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Short Notes Plant Growth and Development - Short Notes for NEET
Seed Germination
| Type | Description | Examples |
|---|---|---|
| Epigeal Germination | Cotyledons come above the soil surface Hypocotyl elongates | Castor, Bean, Cotton, Papaya, Onion |
| Hypogeal Germination | Cotyledons remain below soil Epicotyl elongates | Pea, Gram, Mango, Maize, Rice |
Requirements for Seed Germination
- Water - For imbibition, enzyme activation, and metabolic activities
- Oxygen - For aerobic respiration to provide energy
- Temperature - Optimal temperature (25-35°C for most seeds)
- Light - Required by some seeds (photoblastic seeds)
- Viable embryo - Living and non-dormant
Phases of Plant Growth
| Phase | Characteristics | Features |
|---|---|---|
| 1. Meristematic/Formative Phase | Phase of cell division | • Abundant mitosis • Rich protoplasm, large nuclei • Thin cellulosic walls • Isodiametric cells |
| 2. Elongation/Enlargement Phase | Phase of cell enlargement | • Vacuolation occurs • Cell wall thinning • Maximum growth rate • New cell wall deposition |
| 3. Maturation/Differentiation Phase | Phase of cell maturation | • Thickening of cell walls • Protoplasmic modifications • Cells attain final size and shape • Metabolic maturity achieved |
Plant Growth Rate
Arithmetic Growth
- Formula: L₁ = L₀ + rt
- Where: L₁ = length at time t, L₀ = initial length, r = growth rate, t = time
- Linear growth pattern
- Example: Root elongation at constant rate
- Produces a linear curve when plotted
Geometric/Exponential Growth
- Formula: W₁ = W₀e^(rt)
- Where: W₁ = final size, W₀ = initial size, r = growth rate, t = time, e = base of natural logarithm
- Both progeny cells divide after mitosis
- Produces exponential/J-shaped curve
- Seen in initial stages of growth
Sigmoid Growth Curve
- Most common growth pattern in plants
- Lag phase - Slow initial growth
- Log/Exponential phase - Rapid growth
- Stationary phase - Growth slows down and plateaus
- Represents growth of roots, shoots, and leaves
Conditions of Growth
| Factor | Role in Growth |
|---|---|
| Water | Essential for cell turgidity, biochemical activities, nutrient transport |
| Oxygen | Required for aerobic respiration and energy generation |
| Temperature | Affects enzyme activity; optimal range 28-30°C for most plants |
| Light | Energy source for photosynthesis; affects photoperiodism and phytochrome |
| Nutrients | Macro and micronutrients essential for metabolism and structure |
| Gravity | Affects geotropism in roots and shoots |
Differentiation, Dedifferentiation, and Redifferentiation
| Process | Definition | Example |
|---|---|---|
| Differentiation | Process by which meristematic cells attain permanent shape, size, and function Loss of ability to divide | Formation of tracheids, vessel elements, sieve tubes, fibers from meristematic cells |
| Dedifferentiation | Permanent cells regain capacity to divide Mature cells become meristematic again | Formation of interfascicular cambium, cork cambium, wound healing tissue |
| Redifferentiation | Dedifferentiated cells lose ability to divide and mature again Final maturation after dedifferentiation | Secondary xylem and phloem formation from vascular cambium |
Sequence of Developmental Process in a Plant Cell
- Step 1: Cell division (in meristematic tissue)
- Step 2: Cell enlargement and elongation
- Step 3: Cell differentiation (structural and functional specialization)
- Step 4: Cell maturation (physiological and biochemical maturity)
- Optional: Dedifferentiation → Redifferentiation (in some cases)
- This sequence is open - cells can revert to meristematic condition
Growth Regulators (Plant Hormones)
1. AUXINS
General Features
- First plant hormone discovered by F.W. Went
- Term coined by Kogl and Haagen-Smit
- Main naturally occurring auxin: Indole-3-Acetic Acid (IAA)
- Synthesized in shoot apical meristem, young leaves, developing seeds
- Synthetic auxins: IBA, NAA, 2,4-D, 2,4,5-T
Physiological Effects
| Function | Details |
|---|---|
| Cell elongation | Promotes cell elongation in shoots; basis of phototropism and geotropism |
| Apical dominance | Suppresses lateral bud growth; removal of shoot tip reduces auxin, allowing lateral growth |
| Root initiation | Promotes adventitious root formation; used in vegetative propagation |
| Parthenocarpy | Induces seedless fruit formation |
| Prevents abscission | Delays leaf and fruit drop |
| Herbicide | 2,4-D and 2,4,5-T used as weedicides (kill dicot weeds) |
| Xylem differentiation | Promotes vascular tissue formation |
2. GIBBERELLINS (GA)
General Features
- Discovered by Japanese scientist E. Kurosawa from fungus Gibberella fujikuroi
- Over 100 gibberellins identified (GA₁, GA₂, GA₃... etc.)
