Absorption, Accumulation And Physiological Responses To The Phenanthrene Treatments By Perennial Ryegrass (Lolium Perenne L.)

Polycyclic Aromatic Hydrocarbons (PAHs) pollution is an environmental problem which is focused by many researchers in recent years. But due to the short research history, many aspects in this field such as phytoremediation mechanism and the safety of agricultural products are not sufficiently discussed.In this research, we chose perennial ryegrass (Lolium perenne L.) as our research material and phenanthrene as a typical PAH, and a solution culture experiment was performed to discuss such problems: the absorption, transfer and accumulation of phenanthrene by ryegrass; the response of plant growth, root exudates and detoxification enzymes to phenanthrene. Our main results were listed as below:(1) The absorption, transfer and accumulation of phenanthrene by ryegrass: ryegrass was able to absorb phenanthrene from culture solution, and phenanthrene which had been absorbed by ryegrass root could been transferred to the overground part; the concentrations of phenanthrene in ryegrass root, overground part were related to both the concentration of phenanthrene in culture solution and the exposure time, the higher concentration of phenanthrene in culture solution, the more phenanthrene in plant tissue. After been exposed to phenanthrene for 6 days, the phenanthrene concentrations in plant root and over ground part were all lower then the plant which were exposed for 3 days, considering the plant growth during exposure time, we found that the content of phenanthrene in overground part declined which meant phenanthrene degradation happened in overground part, however, the content in root kept increasing which meant the root was continuously absorbing phenanthrene from culture solution. Plant Concentration Factor and Translocation Factor were all calculated to understand the transfer of phenanthrene in ryegrass; we found that the transfer of phenanthrene in plant was quite limited; the phenanthrene uptake by plant was mainly absorbed in root. Plant uptake and accumulation of phenanthrene was not the main reason which induced the decline of phenanthrene concentration.(2) The impact of phenanthrene on root exudates: phenanthrene affected the organic contents in ryegrass root exudates, relatively lower concentration of phenanthrene stimulated the secretion of amino acids and saccharide, but under high concentration of phenanthrene treatment, the secretion of amino acids and saccharide were restrained. Organic acids were the main part of organic content in ryegrass root exudates, and oxalic acid was the dominant organic acids secreted by ryegrass, under all treatments of phenanthrene in this experiment, oxalic acid secretion were stimulated compared with control. In ryegrass root, oxalic acid was also the dominant organic acid accumulated in root, after calculation we found that the total organic acid ( organic acids in root exudates + organic acids accumulated in root) seemed to be stable under phenanthrene treatments, while the organic acids secretion was enhanced with the phenanthrene concentration increased. The root activity also tested, data showed that, in all treatment the activity of plant root didn't decline. Considering other researches in this field, we speculated that the stimulated organic acids secretion may play an import part in the absorption-desorption of phenanthrene in soil.(3) The effect of phenanthrene on ryegrass growth: the germination rate of ryegrass wasn't affected by phenanthrene when its concentration didn't exceed 8 mg/L, while all concentration of phenanthrene showed delay effect on ryegrass germination at the first 4 days of germination. Phenanthrene had the same effect on the root length and biomass, at relatively low concentrations, the root elongation and seedling growth were stimulated by phenanthrene, but at relatively high concentrations, they were restrained by phenanthrene. Under all treatments, the chlorophyll contents and chlorophyll a/b maintained stability. All these indexes showed that ryegrass had a certain degree of tolerance to phenanthrene.(4) The physiological response related to the phenanthrene metabolism in ryegrass under phenanthrene treatment: the plant could metabolize phenanthrene, and oxygen radicals were the by-product in this procedure. In this study, we found that the SOD activity rose significantly after treated by relatively high concentration of phenanthrene for 6 days. GST activity significantly rose under 8 mg/L treatment for 3 days, after 6 days, GST activity of all the samples treated by phenanthrene were higher than control. Phenanthrene also could induce the increasing of GSH in plant. These physiological responses indicated that the ryegrass could metabolize phenanthrene in its tissues.