A phytohormone is a plant hormone— a chemical substance produced in plants that controls growth and responses to stimuli, even when present in very small amounts.
Definitions [35]
Define the term growth rate.
Growth rate can be defined as increased plant growth per unit time.
Definition: Stimuli
Stimuli are the changes in the external or internal environment of an organism that trigger a response or movement.
Definition: Responses
Responses are the actions or movements shown by an organism as a result of the stimuli.
Define the following term:
Phytohormones
Plant hormones are known as phytohormones. These are organic molecules that serve as mediators, similar to animal hormones, to facilitate the coordination of a variety of cellular activities within a plant.
Definition: Phytohormones
Definition: Hormones
A hormone is a chemical messenger produced in one part of an organism and transported to other parts, where it regulates growth, development, and responses.
Define the following term:
Apical dominance
In the majority of vascular plants, apical buds dominate over lateral buds. The growth of lateral buds occurs only after the removal of the apical buds. This phenomenon of the suppression of the growth of lateral buds by apical buds is called apical dominance.
Definition: Apical dominance
Apical dominance is the phenomenon where the apical (terminal) bud of a plant suppresses the growth of lateral buds. This is mainly due to the action of auxins produced in the apical bud.
Define the following term:
Parthenocarpy
Parthenocarpy is the process by which fruits develop naturally or artificially without the necessity of fertilizing ovules, resulting in seedless fruits.
Definition: Auxin
Auxin is the first discovered plant growth hormone, coined by F.W. Went in 1928. Derived from the Greek word auxein meaning "to grow," auxins are powerful growth stimulants effective even in low concentrations. They are mainly found in actively growing regions like shoot apex, root apex, and meristems. The main natural auxin is IAA (Indole-3-acetic acid).
Definition: Parthenocarpic Fruits
Parthenocarpic fruits are those that develop without fertilization. Auxins can induce such fruit formation in plants like apples, tomatoes, and bananas.
Definition: Parthenocarpy
Parthenocarpy is the process of fruit development without fertilization.
Definition: Gibberellin
Gibberellin is a plant growth hormone found mainly in meristematic tissues like stem apex, root apex, buds, and seeds. It promotes internode elongation, breaks seed dormancy, and supports fruit development. Gibberellic acid (GA₃) is the most studied form of gibberellin.
Definition: Cytokinin
Cytokinin is a plant hormone discovered in the 1950s by Skoog and Miller. It is primarily produced in root tips and transported via xylem. Cytokinin promotes plant growth by stimulating cell division, especially in germinating seeds, developing fruits, and embryos.
Definition: Ethylene
Ethylene is a plant hormone that exists as a gas at room temperature. It is produced in fruits and acts at the same place where it is made. It was discovered by R. Gane and is formed in higher plants, fungi, and meristematic tissues.
Definition: Abscisic Acid (ABA)
Abscisic acid is a plant hormone found in chloroplasts of leaves and in high amounts in fruits and seeds. It acts as a growth inhibitor and is present in angiosperms, gymnosperms, pteridophytes, and some mosses.
Define the following term:
Abscission
Abscission is the process by which plants shed or detach various portions that are no longer necessary for them, such as leaves, fruits, flowers, and seeds.
Define photoperiodism.
The response of plants to the relative length of light and dark periods with reference to the initiation of flowering is called photoperiodism.
Define vernalization.
The low-temperature treatment or chilling treatment of germinating seeds or seedlings to promote early flowering in plants is called vernalization. It was evidenced by Klippart (1918).
Define chemoautotrophs.
Chemoautotrophs are those who make their own food by deriving the energy from chemical reactions. e.g., Nitrifying bacteria, Iron bacteria etc.
Definition: Efficiency Index
The measure that expresses the rate of increase in growth per unit time is called efficiency index.
Definition: Absolute Growth Rate (AGR)
The ratio of change in the number of cells (dn) over a given time interval (dt), representing total growth per unit time, is called absolute growth rate.
Definition: Relative Growth Rate (RGR)
The growth rate of a system per unit time expressed on a common basis, obtained by dividing AGR by the total number of cells present, is called relative growth rate.
Definition: Differentiation
The permanent change in structure and function of meristematic cells leading to their maturation and specialization, with loss of capacity to divide, is called differentiation.
Definition: Vernalin
The hypothetical chemical stimulus produced during vernalization and responsible for flowering is called vernalin.
Definition: Devernalization
The reversal of the effect of vernalization by high temperature treatment is called devernalization.
Definition: De-differentiation
The process by which living, mature, differentiated cells regain the capacity to divide and become meristematic again, is called de-differentiation.
Definition: Day Neutral Plants (DNP)
Plants that flower independent of the duration of light are called day neutral plants.
Definition: Long Day Plants (LDP)
Plants that flower when the day length is longer than the critical photoperiod and require a short dark period are called long day plants.
Definition: Short Day Plants (SDP)
Plants that flower when the day length is shorter than the critical photoperiod and require a long uninterrupted dark period are called short day plants.
