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Article

Effects of Salt Stress on Growth, Proline and Mineral Content in Native Desert Species

by
Majda Khalil Suleiman
,
Arvind Bhatt
*,
Tareq A. Madouh
,
M. Anisul Islam
,
Sheena Jacob
,
Rini Rachel Thomas
and
Mini Thiruthath Sivadasan
Kuwait Institute for Scientific Research, P.O. Box 24885, Safat 13109, Kuwait
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(7), 6232; https://doi.org/10.3390/su15076232
Submission received: 28 December 2022 / Revised: 21 February 2023 / Accepted: 13 March 2023 / Published: 4 April 2023
Browse Figures
Review Reports Versions Notes

Abstract

:
Implementing large-scale restoration programs requires in-depth understanding about the salinity tolerance of native species, especially in the Arabian Peninsula where salinity is one of the most serious problems. Seedlings of four native species, namely Vachillea pachyceras, Haloxylon salicornicum, Rhanterium epapposum, and Farsetia aegyptia, were subjected to different salinity treatments (1.6 (control), 3, 5, 7, and 10 dS/m). Survival, growth performance, biomass and proline accumulation were assessed after six months of salinity exposure. Different mineral concentrations in the shoot and root tissues were assessed before and after the salinity exposure. Our results showed that salinity influenced the growth parameters, enhanced proline accumulation and changed the contents of essential elements. However, the effects of salinity stress on the growth and other parameters were largely species-specific. Proline accumulation increased with an increase in the salinity level in all the species. However, the mineral content in the root and shoot tissues showed variation, depending on the species and the level of salinity stress. Overall. H. salicornicum appeared to be the most tolerant species, as its seedling survival (100% at EC 10), and root and shoot biomass were impacted minimally; meanwhile, F. aegyptia (16.7% seedling survival at EC 10) appeared to be highly sensitive to the salinity. Data generated from this study will be helpful in screening the suitable species according to their salinity tolerance in salt-affected areas.

1. Introduction

The Arabian Peninsula is characterized by extremely high temperatures, scanty and infrequent rainfall (<250 mm), and limited renewable groundwater resources [1,2]. The accumulation of salt in the soil due to the absence of adequate precipitation leads to an increase in salinity [3]. All these environmental factors contribute significantly to limiting plant growth and development [4,5]. Among them, salinity is one of the main abiotic factors that adversely affects the plant growth and development. Salinity causes osmotic stress and ionic imbalance, which lead towards disturbing metabolic processes, nutrient imbalances, membrane dysfunction and thus severe effects upon plant morphology and their physiological processes [6,7,8].
Plants have evolved several strategies that involve alterations at the morphological, physiological, biochemical and genetic level, in order to cope with salinity stress [9]. For example, they maintain homeostasis by (i) osmotic regulation, which transfers out the excessive Na+ ions to the vacuole, and (ii) the synthesis of osmolyte, which helps them to protect against the adverse effects of salinity stress [8,10]. Various plant growth parameters, such as survival, height, plant biomass and yield, have been widely used for screening the salinity tolerance of various agricultural and horticultural crops [11,12,13,14]. Proline accumulation by plants under salinity stress is considered to be a physiological adaptation that helps them to counter oxidative stresses by stabilizing the cellular structure, reducing injury to the photosynthetic apparatus and maintaining the pH and redox status balance, etc. [15,16,17]. Usually, ions (Na+/K+ ratio) and proline accumulation are used for screening the salinity tolerance [18,19,20]. However, salinity usually hampers the root growth through ionic toxicity and osmotic effects, and thus influences the water and nutrient uptake [21]. Therefore, it is important to study the effect of salinity on different essential elements. Minerals not only play an important role in plant growth and development, but also contribute to various physiological and metabolic processes that directly or indirectly affect the salt tolerance. However, salinity causes a nutritional imbalance by affecting the absorption and transport of mineral elements, resulting in mineral nutrient stress and an imbalance in plant ion homeostasis [22]. Adaptation to salinity stress is directly related to the maintenance of intracellular ionic homeostasis. Therefore, understanding the effect of salinity on these nutritional elements is essential and can provide information on the adaptive cellular responses that are related to variations in the degree of salinity tolerance. However, such baseline information still remains unexplored for most of the native plant species of Arabian Peninsula. Understanding the adaptation mechanisms and growth, morphology, physiology and biochemical strategies for surviving with salinity stress could be important for selecting suitable species for restoration based on their salinity tolerance. Salinity tolerance strategies not only vary among species, but also among the different stages of plant growth [7,23]. In general, salinity either hinders the germination of salt-sensitive species (i.e., glycophyte) or delays the germination of salt-tolerant species (i.e., halophyte species) [24,25,26,27,28,29]. Similarly, the seedling stage is also reported to be highly sensitive to salinity stress [30,31]. Therefore, understanding the relationship between different growth, physiological and biochemical parameters, and relating these parameters with salinity tolerance, especially during the seedling stage, would allow us to select suitable species according to the salinity gradient and also predict the different species’ performances during restoration.
Species selection is one of the first and most important steps in restoration and revegetation programs because it plays an important role in determining the long-term success of a restoration. Therefore, the use of native species is highly suggested because they are well adapted to the local environment and thus have higher chances of successful establishment [32,33,34,35]. Moreover, incorporating native species in restoration can offer a complement to conservation efforts, as well as enhance ecosystem services in order to improve livelihoods. Since salinity is one of the main environmental problems in the Arabian Peninsula, understanding the response of different native species to salinity could be crucial for planning, screening, and executing future restoration projects. However, very little is known about the response of native plants to salinity, especially in the Arabian Peninsula. Therefore, it is important to understand the responses of native species to different levels of salinity in the later stages of plant development. In the present study, we attempted to investigate the interspecific differences in growth performance, proline accumulation and mineral contents in response to salinity stress in four native desert species under greenhouse conditions. The findings of the present study provide important information for understanding the salinity tolerance of native plants, and ultimately could be helpful for restoration specialists when wanting to use suitable species according to their salinity tolerance in salt-affected areas.

