Studying the Combined Impact of Salinity and Drought Stress-Simulated Conditions on Physio-Biochemical Characteristics of Lettuce Plant

Abstract

Water scarcity and increasing salinity stress are significant challenges in the farming sector as they often exacerbate each other, as limited water availability can concentrate salts in the soil, further hindering plant growth. Lettuce, a crucial leafy vegetable with high nutritional value, is susceptible to water availability and quality. This study investigates the growth and development of lettuce plants under water scarcity and varying levels of salinity stress to identify effective strategies for reducing water consumption while maintaining or improving plant productivity. Field experiments were designed to simulate three drought levels (50, 75, and 100% of class A pan evaporation) and three salinity stress levels (control, 1500, and 3000 ppm NaCl), assessing their impact on lettuce’s morphological and biochemical parameters. The combination of reduced water supply and high salinity significantly hindered growth, underscoring the detrimental effects of simultaneous water deficit and salinity stress on plant development. Non-stressed treatment enhanced nitrogen, phosphorus, and potassium contents and progressively decreased with the reduction in water supply from 100% to 50%. Interestingly, higher salinity levels increased total phenolic, flavonoid, and antioxidant activity, suggesting an adaptive stress response. Moreover, antioxidant activity, evaluated through DPPH and ABTS assays, peaked in plants irrigated with 75% ETo, whether under control or 1500 ppm salinity conditions. The Yield Stability Index was highest at 75% ETo (0.95), indicating robust stability under stress. The results indicated that lettuce could be cultivated with up to 75% of the water requirement without significantly impacting plant development or quality. Furthermore, the investigation demonstrated that lettuce could thrive when irrigated with water of moderate salinity (1500 ppm). These findings highlight the potential for reducing water quantities and saline water in lettuce production, offering practical solutions for sustainable farming in water-scarce regions.

1. Introduction

Reducing crop losses is crucial to meet the escalating food demand in light of the rapidly growing global population approaching eight billion. Abiotic stress factors such as drought and salinity are the biggest threats, leading to significant crop losses [1]. Climate change has exacerbated these challenges, with reports showing that abiotic stresses can mitigate agricultural productivity by up to 50% [2]. The increasing severity of salinity and drought stresses symbolizes a significant challenge to global agriculture, particularly in regions where water scarcity and soil salinization dominate. These abiotic stresses can negatively impact plant growth, development, and productivity by disrupting physiological and biochemical processes needed for plant survival and yield [3].

The impact of drought and salinity on plants varies from changes in morphology to responses at the molecular level [4,5,6]. The initial impacts of drought and salt stress on plants are similar; however, extended exposure to salt stress can generate toxic ion effects and cause nutritional imbalances in plants—drought and salt stress influence plants at various morphological and molecular levels [7]. Salinity stress usually leads to osmotic stress, ion toxicity, and nutrient imbalances in plants, which can harm growth and reduce yield [8,9]. Increasing the concentration of sodium chloride is accompanied by pronounced toxicity for test plants [10]. When plants are exposed to high concentrations of salinity, they encounter difficulties in water and nutrient uptake due to the lower water potential of the soil solution, leading to dehydration and stunted growth. Additionally, the accumulation of toxic ions, such as sodium (Na⁺) and chloride (Cl⁻), may disrupt cellular homeostasis and interfere with primary metabolic processes, including photosynthesis and protein synthesis [10,11]. The impact of salinity on plant physiology is often manifested through decreased chlorophyll content, impaired photosynthetic efficiency, and altered nutrient uptake, which leads to reduced plant vigor and productivity [12,13].

On the other hand, water deficit stress (drought) primarily impacts plants by limiting water availability, leading to reduced stomatal conductance, decreased photosynthetic activity, and impaired nutrient transport. Under drought conditions, plants often exhibit oxidative stress due to the overproduction of reactive oxygen species (ROS), which can damage cellular components, including lipids, proteins, and DNA. For coping with oxidative stress, plants activate antioxidant defense mechanisms, which include the synthesis of antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX), as well as other molecules such as ascorbate, glutathione, tocopherol, polyphenols, and flavonoids [14]. Plants’ ability to balance ROS production and antioxidant activities is essential for plant survival under drought and salt stress conditions and represents a significant function in determining tolerance to drought stress.

