Bio 207 Botany Projects on Plant Morphogenesis

BACKGROUND: Plant Morphogenesis Projects

In Spring 2009, students in BIO 207--General Botany did a number of experiments designed to demonstrate (or not) examples of plant morphogenesis, i.e. growing in ways that respond to differences in their environment.

Plants can adjust their growth and development in response to light availability, light quality, touch, gravity, grazing, chemicals, etc.

Examination of Phototropism in Zea mays, Poaceae

Background
- The direction of plant growth changes due to the direction of the sunlight (Botany: an introduction, 2009).
- Auxins (a plant hormone) on the side of the plant that is the furthest from the sun are activated which causes the cells to elongate in order for the plant to bend towards the sun ("Phototropism," 2010).
- The auxins release protons which lowers the pH in the cells on the shaded side of the plant. The low, acidic pH activates an enzyme called expansin which is responsible for breaking bonds in the structure of the cell. This makes the wall less rigid and more susceptible to elongation (Campbell, & Reece, 2005).
- Plants and fungi are known to practice phototropism ("Phototropism," 2010).

Hypothesis
- Owing to the theory of phototropism, when the potted Zea mays plants are positioned on their side in the container, the stalk will curve and continue to grow vertically towards the light source.

The Test:
- 8 Zea mays plants - Immediately following germination, four of the eight potted plants will be placed on their side while the other four remain upright. - All plants will receive the same amount of visible sunlight and water. - I predict that the plants will curve and continue to grow vertically instead of horizontally when they are place on their side.

Results
- 7 out of 8 plants germinated. - 4 of the 7 were placed on their side. - The remaining 3 of 7 remained upright. - The four subjects that had their state altered did indeed curve and grow vertically. Auxins and expansins were activated to allow the cell walls on the side of the plant furthest from the sun to elongate to compensate for the bending movement towards the light source. - The response variable measured was a qualitative analysis of the physical appearance of the plants.


Figure 1: The plants prior to tipping them on their side.

Figure 2: The plants after tipping them on their side.


Figure 3: T= ten days. Significant change is seen in the plants that were placed on their side.


Figure 4: Notice the prominent vertical growth rather than lateral which would be expected if a plant were to tip on its side.


Figure 5: No change in the plants that remained upright during the experiment.

Conclusions and Questions
- In Figures 3 and 4, a blatant change and phototropic response may be seen. After t=10 days the plants showed continuous vertical growth despite being tipped on their side.
- If the plants that were tipped were to be placed upright again, would there be another bend in the stem to account for the movement towards the light source once again?
- In an environment where a light source is absent, will this response hold true and cause the plants to continue to grow vertically when tipped on their side?

Further Information
http://apps.isiknowledge.com/full_record.do?product=UA&search_mode=GeneralSearch&qid=2&SID=1A@126bBK9iH5kGd6aj&page=1&doc=2&colname=MEDLINE
http://web.ebscohost.com/ehost/detail?vid=3&hid=5&sid=f5d8063e-4e73-432f-a1b6-faa48fe1d4a2%40sessionmgr14&bdata=JnNpdGU9ZWhvc3QtbGl2ZQ%3d%3d#db=a9h&AN=48223918
Iino, M, Schafer, E, & Briggs, W.R. (1984). Photoperception sites for phytochrome-mediated phototropism of maize mesocotyls. Planta, 162(5), Retrieved from http://journals.ohiolink.edu/ejc/pdf.cgi/Iino_Moritoshi.pdf?issn=00320935&issue=v162i0005&article=477_psfppomm
This paper proves that the site of bending in the Zea mays plant is in the mesocotyls. The researchers performed a very similar experiment to the one stated here and derived the same results.

References
1. Botany: an introduction to plant biology. (2009). Sudbury, Massachusetts: Jones and Bartlett Publishers.
2. Phototropism. (2010, March 24). Retrieved from http://en.wikipedia.org/wiki/Phototropism
3. Campbell, N.A., & Reece, J.B. (2005). Biology. San Francisco: Pearson Benjamin Cummings

Etiolation Exhibited by Zea mays in the Laboratory

Background on Etiolation:
Etiolation occurs when plants are grown in a dark environment or an environment lacking normal light levels. As a result, the plant is not as green but a pale yellow color (this is characterized as chlorosis), it exhibits reduced leaf development, and weak stems. All of these changes in plant physiology conserve energy until the plant reaches sunlight and will then experience rapid growth. The mechanism of etiolation allows plants growing in a dark environment, such as leaf litter, to reach sunlight in a more efficient manner while expending the least amount of energy. Also, the apical meristems on such plants are strongly attracted to light and will exhibit phototropism in order to quickly reach a light source [1].

Zea mays, a member of the family Poaceae, was selected to take part in a study focusing on the physiological effects of etiolation on plant growth. This plant species is a dominant agricultural corn plant and grows throughout the United States. The success of Zea mays is vital for the support of the agricultural industry [2].

