Evaluation of carboxymethyl cellulose-based seed coatings enriched with micro mineral fertilizers for enhancing wheat seed resilience to abiotic stresses

Abstract

This study evaluated carboxymethyl cellulose (CMC)-based films as seed coatings for improving the germination and early growth of wheat (Triticum aestivum L., cv. Mihan) under abiotic stress conditions. Experiments were carried out in three successive stages to evaluate the effects of polymer and fertilizer concentrations under varying levels of drought stress. Germination percentage (GP), germination rate (GR), root and shoot biomass, and their lengths were determined. Moderate CMC concentrations (0.25–0.5%) improved seed germination and vigor by approximately 5–10% compared with the untreated control, whereas higher concentrations slightly reduced growth, likely due to excessive film rigidity. The addition of 0.2% fertilizer increased coating hardness and hydrophobicity, but slightly reduced root weight and shoot length. Films with higher viscosity showed greater strength and toughness, while medium-viscosity films provided the best balance between mechanical stability and biological performance. Overall, CMC-based coatings, especially at 0.25–0.5% and medium viscosity, significantly enhanced wheat seed resilience, highlighting their potential as multifunctional seed-coating materials to improve establishment under abiotic stress conditions.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-025-31372-9.

Introduction

Wheat is vital to global nutrition and is regarded as one of the major cereal crops^1,2. However, wheat production faces numerous challenges. Among these, abiotic stresses such as drought and nutrient deficiencies are primary constraints, causing severe productivity losses worldwide^3,4. Climate change is expected to exacerbate the intensity and frequency of droughts in the coming years, further threatening crop productivity⁵. When plants experience stress, both their growth and the quality of their produce decline⁶.

Traditional agricultural practices have often relied on chemical inputs, including fertilizers and pesticides, to mitigate such stresses⁷. However, these methods pose several challenges, such as low nutrient use efficiency, environmental contamination, and harm to non-target organisms.⁷ Furthermore, direct fertilization methods can lead to the rapid and uncontrolled release of nutrients, thereby exacerbating plant stress⁸. Consequently, alternative strategies are increasingly being explored to improve crop resilience and productivity^9,10.

Among the various polymers investigated for seed coating applications, those that provide high adhesion, water solubility, and environmental safety are particularly valuable for sustainable agriculture^9,10. Carboxymethyl cellulose (CMC), a water-soluble cellulose derivative, demonstrates an exceptional balance of these properties^11,12. Its strong film-forming ability ensures uniform coating and nutrient retention, while its biodegradability and hydrophilic nature enhance soil moisture conservation and seed hydration under drought stress^13–15. In comparison with other natural or synthetic polymers such as chitosan, starch, or polyvinyl alcohol (PVA), CMC offers a superior combination of mechanical integrity, moisture control, and ecological compatibility, making it highly suitable for formulating drought-resilient seed coatings^9,10,12,15. Deficiency of micronutrients like manganese (Mn), copper (Cu), zinc (Zn), and iron (Fe), which are critical cofactors in plant enzymatic defense and stress response systems. Their deficiency can severely impair plant growth and development, ultimately limiting productivity potential¹⁶. Thus, enhancing the availability of these nutrients through seed treatments or coatings can significantly improve crop resilience, especially in nutrient-deficient soils¹⁷. Incorporating micronutrients into seed coatings not only increases their use efficiency, but also directly promotes seedling growth and enhances plant stress tolerance, particularly in nutrient-deficient soils^18,19.

The mechanical integrity of the seed coating is crucial to its functionality. The thickness, adhesive strength, and water uptake capacity of seed coatings must be carefully optimized to ensure that the coating does not inhibit seed germination or root penetration, and yet is robust enough to withstand handling and sowing without abrasion or damage²⁰. In addition, the coating must provide an effective barrier against environmental stressors, such as excessive moisture loss or exposure to chemicals, which can adversely affect seedling growth²¹. A comprehensive evaluation of these mechanical and physical properties is essential for developing an effective coating formulation that supports, rather than hinders, seed performance.

Despite the individual benefits of CMC and micronutrient fertilizers, their combined application in a seed coating to enhance wheat resilience under concurrent drought and nutrient deficiency conditions has remained largely underexplored. Therefore, this research focuses on evaluating the physical and mechanical characteristics of these coatings and their subsequent effect on seed quality and seedling growth under drought stress. We hypothesize that a combined CMC-micronutrient coating will significantly improve wheat seed germination and seedling growth under abiotic stress relative to uncoated seeds or seeds coated with CMC alone. By combining CMC with micronutrients, this research seeks to develop a sustainable and effective solution to enhance wheat production in drought-prone regions.

