Which one of the following plant hormones can retard the ageing of plant organs by controlling protein synthesis?

1. Introduction to Plant Growth Regulators (Phytohormones) (basic)

In the plant world, coordination and growth are not managed by a nervous system, but by a sophisticated system of chemical messengers known as Plant Growth Regulators (PGRs), often called Phytohormones. These are small, simple organic molecules produced in tiny quantities that travel through the plant to signal specific physiological responses, such as bending toward light or shedding leaves in autumn Science, Class X (NCERT 2025 ed.), Chapter 6: Control and Coordination, p.108.

We broadly classify these hormones into two functional groups: Growth Promoters and Growth Inhibitors. Promoters like Auxins and Gibberellins drive the plant's upward and outward expansion, while Cytokinins are particularly crucial for cell division. Because of this role, you will find them in high concentrations in regions of active growth, such as developing fruits and seeds Science, Class X (NCERT 2025 ed.), Chapter 6: Control and Coordination, p.108. On the flip side, inhibitors like Abscisic Acid (ABA) act as the plant's "brakes," signaling it to stop growing or inducing the wilting of leaves to conserve resources during stress.

One of the most fascinating aspects of Phytohormones is their role in Senescence (the biological process of ageing). While some hormones promote ageing, Cytokinins act as an anti-ageing shield. They delay the yellowing of leaves by mobilizing nutrients and maintaining protein levels—a phenomenon known as the Richmond-Lang effect. By preventing the breakdown of chlorophyll, Cytokinins ensure the plant stays green and metabolically active for longer. It is important to remember that these hormonal levels are ultimately controlled by the plant's genes; a gene provides the instructions for the enzymes that produce these hormones, thereby determining traits like height or leaf longevity Science, Class X (NCERT 2025 ed.), Heredity, p.131.

Category	Hormone Examples	Primary Functions
Growth Promoters	Auxins, Gibberellins, Cytokinins	Cell elongation, stem growth, cell division, delaying ageing.
Growth Inhibitors	Abscisic Acid (ABA), Ethylene	Inhibiting growth, inducing wilting, promoting leaf fall.

Sources: Science, Class X (NCERT 2025 ed.), Control and Coordination, p.108; Science, Class X (NCERT 2025 ed.), Heredity, p.131

2. The Role of Auxins and Gibberellins in Growth (intermediate)

In the complex life of a plant, growth is not just a random increase in size but a highly coordinated process managed by chemical messengers called phytohormones. Among these, Auxins and Gibberellins are the primary "growth promoters" that ensure a seedling successfully matures into a reproductive plant Science-Class VII, NCERT (Revised ed 2025), Adolescence: A Stage of Growth and Change, p.73. While both drive growth, they operate through distinct biological mechanisms.

Auxins, such as Indole-3-acetic acid (IAA), are primarily synthesized at the shoot tips (apices). Their fundamental role is cell elongation. A fascinating application of this is phototropism: when a plant detects light from one direction, auxin diffuses toward the shaded side of the shoot. This concentrated auxin stimulates the cells on the dark side to grow longer than those on the bright side, causing the plant to bend toward the light source Science, Class X (NCERT 2025 ed.), Control and Coordination, p.108. Additionally, auxins are responsible for apical dominance, where the main central stem grows more strongly than side branches.

Gibberellins function somewhat differently, focusing on the elongation of the entire internode (the section of stem between two leaves). While auxins help individual cells stretch, gibberellins increase the length of the plant's axis. They are also famous for inducing "bolting"—the sudden, rapid growth of a flower stalk in plants like cabbage. Beyond stem growth, gibberellins are essential for breaking seed dormancy, signaling the seed that it is time to germinate when environmental conditions are right.

To help you distinguish between these two powerhouses, look at their primary roles side-by-side:

Feature	Auxins	Gibberellins
Main Function	Cell elongation and Apical dominance.	Internodal growth and Bolting.
Directional Growth	Mediates tropisms (bending toward light/gravity).	Promotes overall increase in height/axis length.
Seed State	Involved in root initiation.	Breaks seed dormancy to start germination.