Lettuce (Lactuca sativa L.) is a leafy vegetable crop with high value for its nutritional richness, which holds substantial economic importance and is widely enjoyed globally [15]. It belongs to the Compositae family and is a moderately salt-sensitive vegetable crop [16]. Lettuce is a vital salad plant rich in antioxidants, carotenoids, caffeic acid, and flavonoids [17]. Lettuce does not contain high levels of vitamin C. However, a key benefit is that it is eaten fresh, allowing for full utilization of its vitamin C content, which is otherwise significantly reduced during cooking [18]. Due to its shallow taproot system, lettuce needs frequent irrigation. Even a short drought stress can negatively result in low crop productivity [19]. Plants have developed various mechanisms and molecular networks to cope with these environmental stresses, such as closing stomata to reduce water loss, though these responses are often temporary and stress-duration-dependent. Additionally, plants have evolved genes that code for expressing protective compounds to manage stress conditions [20,21].

When plants are subjected to the combined effects of salinity and drought, the interactions between these stresses can exacerbate their negative impact on plant physiology and biochemistry. Plants have developed mechanisms to detect drought and salt stress, enabling them to quickly adapt through intricate physiological and biochemical processes at the cellular, tissue, organ, and whole-plant levels [22,23]. The simultaneous occurrence of osmotic stress from salinity and water deficit from drought can severely limit plant growth and productivity, making it challenging to disentangle the individual contributions of each stress [24,25]. However, studying the combined impact of these stresses on lettuce plants can provide valuable insights into the complex stress tolerance mechanisms and the potential for developing resilient crop varieties.

This investigation hypothesizes that the combined drought and salinity stresses induce various physiological and biochemical responses in lettuce plants, contributing to understanding abiotic tolerance mechanisms. This study seeks to answer the following questions: (1) How do the combined abiotic stresses influence the key physiological parameters of lettuce plants? (2) What specific biochemical pathways are activated in response to combined abiotic stresses? By investigating these responses, we may gain valuable insights into complex stress tolerance mechanisms, identify potential strategies, and improve crop productivity under these challenging conditions.

2. Materials and Methods

2.1. Experiment Location and Design

The experiment was conducted in open fields during the late summer seasons of 2022/2023 and 2023/2024 at a research farm affiliated with the National Research Center, Nubaria area, Beheira Governorate, Egypt. Firstly, lettuce seeds (Balady cultivar) were sown in trays containing (1:1) a mixture of peat moss and vermiculite enriched with macro- and micronutrients. The trays were kept in the greenhouse, and standard agronomic practices were performed to produce lettuce seedlings. Seedlings at 30 days old were planted in the open field. The plants were fertilized with N at 230, P₂O₅ at 45, and K₂O at 70 Kg/acre (0.42 ha). A drip irrigation system and drip irrigation flow rate were used.

Table 1. Physical properties and chemical analyses of the soil taken from the experimental site (National Research Center, Nubaria area, Beheira Governorate, Egypt).

Physical Analysis		Chemical Analysis
		Cations (meq/L)		Anions (meq/L)
		Ca++	8.7	CO3−	0.0
Sand	84.2%	Mg++	4.0	HCO3−	0.52
Silt	11.8%	Na+	2.3	Cl−	11.48
Clay	4.1%	K+	1.0	SO4−	4.0
Texture class:		Loamy sand
Soil pH	7.7	Available N		0.78%
EC (dS/m)	1.6	Available P		0.32%
Organic matter	3.54%	Available K		0.46%

shows the mechanical and chemical characteristics of the experimental soil. The chemical analyses of the irrigation water from the experimental site showed the following characteristics: the water had a slightly basic pH of 7.23; electrical conductivity (EC): The EC is 0.95 dS/m.

