Effects of Mn²⁺ on Cd accumulation and ionome in rice and spinach

Health risks caused by food containing Cd is a concern worldwide. Interaction between Mn and Cd has been widely studied in normal hydroponic solution with high ion activities (e.g., the study on sharing of transporter Natural Resistance-Associated Macrophage Protein 5 between Mn and Cd in rice [Oryza sativa L.]). However, interaction of Mn and Cd in crops like rice and spinach (Spinacia oleracea L.) at field ion activity level is still unknown. Thus, an ethyleneglycoltetraacetate-buffered solution experiment was conducted to explore the effect of Mn on the uptake and accumulation of Cd and other mineral elements in rice and spinach. In rice, antagonism of Mn and Cd was only observed in roots at deficient and toxic levels of external Mn²⁺ . Compared with those at Mn²⁺ sufficiency (pMn²⁺ 6.7-5.3), average root Cd levels were elevated significantly by 1.85-3.05 times at Mn²⁺ deficiency (pMn²⁺ 8.2) but decreased by 1.57-2.59 times at Mn²⁺ toxicity (pMn²⁺ 4.8). The antagonism between Mn and K/Mg in rice shoots might be caused by their common role in physiological processes in plants. Antagonism of Mn/Ni in spinach in this work was consistent with their shared transporters in dicots. Results about the antagonism of root Cd/Mn at Mn²⁺ deficiency suggest that sufficiently available Mn²⁺ is significant to reduce Cd uptake in rice under field levels of ion activity, but it was not for spinach because the change of tissue Cd was insignificant with the increase of Mn²⁺ activity from deficiency to toxicity.