2. Materials and Methods

2.1. Seed Source

Mature seeds of V. pachyceras were collected from Alizzia, Riyadh, Saudi Arabia, F. aegyptia seeds were collected from KSRI, Kuwait, H. salicornicum seeds were collected from the National Park, Kuwait and R. epapposum seeds were collected from Al-Faisaliya Farms, Wafra, Kuwait, at the time of their natural dispersal. Seed viability was determined immediately after seed collection (within a week) for all the species, by staining with 1% tetrazolium (2,3,5-tryphenyl tetrazolium chloride, TTC) [36].

2.2. Seedling Production

Vachellia pachyecras seeds have physical dormancy, so the seeds were scarified using sand paper and then soaked in water (24 h) prior to sowing them in biodegradable jiffy pots (5 cm). Conversely, the capitulli of R. epapposum was soaked in 750 ppm GA3 for 24 h prior to being sown in biodegradable jiffy pots. However, F. aegyptia and H. salicornicum seeds were directly sown in jiffy pots. The native soil (raw sand) was used to fill the pots without adding any fertilizers and nutrients. The pots were placed in a greenhouse at Kuwait Station for Research and Innovation (KSRI), Sulaibiya, Kuwait.
The seedlings of all the species were maintained in a greenhouse for one year. Prior to the initiation of the salinity trials (December 2017 to May 2018), the seedlings were transplanted into five-gallon pots and maintained for three months. Seedlings of each species were irrigated with freshwater before the salinity trials were initiated. However, the seedlings were irrigated with a salinized nutrient solution with four different electrical conductivities (i.e., 1.6, 3, 5, 7, and 10 dS/m) during the salinity trials. The salinity level of 1.6 dS/m was similar to the salinity level of freshwater and hence this treatment was considered as the control. The Hoagland nutrient solution was made by adding different salts, including Ca(NO3)2 4H2O, KNO3, MgSO4 7H2O, K2HPO4, CaCl2, and NaCl (Table S1). However, the modified Hoagland nutrient solution was applied to the seedling, ensuring that the selected level of salinity was achieved and that the pH was adjusted to ~7.0 by adding phosphoric acid 45 v/v [37]. The seedlings were irrigated twice a week using nutrient solutions of different electrical conductivities during warmer months (April–May). However, during winter, they are irrigated once every 10 days (from December to March). For the each irrigation event, the amount of water used was 1 L/plant. The experiment had five levels of salinity (treatment) and three replications, with six plants per species in each replication. Treatments were arranged in a completely randomized design. The trial was run for about 6 months and data on the number of surviving seedlings of each species were counted at the end of the experiment. To keep track of the environmental conditions in the greenhouse, the minimum/maximum air temperature and relative humidity were recorded using a Thermo-Hygrometer (Electronic Temperature Instrument Ltd.; UK). The average temperature of the greenhouse was 34.5 °C (maximum) in May and 23.0 °C (minimum) in January, whereas, the relative humidity varied between 65.4% (May) and 34.0% (January), respectively (Table 1). The physical properties of the soil were analysed before planting. The chemical properties (pH; EC; N; P; K; Ca; Mg; Na; Cl and S) of the soil were analysed before (Table 2) and after (Table 3) terminating the experiment using the appropriate methods [38].
At the end of the experiment, seedlings of each species from their respective treatments were washed free of soil with water and separated into shoots and roots. All of these were oven-dried at 70 °C for 72 h and weighed to determine the shoot and root biomass. Data on growth parameters (seedling height and root collar diameter) and survival were collected at the end of the experiment (6 months after the salinity exposure). Seedling index was calculated by using the following formula:
SI = H × RCD2,
where H = height of the plant at the end of the experiment, RCD = root collar diameter and SI = seedling index.

2.3. Proline Content

The proline content in the leaf tissues was measured as the proline accumulation after salinity exposure, as described by Bates et al. (1973) [39]. Approximately 0.5 g of fresh leaf sample of each species was homogenized in 10 mL of 3% aqueous sulfosalicylic acid and centrifuged at 3000 rpm for 10 min. A reaction mixture containing 2 mL of Glacial Acetic Acid, 2 mL of 6 M Orthophosphoric acid, and 2 mL of acid Ninhydrin solution was prepared, and 2 mL of the filtrate containing the samples was added and incubated at 100 °C for 1 h. Then, 4 mL of toluene was added to the reaction mixture, vortexed for 10 min and left for 5 min on the benchtop. The chromophore containing toluene was transferred to fresh tubes and warmed to room temperature. The absorbance was measured at 520 nm using toluene as a blank under the Spectrophotometer (Shimadzu UV spectrophotometer, UV-1800). Proline concentration was determined using a standard concentration curve and calculated on a fresh weight basis using the following formula:
Proline   concentration   ( µ moles / g   of   fresh   weight   material = [ ( P × T ) / 115.5   µ g / micro   mole ] / ( sample   weight   in   grams / 5 ) ,
where P = Amount of Proline (µg/mL) and T = Amount of Toluene (mL).

2.4. Chemical Analyses of Soil and Plant Tissues (Root and Shoot)

Initial and final composite soil samples were collected from the pots and analyzed for their physical and chemical properties. Standard procedures as per USDA [40] were used to analyze the soil at Soil Chemistry Laboratory and Central Analytical Laboratory of KISR.
Ions and mineral (N, P, K, Mg, Ca and S) analyses were performed on root and shoot tissues. Samples (shoot and root) were collected before and after the salinity treatments. The plant parts of the seedlings grown in soil at the same level of salinity for each species were pooled separately. Plant materials were dried and powdered in a grinder before analysis. Approximately, 5 g each of shoot and root were used for analysis [41,42]. Three subsamples of plant tissues (shoot and root) were analyzed.

