Dynamic Quantification and Prediction of Salt Tolerance Threshold in Summer Maize Under Different Regimes of Brackish Water Irrigation

To investigate how different training modes of salt stress priming affect the dynamic variation of the salt tolerance threshold (STT) in summer maize, a micro-plot experiment with staged brackish water irrigation was conducted. Based on physiological and biochemical parameters, along with shoot and root traits, a dynamic salt tolerance coefficient (αSTT) was defined to quantify STT across growth stages. The results revealed a clear two-stage adaptive response to salt stress, consisting of an initial physiological adaptation phase followed by a phenotypic adaptation phase. Different training modes induced distinct salt stress memory effects by regulating the coordination between these two stages. Among treatments, the S1-2-3 regime—corresponding to mild (2.0 g·L−1), moderate (4.0 g·L−1), and severe (6.0 g·L−1) salinity applied sequentially at the six-leaf, ten-leaf, and tasseling stages—exhibited the most favorable adaptive outcome, with αSTT gradually recovering to 1.0 at later stages and a concomitantly higher STT. Furthermore, a unified predictive framework was established to estimate STT dynamics, within which the process-constrained PCR-STP pathway outperformed purely data-driven pathways. Overall, our study elucidates the dynamic nature of salt tolerance in summer maize and provides a scientific basis for optimizing brackish water irrigation regimes and refining salt stress modules in crop models.