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GNDU Question Paper-2024
Bachelor of Computer Application (BCA) (Hons.)
5
th
Semester (Batch 2024-28) (CBGS)
BOTANY: Paper-V (A)
(Plant Physiology)
Time Allowed: Three Hours Max. Marks:35
Note: Attempt Five questions in all, selecting at least One question from each section. The
Fifth question may be attempted from any section. All questions carry equal marks.
SECTION-A
1. Elucidate transport of water from soil to atmosphere through plants.
2. Discuss the theories explaining the mechanism of stomatal opening and closing.
SECTION-B
3. Write in detail about micro-nutrients and their deficiency and toxic effects in plants.
4. Discuss in detail phloem loading and unloading in plants.
SECTION-C
5. Discuss C4 Cycle and its variation in plants.
6. Explain photorespiration and its importance to plants.
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SECTION-D
7. Elucidate the physiology of flowering and role of dark period.
8. Discuss biosynthesis of auxins and their physiological roles in plant.
GNDU Answer Paper-2024
Bachelor of Computer Application (BCA) (Hons.)
5
th
Semester (Batch 2024-28) (CBGS)
BOTANY: Paper-V (A)
(Plant Physiology)
Time Allowed: Three Hours Max. Marks:35
Note: Attempt Five questions in all, selecting at least One question from each section. The
Fifth question may be attempted from any section. All questions carry equal marks.
SECTION-A
1. Elucidate transport of water from soil to atmosphere through plants.
Ans: 1. Elucidate the Transport of Water from Soil to Atmosphere Through Plants
The movement of water from the soil → roots → stem → leaves → atmosphere is called
the Soil-Plant-Atmosphere Continuum (SPAC) or simply the transpiration stream. This is
one of the most important natural processes because it helps plants survive, grow, make
food, and maintain the Earth's water cycle.
Simple Definition
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Transport of water from soil to atmosphere through plants is the continuous movement of
water from the soil into plant roots, then through the stem to the leaves, and finally into
the air as water vapour through tiny pores called stomata.
Think of a plant as a natural water pump.
Soil = Water tank
Roots = Straw that absorbs water
Stem = Pipeline
Leaves = Water outlet
Atmosphere = Final destination
Simple Diagram
Atmosphere
↑
Water Vapour (Transpiration)
↑
┌───────────┐
│ Leaves │
│ (Stomata) │
└─────↑─────┘
│
Xylem Tissue
│
┌─────┴─────┐
│ Stem │
└─────↑─────┘
│
Xylem
│
┌─────┴─────┐
│ Roots │
│Root Hairs │
└─────↑─────┘
│
Soil Water
Step-by-Step Explanation
Step 1: Water is Present in the Soil
The journey begins in the soil.
Rainwater or irrigation water fills the spaces between soil particles. This water contains
important dissolved minerals such as:
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Nitrogen
Phosphorus
Potassium
Calcium
Magnesium
Plants need both water and minerals for healthy growth.
Step 2: Roots Absorb Water
The roots have millions of tiny structures called root hairs.
These root hairs increase the surface area so the plant can absorb more water.
Water enters root hairs mainly through osmosis.
What is Osmosis?
Osmosis is the movement of water from an area where there is more water (soil) to an area
where there is less water (root cells) through a selectively permeable membrane.
Simply put,
Water naturally moves from the wet soil into the comparatively drier root cells.
Step 3: Water Moves Through the Root
After entering the root hairs, water moves through different layers of root tissues until it
reaches the xylem vessels.
The xylem is a special tissue responsible for transporting water throughout the plant.
You can think of xylem as the water pipes inside a building.
Step 4: Water Travels Up the Stem
Now the water must travel upward, sometimes to trees that are over 50 metres tall.
But plants do not have a heart or pump.
So how does water move upward?
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It moves because of three important forces:
1. Root Pressure
Roots continuously absorb water.
This creates pressure that pushes water upward.
However, root pressure alone is not strong enough for tall trees.
2. Capillary Action
Water molecules stick to the walls of narrow xylem vessels.
This helps water rise upward in thin tubes.
It is similar to how a paper towel absorbs water.
3. Transpiration Pull (Most Important)
This is the main force.
When water evaporates from the leaves, it creates a pulling force.
Imagine drinking juice with a straw.
When you suck from the top, the juice rises.
Similarly,
Leaves "pull" water upward from the roots.
This force is called transpiration pull.
Step 5: Water Reaches the Leaves
When water reaches the leaves, it is used for many important functions.
(a) Photosynthesis
Plants use water and carbon dioxide to prepare food in the presence of sunlight.
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Without water, plants cannot make food.
(b) Cooling the Plant
Water helps cool the plant during hot weather.
It works like sweating in humans.
(c) Maintaining Cell Pressure
Water keeps plant cells firm and upright.
Without enough water, plants wilt and become weak.
(d) Transport of Minerals
Water carries dissolved minerals to every part of the plant.
Step 6: Water Leaves Through Stomata
Leaves contain tiny pores called stomata.
These pores open and close with the help of guard cells.
Most of the water absorbed by plants (about 9599%) is not used for making food.
Instead, it escapes into the atmosphere as water vapour.
This process is called transpiration.
What is Transpiration?
Transpiration is the loss of water from the aerial parts of plants, mainly through the
stomata in the form of water vapour.
It is the final step in the transport of water from soil to atmosphere.
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Complete Journey of Water
Rain / Irrigation
↓
Soil
↓
Root Hairs
↓
Roots
↓
Xylem
↓
Stem
↓
Leaves
↓
Stomata
↓
Water Vapour
↓
Atmosphere
Importance of Water Transport
This process is essential because it:
Helps plants absorb water and minerals.
Supplies water for photosynthesis.
Keeps plant cells turgid and prevents wilting.
Cools the plant through transpiration.
Transports nutrients to different plant parts.
Supports the global water cycle by returning water to the atmosphere.