2.5. Data Analysis

Data on survival percentage, seedling index, biomass (shoot and root) and proline content, and soil and plant nutrients were analyzed using the VSNi software for Bioscientists® Genstat® software—20th Edition). The normality of the data and the homogeneity or equality of variances (homoscedasticity) were assessed using the Shapiro–Wilk test for normality and Bartlett’s test for homogeneity of variances. Wherever necessary, data were log transformed and analyzed using one-way Analysis of Variance (ANOVA), and the significant differences among treatments were ascertained by performing a Bonferroni test with an alpha level of 0.05. Survival data were arcsine transformed prior to analysis. The non-transformed data are presented in the graphs.

3. Results

Seed viability ranged from 72% to 100% among the tested species. Vachillea pachyceras seeds show the highest viability, followed by R. epapposum (90%), F. aegyptia (76%) and H. salicornicum (72%), respectively.

3.1. Soil Characteristics

The sandy fraction in the soil was highest and the clay fraction was the lowest. Overall, the soil was alkaline in nature with a low availability of macro and micronutrients. The soil’s chemical properties (pH and EC) and mineral content were influenced by salinity, depending on the species and salinity concertation. In general, the seedlings of all the species irrigated with the control (EC 1.6) showed a significantly higher pH content in the soil compared to those irrigated with higher salinity (EC 10). However, the EC values of the soil significantly increased with an increasing salinity level. Similarly, the Na and Cl contents in the soil also increased significantly with the increasing salinity level (Table 3). The soil’s S content also increased with the increasing salinity, but it was significant only for R. epapposum and F. aegyptia (Figure 1 and Figure 2). Meanwhile, K content increased in soil with the increasing salinity level for V. pachyceras (Figure 3) and F. aegyptia (Figure 2). The seedling of F. aegyptia irrigated with a higher salinity level showed a significantly higher Ca content in the soil, whereas other species irrigated with different salinity treatments did not show any consistency in terms of Ca content in the soil. Mg contents in the soil reduced marginally with the increasing salinity, especially for V. pachyceras (Figure 3) and R. epapposum (Figure 1), but it did not show any consistency in H. salicornicum (Figure 4) and F. aegyptia (Figure 2). However, N and p values in the soil did not show any significant change concerning the salinity and species.

3.2. Effect of Salinity on Survival and Growth Characteristics

A two way ANOVA revealed that there was a statistically significant interaction between salinity treatments and species on survival percentage (F(12, 40) = 8.16, p < 0.001). A simple main effect analysis showed that both the factors, i.e., species (F(3, 40) = 37.66, p < 0.001) and salinity treatments (F(4, 40) = 24.78, p < 0.001), have a significant effect on the survival percentage.
The salinity treatments had no significant effect on the seedling survival percentage of V. pachyceras. However, the survival percentage was slightly higher under the control (100%) compared to EC 10 (94%). Meanwhile, H. salicornicum seedlings showed 100% survival across all the salinity treatments. In contrast, the salinity treatment had a significant impact on the seedling survival rate of R. epapposum and F. aegyptia (Table 4). The survival percentage dropped to 56% for R. epapposum and 17% for F. aegyptia with the higher salinity treatment at the end of the study period (EC 10) (Figure 5). The seedlings of R. epapposum grown in control (EC 1.6) and those irrigated with moderate salinity (EC 3, EC 5, and EC 7) had a significantly higher seedling survival rate (100%) compared to those irrigated with EC 10 treatment. However, the seedling survival percentage in F. aegyptia gradually declined as the salinity level increased (Figure 5). The onset of the summer months increased seedling mortality in both species (R. epapposum and F. aegyptia).
Different growth characteristics (i.e., seedling index, mean shoot biomass and mean root biomass) were significantly influenced by salinity, depending on the species and salinity concentration. A two-way ANOVA revealed that there was a statistically significant interaction between the salinity treatments and species on the seedling index (F(12, 160) = 3.07, p < 0.001). In addition, both the factors, i.e., the species (F(3, 160) = 118.62, p < 0.001) and salinity treatment (F(4, 160) = 6.06, p < 0.001), had a significant effect on the seedling index.
The seedling indexes of V. pachyceras, R. epapposum and F. aegyptia were significantly affected by treatments with different levels of salinity. The seedling of V. pachyceras, when subjected to a higher salinity treatment (EC 10), exhibited a significantly lower seedling index compared to the control (EC 1.6). However, there were no significant differences in the seedling index across the EC 3, EC 5, EC 7 and EC 10 treatments. Similarly, for R. epapposum, the seedlings subjected to a higher salinity treatment (EC 10) showed a significantly lower seedling index compared to the control (EC 1.6) and EC 7. However, there were no significant differences in the seedling index across the other treatments. F. aegyptia seedlings attained a significantly lower seedling index in EC 10 compared to the other salinity treatments. Although increasing the salinity level reduced the seedling index in H. salicornicum, there was still no significant effect on the seedling index.
A statistically significant interaction between the salinity treatments and species on shoot biomass (F(12, 340) = 8.13, p < 0.001) was observed in the two-way ANOVA. Additionally, both (main factor) species (F(3, 340) = 606.19, p < 0.001) and salinity treatments (F(4, 340) = 20.53, p < 0.001) had a significant effect on the shoot biomass.
Salinity significantly affected the shoot biomass in all species except H. salicornicum (Figure 6). We observed a gradual reduction in the shoot biomass of V. pachyceras, R. epapposum and F. aegyptia with an increasing level of salinity (Figure 7, Figure 8 and Figure 9). V. pachyceras seedlings attained a significantly higher shoot biomass in EC 3 compared to the EC 5, EC 7 and EC 10 treatments. However, the seedlings exposed to the control and EC 3 did not show any significant difference. Similarly, R. epapposum and F. aegyptia seedlings also attained a maximum shoot biomass in EC 3 compared to the EC 7 and EC 10 treatments (Figure 7 and Figure 9). The highest level of salinity (EC 10) was correlated with a significantly lower shoot biomass in both R. epapposum and F. aegyptia. Although H. salicornicum seedlings experienced a gradual reduction in their shoot biomass with an increasing level of salinity, there was no significant effect on the shoot biomass (Figure S1).
The interaction between the salinity treatments and the species on the root biomass (F(12, 340) = 2.50, p < 0.004) was statistically significant. A simple main effect analysis indicated that both species (F(3, 340) = 441.51, p < 0.001) and salinity treatments (F(4, 340) = 3.53, p < 0.008) also had a significant effect on the root biomass.
The salinity treatments had a significant effect on the root biomass of V. pachyceras and the seedlings attained their highest root biomass with the EC 3 treatment, but it was not significantly different from the control. However, increasing the salinity level caused a significant reduction in the root biomass compared to the control. Although R. epapposum and F. aegyptia seedlings attained a maximum root biomass with the EC 3 treatment, it was not significantly different from the other treatments. Similar trends were shown by H. salicornicum seedlings in terms of their root biomass across the salinity treatments (Table 4 and Figure 6, Figure 7, Figure 8 and Figure 9).

