Physiological, Productive, and Nutritional Performance of Tomato Plants Treated with Iron and Zinc Nanoparticles via Foliar Application Under Deficit Irrigation

Abstract

Water deficit in the semi-arid region of Brazil is a critical limiting factor for tomato (Solanum lycopersicum Mill.), plant development and productivity. We evaluated whether foliar zinc (ZnO NPs) and iron (Fe₂O₃NPs) nano-oxides and their conventional salts (ZnSO₄·7H₂O and FeSO₄·7H₂O) mitigate water deficit effects on tomato (hybrid HM 2798). A split-plot field experiment was conducted with two irrigation levels (50% and 100% ETc) and five foliar treatments: control (no application), FeSO₄·7H₂O (T1), Fe₂O₃NPs (T2), ZnONPs (T3), ZnSO₄·7H₂O (T4), with four replications, totaling 40 experimental plots (2 irrigation levels × 5 foliar treatments × 4 replicates). The water deficit significantly reduced the leaf area index, photosynthetic rate, membrane stability, calcium and boron contents in fruits, and total and marketable yield. Foliar application of iron and zinc nano-oxides and their conventional sources had a limited effect on tomato plant growth but increased the photosynthetic rate under both irrigation levels. Under full irrigation, ZnSO₄·7H₂O increased total fruit production by 61% and fruit Zn content by 18.1%. In turn, Fe₂O₃ NPs (T2) led increases in fruit iron content by 117.3% under water deficit and 135.2% under full irrigation. Foliar application of Fe as Fe₂O₃ NPs is promising to promote the biofortification of tomato fruits with this micronutrient, especially in regions with deficiency problems of this micronutrient.

1. Introduction

Tomato (Solanum lycopersicum) is one of the most widely cultivated and consumed vegetables globally, notable for its nutritional value, richness in lycopene, vitamins, and antioxidants, which confer human health benefits [1,2]. In Brazil, tomato ranks among the top vegetables in economic value and is widely consumed fresh in salads, as sauces, and processed into pulps and extracts [3]. In the semi-arid Northeast region of Brazil, where edaphoclimatic conditions favor the cultivation of this vegetable, water scarcity is a major factor contributing to reduced yield and commercial quality [4].

Water scarcity is one of the greatest global challenges of the 21st century, exacerbated by climate change, population growth, and unsustainable water resource use, especially in countries like Brazil, where agriculture accounts for about 70% of freshwater consumption [5]. Reduced water availability directly compromises plant productivity, particularly in sensitive crops like tomato [6,7].

Water deficit significantly compromises gas exchange in tomato plants, reducing the photosynthetic rate due to stomatal closure and decreased leaf conductance, which limits CO₂ assimilation [8,9]. These physiological alterations inhibit vegetative growth, resulting in leaf area and biomass decreases and also affect the synthesis of photosynthetic pigments such as chlorophylls and carotenoids, essential for plant energy efficiency [10]. In fruits, water stress reduces the translocation of photoassimilates and mineral nutrients (such as calcium, potassium, magnesium, iron, and zinc) to the fruits and negatively impacts productivity, reducing the number and size of fruits per plant [11,12].

The decrease in mineral nutrients in fruits also occurs due to reduced diffusion and mass flow processes, which contribute significantly to the quantity of nutrients reaching the roots and their subsequent redistribution to the fruits [13]. These responses highlight the need for water management strategies to mitigate losses under water scarcity conditions.

Foliar application of zinc oxide nanoparticles (ZnONPs) and iron oxide nanoparticles (Fe₂O₃NPs) emerges as a promising strategy to mitigate the effects of water deficit in horticultural crops, as demonstrated by recent advances in agricultural nanotechnology [14,15,16,17]. These nanomaterials have been considered better sources of iron and zinc than conventional sources of these elements due to their high specific surface area and unique physicochemical properties [18,19,20,21]. On the other hand, advances in knowledge regarding the absorption mode and physiological effects of nanoparticles have been made [21,22]. The primary mode of action of ZnONPs and Fe₂O₃NPs would be their stimulatory effect on plant metabolism through interaction with biomolecules, promoting increased enzymatic activity and the synthesis of compounds involved in combating plant oxidative stress [20,21,22,23].

In this sense, NPs appear to modify antioxidant enzyme activity, elevating the activity of antioxidant enzymes like superoxide dismutase and catalase [19,20] while simultaneously inducing the synthesis of osmoprotectants that maintain cellular osmotic pressure under drought conditions [23,24]. NPs containing iron or zinc can also act as efficient sources of Fe and Zn for plants, with slower release than conventional sources like zinc and iron sulfate [25], increasing the bioavailability of these elements, which are important plant and human micronutrients [26,27].

Zinc and iron are vital micronutrients in plants [12]. Zinc participates in the synthesis of various hormones, such as gibberellin, auxin, cytokinin, and abscisic acid, and is involved in chlorophyll production, chloroplast development, and cellular membrane integrity, providing protection, stability, and structure [12,28]. These processes, in turn, are associated with water stress tolerance mechanisms and plant growth and development, with implications for fruit productivity and quality [29,30]. Iron, in turn, acts as an enzymatic cofactor in chlorophyll biosynthesis and chloroplast maintenance [12], is a central component of electron transport chains (cytochromes and ferredoxin), crucial for the photochemical phase of photosynthesis [31], and regulates the activity of antioxidant enzymes (like catalase and peroxidase), protecting photosynthetic pigments from oxidative stress [12], with implications for fruit productivity [32]. Previous studies have investigated the use of iron oxide (Fe) and zinc oxide (Zn) nanoparticles to mitigate water deficit in various crops, including fruit vegetables such as tomato. Nevertheless, the comparison between these nanoscale sources and conventional sources remains poorly explored, particularly under field conditions in a semi-arid tropical climate.

