Freshwater is depleting due to urbanization and industrialization. Irrigation with poor quality (brackish) water to fruit orchards is being practiced in arid and semi-arid regions of the world. The accumulation of salts in the soil revealed induced accumulation and uptake of toxic ions, and decreased uptake of essential minerals which contributed to growth and yield [9]. Salinity and sodicity both had adverse effects on soil and plant health. Sodicity is not only damaging to plants’ health, but it also depleting the soil texture and structure, porosity reduction, and poor water permeability. Under field conditions, irrigated water is recorded as electrical conductivity (EC) or osmotic potential. Pure water exhibited very poor conductance regarding electric current. The conductivity of water samples is mainly based on dissolved ions in water. EC and ESP were recorded higher. Osmotic pressure was measured lower under higher salt levels in water (Tables 1 and 2).

The salt content in the Colorado River about 2000 km downstream was 50 mg L⁻¹, South California 900 mg L⁻¹, and Texas 2000–3000 mg L⁻¹ in the same river. All salinity levels are dangerous for plants, therefore numerous effective practices are necessary for the development and identification of tolerant germplasm of Zizyphus [13]. Disturbances in photosynthesis, metabolism, oxidative injury, and osmotic stress due to access to salts level in fruit trees as well explored (Fig. 1). Modulation of numerous physiological and biochemical mechanisms is necessary for the improvement of plant growth, yield and defense system under salinity stress (Fig. 2).

Excess NaCl level is reducing the growth and yield of Zizyphus germplasm. Induced-salinity in the root zone is causing nutritional imbalances and osmotic stress resulting in poor absorption of water and minerals necessary for sufficient growth, yield, and quality of Zizyphus plants [8]. Zizyphus plants showed stunted leaf and stem elongation, rupturing of cell membranes [8], disturbance in chlorophyll stability [18], increase of lipid peroxidation, low relative water content [18], rupturing of photosynthetic pigments [19], impairing gaseous exchange process [20], enhanced Na⁺ and Cl⁻, and reduction of K⁺ ions [21]. Zizyphus plants exhibited adaptive strategies by improved growth of roots for uptake of minerals and water [22], reduction in uptake of Na⁺ and Cl⁻ [22], regulation of leaf turgidity [20], higher osmolytes generation and antioxidants production for scavenging of toxic ROS against salinity [6]. Therefore, it is more important to explore the importance of biochemical and molecular processes in Zizyphus plants subjected to salinity stress.

ROS production remains continue within plant cell compartments and organelles. The over-generation of ROS to the optimum level becomes more toxic for Zizyphus fruit trees [22]. Plant metabolism and oxidation process of biomolecules are disturbed due to higher concentrations of salts [23]. Over-production of ROS and disturbance in metabolic activities are major causes of oxidative stress. The interruption in the cellular homeostasis mechanism is damaging the biomolecules, cellular organelles, and redox homeostasis of fruit trees [1]. The oxidation process might be triggered due to salinity, damaging numerous proteins, sugars, lipids, and nucleic acids. The disturbed functioning of these bio-molecules due to the occurrence of oxidative stress is causing fruit tree death [24]. Electrochemical induced condition is also a cause of over-production of ROS and H₂O₂ within plants cells and organelles [25]. Hence, ROS over-production is dangerous for plants, therefore different management approaches are necessary to implement for improved fruit yield with superior fruit quality. The optimum production of ROS is contributed to fruit tree growth and is also involved in different developmental processes focusing on the fruit production of Zizyphus germplasm.

