Effects of Radiation in Corn

Published Date: 17 Jan 2018

The Mutagenic and Developmental Effects of Radiation in Corn (Zea mays)

Barrun, Neil Benedict Z.

Institute of Chemistry-College of Arts and Sciences, University of the Philippines Los Baños

College, Laguna

ABSTRACT

In this study, the effects of radiation at 0, 10, 30, and 50 krad were observed in the germination and development of the corn plant (Zea mays) for almost 5 weeks with each of the treatment having 10 corn seeds. The first two weeks showed germination on all treatments and growth was fast on treatments with 0 and 10 krad radiation. The percent germination for the control was 90% and 80% for the 10 krad treatment, and 70% and 30% respectively for the 30 and 50 krad treatment. On the last weeks of the observation and monitoring period, it was observed that the corn exposed to 50 krad didn’t developed and matured into a corn plant suggesting a survival rate of 90%, 80%, 70%, and 0% for the control, the 10krad, 30 krad, and 50 krad treatments respectively. Also, corn grains exposed to 0 and 10 krad showed favourable characteristics based on plant height and overall biophysical appearance as opposed to 30 and most specifically 50 krad which was observed to be deleterious to the corn plant. These observations suggest that increased dosage of radiation causes mutation in the plant that could either be beneficial or fatal to the plant.

Keywords: germination, mutation, radiation

INTRODUCTION

A mutation is any change, however large or small, in DNA. The smallest mutation, affecting the least amount of DNA, is a point mutation in which a single base is converted to another base by any various methods. If a piece of DNA is lost, the mutation is deletion; the addition of extra DNA is insertion. Under some conditions, a piece of DNA becomes tangled and breaks, and during repair it put in backward as an inversion. Mutations happen every time DNA replicates. In addition to DNA replication errors and other spontaneous types of changes, agents called mutagens can alter DNA sequences. Ultraviolet light, ionizing radiation, and certain chemicals act as mutagens. DNA repair enzymes can often find and correct damaged DNA. If left uncorrected, though, a permanent change in DNA sequence, or mutation, occurs. Most mutations are silent, with no visible consequence of the DNA alteration as most DNA in a cell is noncoding. Even altered coding sequences may produce functional proteins (Bidlack et al, 2011).

Mutations are either somatic or germ-line. A somatic mutation occurs in a body cell and will exist in all cells produced by mitosis of the mutant cell. On the other hand, a germ-line mutation occurs in tissue that will produce gametes, or sex cells. Unlike somatic mutations, germ-line mutations will be passed on to future generations through seeds. A germ-line mutation is generally not apparent until it is passed on to offspring, which will carry the altered DNA in all cells. This Mutation has now become a permanent feature of that plant’s lineage. Germ-line mutations are important for the genetic improvement of horticultural and agronomic plants. They are responsible for variability in all traits, including flower colour and fragrance, fruit taste and texture, grain yield and quality, and disease and stress tolerance. While mutations are generally perceived to be bad, mutations are an essential feature of the DNA. All the genetic variability in nature has arisen as a result of mutation. The accumulation of mutations is the mechanism that drives evolution of species (Bidlack et al, 2011).

As mentioned earlier, mutagens are external factors that can cause mutations. One of which is radiation. Radiation is often classified as ionizing or non-ionizing radiation depending on whether ions are emitted in the penetrated tissues or not. X-rays, gamma rays (γ), beta particle radiation (β), and alpha particle (α) radiation (also known as alpha rays) are ionizing form of radiation. On the other hand, UV radiation, like that of sunlight, is non-ionizing. The effects of this radiation types biologically differs on how the energy is distributed in irradiated cell populations and tissues. Take the case of alpha radiation for example wherein ionizations lead to an intense but superficial and localized deposition of energy as opposed to ionization in x rays or gamma radiation wherein it traverses deeper into tissues. Such penetration leads to an even distribution of energy. This principle has been used experimentally to deliver radiation to specific cellular components (lifted from Encyclopedia.com).

