1. List the macro and micronutrients in plants highlighting role of each.
Answer: Plants require various mineral nutrients for growth and development. They are classified into macronutrients (required in large amounts) and micronutrients (required in trace amounts).
Macronutrients:
Carbon (C): Backbone of all organic compounds. Hydrogen (H): Component of water and organic compounds. Oxygen (O): Component of water and organic compounds; used in respiration. Nitrogen (N): Component of proteins, nucleic acids, chlorophyll, and many coenzymes. Phosphorus (P): Component of ATP, nucleic acids, phospholipids, and coenzymes.
Potassium (K): Regulates stomatal opening, enzyme activation, and osmotic balance. Calcium (Ca): Component of cell walls (calcium pectate), involved in cell signaling and membrane stability. Magnesium (Mg): Central atom of chlorophyll; activates many enzymes. Sulphur (S): Component of some amino acids (cysteine, methionine) and coenzyme A.
Micronutrients:
Iron (Fe): Component of cytochromes and ferredoxin; essential for chlorophyll synthesis. Manganese (Mn): Activates enzymes; involved in photosynthesis (water splitting). Zinc (Zn): Activates enzymes; involved in auxin synthesis. Copper (Cu): Component of some enzymes (e.g., cytochrome oxidase). Boron (B): Involved in cell wall formation, pollen germination, and sugar transport. Molybdenum (Mo): Component of nitrogenase and nitrate reductase. Chlorine (Cl): Involved in photosynthesis (water splitting) and osmotic balance.
Each nutrient has specific roles, and deficiencies lead to characteristic symptoms. For example, nitrogen deficiency causes chlorosis and stunted growth, while iron deficiency causes interveinal chlorosis. Understanding nutrient roles is essential for agriculture and horticulture.
2. Explain the role of stomata and palisade tissue in the exchange of gases in plants.
Answer: Gas exchange in plants occurs primarily through stomata and involves the palisade tissue.
Stomata:
Stomata are tiny pores on the leaf surface, primarily on the lower epidermis of dicots and both surfaces of monocots. Each stoma is surrounded by two guard cells that regulate its opening and closing.
When guard cells are turgid, the stomata open, allowing carbon dioxide (CO₂) to enter and oxygen (O₂) to exit. When guard cells lose water and become flaccid, the stomata close, preventing water loss (transpiration). Stomata are the main sites of gas exchange in leaves. They allow CO₂ for photosynthesis and O₂ for respiration.
Palisade Tissue:
The palisade mesophyll is a layer of elongated, closely packed cells just below the upper epidermis. These cells contain numerous chloroplasts and are the primary sites of photosynthesis. During photosynthesis, palisade cells consume CO₂ and produce O₂. The O₂ diffuses out of the palisade cells, through the intercellular spaces, and out of the stomata. CO₂ from the atmosphere diffuses into the leaf through stomata, travels through intercellular spaces, and enters palisade cells.
Coordination:
Stomata and palisade tissue work together to ensure efficient gas exchange. Stomata provide the entry and exit points for gases, while palisade cells are the main consumers and producers of gases. The structure of the leaf (intercellular spaces) facilitates the diffusion of gases between the stomata and the mesophyll cells.
In summary, stomata regulate gas exchange, and palisade tissue performs photosynthesis, consuming CO₂ and producing O₂. Together, they facilitate the exchange of gases necessary for photosynthesis and respiration.
3. Relate transpiration with gas exchange in plants.
Answer: Transpiration and gas exchange are closely related processes in plants, both occurring primarily through stomata.
Transpiration:
Transpiration is the loss of water vapor from the aerial parts of a plant, mainly through stomata. It creates a negative pressure (tension) in the xylem, pulling water upward from the roots. Transpiration also cools the plant and transports minerals.
Gas Exchange:
Gas exchange involves the intake of CO₂ for photosynthesis and the release of O₂, as well as the intake of O₂ for respiration and the release of CO₂. It occurs through stomata and lenticels.
