Effectiveness of Grafting in Enhancing Salinity Tolerance of Tomato (Solanum lycopersicum L.) Using Novel and Commercial Rootstocks in Soilless Systems

Abstract

The scarcity of high-quality water in arid regions like Saudi Arabia necessitates saline water use in irrigation. Sustainable techniques, such as grafting and soilless cultivation, enhance crop resilience and optimize resource use, ensuring long-term agricultural and water sustainability to meet rising food demand. So, this study evaluated grafting’s effectiveness in enhancing the salt tolerance of tomato (Solanum lycopersicum L.) under soilless culture. The experiment tested two salinity levels, two growing media (volcanic rock and sand), and six grafting treatments: the scion ‘Tone Guitar F1’ was cultivated through non-grafting (G1), self-grafted onto itself (G2), and grafted onto the commercial rootstock ‘Maxifort F1’ (G3), which was grafted onto three newly developed rootstocks, namely X-218 (G4), X-238 (G5), and Alawamiya365 (G6). The results indicated that plants performed better at 2 dS m⁻¹, while higher salinity (4 dS m⁻¹) negatively impacted growth. However, grafting under saline stress improved most of the measured traits, excluding fruit quality (vitamin C, titratable acidity, and total soluble sugars). Grafted plants (G2–G6), particularly those grown in volcanic rock at 2 dS m⁻¹, exhibited superior fruit characteristics, yield, water productivity, and leaf calcium (Ca²⁺) and potassium (K⁺) content compared to the non-grafted controls (G1). The sand medium generally produced lower values for all the traits, regardless of salinity or grafting. Moreover, grafting under 2 and 4 dS m⁻¹ reduced leaf sodium (Na⁺) and chloride (Cl⁻). The best overall performance was provided by the rootstocks X-218 and X-238. Grafting onto salt-tolerant rootstocks is a promising strategy for improving tomato yield and water productivity under saline irrigation in arid and semi-arid regions.

1. Introduction

Tomato (Solanum lycopersicum Mill.), a member of the Solanaceae family, stands as one of the most extensively cultivated and economically vital vegetable crops globally [1]. Cultivated annually for both fresh consumption and processing [2], tomatoes are rich in lycopene, phenolics, and flavonoids, recognized for their chemoprotective properties and crucial role in mitigating chronic diseases. Furthermore, they exhibit high concentrations of essential minerals and antioxidants [3]. Its cultivation spans numerous countries, encompassing open-field and greenhouse environments, utilizing soil-based and soilless cultivation techniques [1]. Global tomato cultivation spans approximately 5 million hectares, yielding an average of 36.98 tons of fruit per hectare. In 2021, global tomato production reached a substantial 189 million tons [4]. In 2022, Saudi Arabia allocated approximately 15.208 hectares for tomato cultivation, producing 658.540 thousand tons. Notably, protective cultivation contributed significantly, amounting to 284.981 thousand tons, constituting 43.3% of national tomato production [5].

The global population is projected to reach 9.7 billion by 2050 [6,7], necessitating a 50% increase in food production by 2030 and a 70% increase by 2050 to meet the growing demand [8]. However, this rising demand for food coincides with a steady decline in arable land due to factors such as population growth, industrial expansion, urbanization, desertification, and climate change, with a projected loss of one-third of the currently available agricultural land [9,10,11]. This challenge is particularly pronounced in arid regions, where limited freshwater availability has led to an increasing reliance on saline water for agricultural irrigation. Saudi Arabia exemplifies this trend due to its inherent scarcity of freshwater resources. In such environments, salinity in soil or irrigation water poses a serious threat to sustainable agriculture, affecting approximately 20% of irrigated land worldwide [12]. Furthermore, over 90% of food crops are negatively impacted by moderate levels of salinity [13]. Vegetables, in particular, require more frequent irrigation than many other crops, and their water demands increase significantly in arid and semi-arid regions, due to the combined pressures of harsh environmental conditions and elevated salinity [14]. Among these crops, cultivated tomatoes (Solanum lycopersicum L.), which are classified as moderately salt sensitive, are especially vulnerable to the detrimental effects of salt stress [15,16]. The primary physiological damage caused by salinity includes increased osmotic pressure around the roots, which inhibits water and nutrient uptake by the plant [14]. Additionally, the prolonged accumulation of sodium chloride in plant tissues results in the premature senescence of older leaves [17]. Numerous studies have demonstrated that salinity disrupts key physiological and biochemical processes in plants, leading to morphological changes that ultimately reduce crop productivity [15,18,19].