The hypothesis of the present study is that foliar application of zinc oxide (ZnO) and iron oxide (Fe₂O₃) nanoparticles (NPs) is more effective than conventional sources (salts) in mitigating the negative effects of deficit irrigation on photosynthesis and crop yield. Therefore, this study aims to (i) compare the efficacy of ZnO and Fe₂O₃ NPs with their ionic salt counterparts; (ii) quantify their effects on plant physiological parameters and fruit mineral content; and (iii) evaluate potential interactions between the nutrient source and irrigation regime.

2. Materials and Methods

2.1. Characterization of the Experimental Area

The experiment was conducted under field conditions from October 2023 to February 2024, at the Experimental Farm of the Center for Food Science and Technology (Centro de Ciências e Tecnologia Agroalimentar) of the Federal University of Campina Grande (UFCG), Pombal Campus, Paraíba State (PB). The Experimental Farm is located in the municipality of São Domingos, approximately 30 km from Pombal (PB). The municipality is situated in the Western region of Paraíba State, within the Sertão Paraibano mesoregion and the Sousa microregion. According to the Köppen classification, the predominant climate is type Aw, characterized as hot and humid. Annual precipitation averages around 800 mm, and the thermal amplitude is always less than 5 °C. The Experimental Farm is situated at an altitude of 190 m, at the following coordinates: Latitude −6.83444; longitude −37.88583°. According to the Gaussen classification, the prevailing bioclimatic type is Mediterranean, also known as medium-dry northeastern, featuring a dry season lasting between 4 and 6 months. Area preparation (Figure A1) consisted of one plowing followed by one harrowing using a plow harrow. After this stage, soil sampling was performed in the area, within the 0–20 cm layer, for chemical and physical characterization (

Table 1. Chemical and physical attributes of the soil used in the experiment.

Chemical	Value	Physical	Value
pH (CaCl2)	6.45	Sand (g kg−1)	389
OM (g kg−1)	11.2	Silt (g kg−1)	430
P (mg dm−3)	180.1	Clay (g kg−1)	181
K+ (mg dm−3)	256.8	BD (g cm−3)	1.30
Ca++ (cmolc dm−3)	7.72	PD (g cm−3)	2.59
Mg++ (cmolc dm−3)	2.83	TP (m3 m−3)	0.47
Na+ (cmolc dm−3)	0.06	FC (%)	12.87
H + Al+++ (cmolc dm−3)	1.35	PWP (%)	5.29
Fe (mg dm−3)	18.0	AWC (%)	7.58
Zn (mg dm−3)	1.69	-	-

P, K⁺, Na⁺, Fe, and Zn—Mehlich 1 extractant, H⁺ + Al⁺³—0.5 mol L⁻¹ calcium acetate extractant at pH 7, Ca⁺², Mg⁺²—1 mol L⁻¹ KCl extractant, OM—organic matter, BD—bulk density, PD—soil particle density, TP—total porosity, FC—field capacity, PWP—permanent wilting point, and AWC—available water content.

), following the methodology described in the Manual of Soil Analysis Methods [33]. The climatic conditions recorded from 10 October 2023 to 3 February 2024 were as follows: total rainfall of 51.2 mm; monthly average temperatures of 23.0 °C (minimum), 35.8 °C (maximum), and 29.9 °C (mean); and a mean and minimum relative humidity of 55.5% and 28.5%, respectively [34].

During the experiment, weather data for temperature (°C), relative humidity (%), and precipitation (mm) were collected via the Agritempo website [34] (Figure 1).

2.2. Treatments and Experimental Design

The experiment was arranged in a randomized block design with split plots in space. The main plots consisted of two irrigation levels, denoted as ID (50% and 100% of crop evapotranspiration—ETc), while the subplots comprised four treatments involving foliar application of iron and zinc sources, termed water deficit alleviators (WDAs). The WDAs treatments were as follows: ferrous sulfate (FeSO₄·7H₂O = T1), iron oxide nanoparticles (Fe₂O₃NP = T2), zinc oxide nanoparticles (ZnONP = T3), zinc sulfate (ZnSO₄·7H₂O = T4), and a control (C). The design included four blocks, totaling 40 experimental plots.

The total experimental area measured 40.8 m in length by 18.0 m in width. Plants were spaced at 1.2 m between rows and 0.7 m within rows. Each plot contained four rows (3.6 m) with four plants (2.8 m), totaling 16 plants per plot. The net plot area (2.8 m × 1.2 m) in the center of each subplot included six plants for evaluation. Subplots were separated by 1.5 m, and blocks by 2 m, resulting in a total area of 734.4 m² (Figure S1, Supplementary Materials).

