IONIZING RADIATION (Gamma rays) AND ITS EFFECT ON PLANT MORPHOLOGY, PHYSIOLOGY, AND
CYTOLOGY
1. INTRODUCTION
In molecular
biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome . These random sequences can be defined as
sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic
chemicals, as well as errors that occur during meiosis or DNA
replication. They can also be induced by the
organism itself, by cellular processes such as hypermutation.
Mutation
can result in several different types of change in sequences; these can either
have no effect, alter the product
of a gene, or prevent the
gene from functioning properly or completely. One study on genetic variations
between different species of Drosophila suggests that if a mutation changes a protein produced by a gene, the result is likely to
be harmful, with an estimated 70 percent of amino acid polymorphisms having damaging effects, and the
remainder being either neutral or weakly beneficial.[4] Due to the damaging effects that mutations
can have on genes, organisms have mechanisms such as DNA
repair to
prevent mutations.
Mutations
can involve large sections of DNA becoming duplicated, usually through genetic recombination. These
duplications are a major source of raw material for evolving new genes, with
tens to hundreds of genes duplicated in animal genomes every million years. Most genes belong to larger families
of genes of shared ancestry. Novel genes are produced by several
methods, commonly through the duplication and mutation of an ancestral gene, or
by recombining parts of different genes to form new combinations with new
functions.
Here, domains act as modules, each with a particular and
independent function, that can be mixed together to produce genes encoding new
proteins with novel properties. For
example, the human eye uses four genes to make structures that sense light:
three for color
vision and one
for night
vision; all four arose
from a single ancestral gene.] Another advantage of duplicating a gene (or
even an entire
genome) is that this
increases redundancy; this allows one gene in the pair to acquire
a new function while the other copy performs the original function. Other types of mutation occasionally
create new genes from previously noncoding DNA.
Changes
in chromosome number may involve even larger mutations, where segments of the
DNA within chromosomes break and then rearrange. For example, in the Homininae, two chromosomes fused to produce human chromosome 2; this fusion did not occur in the lineage of the other apes, and they retain these
separate chromosomes. In
evolution, the most important role of such chromosomal rearrangements may be to
accelerate the divergence of a population into new species by making
populations less likely to interbreed, and thereby preserving genetic
differences between these populations.
Sequences
of DNA that can move about the genome, such as transposons, make up a major fraction of the genetic
material of plants and animals, and may have been important in the evolution of
genomes.] For example, more than a million copies of
the Alu
sequence[An Alu
element is a short stretch of DNA originally characterized by the action of the Alu (Arthrobacter
luteus) restriction endonuclease. Alu elements of different kinds occur in large
numbers in primate genomes. In fact, Alu elements are
the most abundant Transposable
elements in the human genome. They
are derived from the small cytoplasmic 7SL RNA, a component of the signal recognition
particle ]are present in the human
genome, and these
sequences have now been recruited to perform functions such as regulating gene
expression. Another effect of these mobile DNA
sequences is that when they move within a genome, they can mutate or delete
existing genes and thereby produce genetic diversity .
Nonlethal mutations accumulate within the gene pool and increase the amount of genetic
variation.[20] The abundance of some genetic changes
within the gene pool can be reduced by natural selection,
while other "more favorable" mutations may accumulate and result in
adaptive changes.
For example, a
butterfly may produce offspring with new mutations. The majority of these
mutations will have no effect; but one might change the color of one of the
butterfly's offspring, making it harder (or easier) for predators to see. If
this color change is advantageous, the chance of this butterfly surviving and
producing its own offspring are a little better, and over time the number of
butterflies with this mutation may form a larger percentage of the population.
Neutral
mutations are defined as mutations whose effects do not influence the fitness of an individual. These can accumulate
over time due to genetic drift.
It is believed that the overwhelming majority of mutations have no significant
effect on an organism's fitness. Also, DNA repair mechanisms are able to mend most
changes before they become permanent mutations, and many organisms have
mechanisms for eliminating otherwise permanently mutated cells. Beneficial mutations can
improve reproductive success.
2. INDUCED MUTATION BREEDING
Plant
breeding requires genetic variation of useful traits for crop improvement.
Often, however, desired variation is lacking. Mutagenic agents, such as
radiation an certain chemicals, then can be used to induce mutations and
generate genetic variations from which desired mutants may be selected.