- GA₃ (Gibberellic acid) most common and studied
- Chemically, they are acidic in nature
Physiological Effects
| Function | Details |
|---|---|
| Stem elongation | Promotes internodal elongation; causes bolting in rosette plants |
| Seed germination | Breaks dormancy; mobilizes food reserves during germination |
| α-amylase production | Stimulates enzyme synthesis in barley seeds for starch hydrolysis |
| Flowering | Induces flowering in long-day plants even under short-day conditions |
| Parthenocarpy | Induces seedless fruit formation |
| Delays senescence | Keeps leaves and fruits active longer |
| Sex expression | Promotes male flower formation in some plants |
3. CYTOKININS
General Features
- Discovered by F. Skoog and C.O. Miller
- Natural cytokinins: Zeatin (from corn kernels)
- Synthetic: Kinetin, BAP (Benzyl Amino Purine)
- Promote cytokinesis (cell division)
- Synthesized in root apices
Physiological Effects
| Function | Details |
|---|---|
| Cell division | Promotes cytokinesis; essential for cell division |
| Delays senescence | Prevents aging in leaves (Richmond-Lang effect); keeps leaves green longer |
| Breaks apical dominance | Promotes lateral bud growth (opposite to auxin) |
| Nutrient mobilization | Directs nutrients to cytokinin-applied area |
| Chloroplast development | Promotes chlorophyll synthesis |
| Morphogenesis | Along with auxin, controls shoot and root formation in tissue culture |
| Overcomes dormancy | Promotes seed germination in some species |
4. ETHYLENE (C₂H₄)
General Features
- Only gaseous plant hormone
- Simple chemical structure: CH₂=CH₂
- Produced in ripening fruits, senescent tissues
- Released during stress conditions
- Precursor: Methionine → ACC → Ethylene
Physiological Effects
| Function | Details |
|---|---|
| Fruit ripening | Promotes ripening; "one rotten apple spoils the barrel" |
| Abscission | Promotes leaf, flower, and fruit abscission |
| Senescence | Accelerates aging in flowers and leaves |
| Triple response | In dicot seedlings: reduced stem elongation, increased radial swelling, horizontal growth |
| Flowering | Induces flowering in pineapple and mango |
| Root/shoot growth inhibition | Generally inhibits growth |
| Sex expression | Promotes female flower formation in cucurbits |
| Breaking dormancy | In potato tubers and some seeds |
5. ABSCISIC ACID (ABA)
General Features
- Also called "Stress hormone" or "Dormin"
- Discovered by Addicott and others
- Acts as growth inhibitor
- Produced in mature leaves, stems, roots, fruits
- Increases under stress conditions (drought, salinity, cold)
Physiological Effects
| Function | Details |
|---|---|
| Stomatal closure | Causes stomata to close during water stress; reduces transpiration |
| Seed dormancy | Maintains seed dormancy; prevents premature germination |
| Abscission | Promotes leaf, flower, and fruit drop |
| Growth inhibition | Inhibits cell division and elongation |
| Senescence | Promotes aging |
| Antagonistic to GA | Opposes gibberellin action in many processes |
| Stress response | Helps plants tolerate drought, cold, and salinity |
Comparison Table: Growth Promoters vs Inhibitors
| Growth Promoters | Growth Inhibitors |
|---|---|
| Auxins, Gibberellins, Cytokinins | Abscisic Acid (ABA), Ethylene |
| Promote growth and development | Inhibit growth; promote dormancy and abscission |
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FAQs on Short Notes Plant Growth and Development - Short Notes for NEET
| 1. What are the primary factors affecting plant growth? |  |
Ans. The primary factors affecting plant growth include light, water, temperature, soil nutrients, and air quality. Each of these elements plays a crucial role in processes such as photosynthesis, respiration, and nutrient uptake, ultimately influencing a plant's overall health and growth rate.
| 2. What is the role of auxins in plant development? | |
Ans. Auxins are a class of plant hormones that regulate various aspects of growth and development, including cell elongation, root formation, and fruit development. They are primarily produced in the stem tips and play a vital role in phototropism and gravitropism, helping plants adjust their growth direction in response to light and gravity.
| 3. How do environmental conditions influence seed germination? | |
Ans. Environmental conditions such as moisture, temperature, light, and oxygen availability significantly influence seed germination. Adequate moisture activates enzymes that begin the metabolic processes necessary for growth, while optimal temperature ensures the right metabolic rate. Some seeds also require specific light conditions to germinate, and oxygen is essential for cellular respiration during this early growth phase.
| 4. What is the significance of photosynthesis in plant growth? | |
Ans. Photosynthesis is the process by which plants convert light energy into chemical energy in the form of glucose, using carbon dioxide and water. This process is essential for plant growth as it provides the necessary energy and organic compounds for cellular functions, supporting growth, development, and reproduction.
| 5. How do nutrients affect plant health and development? | |
Ans. Nutrients are vital for plant health as they are involved in various physiological functions. Macronutrients like nitrogen, phosphorus, and potassium are required in larger quantities and are essential for growth, energy transfer, and overall development. Micronutrients, although needed in smaller amounts, are crucial for processes such as enzyme function and chlorophyll synthesis. A deficiency or imbalance in nutrients can lead to poor growth, reduced yield, and increased susceptibility to diseases.
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