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Control and Coordination - Introduction to plant hormones

Grade 10CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Plants lack a nervous system; instead, they use chemical compounds called phytohormones (plant hormones) for control and coordination.

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Phytohormones are chemical messengers synthesized in one part of the plant and translocated to other parts to trigger specific physiological responses.

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Auxins: Produced at the shoot tips, they promote cell elongation. They are responsible for phototropism (growth towards light) because they diffuse to the shady side of the plant, causing cells there to grow longer.

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Gibberellins: These hormones assist in stem elongation, seed germination, and flowering. They work in conjunction with auxins to promote growth.

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Cytokinins: These promote rapid cell division (cytokinesiscytokinesis) and are found in high concentrations in areas of rapid growth, such as fruits and seeds.

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Abscisic Acid (ABAABA): Known as a growth inhibitor, it signals the wilting of leaves, promotes dormancy in seeds and buds, and helps the plant survive during stress conditions.

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Ethylene: A gaseous hormone primarily responsible for the ripening of fruits and the falling of leaves (abscission).

📐Formulae

Plant Response=f(Hormone Concentration,Target Tissue Sensitivity)\text{Plant Response} = f(\text{Hormone Concentration}, \text{Target Tissue Sensitivity})

Auxin Concentration([Auxin])∝Cell Elongation (in shoots)\text{Auxin Concentration} ([Auxin]) \propto \text{Cell Elongation (in shoots)}

Rate of Growth≈ΔLΔt\text{Rate of Growth} \approx \frac{\Delta L}{\Delta t}

💡Examples

Problem 1:

A plant shoot is illuminated from only one side. Describe the role of Auxin in this scenario.

Solution:

The Auxin hormone diffuses from the illuminated side to the shady side of the shoot tip. The higher concentration of Auxin on the shady side stimulates cells to grow longer (Lshady>LsunnyL_{shady} > L_{sunny}), causing the shoot to bend towards the light source.

Explanation:

This phenomenon is called positive phototropism. The differential growth is a direct result of the chemical gradient of Auxins created by light stimulus.

Problem 2:

Which hormone would be present in high concentrations in a ripening mango and a germinating pea seed?

Solution:

Ethylene would be high in the ripening mango, while Cytokinins and Gibberellins would be high in the germinating pea seed.

Explanation:

Ethylene is the primary gaseous hormone for fruit ripening, whereas Cytokinins promote the rapid cell division required for the embryo to grow during germination.

Problem 3:

Why is Abscisic Acid (ABAABA) often referred to as a 'stress hormone' in plants?

Solution:

Because it inhibits growth and promotes physiological changes that help the plant withstand environmental stress, such as the closing of stomata during water H2OH_2O scarcity.

Explanation:

ABAABA counteracts the effects of growth-promoting hormones like Gibberellins to ensure plant survival during unfavorable conditions.