Mutation induction has become a proven way of creating variation within a crop
variety. It offers the possibility of inducing desired attributes that either
cannot be found in nature or have been lost during evolution. Breeding for
improved plant cultivars is based on two principles: genetic variation and
selection. Induced mutagenesis has been practiced with great success in crop
breeding programmes in developing countries since the 1930s (Ahloowalia,
Maluszynski and Nichterlein, 2004), but its scope and utility have recently
been greatly enhanced and extended by the new molecular-based technology of
Targeting Induced Local Lesions in Genomes (TILLING).
Mutation breeding is the process of exposing seeds to chemicals or radiation in order to
generate mutants with desirable traits to be bred with other cultivars. Plants
created using mutagenesis are sometimes called mutagenic plants or mutagenic
seeds. From 1930–2004 more than 2250 mutagenic plant varietals have been
released that have been derived either as direct mutants (70%) or from their
progenies (30%).Crop plants account for 75% of released mutagenic species with
the remaining 25% ornamentals or decorative plants. However, it is unclear how
many of these varieties are currently used in agricultural production around
the world, as these seeds are not always identified or labeled as being
mutagenic or having a mutagenic provenance. There are different kind of
mutagenic breeding such as using Chemical mutagens like EMS and DMS, radiation
and transposons are used to generate mutants. Mutation breeding is commonly
used to produce traits in crops such as larger seeds, new colors, or sweeter
fruits that either cannot be found in nature or have been lost during
evolution.
Induced mutations on
the molecular level can be caused by:
3. CHEMICALS
Hydroxylamine NH2OH
Alkylating agents
(e.g. N-ethyl-N-nitrosourea) These
agents can mutate both replicating and non-replicating DNA. In contrast, a base
analog can only mutate the DNA when the analog is incorporated in replicating
the DNA. Each of these classes of chemical mutagens has certain effects that
then lead to transitions, transversions, or deletions.
Nitrous acid converts amine
groups on A and C to diazo groups, altering their hydrogen bonding patterns
which leads to incorrect base pairing during replication.
4. RADIATION
Radiation in its passage through matter causes the ejection of electrons
from atoms on which it impinges. The ejection of electrons may result in both
chemical and physical changes in the constituents of cell. These changes are
considered to be primarily responsible for the biological effect of the
radiation.
Ultraviolet radiation (nonionizing radiation).
Two nucleotide bases in DNA – cytosine and thymine – are most vulnerable to
radiation that can change their properties. UV light can induce adjacent pyrimidine bases
in a DNA strand to become covalently joined as a pyrimidine dimer.
UV radiation, particularly longer-wave UVA, can also cause oxidative damage to DNA. Mutation rates also
vary across species.
5. GAMMA RAYS AND ITS EFFECT ON MORPHOLOGY OF
PALNTS
Gamma rays belongs to ionizing radiation
and are the most effective electromagnetic radiation, having the energy level
from around 10kilo electron volts(keV)to several hundred keV. Therefore, they
are more penetrating than other types of radiation such as alpha and beta rays.
There
are several usages of nuclear techniques in agriculture. In plant improvement, the
irradiation of seeds may cause genetic, variability that enable plant breeders
to select\new genotypes with improved characteristics such as precocity,
salinity tolerance, grain yield and quality (Ashraf, 2003). Ionizing radiations
are also used to sterilize some agricultural products in order to increase
their conservation time or to reduce pathogen propagation when trading these
products within the same country or from country to country (Melki &
Salami, 2008).
A
number of radiobiological parameters are commonly used in early assessment of effectiveness
of radiation to induce mutations. Methods based on physiological changes such
as inhibition of seed germination and shoot and root elongation have been
reported for detection of irradiated cereal grains and legumes. Chaudhuri
(2002) reported that the irradiation of wheat seeds reduced shoot and root
lengths upon germination. Gamma radiation can be useful for the alteration of
physiological characters (Kiong etal., 2008). The biological effect of
gamma-rays is based on the interaction with atoms or molecules in the cell,
particularly water, to produce free radicals (Kova´cs & Keresztes2002).
