**5.1 RCF value of PBDEs**

As a result of high bromination, high molecular weight, low mobility and high lipophilicity, BDE-209 as the dominant PBDE in soil is only marginally available for plants at levels of 0.3–0.5% of the initial concentration [36]. Despite the apparently low relevance, the soil – soil moisture – root uptake pathway is still of high relevance as tests with living and non-living roots of different plants showed 3.5–6 times higher BDE-209 levels in the living tissues [49]. Additional analysis of small-scale soil-based BDE gradients within the root plexus underlined the assumption of active BDE-209 uptake by plants [9] and was clearly proved by greenhouse experiments cultivating six different plant species in contaminated and noncontaminated soil in parallel [50]. In comparison to BDE-209 levels in contaminated soil, BDE-209 levels reached 5.2–10.4 ng g DM−1 and, therefore, less than 5%, i.e. more than 95% of BDE-209 contamination in plants could be attributed to root uptake and intrinsic plant transport. Both processes are coupled to plant transpiration elevating PBDE levels in shoots and leaves at dry weather conditions [51].

To increase comparability of PBDE uptake and intrinsic transport, both the root concentration factor (RCF) and the translocation factor (TF) were introduced in literature and correlated to the log KOW value of PBDEs [9]. Both parameters can be correlated in a clearly negative way, i.e. higher RCF values were detected in case of lower brominated PBDEs and, therefore, compounds with lower log KOW values than in case of higher log KOW values [9]. In detail, plant specific RCFs of BDE-209 were up to ten times lower than the RCFs of BDE-28 [7, 9, 19]. This observation may be explained both by the lack of water solubility and thereby restricted root uptake with the soil moisture phase, and the strong adsorptive behavior of higher brominated PBDEs in soil. Hence, with exception of some plants like radish, green squash, and soft-stem bulrush, which are well adapted for phytoremediation processes, RCFs are clearly less than 1 for all PBDE congeners [52]. However, PBDE uptake is significantly affected by the presence of organic solubilizers in the soil like extracts of wheat straw or pig manure, where BDE-47 uptake by wheat as an example was elevated by a factor of 3.1 (wheat manure) and 1.9 (pig maure), respectively [53]. Hence, PBDE uptake increases with increasing surfactant activity, whereas decreases

**71**

**6.2 Plant specifics**

*Plant Uptake, Translocation and Metabolism of PBDEs in Plants*

at weak surfactant activity and thereby increase of the organic content in soil [54]. Furthermore, PBDE uptake and RCFs as consequence are strongly affected by plant species specifics, physical and chemical soil properties, initial concentration levels of PBDEs, relevance of both gaseous and particulate atmospheric uptake, duration of the growth period, and organic soil content. Details will be described in Section 6. In contrast, a positive correlation of both RCF and TF value was observed for maize in case of BDE-15, BDE-28, and BDE-47, which can be explained by an increased transpiration of the plants as these low-brominated congeners reveal

The ratio of PBDE levels in shoots to the levels in roots is defined as translocation factor (TF). In contrast to the RCF, a general statement about the correlation of log KOW and TF is not appropriate, since no clear positive or negative correlation occurs as the TF value depends on numerous parameters like plant species specifics, initial PBDE levels in soil, the lipid content of the shoots, plant age, distance of the plant issue from the root plexus, and the hardly quantifiable effect of the soil-air-plant exposure pathway. In principle, a negative correlation may be assumed as lower log KOW values correlate with higher water solubility and, hence, higher intrinsic transport. As PBDE accumulate in the root area, stem and shoots show significantly lower contamination levels and relevance of atmospheric PBDE uptake significantly increases [51]. Nevertheless, the bioaccumulation and translocation behavior of PBDE in plants is not conclusively clarified and depends on numerous, partially

Numerous studies focused on both physico-chemical and substance specific properties affecting plant uptake and biodegradation behavior of PBDE, where PBDE specifics (vapor pressure, KOW value, air-water distribution KAW value, air-plant distribution KAP value), environmental parameters (temperature, wind rate, precipitation, temporal rain distribution, kinetics of both gaseous and particle-bound deposition), plant properties (species, lipid content, foliage morphology, ratio of non-lipid plant parts, rind thickness, contents of both sugar and fibers), as well as the presence of an microbial active rhizosphere were generally found to be highly germane. For bioavailability and thus biodegradability of PBDEs pH value and soil composition are

Easy metabolizable intermediates as amino acids, organic acids, sugars and exozenzymes are excreted by plants as detoxification strategy to improve microbial bioavailability and biodegradability of PBDE in the rhizosphere [33]. For example, hexose was excreted by *Kandelia obovate* to enhance microbial debromination of

Plant specifics like plant morphology, wax layers of bay leaves and the lipid content of both leaf and roots strongly affect both atmospheric and soil-based uptake of PBDEs. Hence, accumulation of Br3-BDEs to Br10-BDEs in the wax layer

of particular importance [18, 22, 23]. In detail, relevant parameters are:

*DOI: http://dx.doi.org/10.5772/intechopen.95790*

higher water solubility [51].

insufficiently determined parameters.

**6. Factors of PBDE plant uptake**

**6.1 Excretion of plant solubilizers**

BDE-99 to Br2-BDEs and Br3-BDEs in soil [55].

