1. Water Transport in Vascular Plants

2. Key Concepts
Concept.1: Physical forces drive the transport of materials in plants over a range of distances
Concept.2: Roots absorb water and minerals from the soil
Concept.3: Water and minerals ascend from roots to shoots through xylem (木質部)
Concept.4: Stomata help regulate the rate of transpiration (蒸散作用)

3. Non-vascular plants Vs Vascular plants
The evolutionary journey onto land involved the differentiation of the plant body into roots and shoots
Moss are non-vascular plants

4. Vascular tissue
Transports nutrients throughout a plant; such transport may occur over long distances
Figure 36.1

5. Anatomy of a woody stem

6. Water-conducting cells of xylem

7. Transport in vascular plants occurs on two scales
Concept 1: Physical forces drive the transport of materials in plants over a range of distances
Transport in vascular plants occurs on two scales
Short-distance transport of substances from cell to cell at the levels of tissues and organs
Long-distance transport within xylem and phloem at the level of the whole plant

8. A variety of physical processes
are involved in the different types of transport . 5 4
CO2 O2
Light
H2O
Sugar 3 6 . 2 7 1 O2
H2O
CO2
Minerals
Figure 36.2

9. A variety of physical processes
Are involved in the different types of transport
Sugars are produced by
photosynthesis in the leaves. 5
Through stomata, leaves take in CO2 and expel O2. The CO2 provides carbon for
photosynthesis. Some O2 produced by photosynthesis is used in cellular respiration. 4
CO2 O2
Light
H2O
Sugar
Transpiration, the loss of water
from leaves (mostly through
stomata), creates a force within
leaves that pulls xylem sap upward. 3
Sugars are transported as
phloem sap to roots and other
parts of the plant. 6
Water and minerals are
transported upward from
roots to shoots as xylem sap. 2
Roots exchange gases
with the air spaces of soil,
taking in O2 and discharging
CO2. In cellular respiration,
O2 supports the breakdown
of sugars. 7
Roots absorb water
and dissolved minerals
from the soil. 1 O2
H2O
CO2
Minerals
Figure 36.2

10. Selective Permeability of Membranes: A Review
The selective permeability of a plant cell’s plasma membrane
Controls the movement of solutes into and out of the cell
Specific transport proteins
Enable plant cells to maintain an internal environment different from their surroundings

11. Bulk Flow in Long-Distance Transport
In bulk / mass flow (巨流)
Movement of fluid (sap) in the xylem and phloem is driven by pressure differences at opposite ends of the xylem vessels and phloem sieve tubes

12. Absorption of water and minerals from the soil
Concept 2: Roots absorb water and minerals from the soil
Much of the absorption of water and minerals occurs near root tips, where the epidermis is permeable (without cuticle) to water and where root hairs are located
Root hairs account for much of the surface area of roots

13. Effects of Differences in Water Potential
To survive
Plants must balance water uptake and loss
Osmosis
Determines the net uptake or water loss by a cell

14. Water potential
Is a measurement that combines the effects of solute concentration and pressure
Determines the direction of movement of water
Water
Flows from regions of high water potential to regions of low water potential

15. How Solutes and Pressure Affect Water Potential
Both pressure and solute concentration
Affect water potential
The solute potential of a solution
Is proportional to the number of dissolved molecules
Pressure potential
Is the physical pressure on a solution

16. Quantitative Analysis of Water Potential
The addition of solutes
Reduces water potential
0.1 M
solution
H2O
Pure
water
P = 0
S = 0.23
 = 0.23 MPa
 = 0 MPa
(a)
Figure 36.5a

17. Application of Positive physical pressure
Increases water potential
H2O
P =
S = 0.23
 = 0 MPa
 = 0 MPa
(b)
H2O
P =
S = 0.23
 = MPa
 = 0 MPa
(c)
Figure 36.5b, c

18. Negative pressure Decreases water potential Figure 36.5d (d) H2O
 = 0.23 MPa
(d)
P = 0.30
S = 0
 = 0.30 MPa
Figure 36.5d

19. in the form of water vapour
A plant loses water
in the form of water vapour
from the surface of the plant into the atmosphere
This process is called transpiration

