Plants and Water
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Transcript of Plants and Water
![Page 1: Plants and Water](https://reader030.fdocuments.net/reader030/viewer/2022012908/56813696550346895d9e247b/html5/thumbnails/1.jpg)
Plants and Water
Plant Cells and Water
Whole Plant Water Relations
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Physical and chemical properties of water
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MoleculeMass
(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
Water Compared with other liquids
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Water is the universal solvent
• Hydrophobic• Hydrophilic
Capillary action
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
Fick’s Law of Diffusion:
Driving force behind diffusion is the difference in concentration
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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)
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Solute gradients are needed to move water in and out of plant roots
NO-3-----------------------------------
H2O ---------------------------------
Ion pumps bring in nitrate against concentration gradient
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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 forcenecessary to prevent this rise is called osmotic pressure: thegreater the difference, the greater the osmotic pressureneeded Osmotic pressure of an isolated solution is called osmotic orpressure potential (ψp)
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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?
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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
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Measurement of water potential and water status
-Thermocouple psychrometer - water potential (Ψwater) of leaves, soil orsolute 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 weightDW = dry weightTW = turgid weight
-Tissue-volume measurements – water potential of tubers, roots
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Movement of water into, through and out of plants is governed by a water potential
gradient
Soil ------------------ Roots
Atmosphere ←---------Leaf
↑↑
↑
↑
↑
Where will the water potential be the highest (closest to Ψ=0)?
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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-----------
AtmosphereStoma
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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 Ψ and relative humidity
RH = actual water content of air/maximum amount of water that can be held at that temperature
As RH ↑ Ψ ↑
% Ψ100 095 -6.990 -14.250 -93.520 -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)
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Water Transport in the Plant
Xylem – “plumbing” consisting of trachieds and vessel elements
Cross sectionLongitudinal 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 cmFrom tip of root through root hairs
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Water Uptake From Soil
Well-watered soil:Ψ ≈ 0
If Ψ drops to -1.5 MPaplants will wilt
Clay soils high waterretention, low O2
Sandy soils low waterretention, high O2