Factors Affecting Transpiration
Several environmental factors influence the rate of transpiration:
Temperature: Higher temperatures increase transpiration.
Sunlight: Bright sunlight opens stomata and increases water loss.
Wind: Wind removes water vapour around leaves, increasing transpiration.
Humidity: High humidity reduces transpiration because the air already contains
more moisture.
Soil Water Availability: Dry soil reduces water absorption and lowers transpiration.
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Conclusion
The transport of water from soil to atmosphere through plants is a continuous and highly
organized process. Water is absorbed from the soil by root hairs, transported upward
through the xylem with the help of root pressure, capillary action, and transpiration pull,
used in the leaves for photosynthesis, cooling, and growth, and finally released into the
atmosphere through stomata as water vapour by transpiration. This process not only keeps
plants alive and healthy but also plays a vital role in maintaining the Earth's water cycle and
regulating the environment.
2. Discuss the theories explaining the mechanism of stomatal opening and closing.
Ans: Introduction
Plants cannot move from one place to another, but they still need to breathe, take in
carbon dioxide, release oxygen, and control water loss. To perform these important
functions, plants have tiny openings on the surface of their leaves called stomata (singular:
stoma).
Each stoma is surrounded by two guard cells. These guard cells work like the doors of a
room. They decide when the stomata should open and when they should close.
When stomata open: Carbon dioxide (CO₂) enters for photosynthesis, oxygen comes
out, and water vapor is released through transpiration.
When stomata close: Water loss is reduced, helping the plant survive during hot or
dry conditions.
Scientists have proposed several theories to explain how guard cells open and close the
stomata. These theories explain the changes inside guard cells that control their shape.
Structure of a Stoma
Open Stoma
Guard Cell Guard Cell
( ) ( )
\ /
\__________/
Stomatal
Pore
Guard Cells: Kidney-shaped cells (in dicots) surrounding the pore.
Stomatal Pore: Opening through which gases and water vapor move.
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Why Do Stomata Open and Close?
Imagine a door controlled by air pressure.
If the pressure inside increases, the door opens.
If the pressure decreases, the door closes.
Similarly,
When guard cells absorb water, they become turgid (swollen) and the stomata
open.
When guard cells lose water, they become flaccid (shrunken) and the stomata close.
The question is what causes these water movements? Different theories answer this in
different ways.
1. StarchSugar Hypothesis
This is one of the earliest theories proposed to explain stomatal movement.
Main Idea
The opening and closing of stomata depend on the conversion of starch into sugar and
sugar back into starch.
During Day (Opening)
Sunlight starts photosynthesis.
Carbon dioxide inside the leaf decreases.
The pH of guard cells increases.
Stored starch changes into soluble sugar.
Sugar concentration inside guard cells becomes high.
Water enters the guard cells by osmosis.
Guard cells swell.
Stomata open.
During Night (Closing)
Photosynthesis stops.
Carbon dioxide increases.
Sugar converts back into starch.
Water moves out of guard cells.
Guard cells shrink.
Stomata close.
Simple Flow
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Sunlight
↓
Starch → Sugar
↓
Water enters
↓
Guard cells swell
↓
Stomata Open
Limitation
Scientists later found that sugar is not always responsible for stomatal movement.
Therefore, this theory cannot explain all situations.
2. Potassium Ion (K⁺) Theory
This is the most widely accepted theory today.
Main Idea
Opening and closing of stomata mainly depend on the movement of potassium ions (K⁺)
into and out of guard cells.
During Day (Opening)
Light activates proton pumps in guard cells.
Hydrogen ions (H⁺) move out.
Potassium ions (K⁺) move into guard cells.
Chloride ions and malate also accumulate.
The concentration of dissolved substances increases.
Water enters through osmosis.
Guard cells become turgid.
Stomata open.
During Night or Water Stress (Closing)
Potassium ions leave the guard cells.
Water also moves out.
Guard cells lose turgidity.
Stomata close.
Simple Diagram
Day
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K⁺ enters
↓
Water enters
↓
Guard cells swell
↓
Stomata Open
Night
K⁺ leaves
↓
Water leaves
↓
Guard cells shrink
↓
Stomata Close
Importance
This theory is supported by many modern experiments and is considered the best
explanation for stomatal movement.
3. Proton Pump Theory
This theory explains how potassium ions enter guard cells.
Mechanism
Sunlight activates ATP-powered proton pumps.
H⁺ ions are pumped out of guard cells.
This creates an electrical difference.
Potassium ions move inside.
Water follows by osmosis.
Guard cells become swollen.
Stomata open.
At night, the pumps stop working.
Potassium ions leave.
Water leaves.
Guard cells shrink.
Stomata close.
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4. Role of Abscisic Acid (ABA)
ABA is called the stress hormone of plants.
During Drought
When water becomes scarce:
Roots produce ABA.
ABA reaches guard cells.
Potassium ions move out.
Water leaves the guard cells.
Guard cells become flaccid.
Stomata close quickly.
This helps the plant conserve water and survive dry conditions.
Factors Affecting Stomatal Opening and Closing
Several environmental factors influence stomatal movement:
1. Light
Opens stomata during the day.
2. Darkness
Closes stomata at night.
3. Carbon Dioxide
Low CO₂ opens stomata.
High CO₂ closes stomata.
4. Water Availability
Plenty of water keeps stomata open.
Water shortage closes stomata.
5. Temperature
Extremely high temperatures may cause stomata to close to reduce water loss.
6. Humidity
High humidity favors opening.
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Low humidity encourages closing.
