🧠 Plant Hormones

This lesson shows how phytohormones regulate plant development and how their manipulation supports practical crop and horticulture management.

Introduction

Plant hormones (phytohormones) are naturally occurring organic substances that regulate growth, development, and responses to environmental stimuli at very low concentrations. They are produced in one part of the plant and transported to another where they exert their effect. The five major classes of plant hormones are auxins, gibberellins, cytokinins, ethylene, and abscisic acid (ABA). Their agricultural applications are immense, from enhancing crop yield to controlling ripening and managing weeds.

Auxins

Auxins (primarily Indole-3-Acetic Acid / IAA) were the first plant hormones discovered. They are produced mainly in shoot apical meristems, young leaves, and developing seeds.

Functions: Promote cell elongation (growth in length), apical dominance (suppression of lateral bud growth), root initiation from cuttings, phototropism (bending toward light), and gravitropism (root growth downward). They also promote fruit development and prevent premature fruit drop.

Agricultural applications: Synthetic auxins like 2,4-D and NAA are used as selective herbicides (killing broadleaf weeds in cereal fields), rooting hormones (dipping cuttings in IBA/NAA solutions promotes root formation), parthenocarpic fruit production (seedless fruits), and preventing pre-harvest fruit drop in apple and citrus orchards.

Gibberellins

Gibberellins (over 130 types, GA3 being the most studied) were discovered from the fungus Gibberella fujikuroi that caused "foolish seedling disease" (bakanae) in rice.

Functions: Promote stem elongation (internode elongation), break seed dormancy, induce flowering in long-day plants, stimulate enzyme production (alpha-amylase) during seed germination, and promote parthenocarpy.

Agricultural applications: GA3 is commercially used to increase grape berry size and bunch elongation, break dormancy in potato tubers and seeds, promote bolting and seed production in biennial crops (cabbage, beet), and enhance sugarcane internode elongation for higher sugar yield.

Cytokinins

Cytokinins (e.g., zeatin, kinetin, BAP) promote cell division and were discovered during tissue culture research.

Functions: Stimulate cell division (cytokinesis), delay leaf senescence (anti-aging effect), promote lateral bud growth (antagonistic to auxin's apical dominance), and enhance nutrient mobilization toward treated areas.

Agricultural applications: Used extensively in plant tissue culture for shoot multiplication, applied to delay yellowing and extend shelf life of leafy vegetables and cut flowers, and used to break apical dominance for bushier plant growth in ornamental horticulture.

Ethylene

Ethylene (C2H4) is the only gaseous plant hormone, produced in ripening fruits, senescing tissues, and stressed plant parts.

Functions: Promotes fruit ripening, leaf and flower abscission (shedding), senescence, and formation of the abscission layer. It also induces flowering in pineapple and mango, and promotes femaleness in cucurbits.

Agricultural applications: Ethephon (ethrel), which releases ethylene, is used to synchronize fruit ripening in tomato and banana, promote latex flow in rubber trees, induce flowering in pineapple, and defoliate cotton before mechanical harvesting. Controlled atmosphere storage with low ethylene levels extends fruit and vegetable storage life.

Abscisic Acid (ABA)

ABA is known as the "stress hormone" because its levels increase dramatically under drought, salinity, and cold stress conditions.

Functions: Induces stomatal closure to reduce water loss during drought, promotes seed dormancy (preventing premature germination), inhibits growth, and promotes leaf abscission under stress.

Agricultural applications: ABA analogs are being developed as anti-transpirants to improve drought tolerance. Understanding ABA signalling helps breeders develop stress-tolerant crop varieties. ABA also plays a role in fruit maturation and the development of seed dormancy mechanisms important for seed storage and viability.

Summary Cheat Sheet

• Major hormones: auxins, gibberellins, cytokinins, ethylene, and ABA.
• Auxins regulate elongation and rooting; synthetic forms aid weed control.
• Gibberellins promote elongation and dormancy breaking.
• Cytokinins support cell division and delay senescence.
• Ethylene and ABA are central to ripening, stress response, and storage biology.