Silicon Application Methods Differentially Modulate Nutrient Uptake and Morphophysiology in Passiflora edulis Seedlings Under Salt Stress

Abstract

Silicon (Si) is a beneficial element that alleviates the adverse effects of salinity in plants. Despite extensive evidence of Si-mediated stress alleviation in other crops, information for tropical fruit species such as Passiflora edulis remains limited, especially regarding the efficiency of different application methods. This study evaluated two yellow passion fruit cultivars (BRS Sol do Cerrado and BRS Gigante Amarelo) at the seedling stage under five management conditions: irrigation with 1.2 dS m⁻¹ water (control), 4.0 dS m⁻¹ water (salt stress), and salt stress combined with Si applied via soil, foliar spray, or both (soil + foliar), using silicic acid as the Si source. The experiment was conducted in a completely randomized design in a 2 × 5 factorial arrangement with five replicates. High salinity decreased foliar nutrient concentrations, gas exchange, and seedling quality, whereas Si mitigated these effects depending on the application method and cultivar. Combined soil and foliar application increased nutrient contents, biomass, and the Dickson Quality Index, especially in BRS Gigante Amarelo. These findings provide new insights into Si management for P. edulis and offer practical implications for improving nutrient balance, growth, and seedling performance under saline conditions.

1. Introduction

Yellow passion fruit (Passiflora edulis f. flavicarpa Deg.) is one of the most economically important fruit crops in Brazil, with approximately 737 thousand tons harvested in 2024 according to the Brazilian Institute of Geography and Statistics [1]. Its cultivation generates income and employment across tropical and subtropical regions, with strong participation in family-based agriculture and growing demand from the food, beverage, and cosmetic industries [1,2]. The species is ecologically adapted to warm climates with high solar radiation; however, its physiological processes are highly sensitive to saline conditions, with an irrigation water salinity threshold of approximately 1.3 dS m⁻¹ [3]. This sensitivity makes salinity one of the main limiting factors for passion fruit production, especially during the early stages of plant development.

The production of high-quality seedlings is crucial for the success of commercial orchards, as plant uniformity and vigor directly affect productivity and tolerance to abiotic stresses. However, in many producing regions, irrigation water contains elevated salt concentrations, compromising early seedling development and reducing the plants’ productive potential [4,5,6].

Passion fruit is relatively sensitive to salinity [3], which triggers osmotic, ionic, and oxidative stress due to Na⁺ and Cl⁻ accumulation. These effects disrupt mineral nutrition and impair photosynthesis and growth [7,8,9,10]. Such responses, widely documented in cultivated species, highlight the need for effective mitigation strategies.

Among the proposed strategies, silicon (Si) has emerged as a promising agent for alleviating salt stress [4,11,12,13]. Si acts through multiple mechanisms, including restriction of Na⁺ uptake by roots, maintenance of ionic homeostasis, enhancement of nutrient acquisition, reinforcement of cell walls, regulation of water balance, and stimulation of antioxidant activity [12,14,15,16,17,18].

Physiologically and nutritionally, Si modulates nutrient uptake and redistribution among plant tissues, directly influencing morphophysiological traits [19,20,21]. Molecular studies have identified specific transporters, such as Lsi1 and Lsi2, which regulate silicic acid transport and explain interspecific variation in absorption capacity [22,23].

Exogenous Si application methods markedly affect its bioavailability and efficiency in mitigating salt stress. Soil application ensures a continuous supply of silicic acid in the rhizosphere, modifying ionic dynamics and influencing the microbial community, whereas foliar spraying enables direct absorption and rapid physiological responses [13,24,25,26,27]. We hypothesize that combined soil and foliar applications may integrate local and systemic benefits, thereby enhancing nutritional and physiological performance.

In passion fruit, Si application has been reported to improve seedling nutritional status, with increased foliar K, Ca, and Mg and decreased leaf Na in some studies [6,12]. Physiological parameters were also positively affected, including higher relative water content, enhanced gas exchange, and reduced electrolyte leakage [4,11,12]. These nutritional and physiological improvements were subsequently reflected in increased seedling growth and quality. Responses, however, vary according to cultivar, Si source, and application method, and the relative effectiveness of the different application routes in P. edulis remains unclear.

