Foliar Magnesium Application Enhances Fruit External and Interior Quality and Nitrogen Use Efficiency of Tomato (Solanum lycopersicum L.) Simultaneously Under High Nitrogen Supply

Abstract

Magnesium fertilizer application generally improves both the internal and visual quality of tomato fruits grown in magnesium-deficient soils. However, it remains unclear whether similar effects occur in magnesium-sufficient soils under high nitrogen fertilization. A field experiment was conducted in Wuhan, China, using soil with suitable available Mg content (385.97 mg kg⁻¹) and four nitrogen (N) application rates (0, 100, 200, and 300 kg N ha⁻¹) combined with foliar Mg spraying. This study evaluated tomato yield, nitrogen use efficiency, and fruit quality. Nitrogen application combined with foliar Mg significantly increased yield and biomass. The highest fruit yield was achieved with 200 kg N ha⁻¹ plus foliar Mg, showing a 104.9% increase compared with the control, while the greatest biomass was observed under 300 kg N ha⁻¹ with Mg spraying. Foliar Mg application also enhanced leaf nitrogen accumulation, shoot magnesium accumulation, and nitrogen use efficiency. Furthermore, fruit titratable acidity, vitamin C, total phenols, redness, chroma, and yellowness were significantly improved. Fruit redness was positively associated with sugars, amino acids, vitamin C, and phenolic compounds. Overall, foliar Mg application under 200 kg N ha⁻¹ improved tomato yield, nitrogen use efficiency, and fruit quality.

1. Introduction

In tomatoes and other fruit vegetables, fruit quality encompasses not only yield but also various external and internal attributes that affect consumer acceptance, nutritional content, shelf life, and marketability. External qualities such as fruit color, firmness, and uniformity in size, surface smoothness, and overall appearance are key factors in consumer preference and commercial value. For instance, a consistent red color in tomato fruit indicates proper ripening and higher lycopene levels. Internal fruit quality involves physicochemical and nutritional properties, including soluble solids, titratable acidity, sugar-to-acid ratio, and levels of bioactive compounds, all contributing to flavor, nutrition, and overall eating experience. Fruit internal quality involves biochemical and nutritional components, commonly called internal quality, such as soluble solids, titratable acidity, vitamin C, amino acids, phenols, sugars, and the sugar–acid ratio, which determine flavor, taste, nutritional profile, and processing quality. Tomatoes with higher soluble solids and balanced acidity typically have better flavor and sensory appeal, while increased vitamin C and phenolic compounds boost antioxidant capacity and health benefits. These quality indicators are interconnected with photosynthetic efficiency, carbon metabolism, assimilate transport, and nutrient balance, all of which are significantly influenced by magnesium nutrition [1,2,3]. Adequate Mg supply enhances chlorophyll formation, photosynthesis, assimilate transport, and enzyme activation, thereby improving sugar accumulation, organic acid metabolism, and synthesis of antioxidant compounds in fruits [1,4]. Magnesium is also involved in phloem loading and translocation of photo-assimilates from leaves to developing fruits, which is essential for fruit enlargement, sugar accumulation, and maintenance of fruit firmness [1]. In tomato, adequate Mg nutrition can promote lycopene synthesis and improve fruit pigmentation, resulting in brighter, uniform red coloration. Furthermore, Mg contributes to the synthesis of proteins, amino acids, and secondary metabolites associated with improved nutritional quality and antioxidant activity [1,4,5]. Adequate Mg availability, therefore, plays an important role in improving both the external appearance and the internal biochemical quality of tomato fruits. Magnesium (Mg) deficiency impairs the translocation of photoassimilates from source leaves to developing fruits, resulting in reduced sugar accumulation, limited assimilate availability, poor fruit coloration, and overall deterioration of fruit quality [1,2]. These effects negatively influence both the internal fruit quality and the external fruit quality of tomato fruits.

Conversely, excessive nitrogen application may stimulate vigorous vegetative growth at the expense of fruit quality, often resulting in delayed ripening, poor coloration, reduced firmness, lower soluble solids concentration, diluted nutritional compounds, and imbalanced sugar–acid ratios [6,7]. Excessive nitrogen application can also reduce dry matter accumulation and negatively affect flavor and storage quality by promoting excessive water accumulation and altering carbohydrate metabolism [6,8]. In addition, previous studies have suggested that high nitrogen supply may alter nutrient uptake patterns and disrupt nutrient balance within the plant, potentially exacerbating magnesium deficiency through nutrient antagonism [9,10]. Consequently, the decline in fruit quality under excessive N conditions is more likely associated with nutrient imbalance and reduced physiological efficiency than with a direct induction of Mg deficiency [9,11]. Since Mg plays a central role in carbon and nitrogen metabolism, foliar Mg application may help alleviate these negative effects by improving chlorophyll content, photosynthetic performance, assimilate transport, and nutrient-use efficiency, thereby enhancing the nutritional quality, internal fruit quality, and external fruit quality of tomato fruits [1,4,12]. Although Mg fertilization is traditionally recommended for Mg-deficient soils, increasing evidence suggests that crop responses to Mg may also occur under soils with adequate Mg availability [1,10]. High nitrogen inputs stimulate rapid vegetative growth, photosynthesis, and fruit development, thereby increasing plant Mg requirements for chlorophyll synthesis, enzyme activation, carbohydrate transport, and nitrogen metabolism [4,13,14]. In intensive production systems, Mg absorption from soil may not always satisfy the heightened physiological needs, especially during rapid vegetative growth and fruit development. This can lead to short-term mismatches between Mg supply and plant demand, which might influence plant physiological functions and the quality of the fruit overall. Consequently, transient imbalances between Mg supply and plant demand may arise even when soil Mg levels are considered sufficient. Foliar Mg application provides a direct source of Mg to photosynthetically active tissues and may enhance photosynthetic efficiency, assimilate partitioning, and nutrient-use efficiency without necessarily correcting a soil Mg deficiency [11]. Therefore, evaluating the effectiveness of foliar Mg supplementation under Mg-sufficient soil conditions and increasing nitrogen supply remains important for optimizing fruit quality, nutrient utilization, and productivity in intensive tomato production systems. Therefore, a field trial was conducted using suitable available Mg soil and a split-plot randomized complete block design to assess how increasing nitrogen (N) application rates and foliar spraying of Mg fertilizer affect tomato growth in Hubei, China. This study explored whether foliar Mg application combined with increasing N supply could simultaneously enhance nitrogen use efficiency by influencing yield and nutrient uptake; fruit visual quality, such as color, firmness, and appearance, and fruit internal quality, including internal quality of soluble solids, titratable acidity, vitamin C, amino acids, total phenols, and the sugar–acid ratio. Additionally, the research highlights the importance of foliar magnesium in maintaining higher yield, higher quality, and higher efficiency and in supporting tomato productivity and sustainability in systems with intensive nitrogen fertilization on suitable available Mg soils.

2. Materials and Methods

2.1. Plant Materials and Growth Conditions

The experiment was conducted from March to June 2025 at the Vegetable Research Station of Huazhong Agricultural University. The tested soil chemical properties were as follows: pH 5.38, alkaline N 113.2 mg kg⁻¹, available P 117.31 mg kg⁻¹, available K 339.6 mg kg⁻¹, exchangeable Ca 1738.72 mg kg⁻¹, and exchangeable Mg 385.97 mg kg⁻¹ (suitable available Mg concentration in soil).

