Comparative adaptive responses of quinoa cultivars grown hydroponically under saline-sodic stress

Abstract

Background

Pakistan has 22 million hectares (Mha) of cultivated land, of which 6.67 Mha are affected by salinity and sodicity, negatively impacting crop growth and quality. Quinoa, a facultative halophyte, was tested under hydroponic conditions to evaluate the effects of saline-sodic water on the growth, biomass, and ionic concentration of four quinoa cultivars (UAF-Q7, GLN-22, GLN-29, and GLN-33), and to assess their tolerance to salt stress. Five treatments were established: (i) control (no salt), and four saline-sodic irrigation waters with EC: SAR combinations of (ii) 10 dS m^− 1:20 (mmol L^− 1)^1/2, (iii) 10 dS m^− 1:40 (mmol L^− 1)^1/2, (iv) 20 dS m^− 1:20 (mmol L^− 1)^1/2, and (v) 20 dS m^− 1:40 (mmol L^− 1)^1/2. Seeds were germinated in a plant nursery, and uniform seedlings were transplanted to 25-L half-strength Hoagland’s nutritional solution. To prevent osmotic shock, salt stress was gradually applied one week after transplantation.

Results

Plant analysis showed that saline-sodic water negatively affected quinoa, particularly GLN-22. Among the tested cultivars, GLN-33 performed best at all EC: SAR levels, whereas GLN-33 and GLN-29 performed well at higher SAR levels. The GLN-22 cultivar showed sensitivity to all salt stress levels, as indicated by the increased accumulation of Na⁺ by 84% and 72% in the leaves and roots, respectively.

Conclusion

Based on the results, it was concluded that GLN-33 tolerated all stress levels up to EC 20 dSm^− 1 and SAR 40, with significantly better growth, lower Na⁺ accumulation (< 42% increase in leaves), and higher K⁺ retention compared to other cultivars. Thus, GLN-33 has the potential to grow efficiently in soils irrigated with saline water, owing to its effective defense system against salt stress.

Highlights/key points

The performance of four different quinoa cultivars was studied under varying saline-sodic hydroponic culture.

Saline-sodic conditions exhibited negative impacts on the growth and physiology of all quinoa cultivars, especially GLN-22.

GLN-33 emerged as a remarkably salt-tolerant cultivar, showing resilience and robust growth.

GLN-33 could be a promising choice for regions with poor soil quality and water resources

Introduction

Agriculture faces numerous hurdles, and it is a huge challenge to produce 70% more food by 2050, with an ever-increasing world population [1]. However, soil health and productivity are declining in tandem with rising food demand. Approximately 90% of the global cultivable land is reportedly exposed to one or more abiotic stresses, such as salinity, drought, or temperature stress [2], which lowers plant growth and productivity [2, 3]. Fresh water shortages for irrigation are worsening due to low rainfall and high evaporation, and saline water irrigation has become more significant in arid and semi-arid regions worldwide [4]. Soils, particularly in arid regions, tend to be naturally salty, but poor irrigation systems and inadequate drainage cause waterlogging, which raises the water table and brings salts to the soil surface [5]. Saline water irrigation worsens this situation because, as the water evaporates, the salts accumulate in the rhizosphere, inhibiting the plant’s ability to grow. According to Leng et al. [6], drought and salts affect > 800 Mha area of the land and are collectively responsible for 70% yield losses [7]. Furthermore, global warming could double the current annual rate of cultivable land loss by the end of the century [8, 9].

The complex interplay among morphological, physiological, and biochemical processes involved in seed germination, plant growth, and water and nutrient uptake causes the negative impacts of salinity [10, 11, 12]. High quantities of salt can cause osmotic and ionic stress in plants [13]. The osmotic effects of salt can also have deleterious effects on the functioning of numerous stomatal enzymes involved in carbon dioxide (CO₂) reduction. Blocking of electron transport, inactivation of the photosystem-II (PSII) reaction centers, destruction of the oxygen-evolving complex, and reduced electron transfer capacity on the donor side of PSII are some ways in which salts may affect photosynthesis [14, 15]. Additionally, elevated Na⁺ and Cl⁻ concentrations in non-stomatal leaf tissues can significantly impact metabolic processes that limit photosynthesis [16].

