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Article

Morphology and Coating of ZnO Nanoparticles Affect Growth and Gas Exchange Parameters of Bell Pepper Seedlings

by
Eneida A. Pérez Velasco
1,
Luis A. Valdez-Aguilar
2,*,
Rebeca Betancourt Galindo
1,*,
Adolfo Baylon Palomino
3 and
Bertha A. Puente Urbina
1
1
Departamento de Materiales Avanzados, Centro de Investigación en Química Aplicada, Saltillo 25294, Coah, Mexico
2
Departamento de Horticultura, Universidad Autónoma Agraria Antonio Narro, Saltillo 25315, Coah, Mexico
3
Departamento de Biociencias y Agrotecnia, Centro de Investigación en Química Aplicada, Saltillo 25294, Coah, Mexico
*
Authors to whom correspondence should be addressed.
Agronomy 2025, 15(7), 1579; https://doi.org/10.3390/agronomy15071579
Submission received: 27 May 2025 / Revised: 26 June 2025 / Accepted: 27 June 2025 / Published: 28 June 2025
(This article belongs to the Section Horticultural and Floricultural Crops)
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Abstract

The use of zinc oxide nanoparticles (ZnO-NPs) is a promising strategy to enhance zinc availability and promote plant growth due to their physicochemical properties and biocompatibility. This study evaluated the effects of ZnO-NP morphology (spherical and hexagonal), maltodextrin (MD) surface coating, and a concentration range of 0–2000 mg L−1 on growth and gas exchange in bell pepper seedlings. ZnO-NPs increased seedling height, especially at 750 and 1000 mg L−1 for MD-coated spherical NPs and at 250 and 500 mg L−1 for MD-coated hexagonal NPs. Spherical NPs also enhanced stem diameter, root length, and the dry weight of roots and stems. Leaf dry weight was highest with MD-coated spherical NPs, while hexagonal forms had milder effects. Dose–response analysis revealed that, with the exception of hexagonal MD-coated NPs, the total dry weight of seedlings stabilized at concentrations ranging 219.6 to 313.6 mg L−1. Gas exchange parameters were significantly influenced by the evaluated factors. Uncoated hexagonal NPs at 1500 mg L−1 increased the photosynthetic rate by 239%. MD coating improved performance, particularly in spherical NPs at 750 and 1000 mg L−1. The transpiration rate rose with MD-coated hexagonal NPs at 500–2000 mg L−1, and SPAD units increased with both morphologies. Overall, this study confirms that ZnO-NP morphology and surface coating play key roles in enhancing growth and physiological responses in bell pepper seedlings.

1. Introduction

Nanoparticles (NPs) are materials with dimensions ranging from 1 to 100 nanometers, characterized by their unique physical properties such as their small size and high surface-to-volume ratio [1,2]. In terms of chemical properties, NPs exhibit high reactivity due to their large surface area, enabling effective interaction with other chemical compounds, and they can be engineered to transport and release fertilizers or other agrochemicals in a controlled manner, thereby improving efficiency and reducing environmental impact [3,4]. Biologically, NPs can be designed to be biocompatible, minimizing toxic effects on plants, soil microorganisms, and humans [5].
Currently, there are various NPs based on Cu, Ag, Mn, Mo, Zn, Fe, Si, Ti, and their oxides. NPs have shown promising results in seed germination, plant growth, and production [6]. It has been reported that at specific concentrations, NPs have the ability to provide a gradual release, supplying various macro- and micronutrients to plants in a controlled manner [3]. Recently, research has focused on the effects of zinc oxide (ZnO) NPs because Zn represents an essential element in plant development and metabolism. Zinc (Zn) is also involved in cellular division, maintenance of membrane structures, and photosynthesis [7,8]. ZnO-NPs have demonstrated positive effects, significantly increasing root and stem growth as well as seed germination and shoot length [9,10,11]. Moreover, there have been observations of higher accumulation of phenolic compounds, antioxidant activity, and chlorophyll and carotenoid content, thereby enhancing photosynthetic efficiency and overall plant health [12,13,14,15].
However, the beneficial effect of ZnO-NPs on absorption and distribution within plant tissues is affected by issues such as agglomeration and uneven dispersion. To address this problem, ZnO-NPs have been coated with maltodextrin (MD). The application of coatings and/or surface modifications on ZnO-NPs has proven to be an effective method to enhance compatibility with the matrix, prevent agglomeration, and achieve better particle dispersion [16]. MD is of natural origin, biocompatible, and biodegradable, essential in nanotechnology due to its low toxicity. Polysaccharide-based or -coated NPs stand out for their high loading efficiency, rapid release of active ingredients, good targeting, high stability, and low toxicity. They are easy to process and sustainable, making them ideal for NP development [17,18]. Similarly, it has been noted that the morphology of NPs affects crop development differently, influencing their surface area, surface energy, and interaction capacity with their environment, and it also influences how NPs interact with biological systems, such as cells and plant tissues [19,20]. Despite the importance of studying the morphology of ZnO-NPs, research evaluating this effect is limited.
Nanotechnology is a promising technique in agricultural systems given the necessity of increasing food production and quality. Despite the impacts of ZnO-NPs having been positive, research on the effect of the morphology of these NPs on cultivated plants is limited, as the majority of the reported experiments have been conducted in vitro or in highly controlled settings, rather than in greenhouse conditions. Bell pepper (Capsicum annuum L.) is a highly appreciated cultivated plant of high nutritional value as it provides antioxidants, vitamins, carotenoids, and other beneficial substances, contributing to its economic importance [21,22]. Successful cultivation of bell pepper demands the use of high-quality seedlings to ensure a good stand establishment and fruit production [23,24]; however, the optimum concentration of ZnO-NPs for seedling production must be carefully determined as high rates may result toxic, as demonstrated by Pavani [25] in mungo bean (Vigna mungo L) seedlings exhibiting decreased plant growth when treated with ZnO-NPs at 25 mg/100 mL, whereas lower concentrations induced an increased germination index and shoot and leaf growth. For these reasons, the objectives of this study were to investigate the impacts of morphology, MD coating, and ZnO-NP concentration on the growth and quality of bell pepper seedlings.

