Earlier in this booklet radioisotopes were compared to fireflies because
they emit flashes of “light.” Isotopes serve research in another
important way, other than as tracers.
Suppose you collected all the fireflies within a 100-mile radius and put
them into a glass jar. Instead of an occasional twinkle, you would now
have a steady glow of light. Similarly immense numbers of radioactive
atoms can be compressed into a small volume to produce steady, intense
sources of radiation. Agricultural research has answered many questions
with the use of such radiation sources.
Can Radiation Produce New Plants?
Perhaps no biological aspect of atomic energy has so caught the fancy of
the public as the prospect of creating new plant varieties. There is
something mysterious about pouring invisible energy into seeds or buds
and watching for changes in the emerging leaves and flowers. There is
also the challenge of the lottery in being unable to predict where,
when, or in what form the alterations will appear.
Although the claims of over-enthusiastic gardeners and seed dealers
about astonishing new plants “created” by atomic radiation are doubted,
clear proof exists even in the restrained scientific journals of
hereditary changes caused by radiation.
From more than 30 years of scientific study, certain conclusions have
emerged. High-energy radiations can cause sudden hereditary changes
(mutations) in any living thing: man, animal, microbe, or plant. Any
feature of a plant subject to hereditary control—root, shoot, leaf,
flower, or fruit—can be altered by radiation. Most of these changes are
undesirable; they interfere with the normal state of biological affairs.
A very small percentage of mutated organisms is improved in some way. So
far changes cannot be controlled or predicted.
To date fourteen new strains of crop plants improved by radiation have
been put into production in various parts of the world. These varieties
with their places and dates of release follow:
1. “Primex” white mustard, Sweden, 1950
2. “Chlorina Mutant” tobacco, Indonesia, about 1950
3. “Shafer’s Universal” bean, Germany, about 1950
4. “Regina II” summer oil rape, Sweden, 1953
5. “Weibull Stralart” fodder pea, Sweden, 1957
6. “Sanilac” navy bean, Michigan, 1957
7. “Pallas” barley, Sweden, 1958
8. “N. C. 4X” peanut, North Carolina, 1959
9. “Florad” oats, Florida, 1960
10. “Seaway” bean, Michigan, 1960
11. “Alamo-X” oats, Texas, 1961
12. “Gratiot” bean, Michigan, 1963
13. “Pennrad” barley, Pennsylvania, 1963
14. “Yukon-1” carnation, Connecticut, 1963
In these instances no desirable changes appeared in the plant grown
directly from treated seeds, but they appeared several generations
later. In most cases hundreds of thousands of plants were examined
before the desirable ones were found. The desired changes were almost
always accompanied by undesirable ones, and years of cross-breeding and
“purifying” were necessary to obtain usable varieties.
The technique of radiation breeding can be used on any form of life
where large numbers can be grown and discarded at little cost. The
output of penicillin has been increased a thousandfold by repeated
mutations caused in the microorganism producing this antibiotic. Several
studies on radiation breeding of poultry have been started.
An ingenious reverse twist of induced mutation is being applied in the
field of plant diseases. While some scientists are irradiating seeds and
plants in an effort to obtain disease-resistant mutations, others are
irradiating the fungi which cause the diseases. They hope in this way to
foresee the new strains of pathogenic microbes that will occur naturally
in order to breed resistant plants before the new diseases appear.
Some claims have been made that radiation can stimulate plant growth,
germination, earlier maturity, and so on. Similar benefits are sometimes
claimed for human health. These allegations are almost never proved in
reputable laboratories. It seems likely that radiation is stimulating,
in the words of one authority, “only in the sense that a pruning knife
is stimulating.”
Can Radiation Destroy Germs and Insects?
Food technologists have studied ways of preserving food with radiation
for more than ten years. Their findings indicate that complete
sterilization of food with radiation requires doses so high (2 to 6
million roentgens[2]) that cost is prohibitive at present, and the food
often becomes distasteful. These amounts of energy completely destroy
the microbes and enzymes which normally cause food to putrefy.
If radiation is to be used in preserving food, it will probably be as a
supplement to conventional methods of heating and freezing.
“Pasteurizing” with radiation to destroy most (but not all) of the
microbes in meat or fruit or vegetables is accomplished with less than
five per cent of the dosage required for sterilizing. Such treatment
does not alter flavor or texture appreciably and could be used to
prolong the refrigerated “life” of many fresh foods. It is the
responsibility of the Federal Food and Drug Administration to determine
that no threat to human welfare could possibly result before approving
the use of high-energy radiation to preserve foods.