Sustainable intensification represents a promising strategy to address the growing challenges facing global food production. This approach aims to increase agricultural output, while simultaneously preserving the ecological health of farming systems. One effective example of this strategy is the adoption of protected, soilless cultivation systems (SCS), which optimize resource use and reduce environmental pressures [20]. Soilless cultivation is recognized as a cornerstone of sustainable protected horticulture, particularly in resource-limited and high-salinity environments [21].

In this context, grafting is increasingly recognized for its contribution to sustainable crop production. By enabling the cultivation of salt-sensitive, yet high-yielding, cultivars under saline conditions, grafting offers a reliable strategy to maintain productivity in adverse environments [12]. The effectiveness of this technique depends largely on the compatibility between the scion and the rootstock, which enhances the plant’s resilience to stress factors [22]. Its broad adoption underscores its relevance as a sustainable approach for improving crop performance under challenging environmental conditions [23,24].

This practice is widely applied in the cultivation of fruiting vegetables, where both the rootstock and scion collectively contribute to the plant’s ability to withstand salt stress [25,26]. Several studies have demonstrated that grafting in tomato can enhance plant vigor, promote earlier maturity, and improve tolerance to various abiotic stresses, including salinity, depending on the genetic characteristics of the rootstock used [27,28].

Tomato grafting onto intra- or interspecific rootstocks from the Solanaceae family offers a valuable strategy for enhancing plant tolerance to various stresses. This technique benefits from the rich genetic diversity found in wild tomato species, which serve as a reservoir of resistance traits against a broad range of biotic stresses (such as viruses, fungi, bacteria, and nematodes) and abiotic stresses, including salinity and drought [15,29,30]. Notably, while domesticated tomato cultivars exhibit limited genetic variability for salt tolerance, several wild species, such as S. pimpinellifolium, S. habrochaites, S. peruvianum, S. chilense, and S. pennellii, possess valuable traits that confer enhanced salt tolerance [31,32]. Harnessing these wild species as salt-resistant rootstocks to graft onto commercially desirable, yet salt-sensitive, tomato cultivars presents a promising approach for improving salt tolerance in modern tomato production systems [33]. Moreover, the use of indigenous genetic resources in rootstock selection is expected to yield more effective and context-specific solutions to regional agricultural challenges than the use of imported cultivars. Local genotypes often exhibit inherent adaptability to the prevailing environmental conditions, shaped by generations of natural selection. In addition to their ecological suitability, the significant cost advantage of local cultivars over commercial hybrids further underscores the economic feasibility of utilizing native genetic resources [20].

This study aimed to evaluate the potential of grafting tomato scions onto novel and commercial rootstocks to enhance salt tolerance in soilless cultivation systems.

2. Materials and Methods

2.1. Experimental Site

This study was carried out in a greenhouse at the National Center for Agricultural and Animal Research Farm, located in the Al-Kharj Governorate of Saudi Arabia, from November 2022 to June 2023. The farm is located 117 km south of Riyadh (24°201 N, 47°382 E, 438 m a.s.l.; Figure 1).

2.2. Experimental Design

This experiment involved 24 treatment combinations, incorporating two irrigation water quality levels. The two water quality levels were: (1) 2 dS m⁻¹, which was obtained by preparing a nutrient solution using desalinated water, resulting in an EC of 2 dS m⁻¹, deemed ideal for the growth of tomato [34]. (2) The second treatment involved adding NaCl to the same nutrient solution to increase the EC to 4 dS m⁻¹, simulating salt stress, since this salt is one of the dominant salts in deep well water in the study area. The target salinity level of 4 dS m⁻¹ exceeds the known salinity threshold for tomato, as reported by Rosadi et al. [35]. Although an EC of 2 dS m⁻¹ does not induce salinity stress, we refer to both treatments as “salinity levels”, since they differ in terms of their total salt concentration.

