Stage-Dependent Mineral Element Dynamics in ‘Junzao’ Jujube: Ionic Homeostasis and Selective Transport Under Graduated Saline-Alkali Stress

Abstract

Plants dynamically regulate ions in the tree to defend against abiotic stresses such as drought and saline-alkali, However, it is not clear how ‘Junzao’ jujube regulates ions to maintain a normal life cycle under saline-alkali stress. Therefore, in this study, the roots of 10-year old steer jujube trees were watered using a saline and alkaline gradient solution simulating the main salt (NaCl) and alkali (NaHCO₃) of Aral with NaCl:NaHCO₃ = 3:1 gradient of 0, 60, 180, and 300 mM, and three jujube trees with uniform growth were taken as samples in each treatment plot, and the ion contents of potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), zinc (Zn) and carbon (C) in each organ of the fruit at the dot red period (S1) and full-red period (S2) were determined, in order to elucidate the relationship between physiological adaptation mechanisms of saline-alkali tolerance and the characteristics of mineral nutrient uptake and utilisation in jujube fruit. The results showed that under saline-alkali stress, Na was stored in large quantities in the roots, Ca and Mg in the perennial branches at S1, Na and Fe in the leaves at S2, and K, Mg and Mn in the perennial branches. There was no significant difference in the distribution of C content in various organs of ‘Junzao’. Compared with CK (0 mM), under salinity stress, the K content in the leaves was significantly reduced at S1 and S2, and the K/Na ratios remained > 1.0. At S2, under medium and high concentrations of saline-alkali stress (180–300 mM), the K/Na is less than 1, and the ionic homeostasis was disrupted, and the leaves die and fall off, and the Na is excreted from the body. The selective transport coefficients SK/Na, SCa/Na and SMg/Na from root to leaf showed a downward trend at S1, but still maintained positive transport capacity. At S2, this stage is close to leaf fall, the nutrient transport coefficient is less than 1, and a large amount of nutrients are returned to the perennial branches and roots occurred. These results indicated that the mechanism of nutrient regulation and salt tolerance in jujube trees was different at different growth stages.

1. Introduction

Soil salinization has become a major environmental stress factor limiting the sustainable agricultural production, and about 20% of the world’s arable land is suffering from salinization [1]. Through the data of Urumqi Comprehensive Natural Resources Investigation Center of China Geological Survey, it is known that the total area of salinized soil in Xinjiang is more than 14.6 million hectares, accounting for 40.7% of the salinized soil area in China [2], and the southern Xinjiang region is one of the most serious regions of salinization problems in Xinjiang. Jujube (Ziziphus jujuba Mill.) is a characteristic and advantageous economic forest species in southern Xinjiang, which has become the core pillar of Xinjiang’s fruit tree industry, and its cultivation area has been steadily maintained at 4.78 × 10⁵ mu (https://www.163.com/dy/article/JHMVFFGU0511CCT0.html, accessed on 5 April 2025). The cultivation of jujube in the South Xinjiang region is mainly located in the forest and fruit production areas of the Tarim Basin, where soil salinisation is serious, which can affect the growth and development of jujube trees and the quality of jujube fruits [3]. Studies have shown that nutrient uptake, accumulation and transport in the plant body under salinized soil environments change, and this change is on the one hand to resist saline and alkaline stress, and to ensure that the plant body completes its physiological cycle normally [4,5].

The soil of jujube orchards in southern Xinjiang generally showed the characteristics of compound salinization [6], and its salt composition was mainly Na⁺-CO₃²⁻-Cl⁻-mixed type, which met the definition of alkalized saline soil subclass in the Classification Standard for Saline Soil in Xinjiang (DB65/t 602.11-2001) [7]. Higher alkali concentrations in the soil increase pH, which reduces nutrient solubility and reduces the absorption capacity of plant roots, causing deficiencies and imbalances in the plant. If the salt concentration in the soil is too high, the plant will accumulate excessive Na⁺ concentrations, which will cause ionic toxicity to the plant body and intensify the competition between ions, thereby limiting the absorption and transport of other nutrients by the plant body [8,9]. In saline-alkaline environments, Elaeagnus angustifolia [10] blocked Na⁺ outside the root zone and transported K⁺, Ca²⁺, and Mg²⁺ into branches and leaves to maintain ionic balance, physiological activities, and signalling in vivo, so that the plants had high adaptability under high salt stress. In response to NaCl salt stress and NaHCO₃ alkali stress, ‘Jinsixiaozao’ and sour Jujube [11] will maintain a low Na⁺ concentration as well as a stable high K⁺ concentration in the leaves, and maintain a high K⁺/Na⁺ to resist saline-alkali stress with this regulatory mechanism. However, the Na⁺ content in the leaves of the salt-secreting plant Tamarix chinensis Lour. [12] was significantly higher than the content of other elements, and the selective absorption and upward transport of Na⁺ may be the main mechanism of salt resistance. As time goes on, plants will adjust themselves to saline alkali pressure and slowly recover from saline alkali injury [13].

