Physiological Response States Distinguish Contrasting Zinc-Mediated Cadmium Redistribution Across 44 Rice Cultivars

Zinc (Zn) can modify plant growth and stress acclimation under cadmium (Cd) exposure; however, whether Zn consistently mitigates Cd accumulation across rice cultivars remains unclear. Here, 44 rice cultivars were exposed to Cd alone or Cd + Zn under controlled hydroponic conditions to evaluate the effects of Zn on Cd uptake and root-to-shoot translocation. Zn co-exposure resulted in highly divergent Cd responses among the cultivars. The response landscape revealed four interaction types, with response intensities spanning a nearly 200-fold range. Notably, shoot Cd levels increased in 19 of the 44 cultivars, indicating that Zn did not uniformly reduce aboveground Cd accumulation. Random Forest analysis distinguished the response direction with approximately 70% accuracy, whereas the response magnitude was less predictable. Further analysis of 12 representative cultivars identified layer-specific candidate structures, revealing that translocation-related variation was mainly associated with root superoxide/flavonoid traits, shoot Cd accumulation with flavonoid/hormone traits, and total Cd response amplitude with phenolic/flavonoid traits. Together, these results show that Zn-mediated Cd responses in rice are strongly cultivar-dependent and may be linked to distinct physiological response states, particularly root redox-related traits. These findings identify cultivar-specific physiological response states associated with the direction of Zn-mediated Cd redistribution, rather than supporting the universal protective effect of Zn.