2.2. Experimental Design

Tomato seedlings of the commercial hybrid cultivar ‘Hezuo 903’ variety registration number: GPD(2018)310450 were obtained from Shanghai Changzhong tomato seed industry Co., Ltd. (Shanghai, China) Seedlings were transplanted 25 days after sowing at the 4–6 true-leaf stage. The experiment was arranged in a split-plot design, with main plots measuring 6.0 m × 1.9 m and subplots measuring 3.2 m × 1.9 m. Plants were established at a spacing of 47.5 cm × 40 cm, resulting in 64 plants per main plot and 32 plants per subplot. Only the central plants were used for growth, yield, and fruit quality assessments to minimize border effects.

Irrigation was delivered via a sprinkler system and scheduled to match crop water requirements throughout the growing season, with irrigation generally applied after 18:00 h. Nitrogen fertilizer was applied in three split applications, supplying 30%, 40%, and 30% of the total N rate during the vegetative, flowering, and fruit development stages, respectively. Magnesium was applied as a 2% (w/v) MgSO₄ foliar spray on three occasions during the vegetative growth period. Each subplot measured 3.2 m × 1.9 m (6.08 m²). For each application, a 2% MgSO₄ solution was prepared by dissolving 60.8 g of MgSO₄ in 3.04 L of water and uniformly sprayed over each subplot. All other agronomic practices, including weed, pest, and disease management, were implemented uniformly across all treatments in accordance with local production recommendations.

2.3. Sampling and Sample Preparation

From each subplot, four representative plants were randomly selected for fruit sampling. Fruits were harvested at the red-ripe stage from the second to fourth trusses to minimize variation due to fruit position and maturity. A total of 12 fruits per replicate were collected and pooled to form a single composite sample for fruit quality and nutrient analyses. Fruits were selected for uniform size and color and for the absence of visible defects, mechanical damage, or disease symptoms to ensure sample homogeneity and reduce biological variability among treatments.

After harvest, fruits were washed with tap water and then with deionized water and air-dried at room temperature. Each composite sample was divided into three subsamples: one was analyzed immediately for fresh fruit quality attributes, a second was stored at −80 °C for biochemical analyses, and a third was oven-dried at 65 °C for mineral nutrient analysis. The dried samples were weighed, ground to a fine powder, and stored in sealed containers under dry conditions until analysis.

2.4. Analysis of Internal Fruit Quality and Color of Fruits

Fruit quality was evaluated using standard methods. Vitamin C (ascorbic acid) levels were measured via the 2,6-dichloroindophenol titration method [15]. The digital refractometer was used to determine total soluble solids (TSSs), juice yield, and the TSS/acid ratio, while titratable acidity was assessed with NaOH titration [16]. Sugars and organic acids were analyzed as described by [16]. Frozen fruit samples (3 g) were extracted with 80% methanol, then heated, ultrasonicated, and centrifuged. The collected supernatant was pooled and brought to volume with methanol. After adding an internal standard, the samples were derivatized with hydroxylamine hydrochloride, HMDS, and TMCS and then analyzed by GCMS gas chromatography (Agilent 6890N GC (Agilent Technologies, Santa Clara, CA, USA)).

2.5. Determination of Free Amino Acids

In total, 0.50 g of a fresh fruit sample was weighed; then, 5 mL of a 10% C₂H₄O₂ grinding homogenate was added. The mixture was diluted to a final volume of 50 mL with distilled water and filtered. About 1 mL of the filtrate was transferred into a 20 mL dry tube, to which 1 mL of sterile distilled water, 0.5 mL of ascorbic acid, and 3 mL of ninhydrin hydrate were added. The mixture was then boiled in water for 15 min. When the solution turned blue-purple, the volume was adjusted to 20 mL with 60% C2H6O. The absorbance was measured at 570 nm after mixing using a spectrophotometer [17].

2.6. Determination of Total Phenols

The total phenolic content (TPC) was determined via the FolinCiocalteu colorimetric method. The fruit sample extract was mixed with Folin–Ciocalteu reagent and sodium carbonate, and absorbance was measured at 765 nm with a UV–Vis spectrophotometer. Gallic acid served as the standard, and the results were expressed as mg gallic acid equivalents (GAEs) per gram of sample [18].

2.7. Fruit Color Measurement

The external quality of tomato fruit peel color change was measured using a colorimeter (CM-5, Konica Minolta, Osaka, Japan) on targeted fruits from each treatment during ripening. The measurements were expressed in the L (brightness), a (red-green difference), and b (yellow-blue difference) color space. C (color intensity) and H (hue angle) were calculated from a and b and with

$$C=\sqrt{a^2+b^2}$$

(1)

and H = arctangent of ${\displaystyle \frac{a}{b}}$ [19].

2.8. Soil and Plant Nutrient Analysis

Soil alkaline-hydrolyzable nitrogen was measured using the alkaline-hydrolysis diffusion method with 1 mol/L NaOH and 2% H₃BO₃ as described by [20]. Soil available phosphorus was extracted with 0.50 mol/L NaHCO₃ and quantified by spectrophotometry at 700 nm [21] on a Metash UV-5200 instrument (Shanghai Metash Instruments Co., Ltd., Shanghai, China). The levels of exchangeable calcium (Ca), potassium (K), and magnesium (Mg) in soil samples were determined after extraction with ammonium acetate solution. Ca and Mg concentrations were measured via atomic absorption spectrophotometry (AAS), while potassium was analyzed by flame photometry according to standard procedures [10].

Ten disease-free, fully expanded leaves were collected from the middle canopy of each tomato plant at the early fruit-setting stage. Leaves were sampled from the same branch as the corresponding fruits to maintain consistency between vegetative and reproductive tissues. Fully expanded middle-canopy leaves were selected because they represent a physiologically active and relatively stable tissue, thereby minimizing variability associated with leaf age and canopy position. Samples from each plant were pooled to form one replicate. The leaf samples were sequentially washed with 0.1% neutral detergent, tap water, a 0.2% nitric acid solution for 30 s, and deionized water for 2 min. Subsequently, the samples were dried at 105 °C for 30 min and then at 65 °C to constant weight and finally ground into a fine powder using an agate mortar.

Magnesium (Mg) levels were determined through atomic absorption spectrophotometry (AAS) [10]. The dried samples were digested in 50 mL digestion tubes with 10 mL of a mixed HNO₃–HClO₄ solution (4:1, v/v) using an infrared digestion furnace. The initial digestion was performed at 160 °C, and then, the temperature was raised to 240 °C until the samples were completely dissolved. Instrument calibration was performed using a series of commercially prepared Mg standard solutions prior to sample analysis. Reagent blanks and duplicate samples were included as quality-control measures to ensure analytical accuracy and precision. Because the analyses were conducted using standard laboratory protocols, detection limits were not separately determined for this study.