The regulation of stomatal conductance is important to improve the fruit tree’s tolerance against salinity stress. Elevated salinity has been causing stomatal closure and restriction of photosynthesis. Osmotic stress occurs due to alteration in stomatal enzymes and restriction in CO₂ [26]. Photosynthetic pigments and chlorophyll content are involved in the regulation of photosynthesis [27]. The rupturing of photosynthetic reactions like photosystem II also disturbed the photosynthetic pigments and stomatal conductance, and ultimately fruit tree growth and yield. The impairing of electron transfer capacity is also a major cause of imbalance in photosynthetic machinery [28]. The alteration in metabolic processes is causing the restriction in photosynthesis. The optimum production of ROS is necessary for different metabolic activities and their proper functioning in the cells and organelles of fruit trees [29]. However, the over-generation of ROS becomes more toxic to plant’s health. ROS over-production increased the activities of MDA and H₂O₂ content which damages cell membranes [30]. The rupturing of membrane cells is very dangerous for plants’ survival against induced salinity stress [31]. Gaseous exchange and chlorophyll fluorescence are a major indicators of salt stress in plants growing under saline stress. The balance in gaseous exchange and chlorophyll fluorescence is necessary for the normal functioning of fruit trees attaining higher fruit yield with excellent quality fruits.

Soil water potential is reduced due to low osmotic potential. Low osmotic potential is due to the presence of excess dissolved solutes in the root zone [32]. The development of a downhill mechanism between leaves and roots for the uptake of water is necessary for food synthesis in the leaves. The disturbance in the downhill gradient level is resulting in the reduced potential of food synthesis and translocation towards other plant parts [33]. Therefore, low water potential is toxic for sufficient plant growth and yield. The presence of higher dissolved solutes in the root zone are increasing the chances of water deficit conditions [34]. Induced salinity and water stress are reducing the fruit tree growth and poor yield with inferior fruit quality. Moreover, low water potential was also observed in the saline lands [35].

Ion toxicity occurs in the root zone due to increased concentrations of Na⁺ and Cl⁻ in tree cells and organelles. Under normal conditions, the cytosol of higher trees is 1–10 mM Na⁺ and 100–200 mM K⁺. This ionic environment is optimum for the proper functioning of enzyme activities [5]. The increase/decrease from trees level is toxicity/deficit causing alteration in tree defense mechanism by inactivating numerous enzymes and protein biosynthesis. The higher concentration of Na⁺ depletes Ca²⁺ in the root zone which potentially increased the leakage of K⁺ from membrane cells [5]. The permeability of membranes is greatly damaged by induced salinity stress [36].

Higher concentrations of toxic ions such as Na⁺ and Cl⁻ accumulated in chloroplasts under salinity stress. Electrolyte leakage indicates the sensitive/tolerant mechanism of fruit crops due to membrane damage from the over-production of ROS, H₂O₂, and MDA contents. Carbon metabolism/photophosphorylation disturbances in tree cell organelles and compartments were found to be higher in sensitive fruit cultivars [1]. Different antioxidants, osmolytes, and secondary metabolites are indicating tolerance levels off rut tress even at the seedling stage growing under saline conditions. The toxicity of different minerals in the root zone of fruit trees can be replaced through the ions exclusion strategy. Ions exclusion is important for the avoidance of salt injury. Ions exclusion and avoidance are the main approaches to protect fruit trees from the adverse effects of salinity focusing on fruit production [30].

Disturbances in cellular homeostasis are due to the toxicity of ions and the over-generation of ROS. The toxic ROS is a cause of osmotic stress. These ROS are causing the rupturing of membranes, DNA damage, carbohydrate oxidation, photosynthetic pigments breakdown, chlorophyll contents, protein denaturation, disturbance in antioxidant activities, and impairment of osmolytes production [37].

The exclusion of ions is considered one of the more critical aspects of the acclimation and adaptation of Zizyphus trees against salinity stress [16]. The balance through amino acids/carbohydrates utilization is considered a higher energy cost mechanism as compared to the use of ions for the balance of water potential [35]. Ionic balance is involved in the regulation of osmotic potential in Zizyphus trees. However, it has been assessed that higher concentrations of ions are more toxic for numerous cytosolic enzymes. Therefore, ions may possibly be accumulated in the vacuoles of cells to reduce toxic concentrations present in the cytosol [9].