3.3. Effect of Salinity on Proline Accumulation

There was significant effect of species (F(3, 40) = 1033.01, p < 0.001), salinity (F(4, 40) = 714.63, p < 0.001) and their interaction (F(12, 40) = 336.27, p < 0.001) on the proline content. The proline content increased with the increasing salinity level in all the studied species (Figure 6, Figure 7, Figure 8 and Figure 9). The seedlings of V. pachyceras when treated with EC 10 treatment showed a significantly (p = 0.000) high proline content in the leaf tissue compared to those treated with other salinity treatments (EC 3, EC 5 and EC 7). Meanwhile, the seedlings exposed to EC 5 and EC 7 treatments did not show any significant variation in their proline content. The lowest level of proline accumulation in V. pachyceras was observed in the control seedlings. Similar trends were observed in R. epapposum seedlings, in which the EC 10 treatment resulted in a significantly higher proline content in the leaf tissues compared to the other treatments (EC1.6, EC 3, EC 5 and EC 7). However, there were no differences in the proline content between the control and EC 3 treatment. Overall, the lowest proline content was observed in the leaf tissues of H. salicornicum seedlings across all the salinity treatments and when compared to the other species (Figure 6). Similar to other species, H. salicornicum seedlings, when exposed to high salinity (EC 10), showed a significantly higher proline content in their leaf tissues when compared to the control, as well as to the other salinity treatments. However, there were no differences in the proline content between the EC 3 and EC 5 treatments, but they accumulated a significantly higher proline content than the control (EC 1.6) and one lower than the EC 7 treatment. Interestingly, F. aegyptia seedlings did not show much difference in their proline content across the treatments, except for with the control and high salinity (EC 10) treatment.

3.4. Effect of Salinity on Mineral Contents

The mineral contents of the root and shoot tissues showed variation before exposure to the salinity treatments, depending on the species and plant tissue (root and shoot) (Table 5 and Figure 10). Among the species, the root tissues of F. aegyptia showed the highest calcium contents, followed by V. pachyceras, H. salicornicum and R. epapposum, respectively. However, V. pachyceras seems to be rich in potassium compared to the other species. Meanwhile, other mineral contents (N, P, Na, Mg, Cl, and S) did not show much variation in the root tissues. Similarly, shoot tissues also showed variation in terms of their mineral contents. For example, F. aegyptia shoot tissues had the highest N and S contents, whereas V. pachyceras shoot tissues had the highest K contents (Figure 10). However, other mineral contents did not show much variation in the shoot tissues among the tested species.
After salinity exposure, variations in the mineral contents of the plant tissues (root and shoot) were observed, depending on the species. For example, the root tissues of V. pachyceras did not show any significant variation in N and S contents under different concentrations of salinity (Figure 11). Meanwhile, R. epapposum did not show any significant variation in its N, Mg and S contents (Figure 12). Significant variations were observed in the K, Ca and Na contents of root tissues of H. salicornicum (Figure 13). However, F. aegyptia showed a significant variation in all the mineral contents, except N (Figure 14). The different salinity treatments did not show any significant variations in the N, Mg and S contents of the shoot tissues of V. pachyceras (Figure 11). Whereas, the shoot tissues of R. epapposum did not show any significant variation in the N, Ca, Mg and S contents (Figure 12). H. salicornicum also did not show any significant variation in the N, P, K and S contents with an increasing salinity level (Figure 13). Meanwhile, F. aegyptia shoot tissues showed a trend similar to that of the root tissues, where significant variation was observed for all the mineral contents, except N (Figure 14).
The Na content of the root tissues increased with an increasing salinity in all the species. The highest accumulation was observed in H. salicornicum and the lowest in V. pachyceras in EC 10-treated seedlings. The calcium content declined in response to an increasing salinity level in the root tissues of V. pachyceras and R. epapposum, but it increased in H. salicornicum and F. aegyptia. Meanwhile, the chlorine content also increases with an increasing salinity and the EC 10-treated seedlings showed a significantly higher chlorine content in the root tissues compared to the control in all species, except H. salicornicum. Although the highest P content of V. pachyceras root tissue was observed in the EC 10-treated seedlings, other treatments did not show any significant variation in terms of P content. Conversely, a reduction in the P content was observed in root tissues of other species with an increasing salinity. Similarly, the K content also showed a reduction with an increasing salinity level, except in H. salicornicum. However, we did not find any consistent trends for the Mg and S contents of root tissues among the species in response to the different salinity treatments.
Similar to the root tissues, the Na and Cl contents increased significantly in the shoot tissues of all species with an increasing level of salinity. Among the species, the highest Na content was observed in the shoot tissues of H. salicornicum and the lowest was observed in V. pachyceras of the EC 10-treated seedlings, compared to their respective controls. Similarly, the Cl content also significantly increased in the shoot tissues of all species treated with EC 10 compared to the control (EC 1.6). The Ca content in the shoot tissues of V. pachyceras were significantly high at EC 3, EC 5 and 7, compared to the control and EC10 treatment. However, it increased significantly in the H. salicornicum and F. aegyptia seedlings that were exposed to higher salinity levels (EC 10) compared to their respective controls; it declined in R. epapposum. However, the K content in the shoot tissues of V. pachyceras, R. epapposum and F. aegyptia declined especially in the seedlings treated with EC 10 compared to the control, but it increased marginally in H. salicornicum. In response to salinity treatments, the P contents decreased with an increasing salinity level in the shoot tissues of R. epapposum and F. aegyptia, but it increased in V. pachyceras. However, no clear trend was observed for the Mg and S contents of the shoot tissues among all species in response to the increasing salinity level (Figure 11, Figure 12, Figure 13 and Figure 14).