The dissolved cations include calcium (2.9 mEq/L), magnesium (1.5 mEq/L), sodium (4.3 mEq/L), and potassium (0.5 mEq/L). The anions present are bicarbonate (0.9 mEq/L), chloride (5.2 mEq/L), and sulfate (3.1 mEq/L), while carbonate (CO₃⁻²) is not detected in the water samples (

Table 2. Chemical analysis of the water sample was taken from the experimental site (National Research Center, Nubaria area, Beheira Governorate, Egypt).

pH	7.23	Dissolved Cations (mEq/L)		Dissolved Anions (mEq/L)
		Ca++	2.9	CO3−2	-
EC (dSm−1)	0.95	Mg++	1.5	HCO3−	0.9
EC ppm	608.0	Na+	4.3	Cl−	5.2
		K+	0.5	SO4−2	3.1

).

2.2. Irrigation Water Calculation

A field experiment was carried out in a split-plot design (RCBD) with three replicates, where water addition treatments were arranged in the main plots, while salt addition treatments were randomly distributed in the sub-plots. The area of the experimental unit was 30 m², and each plot consisted of three rows, 15 m long and 70 cm wide. The design was used to test different irrigation quantities (ET50, ET75, and ET100) and salt addition treatments (SS0, SS1, and SS2), replicated three times. Therefore, nine plots were arranged in this experiment. Irrigation quantities were adjusted considering the 50, 75, and 100% cumulative evaporation from a class A pan. In the scheduled irrigation period, plants were irrigated considering three different levels (100, 75, and 50%) of water evaporated from the class A pan. A drip irrigation system applied irrigation water. The amount of irrigation water for lettuce was calculated based on the class A evaporation equation for the 100% treatment [26] as follows:

ETo = IBAN × CPP

CU = ETo × Kc

WR = CU × L%

where ETo: evaporation reference; Epan: evaporation pan in mm, and Kp: Pan coefficient (constant, 0.85). CU: Water consumption. L%: Filtration agent (1.25%). WR: water requirements (lm⁻²). Kc: yield coefficient (variable 0.7:1.2 during the growing season [27]. It was subjected to three irrigation treatments: the moderate water system (75%) and the low water system (50%), compared to the high water system (100% basin evaporation, class A). Sodium chloride (coarse salt from Hor Rosetta) was added as a source of salinity to the freshwater to compensate for the three concentrations of salt water, i.e., 1500 and 3000 ppm, compared to the non-salt treatment (control).

2.3. Salinity and Drought Treatments

One-month-old lettuce seedlings were irrigated with a different combination of three water amounts (ET50 = 50, ET75 = 75, and ET100 = 100%; evaporation class A) and three salt treatments (SS2 = 3000 [51.28 mM], SS1 = 1500 [25.64 mM], and SS0= 0 ppm). These were irrigation water treatments provided twice weekly at 3-day intervals. NPK fertilizer was added on time, as recommended for all trial lines. Lettuce seeds were sown in a nursery on 2 October in both seasons, and seedlings were then transplanted approximately 30 days apart on 3 November in the permanent field on either side of the drip lines at a distance of 40 cm between plants. In the harvest stage, 70 days after planting, lettuce plants were harvested, and ten plants were collected as a representative sample to evaluate lettuce productivity and quality during the two agricultural seasons.

2.4. Plant Growth Parameters

The following parameters were measured to assess plant growth: plant height (PH, cm), which was measured from the base of the stem at soil level to the tip of the highest leaf; number of leaves per plant (NLP); leaf area per plant (LAP, cm²), determined using portable laser leaf area meter V. ci-202; leaf fresh weight (LFW, g); and leaf dry matter (LDM, %), which was measured by drying the leaves in the drying oven until reaching a stable weight [28].

2.5. Chemical Analysis of Lettuce

Chlorophyll content (Chl, SPAD) values were determined using a portable Minolta Chlorophyll Meter, model SPAD-501 [29]. Nitrogen (N, %), phosphorus (P, %), potassium (K, %), and nitrate contents (NO₃⁻, mg/kg) were determined [30,31]. Nitrogen content and nitrate levels were analyzed using Kjeldahl Gerhard Vapodest 20s [32]. Potassium content was analyzed with a PFP7 flame photometer [32]. The results were obtained by a standard curve using calcium and potassium chloride as inputs.

