24 February 2015
The Role of Phytochrome in Lettuce Seed Germination
In order for any living plant seed to germinate, the presence of certain molecules are crucial for it to survive. Their functions dictate when conditions are right for optimal and proficient growth. When determining favorable conditions for seed germination, it is important to recognize different environmental factors that trigger new life. By observing the behavior of the molecule phytochrome, which is essential for seed germination, we can examine the molecule’s photoversible behavior in response to different exposure of light treatments. In the experiment, phytochrome germination versus its dormancy rate is studied under red and far-red light using three different varieties of lettuce seeds. Using white light and darkness as control variables, maximum seed germination occurred under red light at 97%, while far-red light exposure showed only 60% germination. This study shows phytochrome’s absorbance of red light to be most efficient causing germination among the lettuce seeds, while far-red light was less absorbed causing seeds to remain in a dormant state.
In order for seeds to germinate and begin their biological processes, they must first receive an additional source of energy to start new life. A proton is a form of energy that is emitted by all light. These protons travel in different concentrations or wavelengths, which depict the amount of energy. Different emissions of light travel on varying wavelengths and biotic organisms in an ecosystem may benefit from varying kinds of light exposure. In seeds, these wavelengths are absorbed by a pigment called phytochrome. This molecule is one of the prime factors responsible for seed germination in lettuce seeds. Phytochrome is a type of pigment inactive in a dormant lettuce seed, which grants it the ability to be light responsive to favorable wavelengths of energy. Because of this genetic trait, Phytochrome has two conformations, Pr and Pfr form (Taj, 1998). The ability to interconvert between these two forms is vital for environmental adaptation, which in fact will determine if the seed will remain in dormancy or begin germination. The first form Pr is triggered between the wavelengths of 650-680, which is red light, while between the wavelengths 710-850 far-red light is absorbed triggering the Pfr form (Taj, 1998). The seed’s photoreversible behavior allows phytochrome to act as a light sensitive mechanism interchanging between Pr and Pfr form.
Under red light, Pr is converted to Pfr form, switching from a dormant to a germination state. But, if far-red light is exposed to the Pfr form, seed germination will cease as it is converted back to the dormancy Pr state. Activation into the Pfr form happens quickly, while conversion back to the Pr form takes much longer. A study at Newcastle University proves in their photomanipulation of lettuce seed experiment that lettuce seeds exposed to red light, then exposed to far-red light had a high percentage of germination when placed in the dark (Kendrick and Russel, 1975). This is important when understanding phytochrome activation with seeds placed in darkness versus seeds exposed to white light, which are then placed into darkness for testing. White light contains both red and far-red light, so germination is triggered and there will be a level of germination when placed in darkness. But seeds placed in darkness should have no germination because they were never exposed to any absorbable light. The purpose of this experiment is to test the hypothesis- maximum seed germination will take place under red and white light exposure versus exposure to far red light and darkness with the null hypothesis being. Germination will not be affected by different types of light exposure.