Nevertheless, to cope with salt stress, plants have developed diverse physiological and biochemical mechanisms, the most complex of which include osmotic fixation [17], osmo-protection and accumulation of compatible solutes, redox control, and scavenging of cytotoxic reactive oxygen species (ROS) through antioxidant defense mechanisms [18, 19, 20, 21]. Extensive plant species have been tested for salt tolerance using these parameters. One of the biochemical responses of plants under salt stress is the accumulation of ROS, i.e., free radicals (superoxide O₂⁻ and hydroxyl radicals OH ⁻), and non-radical forms (hydrogen peroxide H₂O₂ and singlet oxygen ¹O₂) [21, 22].

To counter the problem, the expansion and implementation of the effective irrigation techniques can play a vital role in water conservation, as can the use of non-traditional water sources in reducing the scarcity of clean water [4]. Since ancient times, salts-loaded water has been extensively employed for irrigation in agriculture due to severe water scarcity, but as previously demonstrated, this practice is not sustainable [23]. Sustainable saline irrigation with minimal effects of salts on soil and plant’s health has great potential. In fact, pulse irrigation and other drainage systems are used for active salt leaching to restore and maintain plant growth [24]. The two main techniques for consuming saline water while achieving a manageable, relatively low salinity level are alternating (pulsed) irrigation with fresh and salty water [25], and the fresh-saline water combination irrigation [26]. When freshwater irrigation and saline drainage water were used together, Sharma and Minhas [27] demonstrated that there was no noteworthy buildup of salinity, even though crop yield decreased by 6–18%. According to Malash et al. [28], adjusting the irrigation of fresh-saline water helps lessen the harmful effects of salt on soil and crops. However, plant responses to salinity are determined by the type of salt, its concentration, species of plant, its genotype, and stage of growth [29]. Halophytes have significant genetic variation in their ability to tolerate salt [30]. Therefore, using saline water for irrigation i.e., using high-salinity water without using high-quality water might be a sustainable approach, but it necessitates testing the halophytic potential of plants. Consequently, it seems that a blend of salt-tolerant plants at somewhat low salinity and sodicity level would be a good combination for efficient saline irrigation [29]. One possible option for sustainable agriculture is the use of so-called ‘‘cash crop halophytes’’ irrigated with saline water up to the salinity of seawater [31]. Among the dicotyledonous halophyte, quinoa (Chenopodium quinoa Willd.) exhibits the greatest promise, as it can grow even at 400 mM salinity and yields extremely nutrient-dense seeds [32, 33]. Its broad resilience to abiotic stress conditions like salinity and drought also gives it the capacity to grow on severely salinized soils with inadequate water supplies. Quinoa has the potential to be a high-nutrient human food source due to its exceptional nutritional value and the variety of genotypes it contains [34, 35].

Quinoa has garnered a lot of attention recently as a substitute crop in arid and semi-arid areas of the world [32, 36]. Although many studies have been conducted on quinoa regarding its salt-tolerance ability in different regions with salt-affected soils, no study has reported the evaluation of different quinoa cultivars under a range of salinity: sodicity stress (EC: SAR ratios) in soil-less media. Therefore, the current study was planned with the aims of evaluating the growth, physiological, and ionic responses of four quinoa cultivars grown in hydroponic culture.

Materials and methods

Experiment description

The experiments were conducted at the Institute of Soil and Environmental Sciences (ISES), University of Agriculture Faisalabad (UAF), Pakistan. Seeds of four quinoa (Chenopodium quinoa Willd.) cultivars (UAF-Q7, GLN-22, GLN-29, and GLN-33) were obtained from the King Abdul Aziz University of Science and Technology (KAUST), Saudi Arabia. UAF-Q7 is a Pakistani-adapted line; GLN-22, GLN-29, and GLN-33 are breeding lines selected for varying salt tolerance. The seed color of all cultivars ranged from white to pale yellow, with a maturation duration of 100–120 days. All cultivars are grain-type quinoa, grown primarily for seed production. These four cultivars were chosen primarily for their contrasting salt responses. UAF-Q7 served as a regional check, as it had been previously grown in Pakistan, while GLN-22, GLN-29, and GLN-33 represented potential salt-sensitive and salt-tolerant genotypes. All quinoa cultivar seeds were planted in steel trays lined with polythene, loaded with cleaned sand, and irrigated with canal water. Following four weeks of germination, uniform-sized seedlings of each cultivar, with a plant height of 10–12 cm and four fully expanded leaves, were transplanted into 25-L plastic tubs using foam-plugged holes made in polystyrene sheets. The tubs were filled with Hoagland’s nutrient solution, and aeration pumps were used to aerate the plant roots and growth media. In each tub, three seedlings of each cultivar were planted in separate foam-plugged holes. Each treatment was replicated three times, with each replicate comprising one tube. Thus, per treatment, three tubs × one seedling per cultivar=three biological replicates per cultivar. After one week of plant establishment in hydroponic culture, a mixture of salts, namely sodium chloride (NaCl), sodium sulfate (Na₂SO₄), calcium chloride (CaCl₂), and magnesium sulfate (MgSO₄.7H₂O) were applied to develop different EC: SAR levels, that is, 10:20, 10:40, 20:20, and 20:40, along with a control. The quadratic equation method was used to develop salinity and sodicity levels [37] and the levels were confirmed via analytical procedures after each salinity: sodicity level was developed. The quadratic equation used to develop salinity: sodicity levels is given below:

The EC, pH, and SAR of the water used in the experiment, before the addition of salts, was 0.3 dS m^-1, 7.3, and 1.0 (mmol/L)^1/2, respectively. Salt stress was applied in three split doses to avoid osmotic shock. The pH of the solution was adjusted to 6.5 ± 0.2 using NaOH or HCl, and the Hoagland solution was replaced every week. Each treatment had three replications. Forty-five days after transplantation, the crop plants were harvested and subjected to several agronomic, physiological, and ionic analyses. Throughout the experiment, the environmental conditions in the wire house were maintained at 25 °C temperature, 400 µmol·m^-2s^-1 light intensity and a photoperiod of 16 h of light and 8 h of darkness.

Studied parameters

The LICOR LI-3000 leaf area meter (Lincoln, NE) was used to measure the leaf area of quinoa plants [38]. Forty days after the transplantation of seedlings into the hydroponic culture, physiological attributes such as transpiration rates and photosynthesis were measured using a portable infrared gas analyzer (IRGA) (Analytical Development Company, Hoddeson, UK). Chlorophyll content was analyzed using a SPAD meter (Minolta SPAD-502 DL, Japan). Relative water content (RWC) was calculated using fresh weight (FW), turgid weight (after 4 h in distilled water), and dry weight (after 74 h at 70 °C) following the methodology of Tahjib-Ul-Arif et al. [39]. Agronomic parameters, such as shoot fresh weight (SFW), shoot dry weight (SDW), root fresh weight (RFW), and root dry weight (RDW), were recorded after harvesting. Root attributes, such as root length (RL), root density (RD), and root surface area (RSA), were recorded after washing the roots with tap water, cleaning them, and scanning them using a flatbed root scanner (EPSON GT-X980, Seiko Epson Corp., Indonesia).

For the ionic analysis, leaf and root dry weights were noted following oven drying. After drying, 0.5 g of leaf and 0.5 g of root samples were placed in a digestion flask, filled with 5 mL of HNO₃, and left overnight. The same digestion flasks were filled with 2 mL of HClO₄ after 24 h and placed on a hot plate for 1.0 h until a clear, white solution appeared. The flasks were then removed, cooled, filtered, and the final volume was made up to 50 mL by adding distilled water. The diluted solution was then analysed for Na⁺ and K⁺ using a flame photometer (Sherwood, Japan, Model 410) at 589-nm and 767 nm, respectively, after standardization with standard solutions of different strengths (0, 2, 8, 10, 12, 24, 48, and 100 ppm) [40, 41]. Afterwards, the concentrations of both elements in leaves and roots were determined using a standard-curve equation developed from Na⁺ and K⁺ standard readings and the flame photometer emissions. Although plant height and leaf number were not recorded, shoot and root biomass, leaf area, and physiological parameters provided sufficient indicators of the growth response to salt stress.

Statistical analysis

The experiment followed a completely randomized design (CRD) with two factors: quinoa cultivar (four) and EC: SAR treatments (five levels). Each treatment combination was replicated three times. Two-way ANOVA was used to statistically evaluate the experimental data, and the least significant difference (LSD) test was used to compare the significance of treatments at a 5% probability level using Statistix v8.1 (Statistix Analytical Software, Tallahassee, Florida, USA) [42]. The average values of the three replicates ± standard error was used to display the data. Principal component biplot analysis (PCA) was performed to determine the relationship between the different parameters examined and quinoa cultivars using OriginPro v2021. Principal component analysis (PCA) included the following variables: SFW, SDW, RFW, RDW, LA, photosynthetic rate, transpiration rate, chlorophyll content, RWC, MSI, leaf and root Na⁺ and K⁺, and their ratios, RL, RSA, and RD.