2.6. Determination of Total Phenolic (TP), Flavonoid (TF) Contents, and Antioxidant Activities

The total phenol content in the dry matter was confirmed using a spectrophotometer. Phenolic was extracted using ethanol 80%; then, 1 mL of the extracted sample was diluted with 70 mL distilled water followed by Folin–Ciocalteau reagent and 15 mL of saturated sodium carbonate solution, incubated at room temperature for 30 min [32]. The absorbance was measured at 765 nm, and a calibration curve was developed using gallic acid. Total flavonoid content was calculated using methanol using a 2% aluminum solution—chloride [33]. Quercetin was used as a reference, and absorbance was measured at 368 nm. Antioxidant activities were measured by calculating the ability of extracts to cleanse free radicals. It is evaluated through the use of DPPH. The standard curve was generated using Trolox. The results were measured using milligrams of Trolox equivalent (mg TE/g) of lettuce leaves for the sample [34], ABTS, and ABTS + free radical scavenging [35].

2.7. Stress Tolerance Indices

Drought indices have been used to assay drought-tolerant treatments as they measure drought tolerance based on yield loss under drought conditions compared to non-stressed conditions. The fresh leaves yield data were recorded for each treatment, and the treatment (ET100 + SSO) represented non-stressed treatment. The data were subjected to calculate and analyze different drought selection indices as follows [36,37,38]:

$$\mathrm{Y}\mathrm{S}\mathrm{I}=\mathrm{Y}\mathrm{s}/\mathrm{Y}\mathrm{p}$$

$$\mathrm{T}\mathrm{O}\mathrm{l}=\mathrm{Y}\mathrm{p}-\mathrm{Y}\mathrm{s}$$

$$\mathrm{M}\mathrm{P}\mathrm{I}=0.5\left(\mathrm{Y}\mathrm{s}+\mathrm{Y}\mathrm{p}\right)$$

$$\mathrm{S}\mathrm{T}\mathrm{I}=(\mathrm{Y}\mathrm{s}\times \mathrm{Y}\mathrm{p})/\mathrm{Y}$$

$$\mathrm{H}\mathrm{M}=0.5({\mathrm{Y}\mathrm{s}}^{-1}+{\mathrm{Y}\mathrm{p}}^{-1})$$

where YSI is the yield stability index; MPI is the mean productivity index; HM is the harmonic mean productivity; STI is the tress tolerance index; TOL is the tolerance; Ys is the yield under stress; Yp is the yield under no stress.

2.8. Statistical Analysis

The recorded data were statistically processed using the analysis of variance (two-way ANOVA). The means were compared using Duncan’s multiple range tests and the least significant difference (LSD) at the level of probability p ≤ 0.05%.

3. Results

Data presented in Figure 1 exhibit the interaction effect of three irrigation water quantities (50, 75, and 100% of class A pan evaporation) with three salinity levels (0 (control), 1500, and 3000 ppm) on vegetative growth parameters, i.e., plant height (PH, cm), number of leaves/plant (NFP, Pcs), leaf fresh weight (LFW, g) and leaf dry matter (LDM, g). The results showed that drought and salinity stress significantly diminished lettuce plant growth. The data revealed that lettuce plants grew the longest under optimal water conditions (ETo100), especially without salinity stress (SS0), reaching 41.1 cm. As salinity levels increased (SS1 and SS2), plant height gradually decreased, with the lowest height of 28.35 cm under ETo100 + SS2. This decrease in PH highlights how increased salinity disrupts normal plant growth, even under adequate water supply. Severe drought stress (ETo50), combined with salinity (SS2), resulted in the most significant decrease, with plants growing to only 17.75 cm.

The same trend was observed regarding the number of leaves per plant (NLP). Non-stressed plants (ETo100 + SS0) had the highest number of leaves, 11.7, indicating optimal growth and development. When exposed to drought and salinity, leaf number significantly decreased, with the lowest NLP (6.95) recorded under the most extreme combined stress (ETo50 + SS2). These results suggest that water scarcity and high salinity negatively trimmed leaf growth, which is crucial for whole-plant productivity.