Two growing media, sand and volcanic rock, were used to evaluate tomato plant performance. Five grafting treatments were applied, in addition to a non-grafted control. The commercial tomato variety, ‘Tone Guitar F1’, commonly cultivated in Saudi Arabian greenhouses, was used as the scion. Four different rootstocks were used for grafting: the local variety ‘Alawamiya365’ (provided by the Ministry of Environment, Water and Agriculture, Seed Centre and Plant GenBank, KSA), the interspecific hybrids ‘X-218’ and ‘X-238’ (derived from crosses between Solanum lycopersicum and S. pimpinellifolium, supplied by King Abdullah University of Science and Technology, KSA), and the commercial rootstock ‘Maxifort F1’ (De Ruiter, Bleiswijk, The Netherlands). Additionally, ‘Tone Guitar F1’ was self-grafted, and non-grafted plants served as the control. The experiment followed a factorial randomized complete block design (RCBD), with three replicates. The design included two levels of irrigation water quality, two growing media, and six grafting treatments (including the control). The layout comprised twelve rows, with every three consecutive rows allocated to one of the two growing media. Each experimental unit consisted of five tomato plants grown in Dutch buckets (23 × 31 × 25 cm) and spaced 40 cm apart, within rows that were 1 m apart. The experimental units were randomly distributed within each block (Figure 2). Given that microclimate variability within a greenhouse can influence plant growth and physiological responses, two key strategies were implemented in the experimental design to ensure proper randomization and minimize positional effects. First, the growing media alternated across the three blocks: for example, pots positioned on the right side in one block were placed on the left in the next block, and vice versa. This approach allowed for even distribution of the growing environments throughout the greenhouse and reduced location-based microclimatic variation. Second, grafting treatments, the main experimental factor potentially sensitive to microclimatic differences, were randomly assigned within each block. Each growing medium in each block contained all six grafting treatments, with five replicates per treatment. Randomization was carried out independently in each block. This design ensured that each grafting treatment experienced comparable environmental conditions and satisfied the assumptions of an RCBD, which is commonly employed in controlled-environment studies to account for spatial variability (Figure 2).

2.3. Grafting of the Plants

Successful grafting requires similar rootstock and scion stem diameters [36]. The initial germination studies indicated that the germination rates of local rootstock varieties (‘Alawamiya365’, ‘X-218’, and ‘X-238’) were slower than those of the ‘Maxifort F1’ rootstock and the ‘Tone Guitar F1’ scion [37]. Consequently, local rootstock seeds were sown on 29 November 2022, followed by ‘Maxifort F1’ seeds five days later, and ‘Tone Guitar F1’ seeds three days later, in a polycarbonate-covered greenhouse. Furthermore, the seedlings were initially grown in Giffy-7 discs and then transplanted into peat moss-filled pots, at a height of 10 cm. Then, on 15 December, the simple tube grafting technique was employed to select seedlings with three to four true leaves and uniform stem diameters for grafting [38]. During this process, the lower part of the scion and the upper portion of the rootstock were removed, and the two segments were secured together using a grafting clip (Figure 3) [39]. Lastly, post-grafting, the seedlings were transferred into a healing chamber under controlled conditions (23 °C, 80% relative humidity, and 45% shading) for 10 days to facilitate recovery, following previously established protocols [39,40,41].

2.4. Experimental Site Preparation and Plant Transplanting

The trial site was equipped with bucket pots and a drip irrigation system involving 16 mm GR pipes that delivered 4 L of water per hour. In addition, two 1000 L tanks were added to the irrigation network and controlled by digital timers (model EMT769A, GAO brand, built in China), which were purchased from a local store in the Riyadh region.

Sand and volcanic rocks were used as the native growing media, due to their low cost and ease of accessibility. After sieving the sand to obtain the largest particle size, Virkon S disinfectant was used to wash and sterilize it. These media were then placed in buckets. Vigorous, uniformly sized seedlings (with six leaves) were meticulously chosen and moved to the greenhouse on 8 January 2023. The temperature ranged from 23 to 27 °C during the day and from 18 to 22 °C at night, with 75% relative humidity. It is necessary to add that the temperature and relative humidity inside the greenhouse were regulated using standard climate control systems.

