Tree Peony as an Efficient Organic Selenium Bioreactor: Selenium Uptake, Accumulation, Speciation, and Nutritional Enhancement via Foliar Sodium Selenite Application

Abstract

Selenium (Se) is an essential micronutrient for human health, yet its deficiency remains prevalent worldwide. Biofortification through foliar Se application is an effective strategy to enhance Se levels in crops. Paeonia ostii ‘Fengdan’ is a multifunctional woody plant with potential for Se enrichment, though its Se uptake and transformation mechanisms remain unclear. This study systematically investigated the effects of foliar-applied Na₂SeO₃ (0–200 mg L⁻¹) on Se uptake, accumulation, speciation, and nutritional quality in tree peony. Results showed that Se uptake increased with higher Na₂SeO₃ concentrations, displaying a clear dose-dependent pattern across all organs. Se accumulation significantly enhanced, with a pronounced shift in distribution towards above-ground organs under experimental conditions. Notably, tree peony exhibited strong biotransformation capacity, converting over 73% of Se in leaves and over 81% in seeds into organic forms, primarily SeCys₂ and SeMet, with minor MeSeCys. Comprehensive evaluation indicated that 100 mg L⁻¹ Na₂SeO₃ yielded optimal results, significantly enhancing leaf and seed biomass, increasing seed nutrient contents (soluble proteins, sugars, phenolics), and improving the unsaturated fatty acid profile of seed oil. These findings highlight tree peony’s potential as an efficient bioreactor for organic Se and provide a theoretical foundation for developing Se-enriched products from tree peony.

1. Introduction

Selenium (Se) is an essential trace element for humans and animals [1], and it has various physiological functions such as anti-aging properties, anticancer effects, prevention and treatment of cardiovascular diseases, antioxidant activity, and immune enhancement [2,3,4]. However, more than one billion people worldwide are affected by Se deficiency [5]. China is one of the typical Se-deficient countries globally, with 72% of its land area being Se-deficient regions, and about 70 million people are at health risk [6,7]. Improving dietary Se intake is the most direct and effective way to address Se deficiency. Plant-based foods are the main source of Se for the human body [8]. Therefore, enhancing the Se content in edible parts of plants through biofortification is crucial for improving the human Se nutritional status.

Se biofortification of plants can be achieved through various methods, including soil fertilization [6,9], foliar application [5,8], hydroponics [10,11] and genetic engineering [12,13]. Among these, foliar application is currently a focal point of research and practical implementation, due to its advantages such as rapidity, high efficiency, low environmental risk, and high Se utilization rate [14]. In wheat grains, the Se concentration from foliar Se spray was found to be twice as high as that from soil Se application [15]. Ros et al. [16] observed an eightfold increase in Se enrichment with foliar spray compared to soil application. The form of Se has a significant impact on the uptake, transportation, and transformation of Se in plants. Selenate and selenite are commonly used inorganic Se sources in foliar application. Compared with selenate, selenite is more easily converted into organic Se in plants, resulting in higher accumulation of organic Se in the edible parts, which have higher biological activity and safety for human [17,18,19]. Hence, the utilization of selenite foliar application for Se enrichment presents greater practical advantages.

Se in plants exists in various species, including inorganic Se (such as selenite and selenate) and organic Se (such as selenomethionine SeMet, selenocysteine SeCys₂, and methylselenocysteine MeSeCys) [20]. Different selenium species differ significantly in terms of their bioavailability, toxicity, and health effects on the human body [4,21]. Clinical trial results have shown that SeMet and MeSeCys exhibit promising anticancer effects [22]. SeMet being an authorized food fortificant. The Se species present in plants are crop-specific, with different crops containing different Se species. Understanding the forms and proportions of selenium in edible parts of plants is essential for precise selenium supplementation.

While studies on plant Se biofortification through foliar application of selenite have shown promising results, most research has focused on annual herbaceous crops such as wheat, rice, maize, and soybeans. Recent findings indicate that woody plants also exhibit effective Se enrichment and transformation from exogenous sources. Foliar spraying of Na₂SeO₃ significantly increased the contents of inorganic and organic selenium in kiwifruit, and the organic selenium content in the pulp treated with 50 mg L⁻¹ Na₂SeO₃ was 9.04 times that of the control [23]. Nano-Se foliar application enhanced the plum fruit’s fresh weight, diameter, and plant growth and development by increasing the Se content and amino acids in leaves and fruits [24]. Foliar application of GlcN-Se can increase the organic selenium content in tea leaves, promote photosynthesis, and enhance tea leaves yield [25]. Despite these advancements, research on selenium biofortification in woody horticultural crops remains limited. Given the increasing economic importance of woody plants in agriculture and horticulture, it is crucial to expand this area of study. Woody plants, with their longer growth cycles and distinct physiological characteristics, may exhibit different mechanisms for selenium accumulation and transformation compared to herbaceous plants.

Tree peony (Paeonia suffruticosa Andr.), belonging to the subg. Moutan in the genus Paeonia, family Paeoniaceae [26], is a traditional woody flower, renowned in China for its high ornamental, medicinal, edible, and oil values [27,28]. In recent years, the comprehensive utilization of the entire plant (root bark, stem, leaves, flowers, fruit pods and seeds) of tree peony has received extensive research and attention [29,30,31,32,33]. Paeonia ostii ‘Fengdan’ is a representative variety of tree peony, attracting considerable attention for its flowers and seed oil being approved as new food resources by the National Health Commission of China [29,31], and has been included in the national strategy for new woody oil crops. It has been extensively cultivated in some Chinese provinces, including Shandong, Henan, Anhui, Shaanxi, and other regions, with a planting area exceeding 200,000 hectares by 2024 [34,35], presenting great potential in the fields of functional agriculture. Therefore, taking advantage of the high comprehensive utilization value of tree peony and the continuous expansion of large-scale planting, there is great potential to use Se biofortification to produce Se-enriched tree peony products. Although, exogenous Se has been shown to enhance the total Se content in tree peony seeds [36], the uptake, accumulation, and transformation mechanisms of Se, as well as its effects on the quality of tree peony seeds after Se application, remain unclear.

In this study, ‘Fengdan’ was used as the experimental material, with varying concentrations of Na₂SeO₃ (0, 25, 50, 100, and 200 mg∙L⁻¹) sprayed during the rapid elongation stage of leaves. The total Se content in various organs was measured during the seed developmental stages, along with the content of organic Se and Se speciation in leaves and seeds at the seed harvest stage. The study aims to systematically elucidate the uptake, accumulation, and transformation properties of Se in tree peonies. Furthermore, the optimal Na₂SeO₃ concentration for foliar application will be determined by evaluating its effects on seed growth, nutritional quality, and fatty acid composition of seed oil. This research will establish a technical foundation for the large-scale production of Se-enriched organic products from tree peonies.