Initially, the leaves were weighed precisely using a scale. They were then transferred into a digestion tube where 5 mL of H₂SO₄ and an appropriate amount of H₂O₂ were added, with the exact volume of H₂O₂ determined by the specific protocol or sample size. The tube was heated in a digestion block at 180 °C for several hours to break down organic matter and release nitrogen. After cooling, the sample was diluted with distilled water to a suitable volume for analysis. The total nitrogen content was measured using a semi-micro Kjeldahl distillation, which detected ammonia produced during digestion. The nitrogen content was calculated from the ammonia concentration and sample weight and expressed in units such as percentage. Standard laboratory quality-control procedures, including reagent blanks and duplicate analyses, were applied throughout the analytical process.

2.9. Calculation of Nitrogen Use Efficiency

To evaluate nitrogen use efficiency (NUE) across nitrogen and magnesium treatments, three complementary NUE indices were selected: agronomic efficiency (AE), partial factor productivity (PFP), and recovery efficiency (RE). These indices were chosen because they assess distinct aspects of fertilizer performance and provide comprehensive information to optimize nutrient management strategies. Agronomic efficiency (AE) reflects the yield increase per unit of applied nitrogen; partial factor productivity (PFP) evaluates overall fertilizer productivity; recovery efficiency (RE) estimates the proportion of applied nitrogen recovered by the crop. Together, these indices enable the assessment of crop productivity, nitrogen uptake efficiency, and fertilizer utilization, thereby supporting fertilizer management recommendations aimed at maximizing yield while reducing excessive fertilizer inputs and minimizing environmental impacts.

• Agronomic Efficiency (AE)

To know how much extra yield is produced per unit of applied nitrogen, we used the formula of

AE = (YN − Y0)/(N applied)

(2)

where the variables are defined as follows: YN = yield with nitrogen fertilizer (kg/ha), Y0 = yield without nitrogen (control) (kg/ha); N applied = amount of nitrogen applied (kg/h).

B.

Partial Factor Productivity (PFP) to know the ratio of crop yield to the amount of fertilizer applied, indicating the productivity of a given nutrient input, was calculated by

PFP = YN/(N applied)

(3)

C.

Recovery Efficiency (RE) to measure how much of the applied nitrogen is taken up by the plant was calculated by

RE (%) = (UN − U0)/(N applied) × 100

(4)

where

⮚

UN = plant N uptake with fertilizer (kg/ha);

⮚

U0 = plant N uptake without fertilizer (kg/ha).

2.10. Statistical Analysis

Data were analyzed using SAS version 9.2 (SAS Institute Inc., Cary, NC, USA). The results are presented as means ± standard error (SE) (n = 3). Prior to analysis, data were examined for normality and homogeneity of variances. No data transformation was required because the assumptions of ANOVA were satisfied. Statistical analysis was performed using a split-plot ANOVA model, in which nitrogen treatments were assigned to the main plots and magnesium treatments were assigned to the subplots, with replication treated as a random effect. Mean comparisons were performed using Duncan’s multiple range test at p < 0.05. Figures were generated using GraphPad Prism 9.5.

3. Result

3.1. The Effect of Spraying Magnesium with Nitrogen on the Yield and Biomass of Tomato

Applying nitrogen significantly increased tomato fruit yield (p < 0.001). Similarly, magnesium spraying also had a significant positive effect on yield (p < 0.01). Moreover, the interaction between nitrogen and magnesium was significant (p < 0.05) (

Table 1. Two-way ANOVA analysis of tomato yield and related parameters.

Parameters	Magnesium	Nitrogen	Magnesium × Nitrogen
Fruit yield	***	**	*
Biomass yield kg/plant	***	***	****
Fruit size	*	ns	ns
Single fruit weight	ns	ns	ns

The asterisk (*) indicates significance at p < 0.05, (**) indicates significance at p < 0.01, (***) indicates significant differences at p < 0.001; (****) indicates significant differences at p < 0.0001, and (ns) denotes a non-significant difference.

). Without spraying magnesium fertilizer, the increase in fruit yield (kg/plant) for N1, N2, and N3 was 49.38%, 45.88%, and 51.83%, respectively. With magnesium fertilizer spraying, these increases were significantly higher at 53.78%, 104.90%, and 62.08%, respectively (Figure 1A).The theoretical maximum tomato fruit yield was achieved with Mg spraying combined with N2 (200 kg/ha) (Figure 2).

It showed that nitrogen application with Mg spraying consistently increased yield more than in plants with no Mg spraying and that Mg spraying improved tomato yield regardless of nitrogen fertilizer levels, with significantly higher increases under higher nitrogen levels (N2) than under excess (N3) or lower (N1) levels.

Applying nitrogen significantly increased tomato biomass yield (p < 0.001). Similarly, magnesium application also had a notable positive effect, boosting biomass yield (p < 0.001). Additionally, the interaction between nitrogen and magnesium was significant (p < 0.001) (Table 1).

Without spraying magnesium fertilizer, the biomass (kg/plant) increased by 72.07%, 65.98%, and 95.82% for N1, N2, and N3, respectively. When magnesium fertilizer was applied, this biomass was increased significantly 82.78%, 147.31%, and 99.21% by three nitrogen levels (Figure 1D). It was the same as fruit yield that the nitrogen application with Mg spraying consistently increased more biomass than no Mg spraying plants, and Mg spraying improved tomato biomass regardless of nitrogen fertilizer levels, with significantly higher increases under higher nitrogen levels (N2) than excess (N3) or lower (N1) levels.

A similar pattern was observed in untreated plants (−Mg), with fruit yield initially increasing and then decreasing at the highest nitrogen application rate (R² = 0.89**). A quadratic regression described the relationship between tomato yield and nitrogen application, indicating a maximum yield at approximately 200 kg N ha⁻¹ (Figure 2). However, this estimate should be interpreted cautiously because the regression was based on only four nitrogen application rates. Therefore, the estimated optimum nitrogen rate should be viewed as an approximation of the response trend rather than a definitive recommendation. Future studies incorporating additional nitrogen levels are required to improve the precision and reliability of optimum nitrogen rate estimation.

Similarly, spraying magnesium significantly increased tomato fruit size (p < 0.05). However, nitrogen application and the interaction between magnesium and nitrogen did not significantly affect fruit size. Additionally, magnesium spraying, nitrogen application, and their interaction did not significantly influence the weight of a single tomato fruit (Table 1).

3.2. Spraying Magnesium with Nitrogen Influences N and Mg Accumulation and Nitrogen Use Efficiency

Magnesium spraying, different nitrogen levels, and the interaction between magnesium and nitrogen did not significantly affect nitrogen accumulation in the fruit and roots of tomato, but spraying magnesium and applying different levels of nitrogen significantly affected accumulation of nitrogen in leaves and shoots (p < 0.05); however, the interaction between spraying magnesium and nitrogen was not significant (

Table 2. Two-way ANOVA analysis of N and Mg accumulation in different parts of the tomato.

Parameters	Magnesium	Nitrogen	Magnesium × Nitrogen
Fruit N	ns	ns	ns
Leaf N	*	**	ns
Shoot N	*	*	ns
Root N	ns	ns	ns
Fruit Mg	**	**	ns
Leaf Mg	*	ns	ns
Shoot Mg	***	***	***
Root Mg	ns	ns	ns

The asterisk (*) indicates significance at p < 0.05, (**) indicates significance at p < 0.01, (***) indicates significance at p < 0.001, and (ns) denotes a non-significant difference.