Sources: Science, Class X (NCERT 2025 ed.), Control and Coordination, p.108; Science-Class VII, NCERT (Revised ed 2025), Adolescence: A Stage of Growth and Change, p.73

3. Plant Responses: Tropic and Nastic Movements (intermediate)

Unlike animals, plants do not possess a nervous system or muscles to coordinate their actions. Instead, they respond to environmental triggers through two distinct types of movements: Tropic movements and Nastic movements. These mechanisms allow plants to navigate their environment, optimize light capture, and protect themselves from harm Science, Class X (NCERT 2025 ed.), Control and Coordination, p.105.

Tropic movements (or tropisms) are directional growth responses. In these movements, the plant either grows toward a stimulus (positive tropism) or away from it (negative tropism). For example, in phototropism, shoots typically show positive phototropism by bending toward light, while roots show negative phototropism by growing away from it Science, Class X (NCERT 2025 ed.), Control and Coordination, p.107. Other common tropisms include geotropism (response to gravity), hydrotropism (response to water), and chemotropism (response to chemicals, such as the growth of a pollen tube toward an ovule).

Nastic movements, on the other hand, are non-directional. The direction of the stimulus—such as touch or temperature—does not determine the direction of the plant's response. A famous example is the chhui-mui or "sensitive plant" (Mimosa pudica). When its leaves are touched, they fold up and droop almost instantly Science, Class X (NCERT 2025 ed.), Control and Coordination, p.105. Because these movements must be fast, they do not rely on growth. Instead, the plant uses electrical-chemical signals to move water out of specific cells, causing them to shrink and the leaf to fold.

Feature	Tropic Movement	Nastic Movement
Directionality	Directional (related to stimulus)	Non-directional
Primary Cause	Growth	Changes in water turgor
Speed	Slow (hours or days)	Fast (seconds or minutes)

Sources: Science, Class X (NCERT 2025 ed.), Control and Coordination, p.105; Science, Class X (NCERT 2025 ed.), Control and Coordination, p.107

4. Comparative Study: Human Endocrine System (intermediate)

In our journey through biological coordination, we see a fascinating contrast between how plants and animals manage growth. While plants rely on hormones to dictate directional growth (like bending toward light), animal bodies use the endocrine system to ensure growth happens in a highly proportional and metabolic way. Animals don't grow toward gravity or light; instead, their hormones act as chemical messengers that regulate the rate and balance of internal processes Science, Class X (NCERT 2025 ed.), Control and Coordination, p.109.

Two critical players in this system are the Pituitary gland and the Thyroid gland. The Pituitary, often influenced by the Hypothalamus, secretes Growth Hormone. This hormone is the master regulator of development; a deficiency in childhood leads to dwarfism, while an excess can cause giantism Science, Class X (NCERT 2025 ed.), Control and Coordination, p.110. Nearby, the Thyroid gland produces Thyroxin, which requires Iodine for synthesis. Thyroxin is essential because it coordinates the metabolism of carbohydrates, proteins, and fats, ensuring the body has the right energy balance for growth. This is why we use iodised salt—to prevent conditions like goitre, characterized by a swollen neck Science, Class X (NCERT 2025 ed.), Control and Coordination, p.110.

Endocrine Gland	Hormone Produced	Primary Function
Pituitary	Growth Hormone	Regulates overall body growth and development.
Thyroid	Thyroxin	Regulates metabolism (fats, proteins, carbs) for growth balance.
Pancreas	Insulin	Regulates and maintains blood sugar levels.