Results

Quinoa growth

The growth of quinoa in saline-sodic water culture was negatively affected (Fig. 1A-D). The main effects of cultivar and EC: SAR levels were significant for all growth parameters (p < 0.05). Across all salinity levels, GLN-33 produced the highest shoot and root biomass, followed by UAF-Q7, GLN-29, and GLN-22. Across all cultivars, increasing EC: SAR level from control to 20:40 progressively decreased SFW by 23%, 41%, 58%, and 67% at 10:20, 10:40, 20:20, and 20:40, respectively. The cultivar × treatment interaction was also significant (p < 0.05). Compared to the control, GLN-22 showed the highest reduction (63%) in SFW and SDW (83%) at the highest salinity (20) and sodicity (40) levels (Fig. 1A, B; p < 0.05). RFW declined sharply in GLN-29 (70%) at the EC: SAR level of 20:40, whereas RDW decreased most in UAF-Q7 (63% at 20:40) (Fig. 1C, D; p < 0.01, p < 0.05). Leaf area was reduced, ranging from 11% to 18% across all cultivars, with the maximum reduction recorded in UAF-Q7 at the EC: SAR level of 20:40 compared to the control (Fig. 1E; p > 0.05). Even the most tolerant cultivar, GLN-33, showed significant growth reduction at the highest stress level (e.g., 23% decrease in shoot dry weight at 20:40 compared to control; Fig. 1B), confirming that tolerance is relative rather than absolute.

Fig. 1.

Effect of different levels of salinity/sodicity on; (A) Shoot fresh weight; (B) Shoot dry weight; (C) Root fresh weight; (D) Root dry weight; (E) Leaf area of quinoa cultivars in hydroponic culture. Different letters indicating level of significance among different treatments and cultivars (two-way ANOVA followed by Tukey’s HSD test, p < 0.05), while error bars indicate standard error of the replicates

Physiological attributes

Saline-sodic water culture also had drastic effects on the physiological attributes of quinoa cultivars. The main effects of cultivar and EC: SAR were significant for photosynthetic rate, transpiration rate, and membrane stability index (p < 0.05). Across cultivars, photosynthetic rate declined by 18–62% with increasing salinity and sodicity level, while across stress levels, GLN-33 maintained the highest photosynthetic rate (1.32 mmol m^− 2 s^− 1), but GLN-22 the lowest (0.2 mmol m^− 2 s^− 1).

The cultivar × treatment interaction was significant for photosynthetic rate and MSI (p < 0.05) but not for transpiration and chlorophyll content (p > 0.05). Compared to the control, photosynthetic rate was reduced by 12–68%, with a notable reduction of 68% detected in GLN-22 at EC: SAR level 20:40 (Fig. 2A; p < 0.01). The transpiration rate also declined by up to 51% in GLN-29 (Fig. 2B; p > 0.05). Chlorophyll content decreased under high stress levels but showed a non-significant increase at moderate salt (10:40) (Fig. 2C; p > 0.05). RWC decreased by 2–32%, with GLN-22 most affected at the highest EC: SAR level (20:40) in comparison to the control (Fig. 2D), and MSI declined up to 28% in UAF-Q7 at EC: SAR level (20:40) compared to the control (Fig. 2E; p < 0.05).

Fig. 2.

Effect of different levels of salinity/sodicity on transpiration and photosynthetic rate of quinoa cultivars in hydroponic culture (A) photosynthetic rate, (B) Transpiration rate, (C) Chlorophyll (SPAD value), (D) Relative water contents (RWC), (E) Membrane stability index (MSI). Different letters indicating level of significance among different treatments and cultivars (two-way ANOVA followed by Tukey’s HSD test, p < 0.05),, while error bars indicate standard error of the replicates

