Chapter 3: Plant Water Balance

Question 1

what does water content and the rate of water movement in soil depend on?

Answer 1

soil type and structure

Question 2

what lowers soil water potential?

Answer 2

a negative hydrostatic pressure

Question 3

what is the pressure potential in wet soils? in dry soils?

Answer 3

the pressure potential is close to 0 in wet soils and becomes more and more negative the dryer the soil; water is first removed from the largest spaces between soil particles and subsequently from successively smaller spaces between and within soil particles

Question 4

what is the role of gravitational potential in soils?

Answer 4

Gravitational potential in soil plays an important role in drainage
The downward movement of water is influenced by elevation, faster at higher elevations and vice versa

Question 5

how does water move through the soil?

Answer 5

bulk flow

Question 6

how is pressure formed in soils?

Answer 6

• Pressure in soil water is due to curved air-water interfaces
• Water flows from regions of higher soil water content (water filled spaces are larger and pressure is less negative) to lower soil water content (smaller water filled spaces and more negative)
• As plants remove water from the soil, the water moves into that lower soil water content, forming a pressure gradient

Question 7

what does the rate of water flow in soils depend on?

Answer 7

the size of the pressure gradient through the soil and hydraulic conductivity in soil

Question 8

soil hydraulic conductivity

Answer 8

• a measure of the ease with which water moves through the soil and it varies with the type of soil and its water continent
• Sandy soils have large spaces between particles and have a large hydraulic conductivity when saturated. Clay soils act oppositely
• When water content decreases, hydraulic conductivity decreases
• Due to replacement of water by air
• receeds into smaller and smaller menisci between soil particles
• more difficult to move water because the water potential is lower and hydraulic conductivity is lower

Question 9

root hairs

Answer 9

filamentous outgrowths of root epidermal cells that greatly increase the surface area of the roof, providing a great capacity for absorption of ions and water in the soul
Water enters most readily at the root top
- makes up more than 60% of root surcase area

Question 10

why are mature roots less permeable?

Answer 10

their epidermal layer has been modified to have hydrophobic materials
This is to increase bulk flow

Question 11

how does water move through the roots?

Answer 11

apoplast, symplast, and transmembrane pathways; moves from the epidermis to the endodermis of the roots through these pathways
- roots need direct contact with soil for water and nutrient uptake

Question 12

apoplast

Answer 12

the continuous system of cell walls, intercellular air spaces, and the lumens of nonliving cells (e.g., xylem, conduits and fibers). In this pathway, water moves through cell walls and extracellular spaces without crossing any membranes as it travels across the root cortex

Question 13

symplast

Answer 13

consists of the entire network of cell cytoplasm interconnected by plasmodesmata. In this pathway, water travels across the root cortex via the plasmodesmata.

Question 14

transmembrane pathway

Answer 14

the route by which water enters a cell on one side, exits the cell on the other side, enters the next in the series, and so on. In this pathway, water crosses the plasma membrane of each cell in its path twice (once on entering and once on exiting). Transport across the tonoplast may also be involved.

Question 15

what happens to the apoplast pathway once it reaches the endodermis?

Answer 15

the apoplast pathway is stopped by the casparian strip, and water must enter through the symplast or transmembrane pathways to go into the endodermis

Question 16

casparian strip

Answer 16

a band within the radial cell walls of the endodermis that is impregnated with suberin and/or lignin, two hydrophobic polymers. The Casparian strip forms in the non growing part of the root, several millimeters to several centimeters behind the root tip, at about the same time that the first xylem elements mature.

Question 17

when does water uptake decrease?

Answer 17

when roots are subjected to low temperature or anaerobic conditions, or treated with respiratory inhibitors

Question 18

root pressure

Answer 18

A positive hydrostatic pressure in the xylem of roots that typically occurs at night in the absence of transpiration.
- due to solute accumulation

Question 19

what happens when transpiration is low or absent?

Answer 19

positive hydrostatic pressure builds up in the xylem because roots continue to absorb ions from the soil and transport them into the xylem.
Decrease in xylem osmotic potential and water potential
Gives a driving force for water absorption that can build up positive hydrostatic pressure in the xylem

Question 20

guttation

Answer 20

liquid droplets on the edges of leaves, occurs when root pressure is frequent
Positive xylem pressures causes xylem sap to leave pores called hydathodes that are in the vein endings of the leaf margin
occurs when transpiration is suppressed and relative humidity is high

Question 21

what is the longest part of the water transport pathway?

Answer 21

xylem

Question 22

what are the two types of transport cells in the xylem?

Answer 22

tracheids and vessel elements

Question 23

how are tracheids and vessel elements formed?

Answer 23

Tracheids and vessel elements are formed by making a second cell wall and causing the cell to die by losing the cytoplasm and contents. The cells are then thick hollow tubes that water can easily travel up

Question 24

tracheids

Answer 24

Spindle-shaped, water conducting cells with tapered ends and pitted walls without perforations, found in the xylem of both angiosperms and gymnosperms.
Water flows through the pits in the walls
Pits are microscopic regions where the secondary wall is absent and only the primary wall is present
in both angiosperms and gymnosperms

Question 25

pit pairs

Answer 25

Two pits occurring opposite one another in the walls of adjacent tracheids or vessel elements. Pit pairs constitute a low-resistance path for water movement between the conducting cells of the xylem.

