In Situ Investigate Typical Polycyclic Aromatic Hydrocarbons (PAHs) Distributed On The Root Epidermis And Its Processes Transported Into Tissues

The depuration of PAHs by mangrove plants in the external environment(gas phase,sedimentary phase and water phase)was one of the important ways to remove PAHs.Therefore,there existed many studies focused on investigate a series of related processes such as the transport pathways and distribution of PAHs in plant.Among them,the process and mechanism of PAHs located on the epidermis and transport to tissues were still largely unknown due to the lack of appropriate in situ analytical method.Therefore,the novel methods for in situ determination of the PAHs adsorbed onto the root surface of mangrove was established using LITRF,D-LITRF and MFSA techniques.Then,the established methods were used to investigate the one/two-component PAHs distributed on the root epidermis and its processes transported into tissues.The major research achievements were listed in the follows.(1)Different from the structure of mangrove leaf,the structures of mangrove root were cylindrical.Thus,it is difficult to make the mangrove root at the same position during the in situ determination of Phe,Pyr or B[a]P on K.obovata root epidermis.To lower the error and kept the reliable of this method,the samples holder used were made in this study to fit with the in situ determination of Phe,Pyr or B[a]P on mangrove root epidermis.After using the samples holder,the relative standard deviation of measured values of the fluorescence intensities at the position of the maximum emission peak of Phe(λex = 266 nm,λex = 368 nm)decreased from 10.26%to 2.63%(n=9),which improved the reproducibilityof this method significantly.A novel method for in situ determination of the Phe,Pyr or B[a]P adsorbed onto the root epidermis of K.obovata seedlings was established using LITRF combined with the deduction of short-lifetime fluorescence signal.The linear dynamic ranges for the established method were 1.5-1240.5 ng/g for Phe,1.0-1360.4 ng/g for Pyr and 5.2-1220.2 ng/g for B[a]P.The detection limits of this method were 0.2 ng/g,0.1 ng/g,0.1 ng/g for Phe,Pyr or B[a]P,respectively.Then,the mechanisms of PAHs transported and distributed the K.oovata root epidermis to tissues were investigated.The three-phase dynamic model including fast,slow and very slow fractions was superior to other model to describe the Phe,Pyr or B[a]P transport processes.Moreover,the kinetic of fast fraction of Phe,Pyr or B[a]P transport process was mainly due to passive movement,while the kinetic of slow and very slow fractions were mainly due to the active and passive movement.It can be clearly concluded from the transport kinetic constants that passive movement was the main process of B[a]P adsorbed onto K.obovata root epidermis transport to tissues.In addition,the extents of the Phe,Pyr or B[a]P transport to K.obovata root tissues at different salinity were also evaluated in this section.Compared with the previous laser induced fluorescence spectra method,the established in situ method overcome the affection of the auto-fluorescence of mangrove root epidermis to the fluorescence spectra of Phe,Pyr or B[a]P adsorbed onto mangrove root.(2)The LITRF method established in section 1 was then used to in situ investigate the distribution and transportation of alkyl and N/O/S-containing PAHs on mangrove root epidermis.Our results confirmed that the partition coefficients(Kf)of the Flu retained on the epidermis of mangrove roots were much higher than the Poaceae plants roots.Moreover,to Flu and 1-M-Flu,a well negative correlation was observed between the surface polarity((N+O)/C)of mangrove root and the Kfvalues(p<0.05).To DBT,Car and DBF,these relationships were not obviously due to existing of theπ-π,n-π interactions and hydrogen bonding between the N/O/S-containing PAHs and epidermis.The Flu,1-M-Flu,DBT,Car or DBF retained on the root epidermis formed larger clusters than on Poaceae plants(wheat(Triticum aestivum L.)and maize(Zea mays L.))due to the limitation of the suberization of the root exodermis and endodermis.After exposure of 30 d,rhizo-and endophytic bacteria degraded parts of the DBT,Car and DBF to the corresponding metabolites.(3)A novel method for the simultaneous in situ determination of Phe and fluoranthene(Fla)adsorbed onto K.obovata,B.gymnorhiza and A.corniculatum root epidermis were established using D-LITRF.The selectivity of D-LITRF was much higher than LITRF method,and has ability to distinguish the fluorescence spectra of Phe and Fla with seriously overlapping.The linear dynamic range of Phe and Fla adsorbed onto K.obovata root were 5.3-700.2 ng/g and 1.1-350.3 ng/g,and the detection limits of Phe and Fla adsorbed onto K obovata root were 0.2 ng/g and 0.1 ng/g.The distribution and transportation of Phe and Fla from the mangrove root epidermis to tissues was simultaneously investigated in situ using the established method.The transportation coefficients of the Phe and Fla adsorbed onto the root epidermis showed a good linear relationship with the content of root lipids,while the inhibition rates showed no significant correlation with the content of root lipids(p>0.05).Further studies showed that the interaction between Fla and Phe decreased the transport kinetics,especially the slow and very slow transport kinetics.(4)Mangrove root epidermis was complex and heterogeneous phase matrix,and thus the transport and distribution of PAHs at different micro-zones of mangrove root may significantly different.A novel method for in situ quantitative investigation of PAHs distribution in micro-zones of mangrove root tissues was established using MFSA method.The linear ranges dynamic of the established method were 600.1-1500.2 ng/g.450.4-3000.4 ng/g and 350.6-4200.8 ng/g for Nap,Ant or B[a]P,respectively.The recoveries were 90.6-111.3%,93.6-108.9%and 92.3-106.8%for Nap,Ant or B[a]P,respectively.The spatial and substance resolutions(～1.0 μEm)of the images were enough to investigate the distribution of PAHs in mangrove root tissues.Then,the established MFSA method were used to investigate the the transport of Nap,Ant or B[a]P adsorbed onto micro-zones of K.obovata root and the implications of aging.The distribution of Nap,Ant or B[a]P were in the uneven and large clustered form.The concentrations of Nap,Ant or B[a]P,the highest in the epidermal tissues of K.obovata,were 3729.3 ng/g.1346.6 ng/g 和 813.4 ng/g,respectively(p<0.05).Besides,the uneven distributed extents of Nap,Ant or B[a]P on the epidermis of K.obovata root were 7.63、8.82 和 11.52,respectively.The images of MFSA showed that parts of PAHs enter into the vascular tissue of K.obovata root,which were much different from the wheat(Triticum aestivum L.)and maize(Zea mays L.).Under aged conditions,the mean concentrations decreased from 9713.42 ±266.05 gg/kg.4031.11±94.82 μg/kg and 1298.77±92.28 μg/kg to 4249.52±74.64 μg/kg.2185.34±54.52 gg/kg and 350.21 ±6.09 μg/kg for Nap,Ant and B[a]P,respectively.