United States
Environmental Protection
Agency
Health Effects
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S1-87/005 Nov. 1987
SEPA Project Summary
Availability of Cadmium to
Rats from Crops Grown on
Cadmium Enriched Soil
Donald R. Buhler and Ian J. Tinsley
This research was initiated to
enhance the understanding of the
availability to animals of Cd present in
edible plants. Such information is
important because agricultural crops
can accumulate high concentrations of
the metal when grown in certain soils
or with sewage sludge as a fertilizer.
Edible plants were labeled with 109Cd
by growing them on 109CdCI2-treated
soil. The availability of 108Cd to male
and female rats was determined by
feeding semisynthetic diets containing
either freeze-dried radioactive spinach,
lettuce, soybean, carrots, tomatoes, or
wheat flour, or comparable nonra-
dioactive plant powders spiked with
109CdCI. Retention of 109Cd by liver and
kidney was determined after a 14-day
feeding period.
With the exception of spinach, Cd
accumulation by rats was not found to
be significantly influenced by the form
of Cd in the diet, whether supplied as
plant-bound 109Cd or added to nonra-
dioactive diets as 109CdCI2. The mean
retention of Cd in liver and kidney was
0.17% of the dose consumed for males
and 0.26% for females consuming diets
containing wheat, soybean, carrots,
lettuce, or tomatoes. While uptake and
retention of Cd in rats fed 109Cd-labeled
spinach was generally comparable to
that seen in animals fed the other
labeled plant powders, the retention of
Cd was significantly lower (one-third)
in rats fed nonradioactive spinach diets
spiked with 109CdCJ2. These results
suggest that spinach grown in the
absence of added Cd may contain a
complexing agent which reduces the
availability of added Cd.
A simple linear regression model
involving the log transform of DTPA
soil Cd extraction data was developed
to describe the accumulation of Cd by
plants. The Cd accumulated by spinach
plants was mainly bound to the cell
walls, accounting for 53, 47, and 98%
of the Cd present in leaves, stems, and
roots, respectively. Only 12-39% of the
retained metal appeared in the
100,0000 cytosol fraction in spinach
leaves and stems. In the cytosol Cd was
primarily bound to high molecular
weight proteins with smaller quantities
associated with plant constituents in
the molecular weight ranges of 3,000-
4,000 daltons and greater than 1,000
daltons.
This Project Summary was devel-
oped by EPA's Health Effects Research
Laboratory, Research Triangle Park,
NC, to announce key findings of the
research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering
information at back).
Introduction
Cadmium (Cd) is a highly toxic metal
that is readily accumulated by and
retained in both plants and animals.
Concern over the consequences of Cd
pollution accelerated after chronic expo-
sure of a rural population to the metal
in Japan resulted in severe bone disease.
Some research in the literature focuses
attention on the possible linkage
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between human hypertension and
increased retention of Cd by the kidneys.
Although some Cd is absorbed via
inhalation, especially in smokers, food is
considered to be the major source of
nonoccupational human exposure with
intake via water generally thought to be
of lesser importance. Water, however,
may play a considerably greater rote
when Cd concentrations are naturally
high or are increased by the water
distribution system.
Recent evidence reported in the liter-
ature has shown that edible plants,
among others, can accumulate substan-
tial amounts of Cd from Cd-contaminated
soils and that metal accumulation
depends upon the plant species, soil
composition, and Cd content of the soil.
Various vegetable species, such as corn,
tomato, radish, and Swiss chard, also
efficiently accumulate Cd2+ from nutrient
solutions.
Municipal sewage sludge is the end
product resulting from the digestion and
treatment of municipal wastes that may
contain human excreta, residues from
food processing, and a host of inorganic
and organic constituents generated by
industry. Disposal of the 100 million tons
of municipal sewage sludge generated
annually in this country has led to the
increasing utilization of sludge as a soil
conditioner and a source of nutrients for
agricultural crops. Sewage sludges
typically contain high concentrations of
various heavy metals including Cd, with
concentrations of this metal ranging
between 1 to 1,500/ug Cd/g dry sludge.
When edible crops are then grown on
soils treated with municipal sewage
sludge, they accumulate high concentra-
tions of cadmium and other metals.
