Research progress in uptake of persistent organic pollutants by plants

Abstract

Plants can absorb persistent organic pollutants(POPs) from air and soil. Roots sorption is one of uptake processes,while evidence showed that POPs absorbed by roots was hardly transferred to the aerial tissues of the plant. The POPs re-volatilized from soil may only influence the parts of plants near the surface of the ground. Compared to the absorption from soil,the absorption of atmospheric POPs by foliage raised more concern. The airborne POPs can be accumulated by plant leaves by wet and dry deposition,which is the main pathway of the atmospheric POPs entering the aerial parts of plants. Air-plant exchange equilibrium is an important state that determines the direction of gas exchange between air and plant. The airborne POPs partition onto plant and approach plant gas-phase equilibrium rather rapidly,within less than 6 days of exposure. Besides exposure time,other environmental factors(temperature,wind speed,humidity,etc.) and the shape,areas and life time of the leaves have impacts on the air-plant exchange equilibrium of POPs. Through the air-plant exchange the airborne POPs were fixed by the plants and finally transferred into soil by falling off the leaves. Therefore,the air-plant exchange of POPs will influence the long-range atmospheric transport and the regional,even global distribution of POPs. Plants such as lichens,mosses,grass and forest have been used as passive samplers to monitor the levels of atmospheric POPs on different time scales and reflect the spatial distribution of POPs. Lichens and mosses are extensively used in environmental pollution studies especially in the polar region,since their collection is relatively easy. For grass,different species have the same ability to absorb the airborne POPs,which was used as bio-monitor to compare the POPs in different regions over the world. Numerous studies indicated that plants act as a "sink" for global cycling of POPs. The POPs absorbed by plants will enter the terrestrial food chains through the consumption of plants by animals and will be transported from one trophic level to a higher one,which will result in bio-concentration and even pose potential threat to the health of humans. However,there are a few studies focusing on the bio-concentration of POPs in the terrestrial food chains currently and most of them utilize models to simulate the bio-concentration process. In this paper,the research progress on the accumulation of POPs by plants including the ways of POPs entering the plants and the air-plant exchange of the airborne POPs were reviewed; the advantage and disadvantage of vegetation passive samplers(lichens,mosses and grass) were discussed. We also pointed out the problems needed to be solved in further studies.