2.3. Seedling Transplanting

Tomato seedlings (Solanum lycopersicum Mill.), hybrid HM 2798 (Harris Moran Seed Company Co., Ltd., Campinas, Brazil), were produced in expanded polystyrene trays (Isopor^®), using a sterilized substrate composed of soil, sand, and cattle manure in a 1:1:1 ratio. Transplanting was carried out when the seedlings had four to six true leaves. The seedlings were planted on ridges measuring 0.4 m in width and 0.30 m in height. Based on the soil analysis (Table 1), basal and top-dressing fertilization was performed according to the recommendations of the Fertilizer Recommendation Manual for the State of Pernambuco [35]. For basal fertilization, the following were applied per planting row (four plants) in each subplot: 134 g (398.8 kg ha⁻¹) of NPK fertilizer (10% of N, 10% of P₂O₅ and 10% of K₂O), 35 g (104.2 kg ha⁻¹) of potassium chloride (58% K₂O), and 93 g (276.8 kg ha⁻¹) of single superphosphate (18% P₂O₅). These fertilizers were incorporated into the soil during ridge formation. Additional nitrogen and potassium fertilization was applied via fertigation, consisting of three split applications of 30 g (89.3 kg ha⁻¹) urea (45% N) and 12 g (35.7 kg ha⁻¹) potassium chloride (58% K₂O), applied at 25, 50, and 75 days after transplanting (DAT). All fertilizers used were obtained from Heringer^® Fertilizers, Rio Grande, Brazil.

2.4. Application of Treatments with Iron and Zinc Sources

As the ZnONP source, zinc oxide (ZnO) nanoparticles (544906-10G, batch number MKCP6471, CAS-No 1314-13-2) from Sigma Aldrich^® (Sigma-Aldrich^® Co., Ltd., Darmstadt, Germany) were used, with 97% purity, particle sizes < 100 nm, and a specific surface area of 10.8 m² g⁻¹. Suspensions of this product were prepared at a concentration of 0.25 g L⁻¹, equivalent to 200 mg of Zn L⁻¹. As the Fe₂O₃NP source, iron oxide (Fe₂O₃) nanoparticles (544884-5G, batch number MKCS3534, CAS-No 1309-37-1) from Sigma-Aldrich^® (Sigma-Aldrich^® Co., Ltd., Darmstadt, Germany) were used, with 98% purity, particle dimensions < 50 nm, and an average specific surface area of 50–245 m² g⁻¹. Suspensions of this product were prepared at a concentration of 0.30 g L⁻¹, equivalent to 200 mg of Fe L⁻¹. The definition of the dose of 200 mg L⁻¹ of Zn or Fe as NP was based on Martins et al. [11] and Jafari et al. [15]. For conventional zinc and iron sources, the reagents ZnSO₄·7H₂O (Vetec Fine chemistry Co., Ltd., Duque de Caxias, Brazil)) and FeSO₄·7H₂O (Vetec Fine chemistry Co., Ltd., Duque de Caxias, Brazil)) were employed, respectively, both at a concentration of 4.5 g L⁻¹. This concentration was established such that the Fe and Zn concentrations (approximately 1 g L⁻¹) from conventional sources were approximately five times greater than their nanoparticle counterparts. This approach accounted for both the anticipated higher efficiency and the greater purchase price of the nanoparticle forms. All products/treatments were applied only once, 20 days after transplanting the seedlings. As a solvent for the preparation of nanoparticle suspensions, only distilled water was used. Solutions were applied at a volume of 300 L ha⁻¹, corresponding to approximately 20 mL per plant, using a backpack hand sprayer. To optimize product adhesion to plants, a neutral detergent was used as a spreader–sticker agent at a concentration of 1 mL L⁻¹. The suspensions were prepared moments before application, without the need for storage. In each plot, applications were carried out on each plant individually, which were properly isolated with four polystyrene plates, each measuring 1.0 m × 1.0 m. The applications were carried out in the late afternoon (between 4 pm and 6 pm).

2.5. Irrigation Depth Application

After transplanting, plants were drip irrigated using emitters spaced at 0.20 m intervals with a nominal flow rate of 1.6 L h⁻¹. Following seedling emergence and standardization of plant count and size within subplots, irrigation was applied according to different water depth levels starting 32 days after transplanting (DAT).

The total irrigation requirement (TIR) was calculated using the following expression:

$$\mathrm{TIR}={\displaystyle \frac{\left(\mathrm{Cc}-\mathrm{Pm}\right)\times \mathrm{Z} \times \mathrm{BD} \times \mathrm{f}}{10}}$$

where

TIR (total irrigation requirement)—Corresponds to the total initial water depth applied, in mm;

FC—Soil moisture at field capacity, %;

PWP—Soil moisture at permanent wilting point, %;

Z—Effective root zone depth (30 cm);

BD—Soil bulk density, g cm⁻³;

f—Water depletion factor (0.5).

Water application uniformity tests were conducted using the Christiansen Uniformity Coefficient (CUC) methodology proposed by [36].

The daily water volume for each irrigation depth was controlled at a standardized time, based on the ratio of emitter flow rate to the time required to meet reference evapotranspiration (ETc) proportions. As the scheduled time interval for each depth was reached, the corresponding drip tapes were sequentially shut off. The 100% ETc irrigation depth was calculated using the following expression [37]:

$$\mathrm{ETc}=\mathrm{Kc}\times \mathrm{ETo}$$

where

ETc—Crop evapotranspiration (mm day⁻¹);

ETo—Reference evapotranspiration (mm day⁻¹);

Kc—Crop coefficient (dimensionless).

The Kc values for each crop growth stage and daily ETo data were obtained using the FAO Penman–Monteith model [38]. Meteorological data during the experiment were collected from the automated weather station in São Gonçalo, Paraíba (nearest to the experimental site) via the Agritempo platform [34].