Relationship:
Stomatal Regulation: Stomata are the common pathway for both transpiration and gas exchange. When stomata open for CO₂ intake, water vapor escapes (transpiration). When stomata close to conserve water, gas exchange is reduced. Trade-off: Plants face a trade-off between maximizing CO₂ uptake for photosynthesis and minimizing water loss. In hot, dry conditions, stomata close to conserve water, which reduces CO₂ uptake and can lead to photorespiration. Environmental Factors: Factors that affect transpiration (e.g., light, temperature, humidity, wind) also affect gas exchange. For example, high light intensity opens stomata, increasing both transpiration and CO₂ uptake. Adaptations: Plants have evolved various adaptations to balance these processes. Xerophytes have thick cuticles, sunken stomata, and CAM photosynthesis to reduce water loss. Hydrophytes have large air spaces and stomata on the upper surface.
In summary, transpiration and gas exchange are linked through stomatal regulation. Plants must balance water loss with CO₂ uptake to optimize photosynthesis and survival. Understanding this relationship is crucial for agriculture and ecology.
4. Describe the structure of xylem vessel elements, sieve tube elements, companion cells, tracheids and relate their structures with functions.
Answer: The vascular tissue of plants consists of xylem and phloem, each with specialized cell types.
Xylem:
Tracheids:
Structure: Long, tapered cells with lignified secondary walls. They have pits (thin areas) for water movement. They are dead at maturity. Function: Transport water and minerals; provide structural support.
Vessel Elements:
Structure: Shorter, wider cells with lignified walls. They have perforation plates at their ends, allowing for continuous water flow. They are dead at maturity. Function: Transport water and minerals more efficiently than tracheids; provide support.
Phloem:
Sieve Tube Elements:
Structure: Living cells at maturity but lack a nucleus and most organelles. They have sieve plates at their ends with pores for sap flow. Function: Transport sugars (sucrose) and other organic nutrients from sources to sinks.
Companion Cells:
Structure: Living cells with a nucleus and many organelles, closely associated with sieve tube elements via plasmodesmata. Function: Support the sieve tube elements by providing proteins, ATP, and regulating their activity.
Structure-Function Relationship:
The lignified walls of xylem cells provide rigidity, allowing them to withstand the negative pressure of transpiration. The hollow nature of vessel elements and tracheids provides a low-resistance pathway for water. The perforation plates and sieve plates of phloem cells allow for efficient transport of sap. The lack of organelles in sieve tube elements maximizes space for sap flow, while companion cells provide metabolic support.
In summary, the structure of xylem and phloem cells is highly adapted for their functions: xylem for water transport and support, phloem for sugar transport.
5. Explain movement of water between plant cells, and between the cells and their environment in terms of water potential.
Answer: Water movement in plants is driven by differences in water potential (Ψ). Water always moves from a region of higher water potential to a region of lower water potential.
Water Potential:
Water potential is the potential energy of water per unit volume. Pure water has a water potential of zero. Water potential is affected by solute concentration (solute potential, Ψs) and pressure (pressure potential, Ψp). The addition of solutes lowers water potential (makes it negative). Increased pressure raises water potential (makes it more positive).
Movement Between Cells:
Water moves from a cell with higher water potential to a cell with lower water potential. This can occur through the symplast (cytoplasm via plasmodesmata) or the apoplast (cell walls and intercellular spaces).
Osmosis drives water movement across cell membranes.
Movement Between Cells and Environment:
Root Hairs: Soil water has a higher water potential than the root hair cell cytoplasm (which contains solutes). Water moves by osmosis into the root hair. Leaf Cells: In the leaf, water evaporates from the mesophyll cell walls into the intercellular spaces and then out through the stomata (transpiration). This creates a low water potential in the leaf, pulling water up the xylem. Turgor Pressure: When a plant cell is placed in pure water, water enters the cell, increasing pressure potential (turgor pressure). This pressure opposes further water uptake, and equilibrium is reached when the water potential inside and outside the cell are equal.
