WATER:Properties, Role in Plants,

Watering Strategies

Water Evaporation and Transpiration

• Evaporation - Change of liquid into gaseous state

•Transpiration - Evaporative loss of water from the plant.

•Temperature- measure of the average velocity of molecules (how fast they are moving) a.k.a. HEAT

• Molecule that evaporates from a surface has enough velocity to overcome the attraction of its neighbor.

• When water molecules escape, the temperature of the remaining liquid decreases.

Transpiration & Evaporationand

Temperature

Relevance to Plants

• When water evaporates (transpires) from a leaf, the leaf is cooled.– Much the same as how the evaporation of

perspiration cools us.

• When water evaporates from greenhouse cooling pads, the air is cooled which in turn cools the plants it moves over.

• Condensation is the return of a molecule of water to its liquid form.

• There is an EQUILIBRIUM when the rate of condensation (return) equals the rate of escape (evaporation)

EquilibriumEvaporation = Condensation

No equilibriumEvaporation < or > Condensation

•At equilibrium the atmosphere is saturated

• Relative humidity (RH) =Actual amount of water vapor in the atmosphere

Amount of water vapor the atmosphere when saturated

• RH depends on air temperature

- warm air holds more water than cold air

e.g. 70F air at 100% RH is holding more water than 50F air at 100% RH.

Relevance to Plants and Greenhouses

• The higher the RH, the slower the rate of transpiration from leaf so there is less cooling of the leaf.

• The higher the RH, the less effective evaporative cooling systems are in the greenhouse.

Relative Humidity (RH) =

Can easily measure using a

and a chart

Sling psychrometer

Example: Dry temp 15 wet temp 10 = RH 75%

Reading a psychrometer chart

Reading a psychrometer chart

Constructing your own sling psychrometer:

Tie two thermometers together, wrap the end of one in a wet cloth. Sling around in the air for a minute or so.

Measure air temperature (dry bulb)

Measure cooling effect of evaporation of water (wet bulb)

Compare the readings on a chart to get RH.

Relative humidity is not always a good way to measure the potential for evaporation to occur, because RH is temperature dependent.

A better way is to measure the difference in vapor pressures between the atmosphere and the evaporative surface (leaf or cooling pad).

Vapor pressure - the pressure exerted by a vapor; often understood to mean saturated vapor pressure (the vapor pressure of a vapor in contact with its liquid form). Expressed as kPa

When the vapor pressure of air is less than the surface the air is touching, there is a deficit of air vapor pressure (VPD) relative to the surface.

The greater the deficit, the greater the rate of evaporation from the surface.

VPD determines how fast plants use water and how efficiently wet pads cool greenhouses.

Which in turn determine how often you have to irrigate.

The more you understand VPD, the better you (or the environmental control computer you program) can decide when it’s time to irrigate.

• VPD - Good way to determine watering needs of plants

• The greater the VPD between the leaf and the air, the more likely the plant water use will increase.

Vapor Pressure Deficit (VPD)Relevance to Plants

Understanding VPD also helps you to find out if you are wasting your money (and water resources) using cooling pads to cool your greenhouse.

You could be better off using natural ventilation.

What else makes it beneficial for you to understand VPD?

What else do you have to know to be able to irrigate at the right time?

You have to understand plant water relations and how water moves into and through the plant.

Plant Water Relations

Starts with the concept of

Water Potential

Plant Water Relations

Water Potential () :The difference between the activity of water

molecules in pure distilled water at 1 atm and 30°C (standard conditions), and the activity of water molecules in any other system.

The activity of water molecules in a system may be greater (positive) or less (negative) than the activity of the water molecules under standard conditions.

Plant Water Relations

• Water Potential () Defines how tightly water is held by a

system Determines how easily water move from

one system to another Determines which direction water flows

Plant Water Relations

• Water Potential () summary

units -- atm (atmosphere) or bar or kPa is 1 for pure water at sea level For most systems, is negative Water moves from higher to lower

Think of flow of water from high to low as a waterfall - flowing high to low

is greater at the top of a waterfall than at the bottom.

Plant Water Relations

Implications for plants

Water moves into the root only if in root is lower (more negative) than

the soil.

Water moves through the plant in the from higher to lower .

Components of T = + P + M

Where:T = total potential = osmotic potentialP = pressure potentialM = matric potential

Components of T = + P + M

Osmotic Potential

due to the effect of dissolved solutes

the greater the concentration of solutes, the lower (more negative) the water potential

water moves from an area of low salt concentration to an area of greater concentration.

Components of T = + P + M

Implications for plants

• Generally causes the plant to have more negative than soil/media because of the salts in the plant. This helps water move into the root from the soil.

•Applying liquid fertilizer (a.k.a. salt solution) to a dry soil/media lowers the osmotic potential of the media/soil. If of the soil becomes less than the root, water will leave the root, causing fertilizer burn.

Components of T = + P + M

Pressure Potential P

due to the forces on water from high water concentration in cells

positive value for the most part in turgid (not wilting) plants

early stages of decreasing P =

incipient plasmolysis, useful for controlling length of young shoots stems

- Cell wall

- Cell membrane

When young cells are filled with water, the membrane presses on the growing cell wall. The cell walls elongate and stay relatively thin as the cells grow and divide.

When water is slightly withheld from young plants, the membrane does not press on the growing cell wall (incipient plasmolysis). The cell walls stay more square and thick as the cells grow and divide.

