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GNDU Question Paper-2021

Ba/Bsc 5

Semester

BOTANY :Paper-V(A)

(Plant Physiology)

Time Allowed: Three Hours Maximum 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. Write short notes on the following:

(a) Physical properties of water

(b) Osmosis

2. Define transpiration. Explain the mechanism of transpiration in plants.

SECTION-B

3. Discuss the following:

(a) Difference between macro & micro elements.

(b) Symptoms of Ca & Mg deficiency in plants.

4. Explain the Munch hypothesis of translocation of solutes in phloem. 7

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SECTION-C

5. What do you mean by photophosphorylation? Distinguish between cyclic and non-cyclic

photophosphorylation.

6. Describe briefly:

(a) Red drop effect

(b) Enhancement effect

SECTION-D

7. Explain the physiological role of:

(a) Auxins

(b) Gibberellins

8. Discuss the following

(a) Short day plants

(b) Phytochromes

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GNDU Answer Paper-2021

Ba/Bsc 5

Semester

BOTANY :Paper-V(A)

(Plant Physiology)

Time Allowed: Three Hours Maximum 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. Write short notes on the following:

(a) Physical properties of water

(b) Osmosis

Ans: (a) Physical Properties of Water

Water is one of the most important molecules for life, and its unique physical properties

make it essential for plants. Here's an explanation in simple language:

1. Structure of Water Molecule

Water (H₂O) is made of two hydrogen atoms and one oxygen atom. These atoms are held

together by covalent bonds, where the electrons are shared between the oxygen and

hydrogen atoms. Due to the way the atoms are arranged, water is a polar molecule,

meaning it has a positive and a negative side. The oxygen atom is slightly negative, while the

hydrogen atoms are slightly positive. This causes water molecules to be attracted to each

other, forming hydrogen bonds.

2. Cohesion and Adhesion

• Cohesion: This refers to water molecules sticking to each other. It happens because

of the hydrogen bonds between water molecules. Cohesion is what allows water to

form droplets and also helps plants transport water from their roots to their leaves

through a process called capillary action.

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• Adhesion: Water molecules can also stick to other substances. This property is called

adhesion. In plants, adhesion helps water stick to the walls of cells and travel upward

against gravity, especially in narrow tubes like xylem vessels.

3. Surface Tension

Surface tension is a result of water's cohesive properties. The molecules at the surface of a

body of water stick together more strongly than those below the surface, creating a sort of

"skin." This allows small insects to walk on water and plays a role in water movement in

plants.

4. High Specific Heat Capacity

Water has a high specific heat capacity, which means it can absorb a lot of heat before it

gets hot. This property is important for plants because it helps regulate temperature. When

the temperature outside changes, water inside the plant cells absorbs the heat, protecting

the plant from extreme temperature fluctuations.

5. High Heat of Vaporization

The high heat of vaporization means that water takes a lot of energy to change from a liquid

to a gas (vapor). This is important in plants because it helps them stay cool. When water

evaporates from the surface of the leaves (a process called transpiration), it cools the plant,

much like how sweating cools humans.

6. Density and Expansion on Freezing

Water is unique because it expands when it freezes, which makes ice less dense than liquid

water. This is why ice floats on water. For plants, this property can be both beneficial and

harmful. In cold environments, ice floating on the surface of a body of water can insulate

the water below, protecting plant roots from freezing. However, freezing temperatures can

damage plant tissues because the expansion of ice inside cells can cause them to burst.

7. Solvent Properties

Water is often called the "universal solvent" because it can dissolve many different

substances. In plants, water dissolves nutrients and minerals from the soil, making them

available for the plant to absorb through its roots. This is crucial for the plant's growth and

development.

8. Capillarity

Capillarity is the ability of water to move upward in small spaces, like the tiny tubes in a

plant's roots and stems. This property is a combination of water's adhesive and cohesive

properties. It allows water to defy gravity and move through the plant's tissues, supplying

cells with the water they need to survive.

Importance in Plants

Water is essential for various processes in plants, including photosynthesis, nutrient

transport, and maintaining cell turgidity (firmness). Without water's unique physical

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properties, plants would not be able to survive, as they rely on it for cooling, transporting

nutrients, and maintaining structural integrity.

(b) Osmosis

Osmosis is a process that plays a crucial role in the survival of plants. It refers to the

movement of water molecules from a region of lower concentration of solutes (like salts or

sugars) to a region of higher concentration of solutes, through a semi-permeable

membrane. Let's break this down in simple terms:

1. What is Osmosis?

Imagine you have two solutions: one that is dilute (contains more water and less solute) and

another that is concentrated (contains less water and more solute). If these solutions are

separated by a semi-permeable membrane (a barrier that only allows certain molecules like

water to pass through), water will naturally move from the dilute solution to the

concentrated one. This process is called osmosis.

In plants, the semi-permeable membrane is typically the cell membrane, and the solutes

could be various substances like sugars, salts, or other nutrients. Osmosis helps the plant

cells maintain their internal environment by regulating the amount of water they take in or

lose.

2. Types of Solutions in Relation to Osmosis

• Hypotonic Solution: In this type of solution, the concentration of solutes outside the

cell is lower than inside the cell, so water moves into the cell. As a result, the cell

swells. In plants, this is a good thing because it helps the cell maintain turgor

pressure (the pressure of the cell contents against the cell wall), which keeps the

plant firm and upright.

• Hypertonic Solution: Here, the concentration of solutes outside the cell is higher

than inside, so water moves out of the cell, causing the cell to shrink. If a plant cell

loses too much water, it wilts and can eventually die.

• Isotonic Solution: In an isotonic solution, the concentration of solutes is the same

inside and outside the cell, so there is no net movement of water. The cell remains in

a balanced state.

3. How Does Osmosis Work in Plants?

• Water Absorption by Roots: The process of osmosis is how plants absorb water from

the soil. The soil usually has a higher concentration of water compared to the inside

of the root cells, so water moves into the plant through osmosis.

• Transport of Nutrients: Once water enters the roots, it moves upwards through the

plant via the xylem (vascular tissue responsible for water transport). Along with

water, osmosis helps in the movement of dissolved nutrients.

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• Maintaining Turgidity: Turgor pressure is important for plants because it keeps their

cells firm and helps them maintain their shape. When water enters a plant cell

through osmosis, it fills the central vacuole (a large storage compartment in the cell),

pushing the cell membrane against the cell wall and creating turgor pressure. This

process allows plants to stand upright and maintain their structure.

4. Plasmolysis

Plasmolysis is the process that occurs when a plant cell is placed in a hypertonic solution

(where the concentration of solutes outside the cell is higher than inside). As water moves

out of the cell by osmosis, the cell membrane pulls away from the cell wall, causing the cell

to shrink. If too much water is lost, the plant may wilt and die.

5. Reverse Osmosis

While osmosis is the natural flow of water from a region of lower solute concentration to a

region of higher solute concentration, reverse osmosis is the opposite. In reverse osmosis,

pressure is applied to force water to move from a region of higher solute concentration to a

region of lower solute concentration. This process is often used in water purification

systems to remove impurities.

