Photosynthesis is the chemical reaction which converts the chemical carbon dioxide into different type of organic compounds through solar energy. Photosynthesis can occur in the presence or absence of light; however, the amount of light that is captured by the plant cells can affect plant morphology. Light, therefore, is an important and central factor to the morphology of plant shape, size and development, as well as for the development and function of the ecosystem, as oxygen is an essential element for living things. Therefore, it is important to investigate factors that affect photosynthesis for environmental and ecological reasons. In addition, this type of information could be essential for agricultural companies, as well as greenhouse development.
The amount of light that reaches a plant sets the photomorphogenic pathway that maintains the plant. Wavelengths of light stimulate photosynthesis and change the mechanisms that regulate plant morphology. The amount of light, as well as wavelength can change morphological factors of a plant, such as the length of internodes, leaf position or expansion of leaves. It has been found that there are specific molecules which are responsible for capturing photons of different wavelengths and signaling changes in morphology of the plant. The signaling pathways from the start of photons have been investigated since the 1930s. Lewis H. Flint (1936) studied the light quality effects on plant germination using red light. Further investigations using spectrographs at different wavelengths compared red light with blue and green. It was found that plants exposed to blue and green wavelengths resulted in germination inhibition. (Flint and McAlister, 1937)
The ability for plants to change in morphology in the dark was found through the investigation of phytochromes. Chen et al., (2004), found that phytochromes change between two different conformational states when in the light versus dark. In the dark, the phytochromes were found to be in the form referred to as Pr; whereas, in the light the form is Pfr. It was also found that the phytochromes would switch back and forth when exposed to the differing wavelengths of light. Phytochromes are therefore essential to the development of the plant as they can absorb all wavelengths of lights with the Ultraviolet spectrum, as well as no light. The Phytochromes are responsible for stem elongation, leaf expansion and changes in the plant morphology, as well as flowering. (Najafpour, 2012) Further investigation has found that phytochromes are responsible for the regulation of genes encoding for proteins in the plant when exposed to light or dark environments. Researchers have found that there is a phy gene activation occurring within minutes of exposure to light. (Tepperman et al., 2001; Tepperman et al., 2004)
To evaluate the morphological changes that occur in plants when exposed to light and dark environmental conditions, the current research paper investigates the growth of shoots from different seedlings. Light control starts with seedlings, as seedlings react to light. The purpose of this lab was to compare the shoot growth of seedlings in light versus dark environments at 4, 5, and 6 day intervals. The experimental variable in this experiment is light, with the response variable shoot size. In addition, the controlled variables in this experiment are plant cup size, soil, and temperature. It was hypothesized that the shoots in the dark environment and at 6 days would result in greater shoot sizes due to the differences in light and dark developmental pathways. Light developmental pathways lead to photomorphogenesis of the shoots. Photomorphogenesis results in larger leaves in order to obtain as much sunlight as possible and increase photosynthesis of the plant cells. Dark developmental pathway results in small leaves and elongation of cells. (Armin and Deng, 1996)
Twenty green bean plants seeds were obtained and one seed was placed in a one cup plastic cup containing Miracle-Gro® soil. Each cup contained ½ cup of soil and was labeled with a number starting with “1 Light” or “1 Dark”. Ten cups were used for both the “Light” and “Dark” part of the experiment in order for replication and to have a larger sample size in order to conduct a statistical analysis. In addition, each cup was watered daily with 25 ml of water using a medicine syringe. On day 1, the cups labeled as “Light” were placed on a window sill at room temperature. These plants received direct sunlight for at least 6 hours per day. The cups labeled as “Dark” were placed in the dark at room temperature with no light exposure. The shoot heights of each bean plant were measured using a centimeter ruler and recorded at 4, 5 and 6 days. The average and standard deviation of the shoot length were calculated for light and dark plants at 4, 5, and 6 days. A One-Way ANOVA analysis was performed in order to see if there was a significant difference between the shoots in the light vs. dark environments at the differing days.
The shoot heights for plants exposed to the dark environmental setting were smaller compared to the light environment. As seen in Table 1 and 2, the average height of the shoot at day 4 for the light environment was 0.53 cm versus 1.31 cm for the dark environment. At day 5 the average shoot height of seedlings in light was 0.77 cm versus 2.17 cm and at day 6, the 1.28 cm versus 2.53 cm. This indicates that the seedling shoots had a greater height at each Trial Day when exposed to dark conditions.
A One-Way ANOVA analysis was performed between days 4, 5, and 6 for light versus dark plants. As illustrated in Table 2, there was a significant difference between average shoot heights at day 4, 5, and 6 in light versus dark environments with the P-value less than 0.05 for each Trial day.
The shoot heights for green plant seeds were smaller in the light environment compared to the dark environment. This indicates that the seedling shoots were elongated when exposed to the dark environmental condition. This corresponds to the hypothesis in which it was predicted that the shoots in the dark environment would result in longer shoot sizes. The results of this experiment can be correlated to the developmental pathways in which plants undergo according to light exposure. It can be further indicated that the seedlings that were exposed to direct sunlight conditions for at least six hours a day, most likely underwent the light development pathway. The light development pathway activates photomorphogenesis, which allows for seed morphology to develop or change according to best fit optimization for photosynthesis. For instance, such as growing leaves or elongating cells. The seeds contained in the dark environment experienced the dark developmental pathway, more commonly known as etiolation. Etiolation increases cell elongation in the shoots with little leaves because the plant is elongating in order to reach light environments. (Armin and Deng, 2008; Alabadi, 1996)
The results of this experiment support the hypothesis that the shoot heights of the plant would be longer in dark exposure after six days, further predicting the occurrence of the activation of the light and dark developmental pathways. The activation of these pathways have been previously researched through the study of phytochromes. Two different phytochromes receptors, phyA and phyB have been found to represent stem elongation. Parks et al. (1998) found that these receptors control stem elongation depending upon the wavelength exposure (such as red or blue wavelengths). Further investigations of phyA and phyB receptors resulted in the finding of Phytochrome Interacting Factors (PIFs), which bind to the receptors upon light exposure. In addition, it was found that some PIFs would actually degrade and inhibit photomorphogenesis in the dark. (Leivar et al., 2008).
In order to further research the differences in plant growth in light and dark conditions, more variables should be tested, such as leaf expansion. Shoot height was longer in length for seeds exposed to dark conditions; however, leaf expansion was not tested in order to verify that light exposure causes morphological changes such as expansion of leaves for increased photosynthesis. In addition, other factors such as room temperature, weather and amount of sunlight should be recorded and evaluated. The amount of sunlight each day was not recorded and therefore, it is unknown if there was cloud coverage, or cloudy days that might have factored into the seed shoot growth. Furthermore, the use of specific wavelengths on seedling growth should be tested as well since previous literature has indicated a difference between wavelength exposure and plant growth at different Ultraviolet wavelengths.
Overall, light exposure has shown to be an important physical factor to shoot growth with possible implications at the molecular level, physical and genetic level. More studies on the receptors at different wavelengths could be helpful to industries such as greenhouses or agricultural companies for more efficient and effective means of plant development and morphology.
Table 1. Results of Shoot Height of Green Bean Plant Seeds Grown in the light versus Dark Environment at Days 4, 5, and 6. Average and Standard Deviations are calculated for each day.
Table 2. ANOVA Results for Average Shoots (cm) between Light and Dark Environmental Conditions for Trial Days 4, 5 and 6. *Indicates significantly different. P-Vales < 0.05.
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