Prep Guide: Chemical Coordination in Plants

Key Concepts (Recall & Understanding)

Phytohormones (Plant Hormones): Chemical messengers that regulate plant growth, development, and responses to the environment.
Growth Promoters:
- Auxins: Promote cell elongation, apical dominance, and root initiation. Responsible for tropisms.
- Gibberellins: Promote stem elongation, seed germination, and fruit growth.
- Cytokinins: Promote cell division (cytokinesis) and delay senescence.
Growth Inhibitors:
- Abscisic Acid (ABA): The "stress hormone"; promotes stomatal closure, seed dormancy, and abscission.
- Ethylene: A gaseous hormone that promotes fruit ripening and senescence.
Tropic Movements (Tropisms): Directional growth responses to external stimuli.
- Phototropism: Response to light (shoots are positively phototropic).
- Geotropism: Response to gravity (roots are positively geotropic, shoots are negatively geotropic).
- Hydrotropism: Response to water (roots are positively hydrotropic).
- Thigmotropism: Response to touch (e.g., coiling of tendrils).
- Chemotropism: Response to chemicals (e.g., pollen tube growth towards the ovule).

Application Corner

A gardener wants to create a bushier rose plant with many lateral branches. Should they remove the tip of the main stem or leave it intact? Explain your reasoning in terms of plant hormones.
- Answer: They should remove the tip of the main stem (a practice called decapitation or pinching). The apical bud produces auxins, which cause apical dominance, inhibiting the growth of lateral buds. Removing the source of auxin allows the lateral buds to sprout, resulting in a bushier plant.
Unripe bananas are often shipped in sealed containers. If a ripe banana is accidentally included in the shipment, all the other bananas will ripen very quickly. Which hormone is responsible for this, and what is its state of matter?
- Answer: Ethylene is responsible. It is a gaseous hormone, so it can easily spread from the ripe banana to the unripe ones in the sealed container, triggering a chain reaction of ripening.

Analytical Thinking

Odd One Out: Phototropism, Geotropism, Thigmotropism, Photosynthesis.
- Odd One: Photosynthesis.
- Category: The rest are tropic movements or directional growth responses in plants.
Scenario: A potted plant is placed horizontally in a dark room. After a few days, the shoot starts to bend upwards, and the root starts to bend downwards. Which tropism is being demonstrated, and how does the plant achieve this response in the absence of light?
- Answer: This demonstrates geotropism. The response is to gravity, not light. Gravity causes auxins to accumulate on the lower side of both the shoot and the root. In the shoot, the higher auxin concentration stimulates cell elongation, causing it to bend upwards (negative geotropism). In the root, the higher auxin concentration inhibits cell elongation, causing it to bend downwards (positive geotropism).

Parthenocarpy is the development of fruit without prior fertilization, which results in seedless fruits. This phenomenon can be induced artificially in many crop plants, such as grapes, watermelons, and cucumbers, by applying plant hormones. Auxins and gibberellins are commonly used for this purpose. By spraying these hormones on the flowers, farmers can trigger fruit development without the need for pollination and fertilization. This is commercially valuable because consumers often prefer seedless varieties of fruits. This case study is a prime example of how our understanding of plant hormones can be applied in horticulture and agriculture to modify plant development for human benefit.

Assertion-Reason Practice

Assertion (A): Abscisic acid (ABA) is known as the stress hormone. Reason (R): ABA promotes the closure of stomata during conditions of water scarcity.

(a) Both (A) and (R) are true and (R) is the correct explanation of (A). (b) Both (A) and (R) are true but (R) is not the correct explanation of (A). (c) (A) is true but (R) is false. (d) (A) is false but (R) is true.

Answer: (a) Both (A) and (R) are true and (R) is the correct explanation of (A). One of the primary ways ABA helps a plant cope with stress (like drought) is by triggering stomatal closure, which conserves water. This is a key reason why it is called the stress hormone.

HOTS (Higher-Order Thinking Skills) Question

Auxin and cytokinin have an antagonistic (opposing) relationship in controlling apical dominance. Based on this, predict what would happen if you applied a cytokinin-rich paste to a lateral bud of a plant whose apical bud is still intact. Justify your prediction.

Answer: Applying a cytokinin-rich paste to a lateral bud would likely cause that bud to grow, even with the apical bud intact.
- Justification: The apical bud produces auxin, which flows downwards and inhibits the growth of lateral buds. Cytokinins, on the other hand, promote cell division and lateral bud growth. By applying cytokinin directly to a lateral bud, you are artificially increasing the cytokinin-to-auxin ratio in that specific area. This high local concentration of cytokinin can override the inhibitory effect of the auxin coming from the apex, thus stimulating the lateral bud to break its dormancy and grow into a branch.

Specimen Prep - Chemical Coordination in Plants