Mutagenic effect is a term referring to the relative efficiencies of the different types of radiation to specific cellular components. A cumulative effect of radiation has been observed in animal models. This means that if a population is repeatedly exposed to radiation, the more likely a mutation may occur due to additive effect. The mutagenic effect of radiation, especially ionizing radiation, is generally assumed to be due to direct damage to DNA. In plants, the irradiation of seeds with high doses of ionizing radiation, based on previous studies, tend to have disturbed protein synthesis, hormone balance (Rabie et al., 1996), leaf gas exchange, water exchange and enzyme activity (Stoeva and Bineva, 2001). The morphological, structural, and the functional changes depend on the strength and the duration of the gamma-irradiation stress (Al-Salhi, et al., 2005). Advances in technology has lead to the availability of techniques and tools that allow the precise delivery of small doses of radiation as well as provide better monitoring effects. Reactive oxygen species released in irradiated cells are believed to act directly on nuclear DNA and indirectly by modifying bases that will be incorporated in DNA, or deactivating DNA repair enzymes. There is evidence that radiation induces changes in the cytosol that—in eukaryotes—are transmitted to the nucleus and even to neighbouring cells. The damage to the DNA can be assessed through polymerase chain reaction (PCR) to detect the loss of some marker genes due to large deletions. Damages to the DNA can also be assessed by evaluating the expression level of the stress inducible p21 protein (lifted from Encyclopedia.com).

To determine the effects of high dose of ionizing radiation to plants in a laboratory class set-up, this study dealt with corn seeds (Zea mays) that were pre-treated with different doses of ionizing radiation as well as a control. The seeds were all grown under the same conditions. The study made use of corn as the experimental subject because it’s cheap and is widely available. The objective of this study is to (1) observe the effects of different doses of radiation on plant growth based on plant height and % germination and %survival; (2) explain the mechanisms that resulted to both superficial and cytological changes on the plant.

MATERIALS AND METHODS

The experimental subject was corn and was subjected to varying doses of radiation. Corn seeds were irradiated at varying radiation dosage of 0kR (control), 10 kR, 30kR, and 50 kR with ten seeds for each of this treatment. These seeds were planted 5 cm apart on a garden plot in 4 rows. Each row is designated for a specific treatment. The germination as well as the growth of each plant was observed for a period of 5 weeks. Unfortunately, the observation time was somehow had missing data on due to holidays and non-availability of the designated data gatherers.

During the course of 5 weeks, germination time as well as percent germination was recorded. The height of each of the plant was also recorded. On the last day of data gathering, the percent survival and also the final plant height was noted. The data on plant height (average) is in Table 1. Take note that the last day denoted on the table is Day 17. This is because of the days wherein data gathering was not performed. The data is also presented as a line graph (see Figure 1) to observe the overall trend. Furthermore, the number of germinating (see Figure 2) as well as surviving (see Figure 3) seeds is illustrated in a column graph for comparison.

Table1. Data on plant height of the different treatments in the course of 5 weeks.

Figure 1. Comparison of the overall plant height of the 4 different treatments in the course of 5 weeks.

Figure 2. Comparison of the number of germinated seeds of the 4 different treatments.

Figure 3. Comparison of the number of matured seedlings of the 4 different treatments in the course of 5 weeks.

RESULTS AND DISCUSSION

Taking the data in Table 1 under consideration we can infer that those treated with 0 and 10 kR of radiation is the tallest in terms of plant height. Also, these plants based on careful observation appeared to have fewer signs of deteriorated health such as necrosis or leaf discoloration. The final plant height (average height measured on the 5^th week) for the 4 treatments with increasing radiation dosage of 0 kR, 10kR, 30kR, and 50 kR was found to be 124.18cm, 133.99cm, 69.02cm, and 0cm respectively. In Figure 2, the data suggest high rate of germination (90% and 80% respectively) for both the control (0 kR) and 10 kR treatments. As shown in Figure 3, the germinated plants were observed to have grown to maturity in the control, the 10 kR and 30 kR treatments implying a 90% survival rate for the control, 80% for the 10 kR treatment and 70% for the 30kR treatment. Furthermore the 50 kR treatment had a very low percent germination at 30% (3 out of 10 seedlings germinate) which didn’t grow to maturity on the 5^th week and suggests 0% survival rate.

Based on these results, it can be deduced that the strength of radiation induced to the corn seeds is inversely proportional to the percentage germination and survival as well as the height of the experimental subject. The possible mechanism that could explain such observations is that when the cell is exposed to ionizing radiation, double-stranded breaks occur along the entire length of the DNA. Mutations occur if the repair mechanisms innate to the DNA re-attach the wrong piece of DNA back together. This lead to a missing strand of DNA which led to the deletion of important genes (deletion), or change in the location of a gene within the DNA (insertion). Furthermore, based on the results of the study, increasing the dosage of radiation increases the probability of occurrence of a mutation. The mutation could either be beneficial as observed to the treatment exposed to 10 kR or detrimental as observed in the other treatments other than the control. Mutation as mentioned is beneficial at optimal levels as it is used by plant breeders to induce a mutation that could enhance the plant’s characteristics (Acquaah, 2012). Meanwhile excess of radiation results to something catastrophic to the plant which is not favoured and should be avoided.