Despite these advances, few studies have systematically compared application routes or explored their combined effects on macro- and micronutrient uptake and morphophysiological traits. Such knowledge gaps constrain practical recommendations for seedling production and hinder the broader adoption of Si as a salt stress mitigation strategy. Therefore, this study aimed to evaluate the effects of different Si application methods (soil, foliar, and combined) on the mitigation of salt stress in yellow passion fruit seedlings, focusing on the relationships among nutritional changes, morphophysiological responses, and seedling quality.

2. Materials and Methods

2.1. Experimental Conditions

The experiment was conducted in a greenhouse from August to October 2023 at the Center for Human and Agricultural Sciences, Paraíba State University, located in Catolé do Rocha, Paraíba State, Brazil (6°21′10.295″ S, 37°43′24.029″ W; 252 m a.s.l.). The average air temperature and relative humidity recorded during the experimental period are presented in Figure 1.

Seeds of two yellow passion fruit cultivars (Figure 2a) were sown in polypropylene trays with cells of 0.0125 dm³ (Figure 2b), filled with a substrate composed of soil and bovine manure (1:1, v/v). One seed was placed per cell. Fifteen days after sowing (DAS), seedlings were transplanted into polyethylene bags containing 5 dm³ of the same substrate (Figure 2c). The physical and chemical properties of the soil and manure used in the substrate are shown in Supplementary Material (Table S1).

2.2. Treatments and Experimental Design

The treatments consisted of two yellow passion fruit cultivars (BRS Sol do Cerrado and BRS Gigante Amarelo) combined with five management conditions involving salinity levels and Si application methods: MC₁ = irrigation with water at 1.2 dS m⁻¹ (control); MC₂ = irrigation with water at 4.0 dS m⁻¹ (salt stress); MC₃ = salt stress + soil-applied Si; MC₄ = salt stress + foliar-sprayed Si; and MC₅ = salt stress + combined Si application (50% soil and 50% foliar). The experiment followed a completely randomized design in a 2 × 5 factorial arrangement with five replicates. Each experimental unit consisted of a single plant.

The saline irrigation water for treatments MC₂, MC₃, MC₄, and MC₅ was prepared by dissolving sodium chloride in water from a shallow dug well (locally known as “Amazon well”) until reaching an electrical conductivity (EC) of 4.0 dS m⁻¹. This level was chosen to induce clear salt stress, using a slightly higher salinity than reported by Almeida et al. [4].

Until 14 days after transplanting (DAT), all plants were irrigated with water from the dug well, which naturally had an EC of 1.2 dS m⁻¹ and the following chemical characteristics: pH = 6.9; Ca²⁺ = 2.5, Mg²⁺ = 1.48, Na⁺ = 6.45, Cl⁻ = 8.1, HCO₃⁻ = 2.75, SO₄²⁻ = 0.18 mmol_c L⁻¹; and Na adsorption ratio = 4.57 (mmol L⁻¹)^0.5. Plants in treatment MC₁ continued to receive this water throughout the experiment. High-salinity irrigation (4.0 dS m⁻¹) began at 15 DAT and was applied daily at 16:00 h (Figure 2d).

The irrigation volume at each event was adjusted to meet plant water requirements over a 24 h period using the drainage lysimetry method. The irrigation amount was estimated as the difference between the applied and drained volumes in the previous event, plus a 10% leaching fraction applied fortnightly.

Silicon was supplied using the commercial product Sifol^® (silicic acid) with the following characteristics: 92% SiO₂ (42.9% Si), bulk density 80–140 g L⁻¹, particle size 8–12 µm, and pH 6.0–7.5. The concentrations used were based on previous experiments from our research group demonstrating their effectiveness in mitigating salt stress in seedlings.

Two Si applications were performed at 12 and 27 DAT (Figure 2e). The treatments were conducted as follows: in MC₃, Si was applied to the soil around the plant collar using 150 mL of a solution containing 0.5 g Si L⁻¹ in distilled water; in MC₄, Si was foliar-sprayed using a solution of 0.2 g Si L⁻¹ until runoff; and in MC₅, a combined application was performed, with 150 mL of a 0.25 g Si L⁻¹ solution applied to the soil and a foliar spray of 0.1 g Si L⁻¹ until runoff.

2.3. Experimental Analyses

At 40 DAT, nutritional, physiological, and growth analyses were performed on yellow passion fruit plants (Figure 2f).