2. Materials and Methods

2.1. Location of the Experiment

The synthesis of ZnO-NPs was carried out at the facilities of the Centro de Investigación en Química Aplicada, while bell pepper plants were established in a medium-tech greenhouse at the Departamento de Horticultura in the Universidad Autónoma Agraria Antonio Narro, both located in Saltillo, Coahuila, México.

2.2. Synthesis of ZnO-NPs

For the synthesis of ZnO-NPs, the following reagents (Sigma Aldrich, Darmstadt, Germany) were used: n-propylamine (C3H9N, 99.5%), triethylamine (TEA, 99%), absolute ethanol (99.5%), zinc acetate dihydrate (Zn (CH3COO)2·2H2O, 99%), and deionized water. ZnO-NPs were obtained by precipitation synthesis following the protocol established by Pérez-Velasco (2021) [26]. A total of 26.33 g of zinc acetate dihydrate was dissolved in a solution composed of deionized water (0.3 L), ethanol (1.7 L), n-propylamine (1.42 mL), and triethylamine (TEA, 5.36 mL). The reaction was carried out at a temperature of 65 °C in a reflux system with constant stirring for a period of 6 h to obtain hexagonal NPs and 12 h for spherical NPs.

2.3. Surface Coating of ZnO-NPs with MD

The procedure was carried out using the precipitation method established by Pérez-Velasco (2021) [26], employing the polysaccharide MD as the coating agent. ZnO-NPs and MD were dissolved in ethanol in a 1:1 molar ratio, and the mixture was heated to reflux at a temperature of 65 °C for 6 h. Subsequently, the solution was allowed to cool and washed twice with ethanol to remove residues. The modified ZnO-NPs were then recovered by centrifugation.

2.4. Characterization of ZnO-NPs

To characterize ZnO-NPs, their crystalline structure, morphology, particle size distribution, composition, and purity were examined. The crystalline structure of ZnO-NPs was determined using X-ray diffraction (XRD) with a diffractometer (Siemens D-5000, Munich, Germany). The morphology and particle size distribution were examined through high-resolution transmission electron microscopy (HRTEM) (FEI Titan 80–300 kV HRTEM, Hillsboro, OR, USA). The identification of functional groups and purity of ZnO-NPs was performed using Fourier transform infrared spectroscopy (FT-IR) (Thermo Scientific-Nicolet iS50, Waltham, MA, USA) to identify the corresponding bands of the NPs.

2.5. Treatments and Seedling Establishment

Bell pepper cv. California Wonder seeds were used for this study. The treatments evaluated included nine concentrations of ZnO-NPs (0, 250, 500, 750, 1000, 1250, 1500, 1750, and 2000 mg L−1), two NP morphologies (spherical and hexagonal), and ZnO-NP coating with MD (with and without coatings). ZnO-NP application was carried out in two stages, to the seeds before sowing and to the seedlings 30 days after sowing. For seed application, 100 seeds were placed in a beaker with 50 mL of the corresponding suspension for each treatment and then sonicated (Branson model 2510, Brookfield, CT, USA) at 38% amplitude for 10 min.
Subsequently, the 100 seeds for each treatment were sown in 200-cell polystyrene germination trays. The substrate for germination was a mix (80:20%, v/v) of sphagnum peat (PREMIER Peat Moss, Rivière-du-Loup, QC, Canada) and horticultural-grade perlite (HORTIPERL, Adamas, Milos Island, Greece), with a pH of 6.5 and an electrical conductivity (EC) of 0.8 dS m−1, respectively. The temperature during the study was 28.3 °C (mean maximum), 16.4 °C (mean minimum), and 22.4 °C (average), while the relative humidity was 95% (mean maximum), 38% (mean minimum), and 71.4% (average). The mean seasonal photosynthetically active radiation was 437.4 µmol m−2s−1. For the second application, 10 mL of the ZnO-NP solution was sprayed per plant from each treatment. Throughout the experimental period (60 days after sowing), the seedlings were kept in the germination trays and were irrigated with a nutrient solution [27] containing the following in meq L−1: 12 NO3−, 1 H2PO4−, 7 K, 9 Ca, 4 Mg, and 7 SO42−. Micronutrients were provided at the following levels of mg L−1: 5.3 Fe-EDTA, 0.4 Zn-EDTA, 2.6 Mn-EDTA, 0.5 Cu-EDTA, 0.2 B (Na2 [B4O5 (OH)4]·8H2O), and 0.2 Mo (Na2MoO4). Nutrient solution pH was adjusted to 6.0 ± 0.1 and EC at 2.3 dS m−1.

2.6. Evaluated Parameters

At experiment termination, plant height, stem diameter, root length, shoot and root dry biomass, and SPAD units were measured in 25 plants per each of four replications, whereas photosynthetic rate, stomatal conductance, intracellular CO2, and transpiration rate were measured in one plant per replicate. Plant height and root length were measured from the base of the stem to the apex using a metric ruler (Truper RGL-30, Lerma, Estado de México, Mexico). Stem diameter and root length were measured 60 days after planting using a digital LCD screen caliper (CALIPER, Kawasaki, Kanagawa, Japan, 150 mm) and a metric ruler (Truper RGL-30), respectively. At experiment termination, seedlings were separated into leaves, stems, and roots and placed in labeled paper bags and dried in an oven (Thermo Scientific, model OGS180, Waltham, MA, USA) at 60 °C for 48 h. Subsequently, the dry biomass weight of each plant organ was recorded using an analytical balance (PIONEER model PA214C, Parsippany, NJ, USA). The shoot dry weight was obtained by summing the dry weights of the leaves and stems. SPAD units were determined using a SPAD meter (KONICA MINOLTA, SPAD-502, Chiyoda-ku, Tokyo, Japan). The photosynthetic rate, stomatal conductance, intracellular CO2, and transpiration rate were measured using a portable photosynthesis system (LI-COR 6400XT, Biosciences, Lincoln, NE, USA) on healthy leaves from the middle part of the seedlings and exposed to direct sunlight; the measurement were conducted from 10:00 to 12:00 h. Environmental conditions in the chamber of the portable photosynthesis system were maintained at a CO2 concentration of 390.4 μmol mol−1, air temperature of 34.4 °C, and relative humidity of 58.4%.

