Plants and Water

1
Plants and Water

• Plant Cells and Water
• Whole Plant Water Relations

2
Physical and chemical properties of water
3
Water Compared with other liquids
Molecule Mass (Da) Specific Heat (J/g/C) Heat of vaporization (J/g) Melting Point (C) 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
4
Water is the universal solvent
Capillary action

• Hydrophobic
• Hydrophilic

What is cohesion? What is adhesion? How high in
the tube?
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Water Movement

• Bulk Flow
• Diffusion

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Diffusion
Ficks Law of Diffusion Driving force behind
diffusion is the difference in concentration
7
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
  (?s)

8
Solute gradients are needed to move water in and
out of plant roots
Ion pumps bring in nitrate against concentration
gradient
NO-3-----------------------------------? H2O
---------------------------------?
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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
(?p)
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Osmotic pressure helps to explain why only a
certain amount of water moves into a plant cell
lt----------------Water
Water ------------------------?
Why does water flow into these yeast cells? Why
does this influx eventually stop?
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Water Potential

• Water potential solute potential pressure
  potential
• ? water ?s ?p

Units mPa (megaPascals) pressure ?s 0 or
(pure water 0) ?p 0 or Net difference
determines direction of water movement
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Measurement of water potential and water status
-Tissue-volume measurements water potential of
tubers, roots
-Thermocouple psychrometer - water potential
(?water) of leaves, soil or solute potential (?s)
of leaves -Scholander Pressure Bomb pressure
potential (?p) 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
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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 ?0)?
?
?
?
Soil ------------------? Roots
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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
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Transpiration is driven by a water potential
gradient
Mesophyll Cells (moist cell walls)--------?
Substomatal Cavity-----------?
Atmosphere
Stoma
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Transpiration is about water vaporization
Vapor pressure e As solutes ? e ? As
temperature ? e ? Transpiration
eleaf-eair Transpiration eleaf-eair/rair rleaf
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Relationship between ? and relative humidity
RH actual water content of air/maximum amount
of water that can be held at that temperature As
RH ? ? ? ? 100 0 95
-6.9 90 -14.2 50
-93.5 20 -217.1
As the air dries out, the water potential
gradient between the leaf (in the substomatal
cavity) and air increases increasing
transpiration rate Transpiration can also
continue at 100 RH if the leaf temperature is
higher than the air temperature (see previous
slide)
18
Water Transport in the Plant
Xylem plumbing consisting of trachieds and
vessel elements
Cross section
Longitudinal section
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Evidence for Tension in Stems
Pressure bomb demonstrates tension in cut stems
Where would the tension in the water column be
the highest?
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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
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Water Uptake From Soil
Well-watered soil ? 0 If ? drops to -1.5
MPa plants will wilt Clay soils high
water retention, low O2 Sandy soils low
water retention, high O2