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Questions and Answers
What is cavitation in the context of xylem?
Which of the following factors is not a cause of cavitation and embolism?
How does relative humidity affect the rate of transpiration?
What happens to the vapor pressure inside leaves when atmospheric temperature rises by 10°C?
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Under which condition is the rate of transpiration expected to be the highest?
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What effect does wind movement have on transpiration rates?
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Which statement about the stomata is true?
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What is the primary mechanism by which pathogens contribute to cavitation?
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What negatively affects water uptake due to an increased respiration rate in rhizines?
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What is the optimal range of O2 content for root absorption and plant growth?
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How does transpiration influence water movement in plants?
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What happens when the osmotic potential of root hairs exceeds that of the soil?
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Which process involves the exhalation of liquid along the margins of leaves?
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What role does root pressure play in water movement within plants?
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Which condition is critical for effective transpiration and water movement in plants?
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What effect does temperature between 0-10°C have on plant water uptake?
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What effect does a wind velocity of 40-50 km/hr have on transpiration?
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How does low atmospheric pressure influence transpiration?
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What is incipient wilting?
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During which condition does temporary wilting typically occur?
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What leads to permanent wilting in plants?
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Which type of wilting occurs when lower leaves wilt before upper leaves?
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What factor primarily affects the rate of transpiration related to water availability?
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What does the permanent wilting percentage (PWP) indicate?
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What does the water potential (ψ) of a solution indicate?
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What happens to the water potential (ψ) of a cell when solute molecules are added?
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Which of the following describes turgor pressure in plant cells?
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What is the state of a plant cell when it is at incipient plasmolysis?
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In which solution does plasmolysis occur?
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What drives the movement of water in plants from the soil to the roots?
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How is osmotic pressure produced in plant cells?
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What occurs during deplasmolysis?
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What does a water potential gradient represent?
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What is the primary factor that determines the hydration capacity of a plant cell?
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How does the thickness of the cuticle affect transpiration?
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What is the effect of sunken stomata on transpiration?
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Which statement best describes the relationship between leaf structure and transpiration?
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How does the root/shoot ratio influence transpiration rates?
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What role do hairs on leaves play in relation to transpiration?
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What can be said about the relationship between solutes and transpiration?
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Which adaptation is typical among xerophytes to minimize transpiration?
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What impact does canopy density have on the rate of transpiration?
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Study Notes
Water Potential
- Water potential is the free energy needed to transport a water molecule within a system.
- It is represented by the symbol ψ (psi).
- It is a relative measurement, compared to pure water at atmospheric pressure and temperature.
- Water potential is expressed in units of pressure, such as megapascal (Mpa) with 10 Mpa = 1 bar = 1 atm.
- Pure water has a water potential of 0.
- The maximum value of water potential is 0, and it decreases in aqueous solutions where water is bound physically or chemically.
- Within a cell, water potential is negative because the presence of solute molecules lowers the free energy of water.
- The minus sign signifies that dissolved solutes decrease the water potential of a solution by reducing the concentration of water.
Chemical Potential
- Chemical potential refers to the chemical activity of any substance in terms of free energy per mole of the substance.
- Plant cells must be sufficiently hydrated for proper functioning and processes.
- The hydration capacity of a cell is determined by the chemical potential of water rather than the absolute amount of water present.
Water Movement Between Cells
- Water moves between cells through osmosis, a spontaneous process where movement from a region of high water potential to a region of low water potential releases free energy.
Osmotic Pressure
- Osmotic pressure is generated during osmosis as water freely enters a cell, diluting the cell contents. The cell membrane obstructs the passage of solute molecules, causing the pressure buildup.
Turgor Pressure
- Turgor pressure is the pressure exerted by the protoplast (the cell's living contents) against the cell wall.
- It arises because plant cells have a rigid cell wall that restricts the expansion of the protoplast as water enters via osmosis.
- The cell is at incipient plasmolysis when the cell volume is minimized and the protoplast just touches the cell wall.
- When plasmolyzed, the protoplast shrinks away from the cell wall.
