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RUSSELL
or indirectly via milk, from leaf tissues in which the ratio of the two ions
is similar to that in the rooting medium.
The extent to which stable strontium influences the relationship between
strontium 90 and calcium has been considered. It has been shown both in
water culture and in soil studies that variations in the concentration of
stable strontium do not affect the absorption of strontium 90 provided that
the ratio of stable strontium to calcium is low (10, 25, 26). Entry appears
to be controlled by the total concentration of strontium plus calcium. Since
in nature the ratio of stable strontium to calcium is usually 1 to 100 or less,
and high concentrations of stable strontium are toxic, the carrier ion can be
ignored in studies of plant/soil relations.
Direct contamination of plants—The term “direct contamination” is
used to describe all material which enters plants as a result of lodging on
their above groundtissues, in contradistinction to absorption from the soil.
Translocation within plants may lead to the direct contamination of tissues
which are remote from thesite of initial lodgement.
One of the most important and interesting aspects of food chain studies
has been the investigation of the relative extent to which strontium 90 can
enter plants by direct contamination under conditions of continuous deposi-
tion such as occur with world-wide fallout. The amount which enters in this
way is determined by the deposit only in the recent past while absorption
from the soil will be related to the cumulative total therein. Without knowledge of the relative magnitude of these two processes it was impossible to
form any valid view on the long term consequences of world-wide fallout.
The prediction of the manner in which dietary contamination would change
after a massive accidental release, of short duration, also depends on the
same information.
It has been found necessary to distinguish three types of direct contamination (27, 28) namely: (a) foliar contamination : retention by leaves,
(b) floral contamination: retention by inflorescences, (c) plant base contamination: retention and absorption by the basal parts of plants, or surface
roots, of material which has not been incorporated into the soil.
Strontium 90 which reaches plants by the first two routes will not be
accompanied by calcium; some calcium may enter simultaneously with
strontium 90 from the plant-base but to a much smaller extent than when
strontium 90 is absorbed from the soil. The relative importance of these
alternative routes of entry depends on the growth form of plants. Whereas
foliar absorption can occur in all species, floral absorption is of practical
significance with grain crops only; plant-base absorption is particularly important in perennial pastures though not necessarily confined to them. It is
often difficult to define sharp boundaries between foliar absorption and
either floral or plant-base entry. During rain the downward washing of contamination over the surfaces on the plant will occur concurrently with absorp-
tion; thus foliar and floral absorption merge into plant-base entry. Likewise,