Sustainable Nano‐Fertilizers From Marine Biomass: Codium decorticatum‐Mediated MgO Nanoparticles for Enhanced Legume Productivity

ABSTRACT

The development of eco‐friendly nanofertilizers offers a sustainable strategy to improve crop productivity. In this study, the marine macroalga Codium decorticatum was employed as a green reducing and stabilizing agent for the biosynthesis of magnesium oxide nanoparticles (MgO NPs), and their agronomic potential was evaluated in Vigna radiata and Vigna mungo. The synthesized MgO NPs were characterized by UV–visible spectroscopy, FT‐IR, XRD, FE‐SEM, and EDX, confirming the formation of crystalline, phase‐pure nanoparticles. XRD analysis revealed a cubic crystalline structure with an average crystallite size of 28 nm, while FE‐SEM showed spherical to irregular nanoparticles with an average size of 38 nm. Seed priming and foliar application of MgO NPs (5–160 mg L⁻¹) demonstrated a concentration‐dependent response. The optimum concentration of 20 mg L⁻¹ significantly enhanced seed germination (87% in V. radiata and 84% in V. mungo), chlorophyll content, biomass, and yield. At this concentration, V. radiata produced 40 pods plant⁻¹ (40 g), whereas V. mungo produced 38 pods plant⁻¹ (38 g). In contrast, concentrations ≥80 mg L⁻¹ reduced growth and yield, indicating phytotoxicity. These findings demonstrate that C. decorticatum‐mediated MgO NPs represent an effective and sustainable nanofertilizer for improving legume productivity while reducing reliance on conventional agrochemical inputs.

Codium decorticatum extract was utilized for the green synthesis of MgO nanoparticles (MgO NPs), which were characterized and applied as a foliar nanofertilizer to Vigna radiata and Vigna mungo. An optimum concentration of 20 mg L⁻¹ significantly enhanced seed germination, chlorophyll content, biomass accumulation, biochemical constituents, and pod yield, highlighting the potential of algae‐mediated MgO NPs for sustainable legume production.

1. Introduction

Agriculture is facing the dual challenge of meeting the food demands of a rapidly growing global population while minimizing environmental degradation [1, 2, 3, 4]. Conventional agricultural practices relying on synthetic fertilizers and pesticides have substantially increased crop productivity; however, their excessive and prolonged use has resulted in low nutrient use efficiency, soil degradation, groundwater contamination, and adverse effects on beneficial soil microorganisms [1, 2, 5]. Consequently, sustainable nutrient management strategies that improve crop productivity while reducing environmental impacts have become a major research priority [2, 3, 4, 5]. Nanotechnology has emerged as a promising approach for sustainable agriculture by enabling the controlled delivery of nutrients and agrochemicals, thereby improving nutrient use efficiency and minimizing losses [3, 4, 6]. Owing to their high surface area, enhanced reactivity, and unique physicochemical properties, engineered nanomaterials have shown considerable potential in promoting plant growth, improving stress tolerance, and enhancing crop productivity [7, 8, 9, 10].

Among the various nanomaterials investigated for agricultural applications, MgO NPs have attracted increasing attention because magnesium is an essential macronutrient required for chlorophyll biosynthesis, photosynthesis, enzyme activation, carbohydrate metabolism, and protein synthesis [11, 12, 13, 14]. Magnesium deficiency adversely affects plant growth, photosynthetic efficiency, and biological nitrogen fixation, particularly in leguminous crops [15]. Compared with conventional magnesium fertilizers, MgO NPs provide a larger surface area and improved bioavailability, facilitating efficient nutrient uptake and utilization by plants [12]. In addition, MgO NPs exhibit antimicrobial, antifungal, and antioxidant properties that contribute to improved plant health and resistance to biotic and abiotic stresses [13, 14, 15, 16].

Although several chemical and physical methods have been developed for MgO Nanoparticle (MgO NP) synthesis, these approaches frequently require hazardous chemicals, elevated temperatures, and high energy consumption, thereby limiting their environmental sustainability [17]. Green synthesis has therefore emerged as an environmentally friendly and economically viable alternative [18, 19]. Biological resources, including plants, bacteria, fungi, and algae, contain diverse biomolecules such as polyphenols, polysaccharides, proteins, amino acids, and antioxidants that function as natural reducing and stabilizing agents during nanoparticle synthesis [20]. This approach eliminates the need for toxic chemicals while producing biocompatible nanoparticles suitable for agricultural applications. Recent advances have further highlighted the importance of biologically mediated nanoparticle synthesis for sustainable nanotechnology and agricultural applications [17, 18, 19, 20].

