Impacts Of Polycyclic Aromatic Hydrocarbons On The Root Exudation By Plants

Nowadays, soil organic pollution in China and other countries is of worldwide concerns. Polycyclic aromatic hydrocarbons (PAHs) and other organic pollutants can transport in the soil-plant system, and enter into the agricultural products through the food chains, posing great threats to the ecological and human health. Recently, the study about the environmental behaviors of organic pollutants in rhizosphere has drawn much attention, but mainly focused on the impact of root exudates on the environmental processes of organic pollutants in rhizosphere. There are still limited reports on the effects of organic pollutants on plant root exudation.In this work, the plant root exudate and its effects on the environmental process of organic pollutants in soil were reviewed. On the basis of the investigation of plant uptake of PAHs, the plant root exudation and its influential factors under the PAH pollution were elucidated utilizing the hydroponic and soil culture experimental methods. The gradient distribution of root exudates and PAHs in the rhizosphere were firstly studied. Main novel results are as follows:(1) Plant uptake of PAHs was studied using the hydroponic culture experiment. PAHs were obviously absorbed by ryegrass. Root concentrations and root concentration factors (RCFs) of PAHs were much larger than those of shoots, and the concentrations of four PAHs in plant root or shoot increased with increasing their concentrations in solution.(2) The plant root exudation under the impacts of PAH pollution was elucidated. Compared with the control treatments with no PAHs in solution, PAHs contamination promoted the root exudation of the soluble organic carbons, oxalic acid, and soluble carbohydrate. The amounts of these root exudates increased with increasing the concentrations of phenanthrene, acenaphthene, and naphthalene in solution. In contrast, the enhancement of the amounts of these root exudates increased first and decreased thereafter with the increment of pyrene concentrations in solution. The promotion of root exudation of ryegrass by the PAHs contamination was in the order of pyrene> phenanthrene> acenaphthene> naphthalene. More significant promotion by PAHs was observed for soluble carbohydrate than other root exudates including soluble organic carbons and oxalic acid.(3) The influential factors of plant root exudation under the impacts of PAHs were investigated. The amounts of root exudates differ with the plant uptake time. Under the stress of combined pollution of phenanthrene and pyrene, the amounts of the soluble organic carbon, oxalic acid, and soluble carbohydrate in soil collected after50days was higher than those after40days, and the increase rate of soluble carbohydrate was biggest. Under the same sampling conditions, the amounts of the soluble organic carbon, oxalic acid, and soluble carbohydrate in non-sterilized soil were higher than those in sterilized soil. Root exudates also differed among different kinds of plants under the combined pollution of phenanthrene and pyrene. Compared with clover, the content of soluble organic carbon by ryegrass was less than the clover, but the content of oxalic acid, and soluble carbohydrate were higher than the latter.(4) The gradient distribution of root exudates in the rhizosphere was firstly studied. Under the combined pollution stress of phenanthrene and pyrene, the amounts of the soluble organic carbons, oxalic acid, and soluble carbohydrate by the root exudation of ryegrass all decreased with increasing the distance from the root surface. It was observed that the amounts of root exudates were in negative correlations with the residual concentrations of phenanthrene and pyrene in rhizosphere. The higher contents of root exudates, the less residues and the larger degradation ratios of PAHs in soils.