2. Materials and Methods

2.1. Test Site

The experiment was conducted from March to September 2022 in the Tree Peony Garden of Henan University of Science and Technology, Luoyang, China (34°35′ N, 112°24′ E). The soil at the test site was classified as fluvo-aquic soil. Prior to the experiment, the physicochemical properties of the soil in the 0–20 cm tillage layer were analyzed with reference to methods described in the literature [9]. The results are as follows: pH (H₂O) 7.25, organic matter 15.28 g kg⁻¹, total nitrogen 1.20 g kg⁻¹, available nitrogen 77.32 mg kg⁻¹, available potassium 96.73 mg kg⁻¹, available phosphorus 13.26 mg kg⁻¹, and total Se 0.129 mg kg⁻¹.

2.2. Plant Materials and Experimental Design

In this study, 6-year-old tree peony ‘Fengdan’ with robust growth and no pests or diseases were selected as experimental materials. The plants were spaced at 60 cm × 50 cm intervals, with each experimental plot containing 30 plants. Protective rows were placed between neighboring plots. The experiment was arranged in a randomized complete block design with five treatments: a control (CK, water) and four Na₂SeO₃ (Analytical reagent, purchased from Shanghai Macklin Biochemical Co., Ltd., Shanghai, China, with a purity of ≥98%) foliar concentrations (25, 50, 100, and 200 mg L⁻¹, adjust pH to 5.5–6.0 with 0.1 mol L⁻¹ HCl or NaOH), designated as T1–T4. Each treatment had three replicates. Foliar application was conducted on 26 March 2022 (rapid leaf elongation phase), 16 April 2022 (leaf fixed-length phase), and 6 May 2022 (vigorous leaf photosynthetic function stage). Applications were made between 17:00–18:00 on calm days using electric sprayers (10 L plot⁻¹), with added 0.1% Tuwen-80 as surfactants to enhance adhesion. Plastic film barriers were placed between adjacent plots to prevent cross-contamination. Standard field management practices, including fertilization, irrigation, and weeding, were performed as usual.

2.3. Sample Collection and Processing

Plant samples were collected at four seed developmental stages: seed rapid growth stage (RG, 15 May 2022), seed inclusion enrichment and conversion stage (EC, 25 June 2022), seed dehydration and maturity stage (DM, 20 July 2022), and seed harvest stage (HV, 2 August 2022). At each stage (except HV), three healthy plants per treatment were selected. Branches bearing the uppermost three functional leaves and fruit pods were cut and immediately placed in an icebox for transport to the laboratory. Upon arrival, the leaves, fruit pods, and stems were separated and rinsed with tap water followed by three washes with deionized water. The fruit pods were then opened to separate pod shells from seeds. Part of the samples from all organs were taken, snap-freeze them in liquid nitrogen, and then store them in a −80 °C refrigerator. All the remaining samples were put into an oven at 85 °C to deactivate enzymes, then dried overnight at 60 °C to constant weight, ground, sieved (100 mesh) and stored in self-sealing bags for further analysis.

At HV, three entire plants per treatment were harvested and dissected into roots, stems, leaves, pod shells, and seeds. These were washed, dried to a constant weight, and the biomass of each organ was recorded. The samples were then crushed, sieved, and stored for later use. Additionally, all the fruit pods from ten plants per replicate were harvested and air-dried in a cool and dry place, then threshed. The weight of 100 seeds was recorded, and the seeds were separated into seed shells and kernels. The kernel percentage was calculated, and the kernels were then crushed and stored in a resealable bag at 4 °C for subsequent analysis of crude fat content, nutritional quality, and seed oil fatty acid composition.

2.4. Determination of Total Se Concentrations

Total Se concentrations in tree peony samples was determined using hydride generation-atomic fluorescence spectrometry (HG-AFS 9120, Beijing Puxi, Beijing, China) following acid digestion [18]. Briefly, 0.5 g of sample was accurately weighed and placed in a 100 mL digestion tube, followed by the addition of 6 mL of a nitric acid and perchloric acid (4:1, v/v) solution. The mixture was left at room temperature overnight and then digested in a furnace at 150 °C until the solution became clear and colorless. Subsequently, 2.5 mL of 6 mmol∙L⁻¹ HCl was added, and the tube was returned to the furnace at 100 °C for a reduction reaction until white smoke was produced. After cooling, the solution was diluted to 25 mL with deionized water. The total Se concentrations was determined using a PF3 atomic fluorescence spectrometer (AFS-920, Beijing Puxi Instrument Co., Ltd., Beijing, China) under specific measurement conditions: atomizer height of 9 mm, furnace temperature of 200 °C, carrier gas flow rate of 300 mL min⁻¹, auxiliary gas flow rate of 600 mL min⁻¹, carrier liquid of 5% HCl (v/v), reducing agent of 0.5% KBH₄ and 1.5% NaOH (w/v). The calculation method was based on peak area integration. Se accumulation was calculated by multiplying the total Se concentrations with the biomass of each part [37].

2.5. Determination of Organic Se Concentrations

The organic Se concentrations in tree peony samples was determined using a subtraction method [38]. Specific methods are as follows: A 0.5 g sample was placed in a 50 mL centrifuge tube, with 30 mL of deionized water. After ultrasonic oscillation at 25 °C for 30 min, the mixture was centrifuged at 4200 rpm for 10 min. The supernatant was collected, and the residue was subjected to a second extraction. The combined supernatants were then extracted with 5 mL of cyclohexane in a separatory funnel. The aqueous phase was collected in a 50 mL conical flask, and most of the water was evaporated using a hot plate. Inorganic Se concentrations was determined using the same method as for total Se concentrations. Organic Se concentrations was calculated as the difference between total Se concentrations and inorganic Se concentrations. Organic Se conversion rate (%) = (organic Se/total Se) × 100%. For quality control during digestion and measurement, tea standards (GBW07605, GSV-4, 0.072 mg kg⁻¹ Se, purchased from the National Standard Material Center, China) and blank samples were used. The average recovery rate of Se ranged from 82.2% to 94.5%. Limit of detection (LOD) of the instrument is 0.02 μg L⁻¹ Se, and limit of quantitation (LOQ) was 0.1 μg L⁻¹ Se. Total Se and organic Se concentrations were expressed as dry mass per sample.