Experimental

Materials

CMC in three grades (low, medium, and high viscosity, denoted as L, M, and H, respectively) was purchased from Shandong Province, China. Arjan micro mineral fertilizer (containing 5%, 5%, 1%, and 2.5% Fe, Zn, Cu, and Mn, respectively) was used. Acid Red 14 dye (C₂₀H₁₂N₂Na₂O₇S₂) for marking coated seeds was purchased. The Mihan variety of wheat seeds used in this study was sourced from the Agricultural and Natural Resources Research Center of Kerman, Iran. All materials were used without further purification.

Sample labeling

Treatment codes indicate CMC grade (H, M, L), CMC concentration (%), and fertilizer concentration (%). For example, M0.5f0.1 denotes a sample with 0.5% medium-viscosity CMC and 0.1% fertilizer. The experimental design is summarized in Table S2.

Planting conditions

The study was conducted as a sequential optimization in three stages, with each stage building upon the findings of the previous one. All plant growth experiments were conducted with three biological replicates per treatment (n=3).

First stage planting

In the first planting, the effect of polymer concentration and the presence of dye on seed growth was investigated. For this purpose, CMC was used in three viscosities: high (H), medium (M), and low (L) at concentrations of 0, 0.25, 0.5, 1, and 2%, both with and without the dye (5% w/w relative to the polymer) for seed coating. Fifteen seeds were placed on moist filter paper in 10-cm Petri dishes. The samples were maintained in a growth chamber at 24 °C with a 16/8 h light/dark cycle. The seeds were watered to maintain consistent moisture on the filter paper. All Petri dishes were randomly rearranged every 24 hours. The experiment lasted for 10 days, after which GP, GR, RW, SW, RL, and SL were measured.

Second stage planting

Based on the results of the first stage, this stage evaluated the effect of fertilizer concentration. Mihan wheat seeds were coated with CMC (H, M, L) at 0, 0.25, and 0.5% concentrations, using fertilizer at concentrations of 0, 0.05, 0.1, and 0.15%. Seeds were sown in pots (9 cm height, 14 cm diameter) filled with 500 g of soil (see Table S1 for soil analysis) and watered with 120 ml of distilled water. Fifteen seeds were sown per pot at a depth of 1 cm. The soil was maintained at consistent moisture levels during the experiment. The growth conditions remained the same as the first stage.

Third stage planting

The final stage investigated the performance of optimal coatings under drought stress. The most effective polymers from previous stages (H and M) at 0% and 0.5% concentrations, with and without 0.1% fertilizer, were tested under well-watered and drought-stress conditions. Pot size, soil, seeding depth, and light conditions were identical to the second stage. The ambient temperature was set to 34 °C. All pots received an initial 120 ml of water. Well-watered pots were re-watered every two days based on pot weight, while drought-stressed pots were watered every four days. All pots were randomly rotated every 24 hours.

Solution preparation

Coating solutions were prepared by first dissolving fertilizer in 100 ml of distilled water at 80 °C, followed by the addition of dye. CMC was then added and stirred until the solution became completely homogeneous.

Seed coating

Seeds were coated using a dip-coating method. For each treatment, 17 g of wheat seeds were placed in a Petri dish, mixed with 30 ml of coating solution, and dried at ambient temperature for one week.

Characterization

Viscosity

The viscosity of CMC solutions was measured using a rheometer (MCR300, Anton Paar, Austria).

Tensile test

Free-standing films were prepared and cut into 9 cm × 1.5 cm strips. Tensile tests were performed using a SANTAM universal testing machine (500 kg capacity) at a crosshead speed of 10 mm/min. Five replicates were tested per film type.

FTIR spectra

The Fourier transform infrared (FTIR) spectra were recorded in the range of 400 to 4000 cm⁻¹ wavenumbers.

Differential scanning calorimetry (DSC)

DSC was performed on a DSC 214 POLYMA instrument. Scans were run from 0 to 250 °C at a heating rate of 10 °C /min under a nitrogen atmosphere (flow rate 40–60 mL min⁻¹).

Degree of substitution

The degree of substitution, indicative of the average number of carboxymethyl groups substituted per anhydro glucose unit, was calculated by the acidimetric method²².

Hardness

The specimens were prepared as coatings with a uniform thickness of 2.12 mm on glass slides, and each sample was replicated five times. The hardness was measured using an MH3 microhardness tester with the Vickers method, applying a load of 0.5 kg for 10 seconds on the cross-sectional area of the samples.

Water contact angle

The water contact angle was measured using five replicates per sample.

SEM and X-ray EDX

The seeds were cut transversely and photographed. SEM (Scanning Electron Microscopy) was used to image the samples and determine coating thickness. In this study, both coated and uncoated (control) seeds were analyzed. Seeds were transversely cut and then sputter-coated with gold for SEM imaging. This test was performed using a FE-SEM (MIRA III, Tescan, Czech Republic).