2.7. Experimental Design and Statistical Analysis

The experiment was set under a randomized complete block design with a 9 × 2 × 2 factorial arrangement, with four replications. An analysis of variance (ANOVA) and mean comparison using Tukey’s test (p ≤ 0.05) was conducted with the obtained data using SAS (Statistical Analysis Systems), version 9.0, while a dose–response analysis was performed for the total dry weight using piecewise segmented analysis (NLIN procedure) to define the critical concentration at which the highest effect on plant growth was achieved.

3. Results and Discussion

3.1. Structure and Morphology

The angles detected in the XRD patterns of ZnO-NPs correspond to the wurtzite structure (Figure 1), indicating high purity (JCPDS 36-1451) [28,29]. The micrographs determined the morphology of the ZnO-NPs, showing that spherical-shaped ZnO-NPs had an average particle size of 65.4 nm (Figure 1A), while hexagonal-shaped ZnO-NPs had an average size of 73.3 nm (Figure 1B).
The coating of ZnO-NPs with MD is a method aimed at reducing agglomeration, improving compatibility, and enhancing the dispersion of ZnO-NPs, without compromising their absorbance and purity properties. The obtained results showed that the crystalline structure, purity, and morphology of both spherical and hexagonal ZnO-NPs were not affected by the MD coating (Figure 2). Additionally, the size of MD-coated ZnO-NPs, whether spherical or hexagonal, did not exceed 100 nm. Maintaining the size of ZnO-NPs below 100 nm is crucial due to the enhanced properties they exhibit at this scale, such as increased reactivity and ease of transport. These properties significantly contribute to improving nutrient absorption and utilization, controlled nutrient release, and growth stimulation [30,31].
FT-IR analysis was used to identify the different chemical components present in the ZnO-NP samples and to ensure that they maintain their purity and are free from contaminants. The functional groups in the FT-IR analysis of the powdered sample of ZnO-NPs indicated the typical presence of the band at 510 cm−1 related to the stretching vibration of Zn-O bonds in metal oxides; likewise, the main infrared regions of maltodextrin are found below 1010 cm−1, which characterize the stretching vibrations of anhydroglucose (Figure 3A). Our results are similar to those reported by other FT-IR studies [29,32,33,34,35]. The coating of ZnO-NPs with MD was efficiently carried out on both spherical and hexagonal ZnO-NPs (Figure 3B,C), as evidenced by the clear infrared regions of MD and Zn-O (1010 cm−1 and 547 cm−1, respectively).