Sodium chloride (NaCl) is one of the most abundant salts found in the soil. However, different transport systems are critically involved in the facilitation of Na⁺ into vacuole [26]. Potassium and calcium ions are greatly affecting the intracellular concentrations of Na⁺. High-affinity potassium transporters (HKTs) are greater affinity K⁺ transporters that belong to the class of integrated membrane proteins only present in plants. These HKTs may be varied in the differentiation of Na⁺/K⁺ due to higher selectivity for Na⁺ and a few others for K⁺ [1]. The uptake mechanism of Na⁺ was enhanced due to the inhibition of HKT1 and higher transport affinity of K⁺ and Na⁺ transporter under higher concentrations of Na⁺ and K⁺ [1]. Different mechanisms were involved in tree cells and compartments to increase tree tolerance against adverse growing conditions [31]. However, defense behavior is mainly based on the intensity of stress, type of species, and management practices. Calcium could increase the selectivity of K⁺/Na⁺ resulting in the enhanced tolerance mechanism of fruit trees [24].

Different non-enzymatic activities, i.e., ascorbic acid, different soluble sugars, flavonoids, phenols, and tocopherols are generated in fruit trees under saline conditions. These have excellent capability to scavenge toxic ROS over-produced under salinity. These antioxidants may possibly be varied regarding gene expression, achieved redox buffers, and metabolic boundary to control the optimum stimulus of acclimation reactions as studied by Miller et al. [45].

Ascorbic acid (vitamin C) is effective for the determination of tolerance levels in fruit trees growing under saline conditions. The increased production of ascorbic acid is effective for improved fruit tree tolerance against salinity stress. The improved antioxidants in fruit trees are important for the identification of tolerant rootstocks, and helpful for the restriction of Na⁺ from roots towards leaves. Moreover, the reduction in ascorbic acid was recorded in sensitive fruit trees however improved level was recorded in the tolerant germplasm of Zizyphus like Gola cultivar [46].

Tocopherol is recorded in all the plant parts. These have a good ability to scavenge toxic ROS and restrictlipid radicals. These important biological constituents are contributing to the antioxidant and non-antioxidant functioning. These are four isomers, i.e., δ_, α_, β_ and γ_ related to tocopherols are present in higher plants [47]. However, higher antioxidant activity was recorded in α-tocopherol also known as vitamin E. Moreover, α-tocopherol in considerable amounts is found in fruit trees of chloroplast membranes. Hence, photo-oxidative injury can be eliminated with the generation of tocopherols [48]. Tocopherols and carotenoids had the potential to repair oxidized radicals and restrict transmission chain duration during the auto-oxidation of lipids [49]. These are necessary for the protection of thylakoid membranes under salinity stress conditions in fruit trees. Protection of thylakoid membranes from injury is important for the proper functioning of a plant cell.

Flavonoids are rich in the aerial plant parts, i.e., foliage, pollens, floral, and fruit parts. These are frequently stored in vacuoles of fruit trees in the form of glycosides. These are also good scavengers of ROS in higher plants [30]. Moreover, it is also contributed to lipoxygenases, and reduced lipid peroxidation. Approximately, 1 mM of flavonoids had an excellent ability to restrict the lipoxygenase under saline conditions as studied by plant researchers Potapovich et al. [50].

Zizyphus fruit crop is a rich source of antioxidants and phenolic compounds. The consumption of fruits is important due to their nutritional components contributed to human’s good health [10]. The reduction in phenolic compounds was measured in leaves, roots, and fruits in those plants growing under saline environments. Nutritional characteristics of fruits are drastically disturbed because of the adverse effects of climatic conditions [12]. Oleuropein is a well known phenolic compound which involved in glucose reservoirs. This is necessary for the osmoregulation of fruit trees growing under saline conditions [51]. This is more helpful for the improvement of the fruit tree defense system of higher plants under salinity stress. The injurious due to Na⁺ and Cl⁻ within the cell compartments can be reduced by the generation of these phenolics as oleuropeins in fruit trees [51].

