Lead (Pb) and copper (Cu) contamination seriously threatens agricultural production and food safety. This study aims to investigate Pb and Cu induced hormetic effect and toxicity mechanisms in lettuce (Lactuca sativa L.) and establish reliable empirical models of potentially toxic elements (PTEs) transfer in the soil-plant system. The content and distribution of Pb and Cu at subcellular levels in lettuce plants were examined using inductively coupled plasma-mass spectrometry, differential centrifugation and micro-X-ray fluorescence spectroscopy. The PTE-loaded capacity of Pb that ensures food safety was lower than that of Cu in the studied soil, but the PTE-loaded capacity of Pb that limits yield was higher than that of Cu. Lead in lettuce roots mainly accumulated in the cell wall (41%), while Cu mainly accumulated in the vacuoles (46%). The Pb and Cu were primarily distributed in the radicle of lettuce seeds under severe PTE stress, resulting in no seed development. Iron plaque formed on the root surface of lettuce seedlings and sequestered Pb and Cu via chelation. At the same concentration, lettuce was less tolerant to Cu in contaminated soil than Pb due to the higher activity of Cu ions in the soil. Lead was more phytotoxic to lettuce than Cu, however, since the radicle emerged from the seed under severe Cu levels, while it did not protrude under severe Pb levels. The potentially damaging effect of Pb in the visually healthy lettuce appeared to be higher than that of Cu under the same soil contamination level.

Biochar Engineering Technology Research Center of Guangdong Province School of Environmental and Chemical Engineering Foshan University Foshan Guangdong 528000 China

Biochar Engineering Technology Research Center of Guangdong Province School of Environmental and Chemical Engineering Foshan University Foshan Guangdong 528000 China; College of Tropical Crops Hainan University Haikou Hainan 570228 China

Biochar Engineering Technology Research Center of Guangdong Province School of Environmental and Chemical Engineering Foshan University Foshan Guangdong 528000 China; Key Laboratory of Soil Contamination Bioremediation of Zhejiang Province Zhejiang A and F University Hangzhou Zhejiang 311300 China

College of Oceanology and Food Science Quanzhou Normal University Quanzhou Fujian 362000 China

College of Tropical Crops Hainan University Haikou Hainan 570228 China

Department of Agricultural and Biological Engineering University of Florida Gainesville FL USA

Department of Environmental Geosciences Faculty of Environmental Sciences Czech University of Life Sciences Prague Kamýcká 129 16500 Praha 6 Suchdol Czech Republic

Institute of High Energy Physics Chinese Academy of Sciences Beijing 100039 China

Institute of Soil and Environmental Sciences University of Agriculture Faisalabad Faisalabad 38040 Pakistan; School of Civil Engineering and Surveying University of Southern Queensland Toowoomba 4350 Queensland Australia

School of Water Energy and Environment Cranfield University Cranfield MK43 0AL Bedfordshire United Kingdom

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