These radicals can damage or modify important components of plant cells and
have been reported to affect differentially the morphology, anatomy,
biochemistry and physiology of plants depending on the radiation dose (Ashraf et
al., 2003). These effects include changes in the plant cellular structure
and metabolism e.g., dilation of thylakoidmembranes, alteration in
photosynthesis, modulation of the anti-oxidative system, andaccumulation of
phenolic compounds (Kova´cs & Keresztes 2002; Kim et al., 2004; Wi etal.,
2007; Ashraf, 2009). From the ultra-structural observations of the irradiated
plant cells,the prominent structural changes of chloroplasts after radiation
with 50 Gy revealed thatchloroplasts were more sensitive to a high dose of
gamma rays than the other cellorganelles. Similar results have been reported to
be induced by other environmental stressfactors such as UV, heavy metals,
acidic rain and high light (Molas, 2002; Barbara et al.,2003; Quaggiotti
et al., 2004). However, the low-dose irradiation did not cause
thesechanges in the ultra-structure of chloroplasts. The irradiation of seeds
with high doses ofgamma rays disturbs the synthesis of protein, hormone
balance, leaf gas-exchange, waterexchange and enzyme activity (Hammed et al.,
2008). Due to limited genetic variabilityamong the existing wheat genotypes,
Irfaq & Nawab (2001) opened a new era for cropimprovement and now mutation
induction has become an established tool in plant breedingthat can supplement
the existing germplasm and can improve cultivars in certain specifictraits as
well (Irfaq & Nawab 2001)
Several positive mutations have been created in agricultural crops by
using gamma irradiations Crops with improved characteristics have successfully
been developed by mutagenic inductions (Rehman et al., 1987; Javed et
al., 2000; Gustaffson et al., 1971) developed a high yielding barley
variety with early maturity, high protein contents and stiff straw by mutation
breeding techniques. Khatri et al., (2005) collected three high grain
yielding and early maturing mutants by treating seeds of Brassica juncea L.
cv. S-9 with gamma rays (750-1000KGy) and EMS.
6. REVIEW ON IONIZING RADIATION (Gamma rays) AND ITS EFFECT ON PLANT
MORPHOLOGY, PHYSIOLOGY, AND CYTOLOGY.
The
study of the effects of radiation on plants is a broad and complex field. Gamma
irradiation was found to increase plant growth and development by inducing
cytological, genetical, biochemical, physiological and morphogenetic changes in
cells and tissues depending on the irradiation level (Gunckel and Sparrow,
1961). It is one of the important physical agents used to improve the
characters and productivity of many plants(Jaywardena and Peiris, 1988, Sharma
and Rana,2007). The gamma ray had adverse effect on traits of plants and this
depended on plant species or varieties and the dose of irradiation (Artk and
Peksen 2006).These effects include changes in the plant cellular structure and
metabolism e.g., dilation of thylakoid membranes, alteration in photosynthesis,
modulation of the antioxidative system and accumulation of phenolic compounds
(Kim et al., 2004, Wi et al.,2005). Mokobia and Anomohanran 2005)
found that gamma irradiation were very useful not only for sterilization of
medicine but also for the preservation of food and cereals in nutrition and
agriculture.
7.
EFFECT OF RADIATION ON MORPHOLOGY OF PLANT
7.1
Germination
According to a research conducted by Wilkus,
R.D. (2011)results showed that gamma irradiation can affect the
germination of corn (Zea mays L.)
seeds. It was observed that different doses of gamma rays have various effects
on the total number of germinated seeds and its respective germination rate.,
the control set-up (0 kr) has 10 out of 10 seeds germinated, and has
germination rate of 100 percent. In the 10 kr set-up, 9 out of 10 seeds
germinated with a germination rate of 90 percent. In the 30 kr set-up, 9 out of
10 seeds germinated with a germination rate of 90 percent. In the 50 kr set-up,
7 out of 10 planted seeds germinated with a germination rate of 70 percent.
High stimulatory effect in early growth of
maize was observed in 2Gy irradiation group of Kosungjaerae cultivar and in 12
Gy irradiation group of Youngwoljarae cultivar(Kim sung et al, Korean journal of environmental Agriculture
,19-4, pp 328-331).
7.2
Seedling height
Study revealed that there is significant
effect of low doses of gamma radiation in certain maize cultivars. The seedling
height of maize cultivar kosungjaerae was found more at the low radiation dose of 2
and 4 Gy(Jae-Sung kim,Young-Keun Lee,Hong-sook Park,Myung-Hwa Back,andDong-Hee
Kim,2000).In an publication(Schwartz,1954),a preliminary report was presented
describing the effect of high doses of ionizing radiation on germination and
growth of maize seeds..