Importance in Plants

Osmosis is vital for plant health because it allows them to absorb water, which is essential

for photosynthesis, nutrient transport, and maintaining cell structure. It also helps plants

manage the balance of solutes and water within their cells, which is crucial for growth and

survival. Without osmosis, plants wouldn't be able to maintain the proper water levels in

their tissues, which would lead to wilting, nutrient deficiencies, and ultimately death.

Conclusion

In conclusion, water's physical properties, such as cohesion, adhesion, surface tension, and

its role as a solvent, are fundamental to plant physiology. Osmosis, on the other hand, is

essential for water absorption, nutrient transport, and maintaining turgor pressure. Both of

these processes are interconnected and vital for the survival and functioning of plants.

By understanding these concepts, we can better appreciate the intricate systems that allow

plants to thrive in a variety of environments, and how they manage to sustain life through

the movement and regulation of water within their cells.

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2. Define transpiration. Explain the mechanism of transpiration in plants.

Ans: Transpiration: Definition and Overview

Transpiration is the process by which plants lose water in the form of water vapor through

small pores called stomata, mainly located on the leaves but also present on stems and

flowers. This process is essential for plants, as it helps them in multiple ways, such as cooling

the plant, enabling nutrient transportation from the soil, and maintaining water balance.

However, the loss of water also poses a risk to plants, especially in dry conditions, making

the regulation of transpiration a key process in plant physiology.

To understand transpiration better, let’s break it down into more manageable concepts,

explore how it works, and discuss its significance.

1. What is Transpiration?

Transpiration is simply the movement of water within a plant and its evaporation from

aerial parts, especially from the leaves. The plant absorbs water from the soil through its

roots. This water moves upward through the plant's vascular system, specifically through

structures called xylem, and is eventually released into the atmosphere as water vapor.

Around 90-95% of the water absorbed by plants is lost via transpiration, while the remaining

portion is used for photosynthesis and other metabolic activities.

2. Types of Transpiration

There are three types of transpiration based on where the water vapor escapes from the

plant:

a. Stomatal Transpiration: This is the most common form of transpiration, accounting for 80-

90% of the total water loss in plants. It happens through stomata, the tiny openings

primarily found on the surface of leaves. These stomata can open and close to regulate the

loss of water and the intake of carbon dioxide, balancing transpiration with the plant’s need

to retain water.

b. Cuticular Transpiration: A smaller amount of water is lost directly through the plant's

cuticle, a waxy layer on the leaves that acts as a protective barrier. While the cuticle’s main

role is to prevent excessive water loss, some transpiration still occurs through it. Cuticular

transpiration is more significant in plants with thin or poorly developed cuticles.

c. Lenticular Transpiration: In some plants, water loss occurs through lenticels, which are

small openings on the stems of woody plants. Although lenticular transpiration is relatively

minor, it still plays a part in the overall water regulation of a plant.

3. Mechanism of Transpiration

The process of transpiration can be broken down into several stages that involve water

uptake, movement through the plant, and eventual release into the atmosphere. Let's walk

through the different steps involved:

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a. Water Absorption from the Soil:

Roots absorb water from the soil through a process called osmosis. Osmosis is the

movement of water molecules from an area of low solute concentration (in this case, soil) to

an area of high solute concentration (inside the root cells). The roots of the plant have hair-

like structures called root hairs, which increase the surface area for water absorption.

b. Ascent of Sap:

Once water is absorbed by the roots, it needs to travel upwards to reach the stems and

leaves. This upward movement of water is known as the ascent of sap, which occurs

through the xylem, the vascular tissue responsible for transporting water and nutrients.

Three main forces contribute to the upward movement of water in plants:

1. Root Pressure: This is a positive pressure that builds up in the roots due to the

constant absorption of water. Root pressure can push water up the plant but is

usually only strong enough to move water a short distance.

2. Capillary Action: Capillary action occurs when water molecules move upward

through narrow tubes (like the xylem) due to adhesion (the attraction between

water molecules and the walls of the tubes) and cohesion (the attraction between

water molecules themselves). Capillary action helps move water upward in plants.

3. Transpirational Pull: This is the primary driving force of water movement in tall

plants. When water evaporates from the leaves during transpiration, it creates a

negative pressure or suction force. This pull draws more water upward from the

roots to the leaves. Transpirational pull is the most powerful force that moves water

through plants.

c. Evaporation of Water from Stomata:

Once the water reaches the leaves, it is used for photosynthesis, cell maintenance, and

growth. The excess water is lost to the atmosphere through stomata in the form of water

vapor. The stomata open and close based on environmental conditions, controlled by the

surrounding guard cells, which swell or shrink to regulate the opening.

d. Factors Controlling Stomatal Movement:

The guard cells, which surround the stomata, are responsible for regulating the opening and

closing of these pores. When the plant has enough water, the guard cells fill up and become

turgid, causing the stomata to open. When water is scarce, the guard cells lose water and

become flaccid, closing the stomata to prevent further water loss.

Several factors affect the stomatal movement, and therefore transpiration:

• Light: Stomata generally open in the presence of light, which promotes

photosynthesis and the need for carbon dioxide. This opening also increases water

loss through transpiration.

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• Temperature: Higher temperatures increase the rate of evaporation, causing more

transpiration.

• Humidity: In high-humidity conditions, transpiration decreases because the air is

already saturated with water vapor, reducing the difference in water vapor pressure

between the plant and the atmosphere.

• Wind: Wind can remove the water vapor surrounding the stomata, increasing the

transpiration rate.

• Soil Water Content: When there is a lot of water in the soil, plants can afford to lose

water through transpiration. However, in dry conditions, the plant will close its

stomata to conserve water.

4. Significance of Transpiration

Transpiration is not just about losing water; it plays a crucial role in a plant’s overall

functioning and survival:

a. Cooling the Plant:

Just like sweating helps humans cool down, transpiration helps plants regulate their

temperature. When water evaporates from the leaves, it absorbs heat, cooling the plant.

This process is especially important in hot conditions, where plants can overheat without

transpiration.

b. Nutrient Transportation:

Water serves as a medium for transporting nutrients and minerals from the soil to different

parts of the plant. Through transpiration, the plant continuously pulls water (and the

nutrients dissolved in it) from the roots to the stems and leaves, ensuring a steady supply of

essential substances for growth and development.

c. Maintaining Turgor Pressure:

Turgor pressure is the pressure exerted by water inside the plant cells on the cell walls. This

pressure helps maintain the structure and firmness of plant tissues. By regulating the

amount of water inside the plant, transpiration helps in maintaining this internal pressure,

preventing the plant from wilting.

d. Gas Exchange:

The stomata, which are the main sites of transpiration, are also essential for gas exchange.

While water vapor exits the plant through these pores, carbon dioxide (CO2) enters,

enabling photosynthesis, the process by which plants produce their food.

5. Factors Affecting Transpiration

Numerous environmental and physiological factors influence the rate of transpiration:

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a. Environmental Factors:

1. Light: Plants transpire more during the daytime when stomata are open to absorb

carbon dioxide for photosynthesis.