2.3.1. Foliar Nutrient Content Analysis

For nutritional analyses, leaf samples were oven-dried at 65 °C to constant weight, ground using a knife mill, and analyzed according to the methodologies of Tedesco et al. [28] and Meneghetti [29]. The concentrations of carbon (C), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), zinc (Zn), and manganese (Mn) were determined.

C content was determined using the Walkley–Black method, involving oxidation of organic carbon with 5 mL of potassium dichromate in acidic medium, followed by the addition of 10 mL of sulfuric acid and heating for 30 min. After cooling, 10 mL of phosphoric acid and an indicator were added, and the excess Cr⁶⁺ was titrated. N content was determined by sulfuric acid digestion, distillation, and titration following the Kjeldahl method. For P, K, Ca, Mg, Fe, Zn, and Mn, samples were pre-digested using a nitric–perchloric acid solution. P was quantified spectrophotometrically using the molybdenum blue method, K by flame photometry, and Ca, Mg, Fe, Zn, and Mn by atomic absorption spectrometry.

2.3.2. Physiological Traits Analysis

Physiological measurements were taken on the third fully expanded leaf of each plant in the morning (09:00–11:00 h). Stomatal conductance (g_s) and CO₂ assimilation rate (A) were measured using an infrared gas analyzer (IRGA, model CIRAS-3; PP Systems, Amesbury, MA, USA) under a constant photosynthetic photon flux density of 1800 μmol m⁻² s⁻¹ and an airflow rate of 300 mL min⁻¹.

Relative water content (RWC) and electrolyte leakage (EL) were determined from 1 cm diameter leaf discs. For RWC, 10 discs per plant were weighed immediately to obtain fresh mass (FM), then rehydrated for 24 h in distilled water at room temperature to determine turgid mass (TM), and oven-dried at 65 °C for 48 h to obtain dry mass (DM). RWC was calculated using Equation (1).

$$\mathrm{R}\mathrm{W}\mathrm{C}=\mathrm{ }{\displaystyle \frac{\mathrm{F}\mathrm{M}-\mathrm{D}\mathrm{M}}{\mathrm{T}\mathrm{M}-\mathrm{D}\mathrm{M}}}\mathrm{ }\times 100$$

(1)

For EL, five discs per plant were incubated in 10 mL of deionized water for 90 min, and the initial electrical conductivity (ECi) was recorded. Samples were then heated at 80 °C for 90 min, and final conductivity (ECf) was measured. EL was calculated according to Equation (2).

$$\mathrm{E}\mathrm{L}=\mathrm{ }{\displaystyle \frac{\mathrm{E}\mathrm{C}\mathrm{i}}{\mathrm{E}\mathrm{C}\mathrm{f}}}\mathrm{ }\times 100$$

(2)

2.3.3. Seedling Growth and Quality Analysis

Plant height (PH), root length (RL), and plant dry mass (PDM) were measured to assess growth. PH was measured from the substrate surface to the tip of the highest leaf, and RL from the plant collar to the apex of the main root after substrate removal. For PDM, plants were oven-dried at 65 °C to constant weight and weighed using a precision balance (0.01 g).

Seedling quality was evaluated using the Dickson Quality Index (DQI) [30], which was calculated according to Equation (3).

$$\mathrm{D}\mathrm{Q}\mathrm{I}=\mathrm{ }{\displaystyle \frac{\mathrm{P}\mathrm{D}\mathrm{M}}{\frac{\mathrm{P}\mathrm{H}}{\mathrm{S}\mathrm{D}}\mathrm{ }+\mathrm{ }\frac{\mathrm{S}\mathrm{D}\mathrm{M}}{\mathrm{R}\mathrm{D}\mathrm{M}}}}$$

(3)

where

PDM—plant dry mass (g);

PH—plant height (cm);

SD—stem diameter (mm);

SDM and RDM—shoot dry mass and root dry mass (g), respectively.

2.4. Statistical Analysis

Data were tested for normality (Shapiro–Wilk) and homoscedasticity (Bartlett). When assumptions were met, factorial ANOVA was performed. Means of the cultivars were compared using Student’s t-test (p ≤ 0.05), and means of the management conditions (salinity and Si treatments) were compared using the Scott–Knott test (p ≤ 0.05). Principal component analysis (PCA) was conducted in R version 4.5.1 [31] using the FactoMineR package.