Comparison of Theories
Theory
Main Idea
Status
StarchSugar
Hypothesis
Starch changes into sugar, causing
water entry
Early theory; partly
accepted
Potassium Ion
Theory
Potassium ions control water
movement into guard cells
Most widely accepted
Proton Pump
Theory
Proton pumps help potassium enter
guard cells
Supports K⁺ theory
ABA Theory
Plant hormone causes stomatal closure
during stress
Accepted for drought
conditions
Summary of the Mechanism
DAY
Light
↓
Proton pumps active
↓
K⁺ enters guard cells
↓
Water enters
↓
Guard cells become turgid
↓
STOMATA OPEN
NIGHT / DROUGHT
ABA released or darkness
↓
K⁺ leaves guard cells
↓
Water leaves
↓
Guard cells become flaccid
↓
STOMATA CLOSE
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Conclusion
Stomata act like tiny automatic doors that regulate the exchange of gases and water in
plants. Their opening and closing depend on changes in the water content of guard cells.
Earlier, scientists believed this was mainly due to the conversion of starch into sugar, but
modern research shows that the Potassium Ion (K⁺) Theory, supported by the Proton Pump
Theory, best explains the mechanism. During drought, the plant hormone abscisic acid
(ABA) plays a crucial role in closing stomata to prevent excessive water loss. Together, these
mechanisms help plants balance photosynthesis, respiration, transpiration, and water
conservation, ensuring healthy growth and survival in changing environmental conditions.
SECTION-B
3. Write in detail about micro-nutrients and their deficiency and toxic effects in plants.
Ans: 3. Write in detail about Micronutrients and their Deficiency and Toxic Effects in
Plants
Plants, just like humans, need a balanced diet to stay healthy. Humans need vitamins and
minerals in small amounts, while plants need micronutrients in tiny quantities. Although
plants require only a small amount of these nutrients, they are essential for growth,
flowering, seed formation, photosynthesis, and disease resistance. If these nutrients are
lacking or present in excess, plants become weak and unhealthy.
What are Micronutrients?
Micronutrients are essential mineral elements that plants require in very small amounts
(less than 0.1% of the plant's dry weight). Even though they are needed in tiny quantities,
they perform very important functions inside the plant.
The essential micronutrients are:
1. Iron (Fe)
2. Manganese (Mn)
3. Zinc (Zn)
4. Copper (Cu)
5. Boron (B)
6. Molybdenum (Mo)
7. Chlorine (Cl)
8. Nickel (Ni)
Easy Trick to Remember:
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"Funny Men Zip Copper Bottles Mostly Carefully Now."
F Iron (Fe)
M Manganese (Mn)
Z Zinc (Zn)
C Copper (Cu)
B Boron (B)
M Molybdenum (Mo)
C Chlorine (Cl)
N Nickel (Ni)
Functions of Micronutrients
Each micronutrient has a special role in plant growth.
Micronutrient
Main Function
Iron (Fe)
Helps in chlorophyll formation and photosynthesis
Zinc (Zn)
Produces growth hormones and enzymes
Manganese (Mn)
Helps in photosynthesis and respiration
Copper (Cu)
Needed for enzyme activity and reproduction
Boron (B)
Cell wall formation, flowering, fruit and seed development
Molybdenum (Mo)
Helps plants use nitrogen efficiently
Chlorine (Cl)
Maintains water balance and photosynthesis
Nickel (Ni)
Helps break down urea and supports seed germination
Simple Diagram
MICRONUTRIENTS
│
──────────────────────────────────────
│ │ │ │ │ │ │ │
Fe Zn Mn Cu B Mo Cl Ni
│ │ │ │ │ │ │ │
Green Growth Enzymes Photosynthesis Flowering Nitrogen Water Urea
Leaves Hormones & Respiration Fruits Fixation Balance Metabolism
Deficiency Effects of Micronutrients
When plants do not get enough micronutrients, they show visible symptoms.
1. Iron (Fe) Deficiency
Function
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Helps make chlorophyll.
Essential for photosynthesis.
Deficiency Symptoms
Young leaves become yellow (chlorosis).
Leaf veins remain green.
Poor plant growth.
Example
Rice, citrus, and soybean commonly suffer from iron deficiency.
2. Zinc (Zn) Deficiency
Function
Produces plant growth hormones.
Helps enzyme activity.
Deficiency Symptoms
Small leaves.
Short stems.
Yellow patches between veins.
Slow growth.
3. Manganese (Mn) Deficiency
Function
Helps in photosynthesis.
Activates enzymes.
Deficiency Symptoms
Yellowing between leaf veins.
Brown spots appear later.
Weak plant growth.
4. Copper (Cu) Deficiency
Function
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Helps enzyme activity.
Needed for flowering.
Deficiency Symptoms
Twisted young leaves.
Dry shoot tips.
Poor flowering.
5. Boron (B) Deficiency
Function
Cell wall formation.
Fruit and seed development.
Deficiency Symptoms
Death of growing tips.
Brittle leaves.
Poor fruit formation.
Cracked fruits.
6. Molybdenum (Mo) Deficiency
Function
Helps plants use nitrogen.
Important for nitrate reduction.
Deficiency Symptoms
Yellow leaves.
Poor nitrogen utilization.
Stunted growth.
7. Chlorine (Cl) Deficiency
Function
Maintains water balance.
Helps photosynthesis.
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Deficiency Symptoms
Wilting.
Yellow leaves.
Reduced root growth.
8. Nickel (Ni) Deficiency
Function
Helps convert urea into usable nitrogen.
Deficiency Symptoms
Poor seed germination.
Leaf tip burn.
Slow growth.
Toxic Effects of Micronutrients
Just as too little of a nutrient is harmful, too much can also damage plants. Excess
micronutrients become toxic and interfere with normal plant processes.
Micronutrient
Toxic Effect
Iron
Brown spots on leaves, reduced root growth
Zinc
Iron deficiency symptoms, stunted growth
Manganese
Brown patches and curled leaves
Copper
Root damage and poor water absorption
Boron
Yellow leaf margins and leaf burn
Molybdenum
Rare in plants but may accumulate excessively
Chlorine
Leaf burn, premature leaf fall
Nickel
Poor root growth, reduced photosynthesis
Why Do Deficiency and Toxicity Occur?
