Science · Class 10 · Control and Coordination · Term 3

Plant Hormones: The Chemical Messengers

Discover the role of phytohormones like auxins and gibberellins in controlling plant growth, development, and responses.

TL;DR:Ever wondered how a plant in a dark room can find the smallest crack of light? Let's uncover the secrets of the plant world's own chemical messengers that direct their every move and milestone.

• CBSE Learning Outcomes
• NCERT: Class 10 - Chapter 7 - Hormones in Plants

About This Topic

This topic, 'Plant Hormones', is a fundamental component of the 'Control and Coordination' chapter in the Class 10 science curriculum, as prescribed by NCERT and followed by CBSE and other state boards. It moves students beyond the basic understanding of plant life processes to the intricate chemical regulation that governs them. The lesson introduces phytohormones as the plant's internal communication system, crucial for responding to environmental stimuli. The core of the topic revolves around the five principal classes of plant hormones: auxins, gibberellins, cytokinins, abscisic acid (ABA), and ethylene. It's essential to explain not just their individual functions, like auxin's role in phototropism or ABA's role in stress responses, but also their synergistic and antagonistic interactions. For instance, the balance between auxins and cytokinins determines root and shoot development. This topic provides a strong foundation for understanding plant physiology and has direct applications in agriculture and horticulture, making it relatable and significant for students in the Indian context.

Key Questions

1. Explain the role of auxin in phototropism.
2. Compare the functions of gibberellins and cytokinins in plant growth.
3. Justify why abscisic acid is known as the stress hormone.

Learning Objectives

• Define phytohormones and list the five major types with their primary functions.
• Explain the mechanism of phototropism and geotropism with respect to the action of auxin.
• Differentiate between the roles of growth-promoting (auxins, gibberellins, cytokinins) and growth-inhibiting (abscisic acid) hormones.
• Analyse how plant hormones help plants respond to environmental stresses like drought.
• Relate the application of plant hormones to common agricultural and horticultural practices.

Key Vocabulary

Phytohormone	A chemical messenger produced by plants in very small quantities that regulates their growth, development, and responses to the environment.
Auxin	A plant hormone that promotes cell elongation and is responsible for tropic movements like phototropism.
Gibberellin	A plant hormone that promotes stem elongation, breaks seed dormancy, and stimulates flowering.
Cytokinin	A plant hormone that primarily promotes cell division and is found in high concentrations in growing areas like fruits and seeds.
Abscisic Acid (ABA)	A plant hormone that acts as a growth inhibitor, promotes stomatal closure during water stress, and induces dormancy.
Tropism	The directional growth movement of a part of a plant in response to an external stimulus like light or gravity.

Watch Out for These Misconceptions

Common Misconception

Plants do not move because they are fixed in the soil.

What to Teach Instead

While plants are fixed, they show two types of movements: growth-dependent (tropic) movements like bending towards light, and growth-independent (nastic) movements like the folding of a 'touch-me-not' plant's leaves. Hormones control these movements.

Common Misconception

All plant hormones are 'growth hormones' that make plants bigger.

What to Teach Instead

Not all hormones promote growth. While auxins and gibberellins are growth promoters, hormones like Abscisic Acid (ABA) are growth inhibitors. ABA causes seed dormancy and the closing of stomata, which are crucial for survival during stressful conditions.

Common Misconception

Plant hormones are produced in special glands, just like in humans.

What to Teach Instead

Unlike animals, plants do not have specialized glands to produce hormones. Phytohormones are produced in cells in specific regions, like the tips of shoots and roots, and then transported to other parts where they act.

Active Learning Ideas

Concept Mapping

The Light Maze Challenge

Students grow a bean seedling inside a shoebox with internal partitions creating a maze. A single hole allows light in at one end, and students observe how the plant stem navigates the maze towards the light over a week, demonstrating phototropism.

20 min·Small Groups

Concept Mapping

To Pinch or Not to Pinch

Using two identical potted plants like marigold or coleus, students pinch off the apical bud of one plant. They then observe both plants for two weeks to see how removing the auxin source in the pinched plant promotes the growth of lateral branches, demonstrating apical dominance.

15 min·Pairs

Concept Mapping

Ripen it Right!

Students place an unripe tomato or chikoo in a sealed paper bag with a ripe banana, and another similar unripe fruit in the open. They compare the rate of ripening over a few days to understand the effect of ethylene gas.

10 min·Individual

Real-World Connections

• Using synthetic auxins as a 'rooting hormone' powder to help gardeners grow new plants from cuttings.
• Spraying gibberellins on grapevines to increase the length of the stalks, resulting in larger, less tightly packed grapes.
• The commercial use of ethylene gas in ripening chambers to ripen fruits like bananas and mangoes uniformly after transport.
• The practice of pruning tea plants and trimming hedges, which removes the apical buds and promotes bushier growth by overcoming apical dominance.
• Using cytokinins in tissue culture laboratories to stimulate the growth of plant cells into new plantlets.

Assessment Ideas

Quick Check

Conduct a 'Hormone Hot Seat' activity. One student takes the role of a hormone (e.g., Auxin) and other students ask questions about its functions, location, and effects.

Quick Check

A short test including a diagram of a plant bending towards a window, where students must explain the phenomenon by labelling the distribution of auxin and the resulting cell elongation.

Quick Check

Provide students with a worksheet containing a table with two columns: 'Plant Phenomenon' (e.g., seed germination, fruit ripening) and 'Hormone Responsible'. Students have to fill in the blanks.

Frequently Asked Questions

Why do some fruits we buy from the market ripen so quickly at home?

Many fruits are picked when they are raw and then treated with ethylene, a ripening hormone, just before being sold. This controlled ripening ensures they are perfect for consumption when they reach you.

How do weed killers work without harming the main crop, like wheat or rice?

Many common weed killers are made of synthetic auxins. They are designed to affect only broad-leaved plants (dicots), which most weeds are. When sprayed, they cause uncontrolled, rapid growth that the weed cannot sustain, leading to its death, while narrow-leaved crops (monocots) like wheat and rice remain unaffected.

If auxin promotes growth, why does it move to the shady side of the stem in phototropism?

Auxin promotes cell elongation. When light falls on one side of the stem, auxin accumulates on the opposite, shady side. This higher concentration of auxin causes the cells on the shady side to grow longer and faster than the cells on the sunny side, causing the stem to bend towards the light.