Among biological resources, marine macroalgae have gained considerable interest owing to their abundance, rapid biomass production, renewable nature, and rich phytochemical composition [20]. C. decorticatum was selected in the present study because it is an abundant marine green macroalga rich in polysaccharides, phenolic compounds, proteins, flavonoids, and other bioactive metabolites that serve as natural reducing and stabilizing agents during nanoparticle synthesis [21]. These biomolecules facilitate the efficient formation and stabilization of MgO NPs, while the renewable nature and wide availability of C. decorticatum make it an environmentally friendly and sustainable biological resource for green nanotechnology [22]. Consequently, marine algae are considered highly attractive biofactories for the green synthesis of metal oxide NPs [21, 22]. However, despite increasing interest in algae‐mediated nanotechnology, studies investigating C. decorticatum‐mediated synthesis of MgO NPs and their agricultural applications remain limited.

Legume crops such as V. radiata (green gram) and V. mungo (black gram) are among the most important pulse crops cultivated throughout tropical and subtropical regions [15, 23]. These crops represent valuable sources of dietary protein and contribute significantly to sustainable agriculture through biological nitrogen fixation and improvement of soil fertility [15]. Nevertheless, their productivity is frequently constrained by nutrient deficiencies and environmental stresses [12, 23]. Since magnesium plays a crucial role in photosynthesis and nitrogen metabolism, efficient magnesium supplementation using nano‐enabled fertilizers may substantially improve the growth and productivity of these economically important legumes [11, 13, 23].

Although previous studies have demonstrated the beneficial effects of MgO NPs on plant growth and physiological performance, most investigations have focused on chemically synthesized nanoparticles or plant extract‐mediated synthesis [12, 13, 16]. Comparatively fewer studies have explored marine algae‐mediated MgO NPs synthesis, and systematic comparative evaluations of their effects on different legume crops remain scarce [21, 22]. Consequently, crop‐specific responses to biosynthesized MgO NPs and their potential application as sustainable nanofertilizers remain poorly understood.

The present study differs from previous reports by integrating green synthesis, comprehensive physicochemical characterization, and agronomic evaluation within a single investigation. Unlike earlier studies that largely emphasized nanoparticle synthesis or laboratory‐scale biological activities, this work systematically evaluates the concentration‐dependent effects of C. decorticatum‐mediated MgO NPs on seed germination, biochemical constituents, vegetative growth, biomass accumulation, and yield performance of two economically important legume crops (V. radiata and V. mungo) under field conditions. Furthermore, the responses of both crops were comparatively analyzed to identify the optimum MgO NPs concentration for maximizing plant productivity.

The novelty of the present study lies in the green synthesis of MgO NPs using C. decorticatum and the comprehensive evaluation of their concentration‐dependent effects on two economically important legume crops, V. radiata and V. mungo. The study integrates detailed physicochemical characterization with comparative field evaluation of seed germination, biochemical constituents, vegetative growth, biomass accumulation, and yield performance. Furthermore, the optimum MgO NPs concentration for maximizing crop productivity is identified, providing new insights into the application of marine algae‐mediated MgO NPs as sustainable nanofertilizers for legume cultivation.

Therefore, the objectives of the present study were to (i) synthesize MgO NPs using the marine green macroalga C. decorticatum through an environmentally friendly green synthesis approach; (ii) characterize the synthesized nanoparticles using UV–visible spectroscopy, Fourier transform infrared spectroscopy (FTIR), x‐ray diffraction (XRD), field emission scanning electron microscopy (FE‐SEM), and energy‐dispersive x‐ray (EDX) analysis; (iii) evaluate their effects on seed germination, physiological and biochemical characteristics, growth, biomass, and yield of V. radiata and V. mungo; and (iv) compare the responses of both legume crops to identify the optimum nanoparticle concentration for sustainable agricultural applications.

2. Materials and Methods

2.1. Collection and Extract Preparation

The green seaweed C. decorticatum (Chlorophyceae) was collected from Sethubhavachatram, Tamil Nadu, India (10.9667°N, 79.4167°E) in January 2025. The collected specimen was taxonomically authenticated by Dr. P. Arulmurugan, Assistant Professor, Department of Plant Biology and Plant Biotechnology, Sir Theagaraya College, Old Washermenpet, Chennai, India. A voucher specimen (PHC 787) was deposited in the Pushpam Herbarium Cabinet (PHC), A.V.V.M. Sri Pushpam College (Autonomous), Poondi, Tamil Nadu, India, for future reference (Figure 1).

FIGURE 1.

Collection and taxonomic authentication of C. decorticatum used for the green synthesis of MgO NPs: (A) Collection sample photograph and (B) authenticated herbarium specimen.

The collected algal biomass was thoroughly washed with distilled water to remove adhering sand, salts, epiphytes, and other impurities and then cut into small segments. The cleaned biomass was boiled at 95°C for 10 min. Subsequently, 100 g of the processed seaweed was homogenized with 200 mL of distilled water using a laboratory blender. The homogenate was filtered through muslin cloth to remove coarse particles and then through Whatman No. 1 filter paper. The resulting filtrate was diluted with an equal volume of Millipore water and stored at 4°C until further use.

