Abstract
Germination is an important process that begins plant development. For germination to take place, seeds need to take in water, which initiates a sequence of activities that culminate in the emergence of roots and shoots. The intake of water by the seeds involves the process of osmosis, which is affected by a number of factors. This study aimed at investigating the effect of salts such as KCl on germination.
It was hypothesized that an increase in the concentration of KCl would reduce the rate of germination. It was noted that the rate of germination was higher in the control (deionized water) than in the KCl solutions. It was also realized that the rate of germination decreased with an increase in KCl concentration. The study concluded that salinity lowered the rate of germination by impairing osmosis.
Introduction
Germination, which is the first phase in the life of plants, can be described as the process that enables plants to develop from seeds. Before germination, seeds are in a state of dormancy. Dormancy means that the seeds are unable to germinate because the prevailing environmental conditions such as water availability, oxygen, light, and temperature are unfavorable (Bazin et al. 2011). The intake of water by seeds initiates the process of germination. Water hydrates the embryos and sets off enzymes that catalyze the steps of respiration as well as the breakdown of stored food to release energy for germination (Weitbrecht, Müller and Leubner-Metzger 2011).
The hydrolyzed food substances then move into the actively dividing cells of the seed embryos resulting in cell division and proliferation at those regions. In addition, the presence of water stimulates the production of growth hormones (gibberellins), which facilitate further growth of the actively dividing cells. As a result, the plumule and radical come out from the micropyle of the seeds to form the root and shoot systems respectively. This occurrence is what we observe as germination. Therefore, water is a key requirement for germination because it begins a chain of reactions and activities that result in the germination of seeds.
Osmosis, which is the motion of water molecules from a zone of high water concentration to a zone of low water concentration across a partially porous membrane, facilitates the intake water by seeds. The rate of osmosis is influenced by factors such as temperature, water potential (presence of salts and sugars) and distance to be travelled by the water molecules. The water potential of a solution in relation to the cell environment leads to the categorization of the solution as isotonic, hypertonic or hypertonic. The injurious effects of high salt concentrations are complex and affect plants in various ways.
For example, salinity produces drought strain, a decline in the proliferation and differentiation of cells, ion toxicity, and disorders relating to the nutrition of the plant (Panuccio et al. 2014). Moreover, salinity interferes with the integrity of plant membranes and interrupts metabolism. Salinity causes two key strains in plants namely osmotic strain and ionic strain. Osmotic stress hinders the absorption of water while ionic stress results from the accumulation of noxious ions in plant cells.
This experiment sought to determine the effect of varying salt concentrations on the germination of mung bean seeds, which are examples of dicotyledons. Potassium chloride (KCl) was used as a salt, and its presence in the solutions reduced the water potential of the solutions. It was hypothesized that increasing the concentrations of KCl would lead to a decrease in the rate of germination.
Methods
The effect of salinity on germination was investigated using 20 mung beans. The test solution was 0.5M KCl solution while the control was deionized water. The first ten mung beans were folded in a paper towel, which was placed in a plastic bag that contained 12ml of deionized water. The remaining mung beans were then placed in a second plastic bag that contained 12ml of 0.5M KCl solution. The two plastic bags were then placed near a window at temperatures ranging from 65°F to 70°F.
Daily observations were made at 3:00 pm to check for germination of the seeds. The effect of various concentrations of KCl (0.1M, 0.25M and 0.4M) on the rate of germination of mung bean seeds was also observed. The number of seeds that germinated after each day were counted and recorded for each solution. The percentage germination was computed and recorded in table 1 while the graphical representation of the data was illustrated in figures 1 and 2.
Results
It was observed that deionized water led to the highest germination rate compared to KCl solutions. However, it was noted that among the various concentrations of KCl solutions, 0.1M KCl had the highest rate of germination followed by 0.25M KCl solution and 0.4M KCl solution (table 1).
Table 1: Percentage of seeds that germinated over the seven-day period.
The 0.5M KCl solution, on the other hand, had the lowest rate of germination. The overall trend of germination was that the germination rate decreased with an increase in salt concentration (figure 1).
There was zero germination in the control (deionized water) as well as in all the KCl solutions. Germination was first observed on the second day. In the control, the rate of germination began at 41% and increased to 95% on the third day and 100% by day five. Similarly, germination in 0.1M KCl began on the second day and came to completion by the termination of the seventh day. It was observed that there was complete germination in the control and 0.1M KCl.
However, complete germination was attained at a faster rate in deionized water than in 0.1M KCl. The percentage germination was lower in the 0.25M KCl compared to the rates in the 0.1M KCl solution and the control. By the end of the seventh day, only 73.3% of the seeds had germinated compared to the 100% germination rate that was observed in the control and 0.1M KCl solution (figure 2).
