Effects of Ionizing Radiation on the Growth of Corn (Zea mays)

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Effects of Ionizing Radiation on the Growth of Corn (Zea mays)1

1 A paper submitted in partial fulfillment of the requirements in Biology 30 (Genetics) Laboratory under Prof. Ana Mariel U. Toledo, 2nd Semester, 2014 – 2015.

ABSTRACT

The effects of irradiation on the growth of Zea mays were determined by subjecting corn kernels into different doses of gamma radiation from Co-60. The assumption is that certain doses would promote growth while some would cause terminal mutations. In accordance to the hypothesis, lower dosages of radiation caused an increased growth rate in corn plants, while higher dosages (30 krad and above) were found to be detrimental. Higher doses could have caused nonsense mutations causing the premature termination of mRNA translation. No other phenotypic enhancements were observed except for the increased growth rate in 10 krad. Thus, low dosages of radiation would promote growth, whereas higher dosages would stunt growth or are detrimental.

INTRODUCTION

Corn is one of the most in demand food in the world. Corn is used as an ingredient from staple food of the Americans to the street food of Filipinos. A larger yield should then be observed in response to this demand. Plant breeders have devised a scientific way in order to compensate the call for it. Corn can have an increase in yield if mutation is induced.

Mutations are inherent changes in the genetic material of an organism, which can be transmitted into its offspring. There are two type of mutations, namely, chromosomal and gene mutations. The occurrence of mutations varies greatly as it could be spontaneous, or induced by an external factor, such as mutagens (Mendioro, et al., 2013). Mutations can either be spontaneous, or induced by chemical or physical agents. It is possible to distinguish chemical mutagens through their modes of action. Some of these cause mutations by mechanisms similar to those which arise spontaneously, while others are more like radiation, a physical agent, in their effects (Barrion et al., 2005).

Radiation was the first mutagenic agent known, with its effect on genes was first reported in the 1920's. There are two major types of radiation: EM spectrum and ionizing radiation. Electromagnetic radiation consists of electric and magnetic waves while the ionizing radiation consists of X-rays and gamma-rays which are energetic enough to produce reactive ions that react with biological molecules (Al-Salhi et al., 2005).Ionizing Radiation, at higher doses, may cause both mutation and death of the cell. Cell death may be of two kinds. First, the cell may no longer perform its function due to internal ionization cause by the ionizing radiation; and second, "reproductive death" or mitotic inhibition where the cell can no longer reproduce, but still performs its other functions (Heather, 2000). This could be exhibited by high mortality rate on the seeds and slow growth rate on the irradiated seeds. Mutations by radiation in plants are easily induced at the seed stage, before the plants begin to develop protective mechanisms as they grow (Quastler, 1950). Hence, young plants are susceptible to radiations than older plant.

In the experiment, corn seeds were used to determine and verify the effect of radiations on plants particularly on the seeds and germinating shoots. Corn or Zea mays have 10 chromosomes. The seeds were subjected to different degrees of radiation due to cobalt. Cobalt is a nonradioactive metal found in nature from which radioactive isotopes can be produced by linear accelerators and nuclear reactors (Heather, F. 2000). As stated earlier, ionizing radiations may cause mutations at higher doses. Thus, the higher the degree of ionizing radiation in the corn seed, the higher the probability that mutation will occur. This experiment was designed to determine the effects of different degree of radiation in corn seeds.

The study aimed to determine the effect of increasing strengths of radiation on plant growth in corn and to verify that the amount of damage in the cell is related to the dose of radiation it receives. The specific objectives were:

1. To observe the effects of different doses of radiation on plant growth in terms of height and % germination; and

2. To explain the possible mechanisms behind the observe effect of radiation on plant growth.

MATERIALS AND METHODS

To quantify the effects of different degrees of radiation on plant particularly on the corn, ten seeds were subjected in each degree of radiation measure by kilorad: 50kr, 30kr and 10kr. The seeds were planted and were allowed to grow for 26 days on the plot prepared at the back of the Institute of Biological Sciences. A control set –up was also prepared and planted. The plots were watered and monitored every other day to assure adequate water supply. The height of the germinated seeds, if present was recorded on the scheduled day. To properly assess the effect of radiation on the growth of the corn, these formulas were used:

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The results from the computations were then interpreted.

