Stage-specific regulation of seed germination under saline-alkali stress and implications for sequential emergence capacity

Soil salinization and alkalization severely constrain crop emergence and stand establishment. The seed germination stage involves rapid transitions through imbibition, metabolic reactivation, radicle protrusion, and early seedling establishment. Its responses to saline-alkali stress are stage-specific rather than simply representing an early manifestation of seedling-stage saline-alkali tolerance. This review synthesizes research progress on seed germination under saline-alkali stress, with emphasis on distinguishing the differential constraints imposed by neutral salts versus alkaline salts on germination. It summarizes regulatory mechanisms including early perception, ion and redox homeostasis, autophagy and proteostasis, cell-wall biomechanical remodeling, and the ABA-GA-mediated balance between growth and defense. On this basis, we propose sequential emergence capacity (SEC) as a conceptual framework for process-oriented phenotyping and evaluation. It covers the sequence from early damage buffering to germination progression and early establishment, and includes cross-stage recovery or re-exposure responses when relevant. SEC comprises four dimensions: damage avoidance capacity (DAC), germination progression robustness (GPR), early establishment quality (EEQ), and cross-stage adaptive potential (CAP). DAC, GPR, and EEQ constitute the operational core (SEC_core); CAP is retained only as an exploratory or mechanistic validation dimension. In practice, SEC is meant to be used primarily as a multidimensional phenotypic profile. Composite scores are optional and, if used, must be validated against substrate- or field-level emergence data. Future efforts should develop stepwise, high-throughput phenotyping pipelines based on SEC_core. These could help uncover the genetic control of emergence-related traits, if validated under field conditions, and thereby bridge the gap between laboratory assays and field performance in saline-alkali environments.