NaCl-Induced Dynamic Physiological Response and Growth Stage Sensitivity in Quinoa in Sandy Soils

Salinity strongly limits crop establishment in arid and semi-arid regions, yet the salinity dynamics of sandy soils, including how these dynamics relate to germination and early growth, remain poorly understood. This study integrated multi-depth, in situ bulk electrical conductivity (ECb) monitoring, converted to ECe, to quantify salinity dynamics and assess their effects on two quinoa genotypes (KD and J009) grown under freshwater (TR1), a single 200 mM NaCl pulse (TR2), or continuous 200 mM NaCl irrigation (TR3). Seed-zone salinity increased sharply following saline irrigation and declined with leaching, and actual emergence began when ECe dropped to approximately 8.4–11 dS m−1. KD showed stronger tolerance to rising salinity, maintaining ~85% emergence compared with ~55% in J009 under TR3, and exhibited 17% faster emergence and greater biomass recovery under TR2. Na+ accumulated mainly in leaves in KD but predominantly in stems in J009, which also experienced a 25% reduction in chlorophyll under continuous salinity, indicating greater photosynthetic inhibition. By linking dynamic EC fluctuations with genotype-specific physiological responses, this study provides a novel approach for defining salinity thresholds, identifying growth stage sensitivities, and improving salinity management in sandy, saline-prone soils.