Question 26

pit membranes in angiosperms and gymnosperms and conifers

Answer 26

the water permeable layer between pit pairs, consisting of two primary walls and a middle lamella
In conifers, the pit membranes have a thickening torus surrounded by porous and relatively flexible margo
The torus is a valve that can prevent gas bubbles from spreading to other regions
Pit membranes are important in preventing the spread of gas bubbles - emboli

Question 27

vessel elements

Answer 27

Nonliving water-conducting cells with perforated end walls, found only in angiosperms and a small group of gymnosperms.
Shorter and wider than tracheids
Have perforated end walls that form a perforation plate at the end of each cell
Have pits on lateral walls

Question 28

how do vessel elements differ from tracheids?

Answer 28

Unlike tracheids, vessel elements can be stacked due to these perforation plates, forming one long conduit called a vessel
Vessels are multicellular conduits that vary in length both within and among species.

Question 29

how does water move through the xylem?

Answer 29

Pressure driven bulk flow of water is responsible for long distance transport of water in the xylem
Also accounts for water flow through the soil and cell wall of plants
Independent of solute concentration gradients

Question 30

what does the rate of bulk flow depend on?

Answer 30

The rate of bulk flow depends on the radius of the tube, the viscosity of the liquid, and the pressure gradient
If the radius is doubled, the volume flow rate increases by a factor of 16
Vessel elements of up to 500 micrometers

Question 31

where does the energy that powers water movement come from?

Answer 31

The energy that powers the movement of water through plants comes from the sun, which, by increasing the temperature of both the leaf and the surrounding air, drives the evaporation of water.

Question 32

why do xylem walls not collapse due to pressure?

Answer 32

Xylem walls do not collapse due to the extreme pressure due to the secondary wall and the lignification of them to prevent collapse

Question 33

when do gas bubbles occur?

Answer 33

When a gas bubble grows to a sufficient size that the inward force resulting from surface tension is less than the outward force due to the negative pressure in the liquid phase, the bubble expands.
A bubble can expand until it fills an entire conduit

Question 34

how does the root system try to prevent gas bubbles?

Answer 34

The endodermis acts as a filter, preventing gas bubbles from entering the xylem. Pit membranes also serve as filters as water flows from one xylem conduit to another
When pit membranes are exposed to air on one side (due to an injury such as leaf abscission or a neighboring conduit is filled with gas) an air bubble can occur

Question 35

air seeding

Answer 35

when air enters the xylem due to pressure difference across the pit membrane is sufficient to allow air to penetrate or dislodge the conifer pit membrane

Question 36

cavitation and embolisms

Answer 36

bubble expansion
Breaks the continuity of the water column and prevent the transport of water under tension
Xylem hydraulic conductivity decreases with increasing tension until flow ceases entirely
Embolism: the gas filled void

Question 37

what do vulnerability curves measure?

Answer 37

Vulnerability curves provide a way to quantify a particular species’ vulnerability to cavitation and its impact
Hydraulic conductivity against the imposed level of xylem tension

Question 38

how can embolisms be eliminated/alleviated?

Answer 38

Water conduits are interconnected, one gas bubble could fill an entire network, but typical don’t
This is because the gas cannot fill pass through the small pores of the pit membranes
So water can bypass the bubble by traveling to other conduits
Gas bubbles can be eliminated by increasing root pressure of the xylem
New xylem is grown every year, so if a bubble cannot be resolved then it can be replaced

Question 39

how does water leave the leaf?

Answer 39

Water is pulled from the xylem into the cell walls of the mesophyll where it can be evaporated into the air spaces of the leaf
The water vapor leaves through stomatal pores
Transpiration is controlled by the concentration gradient of water vapor

Question 40

how much water is lost from the cuticle? where else is water lost?

Answer 40

5%
Almost all water lost is lost through the diffusion of water vapor through tiny stomatal pores
In herbaceous plants, stomata are on the upper and lower surfaces on the leaf and more abundant on the lower
In trees, the stomata are only on the lower surface of the leaf

Question 41

what does leaf hydraulic resistance depend on?

Answer 41

Leaf hydraulic resistance thus reflects the number, distribution, and size of xylem conduits, as well as the hydraulic properties of leaf mesophyll cells.
Closely spaced veins in leaves have a lower hydraulic resistance and higher rates of photosynthesis
Hydraulic resistance also depends on growth conditions
leaves of plants growing in shaded conditions exhibit greater resistance to water flow than do leaves of plants grown in higher light.
Leaf hydraulic resistance also typically increases with leaf age

Question 42

what does transpiration in a leaf depend on?