Under saline-alkali environment, few studies have been conducted on nutrient changes and transport in adult jujube trees, and the impact of soil saline environment on jujube cultivation in southern Xinjiang cannot be ignored. Jujube demonstrates inherent salt tolerance attributed to specialized ion exclusion mechanisms and tissue-specific compartmentalization strategies. However, the developmental stage-dependent regulation of these mechanisms during fruit maturation remains poorly understood. S1 is the period of maximum nutrient accumulation in jujube, and S2 is the period when the fruit quality of jujube is formed, which does not need a lot of nutrients. Therefore, it is extremely important to explore the salt tolerance mechanism of nutrient uptake and transport changes of the tree during the important development period of ‘Junzao’ fruit. In this study, the primary ionic components (NaCl and NaHCO₃) of salinization in southern Xinjiang were used to simulate the real environment of ‘Junzao’ trees under salin-alkali stress in this area, and to study the accumulation and transport of nutrients in the organs of adult jujube under different saline-alkali concentrations, and to explain the salt tolerance mechanisms of ‘Junzao’ from the aspect of nutritional physiology, so as to provide theoretical basis for the study of salt tolerance of Xinjiang red jujube under saline-alkali environment.

2. Materials and Methods

2.1. Experimental Site and Materials

The field experiment was conducted at the 7th Company of the 10th Regiment, First Division, Xinjiang Production and Construction Corps. The trial utilized 10-year-old ‘Junzao’ trees with standardized morphological parameters: plant height 2.0–2.5 m, canopy spread 1.8–2.2 m, and planting density maintained at 1.5 m (intra-row) × 3.0m (inter-row). Arboricultural management included an annual rejuvenation pruning protocol to sustain photosynthetic capacity and vegetative productivity.

2.2. Experimental Design

This investigation employed a single-factor completely randomized design with four saline-alkali stress gradients. Based on ionic composition profiling of Alar’s saline soils, we formulated mixed salt-alkali solutions (NaCl:NaHCO₃ = 3:1 w/w) through aqueous dissolution, establishing the following treatments: CK (Control, 0 mM), T1 (Mild stress, 60 mM), T2 (Moderate stress, 180 mM), and T3 (Severe stress, 300 mM).

To preclude hydraulic connectivity between differential saline-alkali treatments, 1-m-depth isolation trenches were excavated between experimental plots prior to stress imposition. These trenches were lined with double-layered polyethylene membranes along vertical walls and basal zones to ensure complete hydrological segregation. A 6-m elevated rainout shelter with lateral ventilation panels was constructed over the experimental orchard to eliminate precipitation interference [14]. In 2021–2022, in order to prevent jujube flower abscission caused by salt alkali stimulation, salt alkali stress was applied through drip irrigation during the phenologically sensitive fruiting period (early July), including four applications every 20 days. Each irrigation event delivered 2 h of treatment solution, 2.76 L of saline solution per tree. Post-biennial stress exposure (soil sampling date: 5 September 2023), rhizosphere soils from the principal root proliferation stratum (20–40 cm depth) were collected for pedological characterization (

Table 1. Fundamental characteristics of soil at a depth of 20–40 cm under saline-alkaline treatments in the experimental field.