Leaf fresh weight (LFW) followed a similar pattern, with the highest fresh weight (1282.5 g) under non-stress conditions (ETo100 + SS0). Drought and increased salinity caused a significant decrease in fresh weight, with the lowest (687 g) observed under the combined treatment of ETo50 + SS2. The decrease in fresh leaf weight under stress conditions reflects the impaired ability of plants to accumulate biomass, which is directly related to the reduction in plant height and number of leaves (Figure 1).

Moderate drought stress (ETo75) combined with regular water irrigation (SS0) resulted in the highest LDM (5.71%), indicating that moderate stress might promote dry matter accumulation. However, under combined severe stress (ETo50 + SS2), LDM decreased to 4.61%, indicating that severe stress limited fresh weight and dry matter production. Overall, the data confirm that while salinity and drought negatively affect lettuce growth, the combined effect of both stresses is particularly detrimental, affecting main growth parameters (PH, NLP, LFW, and LDM) as they are closely correlated, collectively reflecting the full impact of drought and salinity stresses on lettuce plants. Under non-stressed conditions (ETo100 + SS0), the grown plants gave the highest values for all studied parameters, expressing healthy growth with high biomass accumulation. When stress levels increased, both PH and NLP decreased. This decrease in leaf production and subsequent height resulted in a vital decrease in LFW, highlighting the reduced ability of plants to generate fresh biomass. Meanwhile, the LDM ratio fluctuated, with mild drought promoting dry matter accumulation, but the highest stress conditions (ETo50 + SS2) decreased LDM. These results show that drought and salinity negatively impact lettuce plants comprehensively, leading to impaired growth under extreme conditions.

The data shown in Figure 2 show an apparent effect of drought and salinity stresses on leaf area per plant (LAP). The most significant decrease was observed under the severe combined stress and salinity. Under non-stressed conditions (ETo100 + SS0), the leaf area reached 275.3 cm². However, when drought increased (ETo50), leaf area decreased significantly, with ETo50 + SS1 resulting in a 54% decrease (126.425 cm²) compared to the control. The most extreme stress condition (ETo50 + SS2) showed only a slight increase to 131.39 cm², a 52% decrease from the optimum conditions, indicating that combined drought and salinity stress significantly limit lettuce leaf growth. Under moderate drought conditions (ETo75), the leaf area was less affected but significantly decreased with increasing salinity.

In the non-saline treatment (SS0), LAP was 249.52 cm², only 9% lower than the control, but with increasing salinity, leaf area decreased by 16% and 25% under ETo75 + SS1 (209.435 cm²) and ETo75 + SS2 (188.195 cm²), respectively (Figure 2). This indicates that while mild drought limits leaf expansion, salinity stress exacerbates the reduction in leaf area.

Chlorophyll (Chl, SPAD) content was less varied than LAP in the applied treatments. Under the treatment (ETo100 + SS0), chlorophyll content was the highest (55.6 SPAD), while under severe drought conditions (ETo50 + SS1), chlorophyll content decreased modestly by about 7% to 51.85 SPAD. Even under the most extreme stress conditions (ETo50 + SS2), the decrease in chlorophyll content reached about 6.5% compared to the control treatment. This suggests that while physical growth is severely affected, lettuce plants can maintain relatively stable chlorophyll levels and likely continue photosynthetic activity under stress (Figure 2).

The results (Figure 3) showed that the combined effects of drought and salinity strongly influenced nitrogen (N) content. Under optimal conditions (ETo100 + SS0), nitrogen levels peaked at 3.87%, indicating strong nutrient uptake and plant health. As stress levels increased, nitrogen content decreased significantly. For example, under severe drought and high salinity (ETo50 + SS2), nitrogen content decreased to 2.465%, indicating a 36% decrease compared to the control group. This trend is consistent across other stress levels, demonstrating how drought and salinity reduce the plant’s ability to uptake essential nutrients such as nitrogen.

