher in 2021 compared to the control (CK). This advantage further widened to 3.32–7.93% (2020) and 2.80–7.76% (2021) at 40 days after heading, respectively. Notably, at 20 days after heading, the NF₁ and NF₂ treatments induced pronounced enhancements relative to CK, with increases of 4.21–6.04% in 2020 and 4.33–5.90% in 2021.

SPAD values exhibited a similar trend: at the heading stage, only the NF₁ and NF₂ treatments led to significant increments (2.97–4.02% in 2020 and 2.72–3.89% in 2021). By contrast, all Fe₂O₃ NPs treatments increased SPAD values at 20 days after heading (by 2.99–6.74% in 2020 and 2.14–4.80% in 2021) and at 40 days after heading (by 3.62–8.43% in 2020 and 3.90–8.48% in 2021), respectively. At maturity, no significant differences in either net photosynthetic rate or SPAD value were observed between Fe₂O₃ NPs treatments and CK.

3.3. Processing Quality and Appearance Quality

Table 3. Effects of foliar spraying Fe₂O₃ NPs concentration on rice processing quality and appearance quality.

Year	Treatment	Brown Rice Rate (%)	Milled Rice Rate (%)	Head Rice Rate (%)	Chalkiness Grain Rate (%)	Chalkiness Degree (%)
2020	CK	85.50 ± 0.37 c	75.20 ± 0.19 c	63.46 ± 0.18 c	47.64 ± 0.55 a	18.54 ± 0.59 a
	NF0.5	85.79 ± 0.35 b c	75.42 ± 0.22 c	63.67 ± 0.31 b c	46.43 ± 0.33 b	17.41 ± 0.87 b
	NF1	86.08 ± 0.17 a b	75.70 ± 0.17 b	63.95 ± 0.24 b	45.61 ± 0.59 c	16.02 ± 0.31 c
	NF2	86.25 ± 0.14 a	75.89 ± 0.12 a	64.27 ± 0.22 a	44.37 ± 0.55 d	14.40 ± 0.56 d
2021	CK	85.68 ± 0.24 b	75.37 ± 0.18 d	64.05 ± 0.13 c	44.78 ± 0.67 a	16.14 ± 0.51 a
	NF0.5	85.97 ± 0.27 a b	75.63 ± 0.16 c	64.27 ± 0.20 b c	43.57 ± 1.02 a	15.38 ± 0.21 a
	NF1	86.17 ± 0.20 a b	76.02 ± 0.25 b	64.54 ± 0.18 a b	41.78 ± 0.45 b	14.96 ± 0.59 b
	NF2	86.37 ± 0.12 a	76.28 ± 0.14 a	64.75 ± 0.12 a	39.42 ± 0.70 c	13.82 ± 0.45 c

Note: Values within the same column followed by different letters are significantly different at the 0.05 probability level.

demonstrates that foliar application of Fe₂O₃ NPs improved rice processing quality in a concentration-dependent manner, as evidenced by increased brown rice rate, milled rice rate, and head rice rate. Across the two-year data, only the NF₂ treatment significantly increased the brown rice rate, with an average increment of 0.84%. Regarding the milled rice rate, Fe₂O₃ NPs application led to significant increases of 0.29–0.92% in 2020 and 0.34–1.21% in 2021, respectively. For the head rice rate, the NF₁ and NF₂ treatments significantly elevated it by 0.77–1.28% (2020) and 0.76–1.09% (2021), respectively.

Concurrently, both the chalkiness grain rate and chalkiness degree decreased with increasing Fe₂O₃ NPs concentrations. Compared to the control, in 2020, increasing Fe₂O₃ NPs concentrations resulted in marked reductions in the chalkiness grain rate (2.54–6.86%) and chalkiness degree (6.09–22.33%). In 2021, the NF₁ and NF₂ treatments led to significant reductions in these traits, with the chalkiness grain rate decreasing by 6.70–11.97% and the chalkiness degree by 7.31–14.37%, respectively.

3.4. Tasting Quality and Cooking Quality

The protein content in polished rice showed an increasing trend with the elevation of Fe₂O₃ NPs application rates, whereas the amylose content displayed the opposite trend (

Table 4. Effect of foliar spraying Fe₂O₃ NPs concentration on rice nutritional quality and tasting quality.

