Sodium enhances photosynthetic capacity to alleviate potassium deficiency in the euhalophyte Suaeda salsa

Soil salinity, primarily caused by NaCl, disrupts plant Na⁺/K⁺ homeostasis, hindering growth. While euhalophytes like Suaeda salsa thrive in saline conditions, the mechanisms by which Na⁺ promotes their growth, especially under K⁺ deficiency, remain poorly understood. Here, we used transcriptomic, metabolomic, and physiological analyses to investigate how Na⁺ alleviates K⁺ deficiency in S. salsa. Exogenous NaCl application promoted the growth of S. salsa under K⁺ deficiency, which was associated with upregulated expression of genes for photosynthetic subunits, light-harvesting complexes (LHCs), and key Calvin cycle enzymes, including phosphoribulokinase (PRK), phosphoglycerate kinase (PGK), and glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and Rubisco. Consequently, Na⁺ application enhanced the net photosynthetic rate, PSII photochemistry (Fv/Fm), photochemical quenching (qP), non-photochemical quenching (NPQ), and the activities of enzymes like ferredoxin-NADP⁺ reductase (FNR) and Rubisco. Transmission electron microscopy revealed that NaCl treatment increased the number and size of chloroplasts, maintaining their ultrastructural integrity under K⁺ deficiency. Furthermore, the relative contents of L-glutamate, serine, and threonine were elevated, with concurrent upregulation of their biosynthetic genes. These findings demonstrate that Na⁺ enhances photosynthesis and related metabolic pathways to promote biomass accumulation under K⁺ deficiency in S. salsa. Our findings elucidate the key mechanisms underpinning the survival of euhalophytes in environments characterized by high salinity and potassium deficiency.