Several environmental and soil factors can affect micronutrient availability:
Deficiency may occur due to:
o Nutrient-poor soil.
o High soil pH (alkaline soil), which reduces the availability of nutrients like iron
and zinc.
o Heavy rainfall that washes nutrients away.
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o Continuous cropping without replenishing nutrients.
Toxicity may occur due to:
o Excessive use of fertilizers or micronutrient sprays.
o Industrial pollution or contaminated irrigation water.
o Very acidic soils, where some micronutrients become too soluble and are
absorbed in excess.
How Can These Problems Be Managed?
Test the soil regularly before applying fertilizers.
Apply only the recommended dose of micronutrient fertilizers.
Use balanced fertilizers that contain both macro- and micronutrients.
Maintain proper soil pH so nutrients remain available in the right amounts.
Improve soil health by adding organic matter such as compost or farmyard manure.
Avoid overuse of chemical fertilizers to prevent toxicity.
Conclusion
Micronutrients are required by plants in very small amounts, but they are vital for healthy
growth, chlorophyll production, photosynthesis, enzyme activity, flowering, fruit
development, and nitrogen metabolism. A deficiency of micronutrients leads to symptoms
such as yellowing of leaves, stunted growth, poor flowering, and reduced yield, while an
excess causes leaf burn, root damage, and toxicity. Therefore, maintaining the correct
balance of micronutrients through proper soil management and balanced fertilization is
essential for producing healthy plants and achieving higher crop yields.
4. Discuss in detail phloem loading and unloading in plants.
Ans: 4. Discuss in detail Phloem Loading and Unloading in Plants
Plants prepare food in their leaves through the process of photosynthesis. However, all
parts of the plant, such as the roots, flowers, fruits, stems, and growing buds, also need
this food for growth, storage, and energy. Since these parts cannot make enough food on
their own, the plant has a special transport system called the phloem, which carries food
from the leaves to the rest of the plant.
The movement of sugar into the phloem is called phloem loading, and the movement of
sugar out of the phloem into different plant parts is called phloem unloading.
What is Phloem?
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Phloem is a type of vascular tissue that transports organic food, mainly sucrose (a type of
sugar), from the leaves to other parts of the plant.
The main parts of phloem are:
Sieve tube elements Transport food.
Companion cells Help sieve tubes during transport.
Phloem parenchyma Store food.
Phloem fibers Provide strength.
Think of the phloem like a delivery network in a city. The leaves are the factory, the
phloem is the road, and the roots, fruits, flowers, and growing shoots are the customers
receiving the food.
What is Phloem Loading?
Phloem loading is the process of transferring sugar (mainly sucrose) from the leaf cells into
the sieve tube elements of the phloem.
This process mainly occurs in the leaves, which are called the source because food is
produced there.
Steps of Phloem Loading
1. Photosynthesis occurs in leaves, producing glucose.
2. Glucose is converted into sucrose because sucrose is easier to transport.
3. Companion cells actively pump sucrose into the sieve tubes using ATP (energy).
4. The concentration of sugar inside the sieve tube increases.
5. Water enters the sieve tubes from the xylem by osmosis.
6. This creates high pressure inside the sieve tubes, pushing the sugar-rich solution
toward other parts of the plant.
What is Active Transport?
Active transport means moving substances against the concentration gradient, which
requires energy (ATP).
In phloem loading, companion cells use ATP to move sucrose into sieve tubes.
What is Phloem Unloading?
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Phloem unloading is the process in which sucrose leaves the sieve tubes and enters the cells
of roots, fruits, seeds, flowers, or growing shoots.
These parts are called the sink, because they consume or store food.
Steps of Phloem Unloading
1. Sucrose reaches the sink through the sieve tubes.
2. Sugar moves from sieve tubes into nearby cells.
3. The sugar is used for:
o Energy (respiration)
o Growth
o Formation of new tissues
o Storage as starch
4. Water leaves the phloem and returns to the xylem.
As sugar leaves the sieve tubes, the pressure decreases.
Pressure Flow (Mass Flow) Theory
The movement of food in phloem is best explained by the Pressure Flow Theory, proposed
by Ernst Münch.
According to this theory:
High sugar concentration in leaves creates high pressure.
Low sugar concentration in roots and fruits creates low pressure.
Food naturally flows from high pressure to low pressure through the sieve tubes.
This process continues until food reaches all parts of the plant.
Simple Diagram
LEAF (Source)
Photosynthesis produces sugar
│
▼
Sucrose enters Companion Cell
│
(ATP used - Active Transport)
▼
Sieve Tube (Phloem)
│
Water enters from Xylem
│
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High Pressure Developed
│
================================>
Food moves through Phloem
│
▼
ROOT / FRUIT / FLOWER (Sink)
│
Sugar leaves Sieve Tube
│
Used for Growth, Energy & Storage
│
Water returns to Xylem
Difference Between Phloem Loading and Unloading
Phloem Unloading
Takes place in roots, fruits, flowers, and growing
tissues (sink).
Sugar leaves the sieve tubes.
May occur by active or passive transport,
depending on the tissue.
Decreases pressure inside phloem.
Importance of Phloem Loading and Unloading
Distributes food throughout the plant.
Supplies energy for respiration.
Helps roots grow and absorb water.
Supports flowering and fruit development.
Stores food in roots, stems, seeds, and fruits.
Ensures continuous growth and survival of the plant.
Conclusion
Phloem loading and unloading are two essential steps in the transport of food in plants.
During phloem loading, sucrose produced in the leaves is actively transported into the sieve
tubes with the help of companion cells and ATP. Water then enters the phloem, creating
pressure that pushes the food through the plant. During phloem unloading, the sucrose
leaves the phloem and enters roots, fruits, flowers, and other growing parts where it is used
for energy, growth, or storage. This entire process follows the Pressure Flow Theory,
ensuring that every part of the plant receives the nutrients it needs. Together, phloem
loading and unloading maintain healthy growth, development, and survival of the plant.