Daily irrigation depths were applied based on irrigation time, calculated using system and crop characteristics as per the following expression:

$$\mathrm{Ti}={\displaystyle \frac{\mathrm{Eto}\times \mathrm{Kc}\times \mathrm{A}}{\mathrm{Ea}\times \mathrm{n}\times \mathrm{q}}}$$

where

Ti—Irrigation time (hours);

ETo—Reference evapotranspiration (mm day⁻¹);

Kc—Crop coefficient (dimensionless);

A—Area occupied per plant (m²);

n—Number of emitters per plant;

q—Emitter flow rate (L h⁻¹);

Ea—Application efficiency (0.90).

2.6. Cultural Practices and Phytosanitary Control

Weed control was carried out mechanically, either manually or using hand hoes. Pest and disease control, when necessary, was performed using available natural products, such as neem extract, at a dose of 200 mL L⁻¹. For vertical staking, stakes measuring approximately 1.8 m in length were used, alternating between treatments. Following this, string was run from one end to the other of each row, tied horizontally to the stakes to help support the plants during their growth and fruiting.

2.7. Evaluated Variables

2.7.1. Plant Growth

At the pre-flowering stage, the leaf area index (LAI), plant height, and stem diameter of plants within the useful plot area were evaluated. Plant height and stem diameter were measured 2 cm above the ground. LAI values were obtained using a photosynthetically active radiation (PAR) meter (AccuPAR model LP-80, Decagon Devices, Inc., Pullman, WA, USA). Four readings were taken in each useful plot (net plot area) on newly mature leaves in the central position of the plant.

2.7.2. Gas Exchange and Photosynthetic Pigments

Concurrent with the plant growth evaluation, measurements of gas exchange, pigment concentration, and cell membrane integrity were performed. Gas exchange (net photosynthesis, stomatal conductance, transpiration, and intercellular CO₂ concentration) was measured using an infrared gas analyzer (Infra-red Gas Analyzer, Li-Cor 6400, LCpro Analytical Development, Kings Lynn, UK), with a constant light source of 1200 µmol photons m⁻² s⁻¹ and a CO₂ concentration of 370 µmol mol⁻¹, a leaf temperature of 32 ± 1 °C, and water vapor around 20 ± 1 mmol mol⁻¹. Measurements were carried out on newly mature leaves in the central position of the plant.

The evaluations occurred between 07:00 and 9:00 a.m., determining the CO₂ assimilation rate—A (µmol CO₂ m⁻² s⁻¹), transpiration—E (mmol H₂O m⁻² s⁻¹), stomatal conductance—gs (mol H₂O m⁻² s⁻¹), and internal CO₂ concentration—Ci (µmol CO₂ mol⁻¹). Water use efficiency (WUE) was estimated by the A/E ratio, while the A/Gs ratio was used to estimate intrinsic water use efficiency.

The contents of chlorophyll a (chl a), chlorophyll b (chl b), and carotenoid pigments (carotenoids and chlorophylls) were determined according to the methodology described by Lichtenthaler [39]. Pigments were extracted in pure, chilled acetone, subsequently filtered through 0.45 μm filter paper, and quantified by UV/VIS spectrophotometry (KASVI, K37, KASV Co., Ltd., São José dos Pinhais, Brazil), according to the expressions of Lichtenthaler [39]:

$$\mathrm{chl}\mathrm{a}\left(\mathrm{mg}/\mathrm{g}\right)={\displaystyle \frac{\left(12.25\times \mathrm{Abs}663\mathrm{nm}-1.79\times \mathrm{Abs}645\mathrm{nm}\right)\times \mathrm{V}}{1000\times \mathrm{FW}}}$$

$$\mathrm{chl}\mathrm{b}\left(\mathrm{mg}/\mathrm{g}\right)={\displaystyle \frac{\left(21.5\times \mathrm{Abs}645\mathrm{nm}-5.1\times \mathrm{Abs}643\mathrm{nm}\right)\times \mathrm{V}}{1000\times \mathrm{FW}}}$$

$$\mathrm{chl}\mathrm{a}+\mathrm{b}\left(\mathrm{mg}/\mathrm{g}\right)={\displaystyle \frac{\left(7.15\times \mathrm{Abs}643\mathrm{nm}-18.71\times \mathrm{Abs}645\mathrm{nm}\right)\times \mathrm{V}}{1000\times \mathrm{FW}}}$$

$$\mathrm{Car}\left(\mathrm{mg}/\mathrm{g}\right)={\displaystyle \frac{\left(1000\times \mathrm{Abs}470\mathrm{nm}-1.82\times \mathrm{chl}\mathrm{a}-85.02\times \mathrm{chl}\mathrm{b}\right)\times \mathrm{V}}{198\times 1000\times \mathrm{FW}}}$$

where FW and V correspond to the values of leaf fresh mass and extract volume, respectively. Membrane integrity was assessed by estimating electrolyte leakage (Ext) using the methodology suggested by Campos and Thi [40]. For this, eight leaf disks with an area of 113 mm² each were obtained from plants within the useful plot area of each experimental plot, using a copper hole punch. These disks were washed and placed in Petri dishes containing 20 mL of deionized water. The dishes were sealed and maintained at 25 °C for 90 min. Immediately afterwards, the initial electrical conductivity of the medium (Ci) was measured using a bench conductivity meter (MB11, MS Techonopon^®). Subsequently, the dishes containing the leaf disks were subjected to 80 °C for 90 min in a drying oven (SL100/336, SOLAB^® Co., Ltd., Piracicaba, Brazil). After cooling, the final electrical conductivity (Cf) was measured. Electrolyte leakage was expressed as a percentage of the initial electrical conductivity relative to the final electrical conductivity, using the expression below, as described in Juzon-Sikora et al. [41].