Hypothesis:
The plant growing in the dark environment will exhibit all of the normal characteristics of etiolation as opposed to the plants growing in the full sun environment. The lack of sufficient light levels in the dark environment will cause the plant to produce less leaves and have less photosynthetic cells resulting in pale coloration.

Procedures:
Eight corn plants (Zea mays) were used. Four were placed in an environment with full sun and four were placed in a dark environment. All eight plants remained in a green house for the duration of the experiment for exposure to controlled temperature and water treatment. Both sets of plants were germinated and grown in the same soil and type of container. Once all eight of the plants were germinated, four were placed in the full sun environment and four were placed in the dark environment.

Results:
The results supported the hypothesis such that plants growing in the dark environment (Figure 1) had less leaves and were unable to support their weight and remain vertical (Table 1). Plants in the sunlit environment (Figure 2) had more leaves and were able to remain in the upright position without any additional support. A considerable difference was documented in leaf development. Shade plants had fewer leaves compared to the sun plants (Table 2). Also, color variation was present between the two environments. Plants from the dark environment were white to yellow, while the plants grown in the sunlit environment were green.

Table 1: Plant Height.

Table 2: Leaf Development.


Response Variable(s) Measured:
Qualitative and quantitative responses were measured. The color of each plant was compared between the two light environments (Figure 1 and 2) and at the conclusion of the study, plant height (Table 1) and leaf development (Table 2) were documented.

Figure 1: Plants Grown in Dark Environment.


Figure 2: Plants Grown in Light Environment.


Conclusions and Questions:
The conclusion for this study supports the hypothesis. The plants growing in a dark environment were unable to support their weight and were much lighter in color when compared to the plants grown in a sunny environment (Figure 1 and 2). Due to the lack of sufficient light intensity, the corn plants growing in the dark environment did not need to expend energy in the production of photosynthetic cells. Without these chloroplast-containing cells, the corn plants lacked the characteristic green color. Also, the reduced light levels limited the need for leaves—development of leaves would be futile without sufficient light levels to aid in photosynthesis. As a result the shade plants had, on average, half as many leaves as plants growing in the sunlight (Table 2).

Reasoning behind the shade plants’ inability to support their mass is not as well defined. The drooping behavior behind the plants in the light-limited environment may be another mechanism to preserve energy. Since these plants are not able to produce energy through photosynthesis, they are energy-limited. To help conserve their limited resources, less work (i.e. energy) is invested in supporting the plant. Once sunlight is reached, the plant will then be able to grow properly.

Further Research:
Symons, G. M., Smith, J. J., Nomura, T., Davies, N. W., Yokota, T., & Reid, J. B. (2008). The hormonal regulation of de-etiolation. Planta (Berlin), 227(5), 1115-1125.
-This paper focuses on de-etiolation (the change in plant environment from a dark environment to light environment). The focus of this study was spent on the role of hormones, mainly gibberellins, which help regulate the plant to adapt to the new light environment.

Symons, G. M., & Reid, B. (2003). Interactions Between Light and Plant Hormones During De-etiolation. Journal of Plant Growth Regulation, 22(1), 3 - 14.
-This paper also studied the effect of de-etiolation in plants. Specific areas of plant physiology which were analyzed were: rate of shoot elongation, opening of the apical hook, expansion of true leaves and the development of mature chloroplasts. Although the experiment performed separated the two light environments, studying the change in a single plant between various light environments will help aid in the understanding the differences in growth and development between the two environments.

Phaseolus vulgaris Plant Growth in Light and Dark Environments

Introduction

Figure 1:Etiolation in plants

Etiolation occurs in plants that are in minimal to low light. This is where chlorosis, or insufficient levels of chlorophyll, occurs causing the leaves turn a pale-yellow. Loss of chlorophyll, which causes the green coloring in plants, is what causes plants to lose their green coloring. Producing chlorophyll under low light conditions would be a waste of energy because there is too little light for photosynthesis Etiolation has been shows to cause many physiological changes within plants, including loss of chlorophyll, increased succulence, increased internode length and decreased strength of stems and tissues De-etiolation, or changes that prepare the plant for photosynthesis, occurs in plants that are in intense levels of light. It was predicted that the plants grown in the light environment would have more growth than those in the dark environment, if those in the dark had any growth at all.

Methods

Eight bean plants were grown of the species Phaseolus vulgaris. Four of them were grown in a light environment and four of them were grown in a dark environment. The light environment beans were placed in the living room next to the window sill, where they could obtain maximum light. Those in the dark environment were put in the basement where there was little to no light. The plants all started out in the light environment until they all germinated and then they were separated. This experiment was observed over ten days. The plants were watered when needed, which was approximately every few days. The temperature was relatively constant in both environments.