).

Leaf N accumulation was consistently higher in the +Mg treatment than in the −Mg treatment across all nitrogen levels, suggesting a potential association between magnesium application and improved nitrogen use efficiency and leaf biomass production.

Without spraying magnesium fertilizer, the increases in leaf N accumulation for N1, N2, and N3 were 41.90%, 72.94%, and 76.45%, respectively. When magnesium fertilizer was sprayed, leaf N accumulation increased by 65.38%, 160.86%, and 156.77% for N1, N2, and N3, respectively. However, they were not significantly different from each other (Figure 3B,C).

At N1 (100 kg N ha⁻¹), leaf N accumulation increased further, suggesting that moderate nitrogen combined with Mg supported better nutrient assimilation; however, at N2 (200 kg N ha⁻¹) and N3 (300 kg N ha⁻¹), the highest values were recorded under +Mg. At the same nitrogen application rates, leaf N accumulation remained lower in the −Mg treatment than in the +Mg treatment, particularly under N2 (200 kg N ha⁻¹) and N3 (300 kg N ha⁻¹).

Spraying magnesium with different levels of nitrogen significantly affected (p < 0.001) shoot Mg accumulation, and the interaction between magnesium and nitrogen was significantly affected (p < 0.05). Additionally, spraying magnesium at different nitrogen levels significantly affected (p < 0.01) tomato fruit Mg accumulation, but the interaction between magnesium and nitrogen was not significant (Table 2).

The +Mg treatment increased shoot Mg accumulation across all nitrogen levels compared with −Mg, confirming improved soil Mg availability and enhanced uptake and translocation within the plant.

Without spraying magnesium fertilizer, the increase in shoot Mg accumulation for N1, N2, and N3 was 139.1%, 171.7%, and 248.4%, respectively. When magnesium fertilizer was sprayed, these Mg concentrations increased significantly by 222.0%, 357.6%, and 217.5%, respectively, for N1, N2, and N3 (Figure 3G).

The increase from N2 (200 kg ha⁻¹) to N3 (300 kg ha⁻¹) indicates that high nitrogen supply enhances plant growth and biomass, leading to greater Mg uptake and accumulation in shoots. However, excess nitrogen May reduce Mg uptake efficiency.

Fruit Mg accumulation increased with nitrogen application up to the N2 level, at which the highest accumulation was observed, particularly under the +Mg treatment. However, further nitrogen application at the N3 level resulted in a decline in Mg accumulation.

Without spraying magnesium fertilizer, the increase in fruit Mg accumulation for N1, N2, and N3 was 32.00%, 75.04%, 9.83%, respectively. When magnesium fertilizer was sprayed, the Mg concentration in these leaves increased significantly by 50.96%, 273.20%, and 159.75% in N1, N2, and N3, respectively (Figure 3E).

The +Mg treatment significantly increased fruit Mg accumulation across all nitrogen levels, confirming enhanced Mg availability and uptake and improved partitioning of Mg to reproductive organs (fruits).

Fruit Mg accumulation increased up to N2 (200 kg ha⁻¹) and decreased at N3 (300 kg ha⁻¹). This indicates that neither too low nor excessive nitrogen levels promote better plant growth, nutrient uptake, and Mg translocation to fruits. The highest N2 (200 kg ha⁻¹) + Mg levels show that optimal nitrogen and Mg supply maximize nutrient recovery and utilization.

Spraying magnesium at different nitrogen levels cannot significantly affect leaf and root Mg accumulation in tomato (

Table 3. Two-way ANOVA of internal and external quality parameters of tomato.

Parameters	Magnesium	Nitrogen	Magnesium × Nitrogen
Fruit hue	***	***	***
Coloring intensity (C)	***	***	***
a*	***	***	***
b value (yellow-blue difference)	***	***	***
Fruit brightness (L value)	***	***	***
Titratable acidity (TA %)	**	**	*
Ascorbic acid (Vc)	***	***	***
Total soluble solid	**	*	ns
Solid acid ratio	*	ns	ns
Glucose	*	ns	*
Fructose	*	ns	*
Malic acid	*	ns	*
Chla	ns	**	ns
Chlb	ns	*	ns
Carotenoids	ns	ns	ns
Free amino acids	***	***	**
Total phenols	***	***	***

The asterisk (*) indicates significance at p < 0.05, (**) indicates significance at p < 0.01, (***) indicates the significant difference at p < 0.001, and (ns) denotes a non-significant difference.

).

A significant interaction between nitrogen and magnesium was observed for agronomic efficiency (AE) (Figure 4A). Under the −Mg treatment, AE remained relatively low across nitrogen levels and declined at the highest nitrogen rate (N3 (300 kg ha⁻¹)). In contrast, AE values were higher under +Mg, with the greatest difference between Mg treatments observed at N2 (200 kg ha⁻¹). The highest AE was recorded under the N2 + Mg treatment.

Magnesium application (+Mg) consistently led to higher recovery efficiency (RE) (Figure 4B) compared to -Mg across all nitrogen levels. RE increased from N1 (100 kg ha⁻¹) to N2 (200 kg ha⁻¹) but declined at N3 (300 kg ha⁻¹) under both Mg treatments. The greatest difference between +Mg and −Mg was observed at N2 (200 kg ha⁻¹), where RE peaked. This suggests that magnesium promotes nitrogen uptake and utilization at higher nitrogen levels.

PFP (partial productivity factor) decreased with increasing nitrogen rates under both Mg and non-Mg treatments (Figure 4C), indicating diminishing returns at higher N levels. However, magnesium application consistently improved PFP, especially at N2 and N3, emphasizing Mg’s role in sustaining nitrogen productivity during moderate to high N supply. Overall, a balanced combination of N and Mg nutrition enhances fertilizer efficiency compared to applying nitrogen alone.

3.3. Spraying of Magnesium with Nitrogen in the Coloration of Tomato Fruit (Face Score)

Magnesium spraying significantly affected the hue angle of tomato fruits, and a strong interaction between magnesium and nitrogen treatments was observed. Both nitrogen application combined with magnesium spraying and the nitrogen × magnesium interaction were highly significant (p < 0.001) (Table 3).

Across all nitrogen levels, magnesium-sprayed plants (+Mg) consistently exhibited lower hue angle values than non-sprayed plants (−Mg), indicating enhanced fruit coloration and ripening. The lowest hue angle was recorded at N2 under both magnesium treatments, although the reduction was more pronounced in the +Mg treatment (Figure 5A).

The progressive decline in hue angle from N0 to N2 (200 kg ha⁻¹) indicates a transition in fruit color from green/yellow toward deeper, more mature ripening tones as nitrogen availability increased. However, a slight increase in hue angle at N3 (300 kg ha⁻¹) suggests that excessive nitrogen may delay fruit ripening by stimulating vegetative growth, thereby reducing the intensity of color development.

Tomato fruit color intensity (C), as measured by hue angle, was significantly affected by nitrogen levels, magnesium application, and their interaction (p < 0.001) (Table 3). Fruits treated with magnesium spray (+Mg) consistently exhibited higher color intensity than those without magnesium application (−Mg). Color intensity increased progressively from N0 to N2, reaching its maximum at N2 (200 kg N ha⁻¹) before declining slightly at N3 (300 kg N ha⁻¹).

