Question

Phototropism in shoots is attributed to are due to -
a. Auxin
b. Gibberellins
c. Cytokinin
d. Abscisic acid

Answer 1

Hint: Phototropism is the growth of an organism in response to a light stimulus which further helps a plant to grow. If the stem grows towards the direction of light it is called positively phototropic. However, the root is called negatively geotropic if it grows away from the light source.

Complete answer:
Auxin is a plant hormone that plays an important role in phototropism, growing movement in response to light. It promotes cell elongation. It is unequally distributed in the plant cells. When present in higher concentration, it is transported towards the shady side. However, less auxin is transported towards the side exposed to light. Due to this, the plant grows more on the shaded side and bends in the direction of the light source.

Positive phototropism is observed where faster cell elongation is seen on the shaded side and less on the illuminated side. Auxin is the phytohormone which is responsible for apical dominance as well as positive phototropism. Auxin makes the shot to grow positively phototropic.

Hence, the correct answer is option (A).

Additional information:
- Gibberellins stimulate the growth of main stems, especially when applied to the whole plant. They are also involved in the bolting (elongation) of rosette plants (e.g., lettuce) after exposure to certain environmental stimuli such as long periods of daylight.
- Cytokinins are plant hormones that plant hormones that cause increased cell division by stimulating the process of mitosis.
- Abscisic acid (ABA) is a crucial phytohormone that regulates growth of plant, development, and stress responses. For example - seed and bud dormancy, controlling organ size, and stomatal closure.

Note: Auxin is the phytohormone which is responsible for apical dominance as well as positive phototropism. Auxin makes the shot to grow positively phototropic.
Various experiments have been performed by different scientists to explain phototropism, Charles Darwin’s experiment, Boysen Jensen’s experiment.