Interaction Of Polycyclic Aromatic Hydrocarbons And Nitrogen Uptake By Wheat,Soybean And Lettuce Roots

Investigation regarding interaction of root uptake of polycyclic aromatic hydrocarbons (PAHs) and nitrogen is beneficial for safe production of crop foods and enhancement in phytoremediation of PAH-contaminated soil and water. In this study, we assessed the interaction of PAHs and nitrogen (nitrate and ammonium) uptake by wheat, soybean and lettuce roots. The hydroponic experiment was employed to study absorption and accumulation of phenanthrene by wheat, lettuce and soybean roots under different nitrogen conditions. In the presence of different nitrogen and phenanthrene, we measured the activity of six enzymes, i.e., nitrate reductase (NR), plasma membrane H+-ATPase, tonoplast H+-ATPase, pyrophosphatase (PPase), glutamine synthetase (GS) and glutamate synthase (GOGAT) of the different crop roots and the membrane potential changes of the wheat, soybean and lettuce roots. The main results show as follows:Wheat, soybean, and lettuce roots can take up and absorb phenanthrene from Hoagland nutrient solution. The phenanthrene absobed by roots varies with different nitrogen forms. Nitrate inhibits the root uptake of phenanthrene and promotes migration of phenanthrene from roots to shoot. In contrast, ammonium promotes root uptake of phenanthrene and inhibits migration of phenanthrene from root to shoot. Although the transfer factors of soybean and lettuce fed by ammonium are lower than those fed by nitrate, the difference in transfer factor of wheat between treatments with ammonium and nitrate is not significant. In the mixed nitrogen treatments, phenanthrene concentration in wheat and lettuce roots decreases with declining in ratio of ammonium/nitrate, phenanthrene concentration in soybean roots is highest at1:1of ammonium/nitrate ratio. The transfer factors of lettuce and soybean treated with mixture of ammonium and nitrate are higher than those treated with ammonium and there is no significant difference in transfer factors between treaments with mixed nitrogen and nitrate (p>0.05). Wheat transfer factor for mixed nitrogen is not markedly different from that for either ammonium or nitrate treatment. In sole nitrate, sole ammonium or mixed nitrogen treatments, the uptake of phenanthrene in the three crop roots is soybean> Lettuce> wheat, and the soybean shoot transfer factor is the highest. The order of root concentration factors of the three crops is consistent with that of phenanthrene concentration in roots, the order of shoot concentration factors for phenanthrene is soybean>wheat>lettuce. Transfer factor in lettuce is the lowest among the three crops tested for the same treatments. Hence, soybean roots take up phenanthrene strongest. Whether sole nitrate, sole ammonium or mixed nitrogen, ApH, root uptake of phenanthrene, root concentration factor and transfer factor of phenanthrene vary with crop species in the treatments with or without phenanthrene. Root absorption of phenanthrene and ApH can be arranged as soybean> lettuce> wheat. The uptake of phenanthrene and the migration in plants are closely related to nitrogen forms and type of crops.Phenanthrene and nitrogen form have different effects on crop root enzymatic activity. Nitrate promotes NR activity and inhibits glutamine synthetase activity, and the promotion role in GOGAT activity ranges with nitrate concentration. Ammonium can promote NR and GS activity. In ammonium-fed crop roots, the trend in GOGAT activity is similar to that in nitrate-fed crop roots. With decrease in the ratio of ammonium/nitrate, root NR and GOGAT activities increase in wheat and lettuce, but NR and GOGAT activities peak at1:1of NH4+:NO3-. Compared to the treatments without phenanthrene, the presence of phenanthrene can promote NR, GOGAT activity and GS activities, indicating that these enzymes may be involved in the regulation of the H+/phenanthrene symport system. The activities of these enzymes differ with crops regardless of the presence or absence of phenanthrene. NR activity is wheat> soybean> lettuce. GS and GOGAT activities are soybean>lettuce>Wheat. These results are in good agreement with the root uptake of phenanthrene and root concentraion factor.In phenanthrene treatments or non-phenanthrene treatments, different nitrogen forms have different effects on crop root H+-related enzyme systems. Nitrate decreases plasma membrane H+-ATPase activity, tonoplast H+-ATPase activity and PPase activity. The inhibition in the plasma membrane H+-ATPase activity is not obvious. Ammonium obviously promotes plasma membrane H+-ATPase, tonoplast H+-ATPase, and PPase activity. Root plasma membrane H+-ATPase, tonoplast H+-ATPase, and PPase activity rises with an increase in the ratio of ammonium/nitrate for wheat and lettuce. These enzymatic activities are highest at1:1of ammonium/nitrate for soybean. In contrast to the phenanthrene-free treatments, the presence of phenanthrene can promote the activity of plasma membrane H+-ATPase, tonoplast H+-ATPase, and PPase. There are differences in plasma membrane H+-ATPase, tonoplast H+-ATPase and PPase activity for different crops, irrespective of the presence or absence of phenanthrene. The orders of plasma membrane H+-ATPase, tonoplast H+-ATPase and PPase activities are soybean>Iettuce>wheat. The result is in line with the root uptake of phenanthrene, root concentraion factor, NR activity, GS activity, and GOGAT activity.Resting membrane potentials for wheat, lettuce and soybean roots are different. The addition of phenanthrene triggers a membrane depolarization, and the order of the magnitude for the depolarization soybean> Lettuce> wheat, consistent with that of root uptake of phenanthrene. Nitrate reduces the degree of depolarization, the higher the concentration of nitrate, the less membrane potential depolarizes. In contrast, the trend in depolarization for ammonium and mixed nitrogen is reverse, i.e., the higher ammonium concentration or the ratio of ammonium to nitrate, the greater the magnitude of depolarization is. These results are in line with the effect of nitrogen forms on the root uptake of phenanthrene.Therefore, we can conclude that root uptake of phenanthrene is coupled to nitrogen uptake.