In summary, water potential gradients drive water movement in plants. Water moves from soil to roots, up the stem, and out of the leaves, driven by differences in water potential created by solute concentration and transpiration.
6. Explain the movement of water in xylem through roots in terms of symplast, apoplast and vascular pathways.
Answer: Water movement from the soil to the xylem in roots involves three pathways: the apoplast, symplast, and vascular pathways.
1. Apoplast Pathway:
Water moves through the cell walls and intercellular spaces of the root cortex. It does not cross any cell membranes. This pathway offers little resistance to water flow. However, at the endodermis, the Casparian strip (a waxy band in the cell walls) blocks the apoplast pathway, forcing water into the symplast.
2. Symplast Pathway:
Water moves through the cytoplasm of root cells, connected by plasmodesmata. It crosses cell membranes, allowing for selective uptake of ions. This pathway is slower than the apoplast but allows for regulation of ion and water movement. At the endodermis, water enters the symplast and then moves into the xylem.
3. Vascular Pathway:
Once water reaches the endodermis, it enters the xylem (tracheids and vessel elements). The xylem forms a continuous, low-resistance pathway for water to move up the root and into the stem. Root pressure (generated by active ion transport into the xylem) can push water up the xylem, although transpiration pull is the main driving force.
Coordination:
Water enters the root hair (apoplast and symplast). It moves across the cortex via both pathways. At the endodermis, the Casparian strip forces water into the symplast. Water then enters the xylem and is transported upward.
In summary, water movement in roots involves the apoplast and symplast pathways, with the Casparian strip playing a crucial role in regulating entry into the xylem. The vascular pathway (xylem) then transports water to the rest of the plant.
7. Explain the movement of water in xylem through TACT mechanism.
Answer: The TACT mechanism (Transpiration, Adhesion, Cohesion, Tension) explains the upward movement of water in the xylem.
1. Transpiration:
Water evaporates from the leaves through stomata. This creates a negative pressure (tension) in the leaf mesophyll and the xylem. Transpiration is the driving force for water movement.
2. Adhesion:
Water molecules adhere to the walls of xylem vessels (due to hydrogen bonding with cellulose). This adhesion helps to counteract gravity and supports the water column.
3. Cohesion:
Water molecules are cohesive (attracted to each other) due to hydrogen bonds. This cohesion maintains a continuous water column in the xylem, allowing water to be pulled up without breaking.
4. Tension:
The tension (negative pressure) created by transpiration pulls the water column upward. The cohesive and adhesive properties of water allow the column to withstand this tension without breaking.
Mechanism:
Transpiration creates tension in the leaf xylem. This tension is transmitted down the xylem to the roots. Water is pulled up from the roots to the leaves. Cohesion and adhesion maintain the integrity of the water column. Root pressure can also contribute, but transpiration pull is the dominant force.
In summary, the TACT mechanism explains how water moves up the xylem: transpiration creates tension, cohesion maintains the water column, and adhesion prevents it from breaking. This mechanism allows water to reach the top of even the tallest
trees.
8. Describe the mechanisms involved in the opening and closing of stomata.
Answer: The opening and closing of stomata are regulated by changes in the turgor pressure of guard cells.
Opening of Stomata:
Ion Uptake: In response to light (blue light) and low CO₂ levels, guard cells actively transport potassium ions (K⁺) into the cell. Water Uptake: The increased K⁺ concentration lowers the water potential of the guard cells, causing water to enter by osmosis. Turgor Increase: The influx of water increases the turgor pressure in the guard cells. Cell Wall Structure: Guard cells have unevenly thickened cell walls. The inner wall is thicker and less extensible. When turgor increases, the outer wall stretches more, causing the cells to bow outward and the stomatal pore to open.
Closing of Stomata:
Ion Efflux: In response to darkness, water stress, or high CO₂ levels, K⁺ ions are actively transported out of the guard cells. Water Loss: The loss of K⁺ increases the water potential of the guard cells, causing water to leave by osmosis. Turgor Decrease: The loss of water decreases turgor pressure in the guard cells. Pore Closure: The guard cells become flaccid, and the stomatal pore closes.