2.2. Green preparation of MgO NPs

MgO NPs were synthesized using C. decorticatum extract as a reducing and stabilizing agent. Briefly, an aqueous solution of magnesium nitrate hexahydrate (Mg(NO₃)₂·6H₂O) was added dropwise to 50 mL of the prepared seaweed extract under continuous magnetic stirring. The reaction mixture was maintained at 60°C for 30 min. Subsequently, the pH was gradually adjusted to approximately 8 by the dropwise addition of 1 M NaOH, resulting in the formation of a magnesium hydroxide [Mg(OH)₂] precipitate. The precipitate was collected and washed several times with distilled water and ethanol to remove residual salts, nitrate ions, and unreacted biomolecules. The washed precipitate was dried for 2–3 h and then calcined at 400°C for 3 h. During calcination, the Mg(OH)₂ precursor was thermally decomposed to form crystalline MgO NPs, with the release of water. The resulting MgO NPs were collected as a fine powder for subsequent characterization and agricultural evaluation [23]. The formation and purity of the MgO phase were subsequently confirmed by XRD and EDX analyses, which demonstrated the formation of MgO NPs without detectable impurity phases.

2.3. Physicochemical Analysis of MgO NPs

The physicochemical characteristics of the green‐prepared MgO NPs were systematically examined using multiple analytical techniques. Functional groups and surface chemical features were identified by Fourier transform infrared (FT‐IR) spectroscopy (PerkinElmer, ATR mode; Lab India Instruments Pvt. Ltd) over the spectral range of 4000‐400 cm^{− 1}. The crystalline nature and phase composition of the MgO NPs were determined by x‐ray diffraction (XRD) analysis using a Rigaku Miniflex 600 diffractometer operated at 40 kV and 30 mA, with a scanning rate of 2° min^{− 1}. Morphological attributes and surface architecture were investigated using field emission scanning electron microscopy (FE‐SEM; Carl Zeiss EVO 18 Cryo Environmental). In addition, the optical properties of the green‐prepared MgO NPs were assessed using UV–visible spectroscopy to confirm their characteristic absorption behavior.

2.4. Preparation of MgO NPs Dispersion and Seed Priming

Aqueous suspensions of MgO NPs were prepared at different concentrations (5, 10, 20, 40, 80, and 160 mg L^{− 1}) by dispersing the NPs in deionized water, followed by ultrasonication for 30 min to ensure uniform distribution. A control treatment consisting of deionized water without NPs was maintained under identical conditions. Seeds of green gram and black gram were obtained from the Seed Centre, Tamil Nadu Agricultural University (TNAU), Coimbatore, India, and used for all experimental studies. Prior to treatment, the seeds were surface‐sterilized using 3% H₂O₂ and subsequently rinsed thoroughly with deionized water.

For seed priming, the sterilized seeds were immersed in the respective MgO NP suspensions and allowed to soak for 12 h. After incubation, the seeds were removed and air‐dried at room temperature before further analysis.

Both NP‐treated (nanoprimed) and untreated (control) seeds were evaluated for germination following standard procedures (ISTA, 2008). Approximately 15 seeds were placed on germination paper moistened with distilled water, carefully rolled, secured, and incubated in a growth chamber maintained at 25°C and 90% relative humidity under dark conditions. Germination percentage was recorded on the 3rd day. All treatments were performed in triplicate, and the results were expressed as mean values.

2.5. Field Evaluation

Field evaluations were conducted at an agricultural site located at Ayyampet, Thanjavur District, Tamil Nadu, India, to evaluate the agronomic performance of MgO NPs. Seeds were initially germinated in cement pots for three days and subsequently transplanted into prepared field plots. The seedlings were cultivated in a soil matrix consisting of sand and agricultural soil mixed in a 1:1 ratio. During the first 30 days after transplantation, the plants were irrigated once daily with tap water to maintain adequate soil moisture under field conditions. MgO NP formulations were applied at different concentrations as a nanofertilizer through foliar spraying before and after the flowering stage. Before application, the nanoparticle suspensions were ultrasonicated for 30 min to ensure uniform dispersion.

2.6. Quantification of Biochemical Constituents

Biochemical constituents were determined in 30‐day‐old plant samples. Chlorophyll a, chlorophyll b, and total chlorophyll content were estimated using the method described by Arnon [24]. Total carbohydrate content was quantified by the anthrone method described by Hassid, while protein levels were measured following the procedure of Lowry et al. [25]. All analyses were carried out in accordance with standard protocols reported by Vijayakumar et al. [26].

2.7. Data Analysis

All experiments were conducted in triplicate (n = 3), and the results are presented as the mean ± standard deviation (SD). Statistical analyses were performed using one‐way analysis of variance.

3. Results and Discussion

3.1. UV Spectrum

The UV–visible absorption spectrum of the green‐prepared MgO NPs was recorded in the wavelength range of 200–800 nm (Figure 2). The spectrum exhibited a strong and sharp absorption peak centered at approximately 290 nm, which is a characteristic absorption feature of MgO nanostructures and agrees well with previously reported UV–vis spectra of biosynthesized MgO NPs [27]. This absorption band is attributed to the intrinsic electronic transition from the valence band to the conduction band of MgO, confirming the formation of the oxide phase. Similar absorption peaks in the range of 280–320 nm have been widely reported for MgO NPs, indicating their wide band gap nature [28].