2.5. Nutrient and Irrigation Water Solutions

Desalinated water with a salinity of 0.4 dS m⁻¹ was utilized at the experiment site. The nutrient solution was prepared following Cooper’s full-strength formulation, as mentioned by Shah et al. [42], composed of (mg L⁻¹) N236, P 60.0, K 300, Ca 185, Mg 50, S 68, Fe (EDTA) 12, Mn 2.0, Zn 0.1, Cu 0.1, B 0.3, and Mo 0.2. The pH of the nutrient solution was maintained between 5.5 and 6.5 with regular monitoring. To prevent salt accumulation, a leaching protocol was employed, according to which, every fifth day, the systems were irrigated continuously for an entire day, using desalinated water, free of added nutrients. Additionally, the electrical conductivity of the drainage water was routinely monitored to ensure it remained within acceptable thresholds and to mitigate the risk of excessive salt buildup.

2.6. The Measurements

2.6.1. Physicochemical Properties of Tomato Fruits

For each experimental unit, 10 fully ripe tomato fruits were selected at random during the height of the harvest period. A digital weighing scale was used to determine the average fresh and dry weights, while calipers were used to measure the length and diameter of the fruit. The dry weight was measured after oven drying at 65 °C for 72 h, until the weight stabilized [43]. After rinsing with both fresh and distilled water and deseeding, the three fruits were cut into slices. The juice was extracted from the slices with an electric blender. The juice’s total soluble solids (TSS), total acidity (TA), and vitamin C (VC) were measured. The total soluble solids were measured using a digital refractometer (PR-101 model, (PR-101 model, ATAGO CO., LTD., Minato-ku, Tokyo, Japan), ATAGO, and the methods used for determining the vitamin C and total acidity followed the procedure described by A.O.A.C [44]. Vitamin C was determined by titrating it with 2,6-dichlorophenolindophenol dye. As a proportion of citric acid in fruit juice, the titratable acidity was calculated through titration with 0.1 M NaOH to pH 8.1.

2.6.2. Chemical Content of Tomato Leaves

Tomato leaf samples were analyzed for calcium (Ca²⁺), potassium (K⁺), sodium (Na⁺), and chloride (Cl⁻) content, following the digestion procedure [45]. An electric grinder was used to finely grind 100 g of leaf samples, after they had been oven dried. A 100 mL glass tube filled with 10 mL of concentrated H₂SO₄ was filled with 0.2 g of the ground-up material. Hydrogen peroxide (H₂O₂) was then progressively added to the tubes while they were being digested using a heating unit, until the sample turned transparent. Distilled water was added to the digested sample to bring the total volume to 100 mL. The concentrations of calcium (Ca²⁺), potassium (K⁺), and sodium (Na⁺) were determined using a flame photometer (Model 1382/1385, S/No. 1403149, London, UK), as described by Chapman and Pratt [46]. The chloride (Cl⁻) content was determined following the method by Yeo et al. [47].

2.6.3. Yield and Water Productivity (WP)

All the harvested fruits were weighed with a digital scale to estimate the total yield (kg m⁻²) throughout the harvest period. Additionally, the number of fruits was recorded (fruit m⁻²). Water productivity (WP) was calculated as the ratio of the crop yield (Y, kg m⁻²) to the amount of applied water (W, m³ m⁻²) [48].

$$\mathrm{W}\mathrm{P}={\displaystyle \frac{\mathrm{Y}}{\mathrm{W}}}$$

(1)

2.7. Data Analysis

Statistical software was utilized to analyze the data. Variance analysis (ANOVA) was conducted to compare the data statistically, and the least significant difference (LSD) test was performed at a significance level of 0.05, following the procedure outlined by Steel and Torrie [49].