Salinity and Alkalinity Concentration	EC (us·cm)	pH	Alkaline-Hydrolyzable Nitrogen (mg/kg)	Fast-Acting Phosphorus (mg/kg)	Fast-Acting Potassium (mg/kg)	Organic Matter (g/kg)
0 mmol/L	167.25 ± 15.88	7.76 ± 0.78	20.73 ± 3.78	24.35 ± 5.16	80.08 ± 5.59	2.13 ± 0.44
60 mmol/L	245.23 ± 14.89	7.97 ± 1.21	14.33 ± 2.96	8.23 ± 3.14	72.64 ± 3.27	3.43 ± 0.76
180 mmol/L	500.24 ± 19.01	8.26 ± 0.29	10.56 ± 2.45	7.25 ± 0.82	100.02 ± 6.69	2.65 ± 0.26
300 mmol/L	887.23 ± 16.89	8.79 ± 0.66	9.56 ± 2.25	6.89 ± 1.62	115.9 ± 7.92	3.98 ± 0.62

), demonstrating stabilized treatment-specific salinity profiles with minimal inter-replicate variability.

2.3. Measurement Indicators and Methodologies

Sampling was conducted during two phenologically distinct fruit developmental phases: (1) Dot Red period (initial anthocyanin accumulation, 15 September 2023; 110 ± 3 days post-anthesis) [15,16], and (2) Full-Red period (complete color development, 12 October 2023; 135 ± 2 days post-anthesis) [17], following standardized protocols from the ‘Junzao’ Jujube Germplasm Characterization (NY/T 232926-2013) [18], verified through visual assessment of 50 representative fruits per tree. Three representative trees per treatment (plant height 2.2 ± 0.3 m, homogeneous canopy architecture) were selected for organ-specific sampling. From each specimen, 30 fully expanded sun-exposed leaves from mid-canopy positions, paired 2-year-old lignified branches, and absorptive fine roots (<2 mm diameter) were collected. Samples underwent standardized pretreatment: triple rinsing with 18.2 MΩ·cm deionized water for surface ion removal, enzymatic inactivation at 105 °C (30 min), followed by desiccation at 65 °C to constant mass (72 ± 2h). Sample homogenates were prepared using a planetary ball mill with 60-mesh nylon sieves (250 μm aperture) and stored in nitrogen-purged polyethylene bags under desiccated conditions pending ionomic characterization. Carbon content determination was performed via FlashSmart™ Elemental Analyzer (Thermo Fisher Scientific, Waltham, CA, USA) employing the following protocol: calibration with certified reference materials (GBW 07603, GBW 07604, GBW 07605, http://www.ncrm.org.cn/ Search, National Institute of Metrology, Beijing, China) covering 2.36–45.72% carbon range. Thank you for pointing this out and I agree with this comment, so I have made the changes as you suggested. Precisely weighed aliquots (5–6 mg) of homogenized tissue were encapsulated in acid-washed tin crucibles (5 × 9 mm), with procedural blanks systematically integrated for baseline correction. Analytical parameters included ultrahigh-purity helium carrier gas (99.999%), 1150 °C combustion chamber temperature (180 s dwell time), chromatographic separation through Porapak QS columns (80–100 mesh), and thermal conductivity detection at 250 °C. Analytical determinations were performed in triplicate (technical replicates RSD < 1.5%) using inductively coupled plasma optical emission spectrometry (ICP-OES; Avio 200, PerkinElmer, Springfield, IL, USA) for quantification of macro/micronutrients ion content (K, Na, Ca, Mg, Fe, Mn, Zn). Homogenized samples (0.200 ± 0.001 g) underwent microwave-assisted digestion (XT-9916, Xintuo, Nanjing, China) following U.S. EPA Method 3052B with 8 mL concentrated HNO₃ (65% w/w) and 2 mL H₂O₂ (30% w/w). Post-digestion solutions were vacuum-filtered (0.45 μm) and diluted to 25.00 mL with 18.2 MΩ·cm ultrapure water. ICP-OES operational parameters comprised: 1150 W RF power, 0.70 L/min nebulizer gas flow (Ar ≥ 99.999%), and 1.5 mL/min pump rate. Instrument calibration was performed using a multi-element standard solution (PerkinElmer, Springfield, IL, USA, Catalog No. N9303941). The resulting calibration curves exhibited excellent linearity, with correlation coefficients (R²) ≥ 0.9995. Method validation was conducted by analyzing the certified reference material GBW10015. Under rigorously controlled ambient conditions (25 ± 1 °C, 45 ± 5% relative humidity), recoveries ranged from 97.2% to 102.8% with relative standard deviations (RSDs) less than 3% (n = [3], where [3] is the number of replicate analyses). According to the elemental digestion method and ICP-OES ion content determination method, the ion content determined in this study represents the total amount of ions in various valence states of the element. The abbreviations potassium (K), sodium (Na), calcium (Ca), magnesium (Mg), iron (Fe), manganese (Mn), zinc (Zn) are used in the article to indicate the following