Without magnesium spraying, fruit color intensity increased by 11.25%, 17.46%, and 0.87% under N1, N2, and N3, respectively. In contrast, magnesium spraying markedly enhanced fruit color intensity, resulting in increases of 23.59%, 46.77%, and 6.27% under N1, N2, and N3, respectively (Figure 5B).

The improved coloration seen with magnesium application, especially at the N2 level, boosted the visual appeal and market visual quality of the fruits, indicating higher consumer interest and marketability. These findings suggest that applying magnesium alongside the N2 nitrogen level encourages better fruit color development and enhances overall fruit quality in tomatoes.

Magnesium spraying in combination with nitrogen application significantly influenced the a* value of tomato fruits (p < 0.001), indicating an enhancement in fruit redness intensity. A significant interaction between magnesium and nitrogen was also observed (p < 0.001) (Table 3). The a* value increased progressively from N0 to N2, reaching its maximum at N2 (200 kg ha⁻¹) before declining at N3 (300 kg ha⁻¹). Across all nitrogen levels, fruits treated with magnesium (+Mg) consistently recorded higher a* values than those without magnesium application (−Mg).

In the absence of magnesium spraying, the a* values increased by 20.12%, 32.41%, and 9.95% under N1, N2, and N3, respectively. However, magnesium application further enhanced fruit redness, with increases of 38.91%, 68.63%, and 18.98% under N1, N2, and N3, respectively (Figure 5C).

The highest a* value was observed under the N2 (200 kg N ha⁻¹) plus Mg treatment, indicating enhanced red coloration of tomato fruits. The increased redness under Mg application reflects improved fruit color development and visual quality. However, because carotenoid content was not significantly affected by the treatments (Table 3), the higher a* value should not be directly attributed to increased carotenoid accumulation. Therefore, the N2 + Mg treatment improved external fruit color characteristics, although the underlying biochemical mechanisms require further investigation.

The b* value (yellow–blue difference) of tomato fruit was significantly affected by magnesium spraying and nitrogen levels (p < 0.001), and their interaction was significant (p < 0.001) (Table 3). The b* value increased from N0 to N2 (200 kg N ha⁻¹), reached the highest level at N2, and then declined at N3. Across all nitrogen levels, the +Mg treatment consistently produced higher b* values than the −Mg treatment (Figure 5D). The increased b* values up to N2 indicate enhanced yellow coloration due to greater carotenoid accumulation under adequate nitrogen supply. Improved color development under magnesium application contributed to better fruit appearance and a higher market face score for tomato fruits.

Tomato fruit brightness (L* value) was significantly influenced by nitrogen levels and magnesium application, showing a significant N × Mg interaction (p < 0.001) (Table 3). The L* value decreased up to N2 (200 kg N ha⁻¹) and then increased at N3. Across all nitrogen levels, fruits treated with magnesium (+Mg) consistently showed lower L* values than untreated fruits (−Mg), particularly at N2 (Figure 5E). Lower L* values indicate darker fruits, which are associated with greater pigment accumulation during ripening. The darker and more intense fruit color under +Mg treatment improved the visual quality and enhanced the market appeal of tomato fruits.

3.4. The Impact of Spraying Magnesium with Nitrogen on Tomato Internal Quality

3.4.1. Titratable Acidity (TA %) and Total Soluble Solid

Applying magnesium and nitrogen significantly increased tomato fruits’ titratable acidity (TA %) (p < 0.01) (Table 3). Additionally, the interaction between nitrogen and magnesium was significant (p < 0.05) (Table 3).

Titratable acidity (TA %) increased up to N2 (200 kg ha⁻¹), reaching a maximum at N2 (200 kg ha⁻¹), and then declined at N3 (300 kg ha⁻¹). At most nitrogen levels, +Mg treatment resulted in higher TA% than −Mg. This shows that it enhances nutrient balance, supporting better fruit biochemical quality.

Without spraying magnesium fertilizer, the increase in fruit titratable acidity (TA %) for N1, N2, and N3 was 54.63%, 47.18%, and 38.59%, respectively. With magnesium fertilizer spraying, these increased significantly by 87.10%, 128.33%, and 26.47% for N1, N2, and N3 nitrogen levels (Figure 6B).

The increase in TA up to N2 (200 kg ha⁻¹) suggests that adequate nitrogen enhances organic acid synthesis and accumulation, contributing to improved fruit quality. The decline in excess nitrogen (N3) suggests it may dilute organic acids.

Applying nitrogen showed a significant difference between treatments on tomato fruit total soluble solids (TSSs) (p < 0.01) and magnesium spraying (p < 0.05). However, the interaction between nitrogen and magnesium was not significant (Table 3).

TSS increased from N1 and N2, reaching higher values at moderate to higher nitrogen levels (N1 (100 N ha⁻¹) and N2 (200 N ha⁻¹)) and slightly declined at N3 (300 kg ha⁻¹). Across all treatments, the +Mg result showed that +Mg improved over −Mg, indicating improved sugar accumulation.

Without spraying magnesium fertilizer, the increase in fruit total soluble solids (TSSs) for N1, N2, and N3 was 10.39%, 12.47%, and 7.16%, respectively. With magnesium fertilizer spraying at 15.47% for the N1, N2, and N3 nitrogen levels, these total soluble solids (TSSs) increased significantly by 17.78%, 18.59%, and 15.47%, respectively, for the N1, N2, and N3 nitrogen levels (Figure 6C).

At N1 and N2, +Mg produced the highest TSS, significantly greater than the −Mg treatment. The increase in TSS up to N2 (200 N ha⁻¹) indicates that adequate nitrogen levels enhance carbohydrate synthesis and sugar accumulation in fruits, thereby improving sweetness and quality. The decline at N3 (300 kg ha⁻¹) indicates excess nitrogen application, which promotes vegetative growth and reduction in fruit quality.

3.4.2. Ascorbic Acid (Vc)

Spraying magnesium and different nitrogen levels significantly affected (p < 0.001); the interaction between nitrogen and magnesium was also significant (p < 0.001) (Table 3).

Vitamin C content increased from up to N2 (200 N ha⁻¹), reaching the highest value at N2 (200 N ha⁻¹) under +Mg, and then slightly declined at N3 (300 N ha⁻¹). Across all nitrogen levels, the +Mg treatment resulted in higher VC than the −Mg treatment.

Without spraying magnesium fertilizer, the increase in fruit ascorbic acid (VC) for N1, N2, and N3 was 22.24%, 39.92%, and 40.64%, respectively. With magnesium fertilizer spraying, the ascorbic acid (VC) in the fruit increased significantly by 56.58%, 69.80%, and 49.48% for N1, N2, and N3, respectively (Figure 6A).

The vitamin C (VC) content increased progressively up to N2 (200 kg N ha⁻¹) and declined at N3 (300 kg N ha⁻¹). This result indicates that an optimal nitrogen level combined with magnesium application enhanced metabolic activity and promoted the biosynthesis of ascorbic acid, thereby improving the nutritional quality of tomato fruits. In contrast, excessive nitrogen application negatively affected fruit quality and reduced vitamin C accumulation.