3.2. Effect of ZnO-NPs on the Growth and Biomass of Bell Pepper Seedlings

Several studies have reported that ZnO-NPs significantly enhance the growth and development of various species, including as wheat (Triticum aestivum L.), mustard (Brassica juncea L.), and tomato (Solanum lycopersicum L.) [36,37,38,39]. The improvement in the growth of ZnO-NP-treated plants has been associated with increased Zn translocation from the roots to the aerial tissues, as this element is key in auxin synthesis, which induces cell elongation and regulates cell division, promoting plant development [40,41,42]. In other studies, it has been shown that spherical ZnO-NPs coated with MD enhance the concentration of Zn, compared to uncoated NPs, in tomato [26]; the increase in Zn in plants’ tissues could have enhanced the photosynthesis rate observed in the present study, which was probably due to (1) the role of Zn in carbonic anhydrase activity, a metalloenzyme that catalyzes CO2 fixation [43], and (2) the increase in chlorophyll concentration, as suggested by the higher SPAD index observed [44]. ZnO-NPs may have also played a role in increased plant growth as Zn mediates the synthesis of auxins [45] such as indole acetic and indole butyric acids [46], while other reports indicate that they can also enhance zeatin, abscisic acid, and salicylic acid [47]. However, the effect of ZnO-NPs on plants depends on their morphology, surface coating, and the applied concentration [48,49,50,51]. In this study, it was observed that the morphology, MD coating, and the concentration of ZnO-NPs affected the growth of bell pepper seedlings (Table 1), as the height, stem diameter, root length, dry leaf biomass, and total dry weight were significantly affected by the morphology, since spherical NPs resulted in enhanced growth compared to hexagonal NPs. On the other hand, the ZnO-NPs coated with MD resulted in increased roots, leaves, and total dry weight, although the height and length of the shoot and root were higher with no MD-coated NPs. All growth parameters were affected by the concentration of ZnO-NPs and the interaction among the factors (Table 1). It has been shown the NPs of MD are stable under harsh conditions such as high salt content, acidic pH, and thermal resistance [52,53], which suggests that the MD coating of ZnO-NPs used in our study may be stable under greenhouse conditions and probably under open-field production of bell peppers. Previous studies have reported that the coating of NPs with polysaccharides improved the controlled release of Zn, increasing its accumulation in the soil solution as well as in shoots and roots, also indicating compatibility with soil microorganisms [54]. Polysaccharide coatings provide stability, protect against agglomeration, and, more importantly, play a fundamental role in the interaction of NPs with cells [55,56].
According to the interaction, the height of bell pepper seedlings increased when spherical or hexagonal ZnO-NPs were added, whether coated with MD or not, surpassing the height of the respective control plants (Figure 4A). Our results are similar to those reported in Andean lupinus (Lupinus mutabilis Sweet), where plant height increased by 6% with the application of ZnO-NPs at 270 mg L−1 [57]. The beneficial effect of ZnO-NPs on increasing plant height may be related to their ability to efficiently supply Zn to plant tissues and to enhance the action of growth promoters [36,58]. However, when spherical ZnO-NPs were applied, plant height was higher when they were not coated with MD, especially at 250, 500, 750, 1000, and 1500 mg L−1 (Figure 4A). Similarly, hexagonal ZnO NPs enhanced the seedling height at 250 and 500 mg L−1 of uncoated NPs (Figure 4A). Despite the improvement in plant growth when superficially coated ZnO-NPs were applied, there remains a discrepancy in seedling height based on the type of MD coating applied [49,50]. In this study, a significant increase in seedling height was observed when spherical or hexagonal ZnO-NPs coated with MD were applied. The varied effect on plant growth attributed to the morphology of the NPs has been ascribed to biological interactions such as cellular uptake, cellular activation, distribution within cells, as well as the release of active compounds and the surface-to-volume ratio, which increases the surface area of the NPs and, consequently, enhances their reactivity [20,59].
The interaction of factors showed that ZnO-NPs increased the stem diameter of bell pepper seedlings compared to the control (Figure 4B). This effect was observed when applying concentrations ranging from 250 to 2000 mg L−1 of spherical ZnO-NPs, both with and without MD coating. It was also observed that seedlings treated with uncoated hexagonal ZnO-NPs at 250 and 500 mg L−1 exceeded the stem diameter obtained in the corresponding control, whereas among the treatments with uncoated spherical ZnO-NPs, the greatest stem diameter was recorded at 750, 1500, and 1750 mg L−1 (3.82, 3.94, and 3.90 mm per seedling, respectively) and at 500 mg L−1 of MD-coated spherical ZnO-NPs (3.48 mm per seedling). Nevertheless, no significant differences were found compared to the other treatments with lower NPs concentrations (Figure 4B). In contrast, the stem diameter of seedlings treated with hexagonal ZnO-NPs, both with and without MD coating, exceeded that of the control treatments when applied at concentrations from 250 to 1000 mg L−1, as at higher concentrations, uncoated hexagonal ZnO-NPs caused decreased stem diameter (Figure 4B). When the seedlings were treated with uncoated hexagonal ZnO-NPs, the largest stem diameter was achieved at 250 and 500 mg L−1. Similar results were reported in peanut (Arachis hypogaea L.) [44] and in tobacco (Nicotiana tabacum L.) [60] as ZnO-NPs were effective in increasing stem diameter [44]. Similarly, root length was stimulated by increasing concentrations of spherical ZnO-NPs, with or without MD coating; however, a higher root length was observed when spherical ZnO-NPs were uncoated, especially at 1250 and 1500 mg L−1 (Figure 4C). Hexagonal ZnO-NPs, with or without MD coating, also affected the root length, as described for other growth parameters, and applying uncoated hexagonal ZnO-NPs resulted in greater root length at 250 and 500 mg L−1, whereas MD-coated hexagonal ZnO-NPs showed induced higher root length at 1000 mg L−1 (Figure 4C).
