Explain tropic responses in plants, focusing on auxins and gravitropism.

1a) What is meant by tropic response?
A tropic response is a growth movement of a plant part towards or away from an external stimulus, such as light, gravity, or water.

1b) Discuss the role of auxins in a:

• i) shoot receiving light from one side When a shoot receives light from one side, auxins migrate to the shaded side. The higher concentration of auxins on the shaded side promotes cell elongation, causing the shoot to bend and grow towards the light source.
• ii) radicle growing horizontally If a radicle (root) is growing horizontally, gravity causes auxins to accumulate on its lower side. In roots, high concentrations of auxin inhibit cell elongation. This inhibition on the lower side causes the cells on the upper side to elongate more, making the radicle bend and grow downwards into the soil.

2a) Explain what is meant by the term positive phototropism.
Positive phototropism is the growth of a plant part, typically the shoot, towards a light source. This response ensures that leaves are exposed to sufficient light for photosynthesis.

2b) Explain the effect of gravity on the plumule and radicle in germinating seeds.
Gravity influences the direction of growth in germinating seeds:

• Plumule (shoot): The plumule exhibits negative gravitropism, meaning it grows upwards, away from the pull of gravity. Auxins accumulate on the lower side of the plumule, promoting cell elongation and upward growth.
• Radicle (root): The radicle exhibits positive gravitropism, meaning it grows downwards, in the direction of gravity. Auxins accumulate on the lower side of the radicle, but in roots, high auxin concentrations inhibit cell elongation, causing the root to bend downwards.

3) Using the auxin theory, explain the effects of light coming from one direction on the growth of shoots.
According to the auxin theory, when light comes from one direction (unilateral light), auxins produced at the shoot tip migrate to the shaded side of the shoot. The increased concentration of auxins on the shaded side stimulates greater cell elongation there compared to the illuminated side. This differential growth causes the shoot to bend towards the light source, a phenomenon known as positive phototropism.

4a) What is meant by the term hormone?
A hormone is a chemical messenger produced in small quantities by specific cells or glands in one part of an organism, transported to other parts, and capable of influencing the activity of specific target cells or organs.

4b) Describe the function of auxin in plants.
Auxins are a class of plant hormones with several key functions:

• Promoting cell elongation in shoots.
• Establishing apical dominance (inhibition of lateral bud growth by the apical bud).
• Initiating root formation on stem cuttings.
• Influencing fruit development.
• Mediating tropic responses like phototropism and gravitropism.

4c) Relate the effects of auxin in geotropism.
Geotropism (or gravitropism) is the plant's growth response to gravity, mediated by auxins:

• In shoots: Auxins accumulate on the lower side due to gravity. This higher auxin concentration promotes cell elongation, causing the shoot to grow upwards (negative geotropism).
• In roots: Auxins also accumulate on the lower side due to gravity. However, root cells are much more sensitive to auxin; thus, this higher concentration inhibits cell elongation on the lower side. This inhibition causes the root to grow downwards (positive geotropism).

5) Explain phototropism and positive geotropism.

• Phototropism: This is the growth response of a plant towards or away from a light stimulus. Shoots typically exhibit positive phototropism, growing towards light to maximize photosynthesis.
• Positive geotropism: This is the growth response of a plant part towards the force of gravity. Roots typically exhibit positive geotropism, growing downwards into the soil to anchor the plant and absorb water and nutrients.

6) Distinguish between a tropic response and taxic response.

• A tropic response is a growth movement of a plant part in a specific direction relative to a stimulus. It is slow, irreversible, and involves differential growth.
• A taxic response (or taxis) is a locomotor movement of an entire organism (or motile cells within an organism) towards or away from a stimulus. It is typically faster, reversible, and involves the movement of the whole organism.

7) Why are tropic responses important in plants?
Tropic responses are crucial for plant survival and growth because they allow plants to orient themselves optimally in their environment.

• Phototropism ensures shoots grow towards light for efficient photosynthesis.
• Gravitropism ensures roots grow downwards to access water and minerals and anchor the plant, while shoots grow upwards to reach sunlight.
• Thigmotropism (response to touch) allows climbing plants to find support. These responses enable plants to maximize resource acquisition and withstand environmental challenges.