Results

Figure 2: Growth in Light and Dark Conditions

The plants in the light environment grew substantially more than those in the dark environment(fig 2). Plants grown in the light environment were taller, stronger and more green. This suggests that they may contain more chlorophyll than those grown under dark conditions. Not all of the plants had germinated until day seven of ten during this experiment. If this experiment were to be repeated, it would be watched over a longer period of time to give the plant more time to germinate.

Discussion

Etiolation experiments are important to understand how different plants change and adapt under different light conditions. It is important for people who garden to know which plants will do well in shaded conditions and which will not. These experiments also help scientists learn and understand the mechanisms behind the physical changes caused by etiolation, and the hormones that control it. To further understand etiolation, this experiment could be repeated with many different plants with different life histories in order to compare their adaptations.

Conclusion

This experiment is evidence that light is a very important factor in plant growth.

Apical dominance in beans

The Effects of Apical Dominance on Phaseolus vulgaris

Introduction

Apical dominance is the concept of plant growth where the main central stem is dominant over any lateral/axial growth. The apical bud produces the hormone, auxin which promotes the transfer of nutrients for growth into the region. Auxin also inhibits the growth of lateral buds so there will be no competition between the apical growth and lateral growth. If this apical bud is removed, the suppressive auxin is also removed allowing for lateral growth to occur. These lateral branches then compete to become the new apical meristem. In this experiment, the principle of apical dominance is tested using Phaseolus vulgaris from the Family Fabaceae or the common bean plant. The plants that have the apical meristems removed should show growth from the lateral buds, eventually leading to a bushier plant rather than being taller because there will be no apical meristem producing auxin to suppress the lateral growth.

Methods

Eight bean seeds were planted in separate pots and grown in the same conditions in the Ashland University greenhouse. These plants were allowed to grow to germination then to the point where the first two leaves had grown. Only 5 of the 8 planted germinated. The apical meristems were removed from half of a bean plant population after the first two leaves had grown (12 days). The population of bean plants that germinated was 5 plants and 3 of the plants had the apical meristem removed by scissors. The other 2 were allowed to grow normally and were the controls. Figure 2 shows the point at which the apical meristems were removed. Measurements of height were taken at the time of apical meristem removal, 1 week later, and two weeks later in centimeters. These measurements are shown in Figure 1. Pictures were also taken of the change in growth that occurred. Those pictures showing the change in growth are displayed in Figures 2-4.

Results

Fig. 1

Fig. 2

Controls (Initial)(left pic.) Experimental (Initial) (right pic.)

Fig. 3

Control (1 week later)(left pic.) Experimental (1 week later)(right pic.)

Fig. 4

Control (2 weeks later) (left pic.) Experimental (2 weeks later)(right pic.)

Discussion

One week after the apical buds were removed, those 3 plants began to show growth from the axial buds of the plant. The axial growth is the result of the lack of auxin produced by the apical meristem. The controls continued to grow from the apical meristem and were much farther along in their growth than the experimental plants. The reason plants usually suppress the growth of axillary buds is that apical dominance increases the plant’s exposure to sunlight. However, if that apical bud is removed or eaten off by an animal or if sunlight is much stronger on the edges of the plant rather than from straight above, the axillary buds end their dormancy and grow lateral shoots which in turn grow their own apical meristem, leaves, and axillary buds. This is why after 2 weeks the experimental and control plants had returned to near the same height. The original axillary buds on the experimental plants had grown their own apical meristems which resumed apical dominance and grew just as the controls did throughout (Campbell 2005).

Conclusion

Apical dominance can be interrupted through the removal of the apical meristem which produces auxin to suppress the growth of axillary buds. This allows the axillary buds to grow lateral shoots, creating a shorter, but wider plant. However, eventually the axillary buds grow their own apical meristems which resume apical dominance and act just as the unaffected plants do.

Future Experiments

Another experiment could be run over a longer period of time where the experimental plants could have every apical meristem removed as they grew. If the apical meristems were continuously removed as they appeared off of the lateral growth, it should produce a very wide, bushy plant. Also, this experiment could be tested on other plants to see if different plants react differently to the removal of the apical meristem. One more question could be if artificially auxin treated plants with their apical meristem removed would still show lateral growth.

References

¹Cambell, N. A., Reece, J.B. (2005) Biology. Pearson Education, Inc. San Francisco, CA

²http://journals.ohiolink.edu/ejc/pdf.cgi/Cline_Morris_G.pdf?issn=00319317&issue=v90i0001&article=230_trohiaaopipd Accessed 4/6/10 This article looks at the affects of auxin on the lateral growth of plants. The hypothesis is that auxin suppresses the lateral growth from axillary buds, resulting in apical dominance. The conclusion that is reached is that auxin is necessary for the activation of certain genes which prevent lateral growth. However, it is not the only hormone and further research must be done to discover those other hormones and stimuli.