2.6. Se Speciation Analysis

Se speciation was determined using a modified method based on the procedures described in the literature [39,40]. Briefly, 0.5 g samples were extracted in Tris-HCl buffer with ultrasonic assistance for 30 min. Following this, 50 mg of enzyme protease E was added, and the mixture was thoroughly mixed and incubated in an air bath thermostat set to 55 °C and 250 rpm for 24 h. After incubation, the solution was centrifuged at 10,000 rpm and 4 °C for 20 min. The supernatant was collected and filtered through a 0.22 μm filter membrane to obtain the test solution. Se speciation was determined using high-performance liquid chromatography coupled with atomic fluorescence spectrometry (HPLC-AFS, SA-50, Beijing Titan Instrument Co., Ltd., Beijing, China).

2.7. Determination of Nutritional Quality of Tree Peony Seeds

The crude fat content was measured using the Soxhlet extraction method [41]. The soluble protein content was determined using the Coomassie Brilliant Blue G-250 colorimetric method [42]. The soluble sugar, starch, and total phenol contents were quantified using the Anthrone sulfuric acid method [43] and the Folin phenol method [44], respectively.

2.8. Fatty Acid Profiling of Tree Peony Seed Oil

Seed oil was extracted using a supercritical CO₂ extraction device (HA220-50-06, Nantong, China), following the parameters outlined in Wang et al. [27]. The seed oil underwent methyl esterification as described by Li et al. [45] and Liu et al. [46] with the addition of 20 μL of tridecanoic acid methyl ester (10.0 g L⁻¹ in n-hexane) as an internal standard. Fatty acid composition of the tree peony seed oil was analyzed by GC-MS (7890A-5975, Agilent Technologies Inc., Santa Clara, CA, USA) using settings detailed in Wang et al. [27]. Each sample was tested three times, and compound identification was performed using the mass spectra database NIST17. The main fatty acid content was determined using the internal standard method, calculating peak areas of fatty acid methyl ester standards and samples as outlined by Li et al. [45] and Zhang et al. [47].

2.9. Statistical Analysis and Graph Drawing

All data were presented as mean ± standard deviation (n = 3). Microsoft Excel 2013 was used for data collation and chart creation. SPSS Statistics 25.0 (IBM, Armonk, NY, USA) was employed for statistical analysis. One-way ANOVA and Duncan’s new multiple range test were adopted for multiple comparisons (p < 0.05). GraphPad Prism 9.0 was utilized to generate graphs and perform curve fitting between Se application concentration and total Se concentration in various organs.

To comprehensively evaluate the effects of different selenium application treatments, Principal Component Analysis (PCA) and the membership function method were employed [48]. Firstly, 10 indicators of seeds (biomass, total Se content, organic Se content, soluble protein content, soluble sugar content, starch content, total phenolic content, crude fat content, and the content of unsaturated and polyunsaturated fatty acids in the seed oil) were standardized, followed by PCA to extract principal components with eigenvalues greater than 1. The weights (W_i) were calculated based on the variance contribution rates of each principal component. Then, the membership function values (U(X_j)) for each indicator were computed using the membership function formula. Finally, the comprehensive evaluation value (D value) for each treatment was calculated according to the formula D = Σ [U(X_j) × W_i], and the treatments were ranked accordingly. Membership function calculation formula: U(X_j) = (X_j − X_min)/(X_max − X_min). X_j is the j-th index value of a certain treatment, and X_max and X_min are the maximum and minimum values of this index among all treatments, respectively.

3. Results

3.1. Effect of Spraying Na₂SeO₃ on Biomass of Tree Peony Organs

To evaluate the effect of Se on growth performance, the biomass in various organs of tree peony ‘Fengdan’ at HV was examined (Figure 1). Overall, organ biomass showed a concentration-dependent response pattern, with moderate Se levels promoting growth and higher concentrations resulting in a plateau or decline. Specifically, leaf biomass in the 50 and 100 mg L⁻¹ Na₂SeO₃ treatments increased by 14.05% (p < 0.05) and 11.25% (p < 0.05), respectively, compared to CK. Seed biomass also peaked under 100 mg L⁻¹ Na₂SeO₃, exhibiting an 11.21% increase relative to CK (p < 0.05). In contrast, there were no significant differences in biomass of roots, stems, and pod shells between treatments.

Regression analysis was conducted on the biomass of different organs of tree peony in response to varying concentrations of sodium selenite spraying (Figure S1). The results indicated a significant quadratic relationship between leaf and seed biomass and selenium application concentrations, with R² values of 0.3947 (p < 0.05) and 0.5848 (p < 0.01), respectively. However, the variations in root, stem, pod shell, and total plant biomass showed lower fitting degrees with selenium concentration, suggesting a minor response to sodium selenite spraying. These findings suggest that applying 50–100 mg L⁻¹ Na₂SeO₃ during the rapid leaf elongation stage can enhance the growth of tree peony leaves and seeds, consequently increasing their biomass of tree peony.

3.2. Effect of Spraying Na₂SeO₃ on Total Se Concentrations in Tree Peony Organs

Foliar application of Na₂SeO₃ significantly affected the total Se concentrations in various organs of tree peony ‘Fengdan’ across different seed developmental stages (Figure 2). At each stage, the total Se concentrations in various organs gradually increased with the increased Na₂SeO₃ concentration, showing an obvious dose-effect pattern. Under the same conditions, the order of total Se concentrations in various organs of tree peony was: leaves > pod shells > seeds > stems. A correlation analysis was conducted between the total Se concentrations in various organs at different stages and the spraying concentration of Na₂SeO₃ (Figure S2). The results showed that there was a significant linear correlation between the total Se concentrations in leaves and the Na₂SeO₃ application concentrations. The R² values in the RG, EC, DM, and HV stage were 0.9969, 0.9947, 0.9892, and 0.9859, respectively. The total Se concentrations in stems, pod shells, and seeds showed a significant curvilinear correlation with the spraying concentration of Na₂SeO₃ in each stage, with R² ranging from 0.9860 to 0.9960. It can be seen that under the condition of spraying Na₂SeO₃, the leaves of tree peonies are more sensitive to Se and have a higher absorption effect.

Additionally, developmental timing also played a crucial role in Se uptake. Leaves (Figure 2A) exhibited peak Se levels during EC, while stems (Figure 2B) and seeds (Figure 2D) reached maximum Se concentrations at HV. Pod shells showed a biphasic trend, increasing from RG to EC, slightly decreasing at DM, and rising again at HV. These findings indicate that Se uptake in tree peony ‘Fengdan’ is associated with both concentration and developmental stage.