Precision is everything in the endocrine system. Hormones must be released in exact quantities at the right time. This is managed by feedback mechanisms. For instance, when blood sugar rises, the cells of the pancreas detect the change and respond by secreting more insulin. As the sugar level drops to a safe threshold, the secretion is naturally reduced Science, Class X (NCERT 2025 ed.), Control and Coordination, p.111. This "thermostat-like" control ensures the body maintains a stable internal environment, a process known as homeostasis.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 6: Control and Coordination, p.109; Science, Class X (NCERT 2025 ed.), Chapter 6: Control and Coordination, p.110; Science, Class X (NCERT 2025 ed.), Chapter 6: Control and Coordination, p.111

5. Plant Growth Inhibitors: ABA and Ethylene (exam-level)

While hormones like auxins and cytokinins act as the "accelerators" of plant growth, every complex biological system needs a reliable set of "brakes." In the plant kingdom, these brakes are known as Growth Inhibitors. They are not negative forces; rather, they are critical survival mechanisms that allow a plant to endure harsh environments, conserve water, and time its life cycle correctly. The two primary players here are Abscisic Acid (ABA) and Ethylene.

Abscisic Acid (ABA) is often called the "Stress Hormone." Its primary job is to help the plant survive unfavorable conditions by inducing dormancy. For instance, it prevents seeds from germinating during a mid-winter thaw, ensuring they only grow when spring truly arrives. Furthermore, ABA is the master regulator of the plant's water budget. When a plant faces drought, ABA triggers the rapid closure of stomata (the tiny pores on leaves) to prevent water loss via transpiration Science, Class X (NCERT 2025 ed.), Life Processes, p.83. While growth promoters help a plant thrive, ABA helps a plant survive, even if it leads to the wilting of leaves Science, Class X (NCERT 2025 ed.), Control and Coordination, p.108.

Ethylene is unique among plant hormones because it is a gas. It serves as a signal for "maturation and departure." Ethylene is responsible for the ripening of fruits and the process of abscission—the shedding of leaves, flowers, or fruits. In the context of environmental stress, high concentrations of ethylene can cause premature leaf fall and the curling of petals Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.69. While inhibitors like ABA and Ethylene promote aging (senescence), they work in a delicate balance with growth-promoting hormones to ensure the plant’s resources are used efficiently throughout the seasons.

Hormone	Primary Role	Key Action
Abscisic Acid (ABA)	Stress Management	Closes stomata; induces seed dormancy; growth inhibition.
Ethylene	Maturation & Shedding	Fruit ripening; leaf abscission (falling); senescence.

Sources: Science, Class X (NCERT 2025 ed.), Life Processes, p.83; Science, Class X (NCERT 2025 ed.), Control and Coordination, p.108; Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.69

6. Plant Senescence and Nutrient Mobilization (exam-level)

In the lifecycle of a plant, senescence is not merely an accidental death but a highly regulated, programmed physiological process of aging. It marks the stage where an organ, such as a leaf or flower, has fulfilled its utility and prepares to shut down. One of the most visible signs is the yellowing of leaves, which occurs because the plant begins to break down chlorophyll—the pigment essential for photosynthesis—and other macromolecules Science, class X (NCERT 2025 ed.), Life Processes, p.82. However, the plant is remarkably efficient; it does not let these resources go to waste. Through a process called nutrient mobilization, the plant reclaims minerals and organic compounds (like amino acids) from the aging leaf and moves them via the phloem to growing buds, seeds, or storage organs Science, class X (NCERT 2025 ed.), Life Processes, p.95.

The primary regulator of this 'anti-aging' process is a class of hormones called Cytokinins. They are the chief architects of the Richmond-Lang Effect, a phenomenon where the application of cytokinins delays senescence by actively 'pulling' nutrients toward the treated area. By promoting protein synthesis and stabilizing chlorophyll, cytokinins keep the leaf 'young' and metabolically active. In contrast, hormones like Abscisic Acid (ABA) and Ethylene act as triggers for senescence and leaf fall (abscission), often in response to environmental stress or the end of a growing season.

While internal hormones control the pace, external factors can drastically accelerate this process. For instance, environmental pollutants like acid rain can cause premature senescence in trees, particularly conifers, by damaging the protective layers of needles and causing a nutrient imbalance Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.104. This illustrates that while senescence is a natural part of plant anatomy and physiology, its timing is a delicate balance between hormonal signals and environmental health.

