UPTAKE OF RADIOACTIVE NUCLIDES
LL
available to plants (13). A second cause of difficulty in the application of
the OR concept, which may legitimately be ascribed to over-enthusiasm in
its use, has been the assumption that the OR for a given process should be
ces of
sneral also be the main sour
mple, in
unimportant; thus, for exa
it
main dietary source of calcium,
ying
so (2). The importance of stud
numerically constant over all circumstances. The inadequacy of this view
has been demonstrated particularly in animal studies which show that
changes in discrimination may occur with age or from grossly abnormal
physiological conditions such as can readily be arranged in the laboratory.
of attenius recognised. The amount
howot,
cann
90 from this viewpoint
n dose
the magnitude of the radiatio
These variations do not, however, significantly reduce the value of the con-
reby it
sources. The food chains whe
cept for assessing relationships in normal circumstances. In particular the
consideration of the two ions conjointly is appropriate for assessing the
effects of the dietary intake of strontium 90 by man or animals. The level
ides
than those for many other nucl
n reasons
ariable. There are two mai
from the
ed
orb
abs
ably more readily
of radiation to which the body is exposed depends on the concentration of
strontium 90 in bone; in practice this is determined by its ratio to calcium.
Since the OR (bone/diet) is relatively constant (ca. 0.25) it follows that
of its
1 adequately only on the basis
only
es
mov
it
ium
calc
like
econdly,
ch is ab11); thus strontium 90 whi
the radiation dose varies with the ratio of strontium 90 to calcium in the
total daily intake (2).
The OR (plant shoot/rooting medium) is close to one (2, 14 to 18).
eciably
above groundtissue is not appr
90 in
m
ntiu
stro
of
ingly, the levels
tity
quan
l
tota
the
m
fro
be inferred
on
ing
end
dep
r,
occu
may
ns
atio
it situ
tion
ques
s
rough the aerial tissues. Thi
Numerous investigations show that variations in the extent to which differ-
ent species absorb strontium from uniformly contaminated soil are closely
paralleled by variations in their absorption of calcium (19 to 22) though
sium 13/7,
. with nuclides such as cae
some small divergences have been observed (2, 22). Moredetailed studies
of the relative behaviour of the two ions in plants, however, reveal an appreciable differential movement. When both ions enter through roots their
ratio in the aerial organs most remote from the root is in general lower
than the average in the plant. Varying relationships occur depending
possibly on growth rate but the ratio of strontium to calcium in vegetative
tissues is frequently about half that in underground storage organs and
twice that in grain and seeds (21). Stems may show considerably higher
ratios than other above groundtissues (13). The most detailed comparisons
have been made between different tissues of wheat grain; the ratio in the
endosperm has been found to be 0.7 of that in whole grain (23).
accepted
and calcium.—The currently
cium was
s between strontium and cal
Ratio”
ved
ser
introduced the term “Ob
ted by the equation:
Ratio: Sr/Ca in sample
Ratio; Sr/Ca in precursor
which
is through any biological process
that the
12) laid emphasis on the fact
1ons were
if the ratio in which the two
the ratio
as
well
as
wn,
kno
1 system was
lability
avai
tive
rela
the
of
s
mate
ise esti
of the
conclusions as to the magnitude
h the
oug
thr
hed
on occasions been reac
e”
eabl
hang
“exc
to
m
id tracer strontiu
are
the ratio in which’ the two ions
Qh
‘
e NT
Se
Saswe 8 e
See ce geet at ayer
aes RENE aS
mes arisen
ised, difficulties have someti
the precise
cause has been disregard of
eeer | TARE. MR RET el
soil to the
om diet to bone, or from the
“Disber of successive steps; the term
the
of
r
to define the relative behaviou
uct
prod
the
g
of the entire process bein
277
It appears, therefore, that the two ions interact in a series of exchange
reactions in their upward passage through plants, strontium being more
firmly retained. The possible nature of sites involved has been discussed by
Biddulph et al. (24). The fact that despite this discrimination, the ratio
of strontium to calcium in shoots is close to that in the outer medium appears to reflect the fact that the major part of both the calcium and the
strontium in plant shoots is usually located in the foliage. The discriminatory
mechanism in the stem may thus be likened to an ion exchange column
which has “overrun.” Although differential retention occurs in the column,
the ratio of ions in the effluent becomes similar to that in the added solution
after saturation.
These discriminatory processes havelittle effect on the ratio of strontium
90 to calcium in man’s diet. The majority of dietary calcium comes directly,
ce ere Seep Re tte ee ge are ceee
ena
tr
ser oes
so
eee
eyo