Exogenous organic matter modulates cadmium uptake in rice (Oryza sativa L.): Linking root surface chemistry to metal translocation and tolerance

Cadmium (Cd(II)) poses a severe threat to rice production and food safety, and exogenous organic matter (EOM) offers a cost-effective, eco-friendly solution for Cd(II) remediation. Nevertheless, the effect of EON on root electrochemistry and the relationship with Cd(II) uptake and sequestration remains elusive. This study elucidates the Cd(II) tolerance mechanisms in two rice cultivars (Oryza sativa L., Cd(II)-tolerant WYJ24 and Cd(II)-sensitive FD88) in a hydroponic system under biochar and organic acid treatments. The results revealed that WYJ24 exhibited superior Cd(II) tolerance, maintaining higher root elongation (1.35-3.66 times > FD88) and restricting Cd(II) translocation (shoot-to-root ratio < FD88). This observation was associated with enhanced Cd(II) complexation (70.7% vs. 22.9% after 10 d) and fewer root negative charges (-10.6 vs. -13.4 mV at pH 6.73) in the roots of WYJ24 relative to FD88, respectively. Root exudate profiling under Cd(II) stress revealed WYJ24 exhibiting a targeted, transient increase in specific acids (e.g., malic, quinic), while the sensitive FD88 exhibited a significant, dysregulated exudation. Also, biochar-supplemented media significantly increased root negative zeta potential and functional group concentration, which significantly correlated with total absorbed and exchangeable Cd(II). However, organic acids exhibited cultivar-specific responses, with oxalic acid modulating Cd(II) adsorption in FD88 via protonation while enhancing complexation in WYJ24. Therefore, the findings reveal that Cd(II) tolerance is governed by a combination of root surface properties and internal sequestration efficiency, and that organic amendments function by selectively modifying the root interface. However, the translation of these hydroponic insights into field applications requires future validation under realistic soil conditions.