Regardless of the mechanisms involve, it could be concluded that high dosage of radiation is detrimental to the plant as evidenced by its stunted growth and relatively short life span as observed on the 50 kR treatment and that increasing the dosage of radiation has a nearly inverse relation with plant height. The optimal radiation level for the corn plant is 10 kR based on the results of the study.

SUMMARY AND CONCLUSION

Radiation is a mutagen that could either inflict positive or negative effects. In this study, plant pre-treated to varying doses of radiation was observed. It was found out that extremely high dose of radiation, at 50 kR is fatal to the corn plant as 30% only of the seeds germinated. Furthermore, this plant had a stunted growth and relatively short life span as it did not grow to maturity in the 5^th week. Meanwhile, optimal dose of radiation at 10 kR brought about benefits to the corn plant as it had a higher height of 133.99cm as compared to 124.18cm of the control in the 5^th week. Also, it has closely the same germination rate of 80% to the control which is 90%. The germinated seeds on both the treatments managed to survive up until the 5^th week. The 30 kR treatment was observed to both have a 70% germination and survival rate which is lower than the control. The treatment was also found to have the most stunted growth of the three treatments that have surviving seedlings at 69.02cm on the 5^th week.

Based on these results, it can be concluded that the strength of radiation induced to the corn seeds is inversely proportional to the percentage germination and survival as well as the height of the experimental subject. Such observations can be attributed to the double-stranded breaks that occur along the entire length of the DNA. Mutations occur if the repair mechanisms innate to the DNA re-attach the wrong piece of DNA back together. This lead to a missing strand of DNA which leads to the deletion of important genes (deletion), or change in the location of a gene within the DNA (insertion). Furthermore, based on the results of the study, increasing the dosage of radiation increases the probability of occurrence of a mutation. The mutation could either be beneficial as observed to the treatment exposed to 10 kR or detrimental as observed in the other treatments other than the control. Mutation as mentioned is beneficial at optimal levels as it is used by plant breeders to induce a mutation that could enhance the plant’s characteristics. Meanwhile excess of radiation results to something catastrophic to the plant which is not favoured and should be avoided.

REFERENCES/LITERATURE CITED

"Radiation Mutagenesis."World of Microbiology and Immunology. 2003. Retrieved May 26, 2015 from Encyclopedia.com:http://www.encyclopedia.com/doc/1G2-3409800480.html

Acquaah, G. 2012. Principles of Plant Genetics and Breeding. 2^nd Ed. UK: John Wiley and Sons, Ltd.

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 ofcorn. Nahrung., 48: 95-98.

Bidlack, J.E. and S.H. Jansky (Eds.) 2011. Stern’s Introductory Plant Biology. 12^th Edition. New York: The McGraw-Hill Companies.

Itol, M.G.R., 2010. Effect of Ionizing Radiation on the Growth and Germination of Corn (Zea mays).

Mauseth J.D. 1998. Botany: An Introduction to Plant Biology. 2^nd Ed. USA: Jones and Bartlett Publishers, Inc.

Rabie, K., S. Shenata and M. Bondok, 1996. Analysis of agric. science. Cairo, 41, Univ. Egypt, 551-566.

Stoeva, N. and Z. Bineva. 2001. Physiological response of beans (Phaseolus vulgarisL.) to gamma-radiation contamination I. Growth, photosynthesis rate and contents of plastid pigments. J. Env. Prot. Eco., 2: 299-303.

Stoeva, N., Z. Zlatev and Z. Bineva. 2001. Physiological response of beans (Phaseolus vulgaris L.) to gamma-radiation contamination, II. Water-exchange, respiration and peroxidase activity. J. Env. Prot. Eco., 2: 304-308.

ACKNOWLEDGEMENT

I would like to acknowledge first and foremost the Lord Our God, for giving me wisdom and guidance in the making of this scientific paper. I would also like to acknowledge my partner in the data gathering Jake Celeste for his continuous reminder of the date of the data gathering as well as the rest of BIO30 A-4L class who tried to find time to make this scientific paper possible and credible and most of all Prof. Anna Marie U. Toledo for her incessant reminder on the conduct of this study as well as her guidance in the activities performed throughout the semester.

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