2. Temperature: Higher temperatures increase the rate of transpiration as water

evaporates more quickly from leaf surfaces.

3. Humidity: The more humid the atmosphere, the slower the transpiration rate. When

the air is dry, water vapor easily escapes from the plant.

4. Wind: Windy conditions increase transpiration by removing the humid air around

the leaves, allowing more water vapor to diffuse out of the stomata.

5. Water Availability: If there is limited water in the soil, plants close their stomata to

reduce water loss, lowering the rate of transpiration.

b. Plant Factors:

1. Leaf Surface Area: Plants with larger leaves transpire more because they have a

greater surface area for water loss.

2. Stomatal Density: The number of stomata per unit area of leaf surface can influence

the rate of transpiration. Plants with more stomata lose more water.

3. Cuticle Thickness: Plants with thicker cuticles have a reduced rate of cuticular

transpiration.

4. Plant Species: Different species of plants have different adaptations to control the

rate of transpiration. For example, desert plants (xerophytes) have specialized

structures to reduce water loss.

6. Regulation of Transpiration

Plants have developed various mechanisms to regulate transpiration, especially in

extreme conditions:

1. Stomatal Regulation: As discussed, stomata play a key role in regulating

transpiration by opening and closing. When water is abundant, stomata open,

allowing transpiration to proceed. When water is scarce, stomata close to conserve

moisture.

2. Leaf Modifications: Some plants, especially those in arid environments, have leaves

that are reduced to spines (like cacti) or are covered with a thick cuticle to reduce

water loss. Others have hairs on their leaves that trap moisture, reducing

evaporation.

3. Crassulacean Acid Metabolism (CAM) Plants: CAM plants open their stomata at

night when temperatures are cooler and the air is more humid, which minimizes

water loss. They store carbon dioxide and use it for photosynthesis during the day

when stomata are closed.

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7. Effects of Excessive Transpiration

While transpiration is essential for a plant’s survival, excessive water loss can be harmful. In

situations where the water lost through transpiration exceeds the water absorbed by the

roots, plants may undergo wilting and desiccation (drying out), which can eventually lead to

the plant's death if water loss isn’t controlled.

Conclusion

Transpiration is a vital process for plants that ensures the uptake of nutrients, regulation of

temperature, and overall health. Though it involves water loss, plants have developed

efficient mechanisms to balance the need for transpiration with the risk of dehydration. By

understanding how transpiration works and the factors influencing it, we can appreciate its

critical role in the survival and functioning of plants.

Reliable sources for studying plant physiology and transpiration include textbooks like "Plant

Physiology" by Taiz and Zeiger and scientific databases such as JSTOR or PubMed for peer-

reviewed articles.

SECTION-B

3. Discuss the following:

(a) Difference between macro & micro elements.

(b) Symptoms of Ca & Mg deficiency in plants.

ANS: In plant physiology, understanding the role of different nutrients is crucial for healthy growth

and development. Plants need specific elements to perform various biological functions, and these

elements are classified into two broad categories: macro elements and micro elements. Let's break

down this concept into simpler terms and also discuss what happens when plants lack important

nutrients like calcium (Ca) and magnesium (Mg).

(a) Difference Between Macro and Micro Elements

Plants need nutrients to grow, develop, and survive. These nutrients are absorbed through

the roots from the soil. The difference between macro elements and micro elements is

based on the quantity required by the plant and the roles they play in the plant's

metabolism.

1. Macro Elements (Macronutrients)

Macro elements are the nutrients that plants need in larger quantities. They are like the

"main ingredients" in a recipe, crucial for plant structure and overall health.

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Key Characteristics:

• Required in large amounts (more than 100 milligrams per kilogram of plant dry

weight).

• Play a major role in building the plant's physical structure.

• Involved in essential processes like photosynthesis, protein synthesis, and energy

production.

Examples of Macro Elements:

• Nitrogen (N): Important for the growth of leaves and stems. It is a key component of

chlorophyll, which helps plants perform photosynthesis.

• Phosphorus (P): Crucial for root development, flower and fruit production. It is also a

part of ATP, the energy molecule in cells.

• Potassium (K): Helps in regulating water movement and enzyme activity. It supports

photosynthesis and strengthens the plant's immune system.

• Calcium (Ca): Provides strength to cell walls, helps in root and leaf growth.

• Magnesium (Mg): A key component of chlorophyll, essential for photosynthesis.

• Sulfur (S): Vital for protein synthesis and enzyme function.

Role of Macro Elements:

• They are involved in creating the structural parts of the plant, such as the cell walls

and proteins.

• They regulate processes like nutrient transport, water movement, and energy

production.

2. Micro Elements (Micronutrients)

Micro elements are nutrients that plants require in much smaller quantities compared to

macro elements. Though required in tiny amounts, they are just as important for the plant's

health, playing specialized roles.

Key Characteristics:

• Needed in small amounts (less than 100 milligrams per kilogram of plant dry weight).

• Act as catalysts in various biochemical reactions inside the plant.

• Deficiencies, even though less common, can still cause significant issues in plant

growth and development.

Examples of Micro Elements:

• Iron (Fe): Essential for the formation of chlorophyll and energy transfer within the

plant.

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• Manganese (Mn): Plays a role in enzyme activation, photosynthesis, and nitrogen

metabolism.

• Zinc (Zn): Important for enzyme function, hormone regulation, and protein

synthesis.

• Copper (Cu): Helps with photosynthesis, reproductive growth, and cell wall

structure.

• Boron (B): Involved in the formation of cell walls and reproductive structures.

• Molybdenum (Mo): Required for nitrogen fixation and the synthesis of certain

amino acids.

Role of Micro Elements:

• They serve as cofactors or activators of enzymes that regulate metabolic pathways.

• They aid in the plant's ability to use other nutrients effectively.

Major Differences Between Macro and Micro Elements:

Feature

Macro Elements

Micro Elements

Quantity

Needed

Required in large amounts

Required in trace amounts

Role

Form structural components, regulate

vital processes

Act as cofactors or catalysts in

metabolic reactions

Examples

Nitrogen, Phosphorus, Potassium,

Calcium, Magnesium, Sulfur

Iron, Manganese, Zinc, Copper,

Boron, Molybdenum

Deficiency

Effects

Severe and quickly noticeable

Subtle but still significant effects

(b) Symptoms of Calcium (Ca) and Magnesium (Mg) Deficiency in Plants

Plants can show various symptoms when they do not receive enough nutrients, and these

deficiencies can affect their health and growth. Let's explore the effects of calcium and

magnesium deficiency on plants.

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1. Calcium (Ca) Deficiency in Plants

Calcium plays an essential role in maintaining the structural integrity of cell walls and

membranes. It is critical for the growth of roots, leaves, and the overall development of the

plant.

Symptoms of Calcium Deficiency:

• Stunted Growth: Calcium helps with cell division, especially in the root and shoot

tips. Without enough calcium, plants will show poor root development and restricted

growth in the shoot system.

• Leaf Tip Burn: One of the common signs of calcium deficiency is a brown or

blackened appearance at the tips or edges of young leaves. This is known as tip burn.