3. Results

3.1. Physicochemical Characteristics of Tomato Fruits

According to the results of the triple interaction effect between the salinity levels, growing media, and grafting on the physical properties and content of tomato fruits in terms of some chemical compounds (fruit quality characteristics), there was a significant increase in the physical fruit traits (Figure 4a–d) of grafted plants (G2–G6), such as fruit diameter (mm), fruit length (mm), fresh fruit weight (g), and dry fruit weight (g), under both salinity levels (2 and 4 dS m⁻¹) and grown in both growing media (volcanic rock and sand) compared to the non-grafted treatments. The best values were obtained under a salinity of 2 dS m⁻¹, with the highest values observed in plants grown in the volcanic rock medium, with the ‘Tone Guitar’/X-238 grafting treatment (G5). Conversely, the qualitative parameters of tomato fruits, vitamin C (VC%), total acidity (TA%), and total soluble solids (TSS%) declined when the salinity level was lowered to 2 dS m⁻¹ in both growing media (volcanic rock and sand). The non-grafted ‘Tone Guitar—control’ treatment plants (G1) grown in the sand medium showed the highest values for fruit quality characteristics (Figure 5a–c).

3.2. Chemical Content of Tomato Leaves

Increasing the irrigation water salinity to 4 dS m⁻¹ had a negative effect on the calcium (Ca²⁺) and potassium (K⁺) content of tomato leaves grown in both culture media (Figure 6a,b), while conversely, it increased the sodium (Na⁺) and chloride (Cl⁻) content (Figure 6c,d). Grafting, however, reduced Na⁺ and Cl⁻ content and increased Ca²⁺ and K⁺ content in the leaves. Compared to the other grafting treatments, the G5 grafting treatment resulted in the highest Ca²⁺ and K⁺ content and the lowest Na⁺ and Cl⁻ content in tomato leaves grown in the volcanic rock growth medium under both salinity levels (Figure 6).

3.3. Yield and Water Productivity of Tomato Plants

Plants that were self-grafted or grafted onto different rootstocks (G2–G6) in comparison to the non-grafted control group (G1) showed notable increases in yield and water productivity. G5 performed the best in these areas out of all the examined rootstocks (Figure 7). Notably, the G5 grafting treatment under 4 dS m⁻¹ conditions and in the volcanic rock growth medium resulted in the highest average yield (16.9 kg⁻²), the greatest number of fruits (119 m⁻²) (Figure 7a), and the highest water productivity (71.8 kg⁻³) (Figure 7c). This yield demonstrates a significant 50.9% improvement compared to the lowest recorded yield (8.6 kg⁻²), which was observed in the non-grafted control treatment (G1) under saline conditions, and when using sand as an alternative growth medium.

3.4. Pearson’s Correlation Coefficient Between All the Studied Parameters Under Different Salinity Levels

Table 1. Pearson’s correlation matrix for different parameters (Par.) of physicochemical properties of fruits, yield, water productivity, and chemical content of the leaves under the two salinity levels, 2 dS m⁻¹ (upper right) and 4 dS m⁻¹ (lower left).

Variables	1	2	3	4	5	6	7	8	9	10	11	12	13	14
(1) Fruit diameter (mm)		0.95	0.93	0.83	0.01	−0.35	−0.34	0.10	0.91	0.91	0.77	0.70	−0.73	−0.82
(2) Fruit length (mm)	0.91		0.98	0.79	−0.05	−0.33	−0.33	0.09	0.92	0.92	0.75	0.63	−0.70	−0.80
(3) Fresh fruit weight (g)	0.94	0.96		0.85	−0.09	−0.41	−0.41	0.02	0.95	0.95	0.83	0.71	−0.78	−0.87
(4) fruit dry weight (g)	0.80	0.68	0.83		−0.18	−0.73	−0.65	0.47	0.83	0.83	0.98	0.96	−0.96	−0.97
(5) Vitamin C (%)	−0.52	−0.40	−0.59	−0.77		0.66	0.80	−0.12	−0.13	−0.13	−0.16	−0.16	0.13	0.14
(6) Total acidity (%)	−0.19	−0.24	−0.33	−0.29	0.76		0.94	−0.54	−0.31	−0.31	−0.75	−0.80	0.77	0.68
(7) Total soluble solids (%)	−0.32	−0.30	−0.44	−0.56	0.91	0.90		−0.45	−0.27	−0.27	−0.66	−0.69	0.66	0.64
(8) Fruit number (m−2)	0.28	0.08	0.23	0.63	−0.51	−0.07	−0.35		0.11	0.11	0.40	0.58	−0.44	−0.34
(9) Yield (m−2)	0.88	0.90	0.96	0.89	−0.63	−0.28	−0.48	0.39		1.00	0.81	0.72	−0.77	−0.84
(10) WP (kg m−2)	0.88	0.90	0.96	0.89	−0.63	−0.28	−0.48	0.39	1.00		0.81	0.72	−0.77	−0.84
(11) Ca meq/100 g DW	0.77	0.63	0.80	0.98	−0.78	−0.30	−0.58	0.60	0.85	0.85		0.97	−0.99	−0.98
(12) K meq/100 g DW	0.72	0.54	0.70	0.95	−0.75	−0.21	−0.52	0.64	0.78	0.78	0.98		−0.98	−0.94
(13) Cl meq/100 g DW	−0.82	−0.72	−0.84	−0.98	0.66	0.14	0.42	−0.54	−0.89	−0.89	−0.97	−0.94		0.97
(14) Na meq/100 g DW	−0.83	−0.69	−0.84	−0.96	0.81	0.36	0.61	−0.50	−0.88	−0.88	−0.97	−0.94	0.94