FIGURE 2.

UV–visible absorption spectrum of green‐prepared MgO NPs.

The observed peak position around 290 nm is consistent with previously reported studies, where MgO NPs typically exhibit absorption maxima near 290–300 nm due to band‐to‐band transitions and nanoscale effects [29]. The sharpness of the peak suggests good optical quality and uniform particle distribution, while slight broadening may arise from size variation and surface defects.

A gradual decrease in absorbance is observed in the visible region (above 400 nm), indicating that the material is largely transparent to visible light. This behavior is a typical characteristic of MgO, which possesses a wide band gap and therefore absorbs predominantly in the ultraviolet region [30]. The absence of additional absorption peaks in the visible region further confirms the purity of the green‐prepared MgO NPs and indicates minimal presence of impurity phases or defect‐related states.

Overall, the UV–vis results confirm the successful formation of MgO NPs with characteristic optical absorption at approximately 290 nm. The absence of additional absorption bands indicates high optical purity, while the sharp absorption peak suggests good optical quality and relatively uniform nanoparticle formation.

3.2. XRD

The x‐ray diffraction (XRD) pattern of the green‐prepared MgO sample exhibits well‐defined diffraction peaks at 2θ values of approximately 39.6°, 42.7°, 62.3°, 74.7°, and 78.6°, which are indexed to the (111), (200), (220), (311), and (222) crystallographic planes, respectively (Figure 3). These reflections are characteristic of the face‐centered cubic (FCC) periclase structure of MgO and are consistent with standard JCPDS data (No. 45‐0946) [31].

FIGURE 3.

X‐ray diffraction pattern of green‐prepared MgO NPs.

The average crystallite size was estimated from the most intense (200) peak using the Debye–Scherrer relation

$$D=\frac{K\lambda }{\beta \cos \theta },$$

where K is the shape factor (0.9), λ is the wavelength of Cu Kα radiation (0.15406 nm), β is the full width at half maximum, and θ is the Bragg angle. The calculated crystallite size was found to be in the nanometer range (∼28 nm), indicating the formation of nanocrystalline MgO. The observed peak broadening further supports the nanoscale dimension of the particles.

The presence of sharp and intense peaks, particularly the dominant (200) reflection at ∼42.7°, indicates a high degree of crystallinity and preferred orientation along the (200) plane. Such preferential growth has been widely reported for MgO NPs and is often associated with energetically favorable crystal growth directions [32]. No additional impurity peaks were observed in the diffraction pattern, confirming the phase purity of the green‐prepared MgO. The absence of secondary phases suggests that the synthesis and calcination processes effectively converted the precursor into pure MgO without residual intermediates or by‐products [33]. The relatively narrow peak widths indicate the formation of well‐crystalline nanostructures, while slight peak broadening may be attributed to the nanoscale crystallite size and lattice strain effects. In general, broader peaks correspond to smaller crystallite dimensions, which can be further quantified using the Debye–Scherrer equation [34]. Additionally, the observed peak positions closely match previously reported MgO diffraction patterns, confirming the successful formation of cubic MgO nanocrystals with stable lattice parameters [35].

Overall, the XRD analysis confirms the formation of phase‐pure MgO NPs with a face‐centered cubic (FCC) crystal structure. The sharp diffraction peaks and average crystallite size of approximately 28 nm indicate high crystallinity and successful nanoscale synthesis. The absence of additional diffraction peaks confirms that the calcination process effectively converted the precursor into crystalline MgO without detectable secondary phases or residual intermediates.

3.3. FT‐IR

The Fourier transform infrared (FTIR) spectrum of the green‐prepared MgO NPs was recorded in the range of 4000–500 cm^{− 1} to identify the surface functional groups and confirm oxide formation (Figure 4). A broad absorption band centered at 3419.83 cm^{− 1} is assigned to O–H stretching vibrations of surface hydroxyl groups and physically adsorbed water molecules, which is commonly observed in MgO nanomaterials due to moisture adsorption from the atmosphere [36]. The weak band observed at 2924.63 cm^{− 1} corresponds to aliphatic C–H stretching vibrations, indicating the presence of residual organic constituents or biomolecules associated with the synthesis process. The absorption band at 1099.35 cm^{− 1} is associated with C–O stretching vibrations or Mg–O–C interactions arising from residual organic compounds involved in NPs stabilization. The peak observed at 874.88 cm^{− 1} corresponds to carbonate‐related bending vibrations and partially overlaps with Mg–O lattice‐associated vibrations, further supporting the formation of MgO nanostructures [37].

FIGURE 4.

FTIR spectrum of green‐prepared MgO NPs.

