Abstract:

Aims Root exudates have specialized roles in nutrient cycling and signal transduction between a root system and soil, as well as in plant responses to environmental stresses. They are the key regulators in the rhizosphere communications and can modify the biological and physical interactions between roots and soil organisms. Phytoremediation is an important measure to remove organic pollutants from contaminated soil, and root exudates are considered to be closely related to the mechanisms in the phytoremediation of soils contaminated by organic pollutants．This study was designed to determine the characteristics of root exudates in five Festuca species under the stress of benzo [α] pyrene (BaP) and to identify the effects of BaP on the organic compounds of low molecule weight in root exudates.Methods Five Festuca species, which had been tested to be tolerant to the BaP stress, were used in this study. A soil-cultivating test, with rhizobag technique, was conducted to investigate the effect of BaP concentration on the organic compounds of low molecule weight in root exudates at different growth stages (30-70 days). The BaP concentrations in the contaminated soils were set for 10.25 mg·kg^-1, 20.37 mg·kg^-1, 40.45 mg·kg^-1, 80.24 mg·kg^-1, and 161.74 mg·kg^-1(denoted by T₁, T₂, T₃, T₄ and T₅, respectively).Important findings The presence of vegetation enhanced the dissipation of BaP in soils. This effect was especially marked in treatment with F. arundinacea, followed sequentially by that of F. chelungkiangnica, F. rubra subsp. arctica and F. sinomutica; the dissipation of BaP in treatment with F. stapfiwas lowest during the entire experiment. The contents of soluble sugars, organic acids, and amino acids in root exudates were all increased by the BaP treatments. The contents of soluble sugars in root exudates increased notably at relatively low BaP levels (T₁-T₃) or in earlier stress stages (30-40 days), and declined at relatively high BaP levels (T₄-T₅) or in later stress stages (40-70 days), with highest values always occurring in the T₃ treatments on day 50 of the experiments. In the five Festuca species, oxlic acid, acetic acid, lactic acid and malic acid are the main constituents of organic acids in root exudates, at greater than 97.34% in total in all treatments. However, there are traces of fumaric acid in the root exudates of Festuca species with stronger remediation potentials. When the contents of organic acids in root exudates reached the peak, the stronger the remediation potentials of plants were, the higher the concentrations of BaP would be to induce stress. Nineteen types of common amino acids were found in root exudates of Festuca and the proportion of total amino acids in root exudates remain stable under all the BaP stress treatments, albeit varying contents of the 19 types of amino acids under different BaP concentrations. The contents of all amino acid in root exudates increased with increasing BaP concentrations; especially, the contents of secreted threonine, serine, glycine and alanine increased significantly among the 19 types of amino acids and the differences were significant among all treatments with different BaP concentrations (p< 0.05). However, proline, hydroxyproline, and aspartic acid participated in the stress responses of plants almost in the form of additive or synergistic effects, and their contents in root exudates increased markedly with increasing BaP concentrations in soils; the differences among different treatments were significant (p< 0.05). The more constituents of amino acids there were in stress responses, the stronger the remediation potentials of plants would be. All these illustrate that the characteristics of root exudates in Festuca were closely related to their remediation potential under the BaP stress. The greater the remediation potentials were, the more organic compounds of low molecular weight there were and the more complex those compounds would be. Moreover, they also showed a stronger environment adaptability and physiological plasticity.

Key words: benzo [α] pyrene, Festuca, root exudates, organic compounds of low molecule weight, interspecies difference