³http://journals.ohiolink.edu/ejc/pdf.cgi/Naber_Allanah_C.pdf?issn=00030031&issue=v140i0001&article=42_eosarabarivt Accessed 4/7/10 This article looked at the effects on fitness that apical dominance had. The hypothesis was that there is a fitness cost associated with apical dominance. The results showed that both the plants with the apical meristems intact and removed showed an equal amount of biomass and that the only differences were that the seeds on the more heavily branched specimens were larger and the plants with less branching were more susceptible to certain pests.

Phototropism in corn

Background: Plants respond to numerous stimuli within their environment. One particular stimulus they respond to is light and their response/movement to light is known as phototropism. Phototropism is the growth of a particular plant in the direction of sunlight. “When young shoots of plants are exposed to an uneven light environment, they often express bending along their growth axes."¹ The direction of growth is dependent on the light source stimulus. The light interacts with a hormone called Auxin within the plant which stimulates elongation farthest from the where the light source, causing the plant to bend. Positive phototropism is the growth toward a light source whereas negative phototropism is the growth away from a light source.

Figure 1: Auxin regulates many physiological processes, one being phototropism.

Hypothesis/predictions: Plants, such as Maize, that are subjected to sunlight will tend to grow toward a specific light source whereas the same plant with same light source, but covered tip, will continue to grow (straight) independent of the direction of light.

Subject type: Corn (Zea Mayz L.) (8 pots total)

Treatment: Plants were allowed to germinate. The plants were arranged so that light was consistent throughout the experiment (the plants were not moved from the light source). Four of the plants constituted as control models for the experiment and thus did not have covered tips. The remaining four plants did have covered aluminum foil tips in order to block photo-reception.

Results: The corn plants were allowed to germinate under constant light source direction with daily watering as needed. Plant germination did not occur for one week. Only six of the eight plant samples germinated. Immediately, the early development expressed phototropic responses demonstrated with evident bending. Three of the six samples were covered with foil caps and three were uncapped. After one day, the plant samples that were uncovered showed increased growth toward the light source compared to the samples that were covered. Growth continued as such for several days. After day five, the corn had grown out of their caps due to their abundant growth rate. The covered plants had straight growth whereas the uncovered plants still demonstrated movement toward the light. (After a week of growth my cat decided to eat two of the plants, which eliminated two observational sources. This left only four samples to be evaluated for the remainder of the experiment).

Conclusion/Questions: All of the Maize samples were subjected to consistent light source (the plants were not moved throughout the experiment). Only six out of eight plants germinated, leaving only three to be foil covered and three uncovered. The plants that were not covered grew towards the light immediately whereas the plants that were covered continued to grow straight. This growth continued until the plants were too big to fit within the foil caps and grew out of them. After the plants grew out of the foil caps, the plants began to grow toward the light along with the uncovered plants. These results supported the hypothesis that when plants are subjected to uneven light source, they will tend to grow toward the light. Plants that had their tips covered continued to grow straight (without bending) thus the photo-receptors were blocked for light direction detection. If plants can continue proper growth despite light source direction, why (when exposed to light) do they bend toward the light and covered species continue growing straight?

Additional information regarding phototropism can be found in "Correlations between proton-efflux patterns and growth patterns during geotropism and phototropism in maize and sunflower"² and "Phytochrome is required for the occurrence of time-dependent phototropism in maize coleoptiles."³

References

¹ Iino, M. "Phototropism: mechanisms and ecological implications." Plant, Cell & Environment 13.7 (1990): 633-650. Academic Search Complete. EBSCO.
-This article discusses general principals of phototropism and the effects of phytochrome in the response to a light source.

² T.J. Mulkey, K.M Kuzmanoff, and M. L. Evans; Correlations between proton-efflux patterns and growth patterns during geotropism and phototropism in maize and sunflower. Planta (1981). 239-241.
-This article discusses the pH changes in proton-efflux during phototropism which may mediate the development of the displayed bending of the plant.

³ Y.K. LIU, M. IINO; Phytochrome is required for the occurrence of time-dependent phototropism in maize coleoptiles. Plant, Cell and Environment (1996). 1379-1388
-This article discusses the experiment to determine phototropic characteristics in plants during early development.

Etiolation Exhibited by Zea mays in the Laboratory

Background: Etiolation occurs when plants grow in dark environments or an environment lacking normal light levels. As a result the plant is not as green but a pale yellow color (this is characterized as chlororsis), reduced leaf development, and weak stems. All of these changes in plant physiology conserve energy for until the plant reaches sunlight and will then experience rapid growth. The mechanism of etiolation allows plants growing in a dark environment, such as leaf litter, to reach sunlight in a more efficient manner while expending the least amount of energy. Also, the apical meristems on such plants are strongly attracted to light and will exhibit phototropism in order to quickly reach a light source (Etiolation).

Corn (Zea mays), a member of the family Poaceae, was selected to take part in a study focusing on the physiological effects of etiolation on plant growth. This plant species is a dominant agricultural plant and grows throughout the United States. The success of Zea mays is vital for the support of the agricultural industry (Maize).