3.4.3. Solid Acid Ratio

Spraying magnesium significantly affected the solid acid ratio (p < 0.05). However, applying nitrogen and the interaction between nitrogen and magnesium were not significant (Table 3). The highest mean solid acid ratio was recorded in the control treatments without magnesium application. The effect of magnesium spraying varies with nitrogen level, mostly happening at the N2 (200 kg ha⁻¹) level, with the spraying of magnesium decreasing the solid acid ratio further than other combinations (Figure 6D).

3.4.4. Free Amino Acid and Total Phenols

The analysis of variance revealed that magnesium application, nitrogen levels, and their interaction significantly affected amino acid and total phenol contents (p < 0.001) (Table 3). Increasing nitrogen supply significantly enhanced both parameters, with the highest values observed under the N2 and N3 treatments (Figure 6F,G). Moreover, the significant Mg × N interaction indicated that the accumulation of amino acids and total phenols in tomato fruits depended on the combined effect of nitrogen and magnesium supply. These results suggest that adequate nitrogen, together with magnesium application, promoted metabolic activity and the synthesis of bioactive compounds, thereby improving the nutritional and antioxidant quality of tomato fruits.

Without magnesium spraying, fruit free amino acid content increased by 21.94%, 26.14%, and 16.56% under N1, N2, and N3, respectively. However, magnesium application further enhanced free amino acid accumulation, resulting in significant increases of 29.83%, 47.83%, and 57.86% under N1, N2, and N3, respectively (Figure 6F).

Similarly, without magnesium spraying, total phenol content increased by 8.08%, 27.97%, and 41.73% under N1, N2, and N3, respectively. With magnesium spraying, total phenol content increased significantly by 29.12%, 76.62%, and 46.96% under N1, N2, and N3, respectively (Figure 6G). These results indicate that magnesium application, particularly when combined with optimal nitrogen supply, enhanced the accumulation of amino acids and phenolic compounds, thereby improving the nutritional and antioxidant quality of tomato fruits.

3.4.5. Simple Sugar and Organic Acid

Spraying magnesium and its interaction with nitrogen significantly affected glucose and fructose concentrations (p < 0.05). However, applying different rates of nitrogen was not significant (Table 3).

Glucose and fructose contents increased up to N2 (200 kg ha⁻¹), reaching the highest values under the +Mg treatment, and then decreased at N3 (300 kg N ha⁻¹). In most nitrogen levels, magnesium-treated plants (+Mg) showed higher glucose and fructose contents compared with untreated plants (−Mg), except at N3, where the −Mg treatment showed comparable or slightly higher values.

Without magnesium spraying, glucose content increased by 33.69%, 15.20%, and 41.21% under N1, N2, and N3, respectively, while fructose content increased by 42.12%, 36.04%, and 66.35%, respectively (Figure 6H,I). With magnesium spraying, glucose content further increased by 51.16%, 60.25%, and 12.15%, whereas fructose content increased by 68.96%, 99.48%, and 27.39% under N1, N2, and N3, respectively (Figure 6H,I).

The significant interaction between nitrogen and magnesium suggests that the influence of foliar Mg application on fruit sugar composition was dependent on nitrogen nutritional conditions.

Malic acid concentration was significantly affected by magnesium application (p < 0.05), whereas different nitrogen rates showed no significant effect (Table 3). Malic acid content was highest at nitrogen levels up to N1 and gradually decreased with increasing nitrogen supply, reaching the lowest value at N3. Across all treatments, the +Mg treatment consistently resulted in higher malic acid content than the −Mg treatment.

Without magnesium spraying, malic acid content increased by 35.19%, 36.28%, and 13.92% under N1, N2, and N3, respectively. With magnesium spraying, the increases were 52.27%, 45.31%, and 30.83% under N1, N2, and N3, respectively (Figure 6J). However, these differences were not statistically significant compared with the magnesium-sprayed treatments.

The decline in malic acid content with increasing nitrogen levels suggests that excessive nitrogen reduced organic acid accumulation, leading to lower fruit acidity. In contrast, low to moderate nitrogen levels promoted greater organic acid accumulation, contributing to improved fruit acidity and flavor quality.

3.5. Correlation Matrix and Principal Component Analysis (PCA) of Internal and External Quality of Tomato Fruit

A Pearson correlation analysis was conducted to examine relationships between colorimetric parameters (L*, a*, b*, chroma, and hue angle) and biochemical quality attributes of tomato fruit, including vitamin C (VC), titratable acidity (TA), total soluble solids (TSSs), glucose, fructose, malic acid, and amino acids (Figure 7A). Significant correlations were observed between fruit color characteristics and several biochemical traits, indicating that color parameters were closely associated with changes in fruit composition during ripening.

The L* value (lightness) showed variable relationships with fruit quality attributes. Lower L* values were associated with higher concentrations of glucose, fructose, and TSS, whereas higher L* values were generally associated with lower concentrations of these compounds. These relationships suggest that darker fruits tend to have higher levels of soluble sugars and soluble solids.

The a* value, representing the transition from green to red coloration, was positively correlated with glucose, fructose, TSS, VC, and amino acid contents. Similarly, chroma (C*), which reflects color intensity and saturation, showed positive associations with most biochemical quality attributes, particularly soluble sugars, VC, and amino acids. These results indicate that fruits with greater redness and color intensity were generally associated with favorable biochemical quality characteristics.

The b* value showed moderate positive correlations with VC and soluble sugars. However, these associations were weaker than those observed for a* and chroma, suggesting that yellow color development was less closely related to fruit biochemical composition.

In contrast, hue angle (H°) showed strong negative correlations with glucose, fructose, TSS, VC, and amino acids. Lower H° values were associated with higher concentrations of these quality-related compounds, consistent with more advanced fruit ripening.

Titratable acidity (TA) and malic acid showed a negative correlation with a* and chroma and a positive correlation with hue angle. These findings suggest that fruits exhibiting more redness and color vibrancy generally have lower levels of organic acids. Overall, the correlation analysis highlights notable links between external color features and internal quality parameters; nonetheless, these relationships are associations and do not necessarily imply causation.

Principal component analysis (PCA) was conducted to assess relationships among fruit quality traits and treatment responses across nitrogen and magnesium combinations. The first two principal components explained 82.84% of the total variation, with PC1 and PC2 accounting for 72.41% and 10.43%, respectively (Figure 7B). PC1 was positively associated with glucose, fructose, fruit size, titratable acidity (TA), phenols, yield, total soluble solids (TSS), amino acids, and malic acid, indicating that these traits responded in concert to nutrient management and collectively contributed to fruit quality. In contrast, the sugar–acid ratio and color parameters (H* and L*) were negatively associated with PC1, reflecting an opposing response pattern.

From a physiological perspective, the clustering of sugars, amino acids, phenols, yield, and fruit size on the positive side of PC1 suggests coordinated regulation of carbon and nitrogen metabolism. Nitrogen supports protein synthesis and primary metabolism, whereas magnesium is critical for chlorophyll formation, photosynthesis, and assimilate transport. Consequently, treatments with positive PC1 values tended to show enhanced fruit development and biochemical quality.