• Deformed Young Leaves: New leaves may appear distorted, curled, or malformed

because calcium helps in the proper development of tissues in young growing parts.

• Weak Cell Walls: Calcium strengthens the cell walls, so a deficiency can cause weak,

thin cell walls, making the plant more prone to disease.

• Blossom End Rot in Fruits: Calcium deficiency often leads to a condition called

blossom end rot in fruit-bearing plants like tomatoes and peppers. The fruits develop

dark, sunken patches at their ends.

Causes of Calcium Deficiency:

• Acidic Soil: Calcium is less available in soils with low pH levels.

• Excessive Potassium or Magnesium: An excess of other cations like potassium (K) or

magnesium (Mg) can interfere with calcium uptake.

• Water Deficiency: Since calcium moves with water in the plant, insufficient water

can limit calcium uptake.

2. Magnesium (Mg) Deficiency in Plants

Magnesium is a vital component of chlorophyll, the molecule responsible for capturing

sunlight for photosynthesis. It also plays a role in enzyme activation and energy transfer

within the plant.

Symptoms of Magnesium Deficiency:

• Interveinal Chlorosis: One of the most common symptoms of magnesium deficiency

is interveinal chlorosis. This is when the areas between the veins of the leaves turn

yellow, while the veins themselves remain green. This usually appears on older

leaves first.

• Leaf Curling: In some plants, magnesium deficiency can cause the edges of the

leaves to curl upwards or downwards.

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• Premature Leaf Drop: When plants are deficient in magnesium, they may shed older

leaves prematurely. This is because magnesium is a mobile element and can be

moved from older leaves to newer growth, causing the older leaves to die off.

• Poor Growth: Since magnesium is essential for photosynthesis, a deficiency can

stunt the plant's growth, as it won’t be able to produce enough energy.

Causes of Magnesium Deficiency:

• Sandy or Acidic Soils: Magnesium is more likely to leach out of light, sandy soils or

soils with low pH levels.

• Excessive Potassium or Calcium: Just like with calcium, too much potassium (K) or

calcium (Ca) can reduce magnesium availability to plants.

Conclusion

In plant physiology, understanding the difference between macro and micro elements helps

us appreciate the specific needs of plants for optimal growth. Macro elements like nitrogen,

phosphorus, potassium, calcium, magnesium, and sulfur are required in larger amounts and

play essential roles in structure and metabolism. Micro elements, including iron,

manganese, zinc, copper, boron, and molybdenum, are needed in smaller quantities but are

equally important for specific biochemical processes.

When plants lack nutrients like calcium or magnesium, they show clear deficiency

symptoms. Calcium deficiency can lead to poor root and leaf growth, tip burn, and weak cell

walls, while magnesium deficiency causes interveinal chlorosis, leaf curling, and premature

leaf drop. By identifying these symptoms, we can take steps to correct the deficiencies and

ensure healthy plant growth.

These nutrients work together to support the plant's overall health, and understanding their

roles is essential for successful plant cultivation.

4. Explain the Munch hypothesis of translocation of solutes in phloem.

Ans: The Münch Hypothesis of translocation in the phloem, also known as the Pressure Flow

Hypothesis, is one of the most widely accepted explanations of how nutrients (specifically

sugars) are transported through plants. It was proposed by the German plant physiologist

Ernst Münch in 1930. To explain this concept in simple and easy-to-understand terms, we

can break it down step by step:

Basic Concepts

Before diving into the Münch Hypothesis, let's first understand a few key terms:

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• Phloem: Phloem is a type of tissue found in plants that is responsible for

transporting nutrients, particularly sugars (like sucrose), which are produced during

photosynthesis in the leaves, to other parts of the plant like roots, fruits, and storage

tissues. Phloem is the "transport highway" for these nutrients.

• Solutes: Solutes are dissolved substances. In the context of plants, the main solute

we are talking about is sugar, particularly sucrose.

• Source and Sink: In plants, the "source" is the part of the plant where sugars are

produced, primarily the leaves during photosynthesis. The "sink" is the part of the

plant where sugars are used or stored, such as roots, fruits, seeds, or growing

tissues.

What is the Münch Hypothesis?

The Münch Hypothesis suggests that the movement of solutes (mainly sugars like sucrose)

in the phloem happens due to pressure differences between the "source" (where the sugars

are made) and the "sink" (where the sugars are used or stored). This pressure difference

drives the flow of sugars from the source to the sink through a process called bulk flow or

mass flow.

Key Components of the Münch Hypothesis:

1. Source: The leaves of the plant act as the "source" because this is where

photosynthesis takes place. During photosynthesis, plants produce sugars (mainly

sucrose) that are loaded into the phloem cells, also known as sieve tube elements.

2. Sink: The "sink" refers to the areas of the plant that either consume or store the

sugars produced in the leaves. This could be the roots, flowers, fruits, or growing

regions like young leaves or developing seeds.

3. Osmosis and Pressure: At the source, sugars are loaded into the sieve tube elements

of the phloem. This increases the concentration of solutes in the phloem, which

causes water to flow into the phloem from the nearby xylem (another type of

vascular tissue that transports water). This influx of water increases the pressure in

the phloem near the source.

4. Pressure Flow: The high pressure at the source pushes the sugary solution (sucrose

and water) through the phloem toward the sink. As the solution moves, it is

essentially "pushed" from areas of high pressure (the source) to areas of lower

pressure (the sink).

5. Unloading at the Sink: At the sink, the sugars are removed from the phloem and

used by the plant for energy or stored as starch. When sugars are unloaded at the

sink, the solute concentration decreases, causing water to leave the phloem and

flow back into the xylem. This reduces the pressure in the phloem at the sink,

maintaining the pressure gradient that keeps the flow moving.

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How the Münch Hypothesis Works Step-by-Step:

Let’s break this process down step-by-step for easier understanding:

1. Sugar Production (Source):

o In the leaves (the source), plants produce sugars through photosynthesis.

o The sugar, mainly sucrose, is actively transported (loaded) into the sieve

tubes of the phloem.

2. Water Movement:

o As sucrose accumulates in the phloem, the concentration of solutes

increases.

o Water from the xylem (a tissue that carries water from the roots) moves into

the phloem by osmosis to balance the solute concentration.

o This influx of water creates a high pressure in the phloem near the source.

3. Pressure Flow:

o The high pressure in the phloem near the source forces the sugar-water

solution to move through the sieve tubes toward areas of lower pressure (the

sink).

o This movement is called bulk flow or mass flow and is driven by the

difference in pressure between the source and the sink.

4. Sugar Unloading (Sink):

o When the sugar-water solution reaches the sink (like roots or growing

tissues), the sucrose is removed from the phloem to be used or stored by the

plant.

o As sucrose is unloaded, the concentration of solutes in the phloem decreases.

5. Water Movement at the Sink:

o Once the sugars are unloaded at the sink, water leaves the phloem and

returns to the xylem, which lowers the pressure in the phloem at the sink.

o This maintains the pressure difference that drives the flow of the solution

from source to sink.

6. Cycle Repeats:

o The whole process repeats as long as the plant continues to produce sugars

in the leaves and requires them in the sinks (roots, fruits, etc.).

