Dioxins and Plants

[HRVM60A@prodigy.com (MR CARROLL D JOHNSTON)]

In answer to your question about dioxin in plants, the U. S. EPA’s 
1994 Dioxin Reassessment documents address this issue.  I have 
excerpted a couple of quotes from Volumes II and III of those 
documents below.  I hope this helps.
Carroll Johnston

FROM VOLUME III:
2. Vegetation  -  Concentrations in three types of vegetation are 
considered in this assessment: below ground vegetables (carrots, 
potatoes, e.g.), above ground vegetables/fruits (tomatoes, apples), 
and above ground grass and cattle feed which are required for 
estimation of beef and milk concentrations.  Assumptions critical to 
all three include: above ground vegetation is impacted by vapor phase 
transfers and particle deposition - there is no root to shoot 
translocation, outer portions of the vegetation are only impacted 
with minimal within plant translocation, a steady state is reached 
between vapor phase contaminants in air and vegetation, particle 
bound contaminants deposit onto and mix in a vegetative reservoir and 
are subject to a fourteen-day dissipation half-life which represents 
particle washoff, and vegetables/fruits which have an outer 
protective layer (peas, citrus e.g.) are unimpacted by dioxin-like 
compounds.  Below ground vegetable concentrations are estimated from 
soil water concentrations and a Root Concentration Factor, or RCF.  
Above ground concentrations due to vapor phase transfers are a 
function of the vapor phase air-borne reservoir, an air-to-leaf 
transfer factor, Bvpa, and a surface area to volume reduction factor, 
VG, which is equal to 1.00 for grasses and other leafy vegetation and 
less than 1.00 for bulky vegetation.  

FROM VOLUME II:
II.3.3.  Conclusions for Mechanisms of Impact to Food Chain
	CDD/F can enter aquatic systems by either direct effluent discharges 
or atmospheric deposition.  CDD/Fs in the atmosphere can deposit 
directly onto water bodies or onto watersheds and run off into the 
water system.  The mechanism of impact which dominates in aquatic 
systems will depend on site specific conditions. 
	This assessment proposes the hypothesis that the primary mechanism 
by which dioxin-like compounds enter the terrestrial food chain is 
via atmospheric deposition.  Deposition can occur directly onto plant 
surfaces or onto soil.  Soil deposits can enter the food chain via 
direct ingestion (i.e. earth worms, fur preening by burrowing animals,
 incidental ingestion by grazing animals, etc).  CDD/F in soil can 
become available to plants by volatilization and vapor absorption or 
particle resuspension and adherence to plant surfaces.  In addition, 
CDD/F in soil can adsorb directly to underground portions of plants, 
but uptake from soil via the roots into above ground portions of 
plants is thought to be insignificant (McCrady, et al. 1990). 
	Support for this air-to-food hypothesis is provided by Hites (1991) 
who concluded that "background environmental levels of PCD/F are 
caused by PCD/F entering the environment through the atmospheric 
pathway."  His conclusion was based on demonstrations that the 
congener profiles in lake sediments could be linked to congener 
profiles of combustion sources.  Further argument supporting this 
hypothesis is offered below:

§ Numerous studies have shown that CDD/Fs are emitted into the air 
from a wide variety of sources (see Chapter 3 of Volume II).

§ Studies have shown that CDD/Fs can be measured in wet and dry 
deposition in most locations including remote areas (Koester and 
Hites, 1993; Rappe, 1991).

§ Numerous studies have shown that CDD/Fs are commonly found in soils 
throughout the world (see Chapter 4 of Volume II).  Atmospheric 
transport and deposition is the only plausible mechanism that could 
lead to this widespread distribution.

§ Models of the air-to-plant-to-animal food chain have been 
constructed.  Exercises with these models show that measured 
deposition rates and air concentrations can be used to predict 
measured food levels (Travis and Hattemer-Frey, 1991; also see 
Chapter 7 of Volume III).

Alternative mechanisms to the air-to-food hypothesis seem less 
likely:  ...