Measured seed traits

In this study, traits such as germination percentage (Eq. 1), germination rate (Equation 2), seed vigor index (Eq. 3), shoot and root length, and their dry weight were measured. The number of germinated seeds was recorded daily and germination percentage and rate were calculated according to the Eqs. 1 and 2. The criterion for germination was the observation of roots at least 1 mm in length.

Equation 1:²⁰

1

Equation 2:²⁰

2

Ni: number of seeds germinated on day i, Ti: number of days since sowing.

To determine the dry weight of both shoots and roots, after the completion of the planting process and measuring the length, the samples were dried in an oven at 70 °C for 72 hours, then the weight of the samples was measured with a balance with 0.001 g accuracy. The findings were recorded and presented as the sum of root dry weight and shoot dry weight.

The seed vigor index indicates the ability of seeds to germinate and the dry weight of roots and shoots, which was calculated by Equation 3.

Equation 3:²⁰

3

Statistical analysis

All data were subjected to one-way analysis of variance (ANOVA). Significantly different means were separated using Duncan’s multiple range test at a p < 0.05. The statistical analysis was performed using SPSS software (Version 26, IBM Corp., USA).

Results and discussion

Viscosity

Due to its polarity and high solubility in water, CMC (a polymeric substance) exhibits high viscosity when dissolved in water¹². According to the viscosity curve in Fig. 1, the viscosity remained nearly constant with increasing shear rate (µL= 4.5, µM= 15, µH = 73 mPa s). The shear stress of the studied polymers increases linearly and did not pass through the coordinate center (3.39–48.6, 2.8×10⁻³–11.8, and 6.05×10⁻⁴–6.36 Pa for H, M, and L, respectively). In other words, an initial threshold stress is required for flow. These polymers belong to the Bingham plastic fluid category²³. The samples showed relatively high apparent viscosity, which can be attributed to differences in molecular weight and the degree of chain orientation, rather than different macromolecular compounds. The highest shear stress and viscosity correspond to the H sample (3.39–48.6 Pa and 73 mPa s, respectively), and the lowest values are related to the L sample (6.36×10⁻⁴–6.05 Pa and 4.5 mPa s, respectively). These results correlate well with the molecular weight of the CMC samples. Higher molecular weight leads to more extensive chain entanglements and stronger inter- and intramolecular interactions (e.g., hydrogen bonding) both within the CMC chains and with surrounding water molecules.

Fig. 1.

Viscosity and shear stress versus shear rate diagrams of CMC with low (L), medium (M), and high (H) viscosity.

Tensile test

In this study, the mechanical properties (Young’s modulus, tensile strength, elongation at break, and toughness) of films prepared from three CMC samples were evaluated. Figure 2 presents the stress-strain curves for CMC films. As can be seen, all three samples exhibit a maximum strain of approximately 1–1.1%, indicating their brittle nature. T As can be seen, all three samples exhibit a low elongation at break (approximately 1–1.1%), indicating their brittle nature. the addition of fertilizer further increased the brittleness of all samples, making it impossible to prepare intact films for tensile testing. This increased fragility is attributed to coordination between metal ions in the fertilizer and the carboxyl groups of CMC. Li et al. reported CMC coordinates with metal ions through its carboxyl groups²⁴.

Fig. 2.

Stress–strain curves from tensile tests of free-standing films made from low (L), medium (M), and high (H) viscosity CMC.

The average tensile test results of five replicates are listed in Table 1. The modulus is a measure of the material’s resistance to deformation. The modulus is expected to increase with molecular weight, consistent with the viscosity trends observed. As shown, the moduli of H, M, and L samples are 3151.68, 2706.19, and 1691.76 MPa, respectively, which directly correlate with their viscosity values. Toughness represents the total energy required to fracture a material under stress. Strength refers to the maximum stress a material can withstand before rupture. Tensile strength and toughness are closely correlated. Similar to the modulus, both strength and toughness increase with the molecular weight from L to H samples (modulus: 1691.76, 2706.19, and 3151.68 MPa; strength: 17.81, 24.31, and 33.26 MPa; toughness 9.9, 15.1, and 19.6 MPa, respectively, for L, M, and H).

Table 1.

Mechanical characteristics of CMC samples derived from tensile test diagrams.

Sample	Modulus (MPa)	Strain at break (%)	Strength (MPa)	Toughness (MPa)
L	1691.76	1.09	17.81	9.9
M	2706.19	1.06	24.31	15.1
H	3151.68	1.10	33.26	19.6

FTIR spectroscopy

FTIR spectra of pure fertilizer, unfilled M CMC, and M CMC films with different fertilizer contents are presented in Fig. 3.

Fig. 3.

FTIR spectra of pure fertilizer, pure M1 polymer, and M1 films loaded with 0.2% and 0.4% fertilizer.