Diagram of the Münch Hypothesis:

While I cannot create diagrams here, imagine the following visual:

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• On one side (the source), you have the leaves where sugars are being made and

loaded into the phloem.

• Water from the xylem flows into the phloem due to the high concentration of sugar,

creating high pressure.

• The sugary solution moves down the plant toward the sink (the roots or growing

tissues).

• At the sink, the sugars are unloaded, the pressure decreases, and water moves back

into the xylem, completing the cycle.

Why the Münch Hypothesis is Widely Accepted:

The Münch Hypothesis is accepted by many scientists because it explains the movement of

sugars in plants in a way that is simple, efficient, and consistent with what we observe in

nature. It does not require complex energy expenditure from the plant, relying instead on

physical processes like osmosis and pressure differences to drive the flow of sugars from the

source to the sink.

Additionally, experimental evidence supports the hypothesis. For example, when

researchers measure the pressure in the phloem near the source and the sink, they find that

the pressure is indeed higher near the source and lower near the sink, just as the Münch

Hypothesis predicts.

Strengths of the Münch Hypothesis:

1. Simplicity: The Münch Hypothesis is relatively simple and explains how sugars move

in plants using basic principles of pressure and water movement.

2. Efficiency: The system is energy-efficient. Plants don't need to use a lot of energy to

move sugars around because the process relies on natural pressure gradients.

3. Experimental Support: Studies have shown that there is a pressure difference in the

phloem between the source and the sink, which supports the hypothesis.

Limitations of the Münch Hypothesis:

While the Münch Hypothesis is widely accepted, it does have some limitations:

1. Long-Distance Transport: One criticism is that the hypothesis may not fully explain

how sugars move over very long distances, such as from the top of a tall tree to its

roots. Some scientists argue that additional mechanisms might be needed to support

such long-distance transport.

2. Complexity in Real Plants: In real plants, the process may be more complex than

what the Münch Hypothesis suggests. For example, the loading and unloading of

sugars into and out of the phloem can be more complicated, involving active

transport mechanisms that require energy.

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3. Phloem Anatomy: The structure of the phloem (sieve tubes) is also more complex,

and the hypothesis doesn't fully address how the flow of nutrients is regulated

within these structures.

Current Research:

Modern research in plant physiology continues to investigate how exactly phloem transport

works. While the Münch Hypothesis provides a strong foundation, scientists are exploring

other factors that might influence translocation, such as hormonal control, the role of

companion cells in the phloem, and how plants respond to environmental stresses like

drought.

For example, some studies suggest that plants can adjust the rate of phloem transport

depending on how much sugar they need to move. This implies that while pressure flow

might be the basic mechanism, the process could be more dynamic and responsive to the

plant's needs than originally thought.

Conclusion:

In conclusion, the Münch Hypothesis is a widely accepted explanation for how sugars are

transported in plants through the phloem. It is based on the idea that pressure differences

between the source (leaves) and the sink (roots, fruits, etc.) drive the flow of sugars. The

hypothesis explains many of the basic observations about how nutrients move in plants, and

while there are some limitations, it remains a key theory in plant physiology today.

SECTION-C

5. What do you mean by photophosphorylation? Distinguish between cyclic and non-cyclic

photophosphorylation.

Ans: Photophosphorylation:

Photophosphorylation is a process in plants, algae, and some bacteria where light energy is

used to produce ATP (adenosine triphosphate), the energy currency of the cell. This process

takes place during photosynthesis, which is the method by which plants convert sunlight

into chemical energy. In simpler terms, photophosphorylation is how plants capture sunlight

and turn it into energy that can be used for growth and other functions.

How Does Photophosphorylation Work?

During photosynthesis, sunlight hits the chlorophyll molecules in the plant's leaves, which

excites the electrons in these molecules. These high-energy electrons are then used to add a

phosphate group to ADP (adenosine diphosphate), turning it into ATP. This addition of a

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phosphate group is called phosphorylation, and since it happens using light energy (photo),

the process is termed photophosphorylation.

There are two types of photophosphorylation that occur in plants: cyclic and non-cyclic

photophosphorylation. Both of these processes take place in the thylakoid membranes of

the chloroplasts, the organelles responsible for photosynthesis.

Cyclic Photophosphorylation:

In cyclic photophosphorylation, electrons follow a circular path. The electron starts at a

molecule called Photosystem I, gains energy from light, moves through a series of proteins

(called an electron transport chain), and eventually returns to Photosystem I. In this cycle,

ATP is produced, but no NADPH (another energy-carrying molecule) or oxygen is produced.

• Where It Happens: This process mainly takes place when there is a greater need for

ATP than NADPH.

• Why It’s Important: Cyclic photophosphorylation provides additional ATP, which is

needed for various energy-consuming reactions in the cell, especially during the

Calvin cycle (a part of photosynthesis where carbon dioxide is turned into glucose).

Non-Cyclic Photophosphorylation:

In non-cyclic photophosphorylation, electrons do not return to the same starting point.

Instead, they travel from Photosystem II to Photosystem I, and then they are transferred to

NADP+, forming NADPH. This process produces both ATP and NADPH, and as a by-product,

oxygen is released from the splitting of water molecules.

• Where It Happens: This process takes place when both ATP and NADPH are required

by the plant.

• Why It’s Important: Non-cyclic photophosphorylation is the primary pathway for

energy production in plants. It not only generates the ATP necessary for the Calvin

cycle but also produces NADPH, which provides the reducing power to convert

carbon dioxide into sugars.

Detailed Comparison Between Cyclic and Non-Cyclic Photophosphorylation:

Feature

Cyclic Photophosphorylation

Non-Cyclic Photophosphorylation

Electron

Pathway

Electrons follow a circular path

and return to the same molecule

(Photosystem I).

Electrons move in a linear path from

Photosystem II to Photosystem I and

eventually to NADP+.

Photosystem

Involved

Only Photosystem I is involved.

Both Photosystem II and Photosystem

I are involved.

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Feature

Cyclic Photophosphorylation

Non-Cyclic Photophosphorylation

ATP

Production

ATP is produced.

Both ATP and NADPH are produced.

NADPH

Production

NADPH is not produced.

NADPH is produced.

Oxygen

Production

No oxygen is released.

Oxygen is released as a by-product of

water splitting.

Electron

Donor

The same electron returns to the

chlorophyll molecule in

Photosystem I.

Electrons are replaced by those from

water molecules, which are split into

oxygen, protons, and electrons.

Function

Used when the plant requires

more ATP and does not need

NADPH.

Provides both ATP and NADPH for the

Calvin cycle.

End Products

ATP only.

ATP, NADPH, and oxygen.

Importance

Supplements ATP production

when it is required.

Primary pathway for energy

generation during photosynthesis.

Steps Involved in Cyclic Photophosphorylation:

1. Light Absorption in Photosystem I: Light energy is absorbed by Photosystem I (PSI),

causing its electrons to become excited.

2. Electron Transport: These excited electrons are passed along an electron transport

chain, a series of proteins embedded in the thylakoid membrane.