Hypothesis/predictions:I hypothesize the plant growing in the dark environment will exhibit all of the normal characteristics of etiolation compared to the plants growing in the sun environment. The lack of sufficient light levels of the dark environment will cause the plants to produce less leaves and have less photosynthetic cells, resulting in pale coloration.

Treatment:Eight corn plants (Zea mays) were used in this experiment. Four were placed in the sun environment and four were placed in a dark environment.
- All eight plants remained in the greenhouse for the duration of the experiment to control for temperature and water treatment.
- Both sets of plants were germinated and grown in the same soil and container.
- Once all eight plants germinated four were placed in the sun and for were placed in the dark environment.

Results: The results of this experiment supported the hypothesis. Plants growing in dark environments (Figure 1) had less leaves and were unable to support their weight and remain vertical (Table 1). Plants in a sunny environment (Figure 2) had more leaves and were able to remain in the upright position without any additional support. A considerable difference was documented in leaf development. Shade plants had fewer leaves compared to the sun plants (Table 2). Also, color variation was present between the two environments. Plants from the dark environment were white to yellow while plants which grew in sunny conditions were green.

Table 1. Plant height Height (cm) Plant Light Environment Dark Environment 1 40 0 2 57 0 3 50 0 4 48 0 Average 48.75 0

Table 2. Leaf development Number of leaves Plant Light Environment Dark Environment 1 6 4 2 6 4 3 7 2 4 6 2 Average 6.25 3

Response variable(s) measured: Qualitative and quantitative responses were measured. The coloration of each plant was compared between the two light environments (Figure 1 and 2). At the conclusion of the study plant height (Table 1) and leaf development (Table 2) was documented.

File:Dark environment.jpg

Conclusion/Questions: The conclusion for this study supports the hypothesis. The plants growing in a dark environment were unable to support their weight and were much lighter in coloration compared to the plants which grew in a sunny environment (Figure 1 and 2). Due to the lack of sufficient light intensity, the corn plants growing in the dark environment did not need to expend energy in the production of photosynthetic cells. Without these chloroplast containing cells, the corn plants lack the characteristic green coloration. Also, these reduced light levels limited the need for leaves. Development of leaves would be futile without sufficient light levels to aid in photosynthesis. As a result the shade plants had on average half as many leaves as plants growing in sunlight (Table 2).

Reasoning behind the shade plants’ inability to support their mass is not as well defined. The drooping behavior behind the plants in the light limited environment may be another mechanism to preserve energy. Since these plants are not able to produce energy through photosynthesis they are energy limited. To help conserve their limited resources less work is invested in supporting the plant. Once sunlight is reached, then the plant will be able to properly grow.

Phototropism in Sunflowers (Helianthus annuus)

Introduction: Most of us at some time have noticed a houseplant on a windowsill that seems to have all of its thin stems leaning in the same direction, as if it were trying to press itself against the glass. A tropism is the growth of a plant as a response to a stimulus, and phototropism occurs when a plant responds to light by bending in the direction of the light Phototropism is the growth of a plant in the direction of its light source. Plants are very sensitive to their environment and have evolved many forms of "tropisms" in order to ensure their survival.

Hypothesis Plants have a hormone called auxins in them. When a plant is exposed to light and shade these auxins react. For example: if a light source were on the right hand side of the plant, the left hand side would be in the shade. This makes the auxins expand in the shaded area, causing the plant to grow towards the light.

Experiment To test phototropism in plants a simple test was made. First, six sunflower plants (Helianthus annuus) were planted and allowed to germinate until they were about two inches tall. When the plants had reached this “base height” half of the plants were turned on their sides while the other half stayed up right. With the pots in position the plants were allowed to grow normally. Each plant was watered twice a day, in the morning and the evening, and measured every so often. If phototropism worked as it were suppose to then the plants on their sides would grow upward toward the light instead of sideways.

ResultsThe plants that were allowed to sit upright grew regularly and quickly, which was expected for a regular sunflower plant. Figure 1 shows the normal growth of a sunflower. The plants that were set on their sides experienced a great change. The growth immediately took a turn upward staying close to the side of the pot and grew almost more quickly that the regular sunflowers in order to reach the sunlight. In a day and a half the sunflowers on their side had grown tall enough to escape the shadow of the pot and reach the sunlight. Figure 2 shows how the plants on their sides grew. The sunflower plant immediately started to grow upward toward the sunlight rather than horizontally like the pot is set. Figure 3 shows a sunflower plant that underwent two directions of phototropism. This plant started on its side and began to grow upward; however, the pot was then set upright and took a turn upward. The top of the plant changed the growing direction to reach the optimal amount of sun.
Figure 1- Shows the normal growth of a Sunflower plant
Figure 2- Shows a Sunflower plant exhibiting phototropism
Figure 3- Shows a Sunflower that Grew in two directions due to phototropism
The variable was the height of each plant. The measurements started at 2 inches and were measured every so often throughout the experiment.
Table 1: Height Plant 1 2 3 4 5 Normal 1 2.1in 4in 5.2in 6.8in 7.5in 2 2in 4.2in 5.3in 6.5in 7.4in 3 2in 3.8in 5in 6.6in 7.4in Phototropism 1 1.9in 4.8in 6.1in 7.5in 8.6in 2 2in 5in 6.3in 7in 8.5in 3 1.9in 4.5in 5.9in 7.4in 8.6in