3. Results

3.1. Foliar Mineral Element Content

The interaction between passion fruit cultivars and management conditions, including salinity levels and Si application methods, significantly affected foliar concentrations of C (p < 0.0001), N (p < 0.0001), P (p < 0.0001), K (p < 0.0001), Ca (p = 0.0120), Mg (p = 0.0199), Fe (p < 0.0001), Zn (p = 0.0426), and Mn (p < 0.0001), according to ANOVA.

Irrigation with saline water (4.0 dS m⁻¹) generally reduced foliar macronutrient concentrations in both yellow passion fruit cultivars (Figure 3). In BRS Sol do Cerrado, salt stress caused reductions of 6% in C (Figure 3a), 0.3% in N (Figure 3b), 16% in P (Figure 3c), 19% in K (Figure 3d), 13% in Ca (Figure 3e), and 6% in Mg (Figure 3f), relative to the control (1.2 dS m⁻¹). In BRS Gigante Amarelo, the corresponding reductions were 9%, 0.7%, 11%, 29%, 23%, and 5%, respectively.

Certain Si application methods mitigated nutrient losses and promoted macronutrient accumulation under high salinity, with responses depending on the cultivar and application route. In BRS Sol do Cerrado, foliar spraying and combined application (soil + foliar) increased C by 3% and 5%, respectively, compared with plants without Si (Figure 3a). In BRS Gigante Amarelo, all Si application methods enhanced foliar C by 12% (soil), 20% (foliar), and 25% (soil + foliar) relative to non-supplemented plants.

For N (Figure 3b), in BRS Sol do Cerrado under high salinity, only foliar Si application increased foliar N by 0.2% compared with plants without Si. In contrast, in BRS Gigante Amarelo, soil and combined applications increased N by 0.5% and 0.7%, respectively. Regarding P (Figure 3c), in BRS Sol do Cerrado, soil and foliar applications increased P by 36% and 34%, respectively, whereas in BRS Gigante Amarelo, only soil application produced a 27% increase relative to plants without Si. For K (Figure 3d), in BRS Sol do Cerrado, soil, foliar, and combined applications increased K by 21%, 9%, and 10%, respectively, compared with non-supplemented plants. In BRS Gigante Amarelo, only the combined application increased foliar K by 35% relative to the control.

For Ca (Figure 3e), in BRS Sol do Cerrado, soil and combined applications increased foliar Ca by 12% under salt stress compared with plants without Si. In BRS Gigante Amarelo, all Si application methods enhanced foliar Ca by 13% (soil), 19% (foliar), and 20% (combined) relative to non-supplemented plants. Regarding Mg (Figure 3f), soil and combined applications increased foliar Mg in both cultivars under salt stress, by 5% and 8% in BRS Sol do Cerrado and 9% and 13% in BRS Gigante Amarelo, respectively, compared with plants without Si.

Foliar micronutrient concentrations were also influenced by salinity and Si management (Figure 4). In BRS Sol do Cerrado under salt stress, Foliar Fe (Figure 4a) increased by 16% only with soil Si application compared with plants without Si. In BRS Gigante Amarelo under 4.0 dS m⁻¹, foliar spraying and combined application (soil + foliar) increased Fe by 71% and 41%, respectively. For Zn (Figure 4b), all Si application methods increased foliar Zn in both cultivars, by 6%, 13%, and 17% in BRS Sol do Cerrado, and by 12%, 10%, and 8% in BRS Gigante Amarelo for soil, foliar, and combined applications, respectively, relative to non-supplemented plants. Regarding Mn (Figure 4c), only soil application increased foliar Mn by 14% in BRS Sol do Cerrado, whereas in BRS Gigante Amarelo, only the combined application increased Mn by 42% compared with plants without supplementation.

3.2. Physiological Traits

According to ANOVA, the interaction between passion fruit cultivars and management conditions was significant for stomatal conductance (p = 0.0032), CO₂ assimilation rate (p = 0.0067), leaf relative water content (p < 0.0001), and electrolyte leakage (p < 0.0001). All these variables were negatively affected by irrigation with high-salinity water (EC = 4.0 dS m⁻¹) in the absence of Si (Figure 5). In BRS Sol do Cerrado under salt stress, g_s decreased by 58% (Figure 5a), A by 34% (Figure 5b), and RWC by 9% (Figure 5c), while EL increased by 25% (Figure 5d). In BRS Gigante Amarelo, reductions were 52% for g_s, 29% for A, and 10% for RWC, whereas EL increased by 13% compared with the control (1.2 dS m⁻¹).