The
work of Schmidt and Frolik (1951) and of Beard (1955) has shown that plants of
corn (Zea mays L.) grown from seeds treated with
appropriate doses of X-rays or thermal neutrons are greatly reduced in stature
and survival in comparison with control plants.
7.3 Plant height
According to Hartt and Jones (2006), over a wide range
of radiation doses, the frequency of mutation induced by radiation is
proportional to the radiation dose Thus, the higher the dosage of radiation,
the more evident the expression of morphological changes is observed in the
samples. Therefore, low level of radiation produced positive effects such as
increase in plant height of corn plants while high level of radiation caused
detrimental damage to the corn plants and may result to cell death. Thus, low
levels of gamma radiation could be used in agriculture to improve plant cultivation.
The control group
and 10 kr gamma radiation treatment produced the greatest number of germinated
seeds and also the highest values for plant height were obtained from the 10kr
treatment, only minimal plant height for the 50kr gamma radiation treatment was
obtained in maize(Parcon ,2011).
7.4 Root and shoot
length
A general reduction in
the root and shoot length was reported in maize as the radiation dose increased
from 100 Gy to 500Gy,
and the damages caused to a plant by
radiation are determined by the amount of energy that is absorbed by the
chromosome(Ajayi).
7.5 Grain yield
Mohammad Mehdi Rahimi and Abdollah Bahrani(2009-2010)found that among the two varieties,
Zagros cultivar produced more grain yield than Alamot cultivar, grain yield
increased in response to application of gamma irradiation, with the grain yield
of the crop that no received gamma irradiation being 5% more than control i.e.
25 and 50 Gy gamma irradiation produced the highest grain yield.
There was a more concentrated maturity ,with
more ears ready at the time of first harvest in the 1 Gy- and 3Gy treatment in
sweet corn cultivar-sunnyvee.(Canadian journal of plant
science-vol.48,409-410,1968).
However, Highly significant negative
correlations were obtained between high irradiation dosage and percent kernel set( Pfahler,1967).
Pfahler(1967) found that the percent
kernel set was also reduced by increasing the dosage, no difference was
obtained with an increase in dosage from0 to 1 kr,a sharp decrease was
obtained by increasing the dosage from 1 to 2 kr with relatively slight
decreases observed with further increases in dosages above2 kr and no
significant differences were obtained between 3, 4 and 5 kr.
Similarly,Killion et al(1972) found maize variety
Golden Bantam was more sensitive than WF9 X 38-11 as measured by a reduction in
survival and grain yield when given an acute exposure in the seedling stage on
exposure rate below 16R/min.
7.6 Pollen sterility
The
aspects associated with pollen included fertilization
ability, kernel set (dominant lethality), fertility, diameter, in vitro
germination percentage, and in vitropollen tube
length. As indicated by statistical tests, only fertilization ability and in
vitro germination percentage were altered by the irradiation
treatments.Positive relation with the
pollen sterility and radiation dose was observed in rice (Siddiq and S waminathan,1968),sorghum(Gaud et al.,1970),black gram(Gautam et al.,1992),Trigonella foenum-graecun(Rhaghuvanshi and singh,1974)andVinca rosea(Sudhakaran,1971).
In
maize between 1 and 24 kR, fertilization ability was equal to or above that of
0 kR,above 24 kR, fertilization ability was reduced so that at 40 kR the
fertilization ability was about 75% of 0 kR( Pfahler,1967).
Pfahler(1967) observed that
the in vitro germination percentage was reduced with increasing
dosage, so that at 40 kR the percentage was 75 % of 0 kR, female fertility was
the only aspect of female reproduction measured, increasing doses uniformly
decreased female fertility to a marked degree so that at 40 kR the female
fertility was less than 10 % of 0 kR.
The proportionate increase in the pollen sterility
with the radiation dose can be accounted for the cumulative results of the
various aberrant meiotic stages, as well as the physiological and of genetic
damages induced probably by the breakage of chromosomes through the formation
of anti-metabolic agents in the cells(Sudhakaran,1971).
7.7
Leaf size
The
effects of γ-irradiation on the biophysical and morphological properties of
corn plants investigated with irradiation doses of 0, 1, 1.5, 2.5, 5, and 10 krad showed that
corn grains exposed to 1.5 and 2.5 krad showed highly significant changes in
all growth parameters like leaf size and chlorophyll content, the obtained
results give another support via the biophysical properties for the 1.5
krad irradiation dose to be the most favorable one to improve the plant leaf
size(Al-Salhi, M., Ghannam, M. M., Al-Ayed, M. S., El-Kameesy, S. U. and
Roshdy, S.2004).