Seeds that did not germinate were considered dead. Data were then recorded and analyzed in Table 1 and interpreted in Figure 1.

RESULTS AND DISCUSSION

After the 21 days period of observation on the irradiated corn seeds, data gathered were summarized and analyzed on the following figures and tables. The observations measured were the height of the shoots present in each degree of radiation each day.

Table 1. Average height of corn plants (cm).

Dates of Observations

Control

10 krad

30 krad

50 krad

Height (cm)

n

Height (cm)

n

Height (cm)

n

Height (cm)

n

20-March

0

0

0

0

23-March

6.9

9

6.4

9

2.1

5

1.7

5

25-March

7.3

9

10.1

9

-

-

-

-

27-March

22.7

9

21.6

9

9.1

4

2.0

2

30-March

23.5

9

30.1

9

9.1

4

0.9

2

1-April

28.9

9

34.2

9

10.5

4

0

0

6-April

39.2

9

43.9

9

13.8

8

0

0

8-April

48.0

9

50.5

9

17.1

7

0

0

10-April

54.3

9

62.1

9

19.5

8

0

0

13-April

-

-

-

-

-

-

4.9

2

15-April

-

-

-

-

-

-

4.9

2

17-April

-

-

-

-

-

-

4.9

2

20-April

96.3

8

105.4

9

37.4

3

0

0

22-April

97.4

9

108.3

9

55.5

2

0

0

24-April

-

-

-

-

-

-

0

0

27-April

114.8

8

120.5

9

59.5

2

0

0

29-April

116.5

8

122.8

9

62.0

2

0

0

4-May

122.4

8

129.3

9

66.8

2

0

0

7-May

120.0

8

126.4

9

68.5

2

0

0

8-May

123.3

8

131.1

9

69.0

2

0

0

11-May

124.1

8

133.9

9

69.0

2

0

0

*Values were obtained using the formula: total height of plants/ total no. of plants.

It is observable that as the amount of radiation increase, the fewer are the plants that germinate except for the 10 kilorad radiation. Associated with it is the plant height which decreases also as the amount of radiation increases also with the exception of 10 kilorad radiation which showed a great increase in height.

The mean height for the control set-up was 124.1 cm. The mean height for the plants which were subjected to 10 kilorad of radiation is 133.9 cm. The plants under the 30 kilorad treatment only reached a mean height of 69.00 cm. On the other hand, plants under the 50 kilorad treatment had a mean height of only 4.9 cm. In comparison, the treatment under 10 kilorad radiation had the most number of survivors and a large value for the mean height versus to the plants that received the maximum radiation which had a small value for the mean height and zero survivors.

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Figure 2. A line graph comparing the growth rates of Zea mays irradiated with different doses of gamma rays.

The results were in accordance to the growth rate as the corn plants exposed 10 krad of radiation were the tallest. This is followed by the controlled, with very minimal difference. The heights of the corn plants exposed to 30 and 50 krad of radiation, on the other hand, were severely stunted. The theory is that the growth of plants subjected to low doses of radiation, 10 krad, exhibited enhanced growth as their growth was stimulated by altering the hormonal signaling network of the plants’ cells. It might also be due to the increased anti-oxidative capacity of the cells, which renders the stresses (inconsistent light intensity and temperature) that could stunt growth useless (Minisi, et al., 2013).

Table 2. Germination rate of Zea mays subjected to different dosage of radiation.

Treatment

No. of seeds planted

No. of seeds germinated

Germination Success Rate

Control

20

9

45%

10 kilorad

20

9

45%

30 kilorad

20

5

25%

50 kilorad

20

5

25%

The control group also had a final 45% germination and average height of 124.1 cm as shown in Table 1. The presence of the control group in the experiment assures that external factors such as amount of sunlight, water, pH of the medium would be negligible in evaluating both the % germination and length of height in every set-up. One more important aspect to look into is the survival rate. Table 3 shows the data for survival rate.