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SECTION-C
5. Discuss C4 Cycle and its variation in plants.
Ans: C4 Cycle and Its Variation in Plants (Simple Explanation)
The C4 cycle, also called the HatchSlack pathway, is a special method of photosynthesis
used by some plants. It helps plants prepare food more efficiently in hot, dry, and sunny
environments. While most plants use the C3 cycle, C4 plants have developed an additional
step that reduces the loss of energy and increases food production.
Examples of C4 plants include maize (corn), sugarcane, sorghum, millet, and many tropical
grasses.
What is the C4 Cycle?
Plants make food by using carbon dioxide (CO₂), water, and sunlight. In C4 plants, carbon
dioxide is first converted into a 4-carbon compound, which is why it is called the C4 cycle.
The process occurs in two different types of leaf cells:
1. Mesophyll cells where carbon dioxide is first captured.
2. Bundle sheath cells where the Calvin (C3) cycle takes place to produce glucose.
This separation makes photosynthesis more efficient, especially when the weather is hot.
Why Do Plants Need the C4 Cycle?
In hot weather, plants close their stomata (tiny pores on leaves) to prevent water loss.
When stomata are closed:
Less carbon dioxide enters the leaf.
Oxygen concentration increases.
The enzyme RuBisCO starts reacting with oxygen instead of carbon dioxide.
This causes photorespiration, which wastes energy and reduces food production.
C4 plants solve this problem by concentrating carbon dioxide around RuBisCO, almost
eliminating photorespiration.
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Step-by-Step Working of the C4 Cycle
Step 1: Carbon Dioxide Enters the Leaf
Carbon dioxide enters through the stomata and reaches the mesophyll cells.
Step 2: Formation of a 4-Carbon Compound
Inside the mesophyll cells, the enzyme PEP carboxylase combines carbon dioxide with PEP
(Phosphoenolpyruvate).
This forms a 4-carbon compound, usually Oxaloacetic Acid (OAA).
OAA is then converted into Malate or Aspartate.
Step 3: Transport to Bundle Sheath Cells
The 4-carbon compound moves from the mesophyll cells to the bundle sheath cells.
Step 4: Release of Carbon Dioxide
Inside the bundle sheath cells, Malate releases carbon dioxide.
Now there is a high concentration of CO₂ around RuBisCO.
Step 5: Calvin Cycle
RuBisCO uses this carbon dioxide in the Calvin (C3) cycle to produce glucose (food).
Step 6: Regeneration of PEP
The remaining 3-carbon compound returns to the mesophyll cells and is converted back into
PEP, allowing the cycle to continue.
Simple Flow Diagram
Sunlight
│
CO₂ enters leaf
│
Mesophyll Cell
│
CO₂ + PEP --(PEP Carboxylase)
│
Oxaloacetate (OAA)
│
Malate / Aspartate
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│
▼
Bundle Sheath Cell
│
Malate releases CO₂
│
Calvin (C3) Cycle
│
Glucose (Food)
│
3-carbon compound returns
│
PEP regenerated
Kranz Anatomy (Special Feature of C4 Plants)
One of the most important characteristics of C4 plants is Kranz Anatomy.
In these plants:
Bundle sheath cells are large and rich in chloroplasts.
They are surrounded by mesophyll cells in a ring-like arrangement.
This arrangement helps maintain a high concentration of carbon dioxide, making
photosynthesis very efficient.
Mesophyll Cells
○ ○ ○ ○ ○ ○ ○ ○
○ ○
○ Bundle ○
○ Sheath ○
○ Cells ○
○ ○
○ ○ ○ ○ ○ ○ ○ ○
Variations in C4 Plants
Although all C4 plants follow the same basic pathway, they differ in the type of enzyme
used to release carbon dioxide in the bundle sheath cells.
1. NADP-Malic Enzyme (NADP-ME) Type
Carbon dioxide is released using the NADP-malic enzyme.
Examples: Maize and Sugarcane.
2. NAD-Malic Enzyme (NAD-ME) Type
Uses the NAD-malic enzyme.
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Found in several grasses and tropical plants.
3. PEP Carboxykinase (PCK) Type
Uses the enzyme PEP Carboxykinase.
Found in some grasses and other C4 species.
Although these variations use different enzymes, their final goal is the sameto supply
plenty of carbon dioxide to the Calvin cycle and reduce photorespiration.
Advantages of the C4 Cycle
Very little photorespiration, so less energy is wasted.
Performs better in hot and dry climates.
Uses water more efficiently.
Produces higher crop yields.
Continues photosynthesis even when stomata are partially closed.
More efficient under strong sunlight.
Disadvantages of the C4 Cycle
Requires extra ATP (energy) compared to the C3 pathway.
Less beneficial in cool climates.
More complex because it involves two different cell types.
Examples of C4 Plants
Maize (Corn)
Sugarcane
Sorghum
Pearl Millet (Bajra)
Napier Grass
Sudan Grass
Difference Between C3 and C4 Plants
Feature
C3 Plants
C4 Plants
First stable product
3-carbon compound (3-PGA)
4-carbon compound (OAA)
First enzyme
RuBisCO
PEP Carboxylase
Photorespiration
High
Very low
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Efficiency in hot climate
Lower
Higher
Kranz Anatomy
Absent
Present
Water-use efficiency
Lower
Higher
Examples
Wheat, Rice
Maize, Sugarcane
Conclusion
The C4 cycle (HatchSlack pathway) is a specialized photosynthetic pathway that allows
certain plants to survive and grow efficiently in hot, dry, and sunny conditions. These plants
first fix carbon dioxide into a 4-carbon compound in the mesophyll cells, transport it to the
bundle sheath cells, and then release carbon dioxide for the Calvin cycle. This unique
mechanism greatly reduces photorespiration, conserves water, and increases
photosynthetic efficiency. Different C4 plants show variations based on the enzyme used to
release carbon dioxide (NADP-ME, NAD-ME, or PCK type), but all share the same objective
of maximizing food production under challenging environmental conditions.