2.4. Data Processing and Analysis

The ionic selectivity coefficient (SX/Na) quantifies the preferential translocation of a target cation X (potassium ions; calcium ions; or magnesium ions) over sodium ions from roots to leaves [19]. It is calculated as the ratio of the ionic activity ratios in leaf and root tissues: SX/Na = [X/Na] leaf/[X/Na]root. SX/Na: The selectivity coefficient for cation X relative to Na. The subscript X/Na explicitly denotes this comparison between X and Na. [X/Na] represents the ratio of the concentration (or activity) of cation X to that of Na in the specified tissue (leaf or root). X denotes the target cationspotassium ions, calcium ions, or magnesium ions. Elevated SX/Na values signify superior efficiency in the root-to-shoot translocation of the target cation X (K, Mg, or Ca) compared to Na. Crucially, a higher S-value indicates greater selectivity for transporting K, Mg, and Ca to the leaves/stems (the aboveground parts) while effectively restricting Na translocation. This enhanced selectivity is coupled with effective sequestration of Na in the rhizosphere or root tissues, resulting in more Na being retained in the roots. Consequently, plants exhibiting higher S-values demonstrate greater salt tolerance and experience reduced salt injury symptoms, as this optimized ionic homeostasis mitigates the detrimental effects of Na accumulation in photosynthetic tissues. Primary data processing and outlier detection (Grubbs’ test, α = 0.05) were conducted in Microsoft Excel 2021. Statistical analyses employed one-way ANOVA with Duncan’s multiple range test (DMRT; p < 0.05) in SPSS 27.0 (IBM, Armonk, NY, USA) following confirmation of variance homogeneity (Levene’s test, p > 0.10). Data visualization was performed using OriginPro 2021 (Learning Edition) with column chart construction, while spatial correlation analyses via Mantel tests were implemented in RStudio 4.3.1 utilizing the vegan package (v2.6-4). All experimental procedures incorporated triplicate biological, following confirmation of data normality through Shapiro-Wilk testing (p > 0.05). This quality-controlled workflow adheres to ISO/IEC 17025:2017 [20] standards for analytical rigor in horticultural research.

3. Results

3.1. Effects of Saline-Alkaline Stress on Mineral Element and C Content in Various Organs

As delineated in Figure 1, during S1, foliar, perennial branch and root Na concentrations exhibited progressive elevation with increasing stress intensity, whereas S2 leaves demonstrated 121.98–145.23% higher Na accumulation relative to controls. Temporal divergence characterized Ca dynamics: S2 leaves, Perennial branches, and roots showed dose-dependent accumulation patterns, contrasting with S1 leaves, which showed 13.38% Ca depletion under 300mM stress (p < 0.05). Magnesium homeostasis revealed biphasic responses-S1 leaves and S2 branches maintained ascending Mg levels across gradients, while S2 leaves and S1 branches exhibited initial declines at 60–180 mM concentrations followed by recovery under 300 mM stress. Fe redistribution exhibited developmental phase dependency: S2 leaves demonstrated linear accumulation with 94.24% increase under 300 mM treatment, contrasting sharply with the biphasic response observed in S1 leaves and roots-initial reduction (60–180 mM) followed by restoration at 300 mM. Mn accumulation patterns diverged spatially, with perennial branches showing progressive enrichment across stress gradients (60–300 mM), while S1 leaves and roots displayed unimodal distribution peaking at intermediate intensities (180 mM). Zn dynamics revealed organ-specific reciprocity: S1 branches exhibited initial Zn augmentation (60–180 mM) succeeded by depletion at 300 mM, inversely correlated with root accumulation trends. Notably, foliar and root Zn levels consistently surpassed control values under all stress regimes. C content underwent significant depletion in S1 roots and all S2 organs under saline-alkali stress, with 300 mM treatment reduced C content by 17.98% in leaves, 24.23% in branches, and 37.93% in roots relative to controls in S2 leaves, perennial branches, and roots respectively compared to controls. This integrated ionic regulation framework-encompassing Na rhizospheric sequestration, phase-dependent Ca allocation, and stress-intensity-modulated micronutrient partitioning-elucidates ‘Junzao’s evolutionary adaptation to saline-alkali ecosystems through two synergistic mechanisms: (1) toxic ion mitigation via compartmentalized storage, and (2) metabolic sink prioritization during pivotal phenophase transitions.