Ionic analysis

The main effects of cultivar and EC: SAR were significant for Na⁺ and K⁺ concentrations in both leaves and roots (p < 0.05). Across cultivars, leaf Na⁺ increased by 35%, 58%, 72%, and 89% at 10:20, 10:40, 20:20, and 20:40, respectively, compared to the control. Across all stress levels, GLN-22 accumulated the highest leaf Na⁺ (59.23 mg g^− 1 DW), while GLN-33 accumulated the lowest (33 mg g^− 1 DW). Leaf K⁺ declined with increasing stress, with GLN-33 retaining the highest K⁺ levels. The cultivar × treatment interaction was also significant (p < 0.05). The Na⁺ concentration in leaves and roots of all quinoa cultivars was enhanced, while K⁺ concentration was inversely proportional to the Na^+, i.e., decreased with increasing EC: SAR levels (Fig. 3). In leaves, the overall maximum Na⁺ concentration was recorded in GLN-22, and the minimum Na⁺ concentration was observed in GLN-33 at all salinity and sodicity levels. Similarly, the maximum concentration of K⁺ was observed in GN-33, and the minimum concentration was recorded in GLN-22 at the control and all salinity and sodicity levels, resulting in the highest Na⁺/K⁺ ratio in GLN-22 (Fig. 3A-C; p < 0.01). (Fig. 3B; p < 0.05). Comparatively, less Na⁺ and more K⁺ were recorded in the roots of all tested cultivars. In roots, GLN-29 accumulated the most Na⁺ (94% increase) and the least K⁺ at EC: SAR level 20:40, while GLN-33 maintained the lowest Na⁺/K⁺ ratio (0.55 in control) (Fig. 3D-F; p < 0.05). Even in the tolerant cultivar GLN-33, leaf Na⁺ increased at the highest stress level, suggesting that Na⁺ was not completely excluded.

Fig. 3.

Effect of different levels of salinity/sodicity on ionic concentration of quinoa cultivars in hydroponic culture (A); Na⁺ concentration in leaves; (B) K⁺ concentration in leaves ; (C) Na⁺/K⁺ ratio in leaves ; (D) Na⁺ concentration in roots ; (E) K⁺ concentration in roots ; (F) Na⁺/K⁺ ratio in roots. Different letters indicating level of significance among different treatments and cultivars (two-way ANOVA followed by Tukey’s HSD test, p < 0.05),, while error bars indicate standard error of the replicates

Root attributes

The root attributes of quinoa were negatively affected by increasing EC: SAR levels. The main effects of cultivar and EC: SAR levels were significant for all root attributes (p < 0.05). Across cultivars, all root attributes declined progressively with increasing stress levels. The interactive effect of cultivar and salinity: sodicity level was also significant (p < 0.05). Compared to the control, root density decreased by 4–43%, with the maximum decline observed in GLN-22 at EC: SAR level 20:40 (Fig. 4A). The maximum decrease in root length (58%) was recorded in GLN-22 at EC: SAR level of 20:40, followed by 10% at 10:20, 19% at 10:40, and 21% at 20:20 EC: SAR levels (Fig. 4B; p < 0.05). RSA decreased the most in UAF-Q7 (58–69%) (Fig. 4C; p < 0.01).

Fig. 4.

Effect of different levels of salinity/sodicity on root attributes of quinoa cultivars in hydroponic culture (A) Root density; (B) Root length; (C) Root surface area. Different letters indicating level of significance among different treatments and cultivars (two-way ANOVA followed by Tukey’s HSD test, p < 0.05),, while error bars indicate standard error of the replicates

Correlation analysis

Correlation analysis revealed that salt tolerance was strongly associated with root vigor, photosynthetic efficiency, and ion homeostasis (Fig. 5). Root attributes (RSA, RL, and RD) showed strong positive correlations with quinoa growth parameters (r² = 0.83–0.88), whereas photosynthetic rate and transpiration rate were positively correlated with SFW and LA (r² = 0.72–0.78). Conversely, sodium accumulation (NaR, NaL) exhibited significant negative correlations with most growth and physiological parameters (r² = − 0.65 to − 0.83), whereas potassium showed (KR and KL) positive correlation (r² = 0.75–0.88). Consequently, the Na⁺/K⁺ ratio in roots and leaves (Na/KR and Na/KL) was strongly negatively correlated with MSI and RWC, confirming that ionic imbalance is a key driver of stress injury and that salt tolerance depends on maintaining K⁺ homeostasis, photosynthetic activity, and root system vigor.

Fig. 5.

Pearson’s Correlation analysis between plant attributes under study. RD: root density, RL: Root length, RSA: Root surface area, Photosynt.: Photosynthesis, Trans: Transpiration, SFW: Shoot fresh weight, SDW: Shoot dry weight, RFW: Root fresh weight, RDW: Root dry weight, LA: Leaf area, NaR: Na in roots, KR: K in roots, Na/KR: Na/K ratio in roots, NaL: Na in leaves, KL: K in leaves, Na/KL: Na/K ratio in leaves, SPAD: chlorophyll, RWC: Relative water contents, MSI: Membrane stability index. *, ** indicate the significance of the correlation coefficient at p < 0.05 and 0.01, respectively