The plants on their sides are referred to as phototropism plants in Table 1. The plants on their sides hit a quick growth spurt and for a little while were taller than the normal growing plants. However, a side by side comparison would show that the plants were about the same height. The difference comes from the level of the seeds. Since the phototropism plants started lower, because of the turn on their sides, they had to grow taller to remain even with the tops of the normal growing plants.

Conclusion “In nature stem phototropism likely provides plants with an effective means for maximizing photosynthetic light capture and thus may have appreciable adaptive significance.” (Iino, 1990; Liscum and Stowe-Evans, 2000; Iino, 2001, 2006) The results of the test show that phototropism is taking place in the sunflowers. The auxin in the tipped over sunflowers activated and the plants grew upward toward the light. Since the plants that were left upright did not make any dramatic changes in direction it can be concluded that something was activated in the tipped over plants to make them change direction. If more resources were available another test can be done to isolate the auxin hormone and see how much it affects the growth of the plant.

More InformationTakagi, Shingo 23 December 2002, Actin-based photo-orientation movement of chloroplasts in plant cells, Journal of Experimental Biology 206: 1963-1969 This paper is very interesting and explains the chloroplasts roll in Phototropism.

http://abstracts.aspb.org/pb2004/public/S01/9179.html American Society of Plant Biologists This paper gets very in-depth about the processes of phototropism. Citation- (Iino, 1990; Liscum and Stowe-Evans, 2000; Iino, 2001, 2006)

Phototropism in Triticum aestivum

Background This experiment, as first outlined by Charles Darwin and his son Francis, shows that the hormone that controls growth towards sunlight is contained in the tip of the shoot. In order to show this, Charles Darwin capped the tips of some of the growing shoots with solid caps, and others with transparent caps. The transparent caps bent toward the light just like the plants that were uncapped. The capped plants did not elongate toward the light source. Charles Darwin also tested his hypothesis by placing a ring around the area where the plant began to bend toward the light, preventing the elongation toward the sun. This gave further evidence that the hormone responsible for the growth toward the sun is contained in the root tip. This experiment was also the first bit of evidence that plants have hormones. (Darwin, 1880) Phototropism occurs in all plants and some fungi when the light is blocked from all directions except one side (Liscum, 2002). Plants acquire directional light information by sensing internal light gradients created by the scattering and absorption of light (Fukazawa & Setagaya-ku 1990).

Hypothesis Uncapped plants will bend towards the light, but capped plants will not because the hormone controlling growth towards the sun is in the tip of the plant.

Experiment This experiment used 8 Triticum aestivum (wheat). Once they had all sprouted, 4 plants were capped with aluminum foil (inspired by Darwin, 1880), and all of the pots were placed in a brown paper bag with one opening toward the window (for directional light). Plants were watered and photographed every morning.

Figure 1: Day 1 of Capping

Results Half of the plants were capped immediately after sprouting (figure 1). After about one to two days, the uncapped plants were leaning toward the sunlight. The uncapped plants remained straight (figure 2).

Figure 2: Day 2 of Capping

As the wheat grew taller, it was harder to read the results because they were all starting to lean over from being too tall and skinny and trapped in a box. The capped wheat had popped out of their aluminum foil caps, and the results were skewed (figure 3).

Figure 3: Day 3 of Capping

The wheat was recapped with smaller pieces of foil then placed in larger box. The box was placed near a window with more sun in hopes of getting better results (figures 4 and 5)

Figure 4: Destruction of Experiment Samples

Figure 5: Culprit of Destruction

Conclusions The experiment worked best before the wheat got too tall. It was obvious that the uncapped wheat was bending toward the sunlight (figure 2), while the capped tips were not responding. Once the wheat was too tall, they began popping out of their caps or leaning over from the weight of the foil (figure 3). Even the uncapped wheat was showing signs of etiolation. The main problem with this experiment was that the laboratory setting had too little sunlight, because there were too few windows that faced where the sun would be throughout the day. If repeated, this experiment would be done in a greenhouse under a shaded screen or a cardboard box for more directional light. A stronger, slightly shorter plant may also work best to demonstrate phototropism; otherwise, a lighter weight cap can be used to keep the wheat from bending beneath the weight of the aluminum foil. Also, elimination of any pests would have been beneficial to the final product. Further research on the effects of auxin could be done by comparing different plants under the same conditions. Also, experiments under different temperature and moisture conditions (with the control being light) could provide insight into how temperature and water availability play a role in phototropism.

