1. Resource Acquisition and Transport in Vascular Plants
Chapter 36
Resource Acquisition and Transport in Vascular Plants

2. Concept 36.1: Adaptations for acquiring resources were key steps in the evolution of vascular plants
The algal ancestors of land plants absorbed water, minerals, and CO2 directly from the surrounding water
Early nonvascular land plants lived in shallow water and had aerial shoots
Natural selection favored taller plants with flat appendages, multicellular branching roots, and efficient transport
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3. Xylem transports water and minerals from roots to shoots
The evolution of xylem and phloem in land plants made possible the long-distance transport of water, minerals, and products of photosynthesis
Xylem transports water and minerals from roots to shoots
Phloem transports photosynthetic products from sources to sinks
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4. H2O H2O and minerals Figure 36.2-1
Figure 36.2 An overview of resource acquisition and transport in a vascular plant.
H2O and minerals

5. CO2 O2 H2O O2 H2O and minerals CO2 Figure 36.2-2
Figure 36.2 An overview of resource acquisition and transport in a vascular plant. O2
H2O and minerals
CO2

6. CO2 O2 Light Sugar H2O O2 H2O and minerals CO2 Figure 36.2-3
Figure 36.2 An overview of resource acquisition and transport in a vascular plant. O2
H2O and minerals
CO2

7. Adaptations in each species represent compromises between enhancing photosynthesis and minimizing water loss
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8. Shoot Architecture and Light Capture
Stems serve as conduits for water and nutrients and as supporting structures for leaves
There is generally a positive correlation between water availability and leaf size
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9. Root Architecture and Acquisition of Water and Minerals
Soil is a resource mined by the root system
Taproot systems anchor plants and are characteristic of gymnosperms and eudicots
Root growth can adjust to local conditions
For example, roots branch more in a pocket of high nitrate than low nitrate
Roots are less competitive with other roots from the same plant than with roots from different plants
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10. Mutualisms with fungi helped plants colonize land
Roots and the hyphae of soil fungi form mutualistic associations called mycorrhizae
Mutualisms with fungi helped plants colonize land
Mycorrhizal fungi increase the surface area for absorbing water and minerals, especially phosphate
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11. Figure 36.5
Roots
Figure 36.5 A mycorrhiza, a mutualistic association of fungus and roots.
Fungus

12. Water potential determines the direction of movement of water
Water potential is a measurement that combines the effects of solute concentration and pressure
Water potential determines the direction of movement of water
Water flows from regions of higher water potential to regions of lower water potential
Potential refers to water’s capacity to perform work
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13. Ψ = 0 MPa for pure water at sea level and at room temperature
Water potential is abbreviated as Ψ and measured in a unit of pressure called the megapascal (MPa)
Ψ = 0 MPa for pure water at sea level and at room temperature
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14. How Solutes and Pressure Affect Water Potential
Both pressure and solute concentration affect water potential
This is expressed by the water potential equation: Ψ  ΨS  ΨP
The solute potential (ΨS) of a solution is directly proportional to its molarity
Solute potential is also called osmotic potential
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15. Pressure potential (ΨP) is the physical pressure on a solution
Turgor pressure is the pressure exerted by the plasma membrane against the cell wall, and the cell wall against the protoplast
The protoplast is the living part of the cell, which also includes the plasma membrane
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16. Consider a U-shaped tube where the two arms are separated by a membrane permeable only to water
Water moves in the direction from higher water potential to lower water potential
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17. Figure 36.8
Solutes have a negative effect on  by binding water molecules.
Positive pressure has a positive effect on  by pushing water.
Solutes and positive pressure have opposing effects on water movement.
Negative pressure (tension) has a negative effect on  by pulling water.
Pure water at equilibrium
Pure water at equilibrium
Pure water at equilibrium
Pure water at equilibrium
H2O
H2O
H2O
H2O
Adding solutes to the right arm makes  lower there, resulting in net movement of water to the right arm:
Applying positive pressure to the right arm makes  higher there, resulting in net movement of water to the left arm:
In this example, the effect of adding solutes is offset by positive pressure, resulting in no net movement of water:
Applying negative pressure to the right arm makes  lower there, resulting in net movement of water to the right arm:
Figure 36.8 Effects of solutes and pressure on water potential () and water movement.
Positive pressure
Positive pressure
Negative pressure
Pure water
Solutes
Solutes
Membrane
H2O
H2O
H2O
H2O

