Root Surface Electrochemical Properties Correlate with Salt Tolerance and Sodium Adsorption in Miscanthus Accessions: A Hydroponic Study

Soil salinization threatens global agriculture, yet the mechanisms of salt tolerance in bioenergy crops like Miscanthus remain poorly understood. While root surface electrochemistry has been studied in crop species, its effect on salt tolerance among Miscanthus accessions remains underexplored. Therefore, this study examined whether root surface electrochemical properties are associated with differential salt tolerance and Na+ adsorption in eight Miscanthus accessions. Hydroponically grown plants were subjected to 1% NaCl for three weeks. Salt-tolerant accessions (394, 390, 422, 401) showed significantly higher relative growth parameters (81.49–89.21%) than salt-sensitive accessions (403, 1143, 01293, 11024; 61.82–66.87%; p < 0.05). Root surface zeta potential measurements revealed that salt-tolerant accessions carried less negative charge (ζ = −16.30 mV at pH 6.0) than salt-sensitive accessions (ζ = −30.90 mV; p < 0.01). Also, ATR-FTIR spectroscopy revealed a trend toward fewer ionizable functional groups, particularly carboxylates, on salt-tolerant root surfaces. Consequently, salt-sensitive accessions adsorbed significantly more Na+ (6847 mg kg−1) than salt-tolerant accessions (5163 mg kg−1; p < 0.05). Moreover, significant correlations among zeta potential, Na+ adsorption, and growth indices (R2 = 0.56, p < 0.5 for zeta potential vs. Na+ adsorption; R2 = 0.85, p < 0.01 for zeta potential vs. RRDW; R2 = 0.65, p < 0.05 for Na+ adsorption vs. RRDW) were consistent with the mechanistic link. This shows that fewer functional groups result in less negative charge and consequently reduced Na+ adsorption and enhanced salt tolerance. Therefore, salt-tolerant Miscanthus accessions possess fewer ionizable functional groups and less negative root surface charge, resulting in reduced Na+ adsorption, a mechanism not previously described in this bioenergy crop. These findings establish root surface electrochemical properties as a determinant of salt tolerance in Miscanthus and provide a rapid phenotyping tool for germplasm screening.