Questions: 4B: 1. Explain what is meant by the term phototropism. (Student 1) 2. Charles Darwin and his son Francis were very enthusiastic scientists that ran many scientific experiments with plants. Compare their experiment in phototropism with Went's experiment on coleoptile bending and how they can be used to explain phototropism. (Student 2) 3. Explain and the functions of phytochromes and blue light receptors to phototropism. (Student 3) 4. State the color of the light spectrum that seems to cause the bending of the coleoptiles in the Darwins' experiments and explain why this color might initiate such a response. (Student 4)

Okay, I will answer the questions about phototropism.

1. Explanation of Phototropism:

Phototropism is the growth of a plant in response to a light stimulus. Specifically, it refers to the directional growth where a plant organ, such as a stem or leaf, bends towards a light source (positive phototropism) or away from it (negative phototropism). This response optimizes the plant's ability to capture light for photosynthesis.

2. Comparison of Darwin's and Went's Experiments and Their Contribution to Understanding Phototropism:

• Darwin's Experiment: Charles Darwin and his son Francis conducted experiments on phototropism using grass coleoptiles (protective sheath covering young shoots). They observed that coleoptiles bend towards a light source. They found that:

  • The tip of the coleoptile is responsible for sensing the light.
  • If the tip is covered with an opaque cap, the coleoptile does not bend.
  • If the tip is covered with a transparent cap, the coleoptile bends normally.
  • If the stem below the tip is covered, the coleoptile still bends.
  • Their conclusion was that the tip perceives the light and transmits a signal to the bending zone.

• Went's Experiment: Frits Went expanded on the Darwins' work. He:

  • Removed coleoptile tips and placed them on agar blocks.
  • He then placed these agar blocks asymmetrically on decapitated coleoptiles (without tips).
  • The coleoptiles bent away from the side where the agar block was placed, even in the absence of light.
  • Went concluded that the coleoptile tip produces a chemical messenger (later identified as auxin) that promotes cell elongation on the shaded side of the coleoptile, causing it to bend towards the light.

• How they explain phototropism: Darwin's experiment identified that the tip of the coleoptile was responsible for sensing the light. Went's experiment furthered this by discovering that a chemical messenger, auxin, was produced at the tip in response to light. Auxin then asymmetrically distributes itself down the coleoptile to cause cell elongation on the shaded side of the coleoptile, which causes the coleoptile to bend towards the light.

3. Functions of Phytochromes and Blue Light Receptors in Phototropism:

• Phytochromes: Primarily involved in red and far-red light detection and regulate various developmental processes, including seed germination, flowering, and shade avoidance. Their role in phototropism is less direct compared to blue light receptors. However, they can influence phototropism by affecting overall plant development and sensitivity to light.
• Blue Light Receptors (Phototropins): These receptors are the primary mediators of phototropism. Phototropins are kinases that, upon binding to blue light, trigger a signaling cascade that leads to:
  • Asymmetric distribution of auxin. Blue light causes the plant hormone auxin to be transported to the shaded side of the stem. This leads to differential cell elongation, with the cells on the shaded side elongating more rapidly than those on the illuminated side, causing the plant to bend towards the light.
  • Chloroplast movement: Blue light also mediates chloroplast movement within cells to optimize light capture for photosynthesis.
  • Stomatal opening: Blue light regulates the opening of the stomata.

4. Color of Light and Reason for Response:

• Color: In the Darwins' experiments, blue light seems to cause the bending of the coleoptiles. Although they did not isolate specific wavelengths, later research using action spectra has shown that blue light is the most effective at inducing phototropism in many plants.
• Reason: Blue light is a high-energy light, and the pigment that absorbs blue light to initiate phototropism is phototropin. Plants have evolved receptors (phototropins) specifically sensitive to blue light because blue light is abundant in sunlight and is efficient in driving photosynthesis. The bending response ensures that the plant can optimally position itself to capture this essential light energy.