3. ATP Production: As electrons move along the chain, they lose energy. This energy is

used to pump protons (H+) across the membrane, creating a proton gradient. This

gradient powers the enzyme ATP synthase, which synthesizes ATP from ADP and

inorganic phosphate (Pi).

4. Electron Return: After passing through the electron transport chain, the electron

returns to Photosystem I, completing the cycle.

Steps Involved in Non-Cyclic Photophosphorylation:

1. Light Absorption in Photosystem II: Light is absorbed by Photosystem II (PSII), and

its electrons get excited to a higher energy level.

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2. Water Splitting: To replace the excited electrons lost from Photosystem II, water

molecules are split into oxygen, protons, and electrons. This process is called

photolysis, and it is the source of oxygen released during photosynthesis.

3. Electron Transport Chain: The excited electrons move through the electron

transport chain. As they pass through, they release energy, which is used to pump

protons across the thylakoid membrane, helping to produce ATP.

4. Light Absorption in Photosystem I: The electrons reach Photosystem I, where they

are re-energized by light.

5. NADPH Production: The high-energy electrons are then transferred to NADP+ to

form NADPH. NADPH is an energy carrier used in the Calvin cycle.

6. ATP Production: The flow of protons back across the membrane, through ATP

synthase, drives the synthesis of ATP from ADP and phosphate.

Why Are Both Cyclic and Non-Cyclic Photophosphorylation Necessary?

Plants need both ATP and NADPH for the Calvin cycle, where carbon dioxide is converted

into glucose. Non-cyclic photophosphorylation produces both of these molecules. However,

sometimes the Calvin cycle requires more ATP than NADPH. In such cases, cyclic

photophosphorylation occurs to produce additional ATP without making extra NADPH,

ensuring a balance of these energy molecules.

Importance of Photophosphorylation:

Photophosphorylation is essential for life on Earth because it is part of the larger process of

photosynthesis. Without photosynthesis, plants would not be able to produce the sugars

they need to grow. Furthermore, since plants are at the base of the food chain, all other life

forms depend on them for food and oxygen. Here are some of the critical reasons

photophosphorylation is important:

• Energy Production: ATP produced during photophosphorylation powers various

cellular activities in plants.

• Sugar Production: NADPH generated in non-cyclic photophosphorylation is

necessary for the Calvin cycle to convert carbon dioxide into glucose, which serves as

the primary energy source for plants and, indirectly, for animals.

• Oxygen Release: The splitting of water in non-cyclic photophosphorylation releases

oxygen into the atmosphere, which is essential for the survival of aerobic organisms,

including humans.

Final Thoughts:

Photophosphorylation is a crucial process in photosynthesis where light energy is converted

into chemical energy in the form of ATP and NADPH. The two types—cyclic and non-cyclic

photophosphorylation—serve different purposes. Cyclic photophosphorylation helps

produce additional ATP, while non-cyclic photophosphorylation is the main pathway that

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produces both ATP and NADPH, along with oxygen. Together, these processes ensure that

plants have enough energy to carry out the Calvin cycle and produce the sugars needed for

their growth and survival.

In summary, photophosphorylation is the plant's way of capturing sunlight and using it to

fuel the essential chemical processes that sustain life on Earth.

6. Describe briefly:

(a) Red drop effect

(b) Enhancement effect

Ans: To explain these topics from Plant Physiology in a simple and easy-to-understand way,

let's break down each of them. The three concepts—Red Drop Effect, Enhancement Effect,

and Action Spectrum—are closely related to photosynthesis and the way plants use light

energy. I'll explain each concept, their significance, and how they work within the broader

context of plant physiology.

(a) Red Drop Effect

The Red Drop Effect is an observation related to the efficiency of photosynthesis when

plants are exposed to different wavelengths of light, particularly in the red and far-red

regions of the light spectrum.

What is the Red Drop Effect?

In simple terms, the Red Drop Effect refers to a drop in the efficiency of photosynthesis

when plants are exposed to light wavelengths longer than 680 nanometers (nm), which is in

the far-red region of the spectrum.

Discovery

This effect was discovered by Robert Emerson in the 1940s while studying the

photosynthetic activity of plants. He noticed that when the plants were exposed to far-red

light (beyond 680 nm), the rate of photosynthesis significantly decreased compared to when

they were exposed to red light (around 680 nm).

Explanation

• Light is absorbed by chlorophyll, the green pigment in plants that captures light

energy to carry out photosynthesis.

• Chlorophyll absorbs light most effectively in the blue and red regions of the light

spectrum.

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• However, when plants are exposed to light beyond the red region (far-red light), the

photosynthetic efficiency drops. This is what we call the Red Drop Effect.

In photosynthesis, two systems called Photosystem I (PSI) and Photosystem II (PSII) work

together to capture light energy and convert it into chemical energy. PSII absorbs light most

efficiently at 680 nm, while PSI works best at slightly longer wavelengths. The Red Drop

Effect shows that when light is too far into the red spectrum, PSII becomes less efficient,

leading to a drop in the overall rate of photosynthesis.

Why is it important?

The Red Drop Effect is significant because it helped scientists understand that

photosynthesis involves two different photosystems, each with its specific role and optimal

light conditions. This discovery laid the foundation for further research into the mechanisms

of photosynthesis, including the Enhancement Effect (which we will discuss next).

(b) Enhancement Effect

The Enhancement Effect, also known as the Emerson Enhancement Effect, is closely linked

to the Red Drop Effect and further expands on the idea that photosynthesis requires more

than just one type of light absorption process.

What is the Enhancement Effect?

The Enhancement Effect refers to the observation that when light from two different

regions of the spectrum (red and far-red) is provided simultaneously to plants, the rate of

photosynthesis increases significantly compared to when either light source is provided

alone.

Discovery

Robert Emerson, after discovering the Red Drop Effect, went on to test how plants respond

when they are exposed to a combination of red light and far-red light. He found that the

rate of photosynthesis increased dramatically when plants were exposed to both red and

far-red light together, as opposed to each light source individually. This is known as the

Enhancement Effect.

Explanation

Here’s a breakdown of how the Enhancement Effect works:

• Photosystem II (PSII) works best with light at 680 nm (in the red region).

• Photosystem I (PSI) works best with light at wavelengths beyond 680 nm (in the far-

red region).

When both types of light (red and far-red) are given to plants at the same time,

Photosystem I and Photosystem II work together more efficiently, boosting the overall rate

of photosynthesis. This happens because:

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• PSII captures the energy from red light and uses it to split water molecules and

produce oxygen.

• PSI captures the energy from far-red light and helps in the production of energy-rich

molecules like NADPH, which are essential for the Calvin cycle (the process that

converts carbon dioxide into glucose).

In essence, the Enhancement Effect shows that the two photosystems work cooperatively

and are more efficient when exposed to both red and far-red light simultaneously. This

interaction between the two photosystems is crucial for the overall process of

photosynthesis.

Why is it important?

The Enhancement Effect was a critical discovery because it confirmed that photosynthesis

requires the cooperation of two different photosystems. It also provided evidence that

photosynthesis is a complex, multi-step process involving several stages of light absorption

and energy transfer. This understanding has been crucial in improving our knowledge of

how plants use light energy.