Definition: Vernalization
The induction or acceleration of flowering in plants by low temperature (chilling) treatment is called vernalization.
Definition: Photoperiodism
The response of plants to the duration of light that influences flowering is called photoperiodism.
Definition: Phytochrome
The light-sensitive proteinaceous pigment present in leaves that perceives photoperiodic stimulus and regulates flowering is called phytochrome.
Definition: Re-differentiation
The process by which cells produced after de-differentiation once again lose the capacity to divide and become specialized to perform specific functions, is called re-differentiation.
Definition: Growth Rate
The increase in growth per unit time, indicating the efficiency of growth in plant organs, is called growth rate.
Key Points
Key Points: Auxins
- Auxins stimulate stem, root, and fruit growth by enhancing cell elongation, especially in growing regions like shoot and root apices.
- They delay leaf senescence and help prolong foliage life.
- Auxins promote apical bud growth while suppressing lateral bud development (apical dominance).
- They aid in root initiation in plant cuttings like roses and bougainvillea.
- Auxins can trigger parthenocarpic fruit development in crops like apple, tomato, and banana.
Key Points: Gibberellins
- Gibberellins promote internode elongation by enhancing cell growth, especially in meristematic regions.
- They break seed dormancy and stimulate germination.
- These hormones aid fruit growth and can induce parthenocarpy.
- Gibberellins delay ageing in plants, prolonging functional life.
- Widely used in horticulture and industry—for example, to enlarge grapes and apples, and to accelerate malting in brewing.
Key Points: Cytokinins
- Cytokinins promote plant growth by stimulating cell division, even in non-meristematic tissues.
- They help expand cotyledons during seed development.
- Cytokinins break seed dormancy and encourage germination.
- They enhance chlorophyll synthesis and delay leaf senescence.
- Cytokinins inhibit apical dominance, promoting lateral bud growth.
Key Points: Ethylene
- Ethylene is a gaseous plant hormone produced at the site of action, mainly in fruits and meristematic tissues.
- It promotes fruit ripening and is widely used in agriculture for this purpose.
- It reduces stem elongation and accelerates plant senescence.
- Ethylene initiates seed germination (e.g., in peanuts) and sprouting in potato tubers.
- It enhances root growth, root hair formation, and induces flowering in crops like mango.
Key Points: Abscisic Acid (ABA)
- ABA is a growth-inhibiting hormone that slows down plant metabolism.
- It induces seed dormancy and prevents germination under unfavorable conditions.
- ABA promotes senescence and abscission of leaves, flowers, buds, and fruits.
- It causes stomatal closure, helping the plant conserve water.
- ABA enhances stress tolerance in plants, earning it the name "stress hormone."
Key Points: Plant growth
- Plant Growth
Growth is a characteristic feature of living organisms and is defined as a permanent and irreversible increase in size, mass, or number of cells. - Aspects of Growth
Growth has two aspects: quantitative (increase in size, length, volume, dry weight, and cell number) and qualitative (development and differentiation). - Growth and Development
Development is an ordered and progressive change leading to higher complexity, while differentiation leads to specialized structures. - Nature of Growth in Plants
In multicellular vascular plants, growth is indeterminate and continues throughout life. - Meristematic Regions
Plant growth is restricted to specific regions called meristems, where cells divide continuously. - Types of Meristems
Meristems are of three types based on location: apical (increase in length), intercalary (elongation of internodes), and lateral (increase in girth).
Key Points: Types of Growth
| Aspect | Arithmetic Growth | Geometric Growth |
|---|---|---|
| Definition | Growth in which the rate of increase is constant | Growth in which both daughter cells continue to divide repeatedly |
| Cell Division Pattern | Only one daughter cell continues to divide while the other differentiates | Both daughter cells divide mitotically |
| Growth Rate & Curve | Growth rate remains constant and produces a linear curve | Growth rate is slow initially but later becomes rapid, producing an exponential curve |
| Mathematical Expression | Lt = L0 + rt | Wt = W0 ert |
Key Points: Phases of Growth
| Phase of Growth | Cell Characteristics | Major Events | Growth Rate |
|---|---|---|---|
| Cell Division (Formation) | Thin-walled, non-vacuolated cells with prominent nucleus and dense cytoplasm | Meristematic cells divide mitotically; one remains meristematic, other differentiates | Slow (Lag phase) |
| Cell Enlargement (Elongation) | Vacuolated, osmotically active and turgid cells | Cell enlarges in length and breadth; synthesis of new cell wall materials | Rapid (Exponential / Log phase) |
| Cell Maturation (Differentiation) | Fully developed and specialized cells | Cells attain morphological and physiological maturity | Slow and steady (Stationary phase) |
Key Points: Mineral nutrition
- Mineral Nutrition
The process by which plants absorb and utilize inorganic mineral elements from soil, water, and air for growth and development is called mineral nutrition. - Source and Absorption
Plants absorb minerals mainly from the soil in dissolved ionic form (e.g. PO₄³⁻, SO₄²⁻, CO₃²⁻) through roots. - Essential Elements
Plants require about 36–40 elements, of which some are essential for completing the life cycle and performing structural and physiological functions. - Classification of Minerals
Based on quantity required, minerals are classified into macroelements (required in large amounts) and microelements (required in trace amounts). - Deficiency and Critical Concentration
The minimum concentration of an essential element below which plant growth is affected is called critical concentration, and its shortage leads to deficiency. - Deficiency Symptoms
Common mineral deficiency symptoms include chlorosis, necrosis, stunting, mottling, and abscission, depending on the mobility of the element in plants.