The rate of germination further declined in the 0.4M KCl solution with only 7.5% of the seeds sprouting at the end of the third day. Ultimately, only 32.5 % of the seeds germinated. The lowest rate of germination was observed in the 0.5M KCl solution. Germination began on the sixth day and by the conclusion of the experiment, only 3.3% of the seeds had germinated. The general trend observed was that the rate of germination reduced with the increasing concentrations of KCl. The highest rate of germination was observed in the control, which did not contain any salt. On the other hand, the lowest rate of germination was attained in the presence of 0.5M KCl.
Discussion
It was observed that increasing the concentration of KCl in the solutions decreased the rate of germination. These findings were evident in the percentage germination of the seeds in the various concentrations of KCl. Therefore, the results of this experiment supported the hypothesis that raising the concentration of KCl would lower the rate of germination. The above results were observed because water was a crucial requirement for germination that aided in breaking seed dormancy.
Prior to germination, water got into the cells in the seeds through the process of osmosis with the seed coat (testa) acting as the semi permeable membrane. Deionized water was hypotonic with regard to the mung bean seeds.
Consequently, the seeds in deionized water cells took in water, which activated enzymes necessary for the hydrolysis of stored food hence providing energy for germination. The KCl solutions were hypertonic in relation to the cells in the mung beans. It was expected that the seeds would lose water to the salt solutions. However, the dormant seeds were completely dry and did not have any water to be lost. Water was unable to get into the seeds via osmosis. Consequently, seed dormancy was not broken in appreciable quantities, and that was why low rates of germination were witnessed.
It was also observed that there was no germination on the first day in the control and the various KCl solutions on the first day. The lack of growth during that time could be attributed to the irregularity of imbibition, which meant that water was not taken in by the seeds uniformly. In addition, the seed coat offered some resistance to the absorption of water. As a result, the occurrence of activities that led to the breaking of dormancy (activation of enzymes, hormones as well as the growth and proliferation of novel actively dividing cells) did not occur at the same time. If that were not the case, all the seeds would have germinated on the first day.
The results of the experiment agreed with previous studies regarding the effect of salts on the rate of germination. In other similar studies, it was evident that soil salinity affected the rate of germination of seeds. For example, a study by Abari (2011) showed that increasing the concentrations of NaCl and KCl led to a substantial decrease in the rate of germination of Acacia seeds, which also fell under dicotyledons.
The effects observed by Abari (2011) were similar to what was observed with 0.5M KCl in this experiment. A separate study by Li et al. (2010) revealed that sodium chloride and sodium sulfate also inhibited the germination of Medicago sativa L., a forage crop that was classified under monocotyledons. Under natural environmental conditions, the soil contained more than one type of ion, which interacted in causing osmotic stress to plants. Therefore, experiments that investigated the effect of more than one type of salt on germination were representative of the actual situation in the farms.
Soil salinity was a key abiotic factor that limited plant yield and was responsible for unproductive land that accounted for approximately 95 million hectares globally (Mine 2007). The effects of high salt solutions could be categorized into ionic and osmotic effects (Herr 2005). Ionic effects altered enzymatic processes and influenced the transportation of ions within the plant eventually leading to ion imbalance. A number of enzymes required cofactors and metal ions for complete catalytic activity.
The introduction of ions from salt solutions affected such enzymes by interfering with their cofactors. Since enzymatic activity was among the factors that prompted the germination, it was obvious that increasing salt concentrations would have adverse outcomes on seed germination. The various studies that looked at the effect of salts on germination all came to the deduction that an elevation in the concentration of salts inhibited germination. The studies experimented on the two key types of seeds namely monocotyledons and dicotyledons, and found that the effect of salt on germination was the same regardless of the type of seed.
Conclusion
Water was a vital requirement in the germination of plant cells. Osmosis facilitated the movement of water into the seeds in order to break dormancy and set off germination. The precise mechanisms for germination involved enzymatic activity and hormonal activity (gibberellins), which all relied on water. However, the presence of salts in water altered the osmotic pressure of the resulting solutions and influenced the rate of water intake by osmosis, which translated into a low rate of germination.
The magnitude of the salt effects was dependent on the concentration of the salt. High salt concentrations corresponded to lower rates of osmosis and poor germination rates. Conversely, low salt concentrations in solutions recorded higher rates of osmosis and germination. Overall, the concentration of salts was inversely proportional to the rate of germination. The ideal situation for germination was illustrated by the control where deionized water was used. However, it was almost impossible to find such a situation in the field since soil contained a wide array of dissolved salts. Therefore, it could be concluded that an increase in salinity had a negative effect on the germination of seeds.
Works Cited
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