Plants and Water

Plant Cells and Water

Whole Plant Water Relations

Physical and chemical properties of water

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

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

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

NO-3-----------------------------------

H2O ---------------------------------

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 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)

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

-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

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 ≈ 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 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)

Water Transport in the Plant

Xylem – “plumbing” consisting of trachieds and vessel elements

Cross sectionLongitudinal 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 m2 total area

Most water uptake occurs 0.5 cmFrom tip of root through root hairs

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