Phosphorus (P) content also decreased under combined drought and salinity. Under the least stressful conditions (ETo100 + SS0), phosphorus content was 0.33%, but this value decreased to 0.225% under ETo50 + SS2. This represented a significant decrease in phosphorus uptake as plants were exposed to increasing stress. A similar trend was observed under moderate drought conditions (ETo75), where P content peaked at 0.31% in the absence of salinity (SS0) but decreased to 0.235% under ETo75 + SS2, highlighting the adverse effects of stress on P uptake.

Potassium (K) content showed a similar trend (Figure 3), with the highest levels recorded under optimal conditions. For example, under ETo100 + SS0, K content was 4.4% but decreased to 3.37% under ETo50 + SS2, indicating a significant decrease. Even under moderate drought conditions (ETo75), K levels decreased from 4.205% in the absence of salinity (SS0) to 3.75% when salinity increased to SS2. These results emphasize the sensitivity of K uptake to the combined effects of salinity and drought, which compromise plant nutrient uptake mechanisms.

Nitrate (NO₃⁻) levels followed a similar decreasing trend, with the highest nitrate content (1350.5 mg/kg) observed in the least stressful conditions (ETo50 + SS0). As salinity and drought stress intensified, nitrate levels decreased, reaching their lowest value of 1068.5 mg/kg under ETo100 + SS2. This decrease in nitrate levels is consistent with the decreasing nutrient uptake patterns seen for nitrogen, phosphorus, and potassium, reinforcing the conclusion that increased drought and salinity stress severely limits the plant’s ability to uptake essential nutrients (Figure 3).

Total phenolic (TP), total flavonoid (TF), and antioxidant activity in lettuce leaves were gradually reduced due to the gradual decrease in the amount of water supplied from 100% to 50% of pot evaporation during the growth periods after 70 days of cultivation in both seasons. The differences in total phenolic (TP), total flavonoid (TF), and antioxidant activity between 100% and 75% of pot evaporation were not statistically significant at all growth stages (Figure 4).

The lowest water application rate, 50%, of pot evaporation was consistently lower than the other two water treatments regarding antioxidants [39,40]. The DPPH and ABTS determination results collectively demonstrate that antioxidant activities varied significantly across treatments (Figure 5). In the DPPH, ET75 with 1500 ppm and ET100 with control exhibited the highest antioxidant potential concentration, while ET50 with control and ET100 with 1500 ppm had the lowest. The same trend was observed in the ABTS; ET75 with control and ET50 with 1500 ppm showed the most vital antioxidant activity, whereas ET50 with control and ET100 with control had the weakest. Interestingly, applying 3000 ppm did not consistently improve antioxidant levels. Moderate ET with either control or 1500 ppm treatments appears optimal for maximizing antioxidant activity (Figure 5).

The results from the presented data indicate insights into the impact of salinity stress and water irrigation deficit on lettuce growth and yield characteristics as follows: tolerance (TOL): the lowest TOL was observed in the ETo75 + SS0 treatment (0.06), indicating minimal yield reduction under stress, while ETo50 + SS2 showed the highest TOL (0.60), signifying more significant stress-induced yield loss (Figure 6).

Mean Productivity Index (MPI): The ETo75 + SS0 treatment had the highest MPI (1.25), suggesting optimal yield performance under both conditions, while ETo50 + SS2 had the lowest MPI (0.98), indicating reduced productivity. Harmonic Mean Productivity (HM): The HM varied, with ETo50 + SS2 having the highest HM (1.12), indicating better yield stability, while ETo75 + SS0 had the lowest (0.80), suggesting lower stability. Yield Stability Index (YSI): the highest YSI was observed in the ETo75 + SS0 treatment (0.95), indicating high stability under stress, while ETo50 + SS2 had the lowest YSI (0.54), reflecting lower yield stability. Stress Tolerance Index (STI): the ETo75 + SS0 treatment had the highest STI (1.22), indicating better performance under both stress and non-stress conditions, while ETo50 + SS2 had the lowest STI (0.69), signifying a lesser ability to tolerate and perform under stress (Figure 7).

