consisted of particles with a fractured crystal lattice, the citrate
represents a relatively stable soluble complex, and the nitrate an unstable
complex which rapidly hydrolizes on dilution to form colloidal hydroxides.
The order of bioavailability with respect to transport to seed and root
was plutonium nitrate (hydroxide) > Pu citrate > aged oxide > fresh oxide,
An interesting aspect of these data is that maximum TR values are obtained
when the simulated rainfall occurs at day 7 or 14, the time of rapid
seed development.
This is of interest from the standpoint of plutonium
mobility within the plant and its chemical form.
It is generally accepted
that the movement of materials out of mature leaves requires that they be
transported in the phloem.
Entry of molecules into this transport conduit
is metabolically regulated with the loading process being highly specific
for individual organic metabolites and inorganic elements (Crafts and
Crisp, 1971). There is growing evidence that many {inorganic nutrilites,
especially multivalent cations, are transported as organic complexes in
both the xylem (Tiffin, 1971; 1967; Bradfield, 1976) and phloem (van Goor
and Wiersma, 1976).
By analogy to the behavior of nutrilites, plutonium
must be transported out of the contaminated leavea via the phloem.
similarly, it is unlikely that inorganic plutonium could remain soluble at
the pH of phloem cell sap (Ph 7.2-8.5, Ziegler, 1975). Therefore, the
possibility exists that the mobile plutonium which was deposited in seed
and root tissues may be complexed with phloem mobile organic species.
This would serve to explain the apparent increase in TR values seen during
the time of seed development,
During this period, there is a significant
change in both the composition and quantity of specific metabolites being
produced by leaves and being exported to metabolic sinks such as seeds and
roots.
This may increase the potential for soluble species of plutonium
to become complexed with organic metabolites and be subsequently exported
to metabolic sinks.
Although this is a tentative judgment and subject to
substantive studies, this interpretation serves to explain the observed
results based on known metabolic aspects of plant function.
CONCLUSIONS
The ability of terrestrial plants to accumulate potentially hazardous
elements from soils via root absorption and their relative importance in
the food web to man, has prompted numerous studies over the past 25 years.
The vast majority of these investigations have been concerned with soilplant transfer routes, since the soil represents a major repository for
pollutants released to the environment and because the plant root is an
efficient solute absorbing structure. Until recently, the foliar portions
of plants were considered to play a minor, transient role at best, with
respect to dose assessment problems.
Our current understanding of the aerodynamic behavior of particles and
anticipated reductions in particle size distributions of materials like
plutonium through an expanded nuclear energy program, suggests that a
reevaluation of the role of plant foliage in particle interception and
absorption of materials contained on airborne particulates is in order.
344
This need is supported both by early investigations and studies currently
underway. Early studies of worldwide fallout, and current work on wind
resuspended contaminated soils, indicate that foliar retention and foliar
absorption may be as important and in some cases exceed the performance of
roots with respect to food chain transport problems. As regards leaching
of foliar deposits, a critical evaluation of past literature suggests that
aerosol polydispersity and large particle size (e.g., 45 wm, MMD) may
explain the comparatively large degree of leaching or “weathering” reported
for the latter experiments. This view is reinforced by data reported for
well characterized particles of Pb and Pu in laboratory studies and field
observations for fallout Pu.
These latter investigations indicate a
sizeable fraction (>80%) of submicronic particles deposited onto foliage
to be tenactously held on leaf surfaces under varied conditions (e.g.,
simulated rainfall, wind). Aside from the potential health implications
associated with increased foliar retention, the problem of foliar absorption must be considered.
In the reported studies, a substantial fraction
of the foliar plutonium deposits were transported to seed and roots.
Transport ratios were affected by both the presence of a solution vector
(simulated rainfail) and the timing of its application with respect to
stage of plant development.