Unveiling the salinity tolerance potential of Armenian Dandur (Portulaca oleracea L.) genotypes: Enhancing sustainable agriculture and food security

2.1 Experimental sites

In 2022, the Department of Ecology and Nature Conservation in the Faculty of BiologyYerevan State University (40° 18′ 29.74′' N, 44° 52′ 66.55′' E) conducted a controlled trial of our pot experiment.

2.2 Soil and plant material collection

In 2022, soil from the middle Ararat plain in Armenia was collected for experiments, focusing on clay (40° 06.076′, E 44° 22.78′) and clay loam (40° 06.422′, E 44° 19.975) soils due to their abundance and non-saline nature, with ECe values below 2.0 dS/m (Bernstein et al., 1954).

2.3 Growth conditions

The study involved experiments on P. Oleracea (Dandur) plant seeds in a climate-controlled area near the Department of Ecology and Nature Conservation in the Faculty of Biology. The seeds were surface-sterilized in 10 % sodium hypochlorite and rinsed in distilled water. A 20 kg soil sample was divided among sterile PVC pots, and one seed was sown in each. The seedlings were given salt treatments when rapid growth was observed. Soil salinity was modified by adding sodium chloride to varying concentrations [(from 0 to 2 dS m⁻¹ (non-saline) to 2–4 dS m⁻¹ (slightly saline) to 4–8 dS m⁻¹ (moderately saline) to 8–16 dS m⁻¹ (strongly saline) to > 16 dS m⁻¹ (extremely saline)]) in distilled water (Bernstein et al., 1954). Salt concentrations were gradually increased over 15 days to prevent an osmotic shock. Nonperforated plastic pots were used to prevent salt leaching and maintain the soil's consistent electrical conductivity. The trial lasted around ten weeks in a vegetative state.

2.4 Morphological, physiological, and biochemical parameters

3.1 Morphological traits

The study reveals that plants grown in extreme salinity levels experience a decrease in shoot length, with clay soil typically having 14.6 % less height than clay loam soil (Fig. 1A–B). The shoot length of plants in clay soil and clay loam soil decreases by 36.0 % and 26.2 %, respectively, compared to non-saline soil. The NaCl treatment also impacts the stem diameter of P. oleracea, with plants grown in clay and clay loam soils experiencing decreased stem diameters at higher salinity levels (66.7 % and 70.8 %, respectively) (Fig. 1 C-D).

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Fig. 1

Effect of salinity on some morphological and physiological parameters of P. oleracea during salt treatment (A-shoot length of plants in clay soil, n = 5, P < 0.05, B-shoot length of plants in clay loam soil, n = 5, P < 0.05, C-stem diameter of plants in clay soil, n = 5, P < 0.05, D-stem diameter of plants in clay loam soil, n = 5, P < 0.05, E-fresh and dry weight of root of plants in clay soil, n = 5, P < 0.05, F-fresh and dry weight of shoot of plants in clay soil, n = 5, P < 0.05, G-fresh and dry weight of root of plants in clay loam soil, n = 5, P < 0.05, H-fresh and dry weight of shoot of plants in clay loam soil, n = 5, P < 0.05, I-CCI of plants in clay soil, n = 50, P < 0.05, J-CCI of plants in clay loam soil, n = 50, P < 0.05, K-RWC of plants in clay soil, n = 50, P < 0.05, L-SWC of plants in clay soil, n = 50, P < 0.05, M-RWC of plants in clay loam soil, n = 50, P < 0.05, N-SWC of plants in clay loam soil, n = 50, P < 0.05, O-LS of plants in clay soil, n = 10, P < 0.05, P-LS of plants in clay loam soil, n = 10, P < 0.05, Q-REL of plants in clay soil, n = 10, P < 0.05, R-REL of plants in clay loam soil, n = 10, P < 0.05.

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3.2 Physiological traits

3.3 Biochemical traits

4.1 Salt tolerance index

Plants cultivated in two distinct kinds of soil displayed the greatest values of the salt tolerance index at the non-saline level (Fig. 2A–B). The difference was 63.9 % in clay soil and 70.4 % in clay loam soil when comparing plants grown in slightly and extremely salinized soil. As Fig. 2A–B illustrates, the salt tolerance index value considerably plummeted as salinity levels increased.