With agricultural products stored dry, such as grain, tobacco, and wool,
the chief agents of damage are not microbes, but insects. The loss of
stored field crops caused by insects is estimated at $200 million
annually in the United States. Deinfesting such goods with radiation
doses in the “pasteurizing” range promises to be practical and causes no
apparent change in the product.
Like many other foodstuffs, potatoes are often stored for months between
harvest and use. Precaution must be taken to prevent their deterioration
during storage not only from decay but also from sprouting.
Cold storage inhibits sprouting but is costly and has another serious
drawback. In the making of potato chips, tubers held at low temperature
contain excess sugar and result in darkened chips. Storage at higher
temperatures prevents conversion of starch to sugar but encourages
sprouting.
Atomic energy promises to resolve this dilemma. Given low doses of gamma
rays (5000 to 10,000 roentgens), potatoes may be kept at room
temperature for a year or more without sprouting. Similar doses inhibit
sprouting of onions. The estimated cost of irradiating tubers and bulbs
at such doses is as low as 14 cents per ton. No chemical changes have
yet been found in irradiated potatoes that would make them unsafe for
eating. In fact, health officials in Canada have recently approved the
use of gamma rays on potato tubers that will be stored and later used
for human food. Our own Food and Drug Administration has given similar
approval for applying gamma rays to bacon and fast electrons to wheat
(for killing insects).
An ingenious application of atomic energy to agriculture concerns the
screwworm fly, which inhabits large areas of southern United States,
Mexico, and the Caribbean. The fly lays eggs in open wounds of
livestock, including navels of newly born, and the burrowing maggots
inevitably kill the animal. In the southeastern United States, damage
from this insect amounted to $15 to $25 million annually.
In the years 1958 and 1959 more than two billion screwworm flies were
deliberately released from airplanes over the entire state of Florida
and parts of Georgia and Alabama. This astonishing act was a major step
in successful eradication of the pest from southeastern United States.
The entomologists who conceived this remarkable scheme had the following
information from basic studies: the insect produces a generation about
every three weeks. In the pupal stage males can be sterilized by 2500
roentgens of X or gamma rays, females by 5000 roentgens. The insect can
be reared in large numbers. Sterile males are fully competitive with
normal males for mates. And, of course, sterile eggs do not hatch. (It
was helpful, though incidental, that females mate only once.)
After initial tests on an island in the Caribbean, a large fly-producing
plant was set up. Flies were grown to the pupal stage, irradiated with
8000 roentgens of gamma rays, permitted to mature, and released from
airplanes. With 50 million flies being released weekly over Florida,
Georgia, and Alabama, the area was smothered with sterile flies, and the
number of eggs that hatched (from the normal native flies) rapidly
diminished to zero. The program was continued for 18 months, and in this
time the insect was completely eliminated.
Certain other insects are being considered for the sterility-eradication
technique. Among them are the boll weevil, European corn borer,
mosquito, and tsetse fly. Oriental scientists are using gamma rays
instead of the conventional heating to kill silkworms inside cocoons.
How Does Radiation Affect Farm Animals?
At a few colleges of agriculture in this country, radiation effects on
farm animals are being studied.
Although it may not be flattering to be likened to a pig or a donkey,
the fact remains that human beings are physiologically very similar to
swine and burros. These animals are mammals with simple stomachs and
have the same general size, shape, and placement of organs as do humans.
Radiation studies with swine and burros, although slow and expensive,
should give information more applicable to humans than the more rapid
and inexpensive studies with small laboratory animals.
What Else Can Radiation Tell Us?
Two characteristics of soils besides fertility are vitally important and
difficult to measure. These characteristics are moisture and density.
Moisture must be determined frequently for efficient irrigation. Density
controls the pore space available for water and oxygen; the possible
damage to the soil from tillage and harvesting machines is revealed by
before-and-after tests of density.
Both soil moisture and density were formerly determined by laboratory
methods, which had two drawbacks: the methods were laborious, and they
tested soil in an unnatural state. Today a sort of double-barreled
radiation method can be used to measure these two soil characteristics.
Neutrons are readily scattered by water but not by soil; gamma rays are
absorbed by both soil and water. In practice the experimenter drills two
holes in the soil a few feet apart. Into one he puts a gamma-ray source;
into the other, a radiation detector. The reading on his detector dial
tells him the amount of gamma rays absorbed by both soil and water.
Replacing the gamma-ray source with a neutron source, he obtains a
reading on absorption by water only. The difference between the two
readings is ascribed to the density, or degree of compaction, of that
soil in its native state.