6. Explain photorespiration and its importance to plants.
Ans: 6. Explain Photorespiration and Its Importance to Plants
Photorespiration is a natural process in plants that takes place when the enzyme RuBisCO
uses oxygen (O₂) instead of carbon dioxide (CO₂) during photosynthesis. Although this
process does not produce food for the plant, it plays an important role in protecting plants
under certain environmental conditions.
To understand photorespiration easily, let's first understand photosynthesis.
What is Photosynthesis?
Photosynthesis is the process by which green plants prepare their own food using:
Sunlight
Carbon dioxide (CO₂) from the air
Water (H₂O) from the soil
Chlorophyll (green pigment)
During this process, plants produce:
Glucose (food)
Oxygen (O₂)
Normally, an enzyme called RuBisCO captures carbon dioxide and helps convert it into
sugar.
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What is Photorespiration?
Sometimes the weather becomes very hot and dry. To prevent water loss, plants close their
stomata (tiny pores on leaves).
When stomata close:
Less carbon dioxide enters the leaf.
Oxygen produced during photosynthesis builds up inside the leaf.
Now RuBisCO becomes confused because it can react with both CO₂ and O₂. Since oxygen is
more available, RuBisCO combines with oxygen instead of carbon dioxide.
This unwanted reaction starts photorespiration.
Instead of producing sugar, the plant:
Uses oxygen
Releases carbon dioxide
Consumes energy (ATP)
Produces very little or no food
That is why photorespiration is often called a wasteful process.
Why Does Photorespiration Occur?
Photorespiration mainly occurs under the following conditions:
1. High temperature
2. Dry weather
3. Stomata remain closed
4. Low carbon dioxide concentration
5. High oxygen concentration
Under these conditions, RuBisCO reacts with oxygen rather than carbon dioxide.
Step-by-Step Process of Photorespiration
1. Photosynthesis begins normally.
2. Due to heat or water shortage, stomata close.
3. Carbon dioxide inside the leaf decreases.
4. Oxygen concentration increases.
5. RuBisCO combines with oxygen.
6. A harmful compound (glycolate) is formed.
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7. The plant transports this compound through the chloroplast, peroxisome, and
mitochondrion to recycle it.
8. During recycling:
o Energy is consumed.
o Carbon dioxide is released.
o No glucose is produced.
Thus, the plant loses some of the energy it had already captured.
Simple Flow Diagram
Hot and Dry Weather
│
▼
Stomata Close
│
▼
Less CO₂ enters leaf
More O₂ remains inside
│
▼
RuBisCO reacts with O₂
│
▼
Photorespiration Starts
│
▼
Energy Used + CO₂ Released
│
▼
Less Sugar Produced
Why is Photorespiration Considered Wasteful?
Scientists call photorespiration wasteful because it:
Does not produce glucose.
Uses valuable ATP and energy.
Releases carbon dioxide that was previously fixed.
Reduces the efficiency of photosynthesis.
Slows plant growth and crop yield.
Therefore, plants become less productive when photorespiration is high.
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Importance of Photorespiration
Although photorespiration reduces photosynthesis, it is not completely useless. It has
several important functions.
1. Protects Plants from Sun Damage
During intense sunlight, plants absorb more light energy than they can use.
Photorespiration safely uses some of this extra energy, preventing damage to chloroplasts.
2. Prevents Harmful Chemical Formation
Without photorespiration, excess energy could produce harmful molecules called reactive
oxygen species (ROS).
These molecules damage proteins, DNA, and cell membranes.
Photorespiration helps reduce their formation.
3. Helps Plants Survive Stress
During:
Heat stress
Drought
Water shortage
plants cannot perform normal photosynthesis efficiently.
Photorespiration helps plants survive these difficult conditions until the environment
improves.
4. Recycles Carbon Compounds
The harmful glycolate produced during oxygen fixation is recycled through several cell
organelles.
Although some energy is lost, valuable carbon is recovered instead of being completely
wasted.
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5. Maintains Balance Inside Plant Cells
Photorespiration helps maintain the balance of:
Energy
Oxygen
Carbon compounds
Nitrogen metabolism
This balance keeps plant cells functioning properly under changing environmental
conditions.
Which Plants Show More Photorespiration?
C₃ Plants
Examples:
Wheat
Rice
Potato
Soybean
These plants experience high photorespiration, especially in hot weather.
C₄ Plants
Examples:
Maize
Sugarcane
Sorghum
These plants have a special mechanism that concentrates carbon dioxide around RuBisCO.
As a result, they show very little photorespiration and are more efficient in hot climates.
Difference Between Photosynthesis and Photorespiration
Photosynthesis
Photorespiration
Uses carbon dioxide
Uses oxygen
Produces glucose
Does not produce glucose
Stores energy
Consumes energy
Releases oxygen
Releases carbon dioxide
Increases plant growth
Reduces photosynthesis efficiency
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Conclusion
Photorespiration is a natural process that occurs when the enzyme RuBisCO reacts with
oxygen instead of carbon dioxide, mainly during hot and dry conditions. Although it
reduces the efficiency of photosynthesis by consuming energy and releasing carbon dioxide,
it also serves as an important protective mechanism. It helps plants survive environmental
stress, prevents damage from excess sunlight, reduces harmful reactive oxygen species, and
maintains cellular balance. Thus, photorespiration is both a costly and protective process,
enabling plants to survive under challenging environmental conditions.
SECTION-D
7. Elucidate the physiology of flowering and role of dark period.
Ans: Flowering is one of the most important stages in the life of a plant. It is the process by
which a plant changes from the vegetative stage (producing leaves, stems, and roots) to the
reproductive stage (producing flowers, fruits, and seeds). Without flowering, plants cannot
reproduce, and many crops would not produce fruits or seeds.