The peak at 612 cm⁻¹ corresponds to metal–oxygen (M–O) stretching vibrations²⁵. The peak at 1092 cm⁻¹ is attributed to Mn–OH₂ compounds²⁶ while the peak at 1426 cm⁻¹ is attributed to O–H bending vibrations²⁷. The peak at 1669 cm⁻¹ is associated with metal-hydrogen interactions²⁶, and the broad peak at 3297 cm⁻¹ corresponds to O–H groups of adsorbed water molecules on fertilizer particles²⁷.

For the pure M and M samples filled with various percentages of fertilizer, the characteristic peaks are as follows: The peak at 3440–3480 cm⁻¹ indicates the O–H stretching vibrations²⁸. This O–H peak arises from both CMC hydroxyl groups and absorbed moisture. The bands observed at 2856–2924 cm⁻¹ correspond to C–H stretching vibrations²⁸. The sharp peak at 1630 cm⁻¹ corresponds to the C=O stretching vibration²⁹. The peaks associated with the bending vibration of CH₂ and OH appear at 1437 cm⁻¹ and 1320 cm⁻¹, respectively²⁹. The peaks corresponding to C–O–C and CH–O–CH₂ stretching in CMC are observed at 1174 cm⁻¹ and 1041 cm⁻¹, respectively³⁰.

By dividing the area under the peak within the spectral range of 900–400 cm⁻¹ (corresponding to the metal-oxygen bond) by the area under the peak within the spectral range of 2820–2980 cm⁻¹ (C–H bonds), the normalized data for the metal content ratio in each film were obtained. These ratios for the samples M1, M1f0.2, and M1f0.4 were 1.4, 5.1, and 8.1, respectively, confirming the presence of metal particles in the fertilizer. Similarly, by dividing the area under the peak within the range of 3300–3700 cm⁻¹ (related to the oxygen-hydrogen bond) by that of 2820–2980 cm⁻¹ (related to the carbon-hydrogen bond) for samples M1, M1F0.2, and M1F0.4, the respective values of 8.03, 17.37, and 19.39 were obtained, confirming the presence of oxygen-hydrogen bonds in the fertilizer structure.

Differential scanning calorimetry

The samples underwent thermal analysis using DSC to evaluate their properties and the glass transition temperature (Tg) was determined for each sample. The obtained thermograms for pure M and H films, as well as their composites containing fertilizer at concentrations of 0.2 and 0.4 are shown in Fig. 4. Due to the hygroscopic nature of CMC, environmental moisture is trapped within the films; consequently, all films exhibit an endothermic peak near 100 °C, corresponding to solvent evaporation. The peak area for M1 was 263 J/g, which decreased to 96 J/g and 83 J/g, with the addition of 0.2 and 0.4% fertilizer, respectively. The peak area for the H1 was 203 J/g, which decreased to 120 J/g and 86 J/g upon adding 0.2 and 0.4% fertilizer, respectively. As observed, the incorporation of fertilizer into the CMC polymer reduces the peak area, indicating a decrease in water retention. The presence of hydrophilic groups in the fertilizer particles was confirmed by FTIR spectroscopy. The overall moisture absorption decreased with fertilizer addition. In the present thermograms, the glass transition partially overlapped with the solvent evaporation peak, making its precise determination difficult. The area under the peak of M is greater than that of H indicating a higher water absorption capacity for this polymer. The seed germination rate in the first planting and the shoot length in the second planting for seeds coated with M were higher than those coated with the other two polymers. These results can be attributed to the higher water absorption ability of M.

Fig. 4.

Differential scanning calorimetry (DSC) thermograms for CMC films loaded with various percentages of fertilizer.

Degree of substitution (DS)

The degree of substitution (DS) for various samples was determined using the method outlined in the experimental section. The volume of sulfuric acid used for samples H, M, and L was 2.3, 2.2, and 1.07 mL, respectively, and the corresponding degrees of substitution were 0.51, 0.48, and 0.21.

The DS values of all three samples were less than 1, placing them in the low-DS class (0.4–1.0)³¹. According to previous studies, as DS increases, the solubility and transparency of the films increase while opacity decreases^12,31. The transparency of the solutions (Fig. 5) indicates that sample H exhibited higher transparency than the other two samples. As the DS increases, both toughness and tensile strength also increase (DS: 0.21, 0.48, and 0.51; toughness: 9.9, 15.1, and 19.6 MPa; tensile strength: 17.81, 24.31, and 33.26 MPa, respectively, for L, M, and H), which aligns with the findings reported by Yao et al.³¹ Sample H, which had the highest DS (0.51), also exhibited the highest modulus (3151.68 MPa). According to studies, the hydrophilicity of the polymer increases with increasing DS³². Sample H demonstrated a higher viscosity (73 mPa s) than the other two samples. As mentioned in the references, as DS increases, the viscosity of the solution also rises¹².

