Gorecki Center A, b & C, csb center for Global Education

For the observation of gravitropism and phototropism in pea plants we planted eight canisters with three peas each. When the peas began to grow we thinned each canister down to one plant each. After we had grown them for 10 days they were large enough for us to begin recording our time lapse video. We recorded these phenomena in a dark room in order to prevent other light sources from affecting the peas. We recorded the gravitropism in the pea plants by placing two pea plant canisters on their sides under a standard lamp as a light source and observing their growth. For phototropism we set two pea plant canisters in a dark chamber with a light source located to the right of the plants, oriented horizontal to the plants, or at a ninety degree angle from the plants’ stems. We used a digital camera to record our time-lapse video for gravitropism and a digital web cam for the phototropic plants. Each set up was prepared so that the cameras would take a picture every minute for a 24 hour time span. After gathering the data images, we created and analyzed the videos.

The video images showed the pea plants growing towards their light source, with the gravitropic plants curving upward and the phototropic plants curving toward. The time lapse images were used to evaluate the amount and rate of curvature of the peas throughout the 24 hour period. The video clips were edited to use every tenth frame and were played at half speed in order maximize visualization of the plant’s growth and increase video quality. The phototropic peas responded well to the light source, with three main leaf features tilting 62.55 degrees, 70.79 degrees, and 93.65 degrees toward the light source. Average curvature, or tilt change, of the pea plants was 75.66 degrees with an average tilt of 0.051 degrees per minute. For the gravitropic peas the average rate of curvature was 0.106 degrees per minute, curving from 152.14 degrees (almost horizontal) to -16.74 degrees from straight up.

In conclusion, our hypotheses appear correct in that the pea plants curved upward toward the light source when placed on their sides and tilted toward the light source when the light struck from one side. The pea plants responded both gravitropically and phototropically depending on their orientations and the orientations of the light sources. The rates of curvature were different for the pea plants for the different environments, with the gravitropic peas responding faster, 0.106 degrees/min vs. 0.051 degrees/min. Further study could determine whether gravitropic responses would be consistently faster than phototropic responses, or if our data were a result of the specific leaves measured or the individual plants.

Alexa Goetsch & Ali Niesen, Biology Department, College of Saint Benedict/ St. John’s University, Collegeville MN 56321

The purpose of this project was to create and analyze a time lapse movie to measure the rate of phototropic curvature of a specific plant. A time lapse video was used in order to capture the slow moving phenomenon that cannot be captured by the naked eye. By taking multiple pictures over a specific amount of time, one is able to see how the plant responds and is able to measure the rate of phototropic curvature.

The phenomenon we studied was the phototropic curvature of a radish plant. Phototropism is a growth response to light. A plant that grows toward a light source is known as a positively phototactic plant while a plant that grows away from a light source is known as a negatively phototactic plant. Most plants are positively phototactic and grow towards a light source when their chemical auxin reacts in its cells and elongates them. The plant then curves toward the light source due to its light-liking hormone auxin. The auxin hormone is located in areas of the plant that are in the darkness which causes the areas in the light to curve while the rest of the plant is growing in size. Auxin also decreases the pH, making the plant more acidic and easier to break down bonds in the cell. The breaking of bonds affects the cellulose in the cell walls and causes strength of the wall to decrease and eventually curve towards the light source.

We studied this phenomenon by planting three radish containers to film for our time lapse movie. Three radish seeds were placed in each of the three film containers for growth. Fertilizer and soil was added appropriately and they were watered for approximately two weeks to reach germination for our movie. They were thinned to only one seed each and the most successful plant was used for the movie when the stem was two inches in height. Before filming the movie, the radish plant was vertically up and down and there was no curvature seen in the stem. We then placed the radish plant in a dark room that was unoccupied for eight hours. There was a constant light source present that was one foot away from the radish plant. A digital camera took a picture of the radish plant every thirty seconds for eight hours. When we returned to the radish plant, there was evident curvature and the results of the pictures taken supported our phenomenon. We then analyzed our movie using ImageJ to measure the degrees of curvature that the plant exhibited over time. The degrees were then used to create a graph that showed the rate of curvature verses time of our radish plant.

The results of curvature supported our phenomenon of phototropism. We can conclude that radish plants exhibit positive phototropism and that curvature of the stem will occur towards a light source.

In order to complete this study, four sets of two cucumber seeds were plants in film canisters. The cucumber seeds were packaged by the Farmer and Seed Nursery Co. in 2011. The seeds were germinated in after about two days under growth lights and were grown for another five days afterwards. Once the seedlings had sprouted only their cotyledons and were about an inch above the top of the film canister at their tallest point, two seedlings were placed on their sides and held in position by ring clamps. Using a Nikon Coolpix digital camera, photos of the two cucumber seedlings were taken every thirty seconds for fifteen hours, or 900 minutes. These photos were compiled into a time lapse film, which was then analyzed via the Image J program. The angle of curvature, from the base of the seedling to the tip of the hypocotyl, was measured at various intervals using Image J for both seedlings. The curvature (degrees) was averaged between the two seedlings and plotted against time (minutes) and a best-fit trend line was added to the plot.

Our results show that, on average, the cucumber seedlings had grown 81.53 degrees upward after 900 minutes. The rate of curvature, which was determined from the slope of best-fit trend line of the average curvature (degrees) versus time (minutes), was found to be 0.0844 degrees per minute. Moreover, the correlation coefficient, R2, of time and average curvature was 0.78067, a fairly high correlation. This indicates that the seedlings moved at a relatively stable rate. However, according to Figure 1, the seedlings moved more quickly earlier on, and after about 300 minutes, the rate slowed. This likely occurred because, according to the phenomenon of gravitropism, plants generally “want” to grow upwards, towards a light source. Therefore, after the seedlings neared a ninety degree increase, the rate of growth slowed down, as the seedlings were almost growing completely upward.

Our results suggest that cucumber plants clearly exhibit gravitropism. Moreover, they also suggest that cucumber seedlings will grow and move about 0.08 degrees per minute upward when placed on their sides. Furthermore, our results seem to indicate that plants, at least cucumber plants, will grow display gravitropism growth only until their apical meristems are pointing directly upwards, or rather, if they are placed on their sides, they will only grow upwards about 80 to 90 degrees. This is likely due to the fact that, usually, a plant’s light source, whether that is the sun or grow lights, is directly above the plants.

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