Values in bold are different from 0, with a significance level alpha = 0.05.

presents the results of Pearson’s correlation analysis between various physical and chemical characteristics of the fruit, yield, water productivity, and leaf content in terms of selected chemical elements under two salinity levels (2 dS m⁻¹ and 4 dS m⁻¹). All the measured physical characteristics of the fruit under both salinity levels exhibited strong positive correlations with each other (r = 0.68–0.96). These characteristics also showed strong positive correlations with the yield, water productivity, and leaf calcium and potassium content (r = 0.63–0.98). Similarly, the chemical characteristics of the fruit (quality traits) were positively correlated with leaf sodium and chloride content (r = 0.61–0.81). In contrast, these fruit quality traits and leaf sodium and chloride content showed strong negative correlations with the remaining measured traits under both salinity levels.

3.5. Heatmap Analysis of Measured Traits in Tomato Plants

Figure 8 presents a heatmap cluster analysis of the measured traits, revealing two primary groups (1 and 2), based on the treatment responses. In Group 1, corresponding to the low salinity level (S1), the fruit quality parameters (vitamin C, titratable acidity, and total soluble sugars) and leaf sodium (Na⁺) and chloride (Cl⁻) content exhibited lower values (light blue). Conversely, the remaining measured traits showed higher values (orange), with the most pronounced effects observed in treatments M1G4 and M1G5 (3). In contrast, Group 2, representing the high salinity level (S2), showed increased fruit quality parameters and leaf sodium and chloride content. Concurrently, this group’s other measured traits decreased, with treatments M2G1, M2G2, and M2G6 (4) exhibiting the lowest values.

4. Discussion

The study results indicate that the physical characteristics of tomato fruits are significantly influenced by increased irrigation water salinity, particularly at the level of 4 dS m⁻¹. This impact can be attributed to stress induced by salinity. Previous research has demonstrated that elevated salinity levels adversely affect these traits, resulting in a noticeable decline in both fresh and dry fruit weights [50,51,52]. Furthermore, salinity stress has been shown to significantly reduce fruit dimensions, including length and diameter, as reported in several studies [53,54,55,56,57]. The salinity of irrigation water has been shown to positively influence the quality characteristics of tomato fruit. Our findings revealed a significant increase in vitamin C, total acidity, and total soluble solids in tomatoes cultivated under higher salinity conditions (4 dS m⁻¹). Conversely, these quality parameters were significantly diminished when lower salinity irrigation water (2 dS m⁻¹) was employed. This observation aligns with a body of research that corroborates the beneficial effects of irrigation water salinity on tomato fruit quality [54,55,58,59,60].

Grafting’s influence of tomato fruit’s physical characteristics has been documented in various studies. The improvements observed are potentially attributable to rootstock–scion interactions, which can lead to an expanded root system. This enhanced root architecture may increase water and nutrient uptake, promoting photosynthetic activity. Bie et al. [39] suggested that grafting stimulates scion activity and improves resource use efficiency (e.g., water and fertilizer). These physiological enhancements can ultimately contribute to improved fruit characteristics. These findings corroborate previous research demonstrating enhanced fruit characteristics in grafted plants compared to non-grafted controls [61,62,63]. In contrast to its potential benefits, grafting did not improve the fruit quality traits in the current study. Instead, the findings revealed a significant decline in the quality characteristics of fruits from grafted plants compared to those from non-grafted treatments. This observed reduction in quality traits is consistent with previous research, which similarly reported a lack of enhancement in fruit quality as a result of grafting [12,60,64].