3.2. Comparison of K and Na Contents in Leaves, Branches and Roots of ‘Junzao’ at Different Developmental Periods

As delineated in Figure 2, ‘Junzao’ displayed compartmentalized redistribution dynamics of K and Na across fruit developmental phases under saline-alkali stress. During S2, foliar Na accumulation surpassed S1 levels by 121.5–145.2%, while perennial branches and roots exhibited inverse temporal patterns-Na preferentially sequestered in roots during S1 before mobilizing to leaves in S2. Concomitantly, K concentrations in leaves and roots showed 18.7–24.3% elevation during S2 compared to S1, contrasting with perennial branches demonstrating 13.4–17.9% K depletion. These coordinated ionic fluxes underscore the species’ adaptive capacity to dynamically reallocate solutes between source and sink organs during critical phenophase transitions under edaphic stress.

3.3. The K/Na Ratio in Different Organs of ‘Junzao’ Jujube Fruits at Various Developmental Periods Under Saline-Alkali Stress

As illustrated in Figure 3, sustained potassium-to-sodium (K/Na) ratios are critical for mitigating Na toxicity in ‘Junzao’ under saline-alkali stress. K/Na ratios in S1 leaves and perennial branches, along with S2 leaves and perennial branches, progressively decreased with escalating stress intensity. Similarly, S2 roots exhibited concentration-dependent reductions in K/Na ratios. Notably, S2 leaves under 180–300 mM saline-alkali treatments displayed K/Na ratios < 1.0, while all other organs (S1 leaves, branches, roots; S2 perennial branches and roots) maintained ratios > 1.0. These findings emphasize organ-specific thresholds for ionic homeostasis, with foliar K/Na imbalance representing a critical vulnerability during late developmental stages under severe stress.

3.4. Effects of Saline-Alkaline Stress on the Selective Nutrient Transport Capacity in ‘Junzao’ Jujube Trees

As demonstrated in Figure 4, ‘Junzao’ exhibited dynamic ionic selectivity under saline-alkali stress. During S1, the root-to-leaf potassium/sodium selectivity coefficient (SK/Na) displayed a biphasic response characterized by initial reduction (60–180 mM) followed by partial recovery at 300 mM, reaching minimal values under 180 mM treatment. Both calcium/sodium (SCa/Na) and magnesium/sodium (SMg/Na) selectivity coefficients exhibited progressive attenuation across stress gradients, with all S1 values significantly lower than controls (p < 0.05). Notably, S1 roots maintained SK/Na >1.0, indicating preferential K translocation to leaves. Conversely, during S2, SCa/Na and SMg/Na demonstrated U-shaped trajectories-initial decline (60–180mM) succeeded by partial restoration at 300 mM. All S2 selectivity coefficients (SK/Na, SCa/Na, SMg/Na) fell below 1.0 (p < 0.01), indicating compromised ion discrimination capacity under prolonged saline-alkali exposure.

3.5. Correlation Analysis of Mineral Nutrients in Various Organs at Different Developmental Periods Under Saline-Alkali Stress

As demonstrated in Figure 5, Mantel test analyses across saline-alkali gradients revealed distinct mineral nutrient coordination patterns in ‘Junzao’ organs during S1. Leaves exhibited strong positive correlations among C, K, Ca, Zn, and Mg, contrasted by negative associations between Mg and K/Ca, as well as C and Mg/Zn. Na displayed antagonistic relationships with K and Ca, alongside weaker interactions with Mg and Mn. In perennial branches, synergistic correlations were observed between K-Ca, Fe-Mg, and K-Mn, while Zn inversely correlated with Ca and Mn. Na showed pronounced negative associations with K, Ca, Mn, and Zn, coupled with moderate Mg antagonism. Root systems demonstrated coordinated C-K accumulation and Mg-Fe-Mn-Zn co-regulation, contrasting with C/K’s negative correlations to Ca, Mg, Mn, and Zn. These organ-specific nutrient networks highlight differential ionic coordination strategies under saline-alkali stress.