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Fig. 2

Effect of salinity on some stress indices of P. oleracea (A-salt tolerance index of P. oleracea in clay soil, n = 10, P < 0.05, B-Salt tolerance index of P. oleracea in clay loam soil, n = 10, P < 0.05, C-Salt shoot toxicity of P. oleracea in clay soil, n = 10, P < 0.05, D-Salt shoot toxicity of P. oleracea in clay loam soil, n = 10, P < 0.05, E-Shoot Length Stress Index of P. oleracea in clay soil, n = 10, P < 0.05, F-Shoot Length Stress Index of P. oleracea in clay loam soil, n = 10, P < 0.05, G-Shoot Weight Stress Index of P. oleracea in clay soil, n = 10, P < 0.05, H-Shoot Weight Stress Index of P. oleracea in clay loam soil, n = 10, P < 0.05.

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4.2 Salt shoot toxicity %

Salt toxicity on P. oleracea shoots increases with salinity levels, with extreme clay soils enhancing it by 6.5 times and clay loam soils by 3.2 times (Fig. 2C–D).

4.3 Shoot length stress index %

The shoot length stress index, a crucial metric for assessing P. oleracea tolerance, decreases with increased saline levels in both soil types. The decline is more proportionate in clay soil, with a sharp decline in clay loam soil after moderate salinity (Fig. 2 E-F). This difference is 19.8 % in clay loam soil and 16.3 % in clay soil.

4.4 Shoot weight stress index %

The shoot weight stress index declined in both slightly and extremely saline soils as salinity levels increased, with a 72.3 % difference in clay soil and a 67.5 % difference in clay loam soil (Fig. 2 G-H).

6.1 Status of growth, water content and productivity of shoot and root

For water from drainage reuse systems, purslane (P. olereacea) has been widely proposed as a desirable halophytic crop with an excellent nutritional profile (Grieve and Suarez, 1997). It was reported that P. olereacea could develop with greater biomass in low-salinity environments and possibly complete its life cycle in high-salinity environments. Furthermore, the plant's shoots revealed no manifestations of harm or nutrient deficiencies arising from the excess salt (Grieve and Suarez, 1997). This study examined the morphological, physiological, and biochemical changes of the undocumented Dandur (P. oleracea L.) plant genotype under two soil conditions (clay and clay loam) along with different concentrations of salt (0-2 dS m^-1 (non-saline) to 2-4 dS m^-1 (slightly saline) to 4-8 dS m^-1 (moderately saline) to 8-16 dS m^-1 (strongly saline) to >16 dS m^-1 (extremely saline) (Fig. 3). The results demonstrated that shoot length and steam diameter were not much affected at 100 mM salt stress compared to control (Fig. 1 A-B, C-D). Still, these morphological features declined with increasing salinity levels from moderate to extreme in Dandur purslane plants (Fig. 1A–B). Some previous studies also suggested that NaCl above 350 mM affected the growth and development of purslane plants (Grieve and Suarez, 1997). Some reports also support our finding that higher NaCl concentrations at 200 mM negatively impacted some species of purslane plants, decreasing their stem length and diameter (Borsai et al., 2020a; Zaman et al., 2020). Plant biomass, both fresh and dry, was also negatively affected by salinity stress (Singh et al., 2022). Succulent purslane has a water content of 90 % or more with its leaves, steams, and roots. During our experiment, we also analyzed the water content in the root and shoot of Dandur purslane plants (Tables S1 and S2). Therefore, in comparison to its dry weight, the fresh weight of purslane was higher. A sharp decline in purslane's fresh weight was seen in response to the high salinity stress, and this decline was predictable as the salinity enhancement level increased. At 350 mM salinity stress, sugar beetroot cultivars dry matter content decreased significantly (Dadkhah and Grrifiths, 2006). In contrast, hybrid maize varieties' shoot and root dry matter contents decreased significantly at 250 mM salinity (Eker et al., 2006), while in Pennisetum alopecuroide plant found that dry matter content increased at 100 mM salinity level (Mane et al., 2011). The spike in dry matter production in salinity is presumably attributed to the accumulation of inorganic ions and organic solutes for osmotic acclimatization, while the decrease in dry matter content at the most concentrated soil levels could be driven by the inhibition of reserved foods' hydrolysis and vascular translocation to developing shoots (Xu et al., 2008). On the other hand, after applying salt, several of the accessions further increased the fresh weight of purslane. Dandur grown in slightly to moderate NaCl treatments had the highest shoot and root biomass compared to strong and extreme NaCl treatments levels (Fig. 1E–F, G-H and Tables S1–S2). Flowers et al. (Flowers et al., 1986) observed that most halophytes survive in saline environments. We found that undocumented Dandur purslane plant genotypes cultivated with slight NaCl treaments yielded more root and shoot biomass. In addition, we found that Dandur purslane plants can finish their lifespan between slight to moderate NaCl treatments; hence, Dandur may be classified as a halophyte crop (Yuan et al., 2019).