Year	Treatment	Protein Content (%)	Amylose Content (%)	Tasting Value	Appearance Value	Hardness Value	Viscosity Value	Balance Value
2020	CK	7.65 ± 0.11 b	13.57 ± 0.40 a	78.23 ± 0.35 d	7.80 ± 0.06 d	6.03 ± 0.06 a	8.40 ± 0.10 c	7.93 ± 0.06 d
	NF0.5	7.69 ± 0.07 b	12.95 ± 0.19 b	80.87 ± 0.31 c	8.23 ± 0.06 c	5.87 ± 0.10 b	8.67 ± 0.12 b	8.33 ± 0.10 c
	NF1	7.77 ± 0.06 a b	12.27 ± 0.23 c	83.07 ± 0.38 b	8.53 ± 0.15 b	5.80 ± 0.06 b	8.73 ± 0.06 b	8.57 ± 0.12 b
	NF2	7.86 ± 0.06 a	11.99 ± 0.15 c	85.07 ± 0.32 a	8.77 ± 0.06 a	5.67 ± 0.15 c	8.97 ± 0.06 a	8.83 ± 0.10 a
2021	CK	7.60 ± 0.11 b	13.78 ± 0.29 a	80.97 ± 0.21 d	8.17 ± 0.10 d	5.97 ± 0.06 a	8.63 ± 0.06 d	8.33 ± 0.06 d
	NF0.5	7.65 ± 0.08 b	13.27 ± 0.16 b	82.70 ± 0.44 c	8.50 ± 0.06 c	5.77 ± 0.06 b	8.83 ± 0.10 c	8.57 ± 0.06 c
	NF1	7.70 ± 0.09 a b	12.57 ± 0.18 c	84.43 ± 0.50 b	8.83 ± 0.15 b	5.70 ± 0.10 b	9.07 ± 0.15 b	8.90 ± 0.10 b
	NF2	7.80 ± 0.05 a	12.15 ± 0.15 c	87.83 ± 0.47 a	9.07 ± 0.10 a	5.60 ± 0.06 b	9.23 ± 0.12 a	9.23 ± 0.15 a

Note: Values within the same column followed by different letters are significantly different at the 0.05 probability level.

). Over the two-year trial, only the NF₂ treatment resulted in a statistically significant increase in protein content relative to the control, with increments of 2.75% in 2020 and 2.63% in 2021, respectively. In contrast, all Fe₂O₃ NPs treatments significantly reduced the amylose content by 4.57–11.64% in 2020 and 3.70–11.83% in 2021 compared to CK.

Foliar application of Fe₂O₃ NPs also improved rice palatability, as evidenced by significant increases in the overall taste value relative to CK: 3.37–8.74% in 2020 and 2.14–8.48% in 2021, respectively. Decomposition of the taste value index revealed that this improvement primarily stemmed from significantly higher scores for appearance value (5.55–12.40% in 2020 and 4.08–11.02% in 2021), viscosity value (3.18–6.75% in 2020 and 2.32–6.95% in 2021), and balance value (5.04–11.35% in 2020 and 2.81–10.80% in 2021) in 2020 and 2.81–10.80% in 2021, coupled with a significant reduction in hardness value (2.75–6.07% in 2020 and 3.95–12.42% in 2021).

The enhanced taste value induced by foliar Fe₂O₃ NPs application was corroborated by the RVA profile (

Table 5. Effect of foliar spraying Fe₂O₃ NPs concentration on RVA parameters.