Feature	Senescence (Aging)	Nutrient Mobilization
Primary Hormone	Abscisic Acid (ABA) & Ethylene	Cytokinins
Key Action	Breakdown of chlorophyll and proteins.	Transport of amino acids and minerals to 'sinks'.
Outcome	Leaf yellowing and eventual abscission.	Conservation of resources for future growth.

Sources: Science, class X (NCERT 2025 ed.), Life Processes, p.82, 95; Environment, Shankar IAS Academy (ed 10th), Environmental Pollution, p.104; Science-Class VII, NCERT(Revised ed 2025), Life Processes in Plants, p.147

7. Cytokinins: The Anti-Ageing Hormone (exam-level)

In the fascinating world of plant physiology, Cytokinins stand out as the primary defenders against the passage of time. While other hormones focus on elongation or stress response, cytokinins are specialized growth promoters that trigger cytokinesis (cell division). Because of this role, they are found in highest concentrations in regions of the plant where cells are rapidly multiplying, such as in developing fruits and seeds Science, Class X (NCERT 2025 ed.), Control and Coordination, p. 108. They ensure the plant doesn't just grow taller, but remains vital and active at a cellular level.

The most remarkable feature of cytokinins is their anti-ageing property, scientifically known as the delay of senescence. Senescence is the biological process where plant organs, like leaves, begin to age, turn yellow, and eventually die. Cytokinins counteract this by preventing the degradation of chlorophyll and proteins. This phenomenon is often called the Richmond-Lang effect. By stimulating the mobilization of nutrients towards the areas where they are applied, cytokinins ensure that a leaf stays green and functional much longer than it naturally would, effectively "pausing" the clock on its decline.

To truly master this, we must look at the hormonal balance within the plant. While cytokinins work to keep the plant young and vibrant, other hormones like Abscisic Acid (ABA) act as the functional opposites. While cytokinins promote life and division, ABA signals the plant to stop growing and induces the wilting of leaves Science, Class X (NCERT 2025 ed.), Control and Coordination, p. 108. This tug-of-war between growth promoters and growth inhibitors determines the lifespan of every leaf and fruit you see.

Feature	Cytokinins	Abscisic Acid (ABA)
Primary Function	Promotes cell division	Inhibits growth
Effect on Leaves	Delays ageing (stays green)	Promotes wilting and falling
Nutrient Action	Mobilizes nutrients to organs	Accelerates nutrient depletion

Sources: Science, Class X (NCERT 2025 ed.), Control and Coordination, p.108

8. Solving the Original PYQ (exam-level)

Now that you have mastered the individual roles of phytohormones, this question tests your ability to link metabolic regulation to physical outcomes like senescence (aging). You have learned that plant growth is not just about physical expansion, but also about maintaining cellular health. This specific question requires you to identify which hormone acts as a "fountain of youth" by directly influencing protein synthesis and nutrient mobilization to keep tissues functional.

To arrive at the correct answer, look for the hormone that specializes in active cell division and metabolic maintenance. Cytokinin is the primary regulator that delays the breakdown of chlorophyll and proteins, a phenomenon often called the Richmond-Lang effect. By promoting the movement of nutrients into leaves, it prevents them from yellowing and wilting. As highlighted in Science, Class X (NCERT), its high concentration in areas of rapid growth like fruits and seeds underscores its role in life-extension, making (C) Cytokinin the definitive choice.

UPSC often uses Abscisic acid (ABA) as a trap because it is the functional opposite; it is a growth inhibitor that promotes leaf fall and aging. While Auxin and Gibberellin are growth-promoters, their primary functions are cell elongation and stem growth, respectively, rather than the specific metabolic retardation of aging through protein turnover. Distinguishing between general growth and specific anti-aging mechanisms is the key to avoiding these common distractors.