Radish seeds are a good form of experimental seeds. These seeds generally grow in a fairly short amount of time under the proper lighting conditions and with a good hydration supply. When exposed to moderate amounts of radiation, radish seeds tend to exhibit phenotypic changes, such as growth defects and discoloration. All seeds are exposed to natural radiation, as is everything in the world. Consequently, a small amount of radiation is predicted to have null effect on the growth of plant seeds (Health Physics Society). However, the higher the dosage of radiation that seeds are exposed to, the greater the effect the radiation is predicted to have on growth of the seed. In this laboratory experiment, we observed the effect of various dosages of radiation on the growth height phenotypes of radish plants. The radish seeds utilized had received different doses of gamma radiation (50,000 rads, 150,000 rads, 500,000 rads, and 4 million rads). Because gamma radiation is the most penetrating form of radiation, it was the ideal choice to observe the effect of radiation on seed DNA via observations of the plant phenotypes.
Materials and Methods:
8 of each of the following sets of radish seeds treated with different doses of gamma radiation were obtained: 0 rads; 50,000 rads; 150,000 rads; 500,000 rads; and 4,000,000 rads. A planting container and dirt were obtained. Each seed was planted about 1inch down into the dirt and 2 inches apart from each other. All seeds were planted categorically. Marking sticks were labeled with the type of seeds that were planted (0; 50,000; 150,000; 500,000; 4,000,000). The labels were placed next to the appropriate seed categories in the planting containers. The seeds were watered regularly during each weekday.
Approximately three weeks after planting, the grown plants were removed from the planting containers. The leaf, stem, root, and full plant lengths were then measured using a ruler.
The Wilcoxon Rank Sum Test was then performed on the computer using the data collected to allow for a comparison for each phenotype observed and determine whether significant difference in measurements was observed. For each phenotype control (0 rads) was compared with 50,000 rads; 150,000 rads; 500,000 rads; and 4,000,000 rads respectively.
Bar graphs were constructed via Microsoft Excel to show the mean and standard deviation for each phenotype at each dose.
Results and Discussion:
In this laboratory experiment, we observed the effect of various dosages of gamma radiation on the growth height phenotype of radish seeds in an attempt to appreciate radiation effect on organismal DNA.
Mean and Standard Deviation of Plant Lengths At Various Radiation Doses
Table 1. The average and standard deviation data for full plant, leaf, stem, and root lengths at various radiation dosages.
Observations of the average plant length for the full plant, plant leaf, plant stem, and plant root indicate that the effect of gamma radiation on plant growth is dosage dependent, as the greater dose of radiation the seed was exposed to, the shorter the plant, leaf, stem and root height was observe to be. This trend was the same for each group that was observed. The control (0 rads exposure) for all groups, except the leaf group, exhibited the longest height as compared to the data for radiation exposed seeds (Table 1). In the leaf group, the 50,000 rads exposed seeds had a greater height than the control (Table 1); this may be a result of experimental error such as the improper measuring of leaves. The greatest effect of radiation was at 4 million rads (Table 1), as no plant growth was observed at this range of radiation.
Legend for Figures 1-4
Figure 1. Effect of gamma radiation on full plant length.
Full Plant Length Data At Various Radiation Doses
Table 2. Statistical data for full plant length at various radiation dosages
In our observations of radiation effects on full plant length, we observed that plant length decreased as the dosage of radiation to which the seeds were exposed increased. This can be seen by comparing the average and standard deviation of the control, unexposed seeds (76.56 ± 48.52) to seeds exposed to 50,000 rads (69.11 ± 65.93), 150,000 rads (18.44 ± 36.6), 500,000 rads (3.11 ± 6.585), and 4 million rads (0 ± 0),as one can see a decreasing trend in growth (Figure 1, Table 2). For the full plant lengths, the standard deviations did not overlap at all (Figure 1). Additionally, p values calculated using the Wilcoxon Rank Sum Test were 0.86 for control versus 50,000 rads, 0.016 for control versus 150,000 rads, 0.008 for control versus 500,000 rads, and 0.002 for control versus 4 million rads (Table 2). These values suggest that radiation greatly affected the full plant in a dosage dependent manner, as the greater dosage of radiation exposure, the more effect was seen on the full plant length. Since the p value for control versus 50,000 rads was 0.86, the difference in full plant length between the two groups is not statistically different. This indicates that there may be a threshold level of radiation to which seeds must be exposed prior to seeing affects in their growth.
Figure 2. Effect of gamma radiation on plant leaf length.