Silicon application improved gas exchange in both cultivars under salt stress. In BRS Sol do Cerrado at 4.0 dS m⁻¹, g_s increased by 18%, 19%, and 31% with soil, foliar, and combined applications, respectively (Figure 5a). In BRS Gigante Amarelo, g_s increased by 92% with soil application and 49% with foliar spraying compared with stressed plants without Si. Regarding CO₂ assimilation (Figure 5b), all Si application methods enhanced A in both cultivars under high salinity. In BRS Sol do Cerrado, increases in A were 15% (soil), 13% (foliar), and 24% (combined), whereas in BRS Gigante Amarelo, the corresponding increases were 41%, 12%, and 13%.

Relative water content (Figure 5c) and electrolyte leakage (Figure 5d) showed contrasting behaviors between cultivars under saline stress. In BRS Sol do Cerrado, Si application did not significantly change RWC under high salinity, while in BRS Gigante Amarelo, the combined soil + foliar application improved RWC by 17% compared to untreated stressed plants. Regarding membrane stability, in BRS Sol do Cerrado, soil, foliar, and combined Si applications reduced EL by 8%, 10%, and 13%, respectively, compared with non-supplemented plants. In BRS Gigante Amarelo, EL increased by 6% with foliar spraying and 21% with combined application relative to stressed plants without Si.

3.3. Seedling Growth and Quality

Factorial ANOVA revealed that the interaction between cultivars and management conditions significantly influenced plant height (p < 0.0001), root length (p < 0.0001), plant dry mass (p < 0.0001), and the Dickson Quality Index (p = 0.0007). When compared with the control treatment (1.2 dS m⁻¹), both cultivars exhibited marked reductions in all growth and quality parameters under irrigation with high-salinity water (4.0 dS m⁻¹) (Figure 6). In BRS Sol do Cerrado, reductions reached 40% in plant height (Figure 6a), 30% in root length (Figure 6b), 40% in dry mass (Figure 6c), and 43% in DQI (Figure 6d). In BRS Gigante Amarelo, the corresponding decreases were 72%, 27%, 57%, and 27%.

Plant height and root length responded differently to the Si application methods under salt stress (Figure 6a,b). In BRS Sol do Cerrado, soil, foliar, and combined (soil + foliar) applications increased PH by 43%, 67%, and 64%, respectively, compared with high-salinity plants without Si (Figure 6a). In BRS Gigante Amarelo, increases were more pronounced: 206%, 113%, and 68%, respectively, under the same treatments. Similar patterns were observed for root length (Figure 6b), with the combined Si application yielding the greatest increases (32% in BRS Sol do Cerrado and 28% in BRS Gigante Amarelo), followed by soil (10% and 14%) and foliar (8% and 11%) applications.

Plant dry mass (Figure 6c) also improved under salt stress with the application of Si. In BRS Sol do Cerrado, foliar and combined treatments increased dry mass by 49% and 31%, respectively, relative to the saline control. In BRS Gigante Amarelo, Si applied to the soil, leaves, or both increased dry mass by 42%, 54%, and 76%, respectively, showing that combined application provided the greatest benefit for biomass accumulation.

The Dickson Quality Index was consistently affected by salinity and Si application across both cultivars (Figure 6d). Under irrigation with 4.0 dS m⁻¹ water, soil-applied Si increased DQI by 28% in BRS Sol do Cerrado and 41% in BRS Gigante Amarelo relative to non-supplemented plants. When Si was applied via both soil and foliar routes, DQI further increased by 24% in BRS Sol do Cerrado and by 107% in BRS Gigante Amarelo.

3.4. PCA

Principal component analysis explained 61% of the total variance across the first two components (PC1 = 41.3%; PC2 = 19.7%). PC1 distinguished treatments related to improved morphophysiological and nutritional performance (PH, g_s, A, PDM, Ca, N, and Fe) on the positive semiaxis from those associated with salt-induced damage, characterized by higher EL, on the negative semiaxis (Figure 7). PC2 captured a secondary response dimension emphasizing Mg and P on its positive side. Correlation analysis revealed strong positive associations among PH, g_s, A, PDM, Ca, N, and Fe, whereas EL showed negative correlations with these parameters.