The intense absorption peak at 2349.89 cm^{− 1} is attributed to adsorbed atmospheric CO₂ or carbonate‐related vibrations on the MgO surface. Surface carbonation of MgO due to CO₂ interaction has been reported in recent studies [38]. The absorption band located at 1619.38 cm^{− 1} is assigned to H–O–H bending vibrations of adsorbed water molecules, confirming minor surface hydration of the MgO NPs [36]. The peak appearing at 1449.36 cm^{− 1} can be attributed to carbonate (CO₃ ^{2 −}) or C–O vibrational modes formed by atmospheric CO₂ adsorption on the MgO surface, which is commonly observed in alkaline oxide nanomaterials [39]. Most importantly, the strong absorption band at 539.91 cm^{− 1} is assigned to the characteristic Mg–O stretching vibration, confirming the successful formation of MgO NPs. Recent reports show Mg–O vibrational bands in the 500–600 cm^{− 1} region, which is considered the fingerprint region of crystalline MgO [40]. Overall, the FT‐IR analysis confirms the formation of MgO NPs with surface hydroxyl groups, minor carbonate species, and characteristic Mg–O bonding, indicating successful synthesis of chemically stable nanocrystalline MgO.

Overall, the FT‐IR results confirm the formation of MgO NPs through the characteristic Mg–O stretching vibration. The identified surface functional groups indicate the involvement of plant‐derived biomolecules in the reduction and stabilization of the NPs.

3.4. FE‐SEM With EDX

The morphology of the green‐prepared MgO NPs was examined using FE‐SEM, and the micrographs are shown in Figure 5a. The images revealed the formation of nearly spherical to irregularly shaped NPs with slight agglomeration. This aggregation may be due to the high surface energy and strong interparticle interactions of nanosized MgO. The particles appeared to be uniformly distributed, indicating effective nucleation and growth during the synthesis process. Similar morphological characteristics of MgO NPs have been reported in recent studies. The particle size distribution histogram obtained from FE‐SEM images is presented in Figure 5b. The particle size ranged from 33 to 40 nm, with an average size of 38 nm, suggesting relatively uniform particle formation. The elemental composition of the green‐prepared sample was further confirmed by EDX analysis (Figure 5c). The spectrum showed prominent peaks corresponding to magnesium and oxygen, confirming the formation of MgO NPs. No additional impurity peaks were observed, indicating the high purity of the green‐prepared material. Overall, the FE‐SEM, particle size distribution, and EDX results confirmed the successful formation of MgO NPs with nanoscale dimensions and good elemental purity.

FIGURE 5.

(a) FE‐SEM image showing morphology of green‐prepared MgO NPs, (b) Particle size distribution histogram, and (c) EDX spectrum confirming elemental composition of Mg and O.

Overall, FE‐SEM analysis revealed nearly spherical to irregular MgO NPs with nanoscale dimensions and slight agglomeration. EDX confirmed the presence of only magnesium and oxygen, demonstrating the high elemental purity of the synthesized NPs. Furthermore, the EDX spectrum exhibited only Mg and O signals, providing additional evidence for the elemental purity of the synthesized MgO NPs.

3.5. Biochemical Constituents

The effect of green‐prepared MgO NPs on biochemical constituents of green gram and black gram is presented in Figure 11. A clear concentration‐dependent response was observed, with maximum enhancement at 20 mg L^{− 1}, followed by a decline at higher concentrations.

FIGURE 11.

Effect of green‐prepared MgO NPs on biochemical constituents: (a) Chlorophyll a (mg/g), (b) Chlorophyll b (mg/g), (c) Total chlorophyll (mg/g), (d) Total carbohydrate (mg/g), (e) Total protein (mg/g), and (f) Lipid content (mg/g).

3.5.1. Photosynthetic Pigments

Chlorophyll a, chlorophyll b, and total chlorophyll contents increased significantly with MgO NP treatment up to 20 mg L^{− 1} in both crops. In V. radiata, chlorophyll a increased from 1.30 to 2.45 mg g^{− 1}, while total chlorophyll reached 3.438 mg g^{− 1}. A comparable trend was observed in V. mungo. This enhancement can be attributed to the essential role of magnesium as a core element of chlorophyll molecules and its involvement in photosynthetic enzyme activation. Nanoparticles can improve nutrient availability and uptake efficiency, thereby promoting pigment biosynthesis. Similar findings have been reported, where metal oxide nanoparticles enhanced photosynthetic efficiency and chlorophyll accumulation in plants [41]. However, a reduction in pigment content at higher concentrations (≥40 mg L^{− 1}) suggests possible nanoparticle‐induced oxidative stress, which may interfere with chloroplast integrity.

The enhanced chlorophyll content at 20 mg L^{− 1} demonstrates that MgO NPs effectively promoted photosynthetic capacity by improving magnesium availability, which is essential for chlorophyll biosynthesis and photosynthetic enzyme activation. Increased chlorophyll accumulation is expected to enhance light harvesting and carbon assimilation, thereby supporting better plant growth. The decline in pigment content at higher concentrations indicates concentration‐dependent phytotoxicity, likely resulting from oxidative stress and chloroplast damage [42, 43].