Fig. 5.

Visual appearance of 1% w/v aqueous solutions of low (L), medium (M), and high (H) viscosity CMC.

First stage planting

In the first stage of planting, the effects of dye and CMC concentration were investigated. Figures 6 and 7 present the germination rate (GR) and germination percentage (GP) of uncoated control seeds and those coated with CMC. Coatings with low CMC concentrations (0.25% and 0.5%) enhanced both GP and GR, indicating a positive effect of CMC on seed performance.

Fig. 6.

Germination rate (GR) of first planting (Distinct letters signify a statistically significant difference at the 95% confidence level according to Duncan’s multiple range test. Alpha = 0.05).

Fig. 7.

Germination percentage (GP) of first planting (Distinct letters signify a statistically significant difference at the 95% confidence level according to Duncan’s multiple range test. Alpha = 0.05).

The GR of the control seeds was 13.3 ± 3.3, which increased in M0.5 and H0.5 under the colored condition and in H0.25 under the colorless condition to 13.75 ± 0.62, 13.94 ± 0.34, and 13.34 ± 0.99, respectively (p < 0.0001). The GP of the control seeds was 91.1 ± 4.4, which increased in H0.5 under the colored condition and in H0.25 and H0.5 under the colorless conditions to 95.6 ± 2.2, 91.1 ± 5.9, and 93.89 ± 5.9, respectively (n = 3, p < 0.0001).

In contrast, higher CMC concentrations (1% and 2%) inhibited seed growth. GR decreased in L1, M1, H1, L2, M2, and H2 under both colorful and colorless conditions to 11.3 ± 1.41, 12 ± 0, 11.5 ± 1.36, 11.3 ± 1.73, 10.7 ± 1.18, 8.5 ± 0.54, 9.28 ± 0.28, 12 ± 0.57, 11.44 ± 0.29, 12 ± 0.57, 10.07 ± 0.41, and 12.15 ± 1.02, respectively (n = 3, p < 0.0001). These results are consistent with the findings of Behboud et al.³³, who reported that high-concentration polymer coatings reduce water and oxygen permeability to the seeds, thereby limiting seed productivity.

Furthermore, the use of dye did not produce any significant adverse effects on GP (n = 3, p = 0.7646) or GR (n = 3, p = 0.8481), indicating that the dye was effectively utilized to distinguish between control and coated seeds. Additional characteristics of the planted seeds in the first stage of planting are provided in Figures S1–S4 and Table S3.

Second stage planting

Building on the first stage, the second planting incorporated micronutrient fertilizers into the optimal low-concentration CMC coatings to address potential soil nutrient deficiencies. This stage aimed to evaluate the combined effects of polymer concentration, viscosity, and fertilizer concentration on wheat seed growth. The characteristics of the planted seeds during the second stage are presented in Figs. S5–S10 and Table S4. A significant difference in shoot length is highlighted in Figs 8 and S10 (n= 3, p= 0.0217). Consistent with the first stage, CMC was confirmed to enhance seed and seedling growth, even without fertilizer, in agreement with the results reported by Khodadadi et al.³⁴ Among the tested samples, M0.5f0 (21.7±0.92) exhibited the highest shoot length, while samples L0.5f0.1 (15.8±2.02) and L0.25f0.15 (15.9±1.39) showed the lowest values. The probability values (p) for concentration and the interaction of concentration × fertilizer were 0.0337 and 0.0010, respectively. Overall, the performance of seeds coated with polymer M was superior to those treated with the other polymers.

Fig. 8.

Shoot length of wheat seedlings from the second stage of planting, showing the best (M0.5f0) and worst (L0.5f0.1, L0.25f0.15) performing samples.

The results indicated that the impact of fertilizer concentration on growth parameters was minimal. The probability values (p) for GP, SW, RW, SL, and vigor index were 0.8218, 0.5778, 0.6330, 0.0782, and 0.7088, respectively. Fertilizer concentrations of 0% and 0.1% were found to be optimal for seed growth, whereas further increases in fertilizer concentration negatively affected growth. Specifically, at a 0.15% fertilizer concentration, seed growth parameters declined compared to 0.1%, potentially due to an increase in the salinity index (for example, GP in control seeds was 84.4 ± 2.2; with 0.1% fertilizer it increased to 91.1 ± 2.2, then decreased to 84.4 ± 2.2 with 0.15% fertilizer; similarly, GR values for control, f0.1, and f0.15 were 4.98 ± 0.74, 7.21 ± 0.70, and 6.94 ± 0.37, respectively). A higher salinity index may impair water uptake, forcing the plant to expend additional energy to acquire water.

