Liquid water in organisms and environment

1
Liquid water in organisms and environment

• Why do we care?
• Almost all water in organisms is in liquid form
• Water uptake by organisms generally in liquid
  form
• Energy state of liquid water affects evaporation
  rate and vapor exchange

2
Intensive vs. Extensive variables

• 2 types of variables necessary to describe state
  of matter or energy in environment
• Extensive variable describes the amount of
  matter or energy
• Intensive variable describes the intensity or
  quality of matter or energy
• Example Thermal state of a block of plastic
• Heat content
• Temperature

3
Water status

• Can also be described with extensive and
  intensive variables
• Water content tells you what?
• Water potential energy state of water
• Note that water in tissue or soil is very
  different from water in a glass
• Bound to surfaces, diluted by solutes, under
  pressure or tension
• VERY different energy state from free water

4
Water potential

• Definition potential energy per unit mass or
  volume referenced to pure, free water (zero
  potential)
• Units in this class, we will use J/kg or kPa
  (equivalent since density of water is 1Mg/m3)
• Water will always flow from high potential to low
  potential

5
Water Potential

• Water potential has four components

• Defines direction of water flow in soil
• Defines direction of flow in SPAC
• Water is taken up, moves through the plant, and
  lost to the atmosphere on a water potential
  gradient

6
Gravitational Potential
yg gh 9.81 m s-2 100 m 981 J/kg
Flow
100 m
Reference Height
7
?m - Capillary Forces
From Salisbury and Ross. Plant Physiology, Fourth
Edition. 1992
8
Soil Water Retention Curves
9
Pressure potential ?p

• Hydrostatic or pneumatic pressure (or vacuum)
• Positive
• Examples?
• Negative
• Examples?

P pressure (Pa) ?w density of water (Mg/m3)
10
Osmotic potential (?o)

• Arises from dilution effects of solutes dissolved
  in water
• Always negative
• Only affects system if semi-permeable barrier
  present lets through water but not salts
• Examples?

11
Osmotic potential (?o)
C concentration of solute (mol/kg) f osmotic
coefficient - generally 0.9 to 1 ? number of
ions per mol (NaCl 2, CaCl2 3, sucrose 1) R
gas constant T Kelvin temperature
12
Water in plant tissue
13
Magnitude of important water potentials for Env.
Biophysics

• Soil
• Saturated soil 0
• Field capacity -33 J/kg
• PWP -1500 J/kg
• Plant
• Cell sap -1000 to -2000 J/kg
• Leaf tissue 0 at night with wet soil to -2000
  J/kg during day under high transpiration
• Atmosphere
• -100,000 J/kg (depends on humidity)

14
Water potentials in SPAC
Atmosphere -100,000
-1000 -700 -30 -30
-3000 -2500 -1700 -1500
Leaf
Xylem
Root
Soil
Field Capacity (J/kg)
Permanent wilt (J/kg)
15
Relationship between liquid and gas phase water

• To calculate evaporation rate, you must know Cv,s
  at evaporating surface
• Pure, free water
• ea es(Ts) if we know T, we know ea
• Cv,s es(Ts)/Pa
• What if surface isnt pure free water?
• hrs lt1
• Cvs hrs es(Ts)/Pa
• How do we calculate hrs?