1. Water Functions.

Constituent: 80-90% fresh weight herbaceous plants. >50% woody plants.

Solvent/Transport. Substrate/environment for biochemical

reactions. Hydration of organic molecules. Turgor pressure. Temperature regulation.

2. Water Properties.

Attributed to Hydrogen bonding. Specific Heat: 4.184 J g-1

Latent heat of vaporization: 2442 J g-1 at 20°C Latent heat of fusion: 334 J g-1

Cohesion. Adhesion. Surface tension. Tensile strength.

Transparency, incompressibility, density.

3. Water Potential. Definition.

Essential to understand water flow in the system soil--plant--atmosphere.

Useful for evaluating plant water status. Quantitative measure of effect of water stress. Definitions:

Energy: Capacity to do work. Free energy: Portion of a system’s energy that

can perform work when temperature is uniform throughout the system.

3. Water Potential. Definition.

Water Potential (w): A measure of the free energy of water: Symbol: Greek letter psi () Units: bar or Pascal (1 bar = 0.1 MPa). w of pure water is zero (by definition). Solutes and other forces decrease water

potential (w <0)

For living organisms, w will be negative.

3. Water Potential.

0-1-2-3 1 2 3

3. Water Potential. Magnitude.

w = 0 MPaw = 0 MPa Pure WaterPure Water

w = 0 to -1 MPaw = 0 to -1 MPaPlant/Cell in good condition

Plant/Cell in good condition

w < -2 MPaw < -2 MPa Plant/Cell under water stress

Plant/Cell under water stress

w = -1 to -2 MPaw = -1 to -2 MPaPlant/Cell under mild water stress

Plant/Cell under mild water stress

3. Water Potential. Magnitude.

Air at 99% RH = - 1.4 MPa Air at 95% RH = - 7.0 MPa Air at 80% RH = - 30 MPa Air at 10% RH = - 310.8 MPa Sea water = - 2.2 MPa Desert soils can get down to - 6 MPa

3. Water Potential. Flux.

Water will flow from sites of high w (close to zero) to sites of low w (more negative):

Water moves from a wet soil, through the plant, and evaporates (via transpiration) into a dry atmosphere.

Soil Root Stem Leaf Air

-0.3 MPa -1 MPa -2 MPa -30 MPa

3. Water Potential.

0-1-2-3 1 2 3

3. Water Potential. Components.

Matric Potential (m): Represents the effect of insoluble materials (colloids or cell walls). It is negative.

Osmotic Potential (s): Represents the effect of solutes. It is negative.

Pressure Potential (p): Represents the effect of hydrostatic pressure. It is positive.

Gravitational Potential (g): Represents the effect of gravity. It is negative.

3. Water Potential. Components.

w plant = s + p + m

w soil = s + m

3. Water Potential. Example.

w plant = s + p + m

- 0.8 MPa = - 0.9 + 0.3 - 0.2

w soil = s + m

- 0.6 MPa = - 0.2 - 0.4

4. Water movement in the soil.

Water content and rate of water movement in the soil depends mainly on the soil type.

Processes: Infiltration: Water penetration through the

soil surface. (Runoff or evaporation affect). Permeability: Water diffusion among soil

particles. Percolation: Movement of excess water

deeper into the profile due to gravity.

4. Water movement in the soil.

Classification of water in the soil: Hydration water: That chemically bound to

soil particles. Not available to plants. Hygroscopic water: That tightly held by the

soil (>3.1 MPa of suction). Not available. Capillary water: That filling soil micropores.

Most of it is available. Gravitational water: That moving in the soil

by gravity through macropores. Available.

4. Water movement in the soil.

Soil moisture constants: Field Capacity: Moisture left in soil after

gravity has drained macropores; micropores held water at 0.03 MPa of suction.

Permanent Wilting Point: Moisture content at which a plant wilts and does not recover, even when under a humid environment. Water is being held at ca. 1.5 MPa.

Easily Available Water: FC - PWP

4. Water movement in the soil.

PWP FC SP

Non-available Available Excess

Hygroscopic

waterCapillary

water

Gravitational

water

00.031.53.1

MP

a

5. Water movement in the plant.

To the root surface: Soil to root: Diffusion and bulk flow. Roots growing into moist soil.

Inside the plant: Osmosis: Explains water movement across

membranes. Diffusion: Effective at cellular dimensions. Bulk flow: Important for long distance

transport via xylem.

5. Water movement in the plant.

5. Water movement in the plant.

Water movement in the end is a result of differences in water potential.

5. Water movement in the plant.

Transpiration: The loss of water from plants in the form of vapor. Important for water flow, solute transport

and cooling. Occurs through stomata:

Subsidiary Cells.Guard Cells.

Guard cells turgid then stoma is open. Guard cells flaccid then stoma is closed.

5. Water movement in the plant.

5. Water movement in the plant.

= rs + rarH O2

5. Water movement in the plant.

w SOIL

w ROOT

w LEAF

(Slatyer, 1967)

w

5. Water movement in the plant.

LAI

EV

T (

mm

/d)

Safflower

Stern, 1965

80%

5. Water movement in the plant.

Gre

en L

AI

EV

T/o

pen

pan

evap

orat

ion

Days after plantingShaw and Laing, 1966

5. Water movement in the plant.

Factors determining plant water status: Soil:

Texture/structure.w in the soil (moisture content).

Plant:Depth and spread of root system.Root permeability.Stomatal regulation.Species (LAI, Ps. Syndrome, etc.)