Answer 42

Difference in water vapor concentration between the leaf air spaces and the external bulk air and the diffusional resistance of this pathway ; the driving force for water loss from the leaf is the absolute concentration difference and this difference is markedly influenced by leaf temperature

Question 43

stomatal resistance

Answer 43

The resistance associated with diffusion through the stomatal pore
When open, stomatal pores provide a low resistance pathway for diffusional movement of gasses across the epidermis and cuticle
Stomatal resistance is important for regulation of water loss and controlling uptake of carbon dioxide

Question 44

boundary layer resistance

Answer 44

The resistance due to the layer of unstirred air next to the leaf surface through which water vapor must diffuse to reach the turbulent air of the atmosphere

Question 45

how is the boundary layer determined?

Answer 45

Boundary layer is determined by wind speed and leaf size
When surrounding air is still - the boundary layer may be so thick that it is the primary deterrent to water vapor loss from the leaf
Still air - no effect on transpiration rate in regards to stomatal apertures
Closing completely will stop transpiration
When wind velocity is high, the moving air reduces the thickness of the boundary layer of the leaf surface and reduces the resistance of it
Stomatal resistance controls water loss under these conditions

Question 46

what can impact the boundary later?

Answer 46

microscopic hairs on leaves, size of the leaf, and direction of the wind

Question 47

how are stomata controlled?

Answer 47

The control of the stomatal aperture is governed by guard cells, which are specialized epidermal cells that surround the stomatal pore
The stomatal opening is dependent on the special features of the guard cell as well as on leaf water status and light conditions

Question 48

where are guard cells found?

Answer 48

all vascular and some nonvascular

Question 49

what are the two main types of guard cells?

Answer 49

one is typical of grasses while the other is more prominent in flowering plants, mosses, ferns, and gymnosperms

Question 50

describe guard cells in grasses

Answer 50

Dumbbell shape with bulbous ends - located between the handles of the dumbbell
Next to the guard cells are subsidiary cells that help the guard cell control the stomatal pore
Together the guard and subsidiary cells are called the stomatal complex

Question 51

describe guard cells in eudicots

Answer 51

Have an elliptical, almost kidney shaped with the pore in the center
Subsidiary cells are common, but can be absent; if they are absent, then the guard cell is surrounded by normal epidermal cells
Guard cell walls are 5 micrometers across while normal epidermal cells can be 1-2 micrometers

Question 52

how do guard cells function?

Answer 52

• flaccid guard cells close stomata, turgid guard cells open stomata
• swelling is controlled by increasing solute potential in the cell via ion uptake or creating new ions
• For active opening and closing of guard cells, the uptake or creation of ions is stimulated via abscisic acid (ABA) in response to light,
  temperature, water status, or CO2 concentration

Question 53

what is stomata opening dependent on for angiosperms?

Answer 53

• changes in volume and turgor pressure of guard and subsidiary (epidermal) cells
  When solutes enter into the guard cell, subsidiary cells release solutes into the apoplast to decrease turgor pressure and size
  Also causes subsidiary cells closer together to close the guard cells
  Subsidiary cells play a large role in open quickly and largely in angiosperms

Question 54

what light-dependent factors stimulate stomatal opening?

Answer 54

photosynthesis and blue light

Question 55

how does photosynthesis regulate stomatal opening?

Answer 55

When water is abundant, the functional solution to the leaf’s need to limit water loss while taking in CO2 is the temporal regulation of stomatal apertures-open during the day, closed at night
No photosynthesis - no need for CO2
Water loss is abundant when stomatal guard cells are open largely on bright sunny days, but doesnt matter if the water supply is abundant as well
If there is not enough water, the stomata will stay slightly open or be closed - even if its sunny
Helps to avoid dehydration

Question 56

transpiration ratio

Answer 56

the effectiveness of plants in moderating water loss while allowing sufficient CO2 uptake for photosynthesis
the amount of water transpired by the plant divided by the amount of carbon dioxide assimilated by photosynthesis.

Question 57

how many molecules of water are loss for one molecule of CO2 fixed in photosynthesis?

Answer 57

400; Plants with a transpiration ratio of 400 have a water use efficiency of 1/400 0r 0.0025

Question 58

Does CO2 diffuse faster in water or air?

Answer 58

air

Question 59

what is relative water content and what are the weaknesses associated with it?

Answer 59

• simplest index of plant water status
• not very sensitive for measuring drought responses (leaves can show responses to a less than 2% change)
• does not tell about the forces of water movement

Question 60

what is the water potential of well-watered plants?

Answer 60

• -0.2 to -2 MPa

- leaves in dryer climates can be lower due to their ability to accumulate solutes in their cells

Question 61

what is the soil-plant-atmosphere continuum?

Answer 61

• water diffuses from the leaf during transpiration due to the water vapor concentration gradient
• water loss causes a stretching of the air-water interfaces on the surfaces of the interior leaf cells, creating a lot of surface tension in the liquid phase in cells
• tension pulls water through the xylem through bulk flow from the roots
• the tension in the root system pulls water from the particles in the soil

Question 62

vapor pressure deficient

Answer 62

the difference in vapor pressure between the saturated air inside and the ambient air outside the leaf

Question 63

how does water enter into the stele after going past the casparian strip?

Answer 63

via protein channels called aquaporins which can be phosphorylated (opened and closed) and controls what goes into and out of the stele

Question 64

which is faster, tracheids or vessels?

Answer 64

vessels because they are larger