Genotype-Dependent Dual Effects of Zinc on Cadmium Accumulation in Rice Revealed by a Multi-Scale Quantitative Framework

Cadmium (Cd) contamination poses a major risk to rice safety, while zinc (Zn) can modify Cd accumulation in a genotype-dependent manner, with its biological basis remaining incompletely understood. Here, 44 rice varieties were hydroponically cultured under Cd stress either alone or under a near-equimolar Zn + Cd co-exposure treatment, and Zn–Cd interaction patterns were analyzed using an integrated framework combining response-landscape quantification, Random Forest modeling, and sparse partial least squares (sPLS) analysis. The response landscape revealed a four-quadrant distribution of interaction types across cultivars, with response intensities spanning nearly a 200-fold range, indicating that Zn effects on Cd accumulation were highly variable rather than uniformly inhibitory. Random Forest classification distinguished response direction with ~ 70% accuracy using baseline metal status and Zn-induced physiological shifts as predictors, suggesting that non-random information about response direction is recoverable from surface-level phenotypic variation. sPLS analysis of 12 representative cultivars further resolved layer-specific candidate signal structures, with translocation-related variation associated primarily with a root superoxide/flavonoid-centered structure, leaf Cd accumulation with a more flavonoid/hormone-centered structure, and total Cd amplitude with a phenolic/flavonoid-associated structure. Together, these results show that Zn effects on Cd accumulation are genotype-dependent and identify the root redox baseline state under Cd stress alone as a candidate upstream feature associated with response direction. They further suggest that contrasting outcomes across cultivars may reflect different state-dependent response regimes linked to baseline redox status. This framework provides a basis for understanding why the same Zn intervention can yield opposite Cd outcomes across rice cultivars, and it requires broader validation before any screening or management application is proposed.