Chapter 3: Plant Water Balance Flashcards
what does water content and the rate of water movement in soil depend on?
soil type and structure
what lowers soil water potential?
a negative hydrostatic pressure
what is the pressure potential in wet soils? in dry soils?
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
what is the role of gravitational potential in soils?
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
how does water move through the soil?
bulk flow
how is pressure formed in soils?
- 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
what does the rate of water flow in soils depend on?
the size of the pressure gradient through the soil and hydraulic conductivity in soil
soil hydraulic conductivity
- 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
root hairs
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
why are mature roots less permeable?
their epidermal layer has been modified to have hydrophobic materials
This is to increase bulk flow
how does water move through the roots?
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
apoplast
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
symplast
consists of the entire network of cell cytoplasm interconnected by plasmodesmata. In this pathway, water travels across the root cortex via the plasmodesmata.
transmembrane pathway
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.
what happens to the apoplast pathway once it reaches the endodermis?
the apoplast pathway is stopped by the casparian strip, and water must enter through the symplast or transmembrane pathways to go into the endodermis
casparian strip
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.
when does water uptake decrease?
when roots are subjected to low temperature or anaerobic conditions, or treated with respiratory inhibitors
root pressure
A positive hydrostatic pressure in the xylem of roots that typically occurs at night in the absence of transpiration.
- due to solute accumulation
what happens when transpiration is low or absent?
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
guttation
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
what is the longest part of the water transport pathway?
xylem
what are the two types of transport cells in the xylem?
tracheids and vessel elements
how are tracheids and vessel elements formed?
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
tracheids
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
pit pairs
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.
pit membranes in angiosperms and gymnosperms and conifers
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
vessel elements
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
how do vessel elements differ from tracheids?
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.
how does water move through the xylem?
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
what does the rate of bulk flow depend on?
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
where does the energy that powers water movement come from?
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.
why do xylem walls not collapse due to pressure?
Xylem walls do not collapse due to the extreme pressure due to the secondary wall and the lignification of them to prevent collapse
when do gas bubbles occur?
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
how does the root system try to prevent gas bubbles?
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
air seeding
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
cavitation and embolisms
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
what do vulnerability curves measure?
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
how can embolisms be eliminated/alleviated?
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
how does water leave the leaf?
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
how much water is lost from the cuticle? where else is water lost?
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
what does leaf hydraulic resistance depend on?
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
what does transpiration in a leaf depend on?
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
stomatal resistance
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
boundary layer resistance
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
how is the boundary layer determined?
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
what can impact the boundary later?
microscopic hairs on leaves, size of the leaf, and direction of the wind
how are stomata controlled?
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
where are guard cells found?
all vascular and some nonvascular
what are the two main types of guard cells?
one is typical of grasses while the other is more prominent in flowering plants, mosses, ferns, and gymnosperms
describe guard cells in grasses
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
describe guard cells in eudicots
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
how do guard cells function?
- 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
what is stomata opening dependent on for angiosperms?
- 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
what light-dependent factors stimulate stomatal opening?
photosynthesis and blue light
how does photosynthesis regulate stomatal opening?
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
transpiration ratio
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.
how many molecules of water are loss for one molecule of CO2 fixed in photosynthesis?
400; Plants with a transpiration ratio of 400 have a water use efficiency of 1/400 0r 0.0025
Does CO2 diffuse faster in water or air?
air
what is relative water content and what are the weaknesses associated with it?
- 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
what is the water potential of well-watered plants?
- -0.2 to -2 MPa
- leaves in dryer climates can be lower due to their ability to accumulate solutes in their cells
what is the soil-plant-atmosphere continuum?
- 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
vapor pressure deficient
the difference in vapor pressure between the saturated air inside and the ambient air outside the leaf
how does water enter into the stele after going past the casparian strip?
via protein channels called aquaporins which can be phosphorylated (opened and closed) and controls what goes into and out of the stele
which is faster, tracheids or vessels?
vessels because they are larger