Among the treatments, N2Mg was strongly linked to fruit size, TA, phenols, yield, and color parameters (a* and b*), suggesting a positive balance between productivity and fruit quality. This pattern indicates that N2Mg enhances effective assimilate production and allocation, supporting both fruit development and metabolite buildup. Conversely, N3Mg showed a stronger connection with TSS and amino acids, implying that higher nitrogen levels favor the buildup of soluble metabolites over increases in yield-related traits. Treatments without nitrogen (N0Mg0 and N0Mg) clustered on the negative side of PC1 and were negatively correlated with most quality parameters, indicating lower metabolic activity and reduced fruit quality.

Overall, the PCA shows that nitrogen and magnesium together affected fruit composition and yield. The strong link between N2Mg and traits related to both yield and quality indicates that this treatment offered the best balance between fruit production and biochemical quality. Conversely, too much nitrogen led to increased metabolite buildup without corresponding improvements in overall productivity.

3.6. Relationships Between a* Color Value and Biochemical Quality Attributes of the Fruit

The peel color parameter a* showed significant positive correlations with all measured internal quality traits, indicating that as tomato fruits developed deeper red coloration, both nutritional quality and eating quality improved.

Glucose content increased significantly with increasing a* values (R² = 0.351, p < 0.001; Figure 8A), indicating that fruits with stronger red coloration accumulated higher levels of reducing sugars. A similar positive relationship was observed for fructose (R² = 0.290, p < 0.01; Figure 8C), suggesting that sugar accumulation progressed simultaneously with color development during ripening. Total soluble solids (TSSs) were also positively correlated with a* (R² = 0.338, p < 0.001; Figure 8F), indicating that fruits with deeper red color generally possessed sweeter taste and superior flavor quality. These findings demonstrate that enhanced redness not only improved fruit appearance and market face score but also reflected better internal quality and consumer acceptability.

Malic acid showed a moderate correlation with a* values (R² = 0.433, p < 0.001; Figure 8B), while free amino acid content also increased with increasing a* values (R² = 0.480, p < 0.01; Figure 8H). These results indicate that the development of deeper red coloration was closely associated with changes in important metabolites related to fruit flavor and nutritional quality. Therefore, fruits with higher a* values not only exhibited improved visual appearance and market face score but also possessed enhanced internal quality attributes that contribute to better taste and nutritional value.

Vitamin C content increased significantly with increasing a* values (R² = 0.457, p < 0.001; Figure 8D), indicating that fruits with deeper red coloration contained higher levels of ascorbic acid. Among all measured traits, flavonoid content showed the strongest relationship with a* (R² = 0.637, p < 0.001; Figure 8E), demonstrating a close association between peel redness and flavonoid accumulation. Total polyphenol content was also positively correlated with a* (R² = 0.416, p < 0.001; Figure 8G), confirming that redder fruits accumulated greater amounts of antioxidant compounds. These findings suggest that enhanced red coloration not only improved fruit appearance and market face score but also reflected the superior nutritional and antioxidant quality of tomato fruits.

4. Discussion

4.1. Magnesium Spraying with Nitrogen Improved Tomato Yield and Biomass

The beneficial effects of combined nitrogen and magnesium application on tomato biomass and fruit yield may reflect the close interaction between carbon and nitrogen metabolism. Nitrogen is an essential component of chlorophyll, amino acids, proteins, and nucleic acids and therefore plays a central role in photosynthesis, vegetative growth, and biomass production. Adequate nitrogen availability increases leaf photosynthetic capacity and assimilate production, thereby supporting fruit development and yield formation. Similar findings were reported in [22], which demonstrated that optimal nitrogen fertilization enhanced photosynthetic activity, biomass accumulation, and fruit yield in tomato.

Magnesium spray also significantly increased tomato biomass and fruit yield. Magnesium is a central component of chlorophyll and plays a vital role in photosynthesis, enzyme activation, and carbohydrate translocation from leaves to fruit. Improved yield from foliar Mg application indicates enhanced photosynthetic and nutrient-use efficiencies. This result aligns with [2], which reported that magnesium fertilization improves crop productivity by enhancing photosynthesis and assimilate transport.

The benefits of balanced N and Mg nutrition may extend beyond tomato production; however, the underlying physiological mechanisms are likely species-specific [7]. In tomato, productivity is strongly governed by source–sink relationships, whereas other greenhouse vegetables may be more strongly influenced by root activity, nutrient uptake capacity, or ionic homeostasis. Therefore, future studies should investigate N × Mg interactions across vegetable crops to develop crop-specific nutrient management strategies for sustainable greenhouse production.

Magnesium may further enhance these processes because it is the central atom of the chlorophyll molecule and serves as a cofactor for numerous enzymes involved in photosynthetic carbon fixation, ATP synthesis, and energy transfer [14,23]. In addition, Mg plays a critical role in phloem loading and in transporting photoassimilates from source leaves to developing sink organs, thereby facilitating carbohydrate allocation to fruits and supporting yield formation [14]. Improved assimilate transport may also strengthen source–sink relationships, which are particularly important in tomato because fruit growth depends on a continuous supply of carbohydrates from photosynthetically active leaves.

The significant interaction between nitrogen and magnesium suggests close coordination between carbon and nitrogen metabolism. Under high nitrogen supply, rapid vegetative growth and increased sink demand may raise the plant’s requirement for Mg to sustain photosynthesis and assimilate translocation. Magnesium also plays a role in nitrate assimilation by regulating enzymes involved in nitrogen metabolism and maintaining the balance between carbon and nitrogen metabolic pathways [1]. These physiological functions may contribute to improved nitrogen use efficiency (NUE), thereby supporting biomass accumulation and fruit production. Similar findings have been reported, indicating that balanced nitrogen and magnesium nutrition enhances crop growth, nutrient use efficiency, and productivity [1].

4.2. Magnesium Application Combined with Nitrogen Improved the External Quality of Tomato Fruit

The improvement in tomato external fruit quality observed with Mg application may be explained by its central role in photosynthesis, assimilate transport, and source–sink regulation. Magnesium is essential for phloem loading and the long-distance transport of photoassimilates from source leaves to developing fruits, thereby ensuring a continuous carbohydrate supply during fruit growth and ripening [13]. Under Mg deficiency, carbohydrate export from leaves is impaired, leading to assimilate accumulation in source tissues and reduced carbohydrate availability in sink organs [14]. Consequently, adequate Mg supply may enhance fruit development and improve quality attributes associated with ripening.

Tomato productivity is strongly influenced by source–sink relationships because fruit growth depends on the efficient allocation of carbohydrates from photosynthetically active leaves to developing fruits [7]. In addition to facilitating assimilate transport, Mg serves as a cofactor for numerous enzymes involved in ATP synthesis, carbon fixation, and carbohydrate metabolism, thereby supporting photosynthetic efficiency and carbon utilization [14]. These physiological processes may explain the improved fruit development and visual quality observed with the combined application of N and Mg.

Fruit color is an important indicator of ripening, external fruit quality, and marketability in tomatoes. Changes in fruit color parameters under different nitrogen and magnesium treatments suggest that nutrient management influences physiological processes associated with fruit maturation. Balanced N and Mg nutrition may support ripening by maintaining chlorophyll formation, photosynthetic activity, carbon metabolism, and assimilate production [22].