Plant Leaf Length Data At Various Radiation Doses
Table 3. Statistical data for plant leaf length at various radiation dosages
In our observations of radiation effects on plant leaf length, we observed that leaf length decreased as the dosage of radiation to which the seeds were exposed increased. This can be seen by comparing the average and standard deviation of the control, unexposed seeds (6.778 ± 4.764) to seeds exposed to 50,000 rads (15.44 ± 15.02), 150,000 rads (3.778 ± 7.513), 500,000 rads (0.333 ± 1), and 4 million rads (0 ± 0),as one can see a decreasing trend in growth (Figure 2, Table 3). For the plant leaf lengths, the standard deviations did not overlap much (Figure 2). Additionally, p values calculated using the Wilcoxon Rank Sum Test were 0.44 for control versus 50,000 rads, 0.14 for control versus 150,000 rads, 0.003 for control versus 500,000 rads (Table 3). The p value for control versus 4 million rads could not be calculated using this test. These p values suggest that radiation greatly affected the plant leaves in a dosage dependent manner, as the greater the amount of radiation exposure, the shorter the plant leaves were observed as being. Since the p value for control versus 50,000 rads was 0.44 and the control versus 150,000 was 0.14, the difference in plant leaf length between these two groups as compared to the control is not statistically different, indicating that radiation at these levels did not have a large effect on leaf growth. This also indicates that there may be a threshold level of radiation to which seeds must be exposed prior to seeing affects in their growth.
Figure 3. Effect of gamma radiation on plant stem length.
Plant Stem Length Data At Various Radiation Doses
Table 4. Statistical data for plant stem length at various radiation dosages
In our observations of radiation effects on plant stem length, we observed that stem length also decreased as the dosage of radiation to which the seeds were exposed increased. This can be seen by comparing the average and standard deviation of the control, unexposed seeds (38.56 ± 24.6) to seeds exposed to 50,000 rads (26.89 ± 28.12), 150,000 rads (7 ± 14), 500,000 rads (0 ± 0), and 4 million rads (0 ± 0),as one can see a decreasing trend in growth (Figure 3, Table 4). For the plant stem lengths, the standard deviation for control versus 50,000 rads overlapped slightly while the standard deviation for control versus 150,000 did not overlap much (Figure 3). Additionally, p values calculated using the Wilcoxon Rank Sum Test were 0.30 for control versus 50,000 rads, 0.009 for control versus 150,000 rads, 0.002 for control versus 500,000 rads (Table 4). The p value for control versus 4 million rads could not be calculated using this test. These p values also suggest that radiation greatly affected the plant leaves in a dosage dependent manner, as the greater the amount of radiation exposure, the shorter the plant stem length was observed as being. Since the p value for control versus 50,000 rads was 0.30, the difference in plant leaf length between this group as compared to the control is not statistically different, indicating that radiation did not have a large effect on the 50,000 rads exposed seeds. This too indicates that there may be a threshold level of radiation to which seeds must be exposed prior to seeing affects in their growth.
Figure 4. Effect of gamma radiation on plant root length.
Plant Root Length Data At Various Radiation Doses
Table 5. Statistical data for plant root length at various radiation dosages
Our observations of radiation effects on plant root length also indicate that length decreased as the dosage of radiation to which the seeds were exposed increased. This can be seen by comparing the average and standard deviation of the control, unexposed seeds (25.11 ± 18.51) to seeds exposed to 50,000 rads (26.78 ± 28.12), 150,000 rads (6.364 ± 14.22), 500,000 rads (2.556 ± 5.003), and 4 million rads (0 ± 0),as one can see a decreasing trend in growth (Figure 4, Table 5). For the plant root lengths, the standard deviation for control versus exposed radiation level seeds did not overlap much (Figure 4). Additionally, p values calculated using the Wilcoxon Rank Sum Test were 0.96 for control versus 50,000 rads, 0.022 for control versus 150,000 rads, 0.008 for control versus 500,000 rads (Table 5). The p value for control versus 4 million rads could not be calculated using this test. These p values suggest that radiation greatly affected the plant root in a dosage dependent manner also, as the greater the amount of radiation exposure, the shorter the plant root length was observed as being. Since the p value for control versus 50,000 rads was 0.96, the difference in plant root length between this group as compared to the control is not statistically different, indicating that radiation did not have a large effect on the 50,000 rads exposed seeds. This too indicates that there may be a threshold level of radiation to which seeds must be exposed prior to seeing affects in their growth.
Ionizing radiation, such as gamma radiation, in high amounts may inhibit sprouting and cause slow seedling growth. This may be observed in our results of radish seed growth post exposure to various levels of radiation. Our experiment suggests that the effect of radiation is dosage dependent. The greater the dosage of gamma radiation exposed to the seeds prior to germination, the greater the effect radiation will have on the seed phenotypes. This indicates that the more radiation a seed is exposed to, the greater mutation or damage that the DNA of the seed undergoes. Major effect on growth of seeds under the effect of radiation was generally seen in seeds exposed with 150,000 rads of radiation or more; this implies that there may be a threshold level of radiation to which seeds must be exposed prior to seeing affects in their growth.
References:
Baumstark, B. and T. M. Poole. Bio3910/7910 Genetics Laboratory Manual.
saAtlanta, January 2003.
Guidance for Radiation Accident Management Guidance for Radiation Accident
Management. 30 Jan. 2003. < http://www.orau.gov/reacts/guidance.htm > 15 July 2003.
Health Physics Society Radiation and Seeds.
< > 15 July 2003.
South Dakota State Radiation Radiation Overview. <
> 15 July 2003.
Mutagenesis: The Effect of Radiation on Radish Seeds
Sujata A. Sardar
Biol 3910
28 July 2003
