Question
Name the five major plant hormones. For each, state where it is produced, its main function, and one agricultural application.
(NEET + CBSE 11 — appears almost every year)
Solution — Step by Step
| Hormone | Produced in | Main effect | Nature |
|---|---|---|---|
| Auxin (IAA) | Shoot tips, young leaves | Cell elongation, apical dominance | Growth promoter |
| Gibberellin (GA₃) | Root tips, young leaves, seeds | Stem elongation, seed germination | Growth promoter |
| Cytokinin | Root tips (carried upward in xylem) | Cell division, delays senescence | Growth promoter |
| Abscisic acid (ABA) | Mature leaves, root cap | Inhibits growth, closes stomata, dormancy | Growth inhibitor |
| Ethylene | Ripening fruits, senescent tissues | Fruit ripening, leaf abscission | Growth inhibitor (gas) |
- Auxin: Used as weedkiller (2,4-D kills dicot weeds), promotes rooting of cuttings
- Gibberellin: Increases grape size (seedless grapes), breaks seed dormancy
- Cytokinin: Keeps cut flowers fresh longer (delays ageing)
- ABA: Called the “stress hormone” — helps plants survive drought by closing stomata
- Ethylene: Used to ripen bananas and tomatoes artificially in ripening chambers
- Auxin vs Cytokinin: Their ratio determines whether a plant tissue forms roots or shoots in tissue culture. High auxin:cytokinin = roots. High cytokinin:auxin = shoots.
- ABA vs Gibberellin: ABA promotes dormancy, GA breaks dormancy — they antagonise each other in seed germination.
- Ethylene: The only gaseous hormone in plants.
Hormone Function Mapping Flowchart
flowchart TD
A["Plant Hormones"] --> B["Growth Promoters"]
A --> C["Growth Inhibitors"]
B --> B1["Auxin — cell elongation"]
B --> B2["Gibberellin — stem elongation"]
B --> B3["Cytokinin — cell division"]
C --> C1["ABA — dormancy, stomatal closure"]
C --> C2["Ethylene — ripening, abscission"]
B1 --> D["Apical dominance: auxin from tip suppresses lateral buds"]
B2 --> E["Bolting: rapid stem elongation before flowering"]
B3 --> F["Delays leaf senescence"]
C1 --> G["Stress hormone: drought response"]
C2 --> H["Only gaseous plant hormone"]Why This Works
Plant hormones coordinate growth, development, and stress responses without a nervous system. They are produced in one part and act in another (or sometimes locally). Understanding their effects and interactions is crucial because NEET regularly tests hormone identification from a described effect.
The key principle: promoters (auxin, GA, cytokinin) vs inhibitors (ABA, ethylene). But the real complexity is in their ratios and antagonisms, which control precise developmental outcomes.
Common Mistake
Students mix up auxin and gibberellin because both promote “growth.” The distinction: auxin causes cell elongation (existing cells get longer), while gibberellin causes stem elongation (internodal elongation) and is specifically linked to seed germination and bolting. If the question says “seed germination” or “breaking dormancy,” the answer is gibberellin, not auxin.