The timing of flowering is not random. Plants carefully observe their surroundings,
especially the length of day and night, temperature, and seasons. Among these factors, the
dark period (night length) plays a very important role.
What is the Physiology of Flowering?
The physiology of flowering refers to the internal biological and chemical processes that
cause a plant to develop flowers.
These processes involve:
Environmental signals (light, darkness, temperature)
Plant hormones
Genes
Production of flowering signals inside the plant
When the plant receives the correct environmental conditions, it begins producing
substances that trigger flower formation.
Simple Example
Think of a student preparing for exams.
If the exam date is announced, the student starts studying.
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Similarly, when a plant receives the correct signal (such as the right amount of
darkness), it starts preparing to produce flowers.
Factors Affecting Flowering
Several factors influence flowering:
1. Light (Photoperiod)
Plants measure the duration of light and darkness every day.
This response is called Photoperiodism.
Photoperiodism means the response of plants to the relative lengths of day and night.
2. Temperature
Some plants require exposure to low temperatures before flowering.
This process is called Vernalization.
Example:
Wheat
Barley
Cabbage
3. Plant Hormones
Plant hormones regulate flowering.
The important flowering hormone is known as:
Florigen (Flowering Hormone)
Although its exact chemical nature is complex, scientists know that leaves produce a
flowering signal that moves to the shoot tip and initiates flower formation.
4. Genes
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Genes inside plant cells control when flowering should begin.
Modern research has identified several flowering genes that respond to environmental
signals.
Photoperiodism
Photoperiodism is the ability of plants to measure day and night length.
Interestingly, plants actually measure the duration of darkness rather than daylight.
This discovery completely changed scientists' understanding of flowering.
What is the Dark Period?
The dark period is the uninterrupted time during which a plant remains in complete
darkness.
Plants use this dark period like a biological clock.
If the night is too short or too long, flowering may not occur.
Why is the Dark Period Important?
Scientists discovered that continuous darkness is more important than the length of
daylight.
If the required dark period is interrupted by even a short flash of light, some plants fail to
flower.
This shows that plants carefully monitor the length of the night.
Types of Plants Based on Dark Period
Plants are divided into three groups according to their flowering response.
1. Short-Day Plants (Long-Night Plants)
These plants flower only when the night is longer than a critical length.
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They require a long, uninterrupted dark period.
Examples:
Rice
Soybean
Chrysanthemum
Tobacco
If the long night is interrupted by light, flowering is prevented.
2. Long-Day Plants (Short-Night Plants)
These plants flower when the night is shorter than a critical length.
Examples:
Spinach
Radish
Wheat
Lettuce
These plants do not require long darkness.
3. Day-Neutral Plants
These plants flower regardless of day or night length.
Examples:
Tomato
Cucumber
Cotton
Sunflower
Their flowering mainly depends on age and proper nutrition.
Role of Leaves
Leaves are the organs that detect light and darkness.
Inside the leaves is a special pigment called Phytochrome.
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Phytochrome acts like a light sensor.
It detects:
Day length
Night length
Changes in light quality
After detecting suitable conditions, leaves produce the flowering signal (Florigen), which
travels through the phloem to the shoot tip.
The shoot tip then changes into a flower bud.
Physiology of Flower Formation
The flowering process occurs in several steps:
Environmental Signal
│
▼
Leaves detect light and darkness
│
▼
Phytochrome becomes activated
│
▼
Flowering signal (Florigen) produced
│
▼
Signal moves through phloem
│
▼
Shoot apical meristem receives signal
│
▼
Vegetative bud changes into flower bud
│
▼
Flower develops
Importance of the Dark Period
The dark period is essential because it:
Controls the correct season for flowering.
Prevents flowering during unsuitable weather.
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Helps synchronize flowering among plants.
Increases successful pollination.
Improves fruit and seed production.
Helps farmers schedule crop production.
Practical Importance in Agriculture
Knowledge of flowering physiology helps farmers and scientists to:
Produce flowers throughout the year.
Increase crop yield.
Grow ornamental plants in different seasons.
Improve hybrid seed production.
Control flowering in greenhouses using artificial lighting.
For example, greenhouse growers can manipulate light and darkness to make
chrysanthemums bloom even when they normally would not.
Key Points to Remember
Flowering is the transition from the vegetative stage to the reproductive stage.
Photoperiodism is the response of plants to the length of day and night.
Plants primarily measure the length of the uninterrupted dark period, not just
daylight.
Phytochrome in leaves detects light and darkness.
Leaves produce the flowering signal (Florigen), which travels to the shoot apex.
Plants are classified as Short-Day Plants, Long-Day Plants, and Day-Neutral Plants
based on their response to the dark period.
The dark period ensures flowering occurs at the most suitable time for successful
reproduction.
Conclusion
The physiology of flowering is a well-coordinated process involving environmental signals,
plant hormones, genes, and internal biochemical mechanisms. Among all environmental
factors, the dark period (night length) is one of the most important because plants use it to
determine the right season for flowering. Through photoperiodism, leaves detect the
duration of darkness, produce the flowering signal (florigen), and send it to the growing tip,
where flower buds develop. Understanding this process is valuable not only for biology but
also for agriculture, horticulture, and crop improvement, as it helps in controlling flowering
time and increasing agricultural productivity.
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8. Discuss biosynthesis of auxins and their physiological roles in plant.
Ans: Introduction
Plants may look quiet and motionless, but inside them, thousands of chemical reactions take
place every second. These reactions help plants grow taller, produce leaves, flowers, fruits,
and roots. One of the most important chemicals responsible for these activities is a plant
hormone called Auxin.
Auxin is known as the "growth hormone of plants" because it controls many growth and
developmental processes. The most common natural auxin is Indole-3-Acetic Acid (IAA).
To understand auxin completely, we need to learn two things:
1. Biosynthesis of Auxins How plants make auxin.
2. Physiological Roles What auxin does inside the plant.
What is Auxin?