Year	Treatment	Peak Viscosity (cP)	Trough Viscosity (cP)	Breakdown Value (cP)	Final Viscosity (cP)	Setback Value (cP)	Consistence VALUE (cP)
2020	CK	2651.33 ± 19.71 b	1707.33 ± 18.07 a	944.00 ± 15.87 b	2209.67 ± 18.15 a	−441.67 ± 21.50 a	502.33 ± 18.01 a
	NF0.5	2716.00 ± 26.23 a	1715.33 ± 35.25 a	1000.67 ± 29.19 a	2225.00 ± 19.47 a	−491.00 ± 20.07 b	509.67 ± 28.43 a
	NF1	2757.67 ± 38.42 a	1705.00 ± 24.00 a	1052.67 ± 35.13 a	2222.67 ± 49.50 a	−535.00 ± 23.90 c	517.67 ± 39.40 a
	NF2	2771.00 ± 39.28 a	1681.00 ± 34.77 a	1090.00 ± 42.51 a	2200.33 ± 23.03 a	−570.67 ± 20.43 c	519.33 ± 53.16 a
2021	CK	2708.67 ± 17.47 c	1683.00 ± 22.52 a	1025.67 ± 17.16 d	2232.67 ± 33.31 a	−476.00 ± 15.87 a	549.67 ± 24.01 a
	NF0.5	2784.00 ± 28.16 b	1700.00 ± 22.34 a	1084.00 ± 16.37 c	2243.33 ± 40.97 a	−540.67 ± 17.21 b	543.33 ± 25.36 a
	NF1	2815.67 ± 26.92 a b	1686.00 ± 25.24 a	1129.67 ± 28.50 b	2240.67 ± 23.18 a	−575.00 ± 16.58 c	554.67 ± 22.08 a
	NF2	2853.00 ± 30.81 a	1674.67 ± 26.35 a	1178.33 ± 22.78 a	2247.67 ± 31.97 a	−605.33 ± 21.53 d	573.00 ± 33.59 a

Note: Values within the same column followed by different letters are significantly different at the 0.05 probability level.

). Compared to the control, Fe₂O₃ NPs treatments resulted in significantly higher breakdown values (6.00–15.47% in 2020 and 5.69–14.89% in 2021) and significantly lower setback values (11.17–29.21% in 2020 and 13.59–27.17% in 2021). These changes were underpinned by significant increases in peak viscosity under Fe₂O₃ NPs treatments: 2.44–4.51% in 2020 and 2.78–5.33% in 2021, respectively. However, no statistically significant differences were observed in trough viscosity, final viscosity, or consistence value between Fe₂O₃ NPs treatments and CK.

3.5. Fe Content, Distribution, and Bioavailability in Rice Grains

As illustrated in Figure 2, foliar application of Fe₂O₃ NPs significantly increased Fe concentrations in all grain fractions, with the increment positively correlated with the application rate. Relative to CK, the total grain Fe concentration increased by 15.14–54.33% in 2020 and 16.12–55.71% in 2021, respectively. In the edible fractions, brown rice and polished rice exhibited significant increases: 16.13–55.34% and 26.34–89.86% in 2020, and 17.81–62.13% and 26.61–98.16% in 2021, respectively. By contrast, Fe enrichment in non-edible portions was more moderate. Specifically, significant increases were only observed under the NF₁ and NF₂ treatments: glumes and rice bran showed respective increases of 30.53–53.73% and 15.22–22.42% in 2020 and 30.20–34.65% and 18.05–26.32% in 2021, respectively.

The distribution of Fe within rice grains further revealed that foliar application of Fe₂O₃ NPs effectively promoted Fe accumulation in the edible portions of the grains (Figure 3). Relative to the total Fe content in grains, the proportion of Fe sequestered in brown rice significantly increased by 3.46–5.68% in 2020 and 2.34–5.02% in 2021, while that in polished rice rose by 13.40–24.06% in 2020 and 12.81–28.83% in 2021. In contrast, the corresponding proportions of Fe in non-edible glumes and bran significantly decreased by 9.77–14.11% and 11.31–15.54% in 2020, and by 9.24–17.72% and 10.95–18.71% in 2021, respectively. This progressive shift in Fe distribution, consistent across both years, was intensified with increasing Fe₂O₃ NPs concentrations, thereby effectively enhancing the bioavailable Fe fraction in rice.

The enhanced Fe bioavailability induced by Fe₂O₃ NPs was further substantiated by a significant reduction in the phytate-to-Fe molar ratio in both brown and polished rice (Figure 4). With increasing Fe₂O₃ NPs concentrations, this ratio decreased progressively. Relative to the control, the reductions were substantial: in brown rice, the ratio decreased by 19.44–32.36% (2020) and 19.96–34.97% (2021); in polished rice, the decreases were 31.99–44.77% (2020) and 31.11–46.77% (2021), respectively.