Role of Abscisic Acid (ABA):
During water stress, ABA is produced and triggers the closing of stomata by stimulating K⁺ efflux and inhibiting K⁺
influx.
In summary, stomatal opening and closing are driven by changes in guard cell turgor, regulated by ion transport, water movement, and hormonal signals (ABA). This mechanism allows plants to balance CO₂ uptake with water conservation.
9. Explain the movement of sugars within plants.
Answer: The movement of sugars in plants occurs through the phloem, from a source (where sugar is produced or stored) to a sink (where sugar is used or stored). This process is explained by the pressure flow hypothesis.
1. Source (e.g., mature leaves):
Sucrose is actively loaded into the sieve tube elements at the source. This is an active process that requires energy (ATP). The loading of sucrose lowers the water potential in the sieve tubes, causing water to enter from the xylem by osmosis. The influx of water increases the hydrostatic pressure in the sieve tubes at the source.
2. Translocation:
The high hydrostatic pressure at the source drives the flow of phloem sap (sucrose solution) through the sieve tubes to the sink. The flow is from source to sink, down a pressure gradient.
3. Sink (e.g., roots, fruits, seeds):
At the sink, sucrose is actively unloaded from the sieve tubes. This is also an active process. The unloading of sucrose increases the water potential in the sieve tubes at the sink, causing water to leave and
return to the xylem. The loss of water decreases the hydrostatic pressure at the sink, maintaining the pressure gradient.
4. Phloem Loading and Unloading:
Loading: Can be apoplastic (via cell walls and transporter proteins) or symplastic (via plasmodesmata). Unloading: Can be apoplastic or symplastic, depending on the sink.
In summary, sugar movement in plants is driven by a pressure gradient created by active loading at the source and active unloading at the sink. This mechanism ensures that sugars are transported to where they are needed most.
10. Explain movement of water into or out of cell in isotonic, hypotonic, hypertonic conditions.
Answer: The movement of water into or out of a cell depends on the relative solute concentration of the cell and its surrounding environment.
1. Isotonic Conditions:
The solute concentration outside the cell is equal to the solute concentration inside the cell. Water potential inside and outside the cell is equal. There is no net movement of water. The cell maintains its normal shape and turgor pressure.
2. Hypotonic Conditions:
The solute concentration outside the cell is lower than inside the cell (i.e., the external solution is more dilute). Water potential outside is higher than inside. Water moves into the cell by osmosis.
In plant cells, the cell wall prevents bursting; the cell becomes turgid. This is ideal for plant cells. In animal cells, the cell may swell and burst (lyse).
3. Hypertonic Conditions:
The solute concentration outside the cell is higher than inside the cell (i.e., the external solution is more concentrated). Water potential outside is lower than inside. Water moves out of the cell by osmosis. In plant cells, the cell membrane pulls away from the cell wall (plasmolysis), and the cell becomes flaccid. In animal cells, the cell shrinks (crenation).
In summary, water moves from a region of higher water potential to a region of lower water potential. In hypotonic solutions, water enters the cell; in hypertonic solutions, water leaves the cell; in isotonic solutions, there is no net movement.
11. Describe osmotic adjustments in hydrophytic (marine and freshwater), xerophytic and mesophytic plants.
Answer: Plants adapt to their water environments through osmotic adjustments.
1. Hydrophytes:
Freshwater Hydrophytes: Live in hypotonic environments (freshwater). Water tends to enter the cells by osmosis. They have adaptations to prevent bursting, such as rigid cell walls and contractile vacuoles (in some algae). They may have reduced cuticles and stomata. Marine Hydrophytes: Live in hypertonic environments (saltwater). Water tends to leave the cells. They accumulate solutes (e.g., ions, organic compounds) to lower their internal water potential and prevent water loss. They may have salt glands to excrete excess salt.
