Cation-π Interactions with Coexisting Heavy Metals Enhanced the Uptake and Accumulation of Polycyclic Aromatic...

Few studies have considered the effect of co-occurring heavy metals on plant accumulation of hydrophobic organic compounds (HOCs), and less is known about the role of intermolecular interactions. This study investigated the molecular mechanisms of Cu/Zn effects on hydroponic uptake of four deuterated polycyclic aromatic hydrocarbons (PAHs-d₁₀) by spinach (Spinacia oleracea L.). Both solubility enhancement experiment and quantum mechanical calculations demonstrated the existence of [PAH-Cu(H₂O)_0-4]²⁺ and [2·PAH-Cu(H₂O)_0-2]²⁺ via cation-π interactions when Cu²⁺ concentration was ≤100 μmol/L. Notably, PAH-d₁₀ concentrations in both roots and shoots increased significantly with Cu²⁺ concentration. This was because the formation of phytoavailable PAH-Cu²⁺ complexes decreased PAH-d₁₀ hydrophobicity and consequently decreased their sorption onto dissolved organic carbon (DOC, i.e., root exudates), thereby increasing phytoavailable concentrations and uptake of PAHs-d₁₀. X-ray absorption near-edge structure analysis showed that PAH-Cu²⁺ complexes could enter defective spinach roots via apoplastic pathway. However, Zn²⁺ and PAHs-d₁₀ cannot form the cation-π interactions because of the high desolvation penalty of Zn²⁺. Actually, Zn²⁺ decreased the spinach uptake of PAHs-d₁₀ due to the increase of DOC induced by Zn. This work provides molecular insights into how metals could selectively affect the plant uptake of HOCs and highlights the importance of considering the HOC phytoavailability with coexisting metals.