Exogenous Calcium can Promote Nutrient Uptake and Stoichiometric Balance of Mulberry Seedlings Under Salt Stress

Salt stress is a relative concept, excessive sodium ions can inhibit the normal growth and development of mulberry seedlings through osmotic stress, oxidative damage and ion imbalance. Calcium as a second messenger to regulate the process. Therefore, calcium should also be taken seriously in experimental design. The experiment was conducted by setting different salt concentrations (0, 50, 200 mmol L^− 1) and exogenous calcium contents (0, 2.5, 5, 10, 20 mmol L^− 1) on mulberry seedlings in hydroponics. The effects of exogenous calcium on nutrient accumulation, distribution and stoichiometric characteristics of mulberry seedlings under salt stress were studied. Results showed that salt stress reduced mulberry growth, but exogenous calcium reversed this effect, when the concentration of exogenous calcium was 10 mmol L^− 1 the biomass and nutrient accumulation of mulberry seedlings could reach the maximum, and there are significant differences with other treatments (p < 0.05). And salt stress shifted the calcium absorption focus from leaves to trunks. Exogenous calcium mitigation on mulberry stoichiometry under salt stress shows initial decrease then increase, the N: P ratio showed that salt treatment could change the P stress to N stress in mulberry seedlings. Meanwhile, there was a significant positive correlation between the biomass of each organ with C: P and N: P (p < 0.01, 0.001), but a extremely significant negative correlation between the biomass with others stoichiometric ratios (p < 0.001). According to the results of leaf stoichiometric ratio and correlation analysis, the optimum application amount of exogenous calcium in mulberry seedlings under salt stress was 10 mmol L^− 1. This research offers a theoretical foundation for salt-alkali soil fertilization and has significant implications for soil salinization management.