EFB530 Plant Physiology

Plant water status

It is the DY_w that drives water movement through plants

• this is a result of physical forces, no direct energy input into moving water
• rate of water movement depends on DY_w and on conductivity

• leaves of well-watered plants: Y_w = -0.2 - -0.6 MPa
• leaves of plants in arid climates: Y_w = -2 - -5 MPa

1) Psychrometer: determines Y_w

• temperature probe is covered with a drop of solution and is contained inside a chamber
• living or excised plant tissue is put into the chamber with the probe
• water will evaporate from the drop, cooling the probe, until the drop and tissue in the chamber are in equilibrium
• the solute concentration of the drop can be changed and will no longer cool when the water potential matches that of the tissue

2) Cryoscopy: measures Y_s

• plant sap or tissue extract is cooled until freezing
• dissolved solutes cause freezing point depression, which can be used to calculate Y_s

3) Pressure probe: measures Y_p

• a small capillary tube filled with oil with a pressure sensor is used to puncture a cell
• the oil is pushed back into the tube, due to hydrostatic pressure
• equal but opposite pressure is applied to counteract that pressure

4) Pressure bomb (pressure chamber): measures Y_w of whole leaves, stems

• when a stem or leaf is cut off of a plant, the sap is sucked back into the xylem, since it is under tension
• the tissue is sealed inside a steel chamber, with the cut end protruding
• pressure is applied (using compressed nitrogen) until the sap is pushed back up to the top
• this amount of pressure is equal but opposite to the Y_w of the tissue

Root-soil interface

water content of is dependent on soil type

• surface area per gram of soil; sandy soil=low surface area; clay=high surface area
• determines holding capacity (field capacity)

soil water potential-determined by Y_s and Y_p

• soil Y_s is usually high (~-0.01 MPa); except for saline soil (~-0.2 MPa)
• soil Y_p is close to 0 in wet soil, decreases as soil dries out=tension develops
• negative pressure from surface tension (arid soil down to -3 MPa): as air spaces grow, so does the surface tension
• water moves through soil primarily by bulk flow; pressure driven as roots remove the water around them

Y_s is low in the root cells (root Y_w is more negative) - so the primary driving component of the water potential gradient is Y_s

Plants can change the Y_s of the cell (there is an upper limit ~-0.5 MPa)
They can lower Y_s to reduce Y_w to enable them to extract water (root Y_w must be lower than soil )

• this is induced under drought stress
• they synthesize compatible solutes to lower Y_s - proline, glycine betaine, sorbitol (see pg. 596)
• halophytes must maintain lower Y_w than the Y_w of the saline soil they are in

Root hairs increase the surface area of the plant for water absorption (can be 60% of total root surface area)

Root

Water travels into the root by two pathways

• apoplast=non-cytoplasm (cell walls & air space), does NOT cross membranes
• cellular=must cross plasma membrane to enter
  symplast cellular path=crosses a membrane to enter cytoplasm, then moves through plasmodesmata (doesn't need to cross another membrane
  transmembrane cellular path=water is continually crossing plasma membrane, enters cell, exits cell, enters next cell, so on; may also cross the tonoplast to enter vacuoles within those cells

The apoplast is blocked when water hits the endodermis=Casparian strip

• the cell wall spaces on the top, bottom, and sides is impregnated with waxy suberin
• water must cross through the face of the cell - and cross the plasma membrane

Water then exits the symplast when it crosses out of xylem parenchyma cells and into dead xylem vessel members and tracheids

• in the xylem, Y_w is dominated by Y_p (which is typically negative)

water moves very efficiently through xylem tracheary elements

• tracheids=long cells with pits in walls
• vessel members=shorter cells, with perforated end walls (& pits), end-to-end
• average rate of transport is several meters per hour (a few mm per second)
• much less driving force is needed to overcome the resistance of flow in xylem than the resistance of movement across a membrane (0.02 MPa per meter in xylem; 10,000 MPa to cross 1 membrane)

to move water to top of 100 m tall redwood tree

• resistance drag in xylem ~0.02 MPa / m x 100 m = ~2 MPa
• gravity ~0.01 MPa / m = ~1.0 MPa
• TOTAL= ~3.0 MPa

This pressure arises primarily from tension from the top of a plant that pulls water up

• therefore xylem has evolved to tolerate tension (negative pressure)

Water + gas (under tension) = trouble (bubble)

• water has tensile strength, but gas does not
• gas will form a bubble (under tension)=cavitation
• air bubble blocks water movement through the xylem
• gas usually does not squeeze through pits though, so bubbles are localized
• tension goes down at night, bubbles go back in to solution

Root pressure and guttation

• when transpiration is low (moist conditions, like early morning) can get root pressure
• water enters the root because root Y_s is typically low
• then the Y_p in the root xylem increases (and Y_w increases)
• the pressure in the roots can force water up the xylem of the stem
• cut stems can exude sap
• water can be forced out hydathodes (structures on the margins of leaves that form small pores at the ends of veins) in leaf=guttation

the tension to pull water up the xylem develops in the leaf

• water in a film around each cell, with air spaces=surface tension
• xylem venation comes close to nearly every cell in the leaf
• water moves to the air primarily by diffusion
• once in the air, the water is now water vapor and the formula for water potential changes (see below)

the cuticle is an effective barrier to water movement (on average, 5% goes through cuticle)

• water diffuses to the atmosphere through stomata=transpiration
• usually more on the bottom of a leaf

water diffusion out of the leaf is driven by the absolute water vapor concentration gradient, but is limited by stomatal conductance and boundary layer resistance

• because of the air space in a leaf, the surface area for evaporation can be 7-30X the external area of the leaf
• can assume that Y_{w(leaf cell)} = Y_{w(leaf air space)}
• can relate Y_w to relative humidity (RH) [RH=water vapor concentration] & TEMP
• Y_w =RT/(V * (ln(RH));  RH=conc_{water vapor} / conc_{water vapor(when saturated)}

stomatal conductance

• stomata are the sites of water vapor diffusion, pore aperture may be regulated
• stomatal pore is formed by two attached guard cells
• also sites of CO₂ and O₂ exchange, so they balance water loss & gas uptake
• regulate this temporally-open in day; closed at night
• also close under water stress conditions

boundary layer resistance

• thin film of still air hugging the leaf with high water vapor concentration (vs. air)
• the thickness of the boundary layer determines resistance to diffusion
• thickness determined by: wind, trichomes (hairs), sunken stomata, shape of leaf

CAM plants=50-100 g of water lost per g of CO₂ gain
C4 plants=250-300 g of water lost per g of CO₂ gain
C3 plants=400-500 g of water lost per g of CO₂ gain

Cooling

the heat of vaporization of water allows for a great deal of energy release through transpiration

Mineral transport

minerals taken up from the soil are carried through the transpiration stream to the leaf and stem cells

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