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.

Powdery mildew is one of the major diseases of peach. To clarify the relationships between peach powdery mildew resistance and leaf structure and defensive enzyme activities, this study used the resistant genotype ‘Zhou Xing Shan Tao’ (Prunus davidiana) and the susceptible genotype ‘Shache Lürou No.3’ (Prunus ferganensis) as a control. Resistance was evaluated through artificial inoculation with powdery mildew conidia (Podosphaera tridactyla). Key indicators including leaf wax content, stomatal characteristics, leaf transverse structure, and the activities of three defense enzymes were assessed and the relationship between these traits and resistance was analyzed. The results showed that ‘Zhou Xing Shan Tao’ exhibited stronger resistance to powdery mildew than ‘Shache Lürou No.3’. ‘Zhou Xing Shan Tao’ leaves exhibited a significantly higher (p < 0.01) wax content and a significantly lower stomatal density relative to ‘Shache Lürou No.3’. However, the size of individual stomata (long diameter, short diameter, circumference, area) did not differ significantly between the two genotypes. Microstructurally, ‘Zhou Xing Shan Tao’ leaves had significantly greater fenestrated tissue, lower epidermis, palisade cell layers, and overall leaf thickness. Its leaf tissue structure was also significantly more compact, with a higher compactness-to-openness ratio, and significantly less cellular laxity than ‘Shache Lürou No.3’. Following inoculation with powdery mildew, ‘Zhou Xing Shan Tao’ leaves accumulated hydrogen peroxide (H₂O₂) and malondialdehyde (MDA) more rapidly and to a greater extent. The activities of the superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) defense enzymes also increased faster and were significantly higher than in ‘Shache Lürou No.3’. In conclusion, leaf structural traits such as wax content and stomatal density combined with pathogenicity assessments serve as effective phenotypic indicators for evaluating powdery mildew resistance in Prunus species.