L10 - Soil-Plant-Atmosphere Continuum Flashcards
Why are plant-water relations important?
What cues to stomata respond to for balancing CO2 gain and water loss? Give 3.
- Plants require a lot of water. Only small fraction of water absorbed is retained - mostly lost via transpiration.
1) CO2 system: Changes in [CO2] within leaf and carbohydrate accumulation.
2) Water response: Tissue turgor changes, changes in metabolic rate, signals indicating water deficit.
3) Direct sensing: signal sensed in guard cell, e.g. blue light
What is the SPAC?
In which direction does water flow with respect to water potential?
How is 0 water potential set?
What factors reduce potential?
- SPAC: Soil-Plant-Atmosphere-Continuum
- Water flows from less to more negative potential/pressure.
- Pure water at 0˚C and 100 kPa set to have 0 potential.
- Processes that reduce ability of water to do work increase negativity (solutes, tension).
Give the driving forces at each stage of the SPAC and rough water potentials at each stage.
LEAVES TO ATMOSPHERE:
-Vapour pressure difference between sub-stomatal aperture and air.
-Drives whole process.
- 3.0 MPa (leaf) to -30 MPa (air)
ROOTS TO SHOOTS:
- Grad. of negative pressure in xylem from evaporation and tension
- 0.5 to -2 MPa
SOIL TO ROOTS:
- Water potential between soils and roots cells. Water transported through endodermis.
- Soil water potential close to 0 (normally)
Give an equation for water potential:
Ψw = Ψs + Ψp
- Ψs is solute potential (addition of solutes reduces free energy, decreasing Ψs).
- Ψp is turgor potential/pressure. Higher turgor increases Ψp.
-In dry soils/cells Ψm (matric potential) also contributes. Ψm due to adhesion of water to non-dissolved molecules.
How can you measure water potential in the leaf or shoot of a plant?
- Place leaf or shoot in a pressure bomb.
- Pressure bomb compresses air, forcing sap back to cut at the end of the stem.
- Pressure required for this = pressure in stem before cut.
What is Field Capacity?
Describe when plants can extract water from the soil and what happens as water is extracted.
- Amount of water retained within capillaries of soil (space between soil particles) - depends of clay/organic matter ratio.
- Soil water extract possible if matric potential forces are overcome.
- As water is extracted this is increasingly difficuly.
- Eventually wilt point reached where no more water can be extracted
How can root potential be modified to encourage water uptake from the soil?
How does water uptake affect root pressure?
What process can happen upon water uptake? When is this most likely?
- Root stele loaded with inorganic ions by the symplast. Decreases Ψs and Ψw.
- Water absorption causes small positive root pressure.
- Guttation can occur: Positive root pressure causes water to exude through hydathodes.
- Common when transpiration suppressed and high relative humidity - e.g. night
How does plant water and solute potential vary with soil water availability in arid regions?
Give an example and draw the relevant diagram
- Pre-dawn plant Ψw tracks soil water availability on a seasonal basis.
-As soil potential decreases throughout the season with drought, solutes accumulated via osmotic adjustment to maintain turgor. - Midday leaf Ψw is lower than in morning.
- Water transpired more quickly than recharged by xylem.
- No longer tracks soil
- E.g. Hammada scoparia
- In late summer Ψw more negative than Ψs = water deficit
See diagram on pg 29
Outline how plants increase solute potential using osmotic adjustment.
What are some requirements of the solutes moved? Give some example solutes.
How can osmotic adjustment be manipulated for GM crops?
- Solutes accumulated in the cytoplasm and vacuole.
- In the cytoplasm, compatible solutes accumulated, not inorganic ions - would interfere w/ cellular functions.
- Compatible solutes: usually neutral, don’t penetrate hydration shell of protein.
- E.g. proline, polyhydric alcohols.
- Can enhance salt tolerance of crops by accumulating more Na+
How does xylem structure vary between different phylogenys?
Describe the different xylem structures
Gymnosperms (inc. conifers, Cycads, Ginkgo:
Tracheids and fibres .
Angiosperms and Gnetales: Vessels with some tracheids.
Tracheids: elongated, overlapping, water transfer via circular pits in lateral walls.
Fibres: smaller, tracheid like, evolved from tracheids
Vessels: Shorter, wider, perforated end plate. Stacked end to end = more continuous and direct flow (higher cavitation risk).
How does hydraulic conductance depend on the dimensions of a vessel?
Give examples of hydraulic conductance properties.
- Hydraulic conductance ∝ R^4
- Multiple smaller vessels normally less efficient then fewer large diameter vessels.
LOWEST Hydraulic conductance:
-Conifers, drought tolerant schlerophylls (narrow xylem)
Seasonal Variation in Hydraulic conductance:
- Ring Porous trees e.g. Oak, Beech
- Wider, more conductive in spring
- Narrower in late summer.
HIGHEST:
-Lianas
- Trade-off with structural support - very high in lianas which use other tree.
What is an embolism?
How do pits and endplates protect against embolisms?
- Occurs when an air bubble forms in the xylem.
- Water evaporates explosively into the bubble in a cavitation event.
- Restrict water conduction, influencing growth.
How do pits and endplates protect against embolisms?
- Pits and end plates limit sap flow, reducing risk.
- Contain the embolism via surface tension, avoiding runaway cavitation.
- Pits facilitate conductivity around embolised conduit.
Give an example of how different plants respond to embolisms on different timescales on a large scale.
- Zea mays water reduced by 50% daily due to embolisms
- Guttation overnight
- Acer saccharum flow decreased from 31% to 60% - end of summer vs end of winter.
- Rising sap used in spring to unblock flow
Describe the implications of embolism risk on species distribution and growth habits.
- Embolisms more likely during freeze-thaw cycles.
- More likely when transpiring.
Conifers have narrower tracheids
- protects against embolisms due to continuous transpiration through the winter
- allows survival at more northern latitudes and higher altitudes.
Ring-porous (Oak) and semi-ring porous (Beech):
- Overwintering xylem can’t be refilled.
- Transpiration reliant on current year’s xylem.
- Large xylem vessels initially (spring) for adequate water.
- Narrower rings later (to avoid unnecessary embolisms)
- Leaf out later
Diffuse porous (birch, maple):
- No variation in vessel size, lower risk of cavitation.
- Leaf out earlier.