4. Discussion

4.1. Fe₂O₃ NPs Application to Rice Yield and Quality

Iron (Fe) is indispensable to rice physiology, participating in antioxidant defense, chlorophyll synthesis, and photosynthetic electron transport [35,36,37], and chronic Fe excess can impair growth and ultimately depress grain yield [38]. Optimizing Fe nutrition is therefore essential for sustainable rice production [39]. In the present study, foliar application of Fe₂O₃ NPs significantly elevated the net photosynthetic rate and SPAD values, especially from heading to 40 days after heading, moderately delaying leaf senescence and sustaining photosynthetic activity throughout grain filling [40]. Since over 60% of final grain dry matter originates from post-anthesis photosynthates [41,42], the observed increases in filled grain rate and 1000-grain weight translated directly into higher grain yield, corroborating precious research findings [13,16].

Furthermore, enhanced source activity also underpinned improvements in grain quality [43]. Fe₂O₃ NPs treatments significantly raised brown rice rate, milled rice rate, and head rice rate while simultaneously reducing chalkiness grain rate and chalkiness degree. These effects reflect the greater assimilate supply and its more uniform distribution, leading to a denser endosperm structure [44,45]. Flavor, the primary determinant of consumer acceptance [46], is inversely related to grain protein and amylose concentrations, as lower levels confer a softer, springier texture and superior palatability [47,48]. In this study, although Fe₂O₃ NPs application slightly raised protein content, the increment was significant only at the highest dose. By contrast, a pronounced decrease in amylose content was the dominant driver of the improved taste score, as evidenced by higher appearance, viscosity, and balance values and lower hardness, mirrored in the RVA profile by elevated peak and breakdown viscosities and reduced setback value [49]. The amylose reduction may be linked to altered carbon-partitioning efficiencies triggered by the higher post-spray net photosynthetic rate [50], but the underlying mechanisms warrant further investigation.

4.2. Fe₂O₃ NPs Application to Fe Bioavailability

Effective iron enrichment is not simply a matter of elevating total grain Fe, but rather of increasing Fe density in the edible fractions: brown and polished rice [51]. Conventionally, a large proportion of Fe absorbed by the plant is sequestered in the bran and is subsequently lost during polishing [52]. In this study, foliar Fe₂O₃ NPs not only raised Fe concentrations in all grain tissues, but also preferentially enriched the edible portions. With rising Fe₂O₃ NPs rates, the Fe share in brown and polished rice increased concomitantly with declines in the glume and bran fractions. This redistribution is attributed to the higher rate of brown rice and milled rice and to the greater relative gain in Fe in edible versus non-edible tissues. Phytic acid, a potent anti-nutrient, chelates Fe to form insoluble complexes that impede intestinal absorption [53]. Consequently, the phytic acid to iron molar ratio is widely adopted as an index of Fe bioavailability, with values < 1 considered optimal [54]. Fe₂O₃ NPs foliar application markedly lowered this ratio in both brown and polished rice. Notably, the ratio in polished rice fell below the recommended threshold, indicating a substantial enhancement in Fe bioavailability. Collectively, these results demonstrate that foliar Fe₂O₃ NP application can achieve effective Fe biofortification and markedly improve the nutritional value of rice.

5. Conclusions

In this study, we investigated the impact of foliar applications of different concentrations of ferric oxide nanoparticles during the gestation stage of japonica rice. Our findings reveal that the foliar application of Fe₂O₃ NPs presents a significant boost in the net photosynthetic rate and SPAD value in flag leaves during heading maturity stage, ultimately increasing the grain yield for the enhanced filled grain rate and 1000-grain weight. In addition, rice processing and appearance quality treated with Fe₂O₃ NPs foliar spray were improved for the increased brown rice rate, polished rice rate, and head rice rate, as well as the decreased chalkiness grain rate and chalkiness degree. The remarkable improvement in rice taste value, peak viscosity, and breakdown value, along with a significant reduction in setback value, were also observed after Fe₂O₃ NPs application, indicating better flavor. Furthermore, Fe₂O₃ NPs foliar application achieved a notable increase in iron content and proportion in brown rice and polished rice, realizing more effective iron enrichment and higher bioavailability in the edible parts of rice grains. These research results demonstrate that nano-iron oxide has broad application prospects as a foliar fertilizer for synergistically increasing rice yield, improving rice quality, and enhancing iron nutrition.