2. Xerophytes:
Live in dry environments with low water availability. They have adaptations to conserve water and tolerate drought. Osmotic Adjustment: They accumulate solutes (e.g., proline, sugars) to lower their water potential, allowing them to absorb water from dry soil. Other Adaptations: Thick cuticles, sunken stomata, rolled leaves, succulent tissues, and CAM photosynthesis.
3. Mesophytes:
Live in moderate environments with adequate water supply. They do not have extreme osmotic adjustments. They maintain water balance through normal stomatal regulation and root uptake. They may wilt under drought conditions but can recover.
In summary, hydrophytes, xerophytes, and mesophytes have different osmotic adjustments to survive in their respective environments. Hydrophytes face osmotic challenges from their aquatic surroundings, xerophytes conserve water, and mesophytes maintain balance in moderate conditions.
12. Explain the osmotic adjustments in plants in saline soils.
Answer: Plants growing in saline soils (halophytes) face a hypertonic environment, meaning the soil solution has a lower water potential than the plant cells. This makes it difficult for plants to absorb water. To survive, halophytes have evolved osmotic adjustments.
1. Solute Accumulation:
Halophytes accumulate high concentrations of solutes in their cells to lower their internal water potential below that
of the soil. This allows water to enter the roots by osmosis. Solutes include inorganic ions (Na⁺, Cl⁻, K⁺) and organic compounds (proline, glycine betaine, sugars).
2. Ion Compartmentalization:
Toxic ions like Na⁺ and Cl⁻ are sequestered in vacuoles to prevent damage to cytoplasmic enzymes. Organic solutes are accumulated in the cytoplasm to balance the osmotic pressure.
3. Salt Excretion:
Some halophytes have salt glands in their leaves that actively excrete excess salt. Others store salt in older leaves, which are then shed.
4. Succulence:
Many halophytes are succulent, storing water in large vacuoles to dilute salt concentrations.
5. Ion Transport:
They have specialized transport proteins to regulate ion uptake and distribution.
In summary, halophytes survive in saline soils by accumulating solutes, compartmentalizing ions, excreting salt, and storing water. These osmotic adjustments allow them to maintain water uptake and prevent salt toxicity.
13. List the adaptations in plants to cope with low and high temperature.
Answer: Plants have evolved various adaptations to cope with extreme temperatures.
Adaptations to Low Temperature (Cold):
Antifreeze Proteins: Prevent ice crystal formation. Cold Hardening: Increase in solutes (sugars, proline) to lower freezing point. Dormancy: Seeds and buds become dormant to survive winter. Insulation: Thick bark, scales, and hairs. Leaf Abscission: Shedding leaves to reduce water loss and damage. Rosette Growth: Low-growing rosettes to avoid wind and retain heat.
Adaptations to High Temperature (Heat):
Transpiration: Evaporative cooling. Leaf Orientation: Vertical leaves to avoid direct sunlight. Reflective Surfaces: Hairy or waxy leaves to reflect light. Heat Shock Proteins: Protect enzymes from denaturation. CAM Photosynthesis: Stomata open at night to reduce water loss. Dormancy: Summer dormancy (aestivation) in some plants. Small Leaves: Reduce surface area for heat absorption.
In summary, plants adapt to low temperatures by avoiding freezing and conserving energy, and to high temperatures by cooling, reflecting light, and conserving water. These adaptations are crucial for survival in extreme environments.
14. Explain the turgor pressure and explain its significance in providing support to herbaceous plants.
Answer: Turgor pressure is the pressure exerted by the cell contents against the cell wall, caused by the osmotic uptake of water. When a plant cell is placed in a hypotonic solution, water enters the cell, increasing its volume. The rigid cell wall resists expansion, creating turgor pressure.