The PCA identified seven distinct groups, represented by different colors in the biplot. Control treatments (1.2 dS m⁻¹) of both cultivars were located on the positive side of PC1, near vectors representing growth, gas exchange, and the nutrients Ca and N. Treatments exposed solely to high salinity were positioned toward the negative side of PC1, in association with the EL vector.

Silicon-supplemented treatments were distributed between these extremes, with their positioning influenced by cultivar and application method. In BRS Gigante Amarelo, soil application was closely associated with Mg and P and moderately with Zn, whereas combined application shifted samples toward DQI and RWC vectors, indicating strong positive associations with these traits. Additionally, a moderate positive correlation was observed between GA.Salt.Si.Combined and foliar C content. In BRS Sol do Cerrado, Si-treated plants clustered near the plot’s center, exhibiting reduced EL compared with salt-stressed plants lacking Si supplementation.

4. Discussion

Irrigation with high-electrical-conductivity water (4.0 dS m⁻¹) induced a series of imbalances, evidenced by the reduced foliar macro- and micronutrient contents (Figure 3 and Figure 4), compromised gas exchange and water status (Figure 5), and impaired the growth and morphology of P. edulis seedlings (Figure 6). These responses reflect the classic sequence of salt stress effects: an immediate osmotic phase that limits water uptake and causes stomatal closure, followed by ionic stress from Na⁺ and Cl⁻ accumulation that disrupts nutrient homeostasis and metabolic processes, and a secondary oxidative imbalance that damages membranes and photosynthetic machinery [8,10,32]. Specifically, stomatal limitation can explain the reductions in CO₂ assimilation observed here, whereas ionic imbalance likely underpins the declines in leaf nutrient concentrations and the reductions in growth.

In response to these stressors, supplementation with silicon via different application routes mitigated many of these adverse effects, with responses dependent on both the cultivar and application method, which represents an important finding for agronomic management. At the nutritional level, Si applications (soil, foliar spraying, and especially combined) led to the restoration or increase in foliar contents of C, N, P, K, Ca, and Mg compared with salt-stressed plants without Si (Figure 3), with BRS Gigante Amarelo exhibiting the highest capacity for recovery. These findings align with the literature, where Si is a known agent for enhancing nutrient uptake under saline stress. For instance, Sá et al. [12] observed improvements in P. edulis nutrition under salinity with Si, and Rahmani et al. [16] demonstrated that Si can optimize nutrient uptake in black cumin under double stress (drought and salinity). Similarly, Silva et al. [6] reported that Si supplementation improved both nutrition and soil fertility in P. edulis under saline conditions.

Furthermore, the Si application route was a key determinant of Si efficacy. Foliar and combined applications resulted in more pronounced increases in foliar carbon (Figure 3a) and a more substantial recovery of morphophysiological parameters (Figure 5 and Figure 6) than soil-only application. This pattern is consistent with Si uptake kinetics under salt stress [33]. Wadas and Kondraciuk [34] and Queiroz et al. [13] reported effective absorption of soluble Si through the leaf surface, whereas Babu et al. [26] and Tripathi et al. [21] highlighted the complementary benefits of root uptake and rhizosphere modification. Dutra et al. [25] demonstrated that combining soil and foliar applications improved C:N:P stoichiometry and enhanced carbon and phosphorus utilization in field-grown sugarcane. Overall, combined application provides additive benefits [35], with foliar Si delivering immediate protection and root-applied Si promoting long-term effects on root structure and rhizosphere chemistry. These findings are therefore fully consistent with the results of the present study.

Beyond nutrition and uptake dynamics, at the physiological level, increases in stomatal conductance (Figure 5a) and CO₂ assimilation rate (Figure 5b) in salt-stressed seedlings supplemented with Si suggest the preservation of the photosynthetic apparatus, as also reported by Khan et al. [14] in various crops. Maintained stomatal aperture and higher assimilation rates under Si supply likely reflect osmotic adjustment and protection of chloroplast ultrastructure from oxidative damage [14,36]. Regarding membrane stability, the observed reductions in electrolyte leakage (Figure 5d), particularly in BRS Sol do Cerrado, indicate reinforcement of the cell wall through Si deposition associated with hemicelluloses, pectins, and phenolic compounds [17,37], an effect observed only in this genotype. The contrasting EL response observed in BRS Gigante Amarelo, which did not show the same degree of improvement, may reflect genotypic variation in Si accumulation and redistribution or dose- and time-dependent effects, a critical point of divergence that warrants emphasis.

