Differential physiological responses to sodium and potassium chloride in Salicornia europaea, a model euhalophyte

Sodium (Na) and potassium (K) have comparable crustal abundances and similar physicochemical properties. However, due to the predominance of NaCl over KCl in natural salinity, most studies on plant salt responses have focused on NaCl, leaving responses to KCl salinity largely unexplored. To compare plant responses to Na and K salts, the euhalophyte Salicornia europaea was grown under equimolar (100 mM) NaCl and KCl treatments. NaCl salinity enhanced plant dry biomass, succulence, water relations, and the accumulation of organic osmolytes, whereas KCl salinity induced defence responses, lignin deposition, and caused complete plant mortality within 7 weeks. The levels of signalling molecules (H2O2, NO, and H2S) showed distinct patterns between NaCl and KCl treatments. Genes involved in ion compartmentation (HKT1, NHX1, SOS1, and VHA-A) were upregulated only under NaCl salinity. Moreover, NaCl reduced cellular ammonium levels, whereas KCl led to its accumulation, accompanied by declines in amino acid and protein contents, suggesting a disruption of ammonium assimilation under KCl stress. Comparative studies involving two glycophytes and four halophytes revealed that whereas glycophytes exhibited similar or less growth inhibition under KCl than NaCl, halophyte growth was severely inhibited by KCl but stimulated by NaCl. Notably, ammonium accumulation patterns differed between the two plant types; being higher under NaCl in glycophytes, but higher under KCl in halophytes. Overall, despite the chemical similarity between Na+ and K+ ions, our findings demonstrate that K fails to activate adaptive signalling pathways to salt stress, highlighting ion-specific mechanisms underlying halophyte tolerance to NaCl salinity.