Effect of phenanthrene uptake on membrane potential in roots of soybean, wheat and carrot

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants which are toxic for human due to dietary intake of food crops. However, few studies have been conduced to compare the differences in PAH uptake among various crops. In this paper, the effect of uptake of phenanthrene (PHE), a model compound of PAHs, on membrane potential in roots of soybean (Glycine max L.), wheat (Triticum aestivum L.) and carrot (Daucus carota L.) was conducted and the related physiological mechanism was discussed. The electrical responses in roots triggered by PHE consist of two sequential phases: depolarization followed by repolarization. The depolarization of membrane potential is dependent on PHE concentration within the concentration range of 0-1.2 mg L−1. The magnitude of depolarization in root cell membrane follows the order: soybean > carrot > wheat, which is in good accordance with that of PHE uptake. In the pH range of 4.5-6.5, pH 4.5 promotes the uptake of PHE and causes a stronger depolarization. Soybean produces the largest depolarization in membrane potential, carrot gets the similar degree of depolarization at pH 5.5 and 6.5. Vanadate and 2,4-dinitrophenol inhibit the uptake of PHE and reduce membrane potential depolarization. The activity of plasma membrane (PM) H+-ATPase in roots of soybean is significantly higher than that of wheat or carrot at PHE concentrations of 0.8-1.2 mg L−1, which agrees with the changes in membrane potential. Nonetheless, wheat roots have the highest activity of H+-ATPase at PHE concentrations of 0-0.4 mg L−1. It is suggested that the differences in membrane potential responding to PHE uptake among the three crops are due to their different activities of H+-ATPase and affinities of transporters to PHE. Depolarization of cell membrane potential is a potential indicator in selecting and breeding crops that take up less/more PAHs, which would be useful to enhance food safety and improve PAH-contaminated water or soils.