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Fig. 3

The study found that Dandur plants adapt well under 100–200 mM salinity stress concentrations, but experience reduced growth and development when salinity concentrations increase from 300-400 mM.

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6.2 Photosynthesis pigments and efficiency

Purslane, a C₄ plant with palisade layers on both leaves, is affected by water usage efficiency and CO₂ adaptation strategies during photosynthesis in various abiotic stress conditions (Bromham and Biology, 2014; Jin et al., 2016). The purslane plant's cell membrane serves as an essential element for preserving cell turgor pressure and other physiological features against various salt stress scenarios. Plants respond to over-abundance of water or prolonged flooding by eliciting a wide range of biochemical and physiological responses. Salinity stress may impact photosynthetic efficiency, with studies showing that leaves accumulate salt, while total chlorophyll levels decline. Researcher was found that chlorophyll content of Arabidopsis and Thellungiella decreased in response to salt, while chlorophyll content of Thellungiella showed no discernible shift under the same salinity conditions (Stepien and Johnson, 2009). Undocumented Dandur plants exhibited less affected chlorophyll content under salinity stress at concentrations of slight to moderate NaCl saline level, while a substantial decrease was seen at concentrations of strong to extreme NaCl treatments. Research by Rabhi et al. (2012) found that two obligatory halophytes, Tecticornia indica and Sesuvium portulacastrum, have much higher total chlorophyll levels when cultivated in environments with slight to extreme NaCl saline level, respectively (Rabhi et al., 2012). In increasing salinity circumstances, salt-tolerant species display an increase or maintenance of chlorophyll content, while salt-sensitive species exhibit a reduction in chlorophyll levels.

Salinity stress reduces gas exchange properties, impacting plants ability to photosynthesize (Hnilickova et al., 2021). In this study found that Dandur Pn, E, and WUE levels were significantly decreased under conditions of water deficit or osmotic imbalance caused by salinity stress (Fig. 1 I-J, Tables S3 and S4). Stomatal closure, which is the openings for CO₂ and water exchange, is induced by root signals in dry soil, reducing transpirational water loss (Linglan et al., 2008). As transpiration decreases with decreasing the water content in the stem and roots. This decrease in water leads to restriction of stomatal function that directly hindering photosynthesis. A decrease in water results in a decrease in Pn, suppressing overall photosynthesis (Tables S3–S4). This is illustrated in Tables S5 and S6. Stomata shut in response to a fall in water potential and water content in osmotic-stressed leaves, reducing CO₂ efficacy, Pn, and E (Tardieu and Simonneau, 1998; Zhao et al., 2020). The water use efficiency (WUE) of a crop is a measure of its water-saving potential and water productivity, representing the final performance of water consumption and crop production. Water stress enhances WUE across a range of plant species. Stomatal closure reduces transpiration, photosynthesis, and leaf conductivity when plants are under water stress. Water-deficient plants are more likely to use water conservatively, leading to higher WUE (Liu et al., 2016). WUE is a crucial physiological adaptation mechanism that can increase agricultural yields when water is limited (Mashilo et al., 2017). This study found that WUE increased when subjects were subjected to mild stress (slight to moderate NaCl solution) (Tables S3–S4).