During S2, ‘Junzao’ exhibited organ-specific mineral nutrient correlation dynamics. In leaves, strong positive correlations emerged among Mn, Fe, Ca and Mg, alongside Ca-Mg/Fe/Zn synergism. Conversely, Mg displayed negative associations with Ca, Fe, Mn and Zn, while C inversely correlated with Ca and Zn. Na demonstrated pronounced antagonism with K, Mg, Fe, and Mn. Perennial branches showed K-Ca-Mg co-regulation, contrasting with Mn’s negative correlations to Fe and C. Na exhibited significant interactions with Fe, Mn, and C. Root systems manifested Zn-Ca/Mg and Ca-Mg synergies, while K inversely correlated with Ca, Mg, Fe, Mn and Zn. Na further displayed coordinated interactions with Ca, Mg, Zn and C. These periods- and organ-dependent networks highlight ‘Junzao’s adaptive recalibration of nutrient coordination mechanisms under saline-alkali stress.

4. Discussion

4.1. Effects of Saline-Alkaline Stress on the Accumulation of Na and K in Organs of ‘Junzao’ Jujube

Research has established that plants utilize ionic compartmentalization to mitigate saline-alkali stress through differential Na allocation [10,11,21,22,23,24,25,26,27,28]. Salt tolerant varieties of tomatoes [21], dwarf cashew seedlings [22], Elaeagnus angustifolia [10] ‘Jinsixiaozao’ and sour Jujube [23] cope with salt damage by accumulating a large amount of Na underground Conversely, salt-sensitive taxa such as white clover seedlings [24], Atriplex L. [25], Pisum sativum ‘Yinwan 1’/‘737’ [26], Apple rootstocks [27] and pistachio [28] predominantly allocate Na to leaves tissues. These contrasting Na distribution paradigms underscore the pivotal role of tissue-specific ion partitioning in salinity tolerance. In this study, Unlike species with constitutive compartmentalization strategies, ‘Junzao’ exhibits developmental stage-dependent Na allocation. This temporal flexibility reflects evolutionary adaptation to variable salinity environments, optimizing resource allocation based on phenological priorities of ‘Junzao’ at different stages to achieve ion segregation. In S1, ‘Junzao’ plants transport a large amount of Na to the roots and transport other mineral nutrients upwards to meet the nutritional needs of fruit development. In the study, the Na content in various organs during S1 showed that the root > branch > leaf. At S2, there is no longer a need to transport a large amount of nutrients to the leaves for fruit growth and development. ‘Junzao’ transfers a large amount of Na to the leaves to achieve ion segregation. At this stage, the Na content in organs is opposite to that in S1. This study demonstrates that in the salt tolerance mechanism of ion segregation, ‘Junzao’ accumulates a large amount of Na in the roots during S1 and in the leaves during S2. While true halophytes employ specialized excretory structures, jujube lacks such morphological adaptations. Instead, Na⁺ tolerance relies on tissue-specific vacuolar compartmentalization and controlled senescence of Na⁺-loaded organs [28]. In addition, there is also salt secretion through the waxy layer of the leaf epidermis [29], leaf stomata and leaf interstices [30] to adapt to the high-salt environment in which they grow. Research on apple plants has shown that their leaves secrete sodium ions [31]. Therefore, in this study, Na stored in the leaves during the ripening period of Junzao fruit may be partially secreted through special structures or shed from the tree with old leaves to achieve salt excretion. Unlike specialized halophytes, jujube employs tissue-specific vacuolar compartmentalization for Na+ tolerance. The absence of salt glands or bladders in Ziziphus species necessitates alternative strategies, including preferential allocation to expendable organs (leaves) during senescence phases.