Significance in Herbaceous Plants:
Support: Turgor pressure provides mechanical support to herbaceous plants, which lack extensive secondary growth (wood). The turgid cells in the cortex and pith make the stem rigid and upright. Cell Expansion: Turgor pressure is essential for cell expansion and growth. It pushes against the cell wall, allowing cells to elongate. Stomatal Opening: Turgor pressure in guard cells regulates stomatal opening and closing. Movement: Turgor changes drive nastic movements, such as the closing of Mimosa leaves. Wilting: When plants lose water and turgor pressure decreases, they wilt. This is a visible sign of water stress.
In summary, turgor pressure is crucial for the support, growth, and function of herbaceous plants. It keeps them upright and allows for various physiological processes.
15. Describe the structure of supporting tissue in plants.
Answer: Supporting tissues in plants provide mechanical strength and support. They include collenchyma, sclerenchyma, and xylem.
1. Collenchyma:
Structure: Living cells with unevenly thickened primary cell walls. The thickenings are usually at the corners where cells meet. The walls are made of cellulose and pectin. Function: Provides flexible support to growing stems, leaves, and petioles. It allows for growth and elongation. Location: Found in the cortex of stems, petioles, and along veins of leaves.
2. Sclerenchyma:
Structure: Dead cells at maturity with thick, lignified secondary cell walls. They have pits for water and nutrient exchange. Types: Fibers (long, slender) and sclereids (variable shapes, e.g., stone cells).
Function: Provides rigid support and protection. Fibers are strong and flexible; sclereids are hard and brittle. Location: Found in stems, roots, leaves, and fruits (e.g., pear grit).
3. Xylem:
Structure: Composed of tracheids and vessel elements, which have lignified secondary walls. They are dead at maturity. Function: Transports water and minerals; provides structural support. Location: Forms the wood in woody plants.
In summary, collenchyma, sclerenchyma, and xylem are supporting tissues with distinct structures and functions. They provide support for plant growth and development.
16. Define growth and explain primary and secondary growth in plants.
Answer: Growth in plants is an irreversible increase in size, volume, and mass. It involves cell division, cell elongation, and cell differentiation.
Primary Growth:
Definition: Growth in length, occurring at the apical meristems of roots and shoots. Meristems: Apical meristems are located at the tips of roots and shoots. Process: Cells divide rapidly in the meristematic zone, then elongate in the zone of elongation, and differentiate in the zone of maturation. Result: Increases the length of the plant and produces primary tissues (epidermis, cortex, primary xylem, primary phloem). Occurrence: All vascular plants undergo primary growth.
Secondary Growth:
Definition: Growth in thickness (girth), occurring at the lateral meristems (vascular cambium and cork cambium) in woody plants. Meristems: Vascular cambium produces secondary xylem (wood) and secondary phloem. Cork cambium produces periderm (bark). Process: The vascular cambium divides, adding layers of secondary xylem internally and secondary phloem externally. Result: Increases the diameter of the stem and root. Occurrence: Mostly in gymnosperms and woody dicots.
In summary, primary growth increases length, while secondary growth increases girth. Both are essential for plant development.
17. Describe the role of apical meristem and lateral meristem in primary and secondary growth.
Answer: Meristems are regions of active cell division in plants. Apical and lateral meristems play distinct roles in growth.
Apical Meristems:
Location: Tips of roots and shoots. Role: Responsible for primary growth (increase in length). Process: Cells divide rapidly, producing new cells that differentiate into primary tissues (epidermis, cortex, primary vascular tissues, leaves). Hormones: Auxins and cytokinins regulate apical dominance and growth.
Lateral Meristems:
Location: Cylindrical layers in stems and roots (vascular cambium and cork cambium). Role: Responsible for secondary growth (increase in girth/thickness). Vascular Cambium: Produces secondary xylem (wood) internally and secondary phloem externally. Cork Cambium: Produces periderm (bark) to replace the epidermis. Occurrence: Mostly in woody plants (gymnosperms and dicots).
In summary, apical meristems drive primary growth (length), while lateral meristems drive secondary growth (thickness). Both are essential for plant development and adaptation.