3.5.2. Carbohydrate and Protein Content

Total carbohydrate and protein levels showed a marked increase at optimal nanoparticle concentration (20 mg L^{− 1}). In green gram, carbohydrate content increased from 1.761 to 2.540 mg g^{− 1}, while protein content increased from 1.234 to 2.220 mg g^{− 1}. Black gram exhibited a similar pattern. The enhancement in carbohydrate levels is likely due to increased photosynthetic activity, resulting in higher assimilate production. Elevated protein content may be linked to improved nitrogen metabolism and enzyme synthesis. Previous studies have demonstrated that MgO NPs enhance biochemical potency and metabolic activity in plants when applied at suitable concentrations [13].

The increased carbohydrate and protein contents at the optimum MgO NP concentration indicate enhanced photosynthetic carbon fixation, nitrogen assimilation, and enzyme activity. Improved magnesium nutrition may have stimulated metabolic processes involved in carbohydrate synthesis and protein biosynthesis, leading to better plant physiological performance. However, the reduction at higher concentrations suggests metabolic disturbances caused by excessive nanoparticle exposure and oxidative stress [44, 45].

3.5.3. Lipid Content

Lipid content showed a substantial increase at 20 mg L^{− 1} in both species (V. radiata: 0.344 to 1.889 mg g^{− 1}; V. mungo: 0.338 to 1.884 mg g^{− 1}). This suggests improved membrane integrity and metabolic functioning under optimal nanoparticle exposure. At higher concentrations, lipid levels decreased, which may be due to membrane damage caused by reactive oxygen species (ROS) generated under nanoparticle stress conditions. Metal nanoparticles are known to influence lipid metabolism through their interaction with cellular redox systems [46]. The optimal concentration (20 mg L^{− 1}) significantly enhanced all measured biochemical parameters. This trend is consistent with recent reports indicating that MgO NPs act as effective nanofertilizers by improving plant growth, biochemical composition, and productivity when applied at controlled doses.

The accumulation of lipids at 20 mg L^{− 1} suggests improved membrane biosynthesis and cellular stability under optimal MgO NP treatment. Since membrane lipids are essential for maintaining cellular integrity, their increase reflects enhanced physiological activity. In contrast, reduced lipid content at higher nanoparticle concentrations may result from ROS‐mediated lipid peroxidation, leading to membrane damage and impaired cellular functions [43, 47].

Overall, the biochemical analysis demonstrates that green‐synthesized MgO NPs exhibit a concentration‐dependent effect on plant metabolism. The optimum concentration (20 mg L^{− 1}) enhanced photosynthetic pigments, carbohydrates, proteins, and lipids, whereas higher concentrations adversely affected these biochemical constituents due to oxidative stress. These findings highlight the potential of MgO NPs as effective nanofertilizers when applied at appropriate concentrations.

3.6. Growth and Yield Response of Plants Under MgO NPs Evaluation

The impact of MgO NPs on germination, vegetative growth, biomass, and yield traits of V. radiata and V. mungo is presented in Figures 8, 9, 10. The response pattern was clearly dose‐dependent, with the most pronounced enhancement observed at 20 mg L^{− 1}, followed by a decline at higher concentrations.

FIGURE 8.

Effect of green‐prepared MgO NPs on Growth parameters: (a) Germination percentage (%), (b) Total plant height (cm), (c) Shoot length (cm), (d) Root length (cm), and (e) Number of branches.

FIGURE 9.

Effect of green‐prepared MgO NPs on biomass accumulation: (a) Total fresh weight (gm), (b) Shoot fresh weight (gm), (c) Root fresh weight (gm), (d) Total dry weight (gm), (e) Shoot dry weight (gm), and (f) Root dry weight (gm).

FIGURE 10.

Effect of green‐prepared MgO NPs on yield parameters: (a) Leaf area (cm²), (b) Number of pods, (c) Pod length (cm), and (d) Pod weight (gm).

3.6.1. Seed Germination

A noticeable improvement in germination percentage was recorded with MgO NPs treatment. In V. radiata, germination increased from 58% in the control to 87% at 20 mg L^{− 1}, while V. mungo exhibited an increase from 55% to 84%. This enhancement may be attributed to improved seed hydration and activation of metabolic enzymes during early germination. These observations are clearly illustrated in Figures 6a and 7a, showing early germination responses under different MgO NP concentrations [48].

FIGURE 6.

Effect of MgO NPs on germination, growth, and the yield of V. radiata: (a) Early seed germination at 3 days; (b) Growth performance at 60 days; (c) Pod formation and yield attributes; and (d) Variation in harvested seeds under different MgO nanoparticle treatments.

FIGURE 7.

Effect of MgO NPs on germination, growth, and the yield of V. mungo: (a) Early seed germination at 3 days; (b) Growth performance at 60 days; (c) Pod formation and yield attributes; and (d) Variation in harvested seeds under different MgO nanoparticle treatments.