18. Water Movement Across Plant Cell Membranes
Water potential affects uptake and loss of water by plant cells
If a flaccid cell is placed in an environment with a higher solute concentration, the cell will lose water and undergo plasmolysis
Plasmolysis occurs when the protoplast shrinks and pulls away from the cell wall
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19. Figure 36.9 Water relations in plant cells.
Initial flaccid cell: P  S
0.7 
0.4 M sucrose solution:  
0.7 MPa
Pure water: P  P  S 
0.9 S 
Plasmolyzed cell at osmotic equilibrium with its surroundings 
0.9 MPa   
0 MPa
Turgid cell at osmotic equilibrium with its surroundings P  P 
0.7 S
0.9  S 
0.7 
0.9 MPa   
0 MPa
Figure 36.9 Water relations in plant cells.
(a) Initial conditions: cellular   environmental 
(b) Initial conditions: cellular   environmental 

20. If a flaccid cell is placed in a solution with a lower solute concentration, the cell will gain water and become turgid
Turgor loss in plants causes wilting, which can be reversed when the plant is watered
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21. Aquaporins: Facilitating Diffusion of Water
Aquaporins are transport proteins in the cell membrane that allow the passage of water
These affect the rate of water movement across the membrane
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22. Long-Distance Transport: The Role of Bulk Flow
Efficient long distance transport of fluid requires bulk flow, the movement of a fluid driven by pressure
Water and solutes move together through tracheids and vessel elements of xylem, and sieve-tube elements of phloem
Efficient movement is possible because mature tracheids and vessel elements have no cytoplasm, and sieve-tube elements have few organelles in their cytoplasm
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23. Concept 36.3: Transpiration drives the transport of water and minerals from roots to shoots via the xylem
Plants can move a large volume of water from their roots to shoots
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24. Absorption of Water and Minerals by Root Cells
Most water and mineral absorption occurs near root tips, where root hairs are located and the epidermis is permeable to water
Root hairs account for much of the surface area of roots
After soil solution enters the roots, the extensive surface area of cortical cell membranes enhances uptake of water and selected minerals
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25. Pushing Xylem Sap: Root Pressure
At night root cells continue pumping mineral ions into the xylem of the vascular cylinder, lowering the water potential
Water flows in from the root cortex, generating root pressure
Root pressure sometimes results in guttation, the exudation of water droplets on tips or edges of leaves
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26. Figure 36.11
Figure Guttation.

27. Concept 36.4: The rate of transpiration is regulated by stomata
Leaves generally have broad surface areas and high surface-to-volume ratios
These characteristics increase photosynthesis and increase water loss through stomata
Guard cells help balance water conservation with gas exchange for photosynthesis
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28. Figure 36.14
Figure An open stoma (left) and closed stoma (LMs).

29. Stomata: Major Pathways for Water Loss
About 95% of the water a plant loses escapes through stomata
Each stoma is flanked by a pair of guard cells, which control the diameter of the stoma by changing shape
Stomatal density is under genetic and environmental control
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30. Mechanisms of Stomatal Opening and Closing
Changes in turgor pressure open and close stomata
When turgid, guard cells bow outward and the pore between them opens
When flaccid, guard cells become less bowed and the pore closes
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31. Guard cells turgid/ Stoma open Guard cells flaccid/ Stoma closed
Figure 36.15
Guard cells turgid/ Stoma open
Guard cells flaccid/ Stoma closed
Radially oriented cellulose microfibrils
Cell wall
Vacuole
Guard cell
(a)
Changes in guard cell shape and stomatal opening and closing (surface view)
H2O
H2O
H2O
H2O
Figure Mechanisms of stomatal opening and closing.
H2O K
H2O
H2O
H2O
H2O
H2O
(b) Role of potassium in stomatal opening and closing

32. Stimuli for Stomatal Opening and Closing
Generally, stomata open during the day and close at night to minimize water loss
Stomatal opening at dawn is triggered by
Light
CO2 depletion
An internal “clock” in guard cells
All eukaryotic organisms have internal clocks; circadian rhythms are 24-hour cycles
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33. Drought, high temperature, and wind can cause stomata to close during the daytime
The hormone abscisic acid is produced in response to water deficiency and causes the closure of stomata
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34. Effects of Transpiration on Wilting and Leaf Temperature
Plants lose a large amount of water by transpiration
If the lost water is not replaced by sufficient transport of water, the plant will lose water and wilt
Transpiration also results in evaporative cooling, which can lower the temperature of a leaf and prevent denaturation of various enzymes involved in photosynthesis and other metabolic processes
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35. Some desert plants complete their life cycle during the rainy season
Others have leaf modifications that reduce the rate of transpiration
Some plants use a specialized form of photosynthesis called crassulacean acid metabolism (CAM) where stomatal gas exchange occurs at night
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