Zinc is an essential micronutrient responsible for regulating enzymatic activities in plants, enhancing growth and root length [61,62,63]. A study conducted in soybean (Glycine max L.) plants reported a 56% increase in root length when ZnO-NPs were applied at 200 mg Kg−1 of soil [64]. Similarly, in wheat, ZnO-NPs at 62 mg L−1 increased root length by 50% [65], and in cabbage (Brassica oleracea L.), an increase of 10.3% and 1.3% in root length was reported when treated with 270 and 540 mg L−1, respectively [57].
The significant interaction (Table 1) showed that applying 750 mg L−1 of MD-coated spherical ZnO-NPs resulted in a 33% higher root dry weight compared to that of the control plants (0.84 and 0.63 g, respectively) (Figure 5A); the remainder of the treatments showed a root dry weight similar to that of the control (Figure 5A). In line with our results, an increase in root biomass by 10% to 60% in cucumber (Cucumis sativus L.) with the application of ZnO-NPs at 400 and 800 mg per kg−1 of substrate, respectively, has been reported [66], while in chickpea (Cicer arietinum L.), there was a 13% increase [67]. Another study showed that applying ZnO-NPs at rates of 10 and 100 mg L−1 to rice (Oryza sativa L) plants caused an increase in root biomass of 5.2% and 23.9%, respectively [68]. The coating of ZnO-NPs could have enhanced the growth and accumulation of biomass in bell pepper seedlings because MD improves the stability and dispersal of the NPs, and it also reduces the formation of aggregates and also facilitates the release of the internal contained or encapsulated materials [49,50]. When hexagonal ZnO-NPs was applied, regardless of MD coating, no positive effect on root dry weight was observed, as none of the concentrations of NPs surpassed that of the control seedlings (Figure 5A).
An increase in stem dry weight was observed when MD-coated spherical ZnO-NPs were applied at 750, 1000, and 1500 mg L−1 and at 1500 and 2000 mg L−1 of uncoated spherical NPs (Figure 5B). On the other hand, when hexagonal ZnO-NPs were added, stem dry weight was improved only when 1000, 1250, and 1500 mg L−1 of MD-coated hexagonal ZnO-NPs were applied, while the reminder concentrations did not exceed the control plants (Figure 5B). Our results are consistent with findings reported in tomato, as applying ZnO-NPs at 1500 mg L−1 enhanced the stem biomass [69].
Leaf dry weight of plants treated with ZnO-NPs increased by an average of 75% compared to control plants, regardless of the concentration (Figure 5C). However, hexagonal ZnO-NPs, with or without MD coating, had no effect (Figure 5C). Our results are similar to those reported in maize (Zea mays L.), as 2 mg L−1 of ZnO-NPs applied via irrigation increased leaf dry weight indices by 69.7% [63]. Nonetheless, the average leaf dry weight was higher when spherical MD-coated ZnO-NPs were applied compared to hexagonal ZnO-NPs. The different response in dry leaf weight based on the morphology of ZnO-NPs has been attributed to the direct impact of NP shape on their functionality and effectiveness, including their penetration, distribution, reactivity, and biological compatibility with plant cell structures [48,70]. The shape of NPs may activate the genes associated with the reactive oxygen species scavenging system, cell cycle regulation, chloroplast biogenesis, and glucose metabolism [19]. The geometry of NPs also influences the interaction of the NPs with biological systems, impacting their movement and localization within plant tissues, as spherical NPs exhibit a more uniform dispersion and are more readily absorbed by roots compared to hexagonal NPs [71]; the consistent shape of spherical NPs enhances their ability to penetrate cell walls and membranes [72]. Syu [19] indicated that, compared to decahedral NPs, spherical NPs of silver influence the production of antioxidants such as anthocyanins and copper/zinc superoxide dismutase in Arabidopsis (Arabidopsis thaliana (L.) Heynh), enhancing tolerance to abiotic stressors and counteracting ACC-induced root growth inhibition, likely by impeding ethylene perception [19].
All the concentrations of MD-coated spherical ZnO-NPs resulted in enhanced total dry weight, surpassing that of the control seedlings (Figure 5D). However, with MD-coated spherical ZnO-NPs, total dry weight increased significantly starting at concentrations of 250 mg L−1, reaching the highest values between 750 and 1250 mg L−1 (Figure 5D), whereas with hexagonal ZnO-NPs, the response was generally more uniform across the evaluated concentrations; however, it did not surpass the results observed in the control seedlings (Figure 5D). Similar to the leaves (Figure 5C), the total dry weight was higher when spherical ZnO-NPs were applied. Our results align with other research indicating that NPs enhance the accumulation of dry biomass in plants [57,73,74]. The increase in dry biomass of each organ in bell pepper seedlings treated with ZnO-NPs can be mainly attributed to their ability to enhance the availability, release, and absorption of nutrients [75,76]. When NPs are applied to the soil, they can enter the root epidermis through the cell wall and membrane via apoplastic and symplastic pathways; subsequently, the NPs can reach the plant’s vascular bundle and be translocated to the leaves, where they participate in fundamental metabolic processes essential for plant development [51,77,78]. ZnO-NPs can also be directly absorbed by the leaves through stomata, hydathodes, or wounds; in this case, the NPs accumulate in the substomatal region and may be translocated via the phloem to other organs, promoting the bidirectional transport of nutrients [79,80]. This dual absorption pathways, root and foliar, enable a more efficient distribution of nutrients that meets the plant’s nutritional needs and enhances biomass accumulation [73,75,76,81].
The results indicate that ZnO-NP concentrations of 250 mg L−1 and 500 mg L−1 exhibited optimal performance in the assessed parameters, including height, stem diameter, root length, and plant dry weight. According to the dose–response curves (Figure 6), ZnO-NPs at a concentration near 250 mg L−1 yielded the maximum total dry weight of the seedlings, while concentrations of 500 mg L−1, or higher, caused further but marginal enhancements in the performance of plants; however, the response to the hexagonal coated-with-MD NPs was maximum at concentrations near 750 mg L−1 (Figure 6). This additional growth enhancement is insufficient to justify the extra expense associated with the application of higher concentrations. Based on these findings, a concentration of 250 mg L−1 is advised as optimal for use in the greenhouses for seedling production, as it offers an equilibrium between the enhancement in growth and a lower utilization of substances for NP synthesis, rendering it a more accessible and cost-effective choice.

