Change In Intracellular Ph And Its Regulatory Mechanism During Root Uptake Of Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) have been of concern due to their strong carcinogenic, teratogenic, mutagenic properties and recalcitrance to microorganisms. Therefore, it is important to study the characteristics in plant root uptake of polycyclic aromatic hydrocarbons and the changes in root cells for understanding the mechanism of PAH transport into crop roots, and developing strategies for reducing PAH accumulation in agricultural products for safety, secreening the plant species for phytoremediation of PAH-contaminated soils or water. In this thesis, kinetic characterization of naphthalene, phenanthrene and pyrene uptake by wheat roots was studied firstly, then soybean(Glycine max), wheat (Triticum aestivum L.) and lettuce (Lactuca psativap L.) were employed as research materials, and phenanthrene was selected as a model compound for PAHs in hydroponic experiments. The intracellular pH changes of phenanthrene uptake by soybean, wheat and lettuce roots were investigated, the effects of phenanthrene uptake on enzymatic activity in relation to formation and disappearance of proton were evaluated, and the relationship between root organic acids and phenanthrene uptake was also discussed. The main results obtained are shown as follows:Wheat roots can take up polycyclic aromatic hydrocarbons from Hoagland nutrient solution and the polycyclic aromatic hydrocarbons absorbed by roots increase with incubation time. The uptake process consists of two sequential phases:a fast accumulation process followed by a slow one. The ability to take up polycyclic aromatic hydrocarbons for wheat roots can be arranged as pyrene>phenanthrene>naphthalene. Concentration-dependent uptake of the three PAHs by wheat roots can be described with Michaelis-Menten equation and the Km values for naphthalene, phenanthrene and pyrene are759.3,12.89and3.59μmol·L-1, respectively. The affinity of wheat root carrier to the three PAHs can be arranged as pyrene>phenanthrene>naphthalene. The order of the affinity for naphthalene, phenanthrene and pyrene is consistent with that of the Kow of the three PAHs. Solution pH increases due to naphthalene, phenanthrene and pyrene uptake, and the order of increase in pH per molecule of naphthalene, phenanthrene, and pyrene absorbed is pyrene>phenanthrene>naphthalene. The trend in pH increase is in good agreement with that in affinity.Measurements of intracellular pH show that during phenanthrene uptake by roots, cytoplasmic and vacuolar pH values decrease, then gradually rise to even more than their respective initial values and decline to the initial values. The decrease in vacuolar and cytoplasmic pH and the time for drop to the lowest pH in cytoplasma are positively correlated with phenanthrene concentration treated. At different phenanthrene concentrations, wheat roots have the largest decrease in cytoplasmic pH and the decrease in lettuce root cytoplasmic pH is the least. Vacuolar pH decreases largest in wheat roots at lower phenanthrene concentrations and drops largest in lettuce roots at higher phenanthrene concentrations.Phenanthrene uptake can activate NR, H+-ATPase and PPase of soybean, lettuce and wheat roots. The activity of these enzymes positively correlates with phenanthrene concentration treated, but PEPase activity is negatively correlated with phenanthrene concentration treated. These results indicate that proton pumps in cytoplasma membrane and vacuolar membrane are involved in the homeostasis of intracellular pH.Citric acid and malic acid are the dominant organic acids in wheat roots. Lettuce roots have more citric acid, malic acid and tartaric acid. In soybean roots, citric acid, tartaric acid and formic acid are dominant. Phenanthrene uptake can affect the amount of these acids in plant roots. During phenanthrene uptake, citric acid, oxalic acid and pyruvic acid reduce in wheat and lettuce roots, while citric acid, tartaric acid and succinic acid increase. Therefore, wheat and lettuce roots can regulate the intracellular pH by decreasing the amount of citric acid, oxalic acid and pyruvic acid with low pKa and fewer carboxyl groups, and soybean roots by increasing the amount of citric acid, tartaric acid and succinic acid with high pKa and strong buffering capacity.Taken together, it is concluded that acidification will occur in cytoplasmic pH during root uptake of PAHs, and proton pump in cytoplasma and vacuolar membrane and organic acids are involed in the regulation of intracellular pH.