20. Ascend from roots to shoots through the xylem
Concept 3: Water and minerals ascend from roots to shoots through the xylem
Plants lose an enormous amount of water through transpiration, the loss of water vapor from leaves and other aerial parts of the plant
The transpired water must be replaced by water transported up from the roots

21. The Ascent of Xylem Sap Xylem sap
Rises to heights of more than 100 m in the tallest plants
Water-conducting cells

22. Pushing Xylem Sap: Root Pressure
At night, when transpiration is very low
Root cells continue pumping mineral ions into the xylem of the vascular cylinder, lowering the water potential
Water flows in to the xylem from the root cortex
Generating root pressure

23. Guttation- a demonstration of root pressure
Root pressure sometimes results in guttation, the exudation of water droplets on tips of grass blades or the leaf margins of some small, herbaceous plants
Figure 36.11

24. Root pressure
In most plants, root pressure is not the major mechanism driving the ascent of xylem sap.
At most, root pressure can force water upward only a few meters, and many plants generate no root pressure at all.
For the most part, xylem sap is not pushed from below by root pressure but pulled upward by the leaves themselves.

25. Pulling Xylem Sap: The Transpiration-Cohesion-Tension Mechanism
Water is pulled upward by negative pressure in the xylem
Transpirational Pull
starts with water vapor in the airspaces of a leaf diffuses down its water potential gradient and exits the leaf via stomata

26. Water Movement from the Leaf to the Atmosphere
Transpiration = the evaporation of water from leaf surfaces

27. Transpiration produces negative pressure (tension) in the leaf
Which exerts a pulling force on water in the xylem, pulling water into the leaf

28. Hydrogen Bond between water molecules
Water is a polar molecule, In water, the negative regions on one molecule are attracted to the positive regions on another, and the molecules form hydrogen bonds.                    

29. The Process of Transpiration
                                                                                                

30. Movement of Water Up Xylem Vessels
        
When water enters the roots, hydrogen bonds link each water molecule to the next so the molecules of water are pulled up the thin xylem vessels like beads on a string. The water moves up the plant, enters the leaves, moves into air spaces in the leaf, and then evaporates (transpires) through the stomata (singular, stoma).

31. Xylem Sap Ascent by Bulk Flow: A Review
The mechanism of transpiration depends on the generation of negative pressure (tension) in the leaf due to unique physical properties of water.
As water transpires from the leaf, water coating the mesophyll cells replaces water lost from the air spaces..
Adhesion to the wall and surface tension causes the surface of the water film to form a meniscus, “pulling on” the water by adhesive and cohesive forces. 31

32. adhesion and surface tension lowers the water potential
The tension generated by adhesion and surface tension lowers the water potential, drawing water from where its potential is higher to where it is lower.
Mesophyll cells will lose water to the surface film lining the air spaces, which in turn loses water by transpiration.
The water lost via the stomata is replaced by water pulled out of the leaf xylem.

33. Transpiration pull on xylem sap is transmitted all the way….
The transpirational pull on xylem sap is transmitted all the way from the leaves to the root tips and even into the soil solution.
Cohesion of water due to hydrogen bonding makes it possible to pull a column of sap from above without the water separating.
Helping to fight gravity is the strong adhesion of water molecules to the hydrophilic walls of the xylem cells.
The very small diameter of the tracheids and vessel elements exposes a large proportion of the water to the hydrophilic walls.

34. tension within the xylem….
The upward pull on the cohesive sap creates tension within the xylem
This tension can actually cause a decrease in the diameter of a tree on a warm day.
Transpiration puts the xylem under tension all the way down to the root tips, lowering the water potential in the root xylem and pulling water from the soil.