These findings are consistent with prior research by Jiri et al., who reported minimal effects of micronutrient applications on wheat productivity in nutrient-sufficient soils³⁵, and with Zhao et al., who showed that priming tomato seeds with low concentrations of metal nanoparticles enhanced nutrient absorption and improved tomato productivity³⁶.

Third stage planting

Polymer samples H and M, at concentrations of 0% and 0.5%, together with fertilizer treatments at 0% and 0.1%, were evaluated under normal and drought-induced stress conditions. This study aimed to assess the effects of both polymer and fertilizer under contrasting environmental conditions, including stress and normal scenarios³⁷.

When wheat was exposed to drought conditions, a marked reduction occurs in the growth and development of both its seedlings and seeds (Figs. S11–S16 and Table S5). Under normal condition GP, GR, SL, SW, and RW are 75.6 ± 9.68, 2.5 ± 0.19, 14.4 ± 0.78, 125 ± 9.6 and 42.3 ± 6.35 respectively. Under drought conditions these values decreased to 40 ± 6.6, 1 ± 0.21, 1.5 ± 0.26, 29 ± 4.72, and 28.7 ± 4.3 respectively (for all measurements, n = 3, p < 0.0001). A similar trend is observed for coated seeds; for instance, under normal conditions, GP, GR, SL, SW, and RW for the f0.1 sample are 68.9 ± 5.8, 1.9 ± 0.2, 10.7 ± 0.63, 93.7 ± 15.07, and 23.3 ± 1.6, respectively, whereas under drought stress they decrease to 17.8 ± 8.01, 0.37 ± 0.15, 1.9 ± 0.57, 9.3 ± 2.3, and 7 ± 1, respectively (for all measurements, n = 3, p < 0.0001).

For instance, the effect of drought stress on the control sample is shown in Fig. 9. A comparison between samples grown under normal conditions with those from the second-stage planting reveals a reduction in growth parameters. This difference may be attributed to variations in testing temperature, a finding that is consistent with previous studies.

Fig. 9.

Visual comparison of control (uncoated) wheat seedlings grown under normal (right) and drought-stress (left) conditions during the third stage of planting.

Sharma et al. investigated the effects of elevated temperature on the germination and early growth stages of wheat. Their results showed that optimal seedling vigor occurred at 25 °C, with a progressive decline observed as temperatures increased beyond this threshold³⁸. Similarly, Poodle et al. and Lu et al. reported that heat stress negatively affected grain yield, growth duration, growth rate, and quality^39–41.

Hardness

According to hardness data presented in Fig. 10, the incorporation of fertilizer particles into the CMC matrix slightly increased the resistance to indentation. The hardness value for M1 was 53 HV, and it increased to 53.3 HV and 54.2 HV with the addition of 0.2% and 0.4% fertilizer, respectively. Similarly, the hardness of sample H1 increased from 42 HV to 56.6 HV upon the addition of 0.2% fertilizer. The films should possess sufficient mechanical strength to maintain an intact coating around the seeds, ensuring there is no rupture or breakage during handling and transportation. Even a small amount of fertilizer addition increased the hardness. The increase in hardness is due to interactions between the fertilizer particles and the polymer, which increase the cross-linking density within the coating. In addition, adding fertilizer limits the movement of the polymer chains⁴².

Fig. 10.

Hardness of CMC films loaded with different of fertilizer.

Water contact angle

By determining the contact angle of water on the surface of dried films, the degree of hydrophilicity of the film surface was investigated (as shown in Fig. 11). Images of the water contact angle on CMC films are shown in Fig. 12. All the films have an angle much lower than 90°, confirming their hydrophilic character. Sample M has a greater contact angle (60°) compared to H (50.1°), which may be attributed to the difference in the DS. The higher DS appears to reduce the intermolecular polymer chain interactions (cohesive forces) and consequently enhances the polymer–water interaction, thereby increasing hydrophilicity. Lee et al. reported that with an increase in DS of CMC, the interaction between Li₄Ti₅O₁₂ and CMC weakens, leading to enhanced hydrophilicity⁴³.

Fig. 11.

Water contact angle measurements on CMC films.

Fig. 12.

Representative images of water droplets on CMC films for contact angle measurement.

The addition of fertilizer to the CMC samples increased their contact angles (60°, 70.6°, 50.1°, and 68.5° for M1, M1f0.2, H1, and H1f0.2, respectively). The strong interaction between the fertilizer molecules and the hydrophilic groups of CMC leads these functional groups to participate primarily in metal coordination rather than in water adsorption. As a result, the surface hydrophilicity decreases. Studies have shown that incorporating nanofillers like zinc oxide into coatings can reduce their hydrophilicity⁴⁴. The greater hydrophilicity of pure polymers may account for the higher dry weight of the shoot and root, as well as the shoot length in the third stage of planting.