- At maximum turgor, no more water can enter the cell. This state is achieved only when the cell is placed in pure water, where the water potential gradient is 0, with both the inside and outside of the cell having 0 water potential.
Hypertonic Solution
- A hypertonic solution has a higher solute concentration than the cell sap.
- It leads to plasmolysis (shrinking of the protoplast away from the cell wall).
Hypotonic Solution
- A hypotonic solution has a lower solute concentration than the cell sap.
- It leads to deplasmolysis (return of the protoplast to its original position after plasmolysis).
Isotonic Solution
- An isotonic solution has the same solute concentration as the cell sap.
Water Potential Gradient (ψ)
- It is the driving force for water movement.
- Water always moves down a water potential gradient, from a higher to a lower value.
Water Movement from Soil to Root Xylem
- Water is absorbed from the soil into the root system along the water potential gradient between the root hairs (rhizines) and the soil layer surrounding them.
- The water potential of the cell is always lower than the water potential of the soil.
- Water moves from the rhizine cells to neighboring cortical cells, crossing the root radially through the apoplast (cell walls and spaces) or the symplast (cytoplasm and plasmodesmata).
- At the endodermis (innermost layer of the cortex), the apoplastic pathway is blocked by the Casparian strip, forcing water to cross the plasma membrane of the endodermal cells before reaching the vascular tissues.
Factors Affecting Water Uptake by Roots
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Total absorptive surface: A larger surface area of root hairs increases water absorption.
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Rate of rhizine respiration: A higher respiration rate lowers solute concentration, negatively affecting water uptake.
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Osmotic potential of root hairs: It should always be lower than the water potential of the soil.
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Temperature: Water uptake slows down at temperatures below 10°C.
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Oxygen content: Lack of oxygen stops water absorption and plant growth, with an optimal oxygen content of 10-12%.
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Carbon dioxide content: Too low or too high carbon dioxide content (5-15% in soil) inhibits or stops water uptake.
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Rooting depth and profile: The depth and structure of the root system influence water absorption.
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The buildup of solutes in the xylem sap increases the xylem osmotic pressure (Ï€), leading to a decrease in xylem water potential.
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This lowering of water potential provides the driving force for water absorption, generating a positive hydrostatic pressure called root pressure in the xylem.
Guttation
- It is the exhalation of liquid from leaf margins.
- It is exhibited by plants that develop root pressure.
- It is induced by high relative humidity, especially at night or in well-hydrated plants.
Water Movement from Root Xylem to Leaf Xylem
- The transpiration stream is the upward movement of water through the xylem from roots to shoots through dead lignified vessels and tracheids during transpiration.
- Water movement is by mass flow or bulk flow, where the liquid and dissolved or suspended substances move together.
Transpiration-Cohesion-Adhesion Theory
- It explains the negative pressure (tension) gradients that arise in the xylem of transpiring plants and pull the water column up to the treetops.
- Evaporation: Water evaporates from cell walls into air spaces in the leaf and then from the leaf into the atmosphere (transpiration). This creates a water potential gradient between living cells and along cell walls.
- Cohesion and Adhesion: The high surface tension of water allows the cell walls to remain consistently wet. Cohesion (attraction between water molecules) and adhesion (attraction of water to solids, like cell walls) allow water to flow in bulk from the root xylem to the leaf veins. 3.Negative Pressure (Tension): Water removal from the leaf xylem creates negative pressure, a pulling force that drives the transpiration stream. This tension is relayed to the root cells, causing water uptake.
Cavitation
- It is the filling of the xylem conduits with water vapor.
- Under tension, the water column in the xylem can become unstable, and if the pressure drops too low (from sudden jarring, for example), water may vaporize locally.
- This forms an air bubble or embolism, blocking water movement.
Causes of Cavitation and Embolism
- Water stress: High transpiration rates and low xylem pressure, especially in leaves and twigs, can induce cavitation.
- Freezing: Freezing of xylem can lead to extensive air bubble formation.