The Action Spectrum is a graph or chart that shows how effective different wavelengths of

light are in driving a specific biological process, such as photosynthesis.

What is an Action Spectrum?

The Action Spectrum of photosynthesis is a graphical representation that shows the rate of

photosynthesis at different wavelengths of light. It indicates which wavelengths of light are

most effective for photosynthesis.

Explanation

• Photosynthesis relies on pigments, like chlorophyll, to absorb light. Different

pigments absorb light at different wavelengths.

• The Action Spectrum shows the relationship between the wavelength of light and

the rate of photosynthesis.

• The peaks in the Action Spectrum typically occur in the blue region (around 450 nm)

and the red region (around 680 nm), where chlorophyll absorbs the most light.

Chlorophyll, the main pigment involved in photosynthesis, absorbs blue and red light most

effectively. This is why these regions show higher rates of photosynthesis in the Action

Spectrum.

However, plants also have other pigments, like carotenoids, that absorb light in different

regions of the spectrum (such as the green and yellow regions). These pigments extend the

range of light that can be used for photosynthesis, although they are not as efficient as

chlorophyll in driving the process.

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Why is it important?

The Action Spectrum is important because it helps scientists understand which wavelengths

of light are most effective in powering photosynthesis. This knowledge can be used to

optimize growing conditions for plants, especially in controlled environments like

greenhouses or artificial lighting systems for indoor agriculture.

By knowing the most effective wavelengths for photosynthesis, scientists can design lighting

systems that provide the ideal light for plant growth, which is critical for both food

production and environmental sustainability.

Summing It All Up

The Red Drop Effect, Enhancement Effect, and Action Spectrum are all interconnected

concepts that help us understand how plants use light during photosynthesis.

1. Red Drop Effect tells us that the efficiency of photosynthesis decreases when plants

are exposed to light beyond 680 nm (far-red light). This effect helped reveal the

existence of two distinct photosystems (PSI and PSII) in plants.

2. Enhancement Effect shows that when red light (680 nm) and far-red light (beyond

680 nm) are provided together, the rate of photosynthesis increases dramatically.

This discovery proved that PSI and PSII work together, and their cooperation

enhances the overall efficiency of photosynthesis.

3. Action Spectrum provides a graphical representation of how different wavelengths

of light affect the rate of photosynthesis. It highlights that blue and red light are the

most effective for driving photosynthesis, as these are the wavelengths most

efficiently absorbed by chlorophyll.

These concepts are fundamental to understanding the process of photosynthesis and how

plants convert light energy into chemical energy. This knowledge has practical applications

in agriculture, where optimizing light conditions can significantly boost plant growth and

crop yields.

Further Practical Applications

1. Agricultural Optimization: Understanding the Action Spectrum helps in designing

artificial lighting systems, especially in controlled environments like greenhouses or

vertical farms. Farmers can provide the ideal light wavelengths to maximize plant

growth and yield.

2. Environmental Research: By studying how plants respond to different light

conditions, scientists can develop strategies to improve crop resilience to changing

climate conditions.

3. Energy Efficiency in Farming: The Enhancement Effect highlights the importance of

using a combination of light wavelengths, which can lead to more efficient use of

energy in farming practices.

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In summary, the study of light effects on photosynthesis, from the Red Drop and

Enhancement Effects to the Action Spectrum, provides valuable insights into the intricate

mechanisms that power plant growth and survival. These discoveries have had lasting

impacts on biology, agriculture, and environmental science, helping to optimize how we

support and cultivate plant life.

SECTION-D

7. Explain the physiological role of:

(a) Auxins

(b) Gibberellins

Ans: Auxins and gibberellins are two key plant hormones, or phytohormones, that play a

crucial role in regulating various physiological processes essential for plant growth and

development.

(a) Auxins

Auxins are among the most well-known plant hormones. They primarily promote cell

elongation, which leads to the growth of plant tissues. Their major physiological roles

include:

1. Cell Elongation: Auxins help in elongating plant cells by increasing the flexibility of

the cell walls. This allows the cells to expand and grow, especially in the shoot tips

and root tips.

2. Apical Dominance: Auxins are produced in the apical meristem (the tip of the shoot)

and move downward, suppressing the growth of lateral buds. This ensures the plant

grows upwards rather than branching out. When the apical meristem is removed

(e.g., by pruning), lateral buds are released from this suppression, and the plant

becomes bushier.

3. Root Development: Auxins also play a crucial role in root formation. They stimulate

the development of lateral and adventitious roots, which helps in the better

anchorage of the plant and improves its nutrient absorption capacity.

4. Phototropism and Gravitropism: Auxins help plants respond to environmental

stimuli. In phototropism, they cause the plant to grow towards light by accumulating

on the shaded side of the plant, leading to cell elongation on that side. In

gravitropism, auxins help roots grow downwards in response to gravity.

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5. Fruit Development: Auxins can stimulate the development of fruits. In some cases,

they promote fruit set even in the absence of pollination (parthenocarpy), leading to

seedless fruit production.

6. Prevention of Leaf and Fruit Drop: Auxins help in preventing the premature

dropping of leaves, flowers, and fruits by regulating the abscission layer, a group of

cells that form where leaves or fruits attach to the plant.

(b) Gibberellins

Gibberellins are another group of plant hormones that have diverse effects on plant growth.

They were initially discovered because they were responsible for abnormal growth in plants

infected by a fungus. Since then, gibberellins have been found to have several physiological

roles:

1. Stem Elongation: Gibberellins promote the elongation of the internodes (the space

between leaves on a stem). This is particularly important in plants that show

dwarfism. Gibberellin treatment can make these plants grow taller by stimulating

cell elongation and division.

2. Seed Germination: One of the most significant roles of gibberellins is in breaking

seed dormancy and promoting seed germination. They activate enzymes that break

down stored food in the seed, providing the energy needed for the seedling to grow.

3. Flowering: Gibberellins promote flowering in certain plants, especially in long-day

plants (those that flower in response to long periods of daylight). In some cases, they

can induce flowering in plants that would not normally flower under specific

environmental conditions.

4. Fruit Growth: Like auxins, gibberellins also play a role in fruit development. They can

increase the size of fruits, particularly in grapes and apples, making them larger and

more appealing for commercial use.

5. Overcoming Stress: Gibberellins help plants cope with stress conditions, such as low

temperatures and drought. By regulating various genes involved in stress responses,

gibberellins enhance the resilience of plants under adverse environmental

conditions.

6. Bolting: In rosette plants (e.g., cabbage and lettuce), gibberellins are responsible for

bolting, which is the rapid elongation of the stem that occurs just before flowering.

This allows the plant to produce flowers and seeds in a shorter period.

Both auxins and gibberellins have profound effects on the overall development and survival

of plants. They act synergistically in some processes and antagonistically in others, creating

a fine-tuned regulatory network that helps plants grow, reproduce, and respond to

environmental changes. These hormones are widely studied and utilized in agriculture to

enhance crop productivity, fruit size, and resistance to environmental stress

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This simplified explanation of auxins and gibberellins covers their physiological roles in

plants, highlighting their importance in processes like growth, seed germination, fruit

development, and stress response. If you need more detailed or specific examples for

agricultural applications or molecular interactions, feel free to ask!