3.3. Effect of ZnO-NPs on Gas Exchange and SPAD Units of Bell Pepper Seedlings

Some studies have shown that the morphology and coating of ZnO-NPs have a positive effect on the physiological parameters of plants [17,18], which was confirmed in the present study. ZnO-NPs with hexagonal morphology significantly enhanced the transpiration rate, while spherical ZnO-NPs enhanced SPAD units (Table 2). Likewise, MD coating exerted a positive effect on all the physiological parameters, except for the photosynthetic rate. Moreover, all physiological parameters were influenced by the ZnO-NP concentration as well as by the interaction among morphology, coating, and concentration factors, with the exception of the photosynthetic rate, which showed an interaction between coating and concentration factors (Table 2).
Seedlings treated with 1500 mg L−1 of ZnO-NPs without MD coating showed a significant increase of 114% in photosynthetic rate compared to the control (Figure 7A). However, MD-coated ZnO-NPs at 750 mg L−1 also increased the photosynthetic rate by 109% relative to their corresponding control (Figure 7A). Our results are similar to those reported in wheat, where ZnO-NPs at 75 and 100 mg L−1, respectively, improved the photosynthetic rate by 90% and 102%, respectively [82], whereas in bean (Phaseolus vulgaris L.), a concentration of 25 mg L−1 caused a 33.51% increase [83]. The marked increase in the photosynthetic process following the application of ZnO-NPs has been attributed to the effect of ZnO-NPs in accelerating water photolysis and the electron transport chain [43]. Other reports indicate that that the increased photosynthesis rate is associated with an increase in ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) activity with ZnO-NPs exposure [84,85,86].
Stomatal conductance increased when hexagonal ZnO-NPs coated with MD were applied at concentrations of 500 mg L−1 and 1000 mg L−1, representing increases of 90% and 120%, respectively, compared to the corresponding control (Figure 7B). Similarly, MD-coated spherical ZnO-NPs at 500 mg L−1 and 750 mg L−1 increased stomatal conductance by 74% and 61%, respectively, in comparison with the corresponding control (Figure 7B). Published reports show similar results. For example, in wheat, an increase in stomatal conductance was reported with the application of 100 mg L−1 of ZnO-NPs. In soybean plants, increases in stomatal conductance of 48.14% and 103.70% were reported with the supplementation of 50 and 100 mg L−1 of ZnO-NPs, respectively [87]. In coriander (Coriandrum sativum L.), reports indicate that ZnO-NPs at 100 mg L−1 enhanced stomatal conductance compared to untreated plants [88]. Another study conducted on cucumber demonstrated that ZnO-NPs at 100 mg L−1 increased stomatal conductance by 33.73% under drought stress and by 55.20% under normal irrigated conditions, respectively [89]. Zinc is involved in the synthesis and activation of enzymes that participate in the production and regulation of phytohormones, such as abscisic acid, which is essential for controlling the opening and closing of stomata [88]. It has been demonstrated that Zn affects the expression of genes related to stomatal signaling and facilitates the absorption and transport of potassium across cell membranes. This allows guard cells to maintain an optimal potassium concentration to effectively regulate stomatal opening [90].
Hexagonal ZnO-NPs coated with MD applied at 1000 mg L−1 resulted in a 90.8% increase in internal CO2 (Figure 7C), and the remaining concentrations behaved similarly, regardless of whether they were coated or uncoated with MD. These responses were similar with spherical ZnO-NPs, both with and without MD coating, recording an average of 311.58 µmol CO2 mol−1, which represented an average increase of 27.4% (Figure 7C). Our results are similar to those reported in tomato, which showed a 27.9% increase in internal CO2 when applied at 8 mg L−1 [91]. Stomata play a critical role in regulating gas exchange by permitting the influx of the CO2 required for carbon fixation and the efflux of water vapor. Stomatal conductance reflects the ease with which CO2 diffuses into the leaf interior through stomatal pores; higher conductance values generally favor greater CO2 assimilation, potentially enhancing the photosynthetic rate. However, the application of ZnO-NPs has been shown to exert variable effects on stomatal behavior and photosynthetic efficiency. For instance, in contrast to our findings, a study conducted on Arabidopsis reported that treatment with 300 mg L−1 led to reductions exceeding 50% in the net photosynthetic rate, stomatal conductance, internal CO2 concentration, and transpiration rate, indicating that elevated ZnO-NP concentrations may induce stomatal closure, thereby limiting CO2 uptake and impairing photosynthetic performance [92]. Conversely, all the previously cited studies in this manuscript are consistent with our results, demonstrating that ZnO-NP application significantly increased gas exchange parameters in comparison to untreated controls. This apparent contradiction may be attributed to the influence of several factors, including NP concentration, the mode of application, particle size and shape, as well as species-specific responses. The observed enhancement in physiological performance may be explained by the role of Zn as an essential micronutrient. Zinc functions as a cofactor for a wide range of critical metabolic enzymes, including transferases, isomerases, hydrolases, and ligases, which collectively support optimal cellular performance. These enzymes are vital for energy production, cellular homeostasis, stress tolerance, and the maintenance of essential physiological functions [93,94].
The application of MD-coated hexagonal ZnO-NPs increased the transpiration rate at concentrations of 500 mg L−1 and 1000 mg L−1 or higher, resulting in increases of 155% and 122%, respectively, compared to the corresponding control (Figure 8A). However, spherical ZnO-NPs coated with MD at 500 mg L−1 also enhanced the transpiration rate in comparison to the corresponding control (Figure 8A). As previously mentioned, increasing the stomatal conductance allows the diffusion of CO2 into the leaf. However, this also facilitates the simultaneous loss of water vapor; therefore, an increase in stomatal conductance not only enhances CO2 assimilation and raises the photosynthetic rate but is also intrinsically associated with an increase in the transpiration rate [39,43,91].
The SPAD units of the bell pepper seedling leaves showed improvement with the addition of either spherical or hexagonal ZnO-NPs, both uncoated and coated with MD (Figure 7B). The greatest increase was recorded with the application of 1000 mg L−1 of spherical ZnO-NPs coated with MD, showing a 49.1% increase compared to the control. Similarly, hexagonal ZnO-NPs without MD coating at 1000 mg L−1 showed a 47.5% increase (Figure 7B). Our results align closely with those reported in rice, where there was an increase in SPAD values [73]. In maize, an improvement in SPAD units was reported in plants treated with 100 mg L−1 of ZnO-NPs compared to the control [95]. Our results reveal a positive correlation between SPAD values and total dry biomass production in bell pepper seedlings. This pattern suggests that the application of ZnO-NPs promotes an increase in chlorophyll content, as estimated by SPAD measurements, which enhances photosynthetic capacity and consequently leads to greater dry matter accumulation in the plant. Similarly, in rice, the application of 50 mg L−1 of ZnO-NPs increased SPAD units by 40% compared to plants not exposed to ZnO-NPs [96]. Other studies also support our findings; for example, it was reported that at 100 mg L−1, ZnO-NPs increased leaf gas exchange, photosynthetic pigments, and SPAD units in potato (Solanum tuberosum L.) [1], while in maize, the application of 100 mg L−1 of ZnO-NPs enhanced gas exchange [97]. Other studies indicate that the effect of ZnO-NPs on increasing gas exchange is attributed to increased carbonic anhydrase activity and synthesis of photosynthetic pigments. For example, in tomato plants exposed to 10 and 50 mg L−1 of ZnO-NPs, there was a 20% and 30% increase, respectively, in carbonic anhydrase activity [36].

4. Conclusions

The application of ZnO-NPs in bell pepper seedlings elicited a positive response in terms of growth and physiological activity. However, our study shows that the effects of ZnO-NPs depended on the morphology of the NPs, the use of MD coating, and the concentration, as spherical and MD-coated NPs exhibited greater efficacy in enhancing seedling growth by increasing the photosynthetic rate, root development, and biomass accumulation. These effects are attributed to the higher surface area and reactivity of the NPs, as well as the ability of MD to stabilize them and facilitate Zn release into plant tissues. Altogether, the synergistic interaction between the morphology, MD coating, and optimal dose of ZnO-NPs regulates key metabolic pathways related to growth, photosynthesis, and biomass accumulation in bell pepper plants. Considering both growth and physiological parameters, ZnO-NP concentrations of 250, 500, and 750 mg L−1 exhibited the most favorable effects. Specifically, 250 mg L−1 provided a balanced enhancement of both growth and physiological variables, representing a viable option from both agronomic and economic perspectives. Although 500 and 750 mg L−1 improved physiological parameters such as photosynthesis and transpiration, the magnitude of these increases does not necessarily justify the use of higher ZnO-NP concentrations, suggesting that lower concentrations may be preferred in practical application scenarios. We propose that additional research is necessary to determine whether the impact of ZnO-NPs applied during the seedling development phase can extend into the production phase of bell pepper in open-field or greenhouse cultivation systems.