Principal components analysis

Principal component analysis (PCA) clearly separated cultivars based on their response to saline sodic stress (Fig. 6). Across all cultivars, PC1 explained 79–84% of the variance, while PC2 explained 4–7%. GLN-33 was positively associated with most growth and physiological parameters (upper right quadrant), followed by UAF-Q7 under high salinity and GLN-29 under high sodicity. Conversely, GLN-22 was associated with negative traits, particularly leaf and root Na⁺ accumulation (upper left quadrant). Biomass parameters (SFW, SDW, RFW, RDW, RL) were positively associated, whereas the Na⁺/K⁺ ratio showed a negative association, confirming that ionic balance is a key discriminating factor. Collectively, PCA demonstrated that EC:SAR levels strongly influenced agronomic and physiological parameters.

Fig. 6.

Principal Components Analysis (PCA-Biplot) between plant attributes under study showing individual treatments (red color) and variables (blue color arrows) (A = UAF-Q7, B = GLN-22, C = GLN-29, D = GLN-33)

Discussion

In arid regions, due to the use of saline water, salt leaching is required below the root zone through excessive water application. However, water scarcity limits this practice, but it can be managed by installing efficient drainage systems, selecting salt-tolerant cultivars, and monitoring soil salinity. In the present study, four quinoa cultivars (UAF-Q7, GLN-22, GLN-29, and GLN-33) were screened for salinity and sodicity stress under hydroponic conditions by analyzing plant development and physiological characteristics. The results showed that salinity and sodicity stress induced differential effects on plant growth and physiological parameters depending on the cultivar (Figs. 1, 2, 3, 4, 5 and 6). The differential growth responses observed among cultivars indicate genotype-specific adaptive mechanisms. The growth of the quinoa cultivar GLN-33 showed resistance to lower salinity and sodicity levels by increasing extracellular solute concentration, which caused a reduction in cell turgor [43]. Thus, in salt-stressed plants, an increase in osmotic potential is linked to a decrease in growth parameters, namely shoot/root weights under rising salinity and sodicity levels [44, 45]. This response likely results from decreased cell volumes during drought or high extracellular solute concentrations (Fig. 2). According to Jacobsen et al. [46], even under extreme water stress, quinoa plants with small, thick-walled cells that are adapted to water stress do not lose their turgor. Cultivar GLN-33 and UAF-Q7 maintained their plant growth even at the highest salinity and sodicity levels (20:40), indicating that their efficient osmotic adjustment mechanisms preserve cell turgor and division under high external solute concentrations. The performance of cultivar GLN-29 improved as sodicity levels increased, possibly because of the plant’s capacity to retain water status in saline environments, suggesting a specific adaptation to Na⁺-induced alkalinity. In contrast, quinoa cultivar GLN-22 showed more sensitivity even at 10:20 salinity and sodicity levels, indicating a fundamental deficiency in ion exclusion or osmotic adjustment pathways. This decline in growth is likely associated with a limited photosynthetic rate and stomatal conductance under high salt stress [47]. Parallel results were observed in the present study, where the transpiration rate in GLN-33 plants was not much affected at 10:20 level but was reduced with increasing salinity and sodicity stress (Fig. 2B).

Leaf chlorophyll content and RWC were not significantly decreased at lower salt stress levels (EC: SAR 10:20); rather, chlorophyll increased in GLN-29 and GLN-33 at EC: SAR (10:40), confirming the salt-loving nature of both quinoa cultivars (Fig. 2C, D). This increase reflects a hormetic adaptive response, where low-level stress activates protective pathways that enhance photosynthetic pigment synthesis as a protective adaptation to low to moderate salinity stress [48]. Panda et al. [49] observed that even at 400 mM NaCl, the chlorophyll concentrations of Suaeda maritima seedlings remained unaffected. As seen in many other quinoa cultivars, our study revealed that all quinoa cultivars showed a decline in these characteristics at increased salinity and sodicity levels [50, 51]. Degradation of chlorophyll structure may be the cause of this decrease at higher salinity and sodicity levels [52]. These characteristics were more severely reduced in GLN-22 than in other cultivars, suggesting that chlorophyll structure was more stable in other cultivars. This is supported by the decrease in cell membrane integrity observed in all cultivars under increased salt stress, which is a sign of oxidative stress induced by excessive ROS. Many plants, including quinoa, have been shown to suffer from ROS-induced membrane damage due to salinity [50, 53]. The relatively low reduction in MSI for GLN-33 indicates that it is more resistant to oxidative stress induced by salt (Fig. 2E).