35. Cohesion and Adhesion in the Ascent of Xylem Sap
Outside air Y
= –100.0 MPa
Leaf Y (air spaces)
= –7.0 MPa
Leaf Y (cell walls)
= –1.0 MPa
Trunk xylem Y
= – 0.8 MPa
Water potential gradient
Root xylem Y
= – 0.6 MPa
Soil Y
= – 0.3 MPa
Mesophyll
cells
Stoma
Water
molecule
Atmosphere
Transpiration
Adhesion
Cell
wall
Cohesion, by
hydrogen
bonding
Root
hair
Soil
particle
Cohesion
and adhesion
in the xylem
Water uptake
from soil
Figure 36.13
The transpiration pull on xylem sap
Is transmitted all the way from the leaves to the root tips and even into the soil solution
Is facilitated by cohesion and adhesion

36. The Plant – Soil – Atmosphere Continuum
Movement of water from soil through plant to atmosphere involves different mechanisms of transport:
In the vapor phase, water moves by diffusion until it reaches outside air (and convection, a form of bulk flow, becomes dominant)
In xylem, water moves by bulk flow in response to a pressure gradient (ΔΨp)
For water transport across membranes, water potential difference across membrane is driving force (osmosis, e.g. when cells absorb water and roots transport water from soil to xylem)
In all cases: water moves toward regions of low water potential (or free energy)

37. Maple tree Transpiration: 200 liters/day
75 cm/min

38. Stomata: Major Pathways for Water Loss
About 90% of the water a plant loses
Escapes through stomata

39. Stomata Stomata from sedge (Carex) Cytosol and vacuole Pore
Heavily thickened
guard cell wall
Guard cells
Subsidiary cells
Stoma from a grass
Epidermal cell

40. Stomata Stomata from onion epidermis
Outside surface of the leaf with stomatal pore inserted into the cuticle
Guard cells facing the stomatal cavity, toward the inside of the leaf
Stomatal pore
Guard cell

41. The cell walls of guard cells have specialized features
Two main types of guard cells
- kidney-shaped stomata: for dicots, monocots, mosses,
ferns, gymnosperms
- grass-like stomata: grasses and a few other monocots
(e.g. palms)
Kidney-shaped stomata
Grass-like stomata

42. An increase in guard cell turgor pressure opens the stomata
- guard cells function as multisensory hydraulic valves
- guard cells sense changes in the environment: light intensity, light quality,
temperature, leaf water status, intracellular CO2
- this process requires ion uptake and other metabolic changes
in the guard cells (discussed later)
- due to this ion uptake → Ψs decreases → Ψw decreases → water moves
into guard cells → Ψp (turgor pressure) increases → cell volume increases
→ opening of stomata (due to differential thickening of guard cell walls)

43. The cell walls of guard cells have specialized features
Atmosphere
Portions of the guard cell wall are substantially thickened (up to 5 um across)
Pore
Nucleus
Plastid
Vacuole
Substomatal cavity
Inner cell wall

44. Changes in turgor pressure that open and close stomata
Result primarily from the reversible uptake and loss of potassium ions by the guard cells
Role of potassium in stomatal opening and closing.
The transport of K+ (potassium ions, symbolized
here as red dots) across the plasma membrane and
vacuolar membrane causes the turgor changes of
guard cells.
H2O
H2O
H2O
H2O
H2O K+
H2O
H2O
H2O
H2O
H2O

45. 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 absorption through the roots
The plant will lose water and wilt

46. Turgor loss in plants causes wilting
Turgor and wilting
Turgor loss in plants causes wilting
Which can be reversed when the plant is watered
Figure 36.7

47. transport of water and minerals
Importance of transpiration
absorption of water
transport of water and minerals

48. Evaporative cooling?
Transpiration also results in evaporative cooling
Which can lower the temperature of a leaf and prevent the denaturation of various enzymes involved in photosynthesis and other metabolic processes
But recent findings cast doubt on the actual significance of the cooling effects

49. 1. Relative humidity Humidity
Environmental factors affecting the rate of transpiration
1. Relative humidity
relative humidity (%)
rate of transpiration
Humidity
concentration gradient of water vapour between the inside of a leaf and the atmosphere
 more water vapour diffuses out

50.  rate of evaporation of water
Environmental factors affecting the rate of transpiration
rate of transpiration
temperature (oC)
2. Temperature
Temperature
 rate of evaporation of water

51. Wind moves the water vapour away from leaf.
Environmental factors affecting the rate of transpiration
rate of transpiration
wind velocity (km/h)
3. Air movement
Wind moves the water vapour away from leaf.

