Lecture 9- Xylem Flashcards
What cells do the xylem contain?
Tracheary elements- these die (apoptosis) before assuming their function
What are the two types of tracheary elements?
Tracheids (gymnosperms, other vascular plants)
Vessels made of vessel elements (flowering plants)
What is the function of tracheids?
These contain pits which allows water and minerals to move freely through secondary walls
What are the features of vessels?
- Larger diameter pits then tracheids
- Secrete lignin into their secondary walls, then partially break down their end walls and finally die and disintegrate to make a hollow tube- open pipe line for water conduction
What is the function of tracheary elements?
Transport of water and dissolved minerals
During the summer, how many liters of water does a 15m maple tree loose per hour?
220 liters
What are the tallest trees?
Tallest gymnosperms- coast redwoods, Sequoia sempervirens
Tallest angiosperms- Australian Eucalpytus regnans
What early theory was described to explain the rise of sap in the xylem?
A pumping action by living cells in the stem push the sap upwards
Why was pumping action of the xylem ruled out?
- Experiment published in 1983 by Eduard Strasburger
- 20 meter trees
- Sawed through the base of the tree and plunged them into poison (copper sulfate)
- Progressive death of bark and eventually leaves
What three points were established by Eduard Strasburger’s experiments on trees?
- Pumping cells were not responsible for upwards movement because the solution killed all living cell it came into contact with
- Leaves play a crucial role in transport- solution moved only when they were alive
- Movement is not caused by the roots
Root pressure does not account for xylem transport. However, what evidence proves that root pressure does exist?
Guttation
Liquid water is forced out through openings at the margins of leaves
Root pressure is also the source of sap that oozes from stumps of some plants when cut
How was it believed that root pressure led to xylem transport?
Pressure exerted by root tissues would force liquid up the xylem because of the high solute concentration (and a more negative water potential) in the xylem sap then in the soil- water potential draws water into the stele, rising in the vessels and tracheids
Under what conditions does guttation occur?
High atmospheric humidity and plentiful water in the soil- most commonly at night
Why can root pressure not account for xylem transport?
- Only 1-2 atm
- If root pressure drives xylem transport, positive pressure potential would be observed in the xylem all the time
- The xylem sap in most tress is actually under tension (negative pressure potential)
- Strasburger’s experiment- transport occurs even without roots
What is the name of the mechanism that accounts for xylem transport?
Transpiration-cohesion-tension mechanism
What generates tension in the xylem?
Evaporative loss of water from the leaves generates a pulling force
What forces enable tension to be generated?
Hydrogen bonding between water molecules makes the sap cohesive
What is transpiration?
The diffusion of water from intercellular spaces of the leaf through openings called stomata due to the lower concentration of water vapor in the atmosphere
What happens inside a leaf when transpiration occurs?
Water evaporates from the walls of mesophyll cells
The film of water on the cell shrinks
The generates more surface tension (negative pressure potential)
This draws more water into the cell walls to replace what was lost
Define cohesion
The tendency of water molecules to stick to one another through hydrogen bonding
What does tension in mesophyll cells cause?
- Water from the nearest vein is drawn into the mesophyll cell
- Tension in the entire column of water in the xylem
- Water is drawn up
What tubes can withstand greater tension?
Narrower tubes
How else is the integrity of the column maintained?
Adhesion of water to the xylem walls
Define transpiration
The evaporation of water from the leaves
Is the transpiration-cohesion-tension mechanism passive or active?
It is passive- it requires no work/expenditure of energy by the plant
What else may rise in the xylem sap?
Mineral ions
Other then transport, what is the purpose of transpiration?
Temperature regulation
As water evaporates from mesophyll cells, heat is taken up and the leaf temperature drops
This is important for plants living in hot environments
What instrument can measure tension in the xylem?
What is the name of the biologist who demonstrated this?
- Per Scholander
- Measured tension in stems with a pressure chamber
Describe how a pressure chamber can measure the amount of tension in the sap.
- Stem is cut
- The stem is placed in a pressure chamber
- Pressure is raised- the xylem sap is pushed back to the cut surface in the sap sample
- When sap is visible, the pressure is recorded
- Pressure is equal and opposite to the tension
What else did Per Scholander notice during his sap pressure experiments?
Tension dissapeared in some plants at night
In developing vines, sap was under no pressure until leaves formed
Why is the rate at which xylem ascends different at different times?
Temperature, light intensity, wind velocity
Because these factors affect transpiration rate and hence rate of sap flow
Additionally, rate of flow increases with increasing concentration of K+ ions
Why do K+ ions increase rate of sap flow?
It affects the cell wall component in the membrane of pits between vessel elements, changing the size of the pit. This increases rate of water flow through a vessel when a neighboring vessel is blocked
Explain how the hypothesis that K+ ions affects the xylem flow rate can be tested experimentally.
Cut two xylem containing flaps along the stem of a tobacco plant Connect one (the control) to pure water Connect the other to solutions containing known concentrations of K+
What results were seen when xylems were treated with K+ ions?
The addition of K+ ions dramatically increased flow rate.
The rate returned to the control level when the K+ solution was replaced by pure water.
How is water loss controlled?
The waxy cuticle of the leaf and stem epidermis
Stomata open and close by guard cells
Why do stomata open?
To allow CO2 to enter by diffusion
What are guard cells?
A pair of specialised epidermal cells
When do plants open their stomata?
When light intensity is sufficient to maintain a moderate rate of photosynthesis
Why do stomata close at night?
CO2 is not needed and water is conserved
When do stomata close during the day?
When water is being lost too rapidly
What cues cause stomatal opening?
Light intensity, CO2 concentration in intercellular spaces, water
How does water stress control stomatal opening?
If a plant is under water stress (on sunny or windy days), water potential of mesophyll cells act as a cue. The mesophyll cells release a plant hormone called abscisic acid.
Abscisic acid acts on guard cells to close the stomata
By what mechanism do guard cells open and close the stomata?
K+ concentration in guard cells. Blue light, absorbed by pigments in the guard cells plasma membrane, activates a proton pump which actively transports H+ out of the guard cell, drives K+ in.
Increasing concentration of K+ makes water potential more negative
Water enters by osmosis
Cellulose microfibrils in their cell walls causes them to respond to this increase in water by changing shape so a gap appears between them.
How does a stoma close?
The reverse process of stoma opening in the absence of blue light or presence of abscisic acid- potassium diffuses out passively.
How was the amount of K+ ions that move in and out of guard cells measured?
Electron probe microanalyser
How can the hypothesis that guard cells of open stoma contain more K+ ions that do those of closed stoma be demonstrated experimentally?
- Peel strips of epidermis from leaves of broad beans in both the dark (closed stomata) and light (open stomata)
- Examine strips to locate stomata
- Scan across guard cells with electron probe microanaylser set to measure K+ concentrations
In experiments with electron probe microanalyzers, what was the K+ concentration across stomata?
K+ concentration within the guard cells surrounding an open stomata was much greater than that in guard cells surrounding a closed stomata
Why do farmers want to reduce transpiration in their crops?
Reduce the need for irrigation
What is an antitranspirant?
A compound to reduce transpiration without limiting CO2 uptake
Why is abscisic acid not used commerically?
It is too expensive for large scale trials
What plants are more sensitive to abscisic acid?
Transgenic plants with a mutant allele for the era gene are very sensitive- resistant to wilting and droughts.
This could lead to an agricultural application.
Why type of antitranspirant seals off leaves from the atmosphere?
Compounds that form a polymer film around leaves to form a barrier of evaporation by sealing the stomata.
These can have undesirable effects- only used for a short period of time such as transplant of nursery stock.