Key Points: Mineral Salt Absorption
| Type of Absorption | Mechanism | Key Features |
|---|---|---|
| Passive Absorption | Diffusion of ions from higher to lower concentration | No energy required; occurs by diffusion, mass flow, ion exchange, and Donnan equilibrium |
| Donnan Equilibrium | Fixation of anions inside the cell attracts cations from outside | Passive process; ions may move against concentration gradient to maintain electrical balance |
| Active Absorption | Uptake of ions against concentration gradient using ATP | Energy-dependent; requires respiration and oxygen; ions move from root hair to xylem |
Key Points: Nitrogen Cycle
| Process | Nitrogen Form Involved | Organisms / Agents | Site of Occurrence | Significance |
|---|---|---|---|---|
| Nitrogen Fixation | Atmospheric nitrogen (N₂) → Ammonia (NH₃) / Nitrates (NO₃⁻) | Lightning, nitrogen-fixing bacteria (Rhizobium, Azotobacter, Cyanobacteria), Haber–Bosch process | Atmosphere, soil, root nodules | Converts inert atmospheric nitrogen into plant-usable forms |
| Ammonification | Organic nitrogen → Ammonia (NH₃) / Ammonium (NH₄⁺) | Ammonifying bacteria and fungi | Soil | Decomposition of dead plants and animals releases ammonia |
| Nitrification | NH₃ / NH₄⁺ → NO₂⁻ → NO₃⁻ | Nitrosomonas (ammonia → nitrite), Nitrobacter (nitrite → nitrate) | Soil | Converts ammonia into nitrates readily absorbed by plants |
| Assimilation | NO₃⁻ / NH₄⁺ → Amino acids, proteins, nucleic acids | Green plants (animals obtain nitrogen through food chain) | Plant tissues | Incorporation of inorganic nitrogen into organic compounds |
| Denitrification | NO₃⁻ → N₂ (gaseous nitrogen) | Pseudomonas, Bacillus, Paracoccus | Anaerobic / waterlogged soils | Returns nitrogen to the atmosphere, completing the cycle |
Important Questions [29]
- What is photorespiration?
- What is a grand period of growth?
- What is redifferentiation?
- What is differentiation?
- A fine powder of recycled modified plastic is known as ______.
- Name the hormone that is responsible for apical dominance.
- Give the full form of IAA.
- Which is the primary precursor of IAA in plants?
- A farmer wants to remove broad-leaved weeds from the jowar plantation in his field. Suggest any plant hormone to remove such weeds.
- What are gibberellins?
- What Are Gibberellins? Give the Applications of Gibberellins.
- Mention any two applications of cytokinin.
- Write the physiological effects of Cytokinin.
- Which of the following is the only gaseous plant growth regulator?
- Explain the role of chlorophyllase enzyme in banana.
- Describe physiological effects and applications of gaseous hormones in plants.
- Give the chemical features of abscisic acid.
- ______ hormone is responsible for the efflux of K+ ions from guard cells and acts as an antitranspirant.
- Abscisic acid is the common name given to two identical substances isolated separately. Name them.
- Give a reason – ABA is also known as an antitranspirant.
- Identify the growth hormone in plants which causes inhibitory effect.
- Which of the following is not a photosynthetic pigment?
- Give advantages of vernalization.
- Complete the following box with reference to symptoms of mineral deficiency: Abscission Pre-mature fall of flowers,fruits and leaves ______ Appearance of green andnon-green patches on leaves
- Why Chl-a (Chlorophyll-a) is Called Essential Pigment?
- Give graphic representation of cyclic photophosphorylation.
- Give the schematic representation of the TCA cycle.
- Define chemoautotrophs.
- Name the cell which is responsible for nitrogen fixation in cyanobacteria.
Concepts [19]
- Plant Growth
- Phases of Plant Growth
- Conditions Necessary for Plant Growth
- Plant Growth Rate
- Plant Growth Curve
- Differentiation, De-differentiation, Re- Differentiation
- Plant Development
- Plant Plasticity
- Plant Hormones
- Auxins
- Gibberellins
- Cytokinins
- Ethylene
- Abscisic Acid (ABA)
- Photoperiodism
- Vernalization (Yarovization)
- Plant Mineral Nutrition
- Nitrogen Cycle
- Overview of Plant Growth and Mineral Nutrition