Author Contributions

E.A.P.V. contributed substantially to the investigation, methodology, data curation, writing—original draft, and writing—review and editing; L.A.V.-A., writing—review and editing; R.B.G., supervision, validation, and review; A.B.P., B.A.P.U. and A.B.P. contributed to laboratory activities. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this contribution are available in its entirety, upon reasonable request from the corresponding author.

Acknowledgments

The authors are thankful for the support of the Applied Chemistry Research Center (CIQA) and Autonomous Agrarian University Antonio Narro (UAAAN) for supporting this work and the Secretaría de Ciencia, Humanidades, Tecnología e Innovación of México (SECIHTI) for granting a postdoctoral stay to E.A.P.V. (CVU 662436).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. XRD patterns, HRTEM micrographs, and particle size distribution histograms of spherical (A) and hexagonal (B) ZnO-NPs.
Figure 1. XRD patterns, HRTEM micrographs, and particle size distribution histograms of spherical (A) and hexagonal (B) ZnO-NPs.
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Figure 2. XRD patterns of spherical (Sph) or hexagonal (Hex) maltodextrin (MD)-coated ZnO-NPs.
Figure 2. XRD patterns of spherical (Sph) or hexagonal (Hex) maltodextrin (MD)-coated ZnO-NPs.
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Figure 3. FT-IR spectra corresponding to MD (red line), spherical ZnO-NP (blue line), and hexagonal ZnO-NPs (black line) (A); MD-coated spherical ZnO-NP (B); and MD-coated hexagonal ZnO-NP (C).
Figure 3. FT-IR spectra corresponding to MD (red line), spherical ZnO-NP (blue line), and hexagonal ZnO-NPs (black line) (A); MD-coated spherical ZnO-NP (B); and MD-coated hexagonal ZnO-NP (C).
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Figure 4. The effect of the morphology, maltodextrin (MD) coating, and concentration of ZnO-NPs on the height (A), stem diameter (B), and root length (C) of bell pepper seedlings. Each column is the mean of four replications, and the bars represent the standard error of the mean. Different letters indicate significant differences according to Tukey’s multiple comparison test (p < 0.05).
Figure 4. The effect of the morphology, maltodextrin (MD) coating, and concentration of ZnO-NPs on the height (A), stem diameter (B), and root length (C) of bell pepper seedlings. Each column is the mean of four replications, and the bars represent the standard error of the mean. Different letters indicate significant differences according to Tukey’s multiple comparison test (p < 0.05).
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Figure 5. The effect of the morphology, maltodextrin (MD) coating, and concentration of ZnO-NPs on the root dry weight (A), stem dry weight (B), leaf dry weight (C), and total dry weight (D) of bell pepper seedlings. Each column is the mean of four replications, and the bars represent the standard error of the mean. Different letters indicate significant differences according to Tukey’s multiple comparison test (p < 0.05).
Figure 5. The effect of the morphology, maltodextrin (MD) coating, and concentration of ZnO-NPs on the root dry weight (A), stem dry weight (B), leaf dry weight (C), and total dry weight (D) of bell pepper seedlings. Each column is the mean of four replications, and the bars represent the standard error of the mean. Different letters indicate significant differences according to Tukey’s multiple comparison test (p < 0.05).
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Figure 6. Dose–response curves for the total dry weight of bell pepper seedlings as affected by the concentrations of ZnO nanoparticles of either spherical or hexagonal shape and coated or uncoated with maltodextrin (MD). X0 = critical concentration. Bars represent the standard error of the mean.
Figure 6. Dose–response curves for the total dry weight of bell pepper seedlings as affected by the concentrations of ZnO nanoparticles of either spherical or hexagonal shape and coated or uncoated with maltodextrin (MD). X0 = critical concentration. Bars represent the standard error of the mean.
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Figure 7. The effect of the morphology, maltodextrin (MD) coating, and concentration of ZnO-NPs on the photosynthetic rate (A), stomatal conductance (B), and intercellular carbon (C) of bell pepper leaves. Each column is the mean of four replications, and the bars represent the standard error of the mean. Different letters indicate significant differences according to Tukey’s multiple comparison test (p < 0.05).
Figure 7. The effect of the morphology, maltodextrin (MD) coating, and concentration of ZnO-NPs on the photosynthetic rate (A), stomatal conductance (B), and intercellular carbon (C) of bell pepper leaves. Each column is the mean of four replications, and the bars represent the standard error of the mean. Different letters indicate significant differences according to Tukey’s multiple comparison test (p < 0.05).