4. Discussion

Understanding the salinity tolerance could be crucial for species selection for restoration, especially in arid environments where saline soils are common [43,44]. Usually, different species differ distinctly in terms of their salinity tolerance [45,46]. Given this scenario, any expansion in the restoration or revegetation programs of desert environmental conditions requires knowledge about the salinity tolerance of different native species in order to identify the potential species for restoration. Therefore, we explored the salinity responses of four common species from desert habitats in Kuwait.
The soil pH decreased with the increasing salinity, especially at higher salinity levels. The exchange of cations between Na+ and H+ might be responsible for increasing the H+ ion concentration that lowers the soil pH at higher salinity levels [47]. However, the soil EC was low in the control (EC 1.6 dS/m) and increased significantly with an increasing salinity level, indicating that increasing the concentration of NaCl and CaCl2 in Hoagland nutrient solution, which was used for irrigating the seedling, might be responsible for the increasing EC. In the present study, the Na, Cl and S content increased with an increasing EC in the irrigation solution. These findings are in agreement with a previous study, which reported that salinity enhances the concentration of Na, Cl and S in soil [48]. N, P, K, Ca and Mg accumulation in soil showed inconsistency, depending on the salinity level and species. This discrepancy between the salinity level and species could be linked to the variation in ion immobilization in the soil. Such contradictory results have also been reported previously, suggesting that each species under any set of conditions may respond to salinity in a different way [49,50].
Overall, salinity treatments negatively affect seedling survival, growth and biochemical parameters, depending on the species. Survival is one of the important parameters that can be helpful for assessing the potential of a species for restoration. In this study, A. pachyceras and H. salicornicum seedlings exhibited a high survival percentage, irrespective of the salinity treatments, indicating that this species can survive under salinity stress. However, the seedling survival percentage reduced significantly with the increasing salinity level in R. epapposum and F. aegyptia. These results are consistent with previous findings, which reported that different species differ distinctly in terms of salinity tolerance [45,46]. The high seedling survival of A. pachyceras might be due to the presence of salt-tolerant strains of Rhizobium spp. in the root, which help this species to cope with salinity. The N2-fixing bacteria has been reported to be beneficial and to assist the plants to cope with salinity stress [51]. Whereas, the higher seedling survival of H. salicornicum could be linked to its halophytic nature. Generally, halophytes have the ability to grow and proliferate in the presence of a suitable salt concentration, compared to little or no salt [52]. Halophytes are reported to have the ability to grow and survive in saline habitats when compared to glycophytes [53,54]. The seedling survival of R. epapposum and F. aegyptia (glycophytes) was severely affected by the increasing salinity level, indicating their salt sensitivity. Moreover, an increasing seedling mortality in the glycophyte species in the summer months could be linked to the increased ambient and soil temperature that may lead to higher osmotic and ionic stress, which subsequently affects the seedling survival.
Salinity reduced growth parameters such as seedling index and biomass (shoot and root), depending on the species as well as the salinity concentration. For example, an increasing salinity level significantly reduced the seedling index and shoot biomass in V. pachyceras, R. epapposum and F. aegyptia. Meanwhile, different levels of salinity did not affect the seedling index and biomass (root and shot) significantly in H. salicornicum, although seedlings treated with 1.6 dS/m EC (control) irrigation water attained the highest values for these parameters. R. epapposum, and F. aegyptia seedlings did not show any significant difference in terms of root biomass across the salinity treatments. However,, a gradual reduction in the root biomass was observed for both the species with an increasing salinity level. Meanwhile, salinity caused a significant reduction in the root biomass of A. pachyceras. Salinity impairs plant growth and development via osmotic stress that inhibits the uptake of K+ and Ca2+, and enhances Na+ toxicity [55,56,57]. Substantial changes were observed in terms of the growth parameters between the species and salinity treatments. These results are consistent with previous findings, indicating that the severity of salt damage is dependent on the species, salinity level and growth parameters [7,58]. The reduction in the shoot biomass of A. pachyceras, R. epapposum and F. aegyptia with an increasing salinity level might be caused by disturbances in physiological and biochemical activities, inhibiting leaf area and the number of leaves created under salinity conditions [59,60]. However, the root biomass in R. epapposum and F. aegyptia did not show any significant difference across the salinity treatments. The lower sensitivity of the roots of these species to salinity could be a adaptation strategy that helps them to survive in temporary fluctuating salinity exposure. Moreover, roots have a high possibility of detecting new soil zones to avoid salinity if they encounter such conditions [61]. Exposure to different salinity treatments did not affect the biomass accumulation (root and shoot) in H. salicornicum. Being a halophyte, this species is capable of enduring salinity conditions without losing survival and biomass. Therefore, different growth parameters need to be taken into consideration when identifying the suitable species according to the salinity gradients for restoration.
Proline accumulation is correlated with salinity tolerance due to its efficiency in reducing the oxidative stress [16]. Moreover, it also acts as a solute to reduce leaf water potential, enhance water uptake, and decrease transpiration to maintain cell turgor pressure [62,63]. Proline accumulation significantly increased in all the studied species with the increasing salinity level. This is in agreement with previous studies, which reported that proline accumulation is a common indicator of salinity stress [64,65]. However, the extent of proline accumulation varies depending on the species, especially at higher salinity levels. For example, A. pachyceras and R. epapposum seedlings, when exposed to EC 10, showed the highest proline accumulation. Whereas, F. aegyptia did not show much variation in its proline accumulation among the tested salinity treatments. These results indicate that the ability to accumulate proline accumulation may vary from species to species, as well as in the level of stress. These findings are consistent with previous studies in which the same was found [66,67]. In the present study, H. salicornicum showed the least amount of proline accumulation, suggesting that the synthesis of these compounds is constitutive in some halophyte species and, therefore, that they may not show a remarkable enhancement with an increasing salinity level [54,68]. The salinity tolerance and proline accumulation of different species did not show any relation (i.e., halophytic species, H. salicornicum, accumulated lower proline content compared to other glycophyte species in all the salinity levels). These results suggest that proline accumulation is not a main contributing factor of the salinity tolerance mechanisms of these species.