8. Discuss the following

(a) Short day plants

(b) Phytochromes

Ans: (a) Short Day Plants

Introduction

Short day plants (SDPs) are a category of plants that require long nights or short days to

initiate flowering. These plants flower when the day length is less than a critical duration,

typically 12 hours or less. The term "short day" refers to the length of daylight required by

these plants for blooming. They depend on the duration of darkness, rather than the light,

to trigger their flowering process.

How Short Day Plants Work

The flowering of short day plants is controlled by the length of the night. If the night is

longer than a critical threshold, the plant will start to flower. For these plants, uninterrupted

darkness for a specific period is the signal to begin flowering. If the night is interrupted by

even a short flash of light, the plant might not flower because the necessary dark period is

broken.

For example, in some regions, plants like poinsettia and chrysanthemum naturally bloom in

autumn or winter when the days are shorter. In tropical regions, many short day plants may

flower after rainy seasons when the day length shortens.

Examples of Short Day Plants

1. Chrysanthemums: Often used in flower arrangements, they bloom when days

shorten, usually in late summer or autumn.

2. Poinsettias: Popular during Christmas, poinsettias bloom when days are shorter.

3. Soybeans: Certain varieties of soybeans flower when the length of daylight decreases

in late summer.

4. Tobacco: Some varieties of tobacco plants are also sensitive to short days and flower

as the night length increases.

5. Rice: Rice is a short day plant that blooms after exposure to longer nights.

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6. Cotton: Cotton plants are also influenced by day length, specifically shorter days, for

their flowering.

Importance of Short Day Plants

1. Adaptation: These plants are adapted to environments where the changing seasons

and day lengths help them optimize their reproductive cycles. Flowering in autumn,

when the days are shorter, may provide the plant with an advantage by avoiding

competition with other plants for pollinators.

2. Agriculture: Understanding the flowering patterns of short day plants is crucial for

agricultural practices. Farmers can manipulate light exposure in greenhouses to

control flowering times and optimize crop yields.

3. Economic Significance: Many short day plants, like chrysanthemums and poinsettias,

have significant economic value in the floral industry. By controlling light exposure,

florists can make these plants flower at specific times, such as during the holiday

season.

Photoperiodism

The response of short day plants to the length of day and night is an example of

photoperiodism. Photoperiodism is a biological response to the duration of light and dark

periods. In short day plants, the critical factor is the length of darkness. If the plant gets its

required amount of uninterrupted darkness, it will flower.

Critical Day Length

Each short day plant has its own specific "critical day length." This is the maximum duration

of daylight that the plant can be exposed to and still flower. If the day is longer than the

critical day length, the plant will not flower, regardless of other conditions. This means that

short day plants are highly sensitive to day length variations.

(b) Phytochromes

Introduction

Phytochromes are specialized plant pigments that play a vital role in how plants respond to

light. They are light-sensitive proteins that help plants sense changes in light conditions,

such as day length or light quality. This system allows plants to regulate growth,

development, and flowering in response to their environment. Phytochromes are essential

for processes like seed germination, stem elongation, and flowering, as they help the plant

determine whether it's in the shade or receiving direct sunlight.

Structure and Function of Phytochromes

Phytochromes exist in two forms:

1. Pr (Red Light-absorbing form): This form absorbs red light, which is found in sunlight.

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2. Pfr (Far Red Light-absorbing form): This form absorbs far-red light, which is

predominant in shaded environments.

The two forms of phytochrome can convert into each other based on the type of light they

are exposed to:

• When the plant receives red light (sunlight), Pr is converted into Pfr.

• When the plant is exposed to far-red light (shaded areas), Pfr is converted back into

Pr.

The balance between these two forms (Pr and Pfr) is what allows the plant to sense whether

it is in direct sunlight or shade, and whether the day length is increasing or decreasing. This

information is crucial for the plant to decide when to germinate, grow, or flower.

How Phytochromes Work

Phytochromes regulate several important processes in plants:

1. Seed Germination: Many seeds need specific light conditions to germinate. For

example, lettuce seeds require light to germinate. Phytochromes help these seeds

detect the right light conditions, such as the presence of red light, which indicates

that the seed is not buried too deeply in the soil.

2. Shade Avoidance: In a dense plant population, taller plants often overshadow

shorter ones. Phytochromes help plants detect if they are in the shade of another

plant. When shaded, a plant will experience more far-red light than red light, which

signals the plant to grow taller or elongate its stems to compete for sunlight.

3. Flowering: Phytochromes are involved in regulating the flowering process based on

day length. In short day plants, for example, the amount of Pfr is reduced during long

nights, which signals the plant to flower. In contrast, long day plants need more Pfr

to flower.

4. Photomorphogenesis: This refers to the growth and development of plants in

response to light. Phytochromes help regulate the plant’s form, such as leaf

expansion and stem growth, depending on the light conditions.

Phytochrome System and Light Perception

The phytochrome system is not limited to red and far-red light but also interacts with blue

light and UV light receptors. These receptors work together to control plant growth,

development, and adaptation to environmental changes.

The transition between Pr and Pfr forms is influenced by the quality of light. In direct

sunlight, there is more red light, causing the Pr form to convert into Pfr. In shaded

conditions, there is more far-red light, leading to the conversion of Pfr back into Pr. This

reversible system allows plants to accurately measure the length of day and night.

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Significance of Phytochromes

1. Environmental Adaptation: Phytochromes allow plants to adapt to different light

environments. For example, in dense forests where light is scarce, phytochromes

help plants grow taller to reach sunlight.

2. Growth Regulation: By sensing light quality and day length, phytochromes help

regulate when plants should grow leaves, stems, and flowers. This ensures the plant

invests its energy in growth when environmental conditions are most favorable.

3. Seed Germination Control: Phytochromes help seeds decide the best time to

germinate. Seeds exposed to light with more red than far-red light (indicating they

are not deeply buried) will germinate, while seeds in deep soil (far-red light) remain

dormant.

4. Crop Improvement: Understanding phytochromes is important for agriculture.

Scientists can manipulate phytochrome activity in crops to optimize flowering times,

improve yields, and increase adaptability to different environments.

Interaction with Other Plant Hormones

Phytochromes don’t work alone; they interact with other plant hormones such as auxins,

gibberellins, and abscisic acid. Together, they coordinate a plant's response to its

environment by controlling growth, flowering, and other physiological processes.

Conclusion

In conclusion, short day plants are plants that flower when the duration of daylight is

shorter than a specific critical length, relying on long periods of darkness to initiate

flowering. They are critical in both natural ecosystems and agricultural settings, where

controlling day length can optimize flowering times. On the other hand, phytochromes are

light-sensitive proteins that help plants sense their environment and adapt accordingly.

These proteins control key processes such as seed germination, shade avoidance, and

flowering by responding to red and far-red light. Both short day plants and phytochromes

are vital for understanding plant behavior and for improving agricultural practices.

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