18. Explain how annual rings are formed in plants.
Answer: Annual rings are formed due to the seasonal activity of the vascular cambium in woody plants growing in temperate regions with distinct seasons.
Spring (Early Wood):
In spring, water is abundant, and the cambium is highly active. It produces large, thin-walled xylem cells (tracheids and vessel elements) to conduct water efficiently. This early wood is lighter in color and less dense.
Summer (Late Wood):
In late summer, water becomes less abundant, and growth slows. The cambium produces smaller, thicker-walled xylem cells. This late wood is darker in color and denser.
Annual Ring:
The transition from early wood to late wood forms a distinct ring.
One ring typically represents one year of growth. The age of a tree can be estimated by counting the number of annual rings.
Factors:
Annual rings are prominent in trees from temperate climates with distinct seasons. Tropical trees often lack distinct rings due to constant growing conditions.
In summary, annual rings are formed by the differential growth of xylem in spring and summer. They are useful for dendrochronology (tree-ring dating) and understanding past climates.
19. Explain influence of apical meristem on the growth of lateral shoot.
Answer: The apical meristem influences the growth of lateral shoots through a phenomenon called apical dominance.
Apical Dominance:
The apical bud (at the tip of the main shoot) produces auxin (IAA). Auxin is transported downwards and inhibits the growth of lateral buds (axillary buds). This ensures that the main shoot grows taller, competing for light.
Mechanism:
High auxin concentration near the lateral buds suppresses their outgrowth. Cytokinins, produced in the roots, promote lateral bud growth. The balance between auxin and cytokinin determines whether lateral buds grow.
Effects of Removing Apical Bud:
If the apical bud is removed (pruning), auxin levels decrease.
Lateral buds are released from inhibition and start growing. This results in a bushier plant with more lateral branches.
Significance:
Apical dominance is important for plant architecture. It is manipulated in horticulture (pruning) to shape plants and increase yield.
In summary, the apical meristem influences lateral shoot growth via auxin-mediated apical dominance. Removing the apical bud promotes lateral branching.
20. Explain the role of important plant growth regulators.
Answer: Plant growth regulators (phytohormones) are chemical messengers that regulate plant growth and development. The major ones are:
1. Auxins:
Role: Promote cell elongation, apical dominance, root initiation, phototropism, geotropism. Example: Indole-3-acetic acid (IAA).
2. Gibberellins:
Role: Promote stem elongation, seed germination, flowering, fruit development. Example: Gibberellic acid (GA₃).
3. Cytokinins:
Role: Promote cell division, lateral bud growth, delay senescence (aging). Example: Zeatin.
4. Abscisic Acid (ABA):
Role: Inhibits growth, promotes seed dormancy, stomatal closure, stress response. Example: Abscisic acid.
5. Ethylene:
Role: Promotes fruit ripening, leaf abscission, senescence. Example: Ethylene gas.
Interactions:
Hormones often work in concert or antagonistically. The balance of hormones determines the plant's response.
In summary, plant growth regulators control all aspects of plant growth and development. They are used in agriculture to improve crop yield, quality, and stress tolerance.
21. Explain types of movement in plants in response to light, gravity, touch and chemicals.
Answer: Plants exhibit various movements in response to environmental stimuli, classified as tropisms and nastic movements.
1. Phototropism (Light):
Directional growth response to light. Positive Phototropism: Shoots bend towards light (due to auxin redistribution). Negative Phototropism: Roots bend away from light.
2. Geotropism/Gravitropism (Gravity):
Directional growth response to gravity. Positive Geotropism: Roots grow downwards (towards gravity). Negative Geotropism: Shoots grow upwards (against gravity).
3. Thigmotropism (Touch):
Directional growth response to touch. Tendrils coil around a support (e.g., pea plant tendrils).
4. Chemotropism (Chemicals):
Directional growth response to chemicals. Pollen tube grows towards ovule (due to chemical attractants).
5. Nastic Movements (Non-directional):
Nyctinasty: Response to day/night (e.g., closing of petals). Seismonasty: Response to shock (e.g., Mimosa pudica leaves fold).