3.3. Effect of Spraying Na₂SeO₃ on Se Accumulation and Distribution in Tree Peony Organs

At HV, foliar application of Na₂SeO₃ significantly enhanced Se accumulation in all organs of tree peony ‘Fengdan’ (Figure 3A). With increasing Na₂SeO₃ concentrations, Se accumulation rose substantially in roots, stems, leaves, pod shells, and seeds. Compared to CK, plants treated with 25–200 mg L⁻¹ Na₂SeO₃ exhibited 1.14–5.73-fold (roots), 5.86–32.95-fold (stems), 11.22–172.27-fold (leaves), 5.74–38.13-fold (pod shells), and 5.81–32.94-fold (seeds) increases in Se accumulation. Among the organs, leaves have the highest Se accumulation, while roots consistently exhibited the lowest accumulation. Further correlation analysis showed (Figure S3) that there was a positive correlation between the selenium accumulation in various organs of tree peonies and the application concentrations of sodium selenite. Among them, the selenium accumulation in leaves had a significant linear relationship, that is, as the concentration of sodium selenite increased, the accumulation of selenium showed an accelerated growth, with R² being 0.9878 (p < 0.01); the selenium accumulation in roots, stems, pod shells, and seeds showed a curvilinear correlation. As the selenium application concentration increased, the selenium accumulation gradually increased, but at higher selenium concentrations, the growth rate gradually slowed down.

Despite the general increase in Se accumulation, the distribution ratio of Se among organs did not follow a uniform trend (Figure 3B). As Na₂SeO₃ application concentration increased, the proportion of Se distributed to roots sharply declined from 61.26% (CK) to 14.43% (200 mg L⁻¹ Na₂SeO₃), whereas leaf Se distribution increased from 7.70% to 46.70%. The Se allocation to stems, pod shells, and seeds followed a non-linear pattern, peaking under 50, 100, and 25 mg L⁻¹ Na₂SeO₃, respectively. As the spraying Na₂SeO₃ increases the Se accumulation and distribution in the above-ground parts of tree peony ‘Fengdan’ while reducing its distribution in the roots.

3.4. Effect of Spraying Na₂SeO₃ on Conversion of Organic Se in Tree Peony Leaves and Seeds

As shown in

Table 1. Effect of spraying Na₂SeO₃ on organic Se concentrations in tree peony leaves and seeds at HV.

Treatment	Inorganic Se Concentration (mg kg−1 DM)			Organic Se Concentration (mg kg−1 DM)
	Leaf	Seed	Seed Meal	Leaf	Seed	Seed Meal
CK	ND	ND	ND	0.053 ± 0.001 e	0.058 ± 0.003 e	0.081 ± 0.003 e
T1	0.070 ± 0.007 d	0.027 ± 0.003 d	0.038 ± 0.003 d	0.545 ± 0.041 d	0.353 ± 0.002 d	0.495 ± 0.002 d
T2	0.232 ± 0.017 c	0.071 ± 0.008 c	0.102 ± 0.011 c	1.385 ± 0.059 c	0.664 ± 0.058 c	0.949 ± 0.072 c
T3	0.631 ± 0.046 b	0.162 ± 0.004 b	0.229 ± 0.003 b	2.829 ± 0.207 b	1.173 ± 0.052 b	1.655 ± 0.063 b
T4	2.242 ± 0.173 a	0.341 ± 0.022 a	0.476 ± 0.022 a	6.262 ± 0.252 a	1.522 ± 0.038 a	2.128 ± 0.041 a

Note: CK, T1, T2, T3, and T4 means 0, 25, 50, 100, and 200 mg L⁻¹ Na₂SeO₃ spraying. Inorganic Se concentrations in leaves and seeds of CK was below the detection limit and their organic Se concentrations was expressed as the total Se concentrations. ND means not detected. Different small letters in the same column meant significant difference at 0.05 level.

, foliar application of Na₂SeO₃ significantly increased the organic Se concentrations in tree peony leaves, seeds, and seed meal at HV. Compared to CK, the organic Se concentrations in leaves increased by factors of 9.28, 24.57, 52.38, and 117.15 following treatment with 25–200 mg L⁻¹ Na₂SeO₃. Similarly, the organic Se concentrations in seeds increased by factors of 5.09, 10.45, 19.22, and 25.24, respectively. As a by-product of oil extraction, the organic Se and inorganic Se concentrations in seed meal is closely related to the Se concentrations in seeds and seed oil. Given that Se was not detected in seed oil, total Se concentrations of seed meal was consistent with that in seeds. The organic and inorganic Se concentrations in seed meal were obtained by calculation after removal of the crude fat in seeds, and the results showed that the inorganic and organic Se concentrations in seed meal were higher than those in seeds.

Further analysis was conducted on the relationship between the concentrations of inorganic and organic Se in tree peony leaves and seeds and the concentration of Na₂SeO₃ spraying (Figure S4). The results revealed a significant positive correlation between the concentration of inorganic Se in leaves and the Na₂SeO₃ spraying concentration, with an R² of 0.9925 (p < 0.01), indicating a gradual increase in inorganic Se concentrations in leaves with increasing Na₂SeO₃ concentrations. The concentrations of organic Se in leaves showed a linear correlation with the Na₂SeO₃ spraying concentrations, with an R² of 0.9949 (p < 0.01), demonstrating a stable increase in organic Se concentrations with increasing Na₂SeO₃ concentrations. A significant quadratic relationship was observed between organic Se and inorganic Se in leaves, with a fitting R² of 0.9976 (p < 0.01), showing a clear interdependence between the two. In seeds, there was a linear relationship between the concentrations of inorganic Se and the Na₂SeO₃ concentrations, with a fitting R² of 0.9934 (p < 0.01). The concentrations of organic Se in seeds exhibited a quadratic relationship with the Na₂SeO₃ concentration, with an R² value of 0.9949 (p < 0.01). As the Na₂SeO₃ concentration increased, the organic Se concentrations in seeds gradually increased but remained lower than that in leaves and tended to saturate at high concentrations. Additionally, a significant quadratic relationship was observed between organic Se and inorganic Se in seeds (R² = 0.9976, p < 0.01).

Despite the sharp rise in absolute organic Se concentrations, the organic Se conversion ratio declined slightly with increasing Na₂SeO₃ concentrations (Figure 4). Even so, the conversion remained high, over 73% in leaves and over 81% in seeds, indicating efficient biotransformation of inorganic Se into organics, with seeds displaying the stronger conversion capacity. These findings highlight tree peony seed-meal as a promising by-product for Se-enriched functional foods or supplements.

3.5. Effect of Spraying Na₂SeO₃ on Se Speciation in Tree Peony Leaves and Seeds

Se speciation in tree peony leaves and seeds at HV was significantly affected by foliar application of Na₂SeO₃ (

Table 2. Effect of spraying Na₂SeO₃ on Se speciation in tree peony leaves and seeds at HV stage.