6.3 Plant ion toxicity and ion imbalance

Salinity stress impacts plant nutrient concentrations, potentially leading to cytoplasmic toxicity and plant death. Plants regulate homeostasis through ion transport, absorption, and distribution. Halophytes store harmful ions in vacuoles, while glycophytes decrease root-level Na⁺ and Cl⁻uptake. Osmotic adjustment is the primary mechanism for salinity changes in halophytes. Both Na⁺ and Cl⁻ levels increase in response to NaCl concentration suggesting identical Na⁺ absorption and transport from root to shoot in Dandur cultivars (Tables 1 and 2).

The Dandur Na⁺ ion concentration was consistently greater in the stem than the root across all salinity levels, indicating that this cultivar cannot prevent Na⁺ ions from moving from the roots to steam. Cl- and Na + ions had similar distributions despite the greater salinity. Plants quickly absorb sodium and chloride and transfer them to their leaves as osmotica. We found similar results in cotton (Gossypium hirsutum L.) plants (Zhang et al., 2014). Dandur plants can only tolerate slight to moderate salinity levels and Na⁺ and Cl⁻ ions beforehand that photosynthesis and plant growth are impaired, as shown in Tables S6 and S7. In salty environments, Cl/NO₃ antagonism may reduce shoot nitrogen intake due to increased absorption and Cl-accumulation (Muchate et al., 2016). Phosphorus intake, essential for energy transmission, storage, and photosynthesis, is affected by salt stress. Soil with high Na⁺, Cl⁻ and SO₄² concentrations may have decreased phosphorus absorption due to high ionic strength and limited solubility of Ca and P compounds (Borsai et al., 2020). Protein production and water balance need potassium, an inorganic solute. Absorbing too much Na⁺ and Cl⁻ can cause cation imbalance and K⁺ and Ca²⁺ deficiency (Tables 1 and 2). At the same time, Dandur K⁺ and Ca²⁺ levels decreased significantly. However, shoot loss was greater than root loss as NaCl concentration increased (Tables 1 and 2). Due to molecular similarities, large exogenous Na⁺ concentrations restrict K⁺ absorption (Kaya et al., 2007). Plant development in saline soil requires a precise Na⁺/K + ratio in plant cells. In current study analysis show that increasing salinity stress also increased Na⁺ ions which lowered the K⁺/Na⁺ ratio (Tables 1 and 2). Borsai et al. (2020a, 2020b) found that a greater K⁺/Na⁺ ratio influenced K⁺ and other ionic concentrations in Portulaca and Purslane cultivars under salinity stress. Na⁺ cannot replace K⁺ in cellular functions due to their chemical similarities. As Na⁺ increases, K⁺ and Ca²⁺ decrease because they compete for root absorption sites. Na + inhibits plant K + absorption by a competitive mechanism regardless of solution concentration of Na⁺, Cl⁻, or SO₄² salts (Borsani et al., 2003). Salinity decreases wheat, Manihot esculenta, Zea mays, G. hirsutum, and other plant yields and creates nutritional imbalances (Cruz et al., 2018; Hasana and Miyake, 2017). Ramos et al. (2004) observed that membrane integrity worsened with increased salinity, and our work demonstrates that this causes Na⁺ buildup and biomass loss (Ramos et al., 2004). To activate enzymes and synthesize proteins, tissue K⁺ and Ca²⁺ concentrations must be constant because Na⁺ cannot fill the gap of K⁺ and Ca²⁺.

6.4 Salt stress tolerance activity

When exposed to salt, Dandur shows a wide range of morpho-physiological and biochemical responses, and as halophytes, salt-tolerant genotypes often perform well at an initial concentration of salinity. All the morpho-physiological and biochemical variables were shown to be STI-related in this study. The correlation matrix displays the morpho-physiological and biochemical features that correlate significantly with the STI (Fig. 2A–H). For plants to produce new tissues, which involves adjusting osmosis, it is necessary for them to keep the shoot turgor positive. We found that tolerance for salt in saline conditions was negatively connected with increasing concentrations of salinity stress (strong to extreme NaCl saline levels), suggesting that osmotic adjustment is a key component of salt tolerance. There is less STI in the early stages of salinity in the salt-sensitive genotypes when compared to earlier results. However, genotypes that are resilient to salt stress exhibit an increase in shoot length stress index and STI as well as a decrease in salinity shoot toxicity during the early stages of salinity stress (Fig. 2 A-H).