Understanding the differences in the mineral contents of species and various plant parts of the same species could be important for assessing a salinity tolerance strategy. Prior to the salinity exposure, we observed significant variation in terms of the mineral contents of the species, as well as different parts of the same species. The variations in parameters such as root density, the root–soil interaction, and the absorption and translocation mechanisms among the species, might be responsible for such variation [69]. Moreover, variations in the mineral content of individual plant parts usually occurred due to their dependency on translocation, which varies among species and plant tissue [70]. After salinity exposure, variations in the mineral contents of plant tissues (root and shoot) were observed, depending on the species. For example, the N contents in the root and shoot tissues of different species remained unaffected by salinity, suggesting that salinity stress does not influence nitrogen uptake. Previous studies have also shown that the nitrogen content of plant tissues is not affected by an increasing salinity level [71,72]. However, the effects of salinity on other mineral contents are equivocal, depending on the species and on the different parts. These species-specific differences might be associated with variations in the mineral content of different species, as well as with alterations in their movement and preferential accumulation in the plant tissues, especially when the plants are subjected to salinity [73,74,75].
Most of the species showed a reduction in P and K contents in their root tissues, especially at higher salinity levels. However, we did not find any consistent trends for P and K content in shoot tissue. The inhibition of P and K uptake by the root at higher salinity levels is common; this causes a nutritional imbalance and affects plant growth [76]. A reduction in K was observed in the shoot tissues of all the species, except H. salicornicum. This reduction in K with the increasing salinity level might be due to its replacement with a sodium ion, which is elevated with an increase in the Na of nutrient solutions [77]. However, maintaining the K contents among the salinity treatments in the shoot tissue by H. salicornicum could be related to its salinity tolerance trait. Maintaining an appropriate level of K contents in shoot tissues is considered to be a salt tolerance trait in halophytes [7,78].
The increasing salinity level increased the Ca content in the root tissues of A. pachyceras and R. epapposum, but it increased in H. salicornicum and F. aegyptia. These findings are consistent with previous studies that reported that salinity exposure may increase or decrease the Ca content in the root, depending on the species [79,80,81,82]. However, the Ca content in the shoot tissues decreased with the increasing salinity level in V. pachyceras, R. epapposum and H. salicornicum, but it increased in F. aegyptia. Generally, salinity increased the soil osmotic pressure and thus interfered with the mineral contents concentration in a complex manner; this may either increase, decrease, or have no effect. Therefore, this variability could be linked to the genetic differences, as well as to the uptake efficiency, of different plant parts [83]. However, a reduction in the Mg content was observed in the shoot tissues of all the species, especially at a higher salinity level, but we did not find any consistent trends in terms of the Mg content in root tissues among the salinity treatments. High uptakes of toxic ions (i.e., sodium and chloride) at a higher salinity level might be responsible for decreasing the absorption of Mg [84].
The accumulation of Na and Cl in plant tissues is one of the specific features of salt stress [85]. In our case, increasing the salinity level affected the Na and Cl content in both the root and shoot tissues, depending on the species. For example, H. salicornicum root tissues had the highest Na content compared to V. pachyceras. The ability to tolerate salinity depends on the uptake of Na from the soil to the root, and translocating them to the above-ground parts, such as the shoot and leaf tissues. In the present study, H. salicornicum (halophyte), maintained a comparatively low Na content in the root and a high content in the shoot tissues. These results indicate that as a halophyte, this species has a better ability to coordinate the distribution of Na to various tissues and has a more effective process of sequestering Na into vacuoles [86], compared to glycophytes (i.e., V. pachyceras, R. epapposum and F. aegyptia); this is because they do not have the ability to control Na transport. A higher Na concentration, especially in the root compared to the shoot in all the species, indicated its ability to regulate the uptake and translocation of sodium. Usually, roots play an important role in regulating the uptake and translocation of nutrients and salts, and thus consequently determine the salinity tolerance [8]. The accumulation of Na in the roots is reported to be an adaptive response that facilitates avoiding toxicity in the shoots by controlling the root-to-shoot transport of salt [8,87,88].
Halophytes are reported to have succulent leaves (i.e., assist in diluting the salt concentration in the sap of the cell), salt glands and bladders (assist in excreting Na) [54,89]. The chlorine content increased significantly in the shoot tissues of all species, except for H. salicornicum. Increasing the Na content is always accompanied by Cl accumulation [90,91]. Therefore, the accumulation of chlorine content in the shoot tissues at higher salinity levels could be related to the higher chlorine content. However, H. salicornicum did not show any difference in its chlorine content between the treatments, which could be linked to its adaptability to salinity as a halophyte. A previous study also found that salt-tolerant species accumulate a lower amount of chlorine in their shoots than salt-sensitive species [92]. Overall, F. aegyptia showed a remarkably high sensitivity to salinity in terms of its biomass accumulation, survival and inhibition of the uptake of mineral contents. However, H. salicornicum, as a halophyte, was least impacted by its salinity due its adaptability; it is able to adjust osmotically through accumulating and sequestering sodium and chlorine ions in vacuoles [93].