Organs	Treatment	Se Speciation Concentration (mg kg−1 DM)
		Se4+	Se6+	SeMet	SeCys2	MeSeCys
Leaf	CK	ND	ND	ND	ND	ND
	T1	ND	0.063 ± 0.004 d (10.94%)	0.108 ± 0.006 d (18.75%)	0.199 ± 0.013 d (34.55%)	0.206 ± 0.027 d (35.76%)
	T2	ND	0.223 ± 0.010 c (14.86%)	0.273 ± 0.016 c (18.19%)	0.526 ± 0.030 c (35.04%)	0.479 ± 0.044 c (31.91%)
	T3	ND	0.613 ± 0.036 b (19.40%)	0.532 ± 0.030 b (16.84%)	1.165 ± 0.077 b (36.88%)	0.849 ± 0.068 b (26.88%)
	T4	ND	2.159 ± 0.201 a (27.85%)	1.388 ± 0.097 a (17.90%)	2.978 ± 0.124 a (38.41%)	1.228 ± 0.095 a (15.84%)
Seed	CK	ND	ND	ND	ND	ND
	T1	ND	0.021 ± 0.001 d (5.82%)	0.106 ± 0.002 d (29.36%)	0.234 ± 0.002 d (64.82%)	ND
	T2	ND	0.073 ± 0.009 c (10.47%)	0.186 ± 0.009 c (26.69%)	0.438 ± 0.042 c (62.84%)	ND
	T3	ND	0.139 ± 0.009 b (11.25%)	0.348 ± 0.015 b (28.16%)	0.749 ± 0.030 b (60.60%)	ND
	T4	ND	0.248 ± 0.005 a (14.88%)	0.436 ± 0.009 a (26.15%)	0.947 ± 0.036 a (56.81%)	0.036 ± 0.000 (2.16%)

Note: ND, not detected; Se⁴⁺, selenite; Se⁶⁺, selenate; SeMet, selenomethionine; SeCys₂, selenocystine; MeSeCys, methylselenocysteine. CK, T1, T2, T3, and T4 means 0, 25, 50, 100, and 200 mg L⁻¹ Na₂SeO₃ spraying. Data in parentheses are the proportion of each Se speciation to the total Se speciation at that Na₂SeO₃ concentration. Different small letters in the same column for the same organ meant significant difference at 0.05 level.

). In the control group, due to low total Se concentrations, none of the common Se species were detected. Following Na₂SeO₃ application, four Se species were identified in leaves: selenate (Se⁶⁺), selenomethionine (SeMet), selenocystine (SeCys₂), and methylselenocysteine (MeSeCys). In seeds, three major species (SeMet, SeCys₂, and Se⁶⁺) were consistently detected across treatments, while MeSeCys was only observed at 200 mg L⁻¹ Na₂SeO₃ treatment.

Regression analysis was conducted on the relationship between Se speciation in leaves and seeds and the concentration of Na₂SeO₃ spraying (Figure S5). The results indicated that the concentrations of SeVI in leaves exhibited a quadratic correlation with the Na₂SeO₃ application concentrations, with an R² of 0.9897 (p < 0.01). This suggests that as the Na₂SeO₃ concentrations increases, the SeVI concentrations gradually rises, with a faster increase at higher Se concentrations, demonstrating a dose–response relationship of SeVI to Se application concentrations. The concentrations of SeMet and SeCys₂ in leaves showed a linear increase, with R² values of 0.9818 (p < 0.01) and 0.9895 (p < 0.01), respectively. As the Na₂SeO₃ concentration increased, the concentrations of both compounds gradually increased, indicating an influence of Na₂SeO₃ concentration on the synthesis of selenium-containing amino acids. The accumulation of SeMetCys in leaves exhibited a curved correlation, with an R² of 0.9814 (p < 0.01). With an increase in Na₂SeO₃ concentration, the SeMetCys concentrations gradually increased, but the rate of increase slowed at higher Se concentrations. The accumulation of SeVI in seeds showed a linear relationship with Na₂SeO₃ concentration, with an R² of 0.9824 (p < 0.01). As the Se concentration increased, the SeVI concentrations in seeds gradually increased. Despite the clear growth trend, the SeVI concentrations in seeds remained relatively low. The concentrations of SeMet and SeCys₂ in seeds both exhibited significant curved correlations with Na₂SeO₃ concentration, with R² values of 0.9934 (p < 0.01) and 0.9913 (p < 0.01), respectively. The growth in SeMet and SeCys₂ concentrations gradually slowed at medium to high Se concentrations, indicating that the synthesis of selenium-containing amino acids in seeds tends to saturate at higher concentrations.

Among all forms, SeCys₂ was dominant in both leaves and seeds. In leaves, SeCys₂ constitutes the highest proportion of the total Se speciation, ranging from 34–39%, followed by MeSeCys ranging between 15–36%, and SeMet (approximately 18%). In seeds, the proportion of SeCys₂ ranges from 56–65%, SeMet ranges from 26–30%, while MeSeCys makes a minor contribution only under 200 mg L⁻¹ Na₂SeO₃ treatment. Overall, organic Se species (SeMeCys, SeCys₂, and SeMet) represented over 70% of the total Se speciation in leaves and over 85% in seed s under all treatments. These findings indicate that foliar-applied Na₂SeO₃ is effectively absorbed and metabolized into bioavailable organic Se forms, especially SeCys₂ and SeMet, in tree peony.

3.6. Effects of Spraying Na₂SeO₃ on the Nutritional Quality of Tree Peony Seeds

Foliar application of Na₂SeO₃ did not significantly affect basic yield-related traits such as 100-seed weight or kernel percentage in tree peony ‘Fengdan’ (

Table 3. Effects of spraying Na₂SeO₃ on the nutritional quality of tree peony seed.

