Plant Growth Regulators
தாவர வளர்ச்சி சீராக்கிகள்
Do not mistake Plant Growth Regulators for mere stimulants. They are precise chemical signals that dictate the entire life cycle of a plant, from germination to senescence. If you confuse their chemical nature with animal hormones, you will lose marks on the most basic NEET questions.
The idea
Plant Growth Regulators (PGRs) are small, simple molecules with diverse chemical compositions. They function as phytohormones, regulating physiological processes like growth, differentiation, and development. They act as intrinsic factors, controlling plant development alongside extrinsic environmental factors.
Why it matters
They exist to provide internal chemical coordination and regulation of physiological processes in response to both internal genetic programs and external environmental stimuli.
NEET lens
NEET focuses on the chemical nature and specific physiological effects of each PGR. Memorize the classification into promoters (Auxins, Gibberellins, Cytokinins) and inhibitors (Abscisic acid). Expect matching-type questions; know which hormone promotes bolting or delays senescence. Do not skip the 'Discovery' section in NCERT, as it is a frequent source of direct factual questions.
Clinical significance
In humans, endocrine disruption by synthetic chemicals often mimics the role of PGRs, leading to developmental disorders. Just as PGRs maintain homeostasis in plants, hormonal imbalances in humans lead to metabolic failure. Understanding these signaling pathways is fundamental to grasping how complex organisms regulate their internal environment.
Introduction
Plant growth regulators (PGRs) are naturally occurring organic compounds that, in small concentrations, regulate plant growth and development. The five classical PGRs are auxins, gibberellins, cytokinins, abscisic acid, and ethylene.
Core concept
Overview of five PGRs: Auxins — cell elongation, phototropism, apical dominance. Gibberellins — stem elongation, seed germination, flowering. Cytokinins — cell division, delay senescence, lateral bud promotion. Abscisic acid — stress response, stomatal closure, dormancy (stress hormone / inhibitor). Ethylene — fruit ripening, senescence, epinasty (gaseous hormone). General features: effective in trace amounts, chemical messengers, can be promoters or inhibitors, interact with each other.
Go deeper
PGR interactions: Auxin:Cytokinin ratio determines organogenesis in tissue culture — high auxin:cytokinin → root formation, low ratio → shoot formation. Auxin and ethylene: auxin promotes ethylene synthesis at high concentrations → some auxin effects (fruit drop, senescence) are mediated through ethylene.
From Class 10
Plants don't have a nervous system, but they coordinate their growth and responses to the environment using chemical signals — plant growth regulators (PGRs) or plant hormones.