**5.2 TF value of PBDEs**

*Plant Uptake, Translocation and Metabolism of PBDEs in Plants DOI: http://dx.doi.org/10.5772/intechopen.95790*

at weak surfactant activity and thereby increase of the organic content in soil [54]. Furthermore, PBDE uptake and RCFs as consequence are strongly affected by plant species specifics, physical and chemical soil properties, initial concentration levels of PBDEs, relevance of both gaseous and particulate atmospheric uptake, duration of the growth period, and organic soil content. Details will be described in Section 6.

In contrast, a positive correlation of both RCF and TF value was observed for maize in case of BDE-15, BDE-28, and BDE-47, which can be explained by an increased transpiration of the plants as these low-brominated congeners reveal higher water solubility [51].

## **5.2 TF value of PBDEs**

*Flame Retardant and Thermally Insulating Polymers*

**5. Soil-root transport: RCF and TF value**

**5.1 RCF value of PBDEs**

DM), and small quantities of two unidentified hydroxylated BDEs (8 ng∙g−1 DM) with continued decrease over time, similar to the transformation behavior against BDE-28 (Br3). Similar results were observed in plants of pumpkins, rice, wheat and soybean for BDE-47 and BDE-99 with formation of 5-OH-BDE-47, 6-OH-BDE-47, 4'-OH-BDE-49, 4'-OH-BDE-42, 4-MeO-BDE-42, and BDE-28 in case of BDE-47 as parent congener, and with formation of 4-OH-BDE-99 and 4-MeO-BDE-99 in case of BDE-99 as soil contaminant [45, 46]. The total PBDE levels clearly dropped in all studies. In liver cells, transformation of both low and moderate brominated BDEs was shown by cytochrome P450 monooxygenases and glutathione-S-transferase, two enzyme complexes also found in plants, where they can potentially catalyze the same reactions. However, both enzyme sets were induced by BDE-209, but this congener was not converted [47]. In difference, various study showed comparable PBDE patterns both in soil and plant tissues at almost unchanged concentration levels over time, underlining negligible metabolism of PBDEs in plants [6]. In summary, PBDEs might be transformed in plants by debromination, hydroxylation and methoxylation reactions, but transformation behavior strongly depends on the plant species and the established microbial consortium in the rhizosphere [48].

As a result of high bromination, high molecular weight, low mobility and high

contaminated soil in parallel [50]. In comparison to BDE-209 levels in contaminated soil, BDE-209 levels reached 5.2–10.4 ng g DM−1 and, therefore, less than 5%, i.e. more than 95% of BDE-209 contamination in plants could be attributed to root uptake and intrinsic plant transport. Both processes are coupled to plant transpiration elevating PBDE levels in shoots and leaves at dry weather conditions [51].

To increase comparability of PBDE uptake and intrinsic transport, both the root concentration factor (RCF) and the translocation factor (TF) were introduced in literature and correlated to the log KOW value of PBDEs [9]. Both parameters can be correlated in a clearly negative way, i.e. higher RCF values were detected in case of lower brominated PBDEs and, therefore, compounds with lower log KOW values than in case of higher log KOW values [9]. In detail, plant specific RCFs of BDE-209 were up to ten times lower than the RCFs of BDE-28 [7, 9, 19]. This observation may be explained both by the lack of water solubility and thereby restricted root uptake with the soil moisture phase, and the strong adsorptive behavior of higher brominated PBDEs in soil. Hence, with exception of some plants like radish, green squash, and soft-stem bulrush, which are well adapted for phytoremediation processes, RCFs are clearly less than 1 for all PBDE congeners [52]. However, PBDE uptake is significantly affected by the presence of organic solubilizers in the soil like extracts of wheat straw or pig manure, where BDE-47 uptake by wheat as an example was elevated by a factor of 3.1 (wheat manure) and 1.9 (pig maure), respectively [53]. Hence, PBDE uptake increases with increasing surfactant activity, whereas decreases

lipophilicity, BDE-209 as the dominant PBDE in soil is only marginally available for plants at levels of 0.3–0.5% of the initial concentration [36]. Despite the apparently low relevance, the soil – soil moisture – root uptake pathway is still of high relevance as tests with living and non-living roots of different plants showed 3.5–6 times higher BDE-209 levels in the living tissues [49]. Additional analysis of small-scale soil-based BDE gradients within the root plexus underlined the assumption of active BDE-209 uptake by plants [9] and was clearly proved by greenhouse experiments cultivating six different plant species in contaminated and non-

**70**

The ratio of PBDE levels in shoots to the levels in roots is defined as translocation factor (TF). In contrast to the RCF, a general statement about the correlation of log KOW and TF is not appropriate, since no clear positive or negative correlation occurs as the TF value depends on numerous parameters like plant species specifics, initial PBDE levels in soil, the lipid content of the shoots, plant age, distance of the plant issue from the root plexus, and the hardly quantifiable effect of the soil-air-plant exposure pathway. In principle, a negative correlation may be assumed as lower log KOW values correlate with higher water solubility and, hence, higher intrinsic transport. As PBDE accumulate in the root area, stem and shoots show significantly lower contamination levels and relevance of atmospheric PBDE uptake significantly increases [51]. Nevertheless, the bioaccumulation and translocation behavior of PBDE in plants is not conclusively clarified and depends on numerous, partially insufficiently determined parameters.