Miah
and bhatti(1968) reported that the mean values of the irradiated populations
for leaf size in rice to be almost as
same as those of the non irradiated control populations : rather the variance
of the irradiated population was greater in comparison to the corresponding
controls .
8. REVIEW ON EFFECT OF GAMMA RAYS ON
BIOPHYSICAL PARAMETERS
8.1 Chlorophyll
content
Gamma
radiation induces various physiological and biochemical alteration in plants.
The irradiation of plants with high dose of gamma rays disturbs the hormone
balance, leaf gas-exchange, water exchange and enzyme activity (Kiong et al.,
2008). The effects include changes in the plant cellular structure and
metabolism such as dilation of thylakoid membranes, alteration in
photosynthesis, modulation of the antioxidant system, and accumulation of
phenolic compounds. Based on transmission electron microscope observations,
chloroplasts were extremely sensitive to gamma radiation compared to other cell
organelles, particularly thylakoids being heavily swollen (Wi et al.,
2007).
Furthermore,
Kim et al., (2004) have evaluated the chlorophyll content on irradiated
red peper plants; their results showed that plants exposed at 16 Gy may have
some significant increase in their chlorophyll content that can be correlated
with stimulated growth.
The effects of low doses of gamma radiation
on water- or Molybdenum-soaked seeds on the photosynthetic apparatus of the
resultant seedlings showed the increase
in both the content and stability of the chloroplast
pigments, chlorophyll a and b and the carotenoids, moreover, it increased the
protein: chlorophyll ratio, the dehydroascorbic, and to a greater extent, the
ascorbic acids as well as the photochemical activity of the chloroplasts, the
500 R exposure was generally more effective in producing seedlings better
equipped with an active photosynthetic apparatus(Ahmed, . Ashour, El-Basyouni and Sayed,1976).
8.2 Growth physiological indices
Ionizing
radiation like gamma rays can induce some positive effects at lower doses on
the crops physiological indices like CGR(Crop Growth Rate) , LAI(Leaf Area
Indices),NAR(Nate Assimilation Rate).
·
CGR=(W1-W2 T2-T1)*1
GA
·
LAI= LA/GA
·
(NAR) =
LAI × CGR
Where,
W, T and GA refer dry matter weight, time and
ground area, leaf area
respectively.
The highest LAI and CGR obtained in 25 Gy
gamma irradiation with average 4.81 and 25.6, respectively LAI, CGR and NAR decreased with
increasing gamma irradiation . The best
combination in order to obtain the highest LAI and CGR was 50 Gygamma
irradiation and Zagros cultivar The
highest NAR was obtained in Alamot cultivar and 50 Gy, gamma irradiation(World
Applied Sciences Journal 15 (5): 654-659, 2011).
8.3 Protein content
Kushelevskii
and Slifkin (1972)found a cleavage of the peptide linkage after irradiation.At
0.5 Mrad ,the native free organic acids may have been partially destroyed;at
higher doses partial radiolysis may occur in the glyceride ester (Dubravic and
Nawar 1968)or in peptide linkages(Kushelvskii and Slifkin 1972),which might
lead to the liberation of some free fatty acids and amino acids .
Gamma irradiation and interaction between
cultivar and gamma irradiation were significant in grain protein percent of
wheat cultivar, 25 and 50 Gy gamma
irradiation produced the highest grain protein content, increasing in
gamma irradiation more than 50 Gy decreased grain protein about 28% to
67%(World Applied Sciences Journal 15 (5): 654-659, 2011).
The
increase in the soluble nitrogen upto a dosage of 2Mrad could be attributed to
the partial cleavage of peptide linkage ,however, at higher than 2Mrad,the
soluble nitrogen percentage tended to fall,which might be the result of partial
elimination of some amino groups from free amino acids (Simic 1968). Nene et al
(1975) reviewed the literature about the effect of gamma irradiation on
proteins.
8.4 Starch in maize
Many
investigators (Akulova et al 1970; Berger et al 1973,1977;El-Saadany et
al1976;Korotchenko et al 1968,1973;Yakovenko et al1968)have studied the effect
of different gamma ray doses on some physical and chemical properties of corn
starch.