During S1, the K content in the leaves and roots is higher than that in perennial branches. Previous studies have shown that potassium protects nutrient transport pathways by inhibiting the toxic effect of Na on membrane transporters. A high K/Na ratio can reduce the damage of salt stress to plant phloem transport [32]. In this study, at S2, ‘Junzao’ fruit requires a large amount of nutrients. Maintaining K content in roots and leaves is beneficial for the root system to absorb nutrients from the soil and transport nutrients from the underground to the aboveground parts, leaves provide nutrients for fruit development. The K/Na in various organs decreases with the increase of saline-alkali concentration, but remains above 1. Maintaining ion homeostasis can alleviate the damage to plant organs, and the leaves can provide the necessary nutrients for the fruit normally. S2 is at the end of the life cycle of ‘Junzao’, the K/Na in the leaves is less than 1 under the middle and high concentration of saline-alkali (180–300 mM), at this time, the leaves start to senesce, die, and fall off, and the Na is discharged out of the body, so as to achieve the purpose of antisalinity.

4.2. Effects of Saline-Alkaline Stress on the Accumulation of Ca, Mg, Zn, Fe, Mn, and C in ‘Junzao’ Jujube

In this study, at S1 and S2, Ca and Mg contents were elevated in perennial branches and roots under 300 mM mixed saline treatment, except for S1 leaves, which showed a decrease in Ca content. This may be the plant body was subjected to saline and alkaline stress when its own stress, deployment of Ca in the organ, the cell membrane structure of the protective effect, to reduce the damage caused by saline-alkali stress on the plant body [33]. At S1, the leaves need to supply the necessary Ca for fruit development, so under high concentration saline-alkali stress, the Ca content in the leaves actually decreases. At S1, the Mg content in the leaves increases continuously with the concentration of saline-alkali. When encountering saline-alkali stress, the upward transport of nutrients may become slower. Plants need to obtain a large amount of organic matter from the outside to supply fruit development, so they require a large amount of Mg to synthesize chlorophyll for photosynthesis to resist the damage caused by saline-alkali [34]. The effects of different salt alkali concentrations on different nutrient transport systems are different. Under the 60 mM saline-alkali treatment at S1, the Ca and Zn content in ‘Junzao’ roots showed a decreasing trend, similar to the performance of cotton under alkaline environment [35]. At S1, the Fe content in perennial branches and leaves under 60 mmol/L saline alkali treatment was lower than that in roots, while the Fe content in leaves increased under high concentration saline-alkali treatment, Elevated bicarbonate concentrations (>5 mM) form stable Fe-bicarbonate complexes, reducing Fe³⁺ solubility by 60–80% and limiting root-to-shoot translocation. This pH-dependent chelation effect becomes significant only under severe alkaline stress (pH > 8.5) [36], and difficult to transport upwards to the leaves. At higher concentrations of saline-alkali, the buffering capacity of bicarbonate may weaken, thereby reducing its chelation effect on Fe, making it easier for Fe to be transported from roots to aboveground parts. Under S1 saline-alkali treatment, the Mn content in various organs of ‘Junzao’ increased to varying degrees, which may be due to the enhanced activity of the antioxidant enzyme system in the plant to clear excess ROS, and Mn is one of the important components of these antioxidant enzymes [37], Therefore, its content will increase accordingly, and the increase in Zn content in leaves under high concentration saline alkali treatment during the fruit turning red period may also be the reason for this. At S2, the C content in the leaves significantly decreased with increasing saline-alkali concentration under saline-alkali treatment, and the full red period of Junzao approached the end of its physiological cycle, At this point, photosynthesis decreases and mineral elements begin to flow back to perennial branches and roots [38], leading to a disruption of ion balance in leaves and further weakening photosynthesis. Organic matter accumulation decreases and consumption increases. High concentrations of salt can damage the structure and function of plant cell membranes, and membrane damage can further affect carbon fixation and transport in plants [39]. High pH in saline-alkali environments has an inhibitory effect on soil microbial activity, which may lead to short-term accumulation of undecomposed organic matter [40,41].

