Transcript Plants and Water - University Faculty

Plants and Water
Plant Cells and Water
Whole Plant Water Relations
Physical and chemical properties of water
Water Compared with other liquids
Molecule
Mass
(Da)
Specific
Heat
(J/g/C)
Heat of
Melting
vapori
Point
zation
(C)
(J/g)
Boiling
Point
(C)
Water
18
4.2
2452
0
100
Ammonia
17
5.0
1234
-77
-33
Methanol
32
2.6
1226
-94
65
Ethanol
46
2.4
878
-117
78
Water is the universal
solvent
Capillary action
• Hydrophobic
• Hydrophilic
What is cohesion?
What is adhesion?
How high in the tube?
Water Movement
• Bulk Flow
• Diffusion
Diffusion
Fick’s Law of Diffusion:
Driving force behind diffusion is the difference in concentration
Osmosis – a special case of diffusion
•Why does water move?
•Why is the energy of pure water (or with lesser solute
concentration) greater than water with a higher concentration of
dissolved solutes?
•Chemical potential = free energy/mole: as solutes ↑ chemical
potential ↓
•Chemical potential of water = solute potential (ψs)
Solute gradients are needed to move water in and
out of plant roots
Ion pumps bring in nitrate against
concentration gradient
NO-3-----------------------------------
H2O ---------------------------------
Chemical potential of water is also affected by
pressure
Water will rise in tube as a result of solute differences: the force
necessary to prevent this rise is called osmotic pressure: the
greater the difference, the greater the osmotic pressure
needed
Osmotic pressure of an isolated solution is called osmotic or
pressure potential (ψp)
Osmotic pressure helps to explain why only a
certain amount of water moves into a plant cell
<----------------Water
Water ------------------------
Why does water flow into these yeast cells?
Why does this influx eventually stop?
Water Potential
Water potential = solute potential + pressure potential
Ψ water = ψs + ψp
Units = mPa (megaPascals) = pressure
Ψs = 0 or – (pure water = 0)
Ψp = 0 or +
Net difference determines direction of water movement
Measurement of water potential and water status
-Tissue-volume measurements – water potential of tubers, roots
-Thermocouple psychrometer - water potential (Ψwater) of leaves, soil or
solute potential (Ψs) of leaves
-Scholander Pressure Bomb – pressure potential (Ψp) in xylem (stems)
-Relative Water Content (RWC) = water status of all plant tissues
RWC = (FW – DW)/(TW – DW)
FW = fresh weight
DW = dry weight
TW = turgid weight
Movement of water
into, through and
out of plants is
governed by a
water potential
gradient
Atmosphere ←---------Leaf
↑
↑
Where will the water
potential be the highest
(closest to Ψ=0)?
↑
↑
Soil ------------------ Roots
↑
Transpiration: Facts & Figures
1 corn plant: 200 liters/growing season
Maple tree: 225
liters/hour
Appalachian Forest: 1/3 annual
precipitation absorbed by plants
and returned as rainfall
Transpiration is driven by a water potential
gradient
Mesophyll Cells (moist cell walls)--------
Substomatal Cavity-----------
Stoma
Atmosphere
Transpiration is about water vaporization
Vapor pressure = “e”
As solutes ↑ e ↓
As temperature ↑ e ↑
Transpiration ≈ eleaf-eair
Transpiration ≈ eleaf-eair/rair +rleaf
Relationship between Ψ and relative humidity
RH = actual water
content of air/maximum
amount of water that can
be held at that
temperature
As the air dries out, the water
potential gradient between the
leaf (in the substomatal cavity)
and air increases increasing
transpiration rate
As RH ↑ Ψ ↑
%
100
95
90
50
20
Ψ
0
-6.9
-14.2
-93.5
-217.1
Transpiration can also continue
at 100% RH if the leaf
temperature is higher than the
air temperature (see previous
slide)
Water Transport in the Plant
Xylem – “plumbing” consisting of trachieds and vessel elements
Cross section
Longitudinal section
Evidence for Tension in Stems
Where would the tension in the
water column be the highest?
Pressure “bomb”
demonstrates tension in
cut stems
Root Systems are Extensive
Prairie grasses – 1.5 m depth
Corn plant – 6 m depth
Single rye plant – 623 km length
639 m2 total area
Most water uptake occurs 0.5 cm
From tip of root through root hairs
Water Uptake
From Soil
Well-watered soil:
Ψ≈0
If Ψ drops to -1.5 MPa
plants will wilt
Clay soils high water
retention, low O2
Sandy soils low water
retention, high O2