References Darwin, Charles (1880). Movement in Plants. London: John Murray

Charles Darwins experiment that laid the foundation for further research into phototropism and provided evidence that plants had hormones.

http://darwin-online.org.uk/pdf/1880_Movement_F1325.pdf

Fukazawa & Setagaya-ku (1990). Phototropism: mechanisms and ecological implications. Plant, Cell and Environment 13: 633-650.

A very in depth look at the mechanism and hormone (auxin) behind phototropism as well as the ecological implications behind the process. http://www3.interscience.wiley.com/cgi-bin/fulltext/119376133/PDFSTART

Liscum, Emmanuel (2002). Phototropism: Mechanisms and Outcomes. The Arabidopsis Book. Pp 1-21

For more information, you can read about the mechanism of phototropism and its significance and the advances we have made in understanding it since Darwin's experiments in 1880 at: http://www.bioone.org/doi/pdf/10.1199/tab.0042

Phototropism in sunflowers

Introduction

Most of us at some time have noticed a houseplant on a windowsill that seems to have all of its thin stems leaning in the same direction, as if it were trying to press itself against the glass. A tropism is the growth of a plant as a response to a stimulus, and phototropism occurs when a plant responds to light by bending in the direction of the light Phototropism is the growth of a plant in the direction of its light source. Plants are very sensitive to their environment and have evolved many forms of tropisms in order to ensure their survival.

Hypothesis

Plants have a hormone called auxin. When a plant is exposed to varying degrees of light, the auxin reacts. For example: if a light source were on the right side of the plant, the left side would be in the shade. This makes the auxins expand in the shaded area, causing the plant to grow towards the light.

The Test

To test phototropism in plants, a simple test was made. First, six sunflower plants (Helianthus annuus) were planted and allowed to germinate until they were about two inches tall. When the plants had reached this “base height” half of the plants were turned on their sides while the other half stayed up right. With the pots in position the plants were allowed to grow normally. Each plant was watered twice a day, in the morning and the evening, and measured every so often. If phototropism worked as it were suppose to then the plants on their sides would grow upward toward the light instead of sideways.

Results

The plants that were allowed to sit upright grew regularly and quickly, which was expected for a regular sunflower plant. Figure 1 shows the normal growth of a sunflower. The plants that were set on their sides experienced a great change. The growth immediately took a turn upward staying close to the side of the pot and grew almost more quickly that the regular sunflowers in order to reach the sunlight. In a day and a half the sunflowers on their side had grown tall enough to escape the shadow of the pot and reach the sunlight. Figure 2 shows how the plants on their sides grew. The sunflower plant immediately started to grow upward toward the sunlight rather than horizontally like the pot is set. Figure 3 shows a sunflower plant that underwent two directions of phototropism. This plant started on its side and began to grow upward; however, the pot was then set upright and took a turn upward. The top of the plant changed the growing direction to reach the optimal amount of sun.

Figure 1- Sunflower exhibiting regular growth


Figure 2- Sunflower exhibiting phototropism


Figure 3- Sunflower that Grew in two directions due to phototropism

The variable was the height of each plant. The measurements started at 2 inches and were measured every so often throughout the experiment.


Table 1-Shows the varying heights over time of the sunflowers

The plants on their sides are referred to as phototropism plants in Table 1. The plants on their sides hit a quick growth spurt and for a little while were taller than the normal growing plants. However, a side by side comparison would show that the plants were about the same height. The difference comes from the level of the seeds. Since the phototropism plants started lower, because of the turn on their sides, they had to grow taller to remain even with the tops of the normal growing plants.

Conclusion

“In nature stem phototropism likely provides plants with an effective means for maximizing photosynthetic light capture and thus may have appreciable adaptive significance.” (Iino, 1990; Liscum and Stowe-Evans, 2000; Iino, 2001, 2006) The results of the test show that phototropism is taking place in the sunflowers. The auxin in the tipped over sunflowers activated and the plants grew upward toward the light. Since the plants that were left upright did not make any dramatic changes in direction it can be concluded that something was activated in the tipped over plants to make them change direction. If more resources were available another test can be done to isolate the auxin hormone and see how much it affects the growth of the plant.

References

1. Takagi, Shingo 23 December 2002, Actin-based photo-orientation movement of chloroplasts in plant cells, Journal of Experimental Biology 206: 1963-1969
This paper is very interesting and explains the chloroplasts role in Phototropism.

2. http://abstracts.aspb.org/pb2004/public/S01/9179.html American Society of Plant Biologists
This paper gets very in-depth about the processes of phototropism.

Citation-
(Iino, 1990; Liscum and Stowe-Evans, 2000; Iino, 2001, 2006)

References

Etiolation Exhibited by Zea mays in the Laboratory:
[3] Etiolation. (2010, March 8). Wikipedia. Retrieved March 23, 2010.
[4] Maize. (2010, April 5). Wikipedia. Retrieved March 23, 2010.