The improved seed germination at 20 mg L^{− 1} indicates that MgO NPs enhanced water uptake and activated metabolic enzymes required for reserve mobilization during early seedling development. Magnesium supplied by the NPs may also have promoted energy metabolism and enzyme activation, resulting in faster and more uniform germination. The decline observed at higher concentrations suggests that excessive nanoparticle exposure may impair germination through oxidative stress and metabolic disruption [42, 49].

3.6.2. Vegetative Growth Parameters

Growth‐related attributes such as total plant height, shoot length, and root length showed a consistent increase up to the optimal concentration. For instance, plant height in green gram increased from 22 to 32 cm, and in black gram from 19 to 29 cm at 20 mg L^{− 1}. Enhanced root development suggests improved nutrient uptake efficiency. Magnesium plays a central role in photosynthesis and enzyme activation, and its nano‐form enhances bioavailability, thereby promoting cell division and elongation. The improved growth performance is evident in Figures 6b and 7b. Comparable improvements in plant growth under nanoparticle exposure have been documented [50].

The enhanced vegetative growth demonstrates that MgO NPs improved nutrient uptake, photosynthetic efficiency, and chlorophyll‐mediated carbon assimilation, thereby promoting cell division and elongation. Improved root development further facilitated the absorption of water and mineral nutrients, resulting in greater shoot growth and overall plant vigor. Similar mechanisms have been reported for nanoparticle‐mediated growth enhancement in crop plants [42, 49].

3.6.3. Biomass and Morphological Traits

Biomass accumulation, including fresh and dry weights, increased significantly at 20 mg L^{− 1}. In V. radiata, total fresh weight increased from 15 to 21 g, while in V. mungo it increased from 12 to 19 g. Morphological traits such as number of branches and leaf area also showed considerable enhancement at the same concentration, indicating improved vegetative vigor. In addition, both shoot and root biomass (fresh and dry weights) followed a similar trend, with peak values observed at the optimal dose. This trend is consistent with the visual observations presented in Figures 6b and 7b.

These improvements may be linked to increased photosynthetic efficiency and enhanced metabolic activity induced by MgO NPs. Recent reports indicate that nanomaterials can improve biomass accumulation and plant architecture by facilitating nutrient uptake and physiological processes [13]. Recent studies have demonstrated that nanomaterials and seaweed‐derived biostimulants enhance biomass accumulation and plant architecture by improving nutrient uptake, photosynthetic efficiency, and metabolic activity, thereby promoting overall plant growth [51].

The significant increase in biomass and morphological traits at the optimum concentration reflects improved photosynthetic productivity and efficient conversion of assimilates into plant biomass. Enhanced nutrient utilization and metabolic activity promoted by MgO NPs likely contributed to increased leaf expansion, branching, and biomass accumulation. These findings agree with recent reports describing the beneficial effects of NPs on plant growth and biomass production under optimal application rates [49, 52].

3.6.4. Yield Attributes

Yield parameters, including number of pods, pod length, and pod weight, exhibited substantial improvement at 20 mg L^{− 1}. In green gram, pod number increased from 15 to 40, and pod weight reached 40 g, while black gram showed a similar increase from 13 to 38 pods, with pod weight reaching 38 g. The improvement in yield may be attributed to efficient translocation of assimilates and enhanced reproductive development. NPs at optimal concentrations are known to improve flowering and fruiting through better nutrient utilization and hormonal regulation [52]. Enhanced pod formation and yield characteristics are depicted in Figures 6c,d and 7c,d.

The improvement in yield parameters indicates that MgO NPs enhanced reproductive development through efficient assimilate translocation, improved nutrient utilization, and sustained photosynthetic activity. Enhanced flowering, pod formation, and seed development under the optimum nanoparticle concentration demonstrate the potential of MgO NPs as an effective nanofertilizer for increasing crop productivity [49, 52].

3.6.5. Effect of Higher Concentrations

At concentrations above 20 mg L^{− 1} (40–160 mg L^{− 1}), a gradual reduction in most parameters was observed. This decline may be associated with nanoparticle‐induced stress, including oxidative damage and disruption of cellular processes. Excessive nanoparticle accumulation can lead to the generation of ROS, negatively affecting plant growth and productivity [54]. Among all treatments, 20 mg L^{− 1} was identified as the optimal concentration, significantly enhancing germination, growth, biomass, and yield parameters [53].

The reduction in growth and yield at concentrations above 20 mg L^{− 1} confirms the concentration‐dependent response of plants to MgO NPs. Excessive nanoparticle accumulation can induce ROS generation, disrupt photosynthesis and cellular metabolism, and ultimately reduce plant growth and productivity. Similar phytotoxic responses have been widely reported under high nanoparticle exposure [42, 43].