The observed improvement in germination and root/shoot growth at intermediate coating viscosities may be partly explained by the combination of moderate hardness and optimal contact angle of the films, which could have favored water uptake and gas exchange. Nevertheless, this interpretation remains speculative, as direct measurements of water absorption and tissue nutrient content were beyond the scope of this study.

The observed increases in film hardness and reduction in hydrophilicity with fertilizer addition strongly suggest a coordination interaction between the metal ions (e.g., Zn²⁺, Mn²⁺, and Fe²⁺/³⁺) from the fertilizer and the carboxylate groups of the CMC backbone, leading to a more cross-linked and rigid polymer network. This mechanistic reasoning is consistent with previous studies reporting the formation of coordination complexes between CMC and various metal cations, which limit polymer chain mobility and reduce water absorption^24,41. However, it is important to note that this interpretation, while supported by our FTIR, mechanical, and contact angle data, remains somewhat speculative without direct measurement of network properties. Future studies incorporating swelling tests, water uptake kinetics, and more detailed spectroscopic analysis (e.g., XPS) would be invaluable to quantitatively confirm the cross-linking density and the precise nature of the ion–polymer interactions proposed here.

SEM and X-ray EDX

Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) were employed to examine the morphology and distribution of fertilizer particles within the polymer structure and to determine the thickness of the seed coatings.

From control and coated seeds, SEM images were prepared with lower magnification. The thickness of polymer coatings was measured (Fig. 13). Figure 13 suggests a correlation between material parameters, including polymer viscosity, polymer concentration, and fertilizer loading, with the coating thickness. The thickness of the M0.5f0.1 coating ranges from 2.8 to 3.1 µm, decreasing to 2.3–2.6 µm for M0.5f0.05 with lower fertilizer content. Reducing the concentration of the polymer leads to a modest reduction in the coating’s thickness. The coating thickness in M0.25f0.05 varies between 2.2–2.6 µm. Similar trends were observed for the H polymer, with coating thicknesses of 1.1–1.3 µm, 0.8–1.4 µm, and 0.6–1 µm for H0.5f0.1, H0.5f0.05, and H0.25f0.05, respectively. Reducing both the polymer concentration and fertilizer content decreases the solid volume, thereby reducing coating thickness^45,46. When the amount of fertilizer is raised and the polymer coating is made thicker, it becomes more challenging for oxygen to penetrate the coating⁴⁵. This effect may explain the reduced seed growth observed with 0.15% fertilizer addition during the second stage of planting.

Fig. 13.

SEM images with 20 micron magnification and coating thickness measurements on transversely cut wheat seeds.

Elemental analysis of particles was performed by SEM equipped with X-ray EDX. For example, the mass percentage and atomic composition of Fe, Cu, Zn, Mn, carbon, and oxygen for control and M0.5f0.05 samples are given in Table 2. As shown, the fertilizer contains Fe, Cu, Zn, and Mn. It is apparent that the proportion of these elements is elevated in seeds that have been coated. A summary of key correlations between coating physicochemical properties and biological seed performance are provided in Table 3.

Table 2.

Weight and atomic percentage of (a) control and (b) M0.5f0.05 samples.

Elements	Weight %	Atomic %
Control
Carbon	45.50	55.10
Oxygen	43.89	39.90
Manganese	0.17	0.04
Iron	0.00	0.00
Copper	0.00	0.00
Zinc	0.14	0.03
Others	10.3	4.93
M0.5f0.05
Carbon	45.70	56.04
Oxygen	41.15	37.88
Manganese	0.15	0.04
Iron	0.22	0.06
Copper	0.15	0.03
Zinc	0.14	0.03
Others	12.49	5.92

Table 3.

Summary of key correlations between coating physicochemical properties and biological seed performance.