De-etiolation of Solanum tuberosum

De-etiolation involves the changes in a plant shoot undergoes in response to sunlight. This process is informally known as greening. When a plant reaches sunlight its stem elongation slows, leaves expand, roots elongate and the shoots begin to produce chlorophyll in preparation for photosynthesis. De-etiolation is a response triggered by a stimulus which in this case is light. Plants use photoreceptors, such as phytochrome, which is a pigment used to detect red light (Mauseth 331). Energy is focused primarily on developing an efficient structure to maximize photosynthesis. Fully developed leaves have a larger surface area allowing more chlorophyll to photosynthesize. Elongated roots also help the plant maintain a sufficient water supply to undergo photosynthesis.

De-etiolation represents the transition from heterotrophic dependence on stored energy to photoautotrophic independence.

In the case of de-etiolation, because of the abundance of sunlight the potatoes will turn green due to the production of chlorophyll.

Given a bag of potatoes, they were separated into two groups. The first group was placed into bags that would create a dark environment and the second group was left exposed to sunlight. Changes were recorded over time.

Subjects The subjects in this experimental test were potatoes.

Treatments and Controls The treatment in this experiment was the exposure to sunlight. The control group remained enclosed in the dark environment provided by the bag.

Predictions Initial predictions included a color change to the group exposed to sunlight. Also, an increase in growth was predicted since photosynthesis could take place.

Results: The de-etiolation group (Figure 1) initially included brown potatoes with tuber roots already developed. This group was exposed to sunlight as much as possible and responded by producing chlorophyll resulting in green pigments in the potatoes skin as well as in the roots. These green pigments can be seen in Figure 2. Both etiolation and de-etiolation are signal transduction pathways. When the signals are detected by receptors they are then converted into a molecular form; this is known as transduction. This response in molecular form amplifies the signal and transfers it from the receptors to proteins that carry out the response (Cambell 822). For the de-etiolation group, the signal is sunlight. The photoreceptors in potatoes detect the presence of light which stimulates the production of chlorophyll which is responsible for the color change. The etiolation group was placed in a plastic bag that was then placed into a brown paper bag to ensure the potatoes remained in the darkness. These potatoes had no color change, but they did develop elongated tuber roots as seen in Figure 3. In de-etiolation, each activated phytochrome molecule produces hundreds of molecules that lead to a response. The response seen in the experiment was root elongation. Etiolation is a mechanism used to help increase the plants chances of finding sunlight so it is able to survive.

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The experiment showed that potatoes exposed to sunlight respond differently than potatoes deprived of sunlight. The group of potatoes exposed to an abundant amount of light began to develop chlorophyll giving the potatoes a green color on their surface as well as the roots. The group kept in the dark and deprived on sunlight had no color change. The etiolation group did experience root elongation. These responses are examples of how plant cells are able to receive and interoperate signals which can then be transduced into a response.

Follow up questions: What other plants experience similar effects? In what way do the effects vary from plant to plant? How long can specific plants live if deprived on sunlight for a period of time? 1. Middleton, L. 2001. SHADE-TOLERANT FLOWERING PLANTS: ADAPTATIONS AND HORTI-CULTURAL IMPLICATIONS. Acta Hort. (ISHS) 552:95-102. http://www.actahort.org/books/552/552_9.htm. 2. Shade-plants follow strategies of optimize the use of their available energy. Such adaptations include thinner leaves with high chlorophyll contents, epidermal cells that focus incoming light, and a red cell layer that reflects light back through the leaf for a second time. With the use of these mechanisms plants that do not receive a large amount of sunlight can live just as long as plants that receive an abundant amount of sunlight.

Further Information: In order for a plant to survive and colonize in diverse habitats it needs to be able to perceive environmental light signals and respond appropriately through phytochromes. Phytochrome interacting factors 4 and 5 redundantly limit seedling de-etiolation in continuous far-red light http://journals.ohiolink.edu/ejc/pdf.cgi/Lorrain_Sverine.pdf?issn=09607412&issue=v60i0003&article=449_pif4a5sdicfl This paper talks about the important role that phytochrome plays in developing plants. LIGHT AND SHADE SIGNALS REGULATE EOUR PHYTOCHROME A GENES IN STELLARIA LONGIPES http://web.ebscohost.com/ehost/pdf?vid=7&hid=2&sid=69763703-5311-4f02-8596-0f48fd0c3ceb%40sessionmgr4 This study looks at the expression patters of the phytochrome gene in light and dark environments.

'''References''' Campbell, Neil A, and Jane B Reece. Biology. San Francisco: Pearson Benjamin Cummings, 2008. Print. Mauseth, James D. Botany An Introduction to Plant Biology. Sadbury: Jones and Barlett Publishers, 2009. Print.