Higher levels of Na⁺ buildup in leaf tissue and a large decline in stomatal conductance were linked to a significant drop in photosynthetic activity, which in turn greatly decreased the photosynthetic capacity of plants (Fig. 2A). This strong negative correlation between leaf Na⁺ accumulation and photosynthetic rate indicates that Na⁺ toxicity directly impairs the photosynthetic machinery (Fig. 5). A drop in the activity of photosynthetically active enzymes, such as RubisCO, under salt stress conditions could also be the cause of the decrease in photosynthetic capacity [54]. A decline in plant growth under salt stress is frequently linked to the photosynthetic process due to stomatal and non-stomatal restrictions, a drop in chlorophyll content, inhibition of Calvin-Benson cycle enzymes, and membrane-related protein degradation [55, 56, 57]. In contrast, the present study revealed that GLN-29 and GLN-33 cultivars exhibited enhanced chlorophyll content, which consequently led to improved photosynthetic activity and growth restoration under salinity stress (Fig. 2A, C). The delayed onset of photosynthetic impairment in GLN-33 suggests that its ability to compartmentalize Na⁺ into vacuoles protects chloroplast function for longer than in sensitive cultivars. Transpiration rates were significantly lower in GLN-22 plants with increased salinity and sodicity levels. This behavior demonstrates tolerance in quinoa under extremely saline-sodic environments by secreting salt ions and controlling the osmotic energy of the leaves, thereby maintaining turgor and transpiration [58]. Moreover, the tolerant cultivar’s high capacity for regeneration during stress release is suggested by the stimulation of photosynthesis and transpiration rate, which may contribute to both an effective antioxidant system and basic osmotic adjustment, as previously reported by Derbali et al. [59].

A decrease in leaf area is also one of the reasons for the reduction in photosynthesis and transpiration in all cultivars. The maximum reduction in leaf area observed in cultivar GLN-33 (Fig. 1E) represents an adaptive trade-off: smaller leaves reduce transpiration water loss under saline conditions, preserving tissue water status at the cost of reduced photosynthetic surface. This finding aligns with the findings of Netondo et al. [60], who reported that with the increasing salt stress, quinoa cultivars reduced their leaf area to reduce water loss from their surface to combat the drastic consequences of salt stress that ultimately cause drought stress. Salinity causes osmotic stress and specific ion toxicity, while sodicity greatly influences the growth of plant roots [61]. Crop yields are significantly reduced due to the adverse effects of salt on growth and development [62]. In this study, the suppression of root growth in plants under salt stress conditions was due to decreased cellular contents, reduced tissue fragmentation, and an imbalance in essential ion concentrations due to sodicity [63]. The accumulation of salts in the rootzone damages the physiochemical properties of the soil, leading to poor soil aggregation and impairing plant roots. Similarly, the accumulation of salt in the cell wall reduces elasticity and negatively affects root and shoot length, and metabolic pathways are also altered, as recorded in this study. Root length of quinoa cultivars decreased with a constant increment in saline-sodic conditions. The ability of cultivar GLN-33 to maintain greater root length at 10:40 (Fig. 4B) suggests better protection of root meristem activity, likely through more efficient Na⁺ exclusion from dividing tissues, a key trait for sustaining water and nutrient uptake under stress. Root surface area and root density were also adversely affected by saline-sodic water of different concentrations. To respond to the adverse effects of salinity and sodicity, quinoa plants may decrease their surface area and density to minimize the uptake of saline water from their surroundings. This explains why GLN-33 survives efficiently under all such stressed conditions and GLN-29 under sodic conditions (Fig. 4A, C). However, the performance of UAF-Q7 remained less affected under saline conditions, but its roots growth was drastically reduced with increasing sodicity. In contrast, GLN-22 exhibited poorer performance at all levels of saline and sodic water regime.

