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Running head: Phototropism: plants response to external stimuli

A 5 Pages Term Paper on Phototropism: plants response to external stimuli

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    Phototropism is that quality of plants in which different organs of their body bend towards light. It is a common feature of most of the plants but not necessarily all of them. Phototropism was first documented over 100 years ago and since then it is a very common subject to study. However, very little information was gathered about its genetic and molecular determinations.

What actually happens to make plants organs bend towards light?

     Light makes plant grow and for getting light they do whatever they can do. To get as much light as possible, they bend towards the source of light that is ‘Phototropism’. And because many of the plants themselves cannot move towards light, they move parts of their body towards it.

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     When the plant is put in the open light, it will move straight but if the light is coming from one side only like from a window or if the plant is kept in the box and only a side is open, then the organs or majority the upper part of the body will bend towards it. Contrary to what one might think at first, the part away from the light (the DARKER part) grows faster! But this has the effect of getting the rest of the plant closer to the light.

     The way the plant does this is using a plant hormone called auxin. Auxin is produced at the tip of the plant and then goes down causing the cells in the stem to get longer.        When one side is darker than the other, more of the auxin goes to the darker side and it then grows faster, bending the stem toward the light.

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     Here we will do an experiment to show the response of parts of plants towards light. The things required for it are:

  • Seeds.
  • One small jar
  • Soil.
  • Water
  • Put soil mix into both the containers at equal levels and put 3-4 seeds in each container.
  • Allow the plants to grow about an inch tall with sufficient water and light.
  • Place the container on its side and put it in a room with light coming only from the left-hand side.
  • Leave the jar still for about 2-3 days.

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     The observations that made are were the plant changes direction and grows towards the light. Growth in plants takes place with the help of substances called Auxins. In normal conditions, with sufficient light, the plant grows straight up.  Since the auxins are evenly distributed. But in the experiment when the stem starts to grow upwards, the light does not fall evenly on all sides of the plant. Auxins get collected on this dark side, which causes the darker side to grow and elongate. As the other side has a deficiency of auxins, it does not grow. Therefore, the tip of the plant needs and seems to move towards the plant.

Experiment done by Darwin

     Here, we will see the experiment done by Darwin taken from a website …Early in the history of science, Darwin kept birds. To provide the birds with vitamins in those days, one needed to grow sprouts. A common species was Phalaris or canary grass!
Darwin noted that the first leaf (coleoptile) of canary grass was very sensitive and responsive to light. He sprouted the seeds in flats and fed the mature seedlings to the birds. But before they made green leaves, the white first leaf (coleoptile) appeared and grew toward the light coming from the nearby window. Darwin was very curious about this and did a few experiments. Later scientists added to this queue of projects to elucidate the mechanism of phototropism.


Darwin, and others, obviously found that the coleoptile's tip was the light-sensitive part. The growth response to light, however, was produced further down the coleoptile.

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This observation led scientists to think about hormones again. A chemical substance made in the tip would be transported down the coleoptile and the cells some distance away from the tip would respond by growing. (Phototropism)

     The first theory about Phototropism is mentioned below;

     The first theory provided was “acid growth theory” which was explained for the process of cell elongation. This theory tells that Auxins present in plants pump H+ into the cell walls decrease the PH of the cell. The acid inside them contact with walls and the turgor pressure stretches them and that movement produces Phototropism. The color of light that is used has also some effect. Blue, ultraviolet and white light have effects. Green, yellow and red lights usually do not have effects. It has also been proved that the greater is the intensity of light, the greater is the pressure.

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     Phototropic reactions are actually carried out by the growing of tissues. This is sometimes caused by the cells’ loss of plasticity and sometimes by the development of mostly inflexible strengthening elements that set a mechanic resistance against each deformation of the tissue. In the shoot axis are they arranged in the periphery thus bringing about an especially high stability.

     These facts show that we have stimulus perception, stimulus forwarding, the mechanism of the bending, and experimental data for all three-reaction parts, but it is still not understood how these parts are linked. We do not know how the synthesis of the information molecules (the phytohormone auxine) is influenced by the light receptor after stimulation. We do not know either how the decision for a negative or a positive phototropic reaction is made, and we have only rudimentary knowledge about the way in which auxine could stimulate the elongation of the cell walls in the growth zone.

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How Hormones Work - Signal Transduction Pathways

     The diagram and material below are taken from a web site under the topic Plant Behavior (Plant Regulation and Response)

In the diagram above, the receptor is on the surface of the target cell. In other cases, hormones enter cells and bind to specific receptors inside. Environmental stimuli can also initiate signal pathways. For example, phytochrome conversion is the first step in the transduction pathways that lead to a cell's responses to red light.

Plant Movements - Nastic Responses

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Sleep movements

In many plants, during the last glimmerings of day, leaves bend up and together vertically (like hands in prayer). This has been shown to reduce the loss of heat during the night. When light returns the leaves fold down again for maximum exposure to the sun. A special structure at the base of the leaf's petiole, called a pulvinus, contains motor cells specialized in pumping potassium ions into nearby tissues changing the turgor pressure. The result is the nastic movement.

Solar Tracking- "Heliotropism"

    This nastic response, often seen in sunflowers, causes the plant to act as if it were a astronomical tracking device aimed at the sun. In fact the flowers are often parabolic in shape, ideal for receiving the maximum amount of light energy. Growth is not involved in this fast and reversible response (which is repeated in an identical fashion day after day.)

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Thigmonastic response

Some plants have the ability to respond quickly to touch by collapsing their leaves, often together (at the base of the petiole). The sensitivity plant, Mimosa pudica may be responding to hot, dry conditions in order to conserve water or prevent leave damage. Changes in turgor pressure is involved. (Plant Behavior Plant Regulation and Response)

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Phototropism. Retrieved from the World Wide Web on November 29, 2001


Plant Behavior (Plant Regulation and Response). Retrieved from the World Wide Web on November 29, 2001



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