Modulation of root surface charge and functional groups as a key mechanism of aluminum tolerance in centipedegrass...

Determining plant tolerance to toxic metals like aluminum (Al) is crucial for cultivating crops in acidic soils. This study investigates the mechanisms of Al tolerance in centipedegrass (Eremochloa ophiuroides [Munro] Hack.), focusing on the root surface properties that govern initial metal interaction. The result showed that Al-sensitive genotypes maintained a significantly more negative root surface charge (zeta potential) across a pH gradient (pH 4.15-6.57) compared to Al-tolerant counterparts. This translated into a greater surface charge stability across the pH gradient, with only a ≤16.7 % reduction in absolute zeta potential for the Al-tolerant accessions compared to 35.2-54.1 % for the Al-sensitive accessions. Spectroscopic analysis revealed that this heightened negativity was due to a higher density of functional groups (e.g., -COO^-), which provided increased binding sites for toxic Al³⁺ cations. Consequently, Al-sensitive accessions adsorbed respectively 56.3 % and 123.2 % more bioavailable (exchangeable) Al after 2 h and 28 d exposure, leading to severe growth inhibition. In contrast, tolerant genotypes exhibited lower surface charge negativity and a remarkable capacity to transform adsorbed Al into immobilized, non-toxic precipitated forms. Also, while the secretion of citric, malic, and maleic acids is confirmed as part of the tolerance response, they function as a secondary line of defense, supporting the primary advantage conferred by the low-charge root surface. A strong positive correlation was established between zeta potential and plant growth parameters, as opposed to a negative correlation between adsorbed Al and plant growth parameters under Al stress. These findings position root surface charge analysis via zeta potential measurement as a rapid and reliable physiological predictor of Al tolerance. Thus, this study recommends leveraging root electrochemical properties for the rapid screening of Al-tolerant plant genotypes.