The effect of gamma radiation exposure on various biological species is known to vary with the radiation dose. Contrary to earlier theories which predict greater damage with increasing dosage, recent studies have observed a non-linear relationship at relatively low doses (Al-Safadi & Simon, 1996; Bajaj et al., 1970; Mokobia, & Anomohanran, 2005). An experiment to observe this effect on the monocot crop plant Hordeum Vulgare was carried out. Varying doses of 0 Krad, 5Krad, 10 Krad, 25 Krad, 50 Krad, and 100 Krad were used on six batches of seedlings. The aim of this paper is to discuss the methodology and results of this experiment, and to make logical inferences from them.
Existing Theories on Parameters Involved
Germination Rate – This refers to the number of seeds that are likely to germinate in a given time (the typical period for germination in H. Vulgare is 1-3 days). Experiments previously conducted on other species (Amaranth seeds, for instance) show that the germination rate reduces with increasing doses of gamma radiation (Hameed et. al, 2008; Kong et al., 2009).
Seedling Height – Again, research so far, shows that the height of seeds exposed to gamma rays decreases with increasing dosage. The strict relationship between the two has not been published yet, though it is agreed to be non-linear (Sangsiri, 2005; Sparrow & Singleton, 1953; Ananthaswamy et al., 1971).
Coleoptile Cell Volume – A coleoptile is a protective sheath that covers the emerging shoot in most monocotyledons. Coleoptiles are involved in cell enlargement, rather than cell division that take place in other organs. Gamma radiation results in damage to the chromosome structure, thus inhibiting cell division. Cell enlargement, which is independent of this damage, continues to occur. However, studies show that the final height of the coleoptiles is reduced proportionally with increasing irradiation (Miura & Yamaguchi, 1974; Momiyama et al., 1999).
Fifty seeds from each batch were planted in seed trays in a greenhouse. The germination rate, number of leaves, seedling height and coleoptile cell volume were recorded as follows:
1) The germination rate (%) of the seeds (specific batches) for 7, 10, and 14 days, were observed for the different doses.
2) The height of 6 of the seedlings that germinated for each dose was measured at 14 days after planting and 45 days after planting.
3) After 1 week, several of the seedlings from each dosage were removed from trays and dissected. Cell volume of the coleoptile was measured for nine cells at each dose
4) Finally the number of leaves that developed on plants at different doses was counted.
The results recorded for each of these phases, is numbered respectively, in the next section.
Analysis and Discussion
The following figures represent the first 2 results graphically:
1) For all the cases, the germination rate of the seeds decrease initially with increase in dose, reach a local minimum, then again increase sharply. After a point, there is a smooth decrease in the germination rate, which finally increases again smoothly, after a point. In specific, for all three cases (7days, 10 days, and 14 days), the first local minimum occurs at around 5 Krad, and the second one at 25-50 Krad. The first maximum is at around 10 Krad, and the second continuous increase starts after approximately 55 Krad. This shows that there is no consistent relationship between the dose and germination rate. However, the germination rate consistently increases with time (it follows 14 days < 10 days <7 days).
a) Except seed 6, all others have shown a sudden increase in height initially, followed by a rapid decrease which becomes zero at doses above 50 Krad. This implies the height of seedlings is inversely proportional to the dose (though non-linearly).
b) After 45 days, the same seeds’ heights seem to have increased as a whole (vertical shift in respective graphs), though the dose at which the height becomes zero, is much lesser (approximately 25 Krad).
3) The coleoptile cell volume of the 9 seeds observed, show an initial increase, followed by gradual decrease (after 5-10 Krad). This rate of decrease seems to reduce, and reach a more or less stable value after 50 Krad. The final values for all 9 seedlings are more than what is observed with zero irradiation. This coincides with the theory that irradiation does not damage cell enlargement of the coleoptiles.
4) The number of leaves with zero irradiation is less than that with an irradiation dose of 5 Krad, in all cases. However, this value gradually reduces (in most seeds) after 5 Krad, to 10 Krad. Beyond that, there are no leaves developed at all. Though this point of irradiation (value) is not known certainly from the recordings, it can be said that it lies between 10 Krad and 20 Krad, in all cases.
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