Physicochemical property	Trend/effect of formulation	Correlation with biological seed performance	Key example from study
Coating viscosity & molecular weight	H-CMC > M-CMC > L-CMC	Positive correlation with mechanical strength (modulus, toughness). Intermediate viscosity (M-CMC) showed the best balance, promoting seed growth without excessive hardness	M-CMC (medium viscosity) coatings, particularly M0.5f0, resulted in the highest shoot length (21.7 cm)
Degree of substitution (DS)	H (0.51) > M (0.48) > L (0.21)	Higher DS increased solution viscosity, film transparency, and mechanical strength. Also linked to higher hydrophilicity, favoring water uptake	H-CMC with the highest DS (0.51) had the highest tensile strength (33.26 MPa) and viscosity (73 mPa·s)
Fertilizer incorporation	Increase from 0% to 0.4%	Positive: Increased coating hardness and cross-linking Negative: Reduced hydrophilicity (higher contact angle) and increased coating thickness, which correlated with reduced root/shoot dry weight under stress	Adding 0.2% fertilizer to M1 increased hardness from 53 HV to 53.3 HV and the contact angle from 60° to 70.6°, coinciding with reduced seedling biomass in drought trials
Coating thickness	Increased with higher polymer concentration and fertilizer loading	Optimal at intermediate thickness. Thin coatings offer less protection; thick coatings (e.g., from high fertilizer) can impede oxygen diffusion, inhibiting germination	M0.5f0.1 coating (2.8–3.1 µm) performed better than formulations leading to thicker, less permeable layers
Hydrophilicity (water contact angle)	Reduced (higher angle) with fertilizer addition	Decreased hydrophilicity correlated with reduced water uptake, leading to lower seedling vigor and dry weight under drought stress	Pure M1 film (60° contact angle) supported better growth than M1f0.2 (70.6°), especially in drought conditions
Mechanical strength (tensile)	H-CMC > M-CMC > L-CMC	Essential for coating integrity during handling. However, excessive strength from high viscosity/fertilizer must be balanced with flexibility for seed germination	H-CMC had the highest strength (33.26 MPa) but M-CMC provided the optimal balance for biological performance

Comparative analysis and practical considerations

The present study demonstrates that CMC-based seed coatings can effectively enhance early wheat growth, particularly under controlled stress conditions. These findings can be contextualized within the broader landscape of seed enhancement technologies. Similar to biochar coatings that enhance water retention, the CMC coating likely functions as a moisture reservoir⁴⁷. However, unlike some microbial coatings that actively promote nutrient solubilization^48,49, the nutrient delivery here is passive and dependent on the polymer’s degradation. When compared to other biopolymer coatings, CMC offers a distinct balance of properties. Alginate coatings, while excellent at forming robust gels, can be less flexible and more permeable than the more tunable CMC matrix⁹. Polyvinyl alcohol (PVA) coatings often provide superior mechanical strength but lack the inherent natural abundance and biodegradability of CMC²⁰. Starch-based coatings are highly cost-effective but can suffer from lower mechanical integrity and higher susceptibility to microbial degradation⁴⁵. The optimal performance of our CMC coatings at intermediate viscosities and thicknesses (e.g., M0.5) underscores a critical boundary condition: while a thin coating may offer insufficient protection, an excessively thick layer, as observed with higher fertilizer loadings, can impede oxygen diffusion—a drawback also reported for dense alginate hydrogels⁵⁰—and increases the risk of salinity stress. Therefore, the optimal application of this CMC coating requires a precise balance between polymer concentration and fertilizer dose. This ensures that the benefits are not negated by physical hindrance or phytotoxicity, a critical consideration for practical deployment compared to other biopolymer systems.

Limitations and future perspectives

While this study provides valuable insights into the material properties and early-stage efficacy of CMC seed coatings, several limitations must be acknowledged. Firstly, conclusions are based on pot experiments under controlled conditions, which may not capture the full complexity of field environments, including soil heterogeneity, pest pressure, and variable weather. Secondly, the investigation used a single wheat cultivar (‘Mihan’). Genotype-dependent responses are well-documented in seed technology⁵¹, and efficacy should be validated across multiple cultivars. Finally, this study focused on early growth stages (germination and seedling vigor). Long-term effects on crop productivity, grain quality, and soil health remain unknown. Future work should therefore prioritize multi-season field trials, testing diverse genotypes, and a comprehensive life-cycle assessment to evaluate the agronomic and economic viability of this coating technology.

Conclusion

Based on the conducted tests, CMC seed coatings at 0.25–0.5% concentrations significantly enhanced wheat seed germination and early seedling growth. The medium-viscosity (M) polymer generally provided the most favorable outcomes, improving shoot length without the adverse effects associated with higher viscosity formulations. The incorporation of fertilizer, however, altered the coating’s physical properties—increasing thickness, hardness, and hydrophobicity—which correlated with reduced seedling biomass under drought stress and indicated a potential limitation in water availability.

These results highlight the practical potential of CMC-based coatings, particularly at medium viscosity and optimal concentrations, for improving crop establishment in challenging environments. For future research, field-scale trials and evaluations across diverse wheat genotypes are recommended to validate these promising results under real agricultural conditions and assess broader applicability.

Author contributions

Fatemeh Zaimbashi: Writing, methodology, formal analysis, and experimental work. Sina Modiri: Conceptualization, analysis, review, and editing. Hossein Yari: Conceptualization, analysis, review, and editing. Mahboub Saffari: Conceptualization, analysis, review, and editing. Mehdi Rahimi: Conceptualization, analysis, review, and editing.

Funding

Kerman Graduate University of Advanced Technology.

Data availability

All data generated or analyzed during this study are included in this published article and its Supplementary Information files.

Declarations

Competing interests

The authors declare no competing interests.