Treatment	100 Seed Weight (g)	Kernel Percentage (%)	Crude Fat Content (%)	Soluble Protein Content (mg g−1 DM)	Soluble Sugar Content (mg g−1 DM)	Starch Content (mg g−1 DM)	Total Phenols Content (mg g−1 DM)
CK	21.31 ± 0.79	68.77 ± 0.73	28.66 ± 0.37 b	110.42 ± 7.36 b	103.09 ± 3.12 c	151.94 ± 4.30 bc	7.11 ± 0.30 d
T1	21.69 ± 0.70	69.71 ± 1.01	28.74 ± 0.49 b	125.43 ± 4.19 ab	121.15 ± 3.25 b	169.91 ± 6.11 a	20.61 ± 1.22 c
T2	22.35 ± 0.91	68.88 ± 0.86	30.01 ± 0.74 a	133.35 ± 5.06 a	139.11 ± 5.22 a	163.70 ± 6.13 ab	22.69 ± 0.31 ab
T3	22.71 ± 0.31	69.54 ± 1.21	29.14 ± 0.39 ab	135.37 ± 9.54 a	143.40 ± 4.59 a	156.65 ± 3.70 bc	24.17 ± 1.40 a
T4	21.13 ± 0.38	68.59 ± 1.14	28.46 ± 0.43 b	117.27 ± 4.52 b	120.68 ± 2.26 b	147.11 ± 6.37 c	21.00 ± 0.33 bc

Note: CK, T1, T2, T3, and T4 means 0, 25, 50, 100, and 200 mg L⁻¹ Na₂SeO₃ spraying. Different small letters in the same column meant significant difference at 0.05 level.

). However, notable improvements were observed in several nutritional quality parameters of the seed kernels. The crude fat content initially increased, then decreased with rising Na₂SeO₃ concentrations, peaking at 50 mg L⁻¹ Na₂SeO₃ treatment, 4.72% higher than CK (p < 0.05). A similar trend was observed for soluble protein, soluble sugar, and total phenolic content. Specifically, compared to CK, the soluble protein content increased by 13.6%, 20.8%, 22.6%, and 6.2% in the 25–200 mg L⁻¹ Na₂SeO₃ treatments, respectively. The soluble sugar levels rose by 17.1% to 39.1%, with the highest values recorded in 100 mg L⁻¹ Na₂SeO₃ treatment. Total phenolic content increased 2.2–2.4 fold under Se treatments, peaking at 24.17 mg g⁻¹ DM with 100 mg L⁻¹ Na₂SeO₃ treatment. Starch content responded differently, treatments with 25 and 50 mg L⁻¹ Na₂SeO₃ led to slight increase in starch levels by 11.8% and 7.7%, respectively, whereas treatments with 100 and 200 mg L⁻¹ Na₂SeO₃ resulted in slight decrease.

The regression analysis results indicate a significant dose–response relationship between the Na₂SeO₃ application concentrations and the levels of multiple nutrients in tree peony seeds (Figure S6). The seed weight, soluble protein, soluble sugar, and total phenolic content all show significant quadratic correlations with the Na₂SeO₃ concentration, with R² values of 0.4488 (p < 0.05), 0.6495 (p < 0.05), 0.9011 (p < 0.01), and 0.7963 (p < 0.01), respectively. This suggests that appropriate Se application significantly enhances the seed weight, soluble protein, soluble sugar, and total phenolic content, but the enhancement diminishes at higher Se concentrations. There was no significant correlation observed between the seed kernel rate, crude fat, starch content, and Na₂SeO₃ concentration. These results indicate that spraying 50–100 mg L⁻¹ Na₂SeO₃ can significantly improve the nutritional composition of tree peony seeds, particularly in contents of soluble protein, soluble sugar, total phenolic, and also increase the seed weight.

3.7. Effect of Spraying Na₂SeO₃ on the Fatty Acid Composition of Tree Peony Seed Oil

The fatty acid composition of tree peony seed oil exhibited significant alterations in response to Na₂SeO₃ foliar application (

Table 4. Effects of spraying Na₂SeO₃ on the fatty acid composition of tree peony seed oil (%).

Treatment	C16:0	C18:0	C18:1	C18:2	C18:3	UFA	PUFA
CK	6.84 ± 0.09 a	2.61 ± 0.03	25.58 ± 0.10 c	25.42 ± 0.06 b	38.48 ± 0.30 c	89.47 ± 0.46 b	63.90 ± 0.36 c
T1	6.61 ± 0.09 b	2.31 ± 0.34	26.28 ± 0.10 b	25.82 ± 0.10 a	39.31 ± 0.15 b	91.07 ± 0.50 a	65.13 ± 0.21 a
T2	6.47 ± 0.06 bc	2.50 ± 0.02	26.20 ± 0.04 b	24.12 ± 0.11 e	40.26 ± 0.19 a	90.58 ± 0.09 a	64.38 ± 0.08 b
T3	6.82 ± 0.03 a	2.47 ± 0.03	25.90 ± 0.13 c	24.57 ± 0.05 d	39.91 ± 0.14 a	90.39 ± 0.21 a	64.49 ± 0.12 b
T4	6.37 ± 0.06 c	2.57 ± 0.05	26.77 ± 0.02 a	24.78 ± 0.07 c	39.05 ± 0.17 b	90.61 ± 0.11 a	63.84 ± 0.11 c

Note: CK, T1, T2, T3, and T4 means 0, 25, 50, 100, and 200 mg L⁻¹ Na₂SeO₃ spraying. Different small letters in the same column meant significant difference at 0.05 level.

). Among saturated fatty acids (SFA), the content of palmitic acid (C16:0) decreased with increasing Na₂SeO₃ concentrations, reaching the lowest level in 200 mg L⁻¹ Na₂SeO₃ (6.37%), which represented a 6.87% reduction compared to CK (p < 0.05). However, stearic acid (C18:0), remained unaffected by Na₂SeO₃ application. Regarding monounsaturated fatty acids (MUFA), the content of oleic acid (C18:1) showed a progressive increase, peaking at 26.77% in 200 mg L⁻¹ Na₂SeO₃ treatment, a 4.65% rise relative to CK (p < 0.05). For polyunsaturated fatty acids (PUFA), the content of linoleic acid (C18:2) increased under 25 mg L⁻¹ Na₂SeO₃ but declined with further increases, while the content of α-linolenic acid (C18:3) peaked at 50 and 100 mg L⁻¹ Na₂SeO₃ treatments, with increases of 4.63% and 3.85% compared to CK, respectively (p < 0.05). Total unsaturated fatty acids (UFA) and PUFA contents were generally higher in Se-treated plants, with the highest UFA (91.07%) and PUFA (65.13%) content observed in 25 mg L⁻¹ Na₂SeO₃.