$$\mathrm{Ext}(\%)={\displaystyle \frac{\mathrm{Ci}}{\mathrm{Cf}}}\times 100$$

2.7.3. Fruit Yield

During the fruit production phase, two harvests were carried out throughout the experiment, both performed early in the morning, manually, with the aid of bags and cardboard boxes for better storage and transport of the fruits. All fruits at the ripening stage were harvested from each useful plot. The fruits were placed in bags, all identified with their corresponding plot and treatment.

After harvest, the fruits were immediately transported to the Food Analysis and Food Chemistry and Biochemistry Laboratory of CCTA/UFCG. They were then sorted into marketable and non-marketable fruits. Fruits were considered marketable if they had a diameter equal to or greater than 35 mm and were free of any abnormalities or diseases that could compromise their structure and flavor [42]. The following were evaluated: total number of fruits per plant, number of marketable fruits per plant, longitudinal diameter, yield of non-marketable fruits, yield of marketable fruits, and total yield.

2.7.4. Mineral Nutrient Content in Fruits

Six marketable fruits from each subplot were separated to obtain a composite sample of 100 g of fresh fruit mass. These samples were dried in a forced-air oven (65–70 °C) until constant weight was achieved to subsequently determine dry mass and water content. The dried material was then ground in a Wiley-type mill, followed by sulfuric acid digestion according to the methodology described by Tedesco et al. [43]. Digestion was performed in a Kjeldahl microdigestion block (Marconi Co., Ltd., Londrina, Brazil)using 0.2 g samples of dried plant material in 3 mL of concentrated sulfuric acid and 1 mL of hydrogen peroxide, with an initial temperature of 180 °C and a final temperature of 360 °C. In the sulfuric acid digestate, the contents of the macronutrients calcium (Ca), potassium (K), phosphorus (P), and magnesium (Mg), and the micronutrients boron (B), manganese (Mn), iron (Fe), and zinc (Zn), were determined using ICP-OES (Inductively Coupled Plasma Optical Emission SpectrometryThermo Scientific Co., Ltd., Bremen, Germany) at the Mineral Nutrition Laboratory of the Soil Science Department at the Federal University of Lavras (UFLA). The element contents obtained on a dry mass basis were converted to fresh mass basis contents by correcting the values for the fruit water content.

2.7.5. Radar Chart

A radar chart was created using the variables most impacted by the treatments mitigating water deficit (marketable yield, total yield, photosynthetic pigments, photosynthetic rate, transpiration rate, stomatal conductance, and fruit mineral content) to illustrate the magnitude of the positive (increase) and negative (decrease) effects, expressed as relative values compared to the control treatment, under both irrigation levels. The following expression was used:

$$\mathrm{RVi}={\displaystyle \frac{\mathrm{ViTn}}{\mathrm{ViC}}}\times 100$$

where

RVi = Relative value of variable I;

ViTn = Absolute value of variable i obtained in treatment Tn (T1, T2, T3 or T4);

ViC = Absolute value of variable i in the control treatment (C).

2.8. Statistical Analyses

The data were initially subjected to the Shapiro–Wilk normality test [44]. The obtained data were subjected to analysis of variance (see Supplementary Materials in Tables S1–S5), followed by comparison of means (see Supplementary Materials in Tables S6–S10) using Tukey’s test (p ≤ 0.05), as well as a Pearson’s linear correlation analysis (p ≤ 0.05). These statistical analyses were performed using statistical R language version 4.4.1 [45].

3. Results

3.1. Plant Growth

Water deficit (50% ETc) reduced the LAI (leaf area index) by 12.6% (−0.36 m² m⁻²) compared to the full irrigation (100% ETc) (Figure 2a). Conversely, plant height (Figure 2b) and stem diameter (Figure 2c) were unaffected by irrigation depths or WDA treatments, except for the stem diameter under 50% ETc, which was influenced by WDA treatments. Under these conditions (50% ETc), foliar application of ZnSO₄·7H₂O increased the stem diameter by 25.7% (+5.41 mm) relative to the control (15.65 mm), though with statistically similar effects to other treatments (T1, T2, and T3).

3.2. Gas Exchange

Regarding gas exchange, water deficit promoted an average 7.6% decrease (−5.77 µmol m⁻² s⁻¹) in CO₂ assimilation (A, Figure 3a) compared to full irrigation (27.03 µmol m⁻² s⁻¹) without significantly affecting transpiration (E, Figure 3b), stomatal conductance (Gs, Figure 3c), internal CO₂ concentration (Ci, Figure 3d), or the A/E ratio (Figure 3e). Under deficit irrigation, all nutrient treatments—Fe₂O₃NPs (T2), ZnONPs (T3), and their conventional sources (FeSO₄·7H₂O, ZnSO₄·7H₂O)—increased CO₂ assimilation by 25.4% (+5.38 µmol m⁻² s⁻¹) over the control (20.07 µmol m⁻² s⁻¹), an effect that was even more pronounced under well-watered conditions (100% ETc), where a 27.8% increase (+6.81 µmol m⁻² s⁻¹) was observed.