The present study has certain limitations. The release of Mg^{2 +} ions from the synthesized MgO NPs was not directly quantified; therefore, the observed improvements in plant growth and biochemical parameters may result from the combined effects of nanoparticle‐specific properties and the gradual release of Mg^{2 +} ions. Although MgO NPs showed considerable promise as sustainable nanofertilizers, their long‐term environmental fate and safety were beyond the scope of the present study. Following application to soil, MgO NPs may undergo dissolution, aggregation, or interactions with soil minerals and organic matter, influencing their mobility, persistence, and bioavailability. While the optimum concentration (20 mg L^{− 1}) significantly enhanced plant growth and productivity, higher concentrations reduced growth, indicating concentration‐dependent phytotoxicity. Moreover, the potential effects of prolonged nanoparticle application on soil microbial communities, nutrient cycling, and other non‐target organisms were not evaluated. Future studies should quantify Mg^{2 +} ion release using techniques such as ICP‐OES or ICP‐MS, compare biosynthesized MgO NPs with conventional magnesium fertilizers under field conditions, and investigate their long‐term environmental fate, ecotoxicity, agronomic performance, economic feasibility, and large‐scale agricultural applicability [49, 52].

3.7. Mechanistic Insights Into Plant Growth Promotion by Green‐Prepared MgO NPs

A schematic illustration of the proposed mechanism by which C. decorticatum‐mediated MgO NPs enhance plant growth and productivity is presented in Figure 12. Although the underlying molecular mechanisms were not directly investigated, the proposed pathway is supported by the present experimental findings and recent reports on MgO nanofertilizers.

FIGURE 12.

Proposed mechanism of C. decorticatum‐mediated MgO NPs for enhancing nutrient uptake, photosynthetic efficiency, growth, and yield in legume crops.

The gradual dissolution of MgO NPs releases Mg^{2 +} ions, which improve nutrient availability and uptake owing to the high surface area and bioavailability of nanoparticles. Enhanced magnesium supply stimulates root elongation, lateral root formation, and water and mineral uptake, leading to improved plant establishment and biomass accumulation [13, 53, 55]. Magnesium also serves as the central atom of the chlorophyll molecule; therefore, increased Mg availability promotes chlorophyll biosynthesis, photosynthetic efficiency, and CO₂ fixation. This is consistent with the significant increase in chlorophyll content observed at the optimum concentration (20 mg L⁻¹) in both V. radiata and V. mungo [56].

Enhanced photosynthesis subsequently increased carbohydrate production, while improved magnesium nutrition stimulated protein and lipid biosynthesis through activation of enzymes involved in carbon and nitrogen metabolism. These biochemical improvements supported greater vegetative growth, biomass accumulation, flowering, pod development, and seed yield, which were evident in both legume crops under optimum MgO NP treatment [13, 57].

However, concentrations above 20 mg L⁻¹ reduced growth and yield, suggesting concentration‐dependent phytotoxicity. Excess nanoparticle accumulation may induce ROS, impair chloroplast function, and disrupt cellular metabolism, ultimately suppressing plant growth [43, 47, 58].

Overall, the present findings suggest that green‐prepared MgO NPs enhance legume productivity through a coordinated mechanism involving controlled Mg^{2 +} release, improved nutrient uptake, enhanced chlorophyll biosynthesis and photosynthesis, increased carbohydrate, protein, and lipid metabolism, improved root architecture, greater biomass accumulation, and ultimately higher pod formation and seed yield.

4. Conclusion

This study successfully demonstrated the green synthesis of MgO NPs using the marine alga C. decorticatum as an eco‐friendly biological reducing and stabilizing agent. Physicochemical characterization by UV–vis, XRD, FT‐IR, and FE‐SEM with EDX confirmed the formation of highly crystalline, phase‐pure MgO NPs with an average crystallite size of approximately 28 nm and particle size of 38 nm. Application of MgO NPs significantly enhanced seed germination, photosynthetic pigments, carbohydrate, protein, and lipid contents in both V. radiata and V. mungo, indicating improved physiological and metabolic performance. Among the tested concentrations, 20 mg L⁻¹ was identified as the optimum dose, resulting in maximum germination (87% in V. radiata and 84% in V. mungo), increased plant height, biomass accumulation, and improved yield attributes, including pod number and pod weight. In contrast, higher concentrations (≥40 mg L⁻¹) caused a gradual decline in growth and biochemical parameters, suggesting concentration‐dependent phytotoxicity associated with oxidative stress. Overall, the findings demonstrate that marine algae‐mediated MgO NPs can serve as an effective and sustainable nanofertilizer for improving the growth, physiology, and productivity of legume crops while reducing reliance on conventional chemical fertilizers.

Authors Contribution

D. Devadharshini: investigation, formal analysis. P. Srinivasan: data curation. S. Vijayakumar: writing – original draft preparation, review, and editing.

Funding

The authors are grateful to the DBT‐STAR (HRD11011/18/2022‐HRD‐DBT), India, and DST‐FIST (SR/FST/College‐222/2014), India, for financial support.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

On behalf of all the authors, the corresponding author states that our data are available upon reasonable request.