Phototropism is the movement of plants with relation to unidirectional light. Most flowering plants, ferns and conifers exhibit positive phototropism. This means that movement towards the light source is the dominant response. Phototropism mainly enables cotyledons and emerging leaves to absorb adequate light for the process of photosynthesis, before the exhaustion of a seedling’s food reserves. There is evidence of movement of plants to other stimuli including gravity and water. For example, gravitropism is the process whereby a plant grows towards the direction of gravity. This enables especially the root tip quickly to penetrate the soil, giving the plant anchorage and access to water. (Baskin, 1986)
Growth movement is the process whereby the plant grows in the direction, either away from or towards a stimulus. This is usually used concurrently with the term tropism. Positive growth movement occurs when the plant grows toward the stimulus, whereas negative growth movement occurs when the plant grows away from the stimulus. Turgor movements are relatively quick plant movements that result from changes in internal water pressure. Examples include stomatal opening and closure, and the sudden movements of the sensitive plant’s leaves or the Venus flytrap’s insect-capturing leaves. Studying tropisms is important in plant science because it enables us to study plants in a deeper perspective and to understand that the movements plants exhibit are never random, they usually have an underlying mechanism. (Atwell et al, 1999)
In 1880, Charles Darwin and his son Francis used grass seedlings in an experiment to investigate the response of plants to light and what causes the plants to move towards unidirectional light. A similar experiment was designed and conducted to prove the hypothesis that light affects the direction of movement of plants as described below. The experiment was conducted to investigate the response of sunflower meristems to light stimulus. The experiment includes the observation of the meristems exposed to varying conditions and serial measurement of the various angles of deviation of the meristems towards the light source. The results from these observations are then obtained, analyzed and discussed.
Sunflower seedlings were used. Eighteen sunflower seedlings grown in similar conditions consisting of soil rich in nutrients and with adequate water and light were selected. These seedlings were then placed in three sets of six seedlings each. The first set of six seedlings had black plastic wraps around the meristems of each seedling. This set of seedlings was known as the negative control and labelled NC. This set of seedlings were deprived of the normal conditions, in this case, the meristems were deprived of light. The second set of six seedlings had clear plastic wraps around the meristem of each seedling and the seedlings in this set were labelled as the test control (TC).
This set of seedlings were observed to find out if variation of conditions, in this case the presence of a clear plastic wrap around the meristem, would affect the degree of response of plants to light stimulus. The third set of seedlings had no plastic wraps around their meristems. This was known as the positive control (PC). This set had all conditions normal and none altered. Each set was placed in a different box. Each box had only one side open to allow entry of sufficient artificial light. A box for data collection contained three seedlings consisting of one seedling from each of the sets. The seedlings were grown under observation for 16 days. Data was collected by determining the angle of deviation of the meristem of each seedling from each set, towards the light stimulus, using a protractor. Data was collected on days 1, 4, 8, 12 and 16.
The results obtained indicate that the measurements obtained on day1indicate that the meristems of sunflower seedlings from all three sets had no deviation and remained at 90 degrees.
On day 4 however the seedling from the positive control had its meristem deviated towards the open part of the box that let light in and was at 70 degrees. The seedling from the test control set with clear plastic wrap also had its meristem deviated at 70 degrees. The seedling from the set labelled negative control had no deviation remaining at 90 degrees.
On day 8, the seedling from the positive control set had its meristem deviated to 60 degrees, that from the test control set had its meristem deviated at 65 degrees while the seedling from the negative control set did not have its meristem deviated but remained at 90 degrees.
On day 12, the seedling from the positive control set had its meristem deviated to 55 degrees, that from the test control set had its meristem deviated at 55 degrees while the seedling from the negative control set did not have its meristem deviated but remained at 90 degrees.
On day 16, the seedling from the positive control set had its meristem deviated at 30 degrees, the seedling from the test control set had its meristem deviated at 30 degrees and the seedling from the negative control set did not have its meristem deviated at all as it remained at 90 degrees.
The table below shows the results obtained from measurement of the angles of deviation of the meristems of each seedling from each set, serially on different days on days 1, 4, 8, 12 and on day 16.
|Treatment||Census #||Day||Angle (degrees)|
|Positive Control Seedling (PC)||1||1||90|
|Negative Control Seedling (NC)||1||1||90|
|Test Seedling (TS)||1||1||90|
The graph below represents the angle in degrees to which the meristem of each seedling from each set deviated
From the results obtained, it is clearly evident that plants move in response to light, in the direction of the light source. The serial measurements of the angle deviated from the normal 90 degrees indicate that the meristem of seedling from the positive control set had deviated further away from 90 degrees from the previous day when measurements were taken. This seedling was grown under normal general conditions in which plants grow and is a good proof of the hypothesis that plants move in the direction of unidirectional light, as proposed by Charles Darwin and his son Francis.
The results also indicate that the test control seedling which had its meristem wrapped in clear plastic wrap deviated towards the light source. The clear plastic wrap allows light to reach the meristem of the seedling since it is clear and the meristem is therefore able to respond adequately to the light stimulus. It is notable that on day 8, the meristem of the test control seedling had deviated 5 degrees less than that of the positive control. This is probably because the clear plastic wrap alters the wavelength of the light reaching the meristem and therefore limits its response to light stimulus. (Atwell et al, 1999)
The seedling from the negative control set did not have its meristem deviate at all and all measurements obtained had the angle measurement maintained at 90 degrees. This is because the black plastic wrap prevented entry of light and prevented the light stimulus from reaching the seedling. Therefore, there was no deviation of the meristem since there was no source of light.
Charles Darwin and his son Francis concluded that plans move in the direction of light because they have a hormone called auxin which is produced at the tip of the plant and moves to the lower parts of the plant and is especially present in higher concentrations in the side of the plant which is darker, making it grow faster and making the plant to eventually bend towards light Phototropism is said to occur in three stages: light perception, transduction and curvature. In the first stage of light perception, light is perceived and absorbed by the plant. Different plants are also known to absorb light at different gradients and this therefore explains why a different color spectrum of light at the light source is more likely to give a different results (Wada et al, 2007)
Illuminating a seedling from one end creates a light gradient across the width of the stem. Transduction of the light then follows; this facilitates transmission of the light energy throughout the plant stem. The final step seen is then the curvature whereby the plants will the form a curve towards the light to enable it to obtain more light from the same source. This is what occurs in phototropism (Atwell et al, 1999)
In conclusion, the experiment was successful and all our objectives were met. The hypothesis that plants move in the direction of light was proved by the results obtained in the experiment. It was also found out that variations of the general normal conditions present in the environment affects the response of plants to normal stimuli, in this case light. Other parameters of light including spectrum and wavelength were discovered to be of impact in phototropism as well.
Atwell B.J, Kriedemann P .E, Turnbull C.G, 1999, Plants in action, Adaptation in nature,
Performance in cultivation Macmillan Education Australia Pty Ltd, Melbourne, Australia.
Baskin T.I, 1986, Phototropism: Light and growth, Stanford University
Wada M, Shimazaki K, Iino M, 2007, Light Sensing in Plants Springer Science & Business