Abstract
Allelopathy is an interaction between organisms of the same or different species where chemical substances produced by one organism exert an effect on the physiological processes of a plant. The observed effects may be negative or positive. Allelopathy plays a vital role in plant interactions as well as species distribution. This experiment aimed to determine allelopathy in the germination and growth of sunflower seeds (Helianthus annuus).
Sunflower seeds were subjected to density treatments by growing them at a density of one seed per pot in the control and three seeds per pot in the experimental group. A total of 36 seeds were used in each group. The germination rate and mean shoot height were measured after 7 days. The treatment group had a higher germination rate and mean shoot height (84.26% and 9.49 cm respectively) compared to the control (77.77% and 6.47 cm respectively). The differences in the mean shoot heights between the groups were statistically significant (p<0.05). These effects were attributed to the role of gibberellins as allelochemicals in the germination of sunflower seeds.
Introduction
An interesting observation in nature is that specific plants only grow in certain areas. This occurrence could be explained by the fact that environmental conditions and soils in certain areas do not favor the growth of some plants. However, studies have shown that apart from soils and climatic conditions, chemical interactions between different plant species also contribute to this observation (Jabran, Mahajan, Sardana, & Chauhan, 2015).
Allelopathy is a widespread biological occurrence where one living organism secretes chemical substances that affect the development, existence, and propagation of other organisms (Duke, 2015). These biochemicals are referred to as allelochemicals and are often non-nutritive materials secreted by plants as secondary metabolites or decomposition upshots of microorganisms. Their effects on other plants may either be positive or negative.
Therefore, plants that produce these substances are called allelopathic plants. Allelopathy, which falls under chemical ecology, is a sub-discipline that relates to the consequences of chemicals secreted by living organisms on the growth and dissemination of other plants in different ecologies. Consequently, allelopathic interactions are considered among the major aspects that account for species distribution and dominance of intrusive plants (Mod, Heikkinen, Le Roux, Väre, & Luoto, 2016).
The positive effects of plant allelopathy include the regulation of agricultural practices such as weed management, crop defense mechanisms, and re-establishment of crops (Cheng & Cheng, 2015). Therefore, allelochemicals can hypothetically serve as growth control agents, herbicides, antimicrobial agents, and insecticides. In contrast, the adverse consequences of allelopathy include soil disorders, autotoxicity, and invasion by other living organisms. Therefore, sustainable agricultural advancement can be attained by taking advantage of cultivation systems that make the most out of the stimulatory or inhibitory impact of allelopathic plants to control the growth and development of plants as well as to circumvent allelopathic autotoxicity.
To understand the mode of action of allelochemicals, it is important to look into the specific physiological process that is affected during allelopathy. Germination, which is the first stage in the life cycle of plants, is an appropriate starting point to investigate the potential impact of various chemical substances as allelochemicals. Germination is the process whereby organisms grow from seeds following a period of dormancy. It involves three main stages of imbibition, the emergence of the radicle and plumule, and expansion of the cotyledons to form leaves. Germination is known to be inhibited by factors such as water availability, oxygen, and temperatures. However, a detailed look into the detailed mechanisms involved in the three stages reveals the involvement of chemical substances.
The purpose of this experiment was to determine the potential allelopathic effects of sunflower seeds on their germination and growth. It was hypothesized that sunflower seeds would exhibit allelopathic effects on their germination and growth. Sunflower seeds (Helianthus annuus) were grown in different density treatments to ascertain the hypothesized effect.
Methods
A total of 36 seeds were allocated to the control and treatment groups. For the control experiment, a density treatment of one seed per pot was done. The sunflower seeds were planted in holes approximately 1 inch deep in moist potting soil, covered with the soil, and allowed to germinate. Conversely, for the treatment group, three seeds were planted in each pot containing moist soil. The seeds were then observed after 7 days of germination. The total number of seeds that germinated was determined, whereas the shoot heights were measured in centimeters. A two-sample unpaired t-test at p=0.05 was used to analyze the data using the Statistical Package for Social Sciences (SPSS) version 22 software.
Results
It was observed that 84.26% of seeds in the experimental group and 77.77% of seeds in the control group germinated. The control group had a shorter mean shoot height (6.47 cm) than the treatment group (mean shoot height=9.49 cm). The statistical analysis showed that the differences between the mean shoot heights in the two groups were significant (p<0.05). The findings confirmed the hypothesis that sunflower seeds would exhibit allelopathic effects on their germination and growth. Figure 1 provides a graphical representation of the germination rates between the two groups. On the other hand, Table 1 shows the outcomes of the two-sample unpaired t-test.