5. Water movement in the plant.

Evaporative demand of the atmosphere:Solar radiation.Temperature.Relative humidity.Wind.

5. Water movement in the plant.

Water Use Efficiency: Total dry matter produced by plants per unit of water used:

WUE =D

W

Water Requirement: Amount of water used per unit of dry matter produced.

WR =W

D

5. Water movement in the plant.

Monocotyledons Dicotyledons

Species WR Species WR

Sorghum

Corn

304

349

Barley

Wheat

Oats

518

557

583

Pigweed

Thistle

283

314

Cotton

Alfalfa

Lambsquarter

568

844

658

(Black et al., 1969)

6. Precipitation.

Types of rainfall: Convective Orographic Cyclonic

Characteristics: Total annual rainfall Distribution Intensity Relation with evapotranspiration

6. Precipitation. Ames (30 years).

0

20

40

60

80

100

120

140

Pre

cip

ita

tio

n (

mm

)

-20.0-10.00.010.020.030.040.050.060.070.0

Te

mp

era

ture

(°C

)

7. Excess water in the soil.

Reasons: Heavy rains (high intensity). Flooding. Poor soil drainage.

Effects: On absorption and transport. On oxygen availability, then respiration.

Aggravated by high T°

7. Excess water in the soil.

Activation of fermentation:Pyruvate to ethanol or lactate.2 ATP vs. 36 ATP

Anoxia. Growth is depressed. Leaf senescence. Yields are reduced. Plant death is possible.

7. Excess water in the soil.

Adaptations: Fermentation. Aerenchyma. Overwintering.

8. Water deficit. Generalities.

Drought: A meteorological phenomenon: deficiency

of precipitation. Usually associated with:High temperatures.High irradiance.Low relative humidity.

Water Stress: Situation in which a process or reaction is slowed, relative to a control plant, due to a limitation of water.

8. Water deficit. Effects.

f(Duration, R.H., T°, soil, genotype, stage)

Drought effects

Cell growth

Protein synthesis

Stomatal opening

Photosynthesis

Respiration

Pro/sugar accum.

Transport

0 MPa-1 MPa-2 MPa

Hsiao and Acevedo, 1974

8. Water deficit. Effects.

-0.4-1.2 -0.8-1.6

25

50

75

% o

f m

axim

um

Respiration

Enlargement

Photosynthesis

Water Potential (MPa)

8. Water deficit. Effects.

Drought resistance expressed as a reduction in yield

Dro

ught

res

ista

nce

Time

8.Water deficit. Effects.

-0.4-1.2 -0.8-1.6

0.3

0.6

0.9

Rat

e of

elo

nga

tion

(u

ms-1

)

Stem

Root

Leaf

Water Potential (MPa)

Silks

Westgate and Boyer (1985)

JULY AUGUST

75

50

25

100G

rain

yie

ld (

% o

f co

ntr

ol)

Claassen and Shaw (1970)

JULY AUGUST

75

50

25

100%

of

con

trol

Claassen and Shaw (1970)

Grain No.Grain Wt.

JULY AUGUST

75

50

25

100S

talk

dry

wei

ght

(% o

f co

ntr

ol)

Claassen and Shaw (1970)

Stalk

JULY AUGUST

80

60

20

100 (

% o

f co

ntr

ol)

Shaw and Laing (1966)

Seed Wt.

40

Seeds/podSeed yieldPod No.

8. Water deficit. Responses.

Momentary responses. Stop or decrease shoot growth. Expand roots. Osmotic adjustment. Stomatal closure. Organ abscission. Processes are affected during sequence.

8. Water deficit. Osmotic adjustment.

Changes in the solute content of cells (not caused by water loss) by which w can be decreased without a decrease in turgor. Under cell’s control. Fast (3-4 h). s is decreased by 0.2 to 0.8 MPa.

When?: Under slow water stress development

(some spp).

8. Water deficit. Osmotic adjustment.

Why?: Keep a high P in the cell. Maintain enzymes functioning.

How? a) Accumulation of solutes (Glycerol,

proline, mannitol). b) Uptake inorganic solutes/salts; store in

vacuole, use later. c) Slow metabolism.

8. Water deficit. Osmotic adjustment.

w plant = osmotic + pressure + matric

- 1.4 = - 2.9 + 1.5

w plant = osmotic + pressure + matric

- 3.1 = - 4.6 + 1.5

8. Water deficit. Adaptations.

Evolutionary adaptations: Ephemeral cycle. Membrane termoestability. Brightness and reflective characteristics. Decreased leaf area. Large radical system. Leaf movement. Waxy, succulent leaves. C4, CAM.

9. Rainfed agroecosystems.

Long wet season: Conventional: drainage, flood control. Alternative:

Plant around flooded areas, upon drying plant in low-lying areas.

Adapted crops. Alternating wet-dry seasons in tropics:

Conventional: drainage. Alternative: Platforms/canals; intercropping

9. Rainfed agroecosystems.

Seasonal rainfall: Conventional: US agriculture. Alternative: Intercropping.

Dryland farming: Cultivation to promote water penetration

and storage. Use of rest seasons. Water harvesting.

Irrigation; soil coverage.

Definitions.

Specific Heat: Energy required to raise T° of a unit of mass by 1 °C from 14.5 to 15.5 °C

Latent Heat of Vaporization: Energy required to move one molecule from the liquid to the vapor phase at a constant temperature.

Latent Heat of Fusion: Energy required to melt 1 gram of ice at 0 °C