Fruits of two species (Z. mauritiana and Z. jujube) are a rich source of different sugars than other species. However, the reduction in different sugars was also due to the adverse effects of abiotic stresses [52]. The reduction in sugars was observed in the leaves of fruit trees in those plants growing under salinity stress conditions. Sugars level may possibly be based on the type of genotypes and intensity of salt stress. Therefore, salt-resistant germplasm is necessary for superior quality, excellent fruit size, and nutritional profiling. Sucrose and glucose act like osmolytes, and are involved in cellular respiration as substrate [53]. Moreover, fructose is found to be supportive of the generation of secondary metabolites in salinity-stressed plants as studied by Rosa et al. [54]. The increase of sugars was observed in the tolerant plants, while a decreased level of sugars was recorded in the sensitive plants and this phenomenon was recorded in the Troyer citrange as well as in Cleopatra mandarin as reported by Anjum [55].

Over-generation of ROS, H₂O₂, MDA, and lipid peroxidation are considered as stress indicating markers in plants [37]. The timely control of salt-stressed plants is effective for higher production and yield by measuring these activities occurring within plant cells and compartments. The membrane injury was due to exceeding the level of salinity within plants is determined by measuring lipid peroxidation. The increased level of lipid peroxidation was indicating the increased level of injury in the membrane of plant cells [9].

Salinity can be detected by the estimation of stress-signaling molecules produced within cell compartments and organelles. Osmolytes are more effective in indicating salt stress in fruit trees [56]. Osmotic irregularity is found in the root zone of fruit trees due to exceeded concentration of salinity in the root zone and accumulation in leaves. Osmoprotectants have been produced within cell organelles and compartments naturally. However, their accumulation is effective to cope over-generation of toxic ROS and alleviates salt tolerance in fruit trees for better vegetative development, reproductive development, flowering induction, fruit yield, and superior fruit quality [57].

Different osmolytes, i.e., proline, GB, and ascorbates are produced naturally within plant cells to mitigate adverse effects of salinity by scavenging toxic ROS and are helpful for the activation of trees’ defense system [45]. Lipid peroxidation was reduced due to osmolytes generation. These are helpful to protect plant cells from injury to different membranes. Gene expression related to salinity stress is controlled by the production of these osmolytes naturally [58].

The suitable level of compatible osmolytes plays a major role in the regulation of osmotic gradient [59]. The accumulation of osmolytes, i.e., proline, GB, and ascorbates goes higher due to exceeded concentrations of salinity found in the root zone and is further translocated to other plant parts [60]. Even under abiotic stress, production and dramatic accumulation of proline, GB and ascorbates were exceeded naturally to cope with adverse effects that occur from harsh environmental conditions [61].

Proline is involved in stabilizing proteins, subcellular structures, and membranes protecting these from damaging of ROS. It is also known as a scavenger of toxic ROS [62]. The concentration of proline indicates that fruit species are tolerant, while sensitive genotypes had low proline concentration within cell compartments. Hence, proline generation is effective for the identification of tolerant/sensitive genotypes. However, different plant researchers also concluded that proline is a stress indicator which is not contributed to the alleviation of tolerance against salt stress [63].

Glycine betaine is another osmolyte produced in higher amounts in those fruit trees growing under saline stress [64]. It is greatly contributed to protein stability and photosynthetic pigments. The rupturing of photosynthetic machinery is protected by the natural production of GB [65]. Fruit quality and defense systems are also affected due to excess salinity (Table 3). However, its production might be increased by mineral application and cultivation of salt-tolerant germplasm.

Hormonal regulation is necessary to increase the tolerance against salinity stress in Zizyphus as reported in cucumber [66]. Two Zizyphus species (Z. rotundifolia and Z. nummularia) were found to be more tolerant against salinity stress because of better potential in hormonal regulation, stomatal conductance, and photosynthetic pigments [39]. Among exogenous plant hormones, auxin is more effective for the increase of tolerance against salinity stress by increasing the defense mechanism of fruit trees and the similar phenomenon was observed in cucumber [67]. The increased potential of antioxidant profiling is necessary to increase the scavenging capability of fruit trees against toxic ROS [68]. The reduction in MDA and H₂O₂ content is an indication of stress tolerance in fruit trees and the same phenomenon was observed in the Gola cultivar of Zizyphus [69]. Smart reprogramming using molecular basis [70] and hormonal regulation [71] of underutilized fruit crops under salinity stress is a more imperative strategy focusing on fruit production.