Effect of Light Color on Plants

Introduction Living organisms have been classified by humans according to several different characteristics, one of them being the manner in which they obtain nutritive organic molecules (1), in other words, their food. Plants are denominated as autotrophs, organisms that can produce their own food from abiotic sources surrounding them, such as light, carbon dioxide and water (2). Plants use these three factors in photosynthesis to convert light energy into chemical energy, which is then stored until the plant needs it, or used right away (3). The process of photosynthesis takes place on organelles (substructures inside eukaryotic cells) called chloroplasts, more specifically, on the membrane of the thylakoids inside the chloroplasts, where protein complexes known as photosystems are situated. It’s these complexes that are responsible for the photosynthetic processes. Because this is the main way in which plants obtain their food, and light energy is the base of said process, light is absolutely essential for their growth. Sunlight contains many different wavelengths which, when isolated, present different colors of visible light. Plants cannot use all wavelengths of light, however, because the different chlorophylls (pigments in the plants’ chloroplasts, also responsible for photosynthesis) absorb and use only certain wavelengths of light ; the rest is reflected back out, unused (4). Exposed to sunlight, plants have available to them, a range of wavelengths wider than the light spectrum visible to us, from which they utilize only a select range for photosynthesizing. This essay will investigate the effect light color/wavelength has on plant germination and rate of photosynthesis. Cellophane sheets of different colors (red, yellow, blue, green) and clear plastic wrap were used to provide these limited wavelengths . The experiments dealing with germination consisted of different seeds being planted separately from other species and being covered by the different colored cellophane or the clear wrap; they were then watered and monitored for a week, taking note of any growth. The plants used were the common bean (Phaseolus vulgaris), brown mustard (Brassica juncea), and common oats (Avena sativa). For the photosynthesis rate experiment, same sized Brazilian waterweeds (Egeria densa) were inserted in eudiometer tubes filled with water and then placed in a beaker half-filled with water. Each test tube was wrapped with a different color, and the volume of oxygen produced, through photosynthesis, by each of the plants was compared. Beans, specifically all the plants under the genus Phaseolus, are amongst the fastest growing plants and are said to germinate within a week. To make them germinate faster, it’s usually recommended to soak them in water before planting because hydrating the seeds stimulates germination. (5) They also benefit from warm temperatures because they are native to more tropical climates (6). They need to be planted in well drained soil because they aren’t tolerant to water excess. Mustard is also among the fastest sprouting plants, but its growth afterwards is relatively slow as it’s meant to grow flower buds five weeks later. It does better in cool conditions than warm and can usually withstand short periods of mild drought with little to now consequences. In addition, it too, is not tolerant to water clogging in the soil (7). Oats are native to warm, sub-tropical regions and can manage well in poor soils (8) but they also require good water draining . The Experiments Germination Fifteen equally sized plastic containers (cylindrical in shape, approximately 8cm in diameter and 6cm in height) were filled with the same amount of soil. Five of them were allotted to each plant species; making sure to add the same amount of seeds in each segment (comparing to the same species, since the size of the beans and oats is much greater than the brown mustard seeds and would therefore be unreasonable to compare their numbers). Each of the containers was covered with cellophane of a different color (red, blue, yellow, green, or clear plastic wrap) and secured with a rubber band so that each species had a sample exposed to each different light color. They were watered with normal tap water, placed next to the same windowpane for a week and monitored every day. The cellophane secured with the rubber band created a seal that was too tight, preventing excess water from evaporating which overloaded the plants with water and deprived them of enough oxygen. Towards the end of the week, the plants were left covered by the cellophane, but unsecured by the rubber band so that the water was able to evaporate, to provide the dry mass of the samples, which were on average 0. 