Plants and Water Plant Cells and Water Whole Plant Water Relations

Physical and chemical properties of water

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

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

Water Movement Bulk Flow Diffusion

Ficks 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 (ψ s )

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

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 )

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 = ψ s + ψ p 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 -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

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)?

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 AtmosphereStoma

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

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)

Water Transport in the Plant Xylem – plumbing consisting of trachieds and vessel elements Cross section Longitudinal section

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

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

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