4. Discussion

In this study, we aimed to evaluate the combined effects of drought and salinity on lettuce plants. Our findings indicate that these stressors have an additive effect on physiological and biochemical parameters; the interaction between drought and salt stress significantly determines cultivated plants’ growth and physiological responses [41,42]. While individual stressors such as drought or salinity have specific impacts on plant development, our results highlight more complex responses when combined [43]. The combination of reduced water availability (drought) and increased salinity levels significantly affected plant height, leaf area, and fresh weight more than either stressor alone, underscoring the compounding nature of these stresses. Salinity and drought stresses significantly disrupt various plant metabolic processes, reducing yield and compromising quality [44]. These stress factors are increasingly prevalent worldwide due to the scarcity of freshwater resources [45].

Consequently, there is a growing reliance on saline water with high electrical conductivity for irrigation, particularly in regions facing water shortages. This shift exacerbates the challenges of salinity and drought, further threatening agricultural productivity [46]. Water shortage and salinity stress clearly and significantly affected lettuce plant characteristics under simulated conditions. The plant height decreased with increased water loss in the two seasons of study, as did the number of leaves, leaf length, leaf width, and the leaf’s surface area. This is because water shortage reduces the transfer and absorption of nutrients and, thus, has a direct impact on all vegetative characteristics [39]. Water shortage and failure to supply the plant with the water it lost through transpiration also negatively affected the chemical characteristics of the plant.

In particular, the interaction of moderate water deficit (ETo75) and salinity stress (SS1500) showed that the combined effects reduce productivity more dramatically. This is evident in the observed deviation percentages and fitted curves, which indicate a non-linear response when both stressors are applied simultaneously. For example, although plants exposed to moderate drought or salinity alone exhibited minimal changes in antioxidant activity and nutrient content, the combination led to marked reductions in total nitrogen, phosphorus, and potassium, demonstrating the synergistic impact of drought and salinity on nutrient uptake and overall plant health [47,48]. The results show that increasing salinity levels gradually reduce productivity, particularly when combined with water deficits. However, it is essential to highlight that the ETo100 + SS1 treatment (full irrigation with moderate salinity at 1500 ppm) demonstrated good production potential. This observation suggests that under optimal drip irrigation conditions, the negative impact of moderate salinity can be mitigated, allowing plants to maintain relatively high productivity as this type of irrigation (drip) alleviates salt accumulation and reduces salt stress [49].

Regarding providing the necessary water to the plant, the plant obtains all its nutritional needs, which helps it build internal tissues, increase weight, and increase its content of nutritional elements [50,51]. The low content of plants that obtained only 50% of their water needs of nutritional elements is due to the inability of the plant to transfer elements from the soil and the nutrient solution to the inside of the plant [40]. When exposed to drought conditions, the antioxidant enzyme activity in lettuce plants decreased, which may be attributed to the plant’s inability to absorb sufficient nutrients from the environment. Drought stress limits water uptake and impairs nutrient availability and transport within the plant, reducing its ability to maintain optimal antioxidant enzyme levels [52,53]. Antioxidant enzymes indirectly influence nutrient uptake from the soil. Under stress conditions, oxidative stress can damage plant tissues (shoot and root), impairing the physiological functions essential for nutrient absorption [54]. As a result, the plant becomes less capable of mitigating oxidative stress, exacerbating its vulnerability to environmental stresses [55].

When considering the effect of salinity on the vegetative characteristics of the plant, we found that it had a negative effect. Plants respond to salinity stress through two mechanisms: osmotic and ionic stress. Osmotic stress occurs due to the accumulation and high soluble salt concentration in the root zone. As a result, it is difficult for plants to supply water to the soil (physiological drought). As the salt ion increases osmosis, it is not easy to absorb water from the soil and the necessary nutrients [56]. Also, when studying the chemical content of the plant nutrients, it was found that it significantly negatively affected the transfer and formation of elements within the plant. This is due to the difficulty of transferring nutrients from the soil to the plant, which affects all vital processes and the formation of nutrients in the plant [57]. On the contrary, treating plants with water containing high concentrations of salinity increased antioxidant compounds [58]. This occurs as a response to the plant detecting environmental stress, which triggers physiological processes to form these compounds [59].

