UPTAKE OF RADIOACTIVE NUCLIDES
ELL
conditions
xpected under contrasting
t
shaped cavities at leaf axils whenceit is absorbed into the stem. Only a very
small fraction of the strontium 90 which is deposited on the foliage of
potato plants reaches their tubers; in experiments when sprayed plants
were exposed to rain 0.05 per cent or less was found in them (45). However, if sprayed plants were protected from rain negligible levels were
observed in the tubers (11); thus under normal conditions downward
leaching by rain over the surfaces of stems appears to be the main mechanism of transfer.
The absorption of strontium 90 from soil—Laboratory studies of the
absorption of strontium 90 from soil have usually been directed to one or
more of the following objectives: the identification of soil characteristics
which influence the extent to which it is absorbed by plants; the develop-
ween the
as broad similarities bet
Europe are
in North America and
in the United
(2, 27), the situation
90 enters cereal
.m whereby strontium
the observation that
st encouraged by
more times
in flour might be two or
ds, while
foo
ir the majority of other
flour
es that in
often about three tim
by direct flora’
) entered grain largely
strontium 9
iparison of the ratios of
sues of cereal
mtium in the different tis
ment of methods of estimating absorption from soil; the study of the extent
ium, the ratio of
urce of stable stront
same time would
the
tissues formed at
s
ied
iations im thi
Var
.
soil
only from the
‘
mp
unacco
jon with strontium 90
eee
oI
7
n le ~
See
SE RD eee Te Aen
aoe
to which its absorption will change with time. Field experiments have been
carried out both to test the conclusions reached in the laboratory and also to
determine quantitative relationships.
Variations in the extent to which strontium 90 enters plants from different soils depend mainly on their calcium content (2, 16, 21, 22, 46 to 50) ;
absorption is greatest from soils low in that element. However, when the
“exchangeable” calcium content of soils, as measured by conventional
extraction procedures, exceeds a certain limit, often 15 to 20 meq per 100 ¢
soil further increase in soil calcium has little effect on the absorption of
strontium 90, and the ratio in which strontium 90 and calcium are extracted
omta arcane ee 8s Ae
from soil by such proceduresis frequently higher than that found in plants
(21, 51). These observations are in apparent conflict with the finding that
min
JOM Aye serepene rNrern
grain harveste
the different tissues of
that the ratio
wed
countries in 1957 sho
erably lower
sid
con
in flour was always
10 (41). A
ing
eed
exc
es by a factor
had entered by
5 which strontium 90
ontium 90 in the
sy assuming that all str
least two-thirds
at
isis it appeared that
; when result
n
tio
ina
to direct contam
te was about :
e corresponding estima
of the Unite
ions for different parts
in 1959 (42, 43).
'
ted in
has also been demonstra
on
ti
na
er ear
aft
ay
Spr
a
as
im 89 is applied
be some 15 times
> grain at harvest may
the ears emerge
1 is made shortly before
s contamination
3 with small caps reduce
rgence (32).
eme
ris applied before ear
rice (44).
h
wit
o
als
vn to be important
d out in which
riments have been carrie
es, potatoes, an
a fine spray to cabbag
Since the edi 5
their growth (32, 45).
strontium
and
ves
tected by outer lea
ch they are
whi
in
ner
i: plants, the man
ence of direct
ygether o bvious. The occurr
extent than in
, though to a much smaller
studies and
tal
ablished both by experimen
may be
this
that
-). It has been suggested
ging in the saucer| down in rain and lod
Te
cn eee Se dake
283
the OR (plant shoot/outer medium) is relatively constant and close to one.
The reason for this was identified by Schofield (52). The ratio in which
the two ions are absorbed depends on their ratio in the soil solution and
this may be very different from the ratio in the total “exchangeable” fraction which is extracted by conventional procedures, for example with 1 NV
ammonium acetate. In a number of soils, the ratio of the two ions in solution was 0.5 to 0.8 of that in the readily exchangeable ions on surfaces. No
simple procedure is available to measure the absolute concentrations of ions
in the soil solution under equilibrium conditions. However, it has been
shownthat the relative magnitude of the ratio of strontium 90 to calcium in
the solution phases of different soils can be inferred from the extent to
which strontium 90 is removed by equilibration with 0.01 M calcium
chloride (53, 54, 55). This procedure depends on the fact that only small
changes in the concentration of calcium usually occur when this solution
is shaken with soil; even when the readily exchangeable calcium in soil
ranges from less than 1 to over 30 meq per 100 g, the concentration of the
solution may change by not more than 15 per cent and usually considerably
less. Thus the quantities of strontium 90 removed from different soils by
0.01 Af caicium chloride indicate the relative magnitudes of the ratios of the
two ions in the soil solutions, and hence the ratios in which they enter
”
+
+
om
a
-.
-
--—
or
rn
.
ere aparece gn eo
woe
,