Table 1: The Independent-sample t-test.
Discussion
The purpose of this experiment was to investigate the potential allelopathic effect of sunflower seeds on their germination and growth. It was noted that the treatment did not inhibit the germination of H. annuus seeds. These observations corroborated the hypothesis that H. annuus seeds would show allelopathic effects on their germination and growth. However, the allelopathic effect was positive (enhanced germination and growth).
The sunflower seed has a diploid embryonic sporophyte that is capable of germinating following a period of dormancy. However, imbibition, which is a process where seeds absorb large quantities of water, must take place before germination. Sunflower seeds show resilience and can subsequently grow within a range of temperatures and environmental conditions. During imbibition, the embryonic sporophyte enlarges and breaks through its seed coat.
At the same time, increased metabolic and hormonal activities lead to the elongation of the embryo’s cells. In a mature sunflower seed that is not in the dormant state, the multicellular embryonic sporophyte continues to grow and develop when rehydrated.
The rate of growth and development is dependent on the availability of biotic and abiotic factors that stimulate optimum seedling growth. Abiotic factors include temperatures between 70 and 78 ËšF in addition to the availability of adequate soil macronutrients, for example, nitrogen. As the seed gets ready to germinate, the synthesis of gibberellic acid is stimulated in addition to the breakdown of abscisic acid to enhance the disintegration of the food reserve found in the two cotyledons. In the experiment, the seeds in the control and experimental setups were grown under similar environmental conditions. Therefore, it was evident that the observed differences in the germination and growth rates were not due to differences in water, temperature, oxygen, or nitrogen.
Different chemical families are known to be allelochemicals. They include approximately 18 classes according to their chemical resemblances: alcohols, aliphatic aldehydes and ketones, benzoic acid, unsaturated lactones, alkaloids, fatty acids, quinones, phenols, coumarin, cinnamic acid, tannins, flavonoids, water-soluble organic acids, polyacetylenes, terpenoids, and steroids, amino acids, sulfide, and nucleic acids. Furthermore, plant growth regulators such as gibberellic acid, ethylene, and salicylic acid are also regarded as allelochemicals (Cheng & Cheng, 2015).
The germination process of the sunflower seed demonstrates the importance of gibberellins. It has been shown that gibberellins act as chemical messengers by communicating signals within the plant. However, as an allelochemical, it is expected that gibberellins from one plant would influence the biological activities of the other plant. It can be hypothesized that when three sunflower seeds were planted in one pot (the treatment group), the seeds were nearby of each other within the pot.
Gibberellins from one seed generated chemical signals that promoted starch hydrolysis in the cotyledons of adjacent seeds by inducing the production of the alpha-amylase enzyme. Consequently, there was an increased breakdown of stored starch to provide energy for germination. These events could explain why a higher germination rate was observed in the experimental group compared to the control category.
On the other hand, planting the seeds individually led to the physical separation of seeds, which prevented the described allelopathic interaction thus leading to a lower germination rate. This occurrence confirms the assertion of a previous study that chemical substances also influence the rate of germination and growth in plants (Gurung, Swamy, Sarkar, & Ubale, 2014). Gibberellins also play a role in the elongation of cells, which explains why the rate of growth as indicated by the mean shoot length was longer in the experimental group than in the control. Overall, allelopathy in sunflower seeds led to positive effects in the form of increased germination and growth rates.
The findings of this study opposed those reported by Hall, Blum, and Fites (1982) that increasing the density of H. annuus seeds led to inhibited germination and growth. In this study, Hall et al. (1982) attributed the observations to the production of phenolic compounds, which are known allelochemicals. However, in this experiment, the levels of phenolic compounds were not measured. Since a contrasting positive effect was observed on germination and growth, gibberellins were suspected to be the responsible allelochemicals. However, to confirm the involvement of gibberellins in the observed effects, future studies could repeat the experiment and measure the levels of gibberellins in the treatment and control.
Conclusion
Germination is a vital process in the life cycle of plants. However, the success of germination is affected by a combination of physical and chemical factors. Allelopathy occurs when chemical substances produced by one plant influence the growth of another plant. Therefore, this phenomenon contributes to the chemical factors that inhibit or promote germination. In the case of sunflower seeds, gibberellins, which are known to promote germination and growth of seeds, were probably responsible for the increased germination and growth rates when three sunflower seeds were grown in one pot. This experiment showed that gibberellins, as allelochemicals, did not inhibit the germination and growth of H. annuus seeds.
References
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