13.1 Growth — Definition & Features
Growth is an irreversible, permanent increase in size of an organ, its parts or an individual cell. It is accompanied by an increase in dry weight, volume, cell number and protoplasm.
- In plants growth is indeterminate — they grow throughout life due to meristems.
- Growth is measurable: by increase in length, area, volume, fresh/dry weight or cell number.
- Cell division (formation of new protoplasm) is the basis of growth; it is open form of growth in plants.
Meristems
| Meristem | Location | Result |
|---|
| Apical | Root & shoot tips | Primary (length) growth |
| Intercalary | Base of internodes (grasses) | Length after injury/grazing |
| Lateral (vascular & cork cambium) | Sides of stem/root | Secondary (girth) growth |
Key idea: Apical & intercalary meristems are primary meristems (length); lateral meristems are secondary (girth).
13.2 Phases & Rates of Growth
Growth passes through three phases in a cell/tissue:
Meristematic (cell division)→Elongation (cell enlargement)→Maturation (differentiation)
Growth rate
Growth rate = increased growth per unit time. It can be arithmetic or geometric.
| Type | What happens | Graph |
|---|
| Arithmetic | After mitosis only one daughter cell keeps dividing; the other differentiates | Straight line |
| Geometric (exponential) | Both daughter cells keep dividing | Exponential rise |
Arithmetic: Lₜ = L₀ + rt (L₀ = length at start, r = growth rate, t = time)
Geometric/exponential: W₁ = W₀ · e^(rt) (e = base of natural log, r = relative growth rate/efficiency index)
The sigmoid (S-shaped) growth curve
Plotting size against time gives a typical S-shaped sigmoid curve with three phases:
Lag phase (slow)→Log/exponential phase (fast)→Stationary phase (slows, limited)
Absolute vs relative growth rate: absolute = total growth per unit time; relative growth rate (RGR) = growth per unit time expressed per unit initial size — lets us compare a small and a large system fairly.
13.3 Conditions for Growth
Plant growth needs both internal (hormones, genes) and external factors:
- Water: provides turgidity needed for cell enlargement; medium for enzyme activity.
- Oxygen: for respiration → energy (ATP) for growth.
- Nutrients: macro- & micronutrients supply raw material and energy.
- Temperature: an optimum range is needed for the enzymes driving growth.
- Light & gravity: influence growth direction (tropic movements) and development.
Remember: Cell enlargement/elongation depends most directly on water (turgor); cell division depends on nutrients and hormones.
13.4 Differentiation, Dedifferentiation & Redifferentiation
Differentiation is the maturation of cells derived from meristems into specialised structures performing specific functions — e.g. forming a tracheary element: the cell loses its protoplasm and develops a strong, lignified wall.
| Process | Meaning | Example |
|---|
| Differentiation | Meristem cell → mature, specialised cell | Xylem/tracheid formation |
| Dedifferentiation | Mature cell regains the ability to divide | Formation of interfascicular & cork cambium |
| Redifferentiation | Dedifferentiated cells mature again (lose dividing ability) | Secondary xylem/phloem from cambium |
🧠 Memory Hook
"DIFFERENT → DE (back to dividing) → RE (mature again)"
Differentiation = mature; Dedifferentiation = back to meristematic; Redifferentiation = matures once more.
13.5 Development & Plasticity
Development = the sum of growth + differentiation across a plant's life cycle (seed → seedling → mature plant → senescence).
Development = Growth + Differentiation
Plasticity: plants can follow different developmental pathways in response to environment or stage of life, producing different forms.
- Heterophylly — different leaf shapes on the same plant. In cotton/coriander/larkspur, juvenile and adult leaves differ.
- In aquatic buttercup (Ranunculus), submerged leaves are finely divided while aerial leaves are broad.
Controlled by: intrinsic factors (genes, intracellular & intercellular hormones) and extrinsic factors (light, temperature, water, nutrients).
