Hydroponic electrochemical phenotyping of root surfaces for rapid screening of salt-tolerant rice (Oryza sativa L.)

Soil salinization, driven by sodium ion (Na⁺) toxicity, is a major environmental hazard threatening global food security. While research has focused on internal physiological responses, the initial electrochemical interaction between Na⁺ and the root surface remains unexplored. This study hypothesized that the root surface charge (zeta potential) governs Na⁺ adsorption and subsequent toxicity. We employed a hydroponic electrochemical-physiological approach, screening 20 rice varieties of known tolerance and validating findings across 100 cultivars under controlled seedling-stage conditions. Root zeta potential, Na⁺ adsorption/desorption kinetics, tissue ion content, and plant survival under 120 mM NaCl (selected as the optimal screening concentration) were analyzed and correlated. Among the 20 varieties of known tolerance, both japonica and indica types were represented, but no consistent subpopulation-specific pattern emerged; instead, root zeta potential correlated with tolerance irrespective of subspecies. Results revealed that salt-tolerant varieties possessed a significantly less negative mean root zeta potential compared to sensitive ones. This electrochemical trait directly governed Na⁺ dynamics, with the most negatively charged roots adsorbing 127% more exchangeable Na⁺ and exhibiting a strong positive correlation with Na⁺ translocation to shoots and plant mortality ( r ² = 0.70). Receiver operating characteristic analysis confirmed zeta potential as an excellent diagnostic tool (AUC = 0.92), with a threshold of > -20 mV (pH = 6.0) enabling accurate tolerance prediction under the tested conditions. Following these results, we propose an “electrochemical exclusion” model where a less negative root surface reduces initial Na⁺ loading, alleviates stress on cellular detoxification systems, and enables effective root sequestration, thereby enhancing survival. Thus, this study proposes root surface electrochemistry as a fundamental, heritable trait and provides a novel, high-throughput hydroponic phenotyping method for preliminary screening of salt-resilient rice germplasm, contributing to the development of industrial biomass feedstocks on saline marginal lands.