Long-Distance Acropetal Translocation Of Phenanthrene In Plants

The pollution of polycyclic aromatic hydrocarbons(PAHs)in crops has attracted much attention due to its high teratogenicity,carcinogenicity and mutagenicity.However,the specific acropetal translocation of PAHs in plants is still unclear so far In this paper,the dynamic accumulation of phenanthrene in roots,stems,young and mature leaves of wheat and maize seedlings was investigated.Two different transpiration inhibitors were applied on leaves to compare the role of phloem during acropetal translocation of phenanthrene.To verify the transfer of phenanthrene between young and mature leaves of wheat seedling and two sides of castor bean leaves,foliar application of phenanthrene was performed.The xylem and phloem sap of eastor bean seedlings was eollected to analyze its phenanthrene,pH along with the concentration of organic acid and protein under different phenanthrene treatment.This paper was aimed to provide theoretical basis for revealing the possible pathway and mechanism during acropetal translocation of PAHs in plants and improving the phytoremediation of contaminated soil or water.The main results are as follows:(1)Phenanthrene concentration in both roots first rises and then decreases with time,and earlier decline of phenanthrene is occurred in maize roots.Same trend is shown in wheat stem but reaches balance finally in maize stems.Phenanthrene concentration in wheat young leaves is significantly higher than in mature leaves,but the disparity between them turms smaller with time.The difference between maize mature and young leaves is not significant at the first 4 h but becomes the opposite then.Both roots have response to the external change of phenanthrene concentration.The wheat stem is easier to accumulate phenanthrene compared to maize stem,but have no response to the change of phenanthrene concentration.Significant difference only displays under 0.2 and 1.0 mg.L-1 phenanthrene treatment.Only young leaves of maize seedlings have response to concentration variation.The absorbed phenanthrene will first accumulate in wheat stem and then preferential transfer to young leaves.However,the transfer factors will decrease with the increment of concentration.Transfer factors from roots to stems in maize are significantly lower than in wheat,but the transfer factors from roots to mature and young leaves are higher than in wheat under different phenanthrene concentrations.(2)Both ABA and PVA suppress the phenanthrene uptake of plants,and more obviously on leaves with mature xylem.Phloem takes the main role during acropetal translocation when xylems is inhibited or not nature enough,and transport more phenanthrene to the young leaves which results in higher concentration.One-side foliar application of phenanthrene on castor bean leaves indicates that phenanthrene could be transferred to the other leaves through phloem.Transfer from young leaves to mature leaves is 2-3 times of that from mature leaves to young leaves.Xylem sap contains higher concentration of phenanthrene than phloem sap,and ring-girdling treatment could significantly reduce phenanthrene in castor bean leaves.Two-Photon fluorescence Microscope(TPFM)images exhibit that phenanthrene firstly is bounded to root cell wall,then enters into the root xylem by symplast or apoplastic pathway.Xylem pathway plays the main role at stem base,there could be a xylem-to-phloem translocation in the stem,and results in the occurrence of phenanthrene fluorescence in both xylem and phloem.Phloem plays an important role during the acropetal translocation of phenanthrene.(3)The main ingredients of castor bean xylem sap are malic acid and citric acid,and oxaloacetic acid has the lowest concentration.The concentration of malic acid,oxaloacetic acid and succinic acid decreases with time.The decrease in succinic acid concentration has a good linear relationship with time(R2=0.96).The concentration of citric acid and oxalic acid first increases and then decreases with time.The concentration of malic acid and oxalic acid increases with the increment of phenanthrene concentration.The concentration of succinic acid in xylem has no significant change under different phenanthrene concentrations.Citric acid and oxalic acid maybe involved in the acropetal translocation of phenanthrene.Total organic acid decreases over time but increases with the increment of phenanthrene concentration.The SDS-PAGE images displays about 5 clear bands at 80-90,60,50,40,20-23 kDa,and the bands become darker when phenanthrene concentration increases.The protein content in xylem sap can be used to characterize the degree of PAHs pollution or exist as a kind of transporter.In summary,the accumulation of phenanthrene varies much in different organs of wheat and maize seedlings,and young leaves accumulate more phenanthrene than mature leaves.Phloem plays an important role during the acropetal translocation of phenanthrene.Organic acid and protein in plants may involve in the xylem acropetal translocation of phenanthrene.