# Contribution of Atmospheric Fallout to the Soil-Root-Leaf Transfer of PAHs in Higher Plants
## 4 Discussion
### Soil-to-Aerial Parts Pathway  
After the three treatments--on Day 1, Day 8, and Day 15--total PAH concentration in the treated soil was 108 \(\upmu\)g/kg, indicating that wet deposition can be an important source of soil PAH contents. The concentration of \(\Sigma\)PAHs was 143 \(\upmu\)g/kg in the roots, 92.3 \(\upmu\)g/kg in the stems, and finally, 62.5 \(\upmu\)g/kg in the leaves. In an experiment conducted on the uptake of pyrene and phenanthrene by ryegrass, [PERSON] et al. concluded that PAHs first adsorb to plant cell walls, followed by the gradual diffusion into subcellular fractions of tissues [47]. Potential toxicity of PAHs entering the cells can be reduced by metabolic processes, including transformation, conjugation, and compartmentalization [48]. Figure 4: Total PAHs accumulation in different compartments. Figure 3: Concentration of PAHs with different molecular weight in plant compartments, simulating the air-aerial parts–roots pathway. Several studies are available that examined PAH uptake from polluted soils and subsequent partition between different organs of the plant. [PERSON] et al. used fluorescence microscopy [49] to detect the location of selected PAHs in alfalfa plants. The main translocation mechanisms described in the study involved passive diffusion of PAHs into vascular tissues. Assessing the soil-root-aerial parts pathway, results in one hand show bioconcentration between the root-soil compartments and a diminishing tendency in the root-stem-leaves compartments. This trend corresponds well with the published data. While early studies assumed that root uptake was the main uptake pathway of persistent organic pollutants (POPs) [50], some recent findings demonstrated that the transport of PAHs from the roots to the above-ground parts of plants is relatively low [51; 52]. Some studies, however, were not able to determine such clear pathways. [PERSON] et al., for example, cultivated vegetables, including Shanghai green cabbage (_Brassica chintensis_ var.) and Chinese cabbage (_Brassica rapa_ var. ), in experimentally treated soils. They found that the average concentrations of 16 PAHs were 204.7 \(\mathrm{\SIUnitSymbolMicro g}\)/kg in the soils, 83.0 \(\mathrm{\SIUnitSymbolMicro g}\)/kg in the roots, and higher concentrations in the vegetable leaves, at 93.9 \(\mathrm{\SIUnitSymbolMicro g}\)/kg [53]. In the case of soil treatment, uptake and even bioconcentration could be experienced for LMW PAHs (having 2 and 3 rings) and in the case of one 4-ring PAH, Flt. [PERSON] et al. [54] assessed the remediation potential of reed (_Phragmites australis_) in heavily contaminated soils and found effective accumulation of 3- and 4-ring PAHs. [PERSON] et al. found mostly LMW PAHs accumulated in leafy vegetables that were grown in polluted soils [55]. [PERSON] et al. used Chinese cabbage (_Brassica chintensis_ L.), a close relative of rocket, in a pot study when uptake of PAHs with different rings was assessed. The study discussed that uptake of PAHs from experimentally spiked soil showed a negative correlation with the increase in the molecular weight of the PAHs, being 76.55% for LWM PAHs, but only 6.05% for HMW PAHs [56]. Higher bioconcentration factor (BCF) values were reported by [PERSON] et al. for LMW PAHs when accumulation in carrot (_Daucus carota_ L.) and cabbage (_Brassica pekunensis_ L.) was examined [57]. [PERSON] et al. used a pot study to follow migration of PAH congeners in the soil-ryegrass exposure system. It was demonstrated that PAHs with higher molecular weights (molecular weight \(\geq\) B(a)a) mostly adsorbed to soil particles, which reduced their bioavailability and uptake [58]. Statistical evaluation shows that significant differences were found between 3-ring PAH concentrations in roots and leaves (t = 3.6327, df = 4, \(p\)-value = 0.02211), as well as between 4-ring PAH concentrations in roots and leaves (t = 3.8903, df = 3, \(p\)-value = 0.03012).