- Pathogens: Pathogens can release compounds like oxalic acid that lower surface tension, facilitating air seeding into pit membranes.
Transpiration
- Transpiration is the loss of water from a plant as water vapor.
- Over 90% of water vapor loss occurs from the leaves.
Factors Affecting Transpiration
1. Relative Humidity
- Relative humidity is the percentage of water vapor present in the air at a given temperature, compared to the amount needed to make the air saturated at that temperature.
- Transpiration rate is inversely proportional to relative humidity: higher humidity means slower transpiration and vice versa.
2. Atmospheric Temperature
- Higher atmospheric temperature opens stomata, even in darkness.
- Increased temperature not only heats the plant, but also lowers relative humidity and raises the vapor pressure inside the transpiring organ.
- For every 10°C rise in temperature, vapor pressure inside leaves doubles, while relative humidity decreases by 50%, significantly increasing transpiration.
3. Light
- Most plants open stomata in light and close them in darkness.
- Since most transpiration occurs through stomata, transpiration is high in light and falls in darkness.
4. Air Movement (Wind)
- Still air slows transpiration because water vapor accumulates around transpiring organs, reducing the vapor pressure difference.
- Air movement increases transpiration by removing saturated air surrounding the leaves.
- Transpiration increases with wind velocity up to 20-30 km/hr. Wind speeds of 40-50 km/hr decrease transpiration by closing stomata due to mechanical effects and drying/cooling of the transpiring organs.
5. Atmospheric Pressure
- Low atmospheric pressure enhances evaporation, creates air currents, and increases transpiration.
6. Water Availability
- Transpiration rate depends on water absorption by roots, which is influenced by soil factors like water content, particle size, temperature, and air content.
- Decreased water uptake leads to partial dehydration of leaf cells, closing stomata and causing wilting.
Wilting
- Wilting is the loss of turgidity in leaves and soft aerial parts of a plant, causing drooping, folding, and rolling.
- Thick-walled tissues do not show wilting symptoms.
Types of Wilting
- Incipient Wilting: No visible symptoms, but mesophyll cells lose water due to transpiration exceeding water availability. Occurs briefly during midday in most plants, even with sufficient soil water.
- Temporary Wilting (Transient): Temporary drooping of leaves and shoots due to loss of turgidity during the midday, when transpiration is highest. Occurs due to reduced water absorption caused by root shrinkage and depleted water around root hairs.
- Permanent Wilting: Irreversible drooping where leaves do not regain turgidity even in saturated atmosphere. Occurs when soil cannot meet the plant's water needs. Soil contains mostly unavailable water. At permanent wilting percentage (PWP) or coefficient (PWC) the soil retains 10-15% water (around 10% in loam soil). Plant death follows permanent wilting.
7. Leaf Area (Transpiring Area)
- Larger leaf area increases transpiration.
- However, transpiration per unit leaf area decreases in a dense canopy due to shading and reduced air movement.
8. Leaf Structure
- Cuticle Thickness: Cutinization and thicker cuticle reduce cuticular transpiration (water loss directly through the cuticle).
- Stomata Number and Position: Stomata influence transpiration, with most dicots having stomata on the underside of leaves and isobilateral monocots having equal numbers on both surfaces.
- Sunken Stomata: Sunken stomata reduce transpiration by creating a zone of low air movement.
- Stationary Layer and Hair: Leaf hair insulates the leaf surface, holding a stationary air layer that slows transpiration.
- Mesophyll: Compact mesophyll (more palisade tissue, fewer spaces) reduces transpiration, while loose mesophyll (more spongy tissue, larger spaces) increases it.
- Leaf Modifications: Adaptations like prickles, spines, scaly leaves, phyllodes, and phylloclades (modified stems) are found in xerophytes to decrease transpiration. Smaller, leathery leaves help reduce heating and prevent wilting in dry environments.
9. Root/Shoot Ratio
- A low root/shoot ratio reduces transpiration, while a high ratio increases it. This is because a large root system is more efficient in water uptake.
10. Mucilage and Solutes
- These substances decrease transpiration by holding water tightly.
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