The improved red coloration and color intensity observed with Mg application may reflect enhanced photosynthetic performance and more efficient assimilate transport from source leaves to developing fruits. Magnesium plays a key role in phloem loading and the long-distance transport of photoassimilates, thereby supporting carbohydrate supply to sink organs during fruit growth and ripening [13]. Under Mg deficiency, carbohydrate export from leaves can be restricted, leading to sugar accumulation in source tissues and reduced assimilate availability for developing fruits [14]. Therefore, adequate Mg supply may improve fruit maturation by strengthening source–sink relationships and supporting the accumulation of quality-related metabolites.

Tomato productivity and fruit quality depend strongly on source–sink coordination because fruit development requires a continuous supply of carbohydrates from photosynthetically active leaves [7]. Magnesium also serves as a cofactor for enzymes involved in ATP synthesis, carbon fixation, and carbohydrate metabolism, which may further support photosynthetic carbon assimilation and carbon utilization [14]. These functions may explain why Mg application improved fruit coloration and external quality, particularly when paired with an appropriate nitrogen supply.

In contrast, excessive nitrogen supply may promote vegetative growth at the expense of reproductive development, thereby disrupting the source–sink balance, delaying ripening, and reducing color development [2]. Thus, balanced N and Mg nutrition appears important for maintaining proper assimilate partitioning, normal ripening, and desirable external fruit quality. However, because postharvest traits were not directly measured in this study, the potential implications of improved fruit coloration for shelf life, storage performance, and consumer acceptance require further investigation.

4.3. Magnesium Application Combined with Nitrogen Improved the Internal Quality of Tomato Fruit

The improvement in tomato internal fruit quality under combined N and Mg application may be explained by the close coordination between carbon and nitrogen metabolism. Nitrogen is required for the synthesis of amino acids, proteins, and other nitrogen-containing compounds, whereas Mg supports photosynthetic carbon assimilation, ATP production, and carbohydrate translocation [4,14]. The coordinated availability of these nutrients may enhance the accumulation of metabolites associated with nutritional and sensory quality.

The increased vitamin C concentration under balanced N and Mg supply may be associated with enhanced antioxidant metabolism and improved fruit nutritional quality. Similar findings have been reported in tomato, where optimized nutrient management increased ascorbic acid accumulation and improved fruit biochemical properties [24,25]. The significant N × Mg interaction further suggests that adequate Mg availability becomes increasingly important at higher N application rates because of the greater metabolic demand associated with rapid plant growth and fruit development [12].

The reduction in titratable acidity under increasing N supply, particularly with Mg supplementation, may be related to the utilization of organic acids during fruit ripening and respiration. Because the sugar–acid balance is an important determinant of flavor and consumer acceptance, lower acidity at maturity may contribute to improved sensory quality [26]. However, excessive N supply may alter assimilate partitioning and compromise fruit quality if not accompanied by adequate Mg availability.

The increases in free amino acids and total phenolic compounds may also reflect improved coordination between carbon and nitrogen metabolism. Since nitrogen is a structural component of amino acids, increased N availability promotes amino acid biosynthesis, whereas Mg may support these processes by enhancing photosynthetic performance and providing the energy required for protein synthesis and nitrogen metabolism [27]. Similarly, greater phenolic accumulation under Mg application may be associated with enhanced secondary metabolism and antioxidant capacity, particularly under balanced nutrient conditions [28].

Although postharvest traits were not directly evaluated, the observed improvements in fruit composition may have positive implications for storage performance and consumer acceptance. Increased concentrations of sugars, vitamin C, and phenolic compounds may enhance flavor, nutritional value, and oxidative stability during storage, while improved fruit coloration may increase marketability. Nevertheless, these potential benefits should be interpreted cautiously because shelf life, firmness retention, and postharvest physiological changes were not measured in this study. Future studies are needed to directly evaluate the effects of balanced N and Mg nutrition on postharvest quality.

4.4. Magnesium-Mediated Regulation of Nitrogen Assimilation, Amino Acid Biosynthesis, and Nitrogen Use Efficiency in Tomato

Magnesium may regulate nitrogen assimilation by coordinating carbon and nitrogen metabolism. As a central component of chlorophyll and a cofactor for numerous enzymes involved in photosynthesis and ATP production, Mg provides the energy and carbon skeletons required for nitrate reduction and amino acid synthesis [4,14]. Therefore, the beneficial effects of Mg may stem from more efficient physiological use of absorbed nitrogen rather than simply increasing nitrogen uptake.

Nitrogen assimilation depends strongly on the availability of photosynthetically derived carbon skeletons. Magnesium facilitates phloem loading and the transport of photoassimilates from source leaves to developing sink tissues, thereby supporting carbohydrate allocation for amino acid synthesis and plant growth [2,5]. Consequently, increased amino acid accumulation under Mg application may reflect enhanced coordination between carbon and nitrogen metabolism and more efficient incorporation of absorbed nitrogen into organic compounds [27].

The decline in agronomic efficiency, recovery efficiency, and partial factor productivity at the highest nitrogen application rate indicates diminishing returns from excessive nitrogen inputs. Excessive N supply may stimulate vegetative growth, increase nutrient demand, and induce nutritional imbalances that reduce fertilizer use efficiency [6]. In addition, interactions among mineral nutrients may influence Mg availability under intensive fertilization [9]. Although the soil contained sufficient exchangeable Mg, the physiological demand for Mg during rapid biomass accumulation may have temporarily exceeded root uptake capacity. Under these conditions, foliar Mg application may help sustain photosynthesis, nitrate assimilation, and assimilate transport, thereby improving nitrogen use efficiency, particularly under moderate nitrogen supply.

Collectively, these findings suggest that the benefits of Mg are primarily linked to improved physiological nitrogen use and a better carbon–nitrogen balance rather than increased nitrogen uptake alone. These responses may explain the improvements in biomass accumulation, fruit yield, and fruit quality observed under balanced N and Mg nutrition.

5. Conclusions

This study demonstrated that combining foliar Mg application with nitrogen fertilization improved tomato yield, nitrogen use efficiency, and both external and internal fruit quality, even in Mg-sufficient soil. The most favorable responses were observed with the combination of foliar Mg application and N2 (200 kg N ha⁻¹), which increased fruit yield, nutrient accumulation, and fertilizer use efficiency. Magnesium application also improved visual fruit quality by increasing a*, b*, and chroma values while reducing hue angle and L* values. In addition, Mg positively influenced several biochemical quality attributes, including sugars, vitamin C, amino acids, total phenolics, and titratable acidity. Excessive nitrogen application (300 kg N ha⁻¹), particularly without Mg supplementation, negatively affected several fruit quality attributes.

However, the findings of this study should be interpreted in light of its experimental limitations. The experiment was conducted with a single tomato hybrid cultivar ‘Hezuo 903’ at a single location in Wuhan, China, during one growing season. Therefore, the observed responses may be influenced by genotype, environmental conditions, and seasonal variability, and caution should be exercised when extrapolating these findings to other cultivars, production systems, or geographical regions. Additional multi-location and multi-season studies with diverse tomato cultivars are needed to validate the broader applicability of these results and refine nutrient management recommendations.