Potassium Silicate Supplementation Accelerates Recovery from Combined Salinity–Waterlogging Stress in Maize

In reclaimed and poorly drained soils, combined salinity–waterlogging stress markedly inhibits the early vegetative growth of maize. In this study, maize seedlings at 12 days after sowing (DAS) were subjected to combined stress by immersing the entire root system in 200 mM NaCl for 7 d (stress; ST), then transferred to recovery conditions and supplied potassium at equivalent activity (5 mM K+; soil drench) as KH2PO4 (ST + K + P), K2SO4 (ST + K + S), and potassium silicate (ST + K + Si) at 0 and 5 days after treatment (DAT). Morphological traits, chlorophyll fluorescence, and gas-exchange parameters were measured at PreTR (immediately after stress termination), 5 DAT, and 10 DAT. Phytohormone, mineral nutrient profiles, oxidative stress markers and redox status, osmotic and metabolic parameters, and the expression patterns of key ion transport and stress-responsive genes were quantified at 0 and 10 DAT. The effects of K supplementation were evident across the growth- and photosynthesis-related indicators. Treatment groups (ST + K + Si, ST + K + S, and ST + K + P) exhibited significantly higher carbon fixation capacity than ST at 10 DAT. The Na/K ratio was also notably reduced in all K-supplemented groups, indicating that ionic homeostasis was restored with K supplementation through improvements in various stress response indicators such as phytohormones, osmotic adjustment, and antioxidant responses. The potassium- and silicon-treated group showed the greatest recovery effect, which may reflect the physiological characteristics of cereal species. Overall, these findings provide foundational data for the development of cultivation technology to expand the cultivation area of maize.