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Figure 8. The effect of the morphology, maltodextrin (MD) coating, and concentration of ZnO-NPs on the transpiration rate (A) and SPAD units (B) of bell pepper leaves. Each column is the mean of four replications, and the bars represent the standard error of the mean. Different letters indicate significant differences according to Tukey’s multiple comparison test (p < 0.05).
Figure 8. The effect of the morphology, maltodextrin (MD) coating, and concentration of ZnO-NPs on the transpiration rate (A) and SPAD units (B) of bell pepper leaves. Each column is the mean of four replications, and the bars represent the standard error of the mean. Different letters indicate significant differences according to Tukey’s multiple comparison test (p < 0.05).
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Table 1. Effect of the morphology (M), maltodextrin (MD) coating, and concentration (C) of ZnO-NPs on plant growth and dry weight (DW) of bell pepper seedlings.
Table 1. Effect of the morphology (M), maltodextrin (MD) coating, and concentration (C) of ZnO-NPs on plant growth and dry weight (DW) of bell pepper seedlings.
Shoot
Height
(cm)		Stem
Diameter
(mm)		Root Length
(mm)		Root
DW
(g)		Stem DW
(g)		Leaf DW
(g)		Total DW
(g)
Morphology ZnO-NPs
Spherical29.73 a3.36 a65.64 a0.700.760.98 a2.44 a
Hexagonal25.42 b3.15 b63.28 b0.690.750.83 b2.28 b
ANOVA<0.001<0.001<0.0010.8170.294<0.001<0.001
MD-Coated
Without28.46 a3.34 a65.16 a0.68 b0.750.81 b2.25 b
With26.68 b3.17 b63.76 b0.71 a0.761.00 a2.48 a
ANOVA<0.001<0.0010.005<0.0010.1148<0.001<0.001
Concentration (mg L−1)
017.80 e2.52 g51.40 f0.63 b0.67 c0.73 c2.02 d
25030.50 ab3.43 bc68.25 ab0.68 ab0.77 ab0.93 ab2.38 bc
50031.25 a3.80 a65.65 bcde0.73 a0.77 ab0.96 a2.47 ab
75029.65 bc3.39 cd63.30 e0.69 a0.78 ab0.96 a2.44 ab
100030.70 ab3.56 b67.70 abc0.7 a0.77 ab0.93 ab2.42 ab
125026.20 d3.27 cde66.05 abcd0.68 ab0.74 b0.87 b2.29 c
150025.85 c3.12 e68.30 a0.72 a0.80 a0.95 a2.48 a
175026.20 d3.25 de65.15 cde0.73 a0.74 b0.91 ab2.38 bc
200027.05 d2.95 f64.40 de0.7 a0.75 ab0.92 ab2.37 bc
ANOVA<0.001<0.0010.034<0.001<0.001<0.001<0.001
Interactions
M × MD<0.001<0.001<0.0010.06150.6775<0.001<0.001
M × C<0.001<0.001<0.0010.02720.1134<0.001<0.001
MD × C<0.001<0.001<0.001<0.001<0.001<0.001<0.001
M × MD × C<0.001<0.001<0.001<0.001=0.011<0.001<0.001
Different letters within the same column indicate statistical differences according to Tukey’s multiple comparison test (p ≤ 0.05).
Table 2. Gas exchange parameters and SPAD units of bell pepper seedlings in response to the morphology (M), maltodextrin (MD) coating, and concentration (C) of ZnO-NPs.
Table 2. Gas exchange parameters and SPAD units of bell pepper seedlings in response to the morphology (M), maltodextrin (MD) coating, and concentration (C) of ZnO-NPs.
Photosynthetic Rate
(μmol CO2 m−2 s−1)		Stomatic Conductance
(mol H2O m−2 s−1)		Internal CO2 Concentration
(μmol mol−1)		Transpiration Rate
(mmol H2O m−2 s−1)		SPAD
Units
Morphology ZnO-NPs
Spherical11.28 0.37 304.17.7 b51.5 a
Hexagonal11.50 0.36 305.78.1 a50.4 b
ANOVA0.55790.28580.50980.00880.0011
MD-coated
Without11.460.32 b297.6 b7.6 b50.5 b
With11.330.40 a312.3 a8.2 a51.4 a
ANOVA0.7349<0.001<0.0010.00020.0031
Concentration (mg L−1)
06.86 d0.31 c244.6 e3.9 d40.4 e
2509.56 c0.28 c311.4 bc7.83 c49.8 d
50011.76 abc0.43 a319.9 b9.18 ab52.0 c
75011.54 abc0.32 bc296.1 cd7.55 c56.0 ab
100013.16 bc0.42 ab363.9 a8.19 bc56.8 a
125012.74 ab0.31 c301.3 cd9.34 a49.2 d
150013.96 a0.41 ab308.2 bc9.11 ab54.2 b
175010.60 bc0.35 abc290.2 d7.77 c51.0 cd
200012.34 ab0.43 a308.6 bc8.31 bc49.0 d
ANOVA<0.001<0.001<0.001<0.001<0.001
Interactions
M × MD<0.3411<0.001<0.001<0.001<0.001
M × C<0.5487<0.001<0.001<0.001<0.001
MD × C<0.001<0.001<0.001<0.001<0.001
M × MD × C<0.5966<0.001<0.001<0.001<0.001
Different letters within the same column indicate statistical differences according to Tukey’s multiple comparison test (p < 0.05).
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Velasco, E.A.P.; Valdez-Aguilar, L.A.; Galindo, R.B.; Palomino, A.B.; Urbina, B.A.P. Morphology and Coating of ZnO Nanoparticles Affect Growth and Gas Exchange Parameters of Bell Pepper Seedlings. Agronomy 2025, 15, 1579. https://doi.org/10.3390/agronomy15071579

AMA Style

Velasco EAP, Valdez-Aguilar LA, Galindo RB, Palomino AB, Urbina BAP. Morphology and Coating of ZnO Nanoparticles Affect Growth and Gas Exchange Parameters of Bell Pepper Seedlings. Agronomy. 2025; 15(7):1579. https://doi.org/10.3390/agronomy15071579

Chicago/Turabian Style

Velasco, Eneida A. Pérez, Luis A. Valdez-Aguilar, Rebeca Betancourt Galindo, Adolfo Baylon Palomino, and Bertha A. Puente Urbina. 2025. "Morphology and Coating of ZnO Nanoparticles Affect Growth and Gas Exchange Parameters of Bell Pepper Seedlings" Agronomy 15, no. 7: 1579. https://doi.org/10.3390/agronomy15071579

APA Style

Velasco, E. A. P., Valdez-Aguilar, L. A., Galindo, R. B., Palomino, A. B., & Urbina, B. A. P. (2025). Morphology and Coating of ZnO Nanoparticles Affect Growth and Gas Exchange Parameters of Bell Pepper Seedlings. Agronomy, 15(7), 1579. https://doi.org/10.3390/agronomy15071579

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