52. Environmental factors affecting the rate of transpiration
4. Light intensity
rate of transpiration
light intensity
Stomata open wider as light intensity increases. Wider stomata allow more water vapour to diffuse out.

53. Environmental Factors affecting transpiration
Light: stimulates the stomata to open allowing gas exchange for photosynthesis → transpiration increases (plants may lose water during the day and wilt)
Temperature: High temperature increases the rate of evaporation of water from the spongy cells, and reduces air humidity, so transpiration increases.
Humidity: High humidity means a higher water potential in the air, so a lower water potential gradient between the leaf and the air, so less evaporation.
Wind: Blows away saturated air from around stomata, replacing it with drier air, so increasing the water potential gradient and increasing transpiration. 53

54. Water and Plants – Summary
water is the essential medium of life
plants gain energy from sunlight and need to have open pathway for CO2
- plants have large surface area that is not differentially permeable to CO2 vs. water vapor → conflict: need for water conservation and need for CO2 assimilation
- the need to resolve this conflict determines structure of plants: ?

55. Factors affecting transpiration
Plant factors
Leaf structure
Leaf area
Shoot-Root ratio
Adaptations to dry habitats
Plants in different habitats are adapted to cope with different problems of water availability.
Mesophytes plants adapted to a habitat with adequate water
Xerophytes plants adapted to a dry habitat
Halophytes plants adapted to a salty habitat
Hydrophytes plants adapted to a freshwater habitat 55

56. The stomata of xerophytes
Are concentrated on the lower leaf surface
Are often located in depressions (sunken) that shelter the pores from the dry wind
Upper
epidermal
tissue
Cuticle
Trichomes
(“hairs”)
Lower
epidermal
tissue
100 m
Stomata
Figure 36.16

57. Marram grass

58. Marram grass –a xerophyte

59. Rolled leaves of marram grass

60. Internal Factors affecting transpiration
Some adaptations of xerophytes are:
Adaptation
How it works
Example
stops uncontrolled evaporation through leaf cells
most dicots
less area for evaporation
conifer needles, cactus spines
more humid air on lower surface, so less evaporation
reduce water loss at certain times of year
deciduous plants
maintains humid air around stomata
marram grass, pine
marram grass, couch grass
marram grass,
stores water
cacti
maximize water uptake 60 60

61. Internal Factors affecting transpiration
Some adaptations of xerophytes are:
Adaptation
How it works
Example
thick cuticle
stops uncontrolled evaporation through leaf cells
most dicots
small leaf surface area
less area for evaporation
conifer needles, cactus spines
stomata on lower surface of leaf only
more humid air on lower surface, so less evaporation
shedding leaves in dry/cold season
reduce water loss at certain times of year
deciduous plants
sunken stomata
maintains humid air around stomata
marram grass, pine
stomatal hairs
marram grass, couch grass
folded leaves
marram grass,
succulent leaves and stem
stores water
cacti
extensive roots
maximize water uptake 61 61

62. Stomatal Control -- Water conservation vs Leaf Photosynthesis
Problem:
Plants have to take up CO2 from atmosphere, but simultaneously need to
limit water loss – transpiration is a necessary evil
Cuticle protects from desiccation
However, plants cannot prevent outward diffusion of water without
simultaneously excluding CO2 from leaf and concentration gradient for
CO2 uptake is much smaller than concentration gradient that drives water
loss
Solution: 62

63. Stomatal Control -- Water conservation vs Leaf Photosynthesis
Problem:
Plants have to take up CO2 from atmosphere, but simultaneously need to
limit water loss – transpiration is a necessary evil
Cuticle protects from desiccation
However, plants cannot prevent outward diffusion of water without
simultaneously excluding CO2 from leaf and concentration gradient for
CO2 uptake is much smaller than concentration gradient that drives water
loss
Solution: Temporal regulation of stomatal apertures
Closed at night: no photosynthesis → no demand for CO2 → stomatal
aperture kept small → preventing unnecessary loss of water
Open during day: sunny morning; water abundant, light favors
photosynthesis → large demand for CO2 → stomata wide open →
decreased stomatal resistance to CO2 diffusion → water loss by
transpiration also substantial, but water supply is plentiful, i.e. plant trades
water for the product of photosynthesis needed for growth 63