Integrative Evaluation of Salt Tolerance in Cherry Rootstocks Using Phenotypic and Biochemical Markers

The sweet cherry has become a commercially significant fruit crop, yet its cultivation in Xinjiang is severely constrained by saline-alkali soils. To address this, selecting salt-tolerant rootstocks is vital for sustainable crop production in salinized soils. This study investigated the physiological and biochemical responses of five major rootstock cultivars ('Mahaleb CDR-1', 'Gisela 6', 'Colt', 'Daqingye', and 'Krymsk5') to a gradient of NaCl stress (0, 50, 100, 150, 200, 250, 300 mmol·L^-1) under controlled environmental conditions. Key osmoprotectants and antioxidant systems showed a consistent trend across genotypes: the contents of soluble sugars, proteins, and proline, along with the activities of catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD), initially increased under moderate stress but subsequently declined as salinity stress intensified. Specifically, CAT activity peaked at 150 mmol·L^-1 NaCl in most genotypes, with significant increases ranging from 33.9% ('Gisela 6' at 100 mmol·L^-1) to 45.52% ('Colt' at 150 mmol·L^-1) compared to controls. SOD activity also reached maxima at 150 mmol·L^-1 in most cultivars, increasing by 11.30% to 19.38% relative to controls, while POD activity exhibited peak values at 150-200 mmol·L^-1, with increases of 4.12% to 10.45%. Notably, proline (PRO) accumulation peaked at 150 mmol·L^-1 NaCl, with 'Mahaleb CDR-1' demonstrating the highest concentration (29.81 μg·g^-1) and 'Colt' the lowest (25.85 μg·g^-1). Conversely, the malondialdehyde (MDA) content, an indicator of membrane lipid peroxidation, increased progressively with increasing salinity. The cultivar 'Colt' exhibited the most severe membrane damage. Its MDA content under 300 mmol·L^-1 NaCl stress was 80.84% higher than that under the control condition (0 mmol·L^-1 NaCl). These results demonstrate that under moderate salt stress, the rootstocks activated adaptive responses, as evidenced by elevated osmoprotectant levels and enzymatic activity, which were ultimately suppressed under severe conditions. A comprehensive analysis of all physiological and biochemical indices allowed for a clear ranking of salt tolerance: 'Mahaleb CDR-1' > 'Daqingye' > 'Krymsk5' > 'Gisela 6' > 'Colt'. This study provides a robust physiological basis for selecting salt-tolerant rootstocks in saline-alkaline regions and offers valuable insights for breeding programs aimed at enhancing stress resilience.