Specifically, foliar application of Fe₂O₃NPs (T2) increased the transpiration rate (E) by 34.8% (+2.82 mmol H₂O m⁻² s⁻¹) under 50% ETc, compared to the control treatment (4.14 mmol H₂O m⁻² s⁻¹), though it did not differ from the Zn treatments (T3, T4), while under 100% ETc, the highest E values occurred with either Fe₂O₃NPs or ZnONPs, showing an 18% (+1.62 mmol H₂O m⁻² s⁻¹) increase. For stomatal conductance (Gs), treatments T2 and T4 were effective under 50% ETc, whereas only T2 increased Gs under 100% ETc. Internal CO₂ concentration (Ci), when averaged across treatments, was slightly higher under water deficit, and the A/E ratio remained unaffected by either irrigation depth or nutrient application.

3.3. Photosynthetic Pigments and Electrolyte Leakage

Water deficit (50% ETc) significantly increased chlorophyll a (Figure 4a), chlorophyll b (Figure 4b), total chlorophyll a + b (Figure 4c), carotenoid content (Figure 4e), and electrolyte leakage (Figure 4f), but did not alter the chlorophyll a/b ratio (Figure 4d). WDA treatments did not affect chlorophyll a, b, total chlorophyll, or carotenoid content at any irrigation level, nor the a/b ratio under 50% ETc. However, under full irrigation (100% ETc), treatments T1, T2, and T4, the chlorophyll a/b ratio showed similar values—62.9% (+1.48 mg g⁻¹) higher than the control (2.36 mg g⁻¹).

Regarding electrolyte leakage, under water deficit (50% ETc), all Fe/Zn source treatments (T1–T4) reduced leakage by 39.1% (−6.11%) versus the control (15.62%). Under full irrigation, T2 and T4 reduced leakage by 20.8% (−2.0%) versus the control (9.61%). On average, water deficit increased electrolyte leakage (Ext) by 28.0% (+2.35%) compared to 100% ETc (Figure 4f). Under 50% ETc, treatments T1-T4 all reduced Ext versus control but did not differ from each other. Under 100% ETc, treatments T2-T4 lowered Ext versus control but were statistically similar to T1.

3.4. Fruit Productivity

The total number of fruits per plant (Figure 5a) was not significantly affected by the irrigation depths. On the other hand, water deficit reduced the number of marketable fruits per plant (Figure 5b), the transverse diameter (Figure 5c), the longitudinal diameter (Figure 5d), the total yield (Figure 5e), and the marketable fruit yield (Figure 5f). Water restriction decreased total fruit production by 34.9% (−8.00 kg ha⁻¹) and marketable production by 71.2% (−10.58 kg ha⁻¹), compared to full irrigation (14.87 kg ha⁻¹). Under water deficit, treatment T4 increased the total number of fruits by 55.0% (+19.97 fruits per plant), compared to the control (36.33 fruits per plant), but did not alter total fruit yield. Under full irrigation, treatment T4 resulted in a higher number of fruits per plant compared to treatments T1 and T3 (which did not differ from the control and T2 treatments) and increased total yield by 61% (+11.95 kg ha⁻¹) compared to the control (19.54 kg ha⁻¹).

The WDA treatments did not modify the quantity of marketable fruits per plant under the 50% ETc irrigation depth (Figure 5b). However, under full irrigation, treatment T4 outperformed treatments T1, T2, and T3, but not the control treatment. Under both irrigation depths, the WDA treatments did not influence the transverse diameter, the longitudinal diameter of the fruits, or the marketable fruit yield.

3.5. Mineral Element Content in Fruits

The effect of irrigation depths on boron (B, Figure 6a) content in fruits depended on the WDA treatments. On average, across WDA treatments, calcium (Ca, Figure 6b) content was reduced by 16.5% (−31.63 mg 100 g⁻¹) due to water restriction. Water deficit did not affect the content of iron (Fe, Figure 6c), potassium (K, Figure 6d), manganese (Mn, Figure 6e), magnesium (Mg, Figure 6f), phosphorus (P, Figure 6g), or zinc (Zn, Figure 6h), but decreased B levels by 32.9% in the T4 treatment (−0.57 mg 100 g⁻¹).

Under water deficit (50% ETc), WDA treatments did not interfere with B levels. Under the 100% ETc irrigation depth, treatment T4 increased boron content by 65.5% (+0.69 mg 100 g⁻¹) and calcium (Ca) content by 32.8% (+62.66 mg 100 g⁻¹) compared to the control treatment. The WDA treatments particularly affected iron (Fe) content. Under water restriction, treatments T1 (FeSO₄·7H₂O) and T2 (Fe₂O₃NPs) increased the content of this nutrient by 67.5% (+5.73 mg 100 g⁻¹) and 117.3% (+9.96 mg 100 g⁻¹), respectively, compared to the control (8.50 mg 100 g⁻¹). Under full irrigation, treatments T1 and T2 increased iron content by 24.5% (+1.84 mg 100 g⁻¹) and 135.1% (+10.15 mg 100 g⁻¹), respectively, compared to the control treatment (7.51 mg 100 g⁻¹). The WDA treatments influenced zinc (Zn) content only under the 50% ETc irrigation depth, where treatment T4 increased zinc content by 18.5% (+0.26 mg 100 g⁻¹) compared to the control treatment.