06g lighter than at the beginning of the week, when they were planted . The mass measured included the seeds, soil and plastic container. Even though an unknown type of fungus started growing in the containers because of the high humidity and warm temperature, some shoots were visible (Table 1). Cellophane ColorQualitative Data Red Some oat and mustard shoots, fungus growth Yellow Fungus growth, no shoots visible Green Oat and some mustard shoots visible, some fungus growth Blue Some oat and few mustard shoots, fungus growth Clear Fungus growth, no shoots visible These samples were discarded and a new method was devised: A cardboard egg container was cut into fifteen separate segments (for the five colors for each of the three species) which were to be used instead of the plastic containers to allow excess water to evaporate through the porous walls and bottom of the cardboard. These segments were then loosely filled in with cotton-wool. After the first experiment, where seeds drowned and fungus started to grow instead because of the excess of water, cotton seemed the best substitute for soil; it would allow for even dispersal of water, a lot of more light would be able to reach the seeds, it would be easier to observe the germination process, and since the cotton didn’t weigh practically anything, it would be easy to measure the change of plant mass on a scale. The segments were weighed once they had the cotton and seeds in them; their mass was noted down individually. A thin layer of cotton was placed on top of the seeds so they would retain moisture above them as well as below, but still allowing light to reach them. Then, the segments were watered with a handheld sprayer until the cotton was thoroughly moist and then were weighed again. To provide the different light coloring, one sample of each species was placed underneath a handmade cover fashioned out of bamboo and cellophane. These covers (Figure 1) provided the needed space for the plants to grow, free flow of air, and control of light shining on the plants. The cotton wool wasn’t a good substitute for soil since it retained little water and evaporated too quickly for the plants to take it in; it mostly bled into the cardboard, which, because of its porous nature, allowed for further evaporation of water. The results are shown in the following table: Color of CellophaneQualitative Data Red Few mustard shoots + very few oat shoots Yellow Very few mustard shoots Green Mustard shoots* + oat shoots ~4cm Blue Mustard shoots* + oat shoots ~3cm Clear Mustard shoots* + oat shoots ~6cm New cardboard segments were cut out and filled in with soil this time. Care was taken to add the same amount of soil (~ 4. 25g) and water (2. 0g) to each container at the beginning of the experiment. The following three days, the amount of water was altered from day to day to find the adequate amount of water that could be retained by the soil without too much bleeding into the cardboard container (since this would weaken its structure and remain unused by the plant); the conclusion was derived that the amount of water should be of the same mass as half of the soil in the container in order to maximize water intake by the plant: about 2. 5g. The results can be seen in Table 3 on the following page. Color of CellophaneQualitative Data Red Mustard sprouts* Yellow No growth Green Mustard sprouts * + oat shoots ~5cm Blue Mustard sprouts* + oat shoots ~8cm Clear Mustard sprouts* + oat shoots ~8cm Photosynthesis Rate The rate of photosynthesis was measured through the volume of oxygen produced in a set amount of time. To do this, six eudiometer tubes were wrapped with different color cellophane (one was left bare, to be the control) and filled with tap water. Next, six strands of Brazilian waterweeds were cut to the same size, each inserted into an eudiometer tube and then placed in a half filled beaker. The eudiometers were held with clamps on a stand and left next to wide windows to photosynthesize at their own rate. The oxygen produced by the plants floated to the surface creating bubbles that could then be measured and compared to each other. The sunlight provided was insufficient however, and the oxygen produced was too little to be measured accurately, but there were clear differences between the different samples; the waterweed in the red light was the one the produced most oxygen, followed by the green, then the blue, yellow, and clear. The control, the one without any cover, was the one that produced the least oxygen. After the experimentation, the data collected was reviewed to determine the most beneficial color for plant germination and photosynthesis rate. Taking the data from each week, the cellophane colors were arranged in order of effectiveness, to make it easier to compare the results (Table 4). Germination Photosynthesis Rate Week 1 Week 2Week 3 Red Green* Clear* Blue* Green Red* Green* Clear* Blue Blue* Blue* Green* Yellow Clear Red Red* Clear YelllowYellow Yellow No cover