What is the primary difference between plant growth promoters and inhibitors?

Plant growth promoters, such as auxins, gibberellins, and cytokinins, generally stimulate various growth processes like cell division, cell enlargement, stem elongation, flowering, and fruit development. They are essential for active growth phases. In contrast, plant growth inhibitors, primarily abscisic acid and often ethylene, typically suppress growth, induce dormancy, promote abscission (shedding of leaves/fruits), and help plants respond to stress conditions. While ethylene can have some promoter-like effects, its overall role in senescence and ripening often places it in the inhibitor category, especially in the context of growth cessation.

How do auxins contribute to apical dominance, and how can it be overcome?

Apical dominance is the phenomenon where the growth of the apical (terminal) bud inhibits the growth of lateral (axillary) buds. This is primarily due to the high concentration of auxin produced by the apical meristem, which moves downwards and suppresses lateral bud development. To overcome apical dominance, the apical bud can be removed, a process called decapitation. This eliminates the source of high auxin, allowing the lateral buds to grow. This technique is commonly used in horticulture to make plants bushier.

Explain the role of gibberellins in seed germination.

Gibberellins play a crucial role in breaking seed dormancy and promoting germination, especially in cereal grains like barley. Upon imbibition (water absorption), the embryo releases gibberellins, which then diffuse to the aleurone layer (the outermost layer of the endosperm). Here, GAs stimulate the synthesis and secretion of hydrolytic enzymes, particularly $alpha$-amylase. This enzyme breaks down the stored starch in the endosperm into simple sugars, providing the necessary energy and building blocks for the growing embryo to emerge from dormancy and begin growth.

Why is ethylene considered a unique plant growth regulator?

Ethylene is unique among PGRs primarily because of its gaseous nature. Unlike other hormones that are transported through vascular tissues, ethylene can diffuse through air spaces within plant tissues and even into the surrounding atmosphere, affecting nearby plants. This gaseous property makes it highly effective in processes like fruit ripening, where it can trigger a 'climacteric' rise in respiration and accelerate the ripening of an entire batch of fruits. Its simple chemical structure ($C_2H_4$) also sets it apart.

How does Abscisic Acid (ABA) help plants cope with drought stress?

Abscisic Acid (ABA) is often referred to as the 'stress hormone' because of its critical role in helping plants survive adverse environmental conditions, particularly drought. When a plant experiences water scarcity, ABA levels rapidly increase. This surge in ABA signals the guard cells surrounding the stomata to close. Stomatal closure significantly reduces transpiration, the process of water vapor loss from leaves, thereby conserving precious water within the plant. Additionally, ABA can induce dormancy in seeds and buds, allowing the plant to postpone growth until more favorable conditions return.

What is parthenocarpy, and which PGR is commonly used to induce it?

Parthenocarpy is the natural or artificially induced production of fruit without fertilization of ovules, resulting in seedless fruits. This phenomenon is highly desirable in many fruit crops for consumer preference. Auxins are the plant growth regulators most commonly used to induce parthenocarpy. When applied exogenously to flowers, auxins can stimulate the development of the ovary into a fruit even in the absence of pollination and fertilization, leading to the formation of seedless fruits like tomatoes, grapes, and watermelons. This application has significant commercial value in horticulture.

Plant Growth Regulators

Biology

NEET UG

Version 1Updated 22 Mar 2026

Explore This Topic

Definition Deep Explanation Core Principles NEET Importance Problem Strategy Practice Problems MCQ Practice Quick Revision

Plant Growth Regulators (PGRs), also known as phytohormones, are small, simple molecules of diverse chemical composition, which are naturally produced by plants. These chemical messengers act in very low concentrations to influence various physiological processes, including cell division, cell enlargement, pattern formation, tropic growth, flowering, fruiting, and seed formation. They can be broad…

Quick Summary

Plant Growth Regulators (PGRs), or phytohormones, are naturally occurring organic compounds in plants that control growth and development at very low concentrations. They are broadly classified into promoters and inhibitors.

Promoters include auxins, gibberellins, and cytokinins. Auxins, like IAA, promote cell elongation, apical dominance, and root initiation, and are used as herbicides (2,4-D) and for parthenocarpy. Gibberellins (e.

g., GA3) cause stem elongation, break seed dormancy, and promote bolting. Cytokinins (e.g., kinetin, zeatin) are crucial for cell division, lateral bud growth, and delaying senescence. Inhibitors include abscisic acid (ABA) and ethylene.

ABA is a stress hormone, inducing seed dormancy, stomatal closure during drought, and abscission. Ethylene, a gaseous hormone, promotes fruit ripening, senescence, and abscission. PGRs interact synergistically or antagonistically to fine-tune plant responses, making them vital for plant survival and agricultural productivity.

Vyyuha

Your 6-Month Blueprint, Updated Nightly

AI analyses your progress every night. Wake up to a smarter plan. Every. Single.…

Key Concepts

Apical Dominance and its Regulation

Apical dominance is a fundamental growth pattern in many plants where the main central stem (apical bud)…

Ethylene's Role in Fruit Ripening

Ethylene is a unique gaseous plant hormone that plays a pivotal role in the ripening of many fruits,…

Abscisic Acid (ABA) as a Stress Hormone

Abscisic Acid (ABA) is often termed the 'stress hormone' due to its critical functions in helping plants…

Auxins (IAA, IBA, NAA, 2,4-D) — Cell elongation, apical dominance, root initiation, parthenocarpy, herbicide (2,4-D).

Gibberellins (GA3) — Stem elongation (bolting), break seed dormancy, fruit enlargement, malting.

Cytokinins (Kinetin, Zeatin) — Cell division, lateral bud growth, delay senescence, shoot differentiation.

Ethylene ($C_2H_4$) — Gaseous, fruit ripening, senescence, abscission, triple response.

Abscisic Acid (ABA) — Stress hormone, seed dormancy, stomatal closure, abscission, growth inhibitor.

Promoters — Auxins, Gibberellins, Cytokinins.

Inhibitors — ABA, Ethylene (often).

Antagonistic pairs — GA-ABA (dormancy), Auxin-Cytokinin (apical dominance, shoot/root ratio).

A G C E A: All Growing Cells Exhibit Activity.

Auxin: Apical dominance, Absence of seeds (parthenocarpy), Allongation.

Gibberellin: Giant growth (stem elongation/bolting), Germination (breaks dormancy), Grape size.

Cytokinin: Cell division, Counteracts apical dominance (lateral buds), Chloroplasts, Celay senescence.

Ethylene: Every fruit Eats Ethylene (ripening), Everything Ends (senescence/abscission), Epinasty.

Abscisic Acid: All Adverse conditions (stress hormone), Arrests growth (dormancy), Absent water (stomatal closure), Abscission.