Sugar, an osmoregulatory substance used by plants to adapt to abiotic stresses such as drought and salinity, is one of the most important indexes of fruit quality. In this study, 0–150 mM saline–alkali solutions (NaCl:NaHCO₃ = 3:1) were used to irrigate the roots of 10-year-old “Junzao” fruit trees during the growth period to explore the regulation mechanism of different concentrations of saline–alkali stress on sugar and reactive oxygen metabolism in jujube fruit at maturity. The results showed that under low stress (0~90 mM), the contents of sucrose, glucose, and fructose in the jujube fruit and the activities of sucrose phosphate synthase (SPS), sucrose synthase decomposition direction (SS-I), and sucrose synthase synthesis direction (SS-II) increased with increases in stress concentration, results that were consistent with the relative expression trends of the SPS and SS genes; however, the results were reversed under high concentrations (120 and 150 mM). The soluble acid invertase (S-AI) activity decreased with increases in stress concentration under low stress, and the results were reversed with high stress, which was consistent with the relative expression trends of the ZjcINV3, ZjnINV1, and ZjnINV3. Research regarding the response of antioxidant enzymes in fruits under saline–alkali stress showed that only the differences in peroxidase (POD) activity under saline–alkali stress were consistent with sugar accumulation; the proline (PRO), catalase (CAT) decreased and the malondialdehyde (MDA) superoxide dismutase (SOD) increased with increases in saline–alkali stress. These results indicate that the sugar metabolism and antioxidase jointly promote and regulate sugar accumulation in jujube fruits in a low saline–alkali environment.

There have been numerous studies on the effects of salt stress on jujube fruit; however, only a few studies have reported the changes in fruit quality, particularly sugar content, under conditions of combined salt and alkali stress. Therefore, the present study aimed to investigate the performance of jujube fruits under Saline-Alkali stress and the changes in sugar content in fruits. To achieve this, jujube fruits were treated with varying concentrations of salt and alkali during five developmental periods. The content of relevant sugar components was determined, and metabolomics data were analyzed in combination with relevant quantitative gene data. The results indicated that 100 days after flowering, the surface color of jujube fruit gradually turned red, and the cell structure of the fruit gradually loosened with increasing salt and alkali concentration. The content of sugar components at each stage showed that glucose and fructose primarily accumulate in the early stage of development, while sucrose is the main component in the later stage of development. Metabolomic correlation network maps showed that six differentially accumulated metabolites were closely related to specific genes. Among these metabolites, sucrose was identified as the core metabolite in the metabolic pathway. Quantitative analysis of the related genes revealed that ZjvINV2 and ZjHK2 exhibited prolonged adaptability to stress. Additionally, the expression levels of ZjSS1 and ZjSPS2 under Saline-Alkali stress were consistent with the trend of sucrose content during the same period. In conclusion, the variations in sugar content in jujube fruits during different growth stages and under Saline-Alkali treatment conditions were recorded as reference data, and the primary metabolic substances and related regulatory genes produced in jujube fruits under Saline-Alkali stress were preliminarily identified.

The aim of this study was to establish a standardized and unified method for identifying and evaluating jujube fruit texture quality and to provide a theoretical basis for determining the texture quality of jujube germplasm resources. The texture of 56 jujube germplasms was characterized via the texture profile analysis (TPA) method, and the texture and anatomical structure of three each of the hard- and soft-fleshed germplasms were analyzed. Cluster analysis was used to categorize the 56 germplasms into hard- (17) and soft-fleshed jujubes (39). Significant positive correlations were found between all other textural parameters of jujube fruits except adhesiveness. The hardness, springiness, and chewiness of the full-red period hard- and the soft-fleshed jujube fruit were lower than those of the half-red period. The hardness, adhesiveness, and chewiness of the hard-fleshed jujube fruits in the full-red period were 2.13–3.57, 3.00–4.99, and 4.39–9.58 times higher than those of the soft-fleshed jujube fruits, respectively. The findings of this study provide a theoretical foundation for the evaluation and classification of the texture and quality of jujube fruits and a basis for the breeding of new, high-quality jujube varieties with distinctive characteristics.

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.

Saline–alkali stress is an important environmental factor affecting the growth and development of plants. Plants affected by saline–alkali stress can mitigate the damage by regulating the content of osmoregulatory substances such as soluble sugars. Elucidating the regulatory mechanism of the changes in sugar fractions in jujube fruits under saline–alkali stress is crucial for the development of the jujube fruit industry in saline areas. In this study, we investigated the effects of saline–alkali stress on the development and sugar contents of jujube fruits by subjecting jujube trees to low- and high-saline–alkali stress treatments. The result showed that low saline–alkali stress increased the content of each sugar component and total sugar, whereas high saline–alkali stress suppressed their contents. In the early developmental stage, the fruit mainly accumulated fructose and glucose, whereas in the late stage, it accumulated mainly sucrose. We screened various genes, namely trehalose 6-phosphate phosphatase gene (LOC107418410), α-amylase gene (LOC107428855), α-glucosidase gene (LOC107418468), sucrose synthase gene (LOC107416188), and β-amylase gene (LOC107430415, LOC107406235), all of which were highly correlated with sucrose content in saline–alkali stress, indicating that the starch and sucrose metabolic pathways of jujube fruit are the key pathways regulating sugar accumulation in response to saline–alkali stress. To summarize, this study provides a system-level perspective on the dynamic transcriptional regulation of jujube fruits under saline–alkali stress. Additionally, the study preliminarily screened key differentially expressed genes that affect sugar accumulation in response to saline–alkali stress, providing a theoretical basis for the scientific regulation of jujube fruit quality.