Plant stress tolerance indexes are calculated at the end of the experimental period by comparing the weight of control plants against those grown in stressed conditions [60]. Various treatments of salinity and drought had varying impacts on lettuce yield traits, as measured by indices like Tolerance (TOL), Mean Productivity Index (MPI), Harmonic Mean Productivity (HM), Yield Stability Index (YSI), and Stress Tolerance Index (STI). The ETo75 + SS0 treatment consistently showed superior performance across several indices, including the highest MPI, YSI, and STI, suggesting that this treatment offers a balanced approach between yield stability and productivity under stress conditions. This highlights its potential as an optimal strategy for managing lettuce growth under combined salinity and water stress [61,62].

The results show that the interaction between different salinity levels and deficit irrigation (ETo and SS treatments) significantly affects lettuce yield and stress tolerance. The treatment with moderate irrigation (ETo 75%) combined with no salinity stress (SS0) achieved the highest Mean Productivity Index (MPI), Yield Stability Index (YSI), and Stress Tolerance Index (STI). This indicates that a moderate water deficit without salinity stress maintains lettuce productivity and yield stability, suggesting that it is a more resilient approach under these conditions [43,63].

On the other hand, the ETo 50% with the highest salinity stress (SS2) showed the highest Tolerance (TOL) and Harmonic Mean (HM) values but had the lowest YSI and STI, indicating that while it maintained some yield under stress, it was less stable and less productive overall [64,65]. This suggests that higher salinity stress combined with severe water deficit may exacerbate yield losses, making these conditions less favorable for lettuce growth [66,67,68]. Therefore, the data highlight the importance of balancing water and salinity stress to optimize lettuce yield and stability in challenging growing environments [43,69]. Furthermore, the analysis of the mean productivity index (MPI) and stress tolerance index (STI) highlights the negative impact of the combined drought and salt stress on yield stability. Lettuce plants exhibited the least MPI and STI values when subjected to both stresses simultaneously, particularly in treatments with higher salinity and lower water availability (ETo50 + SS2). This reduction can be attributed to the synergistic effects of drought and salinity, which impose a more significant physiological burden on plants than individual stressors.

Drought stress reduces the plant’s ability to uptake water, leading to reduced stomatal conductance and impaired photosynthesis, while salinity exacerbates this by causing ionic imbalance and osmotic stress. Together, these stresses create a hostile environment for plant growth, where nutrient uptake is hindered, reactive oxygen species (ROS) accumulate, and metabolic processes are disrupted. The combined stress conditions intensify oxidative damage and reduce the plant’s capacity to detoxify ROS due to compromised antioxidant enzyme activities. This further diminishes the plant’s resilience to environmental stressors. The observed reduction in productivity and stress tolerance under combined drought and salt stress conditions emphasizes the need for further research on the mechanisms underlying plant responses to multiple stresses and the development of mitigation strategies to enhance crop performance in such challenging environments.

5. Conclusions

The present investigation reveals that while drought and salinity stress individually alleviate lettuce growth, their additive impact disrupts critical physiological and biochemical processes. This leads to observed declines in vegetative traits such as plant height, leaf number, leaf area, and leaf dry matter. The most severe stress conditions, represented by a combination of 50% evapotranspiration (ETo50) and the highest salinity level (3000 ppm NaCl), led to the most significant reduction in the most studied traits, highlighting the vulnerability of lettuce to severe environmental stress. This adverse effect is attributed to the restriction of nutrient uptake and poor water absorption caused by osmotic and ionic stresses, which collectively hinder the plant’s ability to maintain normal growth and development. Moderate water deficit (ETo75) and non-stressed water (SS0) emerged as the most effective treatment, showing the highest mean productivity index, yield stability index, and stress tolerance index. This treatment effectively balanced the tradeoffs between water conservation and yield maintenance, indicating that lettuce can maintain its productivity and more stable growth patterns under moderate stress conditions. These results underscore the urgent need for careful management of drought and salinity levels in lettuce cultivation, especially in areas where water scarcity and salinity are increasing concerns, to improve crop productivity in the face of increasingly challenging environmental conditions.