The effect of irrigation water salinity on the tomato yield revealed a significant decline in the key yield parameters at the 4 dS m⁻¹ salinity level. This observation aligns with the literature documenting yield reductions under elevated salinity conditions, primarily due to a decrease in fruit number and overall productivity [53,55,64,65]. As Passam et al. [66] mentioned, tomatoes can withstand salt concentrations in the root zone of up to 2.5–2.9 dS m⁻¹ without affecting their yield losses; our findings corroborate the detrimental effects of exceeding this range. Salt stress, particularly during critical reproductive stages like flowering and fruiting, has been linked explicitly to a reduced fruit set [67]. Furthermore, the mechanisms by which salinity stress compromises yield have been explored. Sun et al. [68] highlighted the role of increased ovule abortion and premature embryo senescence in reducing the fruit number, thus directly impacting overall productivity. Salt stress also disrupts fundamental physiological processes, including photosynthesis, nutrient and water uptake, and root development, all of which contribute to yield reductions [69]. Conversely, plants irrigated with 2 dS m⁻¹ water exhibited significantly improved performance, likely because this salinity level falls within the acceptable tolerance range for tomatoes. This observation supports the findings by R. A. Bustomi Rosadi [35], who reported a salinity tolerance threshold of 3 dS m⁻¹ for this crop. Our research also demonstrated a negative correlation between salinity and water productivity, consistent with the findings by Al-Harbi et al. [70], who observed reduced fruit yield and water use efficiency with saline irrigation. Other researchers have further substantiated this decline in water use efficiency under increasing salinity [20,71,72].

Grafting tomato plants has been shown to enhance both productivity and water productivity compared to non-grafted plants. This improvement in productivity traits under salt stress conditions includes an increased fruit number and total yield [73]. The enhancement in water productivity aligns with findings from previous studies that have reported a positive impact of grafting on water use efficiency in tomato plants [36,74]. In a study by Madugundu, Al-Gaadi, Tola, Patil, and Sigrimis [72], the effects of varying salinity levels on three tomato varieties grafted onto Maxifort rootstock were studied. It was found that grafting, when paired with an appropriate nutrient solution, boosts water use efficiency and productivity, helping to alleviate the negative impact of salt stress on hydroponic tomatoes. Singh, Kumar, Kumar, Kyriacou, Colla, and Rouphael [12] suggested that grafting salt-sensitive, high-yielding tomato varieties onto salt-tolerant or salt-resistant rootstocks represents a sustainable approach to managing salt stress.

The results reveal a significant impact of irrigation water salinity on leaf chemical composition, specifically concerning calcium (Ca²⁺), potassium (K⁺), sodium (Na⁺), and chlorine (Cl⁻) concentrations. A salinity level of 4 dS m⁻¹ induced a significant decrease in both the Ca²⁺ and K⁺ content, while conversely, the Na⁺ and Cl concentrations increased substantially in the leaves of plants subjected to this salinity level. The elevated Na⁺ and Cl⁻ levels observed in plants irrigated with saline water (4 dS m⁻¹) can be attributed to their increased availability in the irrigation water, leading to enhanced plant uptake. This observation aligns with previous research demonstrating a significant increase in leaf Na⁺ and Cl⁻ concentrations under salt stress conditions [55,56,75,76,77]. The concomitant reduction in K⁺ and Ca²⁺ concentrations at 4 dS m⁻¹ salinity may result from the increased Na⁺ concentration, potentially due to antagonistic relationships between Na⁺ and these two essential nutrients. This antagonistic interaction, where Na⁺ can impede K⁺ and Ca²⁺ uptake, especially under salt stress, has been reported in numerous studies [56,69,77,78,79]. Furthermore, Malone and Andrews [80] suggested that decreased Ca²⁺ content in the leaves of salt-stressed plants may be linked to increased hydraulic resistance under saline conditions. This increased resistance can hinder water and Ca²⁺ transport to the leaves, ultimately reducing Ca²⁺ concentrations.

