MCQs on Plant Hormones

Ethylene is a unique gaseous hormone that regulates several key processes in a plant's life cycle. Its primary functions include:

• Fruit Ripening: It triggers and accelerates the ripening of climacteric fruits, such as bananas, apples, and tomatoes, making them softer and sweeter.
• Senescence and Abscission: It promotes the natural aging (senescence) and shedding (abscission) of plant parts like leaves, flowers, and fruits.
• Breaking Dormancy: In certain species, ethylene helps break the dormancy of seeds and buds, allowing them to germinate or grow.
• Apical Dominance: It contributes to maintaining the dominance of the main stem by suppressing the growth of lateral buds.

Abscisic acid (ABA) is called the 'stress hormone' because its production significantly increases when a plant faces adverse environmental conditions. Its main role is to help the plant survive by initiating protective responses. For instance, during a drought, high levels of ABA cause the stomata (pores on leaves) to close, which drastically reduces water loss through transpiration. It also promotes dormancy in seeds and buds, preventing them from growing until conditions become favourable again.

Auxins and cytokinins have an antagonistic (opposing) relationship that maintains apical dominance, which is the tendency for a plant's main central stem to grow more strongly than its lateral stems.

• Auxins, produced at the apical bud (tip of the main stem), flow downwards and inhibit the growth of nearby lateral buds. This encourages the plant to grow taller.
• Cytokinins, primarily produced in the roots, move upwards and promote the growth of these lateral buds.

Gibberellins, specifically gibberellic acid (GA), play a crucial commercial role in the malting process used in brewing. Malting involves the controlled germination of barley grains. Applying GA speeds up this process significantly by stimulating the aleurone layer of the grain to produce key enzymes, most notably α-amylase. This enzyme breaks down the complex starches stored in the grain's endosperm into simple sugars, which are then fermented by yeast to produce alcohol.

As per the CBSE Class 10 syllabus for 2025-26, students should focus on four main types of hormones for the chapter on Control and Coordination:

• Auxin: Promotes cell elongation and growth, and is responsible for phototropism (the bending of plants towards light).
• Gibberellin: Promotes stem elongation and fruit growth.
• Cytokinin: Promotes cell division and is found in high concentrations in rapidly growing areas like fruits and seeds.
• Abscisic Acid (ABA): A growth inhibitor that causes the wilting of leaves and manages stress responses.

The effect of a hormone like auxin depends on its concentration and the sensitivity of the target tissue. For example, a concentration of auxin that stimulates stem elongation is high enough to inhibit root growth. This is because shoots are less sensitive and require higher auxin levels for growth, while roots are highly sensitive and are inhibited by those same levels. This principle of differential sensitivity allows auxin to orchestrate complex growth patterns, ensuring roots grow down (positive geotropism) and shoots grow up (negative geotropism).

Plant hormones, also called plant growth regulators, are broadly classified based on their primary function:

• Growth Promoters: These hormones stimulate activities like cell division, cell enlargement, flowering, fruiting, and seed formation. The main growth promoters are Auxins, Gibberellins, and Cytokinins.
• Growth Inhibitors: These hormones are mainly involved in processes like dormancy and abscission (shedding of parts) and responding to environmental stresses. The primary growth inhibitors are Abscisic Acid (ABA) and Ethylene.

Yes, plant hormones can be used to produce seedless fruits through a process called artificial parthenocarpy. This is the development of fruit without fertilisation. By applying hormones like auxins and gibberellins to the unpollinated flowers of plants such as grapes, cucumbers, and tomatoes, the ovary is stimulated to develop into a fruit. These hormones effectively trick the plant into forming a fruit, even though the ovules inside have not been fertilised to become seeds. You can test your knowledge with these MCQs on Parthenocarpy.

This phenomenon, called etiolation, is a survival strategy primarily driven by auxins and a lack of light. In darkness, the plant prioritises reaching a light source above all else. High levels of auxin promote rapid stem elongation, causing the plant to grow tall and spindly. It conserves energy by not producing chlorophyll (which makes it appear yellow) or fully developed leaves. The advantage is that this rapid upward growth increases the seedling's chance of breaking through soil or emerging from shade to find sunlight, which is essential for photosynthesis and survival.