Chemical Hydrophobicity and Uptake by Plant Roots

Abstract

The transpiration stream concentration factor (TSCF), the ratio between a compound's concentration in the xylem to that in the solution adjacent to the roots, is commonly used to describe the relative ability of an organic compound to be passively transported from root to shoot. Widely cited bell-shaped curves relating TSCFto the octanol/water partition coefficient (log Kow) imply that significant root uptake and transfer into shoot tissues occurs only for compounds falling within an intermediate hydrophobicity range. However, recent laboratory and field data for relatively water soluble compounds such as sulfolane, methyl tert-butyl ether (MTBE), and 1,4-dioxane suggest that these relationships are not universally applicable, especiallyfor nonionizable, highly polar, water soluble organics. To re-evaluate the relationship between root uptake and chemical hydrophobicity, TSCFs were measured for 25 organic chemicals ranging in log Kow from -0.8 to 5 using a pressure chamber technique. Using the TSCF values measured in this study, a new empirical relationship between TSCF (0 and 1) and log Kow (-0.8 to 5) is presented that indicates that nonionizable, polar, highly water soluble organic compounds are most likely to be taken up by plant roots and translocated to shoot tissue.

Could Uptake and Acropetal Translocation of PBDEs by Corn Be Enhanced Following Cu Exposure? Evidence from a Root Damage Experiment

“…13 Although some theories suggested that a bell-shaped curve represents the relationship between log K ow and the TSCF, 11,41 more evidence from over 190 literature values did not support such behavior. 14 In addition, both BDE47 (log K ow = 5.87−6.16) and BDE209 (log K ow = 8.18− 8.27) in the present study displayed the potential for transposition in corn tissue when the root system was damaged. It has been shown previously that nightshade (Solanum nigrum) and tobacco (Nicotiana tabacum) grown on undiluted biosolids (containing 334 μg/kg penta-BDE) can accumulate up to 15.4 and 76.6 μg/kg penta-BDE, respectively, with the highest levels in the stems rather than the roots or leaves.…”

Relationship between plant uptake of pesticides and water-extractable residue in Japanese soils

“…In general, it has been postulated that translocation of organic chemicals to shoots is influenced by hydrophobicity. Briggs et al 23) found that the relationship between the transpiration stream concentration factor (TSCF), defined as the concentration of chemicals in the transpiration stream divided by the concentration in the external solution (i.e., hydroponic medium), and the log K ow could be represented by a bell-shaped curve with a maximum TSCF at a log K ow of 1.78, while Dettenmaier et al 24) reported a negative sigmoidal relationship between the TSCF and the log K ow . In the present study, however, the relationship between the BCFwater and the log K ow was quite different from either of the above.…”

“…For example, Fujisawa et al 25) improved the plant uptake models of nonionized pesticides using a metabolic parameter, i.e., the degradation rate constant in plants. Briggs et al 23) and Dettenmaier et al 24) studied the plant uptake of pesticides in hydroponic culture, and their experimental periods were 24-48 hr and 5-50 hr, respectively. In addition, the TSCF values of Briggs et al 23) were corrected for degradation of chemicals in plant shoots.…”

A review of measured bioaccumulation data on terrestrial plants for organic chemicals: Metrics, variability, and the need for standardized measurement protocols

“…Once a chemical passes through the root membrane, it can be transported to other parts of the plant, depending on its properties. For neutral compounds, hydrophobicity is one of the key transport factors [8][9][10], whereas for ionizable organics, the movement and distribution also depend on the dissociation constant (pK a ), charge of the chemical, and pH of the various plant compartments [11]. Xylem channels conduct the flow of water, nutrients, and contaminants from roots to the photosynthetic sections of the plant, whereas phloem distributes sugars and other photosynthetic products throughout the plant.…”