Under these circumstances, leafy plants,
such as lettuce, spinach, and turnip
greens (tops), can accumulate Cd con-
centrations as high as 175 to 354 fjg/
g tissue. Fruit and seed tissues of plants,
including turnip tuber, tomato, wheat,
radish, and squash, concentrate lesser
amounts of the metal, ranging from 10
to 15 fig Cd/g tissue. The Cd content of
soybeans and carrots reaches 30 fjg/g
tissues while maximum levels in corn
and rice are below 5//g Cd/g tissue.
Because of the accumulation of high
concentrations of toxic metals, especially
Cd, in edible plants, the disposal of
sewage or sewage-sludge on croplands
or the use of high Cd fertilizers may pose
a serious hazard to human or animal
health. Similarly, crops grown on soils
naturally high in Cd could also concen-
trate dangerously high levels of the
metal. To assess the degree of hazard
from such accumulations, however, it is
first necessary to determine the biolog-
ical availability of Cd to animals fed diets
containing various grains and vegeta-
bles. This project was, therefore, initiated
to compare the absorption of 109Cd by rats
fed diets containing six species of edible
plants grown on CD-treated soils, fed
similar diets spiked with 109Cd2+, or
supplied with l69Cd2+ in their drinking
water. In other experiments the distri-
bution and nature of the binding sites
for 109Cd in spinach was examined.
Conclusions
The results of experimental studies
performed with 109Cd-labeled plants
indicate that Cd retention by rats fed diets
containing low levels of Cd is generally
not influenced by the form of Cd, whether
plant-bound or added as inorganic Cd*+
(CdCI2). Cd retention is influenced by
plant species, but the difference was not
large between animals fed diets contain-
ing freeze-dried wheat, soybean, carrot,
lettuce, or tomato powders. As observed
by other researchers previously, reten-
tion of Cd by female rats was generally
higher than by males. In rats fed a freeze-
dried spinach diet spiked with CdClz,
retention of Cd was significantly reduced
compared with that seen in rats fed
CdClz-spiked semisynthetic diet or a Cd-
bound spinach diet. These latter results
suggest that spinach grown on low Cd
soils may contain metal complexing
agents capable of binding added inor-
ganic Cd and influencing its availability.
Addition of calcium oxalate, which is
present in high concentrations in spin-
ach, to a semisynthetic diet spiked with
CdCI2 significantly reduced Cd retention
in rats.
Accumulation of Cd by container-
grown plants can be adequately des-
cribed by a simple linear regression
model involving the log transformation
of D PTA soil Cd extraction data. Gener-
ation of regression coefficients specific
to the plant species and cultural condi-
tions used permitted the use of published
data to predict Cd concentrations in five
plant species grown to maturity in the
present study.
In spinach plants, most of the accum-
ulated Cd is bound to the cell wall
fraction, especially in roots. A relatively
small percentage of the Cd, about 40%,
is present in the cytosol fractions of
leaves and stems. This soluble Cd is
mainly bound to high molecular weight
proteins but some of the metal is asso-
ciated with plant constituents in the
3,000-4,000 dalton and greater than
1,000 dalton molecular weight ranges.
Recommendations
Although the results of the present
study generally show little difference in
the availability to rats of bound Cd in
different edible plants, it is possible that
some vegetables, fruits, or grains contain
binding or chelating constituents that
markedly influence the absorption of Cd
by animals. Additional research is,
therefore, needed to extend these studies
to include additonal species of edible
plants. Of perhaps greater importance,
however, is the need for subsequent
studies to demonstrate conclusively that
the relative availability of plant-bound Cd
to rats accurately reflects the degree of
absorption of the metal by man. Since
published values on the percentage Cd
uptake by man are generally much higher
than those found in the rat, it may turn
out that the rat is a poor model for Cd
absorption in man and that some other
laboratory animal species should be
used.
Additional research is also needed on
the nature of Cd binding ligandsin plants,
particularly the low molecular weight
constituents. The binding components
from several plant species need to be
isolated, characterized and identified. A
better understanding of how Cd is bound
in plants could help considerably in
explaining the small but significant
differences seen in Cd retention in
animals fed different Cd-bound plant
diets.
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Donald R. Buhler and Ian J. Tinsely are with Department of Agricultural
Chemistry and Environmental Health Sciences Center, Oregon State
University, Corvallis, OR 97331.
Elmer W. Akin is the EPA Project Officer (see below).
The complete report entitled "Availability of Cadmium to Rats from Crops Grown
on Cadmium Enriched Soil." (Order No. PB 87-212 353/AS; Cost: $18.95,
subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Health Effects Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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EPA/600/ S 1-87/005
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