Plant Growth & Development – Advanced Study Notes
(Prepared for IIT‑JEE / Competitive Exams – KVPY Historic Theme)
Goal – Provide a concise, rigorous, exam‑oriented guide that covers the core concepts, mechanistic details, key equations, and subtle points that frequently appear in competitive biology questions.
1. Overview
| Concept | Definition | Key Process |
|---|---|---|
| Growth | Increase in size (mass/length) | Cell division (mitosis), cell expansion |
| Development | Acquisition of new structures/functions | Differentiation, organogenesis, programmed cell death |
| Growth–Development Coupling | Hormones coordinate both | Auxin → elongation, cytokinin → division |
| Plasticity | Ability to change in response to environment | Photomorphogenesis, tropisms, vernalization |
Tip – In exam questions, differentiate growth (quantity) from development (quality).
Example: “What is the difference between primary and secondary growth?” – primary: elongation of shoots/root; secondary: thickening via vascular cambium.
2. Phases of Plant Growth
| Phase | Main Events | Key Hormones |
|---|---|---|
| Seed Germination | Water uptake, activation of metabolism | Abscisic Acid (ABA) ↓, Gibberellins (GA) ↑ |
| Primary (Apical) Growth | Shoot & root elongation | Auxin (elongation), Gibberellins (stem) |
| Secondary Growth | Vascular cambium and cork cambium activity | Cytokinin, Auxin |
| Reproductive Development | Flowering, fruiting | Gibberellins, Ethylene, Brassinosteroids |
Tricky Point – Primary growth is driven by the apical meristem, whereas secondary growth involves vascular cambium. Many questions test this distinction.
3. Hormonal Regulation – Detailed Mechanisms
3.1 Auxin
| Feature | Details |
|---|---|
| Synthesis | Indole‑3‑acetic acid (IAA) from tryptophan via IPyA pathway |
| Transport | Polar auxin transport via PIN proteins (efflux) and AUX1/LAX (influx) |
| Signal Transduction | TIR1/AFB F‑box receptor → Aux/IAA degradation → ARF transcription factors |
| Effect on Growth | Stimulates cell elongation by loosening cell wall (expansins) and increasing proton pumping (acid growth) |
Derivation – Acid growth model:
( \text{Cell elongation rate} = k \times \Delta pH )
where (k) is a constant, and (\Delta pH) is the difference between apoplastic acidification and cytoplasmic pH.
3.1.1 Polar Transport Equation (Diffusion + Advection)
The net auxin flux (J) in a root segment:
[ J = -D \nabla C + v C ]
- (D) – diffusion coefficient
- (C) – auxin concentration
- (v) – effective velocity due to PIN‑mediated active transport
At steady state (( \frac{dJ}{dx}=0 )) one obtains an exponential concentration profile:
[ C(x) = C_0 , e^{-v x / D} ]
Exam Hint – Questions may ask to discuss how this gradient leads to differential cell elongation on the lower vs. upper side of a root after gravistimulation.