5. Conclusions

This work analyzes the responses of four native species to salinity stress. Various growth parameters, such as survival percentage and biomass (root and shoot). were affected by salinity, depending on the species and the salinity level. Among the studied species, F. aegyptia showed remarkably high sensitivity and H. salicornicum showed the least sensitivity to salinity. Proline accumulation seems to be unrelated to salinity tolerance, indicating that proline alone is not a main contributing factor to salinity tolerance mechanisms in these species. The variation in the mineral contents of plant tissue (root and shoot) appeared to be dependent on species as well as the salinity level. These species-specific differences might be related to (i) variability in the mineral contents of species and (ii) changes in their movement and preferential accumulation in the plant tissues after salinity exposure. Higher salinity caused a reduction in the plants’ P and K content, especially in root tissues. Finally, high interspecific variability was observed in growth, ion and mineral contents in response to salinity stress. These variations can be useful in early screening for salt tolerance and for recommending specific species for restoration based on their salinity tolerance.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su15076232/s1, Table S1: Composition of Hoagland Solution; Figure S1: Growth overview of test species subjected to various salinity levels.

Author Contributions

Conceptualization, and project administration, M.K.S.; methodology, M.K.S., M.A.I. and T.A.M.; Data collection and analysis, S.J., R.R.T. and M.T.S., writing original draft, A.B., M.K.S., S.J. and R.R.T., writing—review and editing, A.B., M.K.S., S.J. and R.R.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by, Kuwait Foundation for the Advancement of Sciences (KFAS) and Kuwait Institute for Scientific Research (KISR).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The datasets generated during and/or analyzed during this study are available from the corresponding author on request.

