Copper pre-exposure reduces AgNP bioavailability to wheat.

Heavy metals in contaminated sites can affect plant responses to emerging contaminates such as engineered silver nanoparticles (AgNPs), but the underlying mechanisms are poorly understood. After 4-day exposure to 0-2.5 mg Cu L-1 hydroponically, Cu concentrations in roots of wheat seedlings (Triticum aestivum L.) increased from 20 ± 3 to 325 ± 58 mg kg-1. Meanwhile, the cell death in root tips, as measured by the uptake of Evans blue stain, increased 1.8-2.8 times in response to Cu exposure. Total thiol contents in roots (including glutathione, cysteine and phytochelatins), as measured by high performance liquid chromatography, increased 1.4 times upon low Cu exposure but decreased 2.2 times upon high Cu exposure. After those wheats were exposed to 10 mg L-1 AgNPs for 8 h, the Ag influx rates decreased 1.3-3.9 times in Cu pre-exposed plants. Together, the cell death in root tips and thiol levels in roots could explain the decreased Ag influx rates of Cu pre-exposed plants. These findings indicate that the bioavailability of AgNPs without consideration of pre-existing metals could be over-estimated.