The increases in K (Figure 3d), Ca (Figure 3e), and Mg (Figure 3f) in Si-supplemented plants, especially in BRS Gigante Amarelo, suggest an ionic regulatory mechanism where Si favors K⁺ retention and mitigates Na⁺ competition, thereby enhancing ionic homeostasis under salinity [38,39]. Although the Na⁺/K⁺ ratio was not directly quantified, the observed increase in K along with other physiological improvements implies a potential enhancement of the Na⁺/K⁺ balance that is vital for ionic regulation. Foliar K enrichment is crucial for stomatal regulation, protein synthesis, and turgor maintenance [40], while higher Ca and Mg levels stimulate enzyme activity, support protein and nucleic acid synthesis, and enhance Rubisco performance, contributing to greater salt tolerance and photosynthetic efficiency [41,42,43].

Collectively, the integrated nutritional and physiological responses explain the observed increases in plant height (Figure 6a), root length (Figure 6b), plant dry mass (Figure 6c), and Dickson Quality Index (Figure 6d), particularly under combined Si application. We hypothesize that enhanced root growth under Si improved water and nutrient foraging capacity, while foliar protection sustained carbon assimilation, resulting in higher biomass accumulation. Similar outcomes were reported by Souza et al. [44], Sá et al. [12], and Silva et al. [6] in passion fruit under saline conditions, reinforcing Si’s role as an effective mitigating agent.

The PCA (Figure 7) validated and synthesized these trends, demonstrating that salinity imposes a multivariate stress pattern characterized by nutritional, physiological, and growth impairments. The shift of Si-supplemented treatments toward positive PC1 values reflects the simultaneous recovery of nutrition, gas exchange, and growth, emphasizing the superior integrative effect of the combined application. The association of soil-applied Si with Mg, P, and Zn in BRS Gigante Amarelo suggests a specific rhizospheric mechanism for this cultivar that complements rapid foliar absorption, explaining the superior performance of combined applications over single routes. This multivariate evidence supports a mechanistic model where root- and leaf-targeted Si interventions act through distinct yet complementary pathways to restore homeostasis under salinity.

The proximity of combined Si application treatments to RWC and DQI vectors clearly indicates that integrating foliar and root pathways improves water balance and overall seedling quality. In BRS Sol do Cerrado, the marked reduction in EL demonstrates that maintaining membrane integrity is a key Si-mediated response only for this genotype. In contrast, BRS Gigante Amarelo did not exhibit improved membrane stability; instead, nutritional recovery was more pronounced. Such diferences in EL response underscore the need for genotype-specific recommendations and for future work to quantify tissue Si, transporter expression, and temporal dynamics of response.

Overall, Si proved to be an effective strategy for alleviating salinity effects in P. edulis, maintaining nutritional balance, cellular integrity, water status, and vegetative growth. Among the evaluated methods, combined Si application achieved the best performance by integrating fast foliar protection with sustained root-zone benefits. However, as the present study was conducted under greenhouse conditions and at the seedling stage, field trials and longer-term assessments are required to test persistence of benefits through reproductive stages and fruit production. Future research priorities include direct quantification of Si uptake and partitioning, biochemical assays of oxidative status, and molecular analyses of Si transporters to establish causal mechanisms and to enable genotype-tailored Si management.

5. Conclusions

Irrigation with saline water (4.0 dS m⁻¹) reduced foliar nutrient concentrations and impaired the morphophysiological performance and seedling quality of P. edulis. However, exogenous silicon application mitigated these adverse effects by enhancing nutrient accumulation, improving gas exchange, and increasing growth and quality, with the combined method (soil + foliar spraying) being the most effective. Genotypic variation was evident, as BRS Gigante Amarelo responded more strongly than BRS Sol do Cerrado, indicating that both cultivar and application route must be considered in Si management during seedling production. Future studies should investigate the molecular mechanisms underlying Si-mediated tolerance to salt stress, as well as the long-term effects of Si application under field conditions and different salinity levels. In practical terms, the combined soil and foliar Si application can be recommended as an efficient strategy to improve the salt stress tolerance and overall quality of passion fruit seedlings.