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Results and
Discussion
Experimental results revealed that germination rate in all the
varieties showed a slight increase at lower doses (1 - 3 kR) as
compared to the control, whereas it decreased at higher doses
(Table
1). More
reduction was seen in durum wheats.
Seedling height is often used as a measure of radiation damage in
seeds. In all the varieties there was a stimulation in seedling
growth at lower doses (1 - 3 kR) as compared to the control, whereas
seedling height decreased as the dose increased from 5 kR to 30 kR
(Table
1). Similar
results were observed for mitotic index.
Stimulation in germination may be due to the formation/activation of
some stimulatory substances e.g. auxins, which take active part in
germination (Simonis 1966), whereas the reduction may be caused by
radiation induced damage through the alteration in endogenous
hormonal levels (Mishra et al 1980). Some of the damage could also be
due to a direct effect of radiations in killing meristematic cells.
Stimulation in seedling height may be due to faster cell division,
change in auxin balance or enhancement in enzyme activity. Radiations
are capable of considerably changing the type of information of
certain DNA molecules (Simonis 1966). This may be the reason for the
increase in mitotic index at lower doses and a decrease at higher
doses.
Various types of abnormalities observed during mitosis in somatic
cells of root tip were in the form of laggards, bridges and chromatin
stickiness (Table
2). These
increased with an increase in radiation dose. The most frequent
abnormality was in the form of laggards at anaphase, and the second
was bridges. During meiosis cells with univalents at metaphase was
observed most frequently, and the second was the cells with laggards
at anaphase (Table
3). Univalents
may result either from the failure of chromosomes pairing at zygotene
(asynapsis) or from the disjunction of homologous chromosomes at
diplotene (desynapsis), because chiasma formation did not occur.
Possible explanation for the formation of bridges may be that broken
ends containing centromere from two different chromosomes unite to
form dicentric chromosome, or that chromosomes are clumped together
due to stickiness and are often unable to separate compelety at
anaphase. Chromatin stickiness may be caused by some changes in the
surface properties of the chromosomes which caused them to adhere to
each other on coming in contact (Khanna 1986).
Considering the overall data, it appears that hexaploid wheats seem
to be a little more radiosensitive as compared to the tetraploid
wheats. This could be due to a greater chromosomal volume in the
hexaploid Wheat. Gupta and Roy (1985) reported that gamma-rays
induced more meiotic abnormalities in tetraploid Physalis
peruviane as compared to the diploid P.
ixocarpa.
References
Gupta SK and Roy SK (1985) Comparison of meiotic abnormalities
induced by gamma rays between a diploid and tetraploid species of
Physalis. Cytologia 50: 167-175.
Khanna VK (1986) A comparison of radiosensitivity of wheat and
triticale. Acta Bot Indica 14: 110-114.
Mishra SD, Mathew T, Joshi RK and Gaur BK (1980) Radiation induced
delay in organogenesis of Kalanchoe daigremontiana leaves.
Env and Exptl Bot 20: 213-215.
Simonis W (1966) Physiological problems related to the effects of
small doses of radiation on plants. In: Effects of low doses of
radiation in crop plants, IAEA, Vienna.
Swaminathan (1964)
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