Grafting demonstrably influenced the leaf mineral composition. Specifically, the calcium and potassium concentrations were elevated in grafted plants, a finding concordant with previous research [64]. These authors also reported increased levels of these elements in grafted plants. Conversely, foliar chlorine and sodium levels were reduced compared to their non-grafted counterparts in grafted treatments. This reduction is likely to be attributed to the ameliorative effects of grafting on salt stress. Grafting has been shown to mitigate the negative impacts of salinity by limiting the uptake and accumulation of detrimental ions, such as sodium and chlorine [81,82]. This selective ion exclusion mechanism offers an avenue for enhancing plant performance under saline conditions. Martinez-Rodriguez, Santa-Cruz, Estan, Caro, and Bolarin [28] reported that grafting tomato cultivar UC-82B onto the Futuria rootstock enhanced growth under salt stress, after 35 days of salt stress exposure. This improvement was due to reduced sodium and chloride accumulation in the leaves, observed within 15 days. These results underscore the potential of grafting to mitigate salt-induced stress and optimize plant growth in saline environments. In response to salt stress, plant rootstocks utilize multiple adaptive strategies, restricting their transfer to aboveground tissues [83]. Grafted plants have also been observed to exhibit lower Na⁺ and Cl⁻ accumulation in shoots compared to non-grafted plants, indicating the importance and role of grafting in modulating ion homeostasis under salinity.

The cultivation of plants in a sand growth medium resulted in a significant reduction across all the measured growth parameters, except fruit quality characteristics. This observed decline can likely be attributed to the inherent chemical instability of sand as a growth substrate and its limited porosity. These characteristics may have contributed to an elevated concentration of salts in the rhizosphere, potentially inducing detrimental effects on plant development [20]. Conversely, plant performance was found to be better in volcanic rock. This is likely due to its chemical composition, in contrast to sand, which is often described as less stable as a medium. Volcanic stone has favorable drainage properties, chemical inertness, a near neutral pH, and provides adequate aeration for the roots [84]. A study by Ortega Martínez et al. [85] investigated different growing media for covered tomato cultivation, including volcanic rock, coconut coir, artificial agricultural soil, sawdust, and a 1:1 mixture of sawdust and sheep dung. The results showed that volcanic rock produced the highest fruit yield, reaching 252 tons per hectare. In comparison, the yields from coconut coir, artificial agricultural soil, sawdust, and the sawdust–sheep dung mixture were 168, 194, 221, and 233 tons per hectare, respectively.

More in-depth structural and chemical analysis of the substrate properties would provide valuable context, but this represents a separate line of inquiry that could be pursued in future studies.

These findings underscore the importance of substrate selection in soilless cultivation systems, emphasizing the critical role of factors such as biological and chemical inertness, appropriate porosity, and desirable capillary action [86].

5. Conclusions

This study investigated how grafting impacts the yield of tomatoes (Solanum lycopersicum L.) under salty irrigation, which is a crucial problem in satisfying Saudi Arabia’s expanding tomato production demand under saline irrigation. Specifically, we examined the effects of grafting on the physicochemical properties of tomato fruits, yield, water productivity, and leaf mineral composition (Ca²⁺, K⁺, Na⁺, and Cl⁻) across varying salinity levels and growing media. Our results demonstrate that grafted plants generally performed better than non-grafted controls across most of the measured parameters, except for fruit quality characteristics. Optimal growth and productivity were observed in plants cultivated in volcanic rock medium at the lower salinity level (2 dS m⁻¹). Crucially, grafting the commercial ‘Tone Guitar’ scion onto the novel local rootstock X-238 (G5) led to significant improvements in fruit characteristics, yield, water productivity, and leaf Ca²⁺ and K⁺ concentrations, while simultaneously mitigating Na⁺ and Cl⁻ accumulation. These beneficial effects were evident under saline stress in the volcanic rock and sand-growing media. These results highlight grafting as a practical approach to improve sustainable tomato cultivation in soilless environments, particularly in regions challenged by salinity and water shortages.