Table 3. Computed survival rate of Zea mays subjected to Cobalt-60 radiation.

Treatment

Highest number of survivors

No. of seeds at the last day

Survival Rate

Control

9

8

88.9%

10 kilorad

9

9

100.0%

30 kilorad

8

2

25%

50 kilorad

5

0

0%

The computations Table 3 suggest that a plant or crop devoid of radiation, which induces mutation, is advantageous to its growth. The 10 krad set-up had the highest survival rate of 100.0% then followed by the control set-up which is 88.9%. The 30 kilorad had a survival rate of 25% and 50 kilorad treatment (0%) did not survive until the last day.

The theory for the death and stunted growth of corn plants is that increased doses of radiation causes the cell cycle to arrest at the G2 phase causing a reduced growth among the corn plants. The eventual death of the said plants can be attributed to damages in the corn plants’ genetic material (Minisi, et al., 2013). The possible kinds of gene mutations that can cause death are nonsense mutations and frameshift mutations (frameshift mutations could cause changes in the reading frame of the genetic material resulting to nonsense mutations). These kinds of mutations could cause death as certain codons are transformed into stop codons resulting the termination of mRNA translation (Ramirez, Mendioro and Laude, 2013).

CONCLUSION

The effect of varying doses of radiation on the growth of corn (Zea mays) was determined by subjecting corn kernels into 3 different doses of radiation from gamma rays emitted during the decay of Co-60. For 21 days, the plants were observed thrice a week and the % germination, % survival, average plant height and notable phenotypic changes were noted.

The data obtained from the experiment showed that a minimal dosage of radiation, 10 krad, causes an increase in the growth rate of corn, whereas greater dosages such as 30 and 50 krad are detrimental. The death of the corn plants in the higher dosages could have been caused by nonsense and frameshift mutations, although, it would require genotypic analysis to confirm this. There was also no observed enhancement in the phenotypic traits among the corn plants in all dosages of radiation with the exception of the plant height, where the corn plants from the 10 krad plot had a greater final height, 133.9 cm, followed by the controlled, 124.1 cm. The corn plants subjected to higher dosages were observed to have a severely stunted growth, as the final heights from the 30 and 50 krad plots were 69.0 and 4.9 cm, respectively.

In this experiment, some of the data gathered could have been erroneous because different people did the measurements and there were missing datas. In future replications, it would be recommended to have only a single person doing the measurements. If the previous isn’t possible, the people who will take the measurements must have a common agreement on how they would be performing the measurements.

LITERATURE CITED

Al-Salhi, M., M.M. Ghannam, M.S. Al-Ayed, S.U. El-Kameesy and S. Roshdy. 2004. Effect of gamma-irradiation on the biophysical and morphological properties of corn. Nahrung., 48: 95-98.

Barrion, A.A., Bebbing, N.N., Diaz, M., Laude, R.P., Ramirez, D.A., and Villafuerte, L.S. 2005. Genetics: A Laboratory Manual. 12th ed. San Pablo City, Laguna. 7 Lakes Printing Press. 101 p.

Heather, F. 2000. Effects of Radiation on the Germination and Growth of Seeds. Canada

Mendioro, et al., 2013. Genetics A Laboratory Manual. 13th revision. San Pablo, Philippines: 7 lakes Printing Press.pp. 107-111.

Minisi, et al. 2013. Effects of Gamma Radiation on Germination, Growth Characteristics and Morphological Variations of MolucellalaevisL. Egypt: IDOSI Publications. pp. 696-704.

Ramirez, D., M.S. Mendioro and R.P. Laude. 2013. Lectures in Genetics. 10th ed. San Pablo, Philippines: 7 lakes Printing Press.pp. 171-174.

Quastler, H. and Baer, M. 1950. Inhibition of plant growth by irradiation. Retrieved Febuary 07, 2010 from http://www.cancerreasearch.com

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