13.6 Plant Growth Regulators — Auxins, Gibberellins, Cytokinins
Plant growth regulators (PGRs) are small chemical signals. They fall into promoters (auxins, gibberellins, cytokinins) and inhibitors (abscisic acid; ethylene acts both ways).
Auxins (e.g. IAA — indole-3-acetic acid)
- Discovered through Charles & Francis Darwin (coleoptile bending to light) and isolated by F.W. Went from oat (Avena) coleoptile tips. The Avena curvature test is its bioassay.
- Cause cell elongation; maintain apical dominance (suppress lateral buds); promote rooting (adventitious roots); induce parthenocarpy; prevent fruit & leaf drop early; herbicide 2,4-D kills dicot weeds.
Gibberellins (e.g. GA₃)
- First from the fungus Gibberella fujikuroi (causes "bakanae"/foolish-seedling disease of rice).
- Cause bolting (internode elongation in rosette plants); increase fruit length (e.g. apple); speed up malting in brewing; break seed/bud dormancy; delay senescence.
Cytokinins (e.g. kinetin, zeatin)
- Kinetin came from herring sperm DNA (not natural in plants); zeatin is a natural cytokinin from maize kernels & coconut milk.
- Promote cytokinesis (cell division); help new leaf/chloroplast formation; delay senescence (Richmond–Lang effect); overcome apical dominance.
🧠 Memory Hook
"Auxin = APICAL; Gibberellin = GROW TALL; Cytokinin = CYTOKINESIS"
A→apical dominance, G→bolting/elongation, C→cell division. Promoters = A, G, C.
13.7 Ethylene & Abscisic Acid (the brakes)
Ethylene (a gaseous PGR)
- Promotes fruit ripening (climacteric fruits), senescence & abscission of leaves/flowers.
- Breaks seed & bud dormancy; promotes root-hair & root growth; induces female flowers in cucurbits; causes the "triple response" in seedlings.
- Source compound used in fields: ethephon.
Abscisic acid (ABA — the "stress hormone")
- General growth inhibitor; promotes seed & bud dormancy.
- Causes stomatal closure under water stress → reduces transpiration.
- Acts as an antagonist of gibberellins.
| Regulator | Headline role |
|---|
| Auxin | Apical dominance, rooting, parthenocarpy |
| Gibberellin | Bolting, breaks dormancy, malting |
| Cytokinin | Cell division, delays senescence |
| Ethylene | Ripening, abscission, senescence |
| ABA | Stress, dormancy, stomatal closure |
🧠 Memory Hook
"ABA = Awfully Bad Atmosphere → close stomata, sleep (dormancy)"
ABA = stress hormone; ethylene = "ripe & rot" (ripening, senescence, abscission).
13.8 Photoperiodism & Vernalization
Photoperiodism
The response of flowering to the relative lengths of day and night (discovered by Garner & Allard). It is actually the length of the dark (night) period that is critical.
| Type | Flowers when | Examples |
|---|
| Short-day plants (SDP) | Night longer than a critical length | Rice, soybean, chrysanthemum, tobacco, Xanthium |
| Long-day plants (LDP) | Night shorter than a critical length | Wheat, spinach, henbane, radish |
| Day-neutral (DNP) | Independent of photoperiod | Tomato, cucumber, maize, sunflower |
Light/dark is perceived by the leaves (pigment phytochrome); a hypothetical flowering hormone florigen is thought to move to the shoot apex.
Vernalization
The promotion of flowering by a period of low temperature (cold). It prevents premature flowering and lets plants flower in the right season.
- Seen in winter varieties of wheat, rye, barley and in biennials (sugar beet, cabbage, carrot).
- High temperature can reverse it (devernalization).
🧠 Memory Hook
"SDP = needs a LONG NIGHT; Vernalization = VERY cOld → flower"
Short-day = long-night plants. Vernalization = cold-induced flowering. Leaves perceive photoperiod; florigen carries the signal.