Auxin is a naturally occurring plant hormone mainly produced in:
Young shoot tips
Young leaves
Developing seeds
Fruits
Root tips (small amount)
After being produced, auxin moves from the top of the plant downward, carrying growth
signals to different parts.
Biosynthesis of Auxins
Biosynthesis simply means the process by which living organisms produce a substance
inside their bodies.
So,
Biosynthesis of Auxin = The process through which plants produce auxin naturally.
The main natural auxin is Indole-3-Acetic Acid (IAA).
Source of Auxin
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Plants prepare auxin mainly from an amino acid called Tryptophan.
Think of it like this:
Tryptophan is the raw material, while Auxin (IAA) is the finished product.
Several enzymes convert tryptophan into auxin through different chemical reactions.
Simplified Biosynthesis Process
Tryptophan
│
▼
Indole-3-Pyruvic Acid (IPA)
│
▼
Indole-3-Acetaldehyde
│
▼
Indole-3-Acetic Acid (IAA)
(Auxin)
Explanation of the Steps
Step 1: Tryptophan
Plants absorb nutrients from the soil.
They use them to produce amino acids.
One important amino acid is Tryptophan.
↓
Step 2: Formation of IPA
Special enzymes convert tryptophan into
Indole-3-Pyruvic Acid (IPA).
↓
Step 3: Formation of Indole-3-Acetaldehyde
IPA undergoes another reaction to form
Indole-3-Acetaldehyde.
↓
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Step 4: Formation of IAA
Finally,
Indole-3-Acetaldehyde is converted into
Indole-3-Acetic Acid (IAA),
which is the active natural auxin.
Simple Flow Diagram
Young Leaves
Shoot Tip
Developing Seeds
│
▼
Tryptophan
│
▼
Indole-3-Pyruvic Acid
│
▼
Indole-3-Acetaldehyde
│
▼
Indole-3-Acetic Acid (IAA)
│
▼
Moves to Different Plant Parts
and Controls Growth
Physiological Roles of Auxin
The word physiological means the normal functions performed inside a living organism.
Auxin performs many important functions in plants.
1. Cell Elongation (Increase in Plant Height)
This is the most important function of auxin.
Auxin makes plant cells longer by loosening their cell walls, allowing them to absorb water
and expand. As millions of cells elongate, the stem grows taller.
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Example: A sunflower becomes taller because auxin promotes stem elongation.
2. Root Formation
Auxin stimulates the formation of new roots, especially adventitious roots that arise from
stems or leaves.
This is why gardeners often dip stem cuttings in rooting hormone before planting.
Example: Rose and money plant cuttings develop roots more quickly when treated with
auxin.
3. Apical Dominance
The apical bud (tip of the plant) produces auxin, which suppresses the growth of side buds.
As long as the top bud is present, the plant mainly grows upward.
When the tip is removed, side branches begin to grow.
Example: Pruning a plant encourages it to become bushier because the inhibitory effect of
auxin is reduced.
4. Phototropism (Growth Towards Light)
Auxin moves to the shaded side of a stem.
Cells on the shaded side elongate more than those on the lighted side.
As a result, the stem bends toward the light.
Example: A potted plant near a window leans toward sunlight.
5. Geotropism (Response to Gravity)
Auxin helps roots and shoots respond differently to gravity.
Shoots: Auxin promotes elongation, causing shoots to grow upward.
Roots: Higher auxin concentrations inhibit elongation, causing roots to grow
downward.
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This helps the plant maintain proper orientation.
6. Fruit Development
After fertilization, auxin stimulates the ovary to develop into a fruit.
Applying auxin artificially can also produce seedless fruits (parthenocarpy).
Examples:
Seedless tomato
Seedless cucumber
7. Prevention of Premature Leaf and Fruit Drop
Auxin delays the formation of the abscission layer, the weak region where leaves or fruits
detach.
This helps leaves and fruits remain attached until they mature.
8. Vascular Tissue Differentiation
Auxin promotes the formation of xylem and phloem, the tissues responsible for
transporting water, minerals, and food throughout the plant.
9. Wound Healing
When a plant is injured, auxin helps nearby cells divide and develop into new tissues,
promoting healing and regeneration.
10. Cell Division (with Cytokinins)
Although cytokinins are the primary hormones for cell division, auxin works together with
them to stimulate the formation of new cells, especially in tissue culture.
Summary Diagram of Auxin Functions
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AUXIN (IAA)
│
┌──────────────────┼──────────────────┐
│ │ │
Cell Elongation Root Formation Apical Dominance
│ │ │
├──────────────────┼──────────────────┤
│ │ │
Phototropism Geotropism Fruit Development
│ │ │
├──────────────────┼──────────────────┤
│ │ │
Prevent Leaf Vascular Tissue Wound Healing
Fruit Drop Formation Cell Division
Importance of Auxin in Agriculture
Auxins are widely used in agriculture and horticulture because they:
Help stem cuttings develop roots.
Increase fruit production.
Produce seedless fruits.
Prevent premature fruit drop.
Control weed growth (some synthetic auxins act as selective herbicides).
Improve propagation of ornamental and fruit plants.
Conclusion
Auxin is one of the most essential plant hormones and is often called the master growth
regulator. It is synthesized mainly from the amino acid tryptophan in young shoots, leaves,
seeds, and fruits, producing Indole-3-Acetic Acid (IAA) through a series of enzymatic
reactions. Once formed, auxin moves throughout the plant and regulates numerous
physiological processes, including cell elongation, root initiation, apical dominance,
phototropism, geotropism, fruit development, vascular tissue formation, wound healing,
and the prevention of premature leaf and fruit drop. Because of these diverse roles, auxin
is indispensable for healthy plant growth and is extensively used in modern agriculture and
horticulture to improve crop productivity and propagation.
This paper has been carefully prepared for educational purposes. If you notice any mistakes or
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