Plant growth and CO₂ exchange rates are reduced due to several factors, including the negative effects of salts on the activity of photosystem II, inhibition of the electron transport chain, and destruction of the oxygen-evolving complex. However, among these factors, salt-induced ion exchange is particularly significant [64]. Because of their similar physiochemical characteristics, K⁺ and Na⁺ compete, with Na⁺ known to compete with K⁺ at numerous binding sites [65]. Consequently, the leaf K⁺/Na⁺ ratio is significantly reduced by the buildup of high levels of Na⁺, which was detected in all cultivars (Fig. 3A-C). The loss of K ⁺ in the leaves under salt stress can lead to the death of target cells by triggering enzymes such as proteases [66], thereby enhancing the shedding of older leaves. Variations in growth responses among quinoa plants during the stress recovery phase highlight the need to investigate ion accumulation in plant tissues. In the current study, GLN-22 experienced decreased K⁺ accumulation and excessive Na⁺ buildup under salt stress conditions, disrupting the Na⁺/K⁺ ratio and ion homeostasis [67]. Depending on the duration and intensity of the stress, as well as the stage of exhaustion, the plant may establish a new physiological equilibrium following the removal of the stressor [68]. The more tolerant cultivar GLN-33 achieved restriction of Na⁺ accretion in roots and shoots and rapid renewal of ion status (particularly K⁺), which is vital for plant growth (Fig. 3). In contrast, the salt-sensitive cultivar (GLN-22) stored high levels of Na⁺ in its roots and even higher levels in its leaves, which likely impeded its return to normal growth. In the cultivar UAF-Q7, K⁺ concentration increased up to the 10:20 level and then declined at 20:40 level. To counteract the enhanced Na⁺ contents, a suitable K⁺/Na⁺ concentration in leaves must be achieved by increasing K⁺ transport from root to shoot. This may be because quinoa plants deposit Na⁺ that is easily accessible for cytosolic osmotic adjustment and to maintain turgor pressure [69, 70]. K⁺ is thought to activate approximately 50 enzymes, including those involved in the synthesis of chlorophyll and RubisCO [70]. Hence, Na⁺ cannot replace the specific role of K⁺. To maximize intake of water (as an osmotic adjustment process), conserve growth potential during a stress period [71], and boost growth during the recovery period, high K⁺ uptake must be established.

Despite its superior performance, GLN-33 showed a significant reduction (23% in SDW and RL) and increased leaf Na⁺ contents at the highest EC: SAR level (20:40), confirming that tolerance is relative rather than absolute. This residual reduction likely reflects the energetic costs of osmotic adjustment and ion homeostasis, resources that are diverted from growth to stress adaptation [17]. The primary mechanisms conferring salinity resistance in quinoa are leaf osmoregulation, K⁺ retention, Na⁺ elimination, and ion homeostasis rather than solely antioxidant capacity. Nevertheless, GLN-33 outperformed all other cultivars across all stress levels, maintaining a higher photosynthetic rate, greater K⁺ retention, and lower Na⁺ accumulation than GLN-22, GLN-29, or UAF-Q7. Thus, while absolute tolerance was not achieved, selecting the least affected cultivar (GLN-33) remains the best viable strategy for saline-sodic environments.

Conclusion

Salt tolerance in quinoa is cultivar-dependent, with GLN-33 showing the highest resistance through Na⁺ exclusion and K⁺ homeostasis, mechanisms absent in the most sensitive cultivar, GLN-22. However, tolerance is relative rather than absolute; even GLN-33 exhibited a 23% decline in growth at the highest stress level, whereas GLN-22 failed even at moderate levels, despite all cultivars surviving. This highlights a significant distinction: survival does not imply performance tolerance when selecting genotypes for saline-sodic conditions. Practically, GLN-33 is the most feasible choice among the tested cultivars, although hydroponic conditions circumvent soil restrictions (such as SAR-induced infiltration loss and structural degradation). Therefore, field validation is required before large-scale recommendations can be made. Future studies should focus on (i) multi-location field trials of GLN-33 in real-world saline-sodic soils and (ii) molecular characterization of its ion transport pathways to better understand the genetic basis of its improved homeostasis.

Authors’ contributions

**Asma Zafar: ** Writing – original draft, Formal analysis, Conceptualization, Methodology, Data curation. Writing–review and editing. **Muhammad Munir: ** Formal analysis, Data Curation, Funding acquisition. **Ghulam Murtaza: ** Supervision, Visualization, Investigation, Resources. **Abdul Ghafoor: ** Investigation, Validation, **Babiker M.A Abdel-Banat: ** Data curation. **Irfan Afzal: ** Investigation, Visualization. **Hassan Ali-Dinar: ** Software, Visualization. **Zia Ur Rahman Farooqi: ** Writing – review and editing, Software, Formal analysis. **Abdullah Sheikh: ** Visualization. **Weibo Shen: ** Supervision, Project administration.

Funding

This research was funded by the Deanship of Scientific Research, Vice Presidency for Graduate Studies and Scientific Research, King Faisal University, Kingdom of Saudi Arabia, under the Ambitious Researcher program (KFU262398).

Data availability

The data set used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.