The regression analysis revealed distinct patterns of correlation between different fatty acid components and Na₂SeO₃ application concentrations (Figure S7). In tree peony seed oil, C16:0 significantly decreased with increasing Na₂SeO₃ concentration, with R² value of 0.3006 (p < 0.05), while C18:1 showed a significant positive correlation with selenium concentration, with R² value of 0.5526 (p < 0.01), indicating that Na₂SeO₃ application promotes oleic acid accumulation. Both C18:3 and PUFA exhibited quadratic relationships with Na₂SeO₃ concentration, with R² values of 0.7926 (p < 0.01) and 0.3876 (p < 0.05), respectively, showing higher levels of these components at moderate concentrations. The levels of C18:0, C18:2, and UFA were less affected by Na₂SeO₃ application. These results suggest that Na₂SeO₃ application reduces the content of saturated fatty acid (C16:0) in tree peony seed oil, significantly increases C18:1 and C18:3 levels, and enhances the proportion of polyunsaturated fatty acids, with moderate Na₂SeO₃ application concentrations (50–100 mg L⁻¹) being most beneficial for improving the quality of tree peony seed oil.

3.8. Comprehensive Evaluation of the Biological Effects of Spraying Na₂SeO₃ as an Agronomic Measure

The analysis (

Table 5. Score coefficient and contribution rates of principal components under different indexes.

Character	F1	F2	F3
Seed biomass (g plant−1 DM)	0.288	−0.003	−0.099
Total Se (mg kg−1 DM)	−0.056	0.337	−0.023
Organic Se (mg kg−1 DM)	−0.037	0.330	−0.027
Soluble protein (mg g−1 DM)	0.245	−0.015	0.006
Soluble sugar (mg g−1 DM)	0.261	0.050	−0.066
Starch (mg g−1 DM)	−0.006	−0.167	0.301
Total phenols (mg g−1 DM)	0.027	0.184	0.156
Crude fat (%)	0.408	−0.187	−0.218
UFA (%)	−0.215	0.178	0.418
PUFA (%)	−0.097	−0.066	0.382
Eigenvalue	5.375	2.879	1.314
Contribution ratio (%)	53.745	28.786	13.143
Cumulative contribution ratio (%)	53.75	82.53	95.67

) revealed that the cumulative variance contribution rate of the three principal components (PCs) was 95.67%, encompassing most of the information from each parameter. PC1 primarily represented biomass, crude fat, and soluble nutrients, with a variance contribution rate of 53.75%, reflecting productivity and nutritional enhancement of seeds. PC2 (28.79%) was driven by total and organic Se concentrations, corresponding to the degree of Se enrichment. PC3 (13.14%) was mainly associated with UFA and PUFA, reflecting improvements in oil quality.

Based on PCA-derived score coefficients and standardized indicator values, a comprehensive score was calculated for each treatment (Figure 5). 100 mg L⁻¹ Na₂SeO₃ achieved the highest overall performance, indicating it offers the best balance between Se enrichment, nutritional enhancement, and seed oil quality. Treatments with 50 and 100 mg L⁻¹ Na₂SeO₃ also showed positive impacts compared to CK, while 200 mg L⁻¹ Na₂SeO₃ treatment showed diminished benefits or possible negative effects. These results suggest that a moderate application of Na₂SeO₃, especially at 100 mg L⁻¹, is optimal for simultaneously improving Se enrichment, nutritional value, and oil quality in tree peony seeds.

4. Discussion

4.1. Foliar Application of Na₂SeO₃ Enhances Biomass Accumulation in Tree Peony

While Se is not essential for plant growth, it has been demonstrated to confer multiple beneficial effects, including growth promotion, enhanced stress tolerance, and improved nutritional characteristics [37,49,50]. Particularly, foliar application of Se has been widely recognized for enhancing crop yield and biomass accumulation. For instance, foliar application of 0.75 mg kg⁻¹ Na₂SeO₃ increased potato tuber yield by 4% [51], while 5 g ha⁻² Na₂SeO₃ increased grain yield in buckwheat [52]. Similarly benefits have been reported in citrus fruit [53], broccoli [54] and cabbage [55], where Se treatments enhanced organ development and biomass distribution.

In the present study, foliar application of 100 mg L⁻¹ Na₂SeO₃ significantly increased leaf and seed biomass of tree peony, while the biomass of roots, stems, and pod shells showed no significant changes. This may be attributed to Se-enhanced stimulation of antioxidant enzymes activities and enhanced photosynthetic efficiency, leading to more efficient nutrient partitioning towards actively growing tissues such as leaves and seeds [36,56]. These results demonstrate the potential of Se foliar application as an agronomic tool to enhance the growth performance of tree peony. Importantly, the pronounced accumulation in reproductive organs further supports the feasibility of using Se foliar application as a biofortification strategy in tree peony.

4.2. Foliar Application of Na₂SeO₃ Enhances Nutritional and Functional Quality of Tree Peony Seeds

Enhancing the nutritional quality of crops and improving their functional ingredients are key areas of focus in current agricultural research. Numerous studies have demonstrated that appropriate Se concentrations can positively influence plant nutritional quality. Several studies report that foliar Se treatments elevate soluble sugars and proteins in diverse crops, such as table grapes [57] and cowpeas [58]. Similarly, Na₂SeO₃ spraying increased total sugar and vitamin levels in citrus fruits [53]. Beyond primary metabolism, Se also stimulates the biosynthesis of plant secondary metabolites, including flavonoids, phenolics, and terpenoids, which are vital for both plant defense and human health benefits [54,59]. Experimental results suggest foliar application of Na₂SeO₃ at 50 and 100 mg L⁻¹ significantly increased the content of soluble protein, soluble sugar, and total phenolic content in tree peony seeds, consistent with previous studies. The observed enhancement in protein levels is likely due to Se’s role in the synthesis of selenoproteins, which are essential for redox homeostasis and protein assembly [60]. The elevation in soluble sugars and phenolics may result from Se-induced stimulation of photosynthesis and activation of the phenylpropanoid pathway, as supported by recent findings in other species [61,62,63].

Furthermore, appropriate Se application modulated lipid metabolism in the seeds, leading to an increase in crude fat content and a favorable shift in fatty acid composition. Specifically, the relative abundance of unsaturated and polyunsaturated fatty acids, such as oleic and α-linolenic acids, was enhanced in seed oil. Similar effects were observed in walnuts, where Se application improved fruit and kernel quality, elevating linoleic acid levels while reducing saturated fatty acids [64]. Spraying nanoparticles on peanuts also increased oil yield [65]. These findings may be attributed to Se’s impact on protein-fat interaction or enzyme activity in lipid metabolism [66]. These compositional changes align with dietary recommendations to increase unsaturated fatty acids and limit saturated fatty acids for cardiovascular health [67]. Consequently, Se biofortification in tree peony not only enhances basic nutritional parameters but also contributes to the functional properties of tree peony seeds. The findings from this study provide a scientific basis for further exploration of Se’s role in improving crop nutritional quality.

