[Effects of Pyrene on Low Molecule Weight Organic Compounds in the Root Exudates of Five Species of Festuca].

Phytoremediation is an important measure to remove organic pollutants from contaminated soil, and the root secretion of plant is considered to be closely related to the mechanisms of phytoremediation of organic pollutants. It is in favor of revealing the mechanisms of remediation by studying the characteristics of root exudates of plants with phytoremediation potential under the stress of pollutants. In the present research, pyrene and five species of Festuca which have been testified to be tolerant to pyrene stress were selected as studied objects. A soil-cultivating experiment with rhizobag technique was conducted to investigate the effects of pyrene on low molecule weight organic compounds in the root exudates of plant species under five concentration levels of pyrene (10.19, 20.32, 40.36, 79.94,and 160.68 mg·kg^-1,denoted by C1, C2, C3, C4 and C5,respectively) on day 30, 40, 50, 60 and 70 of experiments. The results showed that the presence of vegetation significantly enhanced the dissipation of pyrene in the soil environment. This effect was especially marked with Festuca arundinacea, followed by those with Festuca mazzetiana, Festuca pubiglumis, and Festuca longiglumis, and that with Festuca stapfii was the lowest. During the whole experiments, the amounts of soluble sugar excreted by the five species of Festuca tested in root exudates were promoted with pyrene stress, then fluctuated with a stable trend along with the increase of stress concentration or the extension of stress period, which appeared to rise appreciably at relative low pyrene spiked (C1-C3) or earlier stress stage (30-40 d) and reduce at relative high pyrene spiked level (C3-C5) or later stress stage (40-70 d), and the highest amount of soluble sugars in root exudates occurred on day 50 of experiments with 40.36 mg·kg^-1 pyrene treatment. The greater the phytoremediation potential of the plant species tested, the more obvious this trend wads. Compared with the control treatment (CK), pyrene stress promoted the root system of all five species of Festuca tested to release more low molecular weight organic acids, the stronger the restoration potential of plant species, the higher the concentration of pyrene stress under which the amount of organic acids in root exudates was increased to the peak value. Among the five species of Festuca tested, oxlic acid, acetic acid, lactic acid and malic acid were the main components of organic acids in root exudates, with a percentage of greater than 98.15% in all pyrene stress treatments, but there were traces of fumaric acid in the roots secretion of plant species with the stronger restoration potential. Data also indicated that 19 types of amino acids were found in root exudates of Festuca and the composition of amino acids in root exudates of Festuca was stable under all pyrene stress treatments, but the amino acid amount was different in root exudates under pyrene stress. The amount of all amino acids in those root exudates increased with increasing pyrene concentration, especially, the amount of threonine, serine, glycine, and alanine increased significantly among the 19 types of amino acids and the differences were significant among different treatments with different pyrene concentrations (P<0.05). However, proline, hydroxy proline and aspartic acid were always released in the form of functional group as a response to the pyrene stress, their contents soared quickly with the increase of stress concentration in soils, and the difference was significant among different treatments with different pyrene concentrations (P<0.05); the more the components of functional group participated in stress response, the stronger the restoration potential of plant species. These results indicated secretion characteristics of soluble sugar, low molecular weight organic acids and amino acids in the root system were closely related to their phytoremediation potential under the pyrene stress, the greater the phytoremediation potential, the more the amount of these low molecular weight secretions and the more complex these components, and the stronger the adaptability to polluted environment and the physiological plasticity to adapt to these contaminants.