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Plants and Water Plant Cells and Water Whole Plant Water Relations.

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Presentation on theme: "Plants and Water Plant Cells and Water Whole Plant Water Relations."— Presentation transcript:

1 Plants and Water Plant Cells and Water Whole Plant Water Relations

2 Physical and chemical properties of water

3 Molecule Mass (Da) Specific Heat (J/g/C) Heat of vapori zation (J/g) Melting Point (C) Boiling Point (C) Water Ammonia Methanol Ethanol Water Compared with other liquids

4 Water is the universal solvent Hydrophobic Hydrophilic Capillary action What is cohesion? What is adhesion? How high in the tube?

5 Water Movement Bulk Flow Diffusion

6 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 (ψ s )

8 Solute gradients are needed to move water in and out of plant roots NO H 2 O Ion pumps bring in nitrate against concentration gradient

9 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 (ψ p )

10 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?

11 Water Potential Water potential = solute potential + pressure potential Ψ water = ψ s + ψ p Units = mPa (megaPascals) = pressure Ψs = 0 or – (pure water = 0) Ψp = 0 or + Net difference determines direction of water movement

12 Measurement of water potential and water status -Thermocouple psychrometer - water potential (Ψ water ) 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 -Tissue-volume measurements – water potential of tubers, roots

13 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)?

14 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

15 Transpiration is driven by a water potential gradient Mesophyll Cells (moist cell walls) Substomatal Cavity AtmosphereStoma

16 Transpiration is about water vaporization Vapor pressure = e As solutes e As temperature e Transpiration e leaf -e air Transpiration e leaf -e air /r air +r leaf

17 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 % Ψ 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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20 Evidence for Tension in Stems Pressure bomb demonstrates tension in cut stems Where would the tension in the water column be the highest?

21 Root Systems are Extensive Prairie grasses – 1.5 m depth Corn plant – 6 m depth Single rye plant – 623 km length 639 m 2 total area Most water uptake occurs 0.5 cm From tip of root through root hairs

22 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 O 2 Sandy soils low water retention, high O 2


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