Figure 7 below presents the relative values (Rv) of the selected variables to illustrate the magnitude of the effects of the WDA treatments under both tested irrigation depths. Iron content was excluded due to the high percentage increase in treatments T1 and T4, which would obscure visualization of effects on other treatments for this variable.

Under water deficit, treatment T1 enhanced the photosynthetic rate (A), transpiration rate (E), carotenoids (Carot), and calcium (Ca) content in fruits (Figure 7a). Under full irrigation, T1 increased values of A, E, total chlorophyll content (Chlt), chlorophyll a, and carotenoids (Figure 7b). Under both irrigation depths, treatment T2 elevated values of A, E, stomatal conductance (gs), Ca content, and total (TYF) and marketable (YMF) fruit yield, in addition to increasing chlorophyll a and carotenoids under 100% ETc. Under water restriction, treatment T3 most markedly increased values of A, E, and Ca content in fruits, while under full irrigation, it elevated A, E, and chlorophylls (a and b). Conversely, treatment T4 promoted increases in Gs, A, E, and Ca and Zn content under water deficit. Under full irrigation, treatment T4 enhanced A, E, chlorophylls (a and b), Ca and B content, and fruit yield (TYF and YMF).

Pearson’s linear correlation analysis (Figure 8) revealed significant positive correlations between A and Gs, the number of marketable fruits and B content (r = 0.57), Fe content and physiological parameters A/Ci (r = 0.75), A (r = 0.68), E (r = 0.46), gs (r = 0.62), and Mn content (r = 0.50). Additionally, B content correlated positively with total fruit production (r = 0.55), K content showed direct relationships with Mg (r = 0.52) and P (r = 0.54), and P content strongly correlated with Mn content (r = 0.86). Conversely, electrolyte leakage exhibited negative correlations with E (r = −0.52), A/E (r = −0.76), and A/Ci (r = −0.69).

4. Discussion

4.1. Plant Growth

This study evaluated the potential of iron (Fe₂O₃NPs) and zinc (Fe₂O₃NPs) nano-oxides, as well as their conventional forms (FeSO₄·7H₂O and ZnSO₄·7H₂O), to mitigate the negative effects of water deficit (50% ETc) on growth traits, gas exchange, photosynthetic pigments, and nutrient content in tomato fruits. Regarding plant growth, the results demonstrate that water deficit did not affect the length of the plant or the diameter of the stem but reduced the leaf area index (LAI). Generally, water stress decreases cell turgor, essential for proper cellular metabolism such as photosynthesis [46], enzymatic activity, and pigment contents [15,46]. The 12.6% reduction in LAI under 50% ETc is attributed to decreased cell expansion and leaf growth [47]. Under drought conditions, plants trigger a series of physiological responses aimed at conserving water [48,49]. A key adaptation is the reduction in leaf area development, which directly limits water loss through evapotranspiration [50]. Foliar application of Fe₂O₃NPs, ZnONPs, FeSO₄·7H₂O, and ZnSO₄·7H₂O did not mitigate the negative effect of water deficit on LAI but favored stem diameter. However, the 25.7% increase in the stem diameter with T4 under water deficit reinforces the role of Zn in auxin synthesis and structural integrity [51,52].

4.2. Gas Exchange, Photosynthetic Pigments, and Electrolyte Leakage

Water deficit reduced CO₂ assimilation (A), but treatments with Fe₂O₃NPs and ZnO NPs or their respective conventional sources (FeSO₄·7H₂O and ZnSO₄·7H₂O) increased A by up to 25.4% under 50% ETc and 27.8% under full irrigation, while Fe₂O₃NPs increased stomatal conductance (Gs) by 36.9% under full irrigation. In terms of Gs, Fe₂O₃NPs and ZnSO₄·7H₂O performed better than the control under water deficit, while T2 was more efficient under full irrigation. The increase in A may have been a result of improved chloroplast function [50,51,52]. These Zn and Fe sources may have enhanced Rubisco activity and electron transport [27]. Iron is crucial for light-dependent reactions, forming components of photosystems and electron transport chains to generate energy [47]. Zinc plays a fundamental role in hormone synthesis and cell division, which contributes to increased root growth and, consequently, greater water absorption; in addition, it also helps stabilize Rubisco itself and thus contributes to increasing the photosynthetic rate [50,51]. The increase in transpiration (E) with Fe₂O₃NPs under 50% ETc may link to nanoparticle-mediated stomatal regulation, balancing water loss and photosynthesis [16,27].

Photosynthetic pigments are key cellular components affected by water deficit [31,53]. Water restriction typically reduces pigment content, especially chlorophyll [46,54], due to oxidative damage from reactive oxygen species and increased activity of oxidases (e.g., catalase, peroxidase, SOD) [54]. However, chlorophyll levels may increase or decrease depending on stress severity and plant species [49,54]—e.g., moderate stress elevated chlorophyll in Lonicera caerulea. Here, increased chlorophyll/carotenoid content under deficit may indicate an adaptive response to maximize light capture [48], compounded by reduced leaf area concentrating pigments per unit area.

The reduction in electrolyte leakage with T1–T4 indicates that NPs and conventional Fe and Zn sources strengthened cell membranes, likely via antioxidant induction [28]. Zinc activates oxidative stress defense enzymes (e.g., SOD), aids tryptophan (IAA precursor) synthesis, and supports cell division and membrane integrity [28,30].