4.3. Effects of Saline-Alkaline Stress on the Transport of K, Ca, and Mg in the ‘Junzao’ Jujube

The differential binding affinities of ions to enzymatic sites fundamentally regulate their absorption and translocation dynamics in plants under saline-alkali stress [42]. Excessive Na accumulation disrupts normal ion transport processes, with selectivity coefficients (SK/Na, SMg/Na, SCa/Na) serving as quantitative indices of preferential cation translocation capacity-higher values denote superior ionic discrimination against Na. During S1, these coefficients showed significant attenuation under saline-alkali stress relative to controls (CK), mirroring responses observed in NaCl-stressed Ziziphus spinosa seedlings [43]. This phenomenon underscores a conserved regulatory mechanism for ionic prioritization within Ziziphus species under osmotic stress conditions. This decline reflects compromised membrane stability and increased membrane permeability under stress conditions, disrupting active ion transport and selective absorption mechanisms. Although photosynthetic inhibition further reduced upward ion translocation capacity, all selectivity coefficients remained > 1, demonstrating sustained preferential transport of K, Mg, and Ca over Na. Similar adaptive responses have been documented in salt-tolerant species including Amorpha fruticosa and Tamarix chinensis [44], which preserve ionic discrimination capabilities despite membrane dysfunction. During S2 phase, saline-alkali stress caused a significant reduction in selectivity coefficients (SK/Na, SMg/Na, and SCa/Na) to values < 1, indicating diminished preferential transport efficiency of K, Mg, and Ca relative to Na. This phase likely triggers nutrient remobilization from senescing leaves to perennial branches and roots prior to abscission. Notably, while all coefficients remained < 1 across stress gradients, their specific trends exhibited distinct patterns: SK/Na displayed a U-shaped response, SCa/Na showed linear decline, and SMg/Na stabilized at elevated concentrations.

During S1, ‘Junzao’ prioritizes fruit development through selective nutrient transport while sequestering toxic ions in root vacuoles. This represents a resource allocation strategy rather than simple ionic regulation, involving coordinated expression of membrane transporters and metabolic adjustments. Significant positive correlations among leaf C, K, and Ca concentrations indicate synergistic maintenance of ionic homeostasis, facilitating mineral nutrient translocation to fruits. The observed Na-K correlation likely stems from ionic antagonism caused by their comparable hydrated radii and competitive binding to membrane transporters [45]. Notably, sodium is predominantly sequestered in roots (root Na >> leaf Na), while elevated foliar K/Na ratios are maintained through selective potassium transport. This adaptive strategy minimizes sodium interference in foliar tissues, ensuring uninterrupted phloem-mediated nutrient supply crucial for metabolic stability. While S2 shifts to foliar compartmentalization as part of senescence-mediated detoxification strategies, reflecting fundamental changes in source-sink dynamics and developmental priorities.

5. Conclusions

‘Junzao’ exhibits developmental stage-dependent Na partitioning: root sequestration during S1 preserves fruit development processes, while foliar accumulation during S2 facilitates senescence-mediated detoxification, demonstrating the dynamic compartmentalization regulation mechanism of Na. At S1, although K/Na in various organs decreased, it was still greater than 1, which could maintain ion homeostasis. Selective transport coefficients (SK/Na, SCa/Na, SMg/Na) declined by 25–40% under stress but remained > 1.0 during S1, indicating preserved preferential cation transport. However, values < 1.0 during S2 under severe stress (≥180 mM) signaled compromised ionic selectivity. At S2, under medium and high concentrations of saline-alkali stress (180–300 mM), Foliar K/Na ratios < 1.0 under severe stress disrupted cellular homeostasis, triggering programmed senescence and leaf abscission. This represents controlled disposal of Na-loaded tissues rather than active excretion, as jujube lacks specialized salt elimination structures. At this time, SK/Na, SK/Ca, and SK/Mg are less than 1, and nutrients flow back from the leaves to the perennial branches and roots. It can be seen that under saline-alkali, ‘Junzao’ Patterns of salt-tolerant Resource Allocation Across Growing Periods through Nutrient Dynamic Regulation. This developmental flexibility in ionic regulation provides adaptive advantage under variable salinity conditions, representing an evolutionary compromise between stress tolerance and resource allocation efficiency optimized for arid region fruit production (see Figure 6).

Our findings on Na compartmentalization and nutrient dynamics stem from controlled conditions; validation under complex field scenarios is essential. The molecular mechanisms governing the developmental stage-dependent ion selectivity and senescence signaling require elucidation. Furthermore, the long-term sustainability of this resource allocation strategy—particularly regarding perennial organ salt accumulation, yield stability, and soil health under recurring high salinity—needs multi-season field assessment.