The research indicated that the combination of
20 Gy of gamma-ray and 1 mmol/L of NaN3 is the most effective for mutation
inducement of maize calli, three endosperm mutant lines with “super sweet”
phenotype were derived from the mutated offspring. By complementation test and
DNA sequence analysis, their mutation site was found in exon 14 of gene sh2 that encodes adenosine diphosphate
glucose pyrophosphorylase(African Journal of Biotechnology Vol. 10(76), pp. 17490-17498,
30 November, 2011).
8.5
Proline content in wheat
Biochemical
differentiation based onproline content revealed that seedlings irradiated at
100, 200 and 300 Gy, exhibitedproline content of 1.4 mg/g FW, 1.7 mg/g FW and
1.5 mg/g FW, respectively which werenot significantly different as compared to
those in non- irradiated seedlings, However, there was no significant
difference among the seedlings irradiated with 100,
200
and 300 Gy. In T-65-58-8 genotype, proline contents were slightly increased afterimposing
different levels of gamma radiation of seeds as compared with
non-irradiatedcontrol , however, in cv. Roshan, proline contents were higher
after irradiationwith 100, 200 and 300 Gy as compared to non-irradiated
control. A maximum increase in proline contents was observed after 200 Gy dose
in Roshan genotype ( BORZOUEI ET AL.,20100).
Gamma
radiation was reported to induce oxidative stress with overproduction
ofreactive oxygen species (ROS) such as superoxide radicals, hydroxyl radicals
andhydrogen peroxide, which react rapidly with almost all structural and
functional organicmolecules, including proteins, lipids and nucleic acids
causing disturbance of cellularmetabolism (Al-Rumaih & Al-Rumaih, 2008;
Ashraf, 2009; Noreen & Ashraf, 2009). Toavoid oxidative damage, plants have
evolved various protective mechanisms tocounteract the effects of reactive
oxygen species in cellular compartments (Kiong et al.,2008). This
defense was brought about by alteration in the pattern of gene expression.This
led to modulation of certain metabolic and defensive pathways. One of the
protective
mechanisms in the synthesis of osmolytes which is essential to plant growthwas
proline synthesis (Esfandiari et al., 2008).
9. EFFECT OF GAMMA IRRADIATION ON GENETIC COMPOSITION
OF PLANTS
9.1 Soyabean genetic diversity
Various
studies showed that the genetic
diversity in soybean (Glycine
max L.) germplasm is limited. There is insufficient data on molecular
analysis of soybean collections from Africa. The main objectives of the present
study were (1) to analyze the genetic
diversity and relationships
among soybean accessions identified in the DR-Congo gene pool and (2) to determine
the effect of gamma radiation on genetic
variability.
Genomic DNA from several cowpea accessions were analyzed using Inter-simple
Sequence Repeat (ISSR) system. The level of polymorphic loci among the soybean
varieties was high, varying from 70 to 90% based on ISSR primers used. The
soybean varieties analyzed were genetically closely related with several
accessions exhibiting similar ISSR amplification profiles. The genetic
distance among the soybean
accessions varied from 0.00 to 0.46. Some accessions from the International
Institute of Tropical Agriculture (ITTA) revealed identical ISSR amplification
profile. Seed treatment with gamma-rays at 0.2 KGy (20 Krad) increases the level of polymorphism in
progenies by 10%. Low genetic
diversity observed within
varieties was increased with gamma-ray treatment at 0.1 KGy (10 Krad) and 0.2
KGy (20 Krad) dose.
9.2 Chromosome abnormalities in Allium cepa
Bulbs
of A. cepa were treated with different doses of gamma rays. Chromosomal
behaviour and antioxidant enzymes status were studied on 3rd and
30th day respectively after irradiation to
understand the level of radiosensitivity. A positive correlation between
chromosomal abnormalities and antioxidant enzymes related defence mechanism of
cell has been established.
9.3 Abnormalities in maize
Maize seeds were pre-soaked in five
concentrations of metronidazole and then submitted to four radiation dosages.
Part of the seeds was used for cytogenetical analysis and another planted for
survival analysis to obtain M2 seeds. Cytogeneticalanalysis showed a radio
sensitizing effect of metronidazole mainly in the dosages of 30 and 60 Gy. At
90 Gy, the harmful effect of the radiation hinderedthe analysis of the radio
sensitizing effect. The alterations found in M1 reappearedin M2, indicating
that those abnormalities are not in their totality due to somatic origin. The
use of a radiosensitizing compound can be useful tool for mutant production in
plants.
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