4.3. Tree Peony as a Promising Se-Enriched Crop for Biofortification

Foliar application of exogenous Se is widely recognized as an effective strategy for enhancing Se content in plants, forming the foundation for Se biofortification in agricultural systems [68,69,70]. Previous studies have demonstrated clear dose-dependent relationships between Se fertilizer application and Se accumulation in staple crops such as maize [71], rice [18], and millet [72]. This study reinforces this paradigm, revealing that increasing Na₂SeO₃ concentrations significantly elevated Se content across multiple organs of tree peony at different seed developmental stages. A strong positive correlation between Se concentration in plant tissues and the level of Na₂SeO₃ application was observed at the seed harvest stage, underscoring the potential of tree peony as a Se-enriched horticultural species.

Se accumulation and distribution in plant organs are crucial factors in assessing the effectiveness of biofortification [73]. This study showed that Se accumulation in tree peony progressively shifted toward above-ground tissues (leaves, stems, pod shells, and seeds) with increasing Na₂SeO₃ concentrations. This trend is consistent with observations in other crops. For example, in peanuts treated with sodium selenite, Se accumulation is highest in the kernels, followed by the leaves, stems, husks, and roots [74]. In spring maize, Se is primarily distributed in the leaves, followed by the roots, grains, and stems [75]. Similarly, foliar Se application in apple trees led to an accumulation pattern of leaves > branches > fruits > roots [76]. This distribution can be attributed to the foliar application of Se, which is initially absorbed by the leaves and then preferential transported to actively growing and reproductive organs [5,77]. Therefore, the findings confirm that foliar Na₂SeO₃ application effectively promotes Se accumulation in the economically valuable above-ground organs of tree peony, representing a practical and efficient biofortification strategy.

Moreover, the timing of foliar application plays a critical role in maximizing Se accumulation. The highest Se concentrations in leaves was observed at the seed inclusion enrichment and conversion stage, likely correlating with the plant’s distinctive physiological. This stage marks the peak physiological metabolism of tree peony leaves during their entire growth period, resulting in escalated nutrient requirements and consequently, maximized Se uptake. Comparable developmental phase-specific Se peaks have been reported in other crops such as garlic [78], blueberry [79], and pear [80], suggesting a universal trend where active growth phases favor Se absorption and assimilation. Based on these observations, it is recommended to apply foliar Na₂SeO₃ prior to or during the early stages of seed inclusion enrichment and conversion to achieve optimal Se biofortification in tree peony.

4.4. Tree Peony Is an Efficient Bioreactor for Organic Se

While total Se concentration serves as a key index for evaluating the effectiveness of biofortification in edible plant parts, the chemical speciation of Se determines its nutritional value and bioavailability. Organic Se species, such as selenomethionine (SeMet), selenocysteine (SeCys₂), and methylselenocysteine (MeSeCys)—exhibit higher biological activity and lower toxicity compared to inorganic forms, making their enrichment a primary objective in agronomic Se fortification strategies [38,61,81].

In this study, foliar application of Na₂SeO₃ significantly increased both total and organic Se concentrations in tree peony leaves and seeds. Despite a slight decline in the organic-to-total Se ratio at higher application rates, the conversion efficiency remained consistently high, exceeding 73% in leaves and 81% in seeds. These findings underscore tree peony’s robust capability to convert inorganic Se into bioavailable organic forms, with transformation efficiency modulated by the applied Se concentration. Similar transformation trends have been reported in other crops. In Se-enriched rice, more than 75% of total Se exists in organic forms, though the relative proportion decreases as application rates rise [18]. In peanuts, the proportion of organic Se decreases with increasing Se fertilizer concentration within the range of 50–100 g ha⁻² [66]. In contrast, wheat [82] and golden needle mushroom [83] exhibit relatively stable organic Se content irrespective of Se supply. This variability may be attributed to differences in plant Se tolerance and metabolic conversion mechanisms.

Se speciation in plants varies significantly across species and tissues. While SeMet dominates in cereals such as wheat and rice [84], SeCys₂ and MeSeCys are prevalent in Se-rich vegetables, particularly Allium and Brassicaceae species [85]. Studies on Auricularia auricular, Solanum tuberosum L., and Lentinula edodes confirm SeMet as the primary organic form in these systems [86,87,88]. In the present study, SeCys₂, MeSeCys, and SeMet were identified as the major organic Se species in tree peony leaves, whereas seeds predominantly accumulated SeCys₂ and SeMet. The presence of MeSeCys is particularly noteworthy due to its recognized anticancer activity, while SeMet is widely used as a safe and effective dietary Se supplement [89]. This biochemical profile suggests that tree peony tissues could serve as high-value raw materials for functional food development and nutritional fortification.

Moreover, with the implementation of the national woody oil development strategy [28], the cultivation of oil tree peonies has expanded rapidly in recent years [34,35,45]. However, the production of tree peony seed oil results in significant amounts of agricultural by-products, such as leaves, stems, pod shells, and seed meal, many of which are underutilized, causing wastage of valuable natural resources and exacerbating environmental pressures [29,30,45,90,91,92]. Our research indicates that tree peonies exhibit a notable capacity for Se enrichment. By implementing Se biofortification, they can accumulate Se in all plant organs and effectively convert inorganic Se into biologically accessible organic forms. These by-products could be repurposed as Se-enrich raw materials for various applications in food, nutraceuticals, and pharmaceuticals. Taken together, the comprehensive utilization potential of tree peonies, along with their efficient Se absorption, transformation, and accumulation abilities, make them an ideal organic Se bioreactor. Integrating Se biofortification into the tree peony value chain may not only open up new avenues for developing Se-enriched products but also provide insights for extending the tree peony industry chain and enhancing its economic, nutritional, and ecological contributions.

5. Conclusions

In summary, tree peony ‘Fengdan’ exhibits remarkable efficiency in Se absorption and accumulation, coupled with a strong capacity to biotransform inorganic Se into organic forms—predominantly SeCys₂ and SeMet. These organic Se species, characterized by high bioavailability and safety, highlight the significant potential of tree peonies as bioreactors for producing selenium-enriched products. The study further confirms that foliar application of Se at an appropriate concentration can not only significantly increase the biomass of tree peony leaves and seeds but also improve the nutritional quality of seeds by increasing key components such as crude fat, soluble protein, soluble sugar, total phenols, and unsaturated fatty acids. Based on these findings, targeted practical recommendations are proposed for selenium biofortification in tree peonies: foliar spraying should be implemented during the rapid growth stage, using Na₂SeO₃ at a recommended concentration of 100 mg L⁻¹. This simple effective approach maximizes selenium accumulation in tree peony organs and significantly improves the quality, providing a solid theoretical foundation for the future application of selenium biofortification in the tree peony industry.