Acknowledgments

We would like to thank Processo Ramos for his continuous support and guidance.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Average final soil nutrient content in Rhanterium epapposum. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
Figure 1. Average final soil nutrient content in Rhanterium epapposum. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
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Figure 2. Average final soil nutrient content in Farsetia aegyptia. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
Figure 2. Average final soil nutrient content in Farsetia aegyptia. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
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Figure 3. Average final soil nutrient content in Vachellia pachyceras. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
Figure 3. Average final soil nutrient content in Vachellia pachyceras. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
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Figure 4. Average final soil nutrient content in Haloxylon salicornicum. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
Figure 4. Average final soil nutrient content in Haloxylon salicornicum. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
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Figure 5. Effect of different salinity treatments on survival of (a) Vachellia pachyceras; (b) Rhanterium epapposum; (c) Haloxylon salicornicum; and (d) Farsetia aegyptia. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
Figure 5. Effect of different salinity treatments on survival of (a) Vachellia pachyceras; (b) Rhanterium epapposum; (c) Haloxylon salicornicum; and (d) Farsetia aegyptia. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
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Figure 6. Effect of different salinity treatments on plant parameters of Haloxylon salicornicum: (a) Seedling Index; (b) Root Biomass; (c) Shoot Biomass; (d) Proline Content. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
Figure 6. Effect of different salinity treatments on plant parameters of Haloxylon salicornicum: (a) Seedling Index; (b) Root Biomass; (c) Shoot Biomass; (d) Proline Content. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
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Figure 7. Effect of different salinity treatments on plant parameters of Vachellia pachyceras: (a) Seedling Index; (b) Root Biomass; (c) Shoot Biomass; (d) Proline Content. The means followed by the same letter are not statistically different at p 0.001.
Figure 7. Effect of different salinity treatments on plant parameters of Vachellia pachyceras: (a) Seedling Index; (b) Root Biomass; (c) Shoot Biomass; (d) Proline Content. The means followed by the same letter are not statistically different at p 0.001.
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Figure 8. Effect of different salinity treatments on plant parameters of Rhanterium epapposum: (a) Seedling Index; (b) Root Biomass; (c) Shoot Biomass; (d) Proline Content. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
Figure 8. Effect of different salinity treatments on plant parameters of Rhanterium epapposum: (a) Seedling Index; (b) Root Biomass; (c) Shoot Biomass; (d) Proline Content. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
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Figure 9. Effect of different salinity treatments on plant parameters of Farsetia aegyptia: (a) Seedling Index; (b) Root Biomass; (c) Shoot Biomass; (d) Proline Content. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
Figure 9. Effect of different salinity treatments on plant parameters of Farsetia aegyptia: (a) Seedling Index; (b) Root Biomass; (c) Shoot Biomass; (d) Proline Content. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
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Figure 10. Initial Plant Nutrient Content (N—Nitrogen, P—Phosphorous, K—Potassium, Ca—Calcium, Mg—Magnesium, Na—Sodium, Cl—Chloride, and S—Sulphur, in (a) Root and (b) Shoot of the test species.
Figure 10. Initial Plant Nutrient Content (N—Nitrogen, P—Phosphorous, K—Potassium, Ca—Calcium, Mg—Magnesium, Na—Sodium, Cl—Chloride, and S—Sulphur, in (a) Root and (b) Shoot of the test species.
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Figure 11. Final Nutrient Content (N—Nitrogen, P—Phosphorous, K—Potassium, Ca—Calcium, Mg—Magnesium, Na—Sodium, Cl—Chloride, and S—Sulphur, in (a) Root and (b) Shoot of Vachellia pachyceras. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
Figure 11. Final Nutrient Content (N—Nitrogen, P—Phosphorous, K—Potassium, Ca—Calcium, Mg—Magnesium, Na—Sodium, Cl—Chloride, and S—Sulphur, in (a) Root and (b) Shoot of Vachellia pachyceras. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
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Figure 12. Final Nutrient Content (N—Nitrogen, P—Phosphorous, K—Potassium, Ca—Calcium, Mg—Magnesium, Na—Sodium, Cl—Chloride, and S—Sulphur, in (a) Root and (b) Shoot of Rhanterium epapposum. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
Figure 12. Final Nutrient Content (N—Nitrogen, P—Phosphorous, K—Potassium, Ca—Calcium, Mg—Magnesium, Na—Sodium, Cl—Chloride, and S—Sulphur, in (a) Root and (b) Shoot of Rhanterium epapposum. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
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Figure 13. Final Nutrient Content (N—Nitrogen, P—Phosphorous, K—Potassium, Ca—Calcium, Mg—Magnesium, Na—Sodium, Cl—Chloride, and S—Sulphur, in (a) Root and (b) Shoot of Haloxylon salicornicum. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
Figure 13. Final Nutrient Content (N—Nitrogen, P—Phosphorous, K—Potassium, Ca—Calcium, Mg—Magnesium, Na—Sodium, Cl—Chloride, and S—Sulphur, in (a) Root and (b) Shoot of Haloxylon salicornicum. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
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Figure 14. Final Nutrient Content (N—Nitrogen, P—Phosphorous, K—Potassium, Ca—Calcium, Mg—Magnesium, Na—Sodium, Cl—Chloride, and S—Sulphur, in (a) Root and (b) Shoot of Farsetia aegyptia. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
Figure 14. Final Nutrient Content (N—Nitrogen, P—Phosphorous, K—Potassium, Ca—Calcium, Mg—Magnesium, Na—Sodium, Cl—Chloride, and S—Sulphur, in (a) Root and (b) Shoot of Farsetia aegyptia. (Control: EC 1.6). The means followed by the same letter are not statistically different at p 0.001.
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Table
Table 1. Average monthly temperature and relative humidity in the greenhouse.
Table 1. Average monthly temperature and relative humidity in the greenhouse.
MonthTemperature
(°C)		Relative Humidity
(%)
December, 201725.058.0
January, 201823.045.0
February, 201824.862.2
March, 201828.248.8
April, 201826.265.4
May, 201834.534.0
Table
Table 2. Average soil nutrient content before initiating salinity treatments.
Table 2. Average soil nutrient content before initiating salinity treatments.
ParametersUnitMean
pH 8.70 ± 0.009
Electrical Conductivity (EC)mS/cm0.193 ± 0.007
Total Nitrogen (N)mg/g0.033 ± 0.001
Phosphorus (P)mg/g0.011 ± 0.000
Potassium (K)mg/g0.120 ± 0.006
Calcium (Ca)mg/g8.45 ± 2.795
Magnesium (Mg)mg/g0.245 ± 0.013
Sodium (Na)mg/g0.006 ± 0.002
Chlorine (Cl)mg/g0.033 ± 0.007
Sulphur (S)mg/g0.002 ± 0.001
Table
Table 3. Results of Analysis of Variance (p values) testing for the effect of salinity treatment on soil in selected native plant species, six months after the initiation of treatments in the greenhouse study.
Table 3. Results of Analysis of Variance (p values) testing for the effect of salinity treatment on soil in selected native plant species, six months after the initiation of treatments in the greenhouse study.
ParametersSalinitySpeciesSalinity * Species
pH<0.001<0.0010.001
Electrical Conductivity (EC)<0.0010.4070.938
Nitrogen (N)0.549<0.001<0.001
Phosphorus (P)0.718<0.0010.341
Potassium (K)0.0270.2340.096
Calcium (Ca)0.007<0.0010.015
Magnesium (Mg)<0.0010.0180.036
Sodium (Na)<0.0010.2140.818
Chlorine (Cl)<0.0010.3270.798
Sulphur (S)<0.001<0.0010.09
*—Denotes interaction.
Table
Table 4. Results of Analysis of Variance (p values) testing for the effect of salinity treatment on the plant parameters of selected native plant species six months after the initiation of treatments in the greenhouse study.
Table 4. Results of Analysis of Variance (p values) testing for the effect of salinity treatment on the plant parameters of selected native plant species six months after the initiation of treatments in the greenhouse study.
ParametersSalinitySpeciesSalinity * Species
Survival%<0.001<0.001<0.001
Seedling Index<0.001<0.001<0.001
Shoot Biomass<0.001<0.001<0.001
Root Biomass<0.0010.0080.004
Proline Content<0.001<0.001<0.001
*—Denotes interaction.
Table
Table 5. Results of Analysis of Variance (p values) testing for the effect of salinity treatment on root and shoot tissues of selected native plant species six months after the initiation of treatments in the greenhouse study.
Table 5. Results of Analysis of Variance (p values) testing for the effect of salinity treatment on root and shoot tissues of selected native plant species six months after the initiation of treatments in the greenhouse study.
NutrientsRootShoot
SpeciesSalinitySpecies * SalinitySpeciesSalinitySpecies * Salinity
Ca<0.001<0.001<0.001<0.001<0.001<0.001
Cl<0.001<0.001<0.001<0.001<0.001<0.001
C0.6290.5960.9820.790.1560.997
K<0.001<0.001<0.001<0.001<0.001<0.001
Mg<0.001<0.001<0.001<0.001<0.0010.002
Na<0.001<0.001<0.001<0.001<0.001<0.001
N0.5340.0110.9660.5130.2230.986
P0.015<0.001<0.001<0.001<0.001<0.001
S0.359<0.0010.5940.784<0.0010.632
*—Denotes interaction.
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Suleiman, M.K.; Bhatt, A.; Madouh, T.A.; Islam, M.A.; Jacob, S.; Thomas, R.R.; Sivadasan, M.T. Effects of Salt Stress on Growth, Proline and Mineral Content in Native Desert Species. Sustainability 2023, 15, 6232. https://doi.org/10.3390/su15076232

AMA Style

Suleiman MK, Bhatt A, Madouh TA, Islam MA, Jacob S, Thomas RR, Sivadasan MT. Effects of Salt Stress on Growth, Proline and Mineral Content in Native Desert Species. Sustainability. 2023; 15(7):6232. https://doi.org/10.3390/su15076232

Chicago/Turabian Style

Suleiman, Majda Khalil, Arvind Bhatt, Tareq A. Madouh, M. Anisul Islam, Sheena Jacob, Rini Rachel Thomas, and Mini Thiruthath Sivadasan. 2023. "Effects of Salt Stress on Growth, Proline and Mineral Content in Native Desert Species" Sustainability 15, no. 7: 6232. https://doi.org/10.3390/su15076232

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