In summary, plants respond to light, gravity, touch, and chemicals through tropisms and nastic movements. These movements are essential for survival and adaptation.
22. Classify plants on the basis of photoperiodism and give examples.
Answer: Photoperiodism is the response of plants to the relative lengths of day and night (photoperiod), which controls flowering and other developmental processes. Plants are classified into three main categories:
1. Short-Day Plants (Long-Night Plants):
Flower when the night length exceeds a critical minimum (e.g., when days are short).
Examples: Chrysanthemum, soybean, poinsettia, strawberry.
2. Long-Day Plants (Short-Night Plants):
Flower when the night length is shorter than a critical maximum (e.g., when days are long). Examples: Spinach, lettuce, radish, wheat, barley.
3. Day-Neutral Plants:
Flowering is not dependent on photoperiod. Examples: Tomato, cucumber, sunflower, dandelion.
Significance:
Photoperiodism ensures that flowering occurs at the optimal time for pollination and seed production. It is important in agriculture for controlling flowering times.
In summary, plants are classified as short-day, long-day, or day-neutral based on their flowering response to photoperiod. This classification is important for understanding plant adaptation and for agricultural practices.
23. Explain the mechanism of photoperiodism with reference to the mode of action of phytochrome.
Answer: Photoperiodism is regulated by a pigment called phytochrome, which exists in two interconvertible forms:
1. Pr (Phytochrome Red):
Inactive form, absorbs red light (660 nm). Synthesized in the dark.
2. Pfr (Phytochrome Far-Red):
Active form, absorbs far-red light (730 nm). Formed when Pr absorbs red light.
Interconversion:
Pr [source symbol unavailable] Pfr: When exposed to red light. Pfr [source symbol unavailable] Pr: When exposed to far-red light or in darkness (slow conversion).
Mechanism:
The ratio of Pfr to Pr determines the plant's response. In short-day plants, high Pfr levels inhibit flowering. Flowering occurs when Pfr levels drop below a threshold during long nights. In long-day plants, high Pfr levels promote flowering. Flowering occurs when Pfr levels remain high due to short nights. Phytochrome acts by regulating gene expression, leading to the production of flowering hormones (florigen).
Significance:
Phytochrome allows plants to sense the length of day and night. It also regulates other processes like seed germination, stem elongation, and leaf expansion.
In summary, phytochrome is the molecular switch for photoperiodism. Its interconversion between Pr and Pfr forms allows plants to measure night length and trigger appropriate developmental responses, such as flowering.
24. Define vernalization and explain the role of low temperature treatment on flower production.
Answer: Vernalization is the process by which exposure to prolonged cold temperature induces or accelerates flowering in certain plants. It is a mechanism that ensures flowering occurs in the appropriate season (spring), after the cold of winter has
passed.
Role of Low Temperature:
Induction of Flowering: Many plants (e.g., winter wheat, biennials like cabbage, carrots) require a period of cold to become competent to flower. Without vernalization, they remain vegetative. Biochemical Changes: Cold treatment induces the synthesis of specific proteins (e.g., vernalization proteins) and changes in gene expression. It also affects the levels of growth regulators like gibberellins. Epigenetic Changes: Vernalization involves stable epigenetic modifications (e.g., histone modifications) that repress flowering repressor genes (like FLC in Arabidopsis), allowing flowering to proceed. Perception Site: The cold is perceived by the shoot apical meristem. Timing: The cold treatment must be of sufficient duration (e.g., several weeks) and at a specific temperature range (e.g., 0-10°C).
Significance:
Vernalization ensures that flowering occurs in spring, when conditions are favorable for pollination and seed production. It is important in agriculture for synchronizing flowering and maximizing yield. It prevents premature flowering before winter, which would be fatal.
In summary, vernalization is the cold-induced promotion of flowering. It involves biochemical and epigenetic changes that allow plants to flower at the optimal time. Understanding vernalization is crucial for crop management and breeding.