4.3. Yield, Fruit Nutrient Content, and Variable Correlations

Water deficit’s negative impact on tomato yield aligns with prior studies [27,32,49]. The 47.9% decrease in marketable yield under 50% ETc reflects stress-induced reductions in fruit number and size [32,55], consistent with a lower LAI, photosynthetic rate, and higher electrolyte leakage. However, T4 (ZnSO₄·7H₂O) increased total yield by 61% under full irrigation, underscoring zinc’s role in cell division and fruit development [12].

Positive correlations between fruit boron (B) content and fruit number (r = 0.57) or marketable yield (r = 0.55) highlight the B importance in cell wall integrity and postharvest quality [28]. Though fruit calcium (Ca) content did not correlate significantly with yield, the deficit reduced Ca. Both B and Ca directly affect cell wall plasticity/elongation [55,56]. These nutrients move via diffusion and mass flow to roots. Water deficit impedes their uptake and phloem-immobile translocation to fruits [12,55].

Fe₂O₃ NPs (T2) increased fruit iron content under deficit, confirming Fe oxide NPs’ efficacy in nutrient delivery under low water availability, as seen in wheat [56]. Chlorophyll content remained unchanged with Fe treatments, but Fe correlation with photosynthetic parameters (A, gs) confirms its central role in chlorophyll synthesis/electron transport [27,57,58]. Under water stress, ZnO NPs did not alter fruit zinc content, while ZnSO₄·7H₂O increased it. Performance differences between NPs or between ZnO NPs and ZnSO₄·7H₂O sources may stem from physical properties: Fe₂O₃ NPs (<50 nm; 50–245 m² g⁻¹ surface area) vs. ZnO NPs (<100 nm; 10.8 m² g⁻¹). The mechanisms governing foliar uptake of NPs are complex and not yet fully elucidated. Large-sized nanoparticles (for example: 100 nm) may have difficulty penetrating leaf tissue cells, promoting leaf retention, and thus not producing the expected physiological, nutritional, and fruit yield effects [21]. On the other hand, its foliar retention can promote a slow release of Zn, which may stimulate enzyme activity without, however, meeting the Zn requirements of the tomato plant sufficiently to increase fruit yield [24].

The increased efficacy of Fe₂O₃ nanoparticles over conventional FeSO₄·7H₂O in boosting tomato fruit iron content may result from their unique properties via direct or indirect mechanisms. The direct mechanism involves a gradual foliar iron release, enhancing bioavailability [21,26], while the indirect mechanism may involve nanoparticle-induced synthesis of organic acids that chelate iron, promoting its translocation to fruits [56,57]. It has been proposed that NP efficacy depends on cellular uptake routes: passive transport (symplastic and apoplastic pathways), endocytosis, or transmembrane protein-mediated transport [59,60]. Internalized Fe₂O₃ and ZnO NPs may reach organelles (chloroplasts and mitochondria), releasing Fe and Zn to modulate electron transport chains and associated processes (e.g., oxidative phosphorylation) [15]. However, NP entry is limited by pore size selectivity and accessibility [24,59,60].

The nutritional enhancement of fruits and vegetables is particularly critical in the Brazilian semi-arid region. These areas typically feature soils with inherently low availability of micronutrients such as iron and zinc, a characteristic attributed to the region’s soil composition [61]. This scarcity can lead to human malnutrition, notably iron deficiency. According to NIH [62], the daily Fe requirements for men and women over 19 years of age are 8 and 18 mg day⁻¹, respectively. Thus, the consumption of 30 g of fresh tomatoes Fe biofortified (+9.96 mg 100 g⁻¹ or +117%) could, theoretically, represent up to 37.3% and 17.6% of the daily reference values, for males and females, respectively. The efficient foliar application of these two nutrients presents a highly effective method to increase their concentration in these vegetables, thereby improving the nutritional quality of the harvested produce [11,13,16]. Furthermore, Fe and Zn nanoparticles are environmentally friendly. Due to the low concentrations applied via foliar spray, when they reach the soil through spray drift, these materials do not leave toxic residues, but only Fe, Zn, or sulfate that can be taken up by plant roots or soil microorganisms [63,64].

5. Conclusions

Water deficit (50% ETc) significantly reduced the tomato leaf area index (LAI), photosynthetic rate, and total and marketable yield. The foliar application of both nano-oxides (Fe₂O₃ and ZnO NPs) and conventional sources (FeSO₄·7H₂O and ZnSO₄·7H₂O) positively influenced several physiological, productivity, and nutritional quality parameters under both irrigation regimes. Notably, ZnSO₄·7H₂O was particularly effective under full irrigation, increasing the stem diameter by 25.7% and total yield by 61%. Across both water conditions, all Fe and Zn sources enhanced CO₂ assimilation and reduced electrolyte leakage, suggesting a role in protecting cellular membrane integrity. Foliar application of iron and zinc nano-oxides and their conventional sources had a limited effect on tomato plant growth but increased the photosynthetic rate under both irrigation levels. Under full irrigation, ZnSO₄·7H₂O increased total fruit production by 61% and fruit Zn content by 18.1%. In turn, Fe₂O₃ NPs (T2) led to increases in fruit iron content by 117.3% under water deficit and 135.2% under full irrigation. Conversely, foliar application using Fe₂O₃ nanoparticles emerged as a promising strategy for the biofortification of tomato fruits with iron, indicating a specific utility for addressing dietary deficiencies in regions where they are prevalent. However, there is a need to carry out further work in field conditions, under a semi-arid climate with different seasons and tomato cultivars to consolidate these findings.