DISCUSSION AND CONCLUSTONS
There is a large literature, too large for citing here, showing that ionizing
radiation produces detectable changes in chromosomes.
The following discussion therefore assumes that ionizing radiation causes chromosomal aberrations
and attempts to address the possibility that the low-level chronic radiation
doses at Site D, Area 11 may have produced an increase in chromosome aberrations in Artemista spineseens there.
The occurrence of chromosome aberrancies
level radiation exposure of Site D, Area
in A. sptnercens
11, NTS, is wel!
in the chronic, low
substantiated.
The
fact that the percentages of aberrancies do not meet generally accepted levels
of statistical significance for a conclusive difference between the Site D
population and our control population raises several questions in interpreting
these data,
The first question concerns the uncertainty of dose, already
mentioned, for those shrubs in Site D.
In light of what appears to be a
relatively high aberrancy rate in those shrubs where background radiation is
"normal," the question of dose must also be considered for those shrubs
selected as controls.
Since we were not able to obtain, within the time for
this work, similar cytological material from regions outside areas of possible
radiation doses from nuclear tests in the last 20 years, we have no basis for
knowing what the normal frequency of chromosomal aberrancies may be.
In
addition, there appears to be nothing in the literature concerning the frequency
of
chromosomal aberrations
in plants
in nature.
Giles
(1940)
studied species
of Tradescantia, a herbaceous genus, {tn the laboratory which "have shown a
comparatively high frequency of spontaneous chromosome alterations ..." A
more sensitive hybrid used by him had a frequency of 0.3% and the mean for
all Tradescantta was 0.11%.
Steffensen (1964) also reported on the spontaneous
rate of chromosomal aberranctes in Tradescantia.
We found a 1.3% rate of
occurrence (13 aberrants in 997 cells) and that the radiation dose which
doubled this rate was only 0.75 R.
(His were acute doses and the effectiveness of acute doses in causing radiation damage at the phenological level and
presumably at the chromosomal level is known to be higher than for chronic
doses [Sparrow and Puglielii, 1969}]).
Nichols, 1941 studied Spontaneous
chromosome aberrancies itn onion seedlings where the frequency was variable
and high, however "No chromosome aberrations were found in the primary root
tip divisions
in onion bulbs
It
interest
.""
In short,
there
appears no
Firm basis
for
evaluating what the spontaneous aberrations rate in Artemtsia might be unless
one is willing to compare a woody shrub with herbaceous species in the
laboratory.
Steffensen's radiation dose for doubling the rate of occurrence
of chromosome aberrants, 0.75 8, even though it is an acute dose, is of
interest, however, since it suggests that chromosomes are sensitivie to low
radiation dose exposures.
ts atso of
that woody species,
generally,
are more sensitive to
radiation damage than herbaceous species by a factor of about two (Sparrow
and Sparrow, 1965).
The large variability in the frequency of occurrence of chromosome aberrancies
in both the Site D population and the control population also raises some
questions as to precisely what the variability in doses may have been to the
tissues we have collected for examination, a variability beyond that contributed
by the uncertainty of dose exposures to a particular shrub at a particular
location with the irregular and uncertain distribution of Pu contaminating the
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areas around shrubs already mentioned.
(It should be pointed out that the
possibility radiation exposure from external radioactive contamination of the
cells themselves is quite unlikely because of the use of cells from within
anthers taken from unopened flower buds.
Such cells had never been exposed
to the open environment.)
If for example a shrub produced branches from its
base or from underground, the branches’ exposure to ionizing radiation will
have been only for the time it has existed above the surface; and this will
determine the dose it will have received.
It has, therefore, to be recognized
that we are dealing with large variations in radiation doses even from one
part of a shrub to another part.
From this it follows that a large variability
is a likely characteristic of the occurrence of any aspects of radiation effects
under conditions of long-term low-level chronic radiation, particularly where
the term of
the radiation exceeds
the
life span of
the shrub
itself,
or parts
of it. Under these conditions, even if the radiation field were extremely
untform, the exposure doses to particular samples taken from vegetation within
such a field would undoubtedly be quite variable; hence, variability in any
manifestation of radiation effects probably ought to be expected.
This kind
of variability, of course, does not lend itself to statistical treatment
except where very large samples can be taken; and the cytological examination of large samples demands a large expenditure of time and money.
It seems
probable it is these variabilities that do not allow statistical support for
a conclusion that chronic low-level exposures produced increased chromosomal
aberrancies. We feel, however, the evidence firmly supports the hypothesis
that such has occurred at Site D.
The hypothesis that chromosomal aberrancies are indeed caused by ionizing
radiation in the field even where morphological changes are not apparent is
supported by the occurrence of similar aberrancies in the two other shrubs,
Machaeranthera and Krameria from the Rock Valley, NTS experimental ly-irradiated
shrub populations.
It should be noted that both these shrubs received radiation exposures much higher than the Site D shrubs.
Again, however, it can
be anticipated that the radiation doses for particular samples taken from the
field will be quite variable for different reasons to those for the Site D
plants. The Rock Valley samples were irradiated from a central source, a high
energy gamma emitter in a stationary position 15 meters above the surface
(French, et al., 1974).
From this point, exposure doses to shrubs at the
ground surface will be varied by shadow effects imposed by other shrubs
between the sampled shrubs and the central source, This is particularly the
case of lower-growing shrubs like Krameria or Machaeranthera,
Although the
dose rates in the air above a particular shrub can be predicted and measured
with relatively great accuracy and will have a uniformity varied primarily
by atmospheric conditions, the ground level doses will no doubt vary widely
and wili depend not only on the atmospheric conditions but on the mass of
vegetation interposed between the sampled shrub, or parts of it, and the
radiation source,
The variability in occurrence of aberrancies among our shrub spectes is likely
to be different for still other reasons, in part at least, beyond arguments
presented so far.
The difference in variability between Macharranthera and
Krameria, as possibly reflected in the statistical analysis, may be attributable
to the phenology of the two shrubs. Machaeranthera, a suffruticose perennial,
develops flower heads from tissue which arises, apparently, annually from a
low woody base which is itself of varied longevity, and may arise from underground.
Kramerta, on the other hand, produces branches, also from underground,
but much less frequently on which Flower are borne for several successive years.
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