If these were stem cells, which would provide the strongest and shortest stems which usually produce the most durable and probably attractive plant?

If you know what you are doing, “drying down” is one of the most effective and cheapest ways to regulate plant height.

Have to be careful, “drying down” is only a few minutes away from “drying up”.

Drying up can cause irreparable damage to plants.

You do not want the plant and its young cells to become desiccated.

Components of T = + P + M

Matrix Potential M

the adhesion of water to particles

the stronger the adhesion of water to a particle, the lower the matrix potential

Components of 3. Matrix Potential M

involves potential of solid components (including soil)

the stronger the adhesion of water to a particle, the lower the matrix potential

Implications for plants

The lower the M in the soil or growing media, the more tightly the water is held by the media –

When you irrigate you are raising the M of the media and in turn you are making it easier for water to enter the plant.

Total water potential

T = + P + M

T determines how much water enters, leaves, and stays inside the plant.

That in turn determines how the plant grows. You can control much of a plant’s growth by

controlling any of the T components.

Triphasic growth pattern:

Typical for most greenhouse plants.

Characterized by:

1. Slow initial growth

2. Rapid vegetative growth and elongation

3. Slow reproductive growth. Growth regulation is most

effective between low and mid-portions of rapid growth phase

Timing when water is withheld, as with every growth regulation technique, is very important.

AirTemp = 20°C = 68°F

RH 5% -2547 bar

30% -1634 bar50% -943 bar75% -390 bar90% -142 bar95% -70 bar98% -27 bar

100% 0 bar

Note that even at near 100% RH, air still more negative than leaf

Thus: water flows from leaf to air

However, even at air RH 100%, the slightest air movement across the leaf

lowers air to less than in leaf so water flows from leaf to air

Cohesion-Tension TheoryMechanism of water movement in xylem is

driven by changes in from soil through plant to air

During all this pulling, hydrogen bonds hold water molecules together in columns inside xylem tubes = cohesion

The very negative of the air tugs onthe water column,causing the H2Omolecules to move up throughthe plant.

Rhizoshere (rootzone)

(Water molecules, not Disney symbols)Air

Cohesion/tension explains how water can travel upwards against gravity in a plant.

Transpiration at leavesWater molecules pulled up stem to replace molecules lost to air

Tension on water in xylem

Water pulled into roots

Water into the Root

Roots have evolved to increase water absorption area by formation of root hairs.

New root hairs have to be constantly produced to have water uptake.

Damaged or diseased roots do not produce root hairs, severely limiting their ability to take up water.

Disease and Water MovementMany fungal or bacterial pathogens cause diseases with a characteristic symptom of wilt. The wilting comes because the pathogen enters the vascular tissue and as it grows, it clogs the water-conducting vessels.

Cutting a stem and seeing discolored vascular tissue is a good “clue” that helps diagnose disease.

In herbaceous stems a vertical cut is made just under the epidermis of the stem. If there is an infection, you can see a “streaking” in the vascular tissue.

Disease-clogged xylem

Cavitation or Embolism Air bubble (vapor lock) in the xylem, break in the water chain NOT GOOD - stops water flow through that column in its tracks and often forever

Practical application:

Cut flowers often can’t take up water because of cavitation at cut ends of xylem - leads to the idea of cutting stems underwater.

Water Loss from the Leaf

• Stomates- pores in the leaves, primary way plants regulate transpiration (water loss)

Stomatal Control1. Light

• signal stomata to open

2. Internal [CO2] (of leaf)

• independent of light• increase in [CO2] → stomata close

• decrease in [CO2] → stomata open

CO O

Stomatal Control3. Plant Water Status

• sensed by the roots

• when soil dries and soil approaches root , roots cannot take up water to meet plant demands, plants begin to loose water faster than it is taken up

• in response to water loss, roots then synthesize ABA

• ABA signals stomata to close to decrease water loss

• water status is the overriding environmental factor that controls stomatal opening/closing

Plant Adaptations to Save Water

1. Sunken StomatesArea of higher RH developsin the “pit” which reduces the VPD between leaf and air.

2. Stomates on underside of leaves

The upper side of leaves are exposed to light which warms the leaf and increases VPD causing more water evaporating if stomates are on the upper surface.

Plant Adaptations to Save Water3. Hairy leaves

• hairs serve as a wind break to maintain an undisturbed layer of air around the leaf (boundary layer)

• reduces VPD at leaf surface

4. Osmotic adjustment

• plant will automatically add solutes to cells which causes to drop which draws water into the cell.

Plant Adaptations to Save Water 5. CAM Metabolism (succulents and some orchids)

• stomates closed during day, open at night

• at night CO2 enters the leaves

• CO2 then converted and stored as an acid

• during day, CO2 released and used in photosynthesis

6. C4 Metabolism (warm-season grasses such as corn, turfgrasses)

• CO2 converted to acid

• acid ‘shuttled’ to special cells for photosynthesis

• CO2 released for photosynthesis

• location of special cells reduces photorespiration which ‘wastes’ CO2 in non-C4 plants

Plant Adaptations to Save Water

Functions of Transpiration

1. Mineral Transport

• rate of transpiration influences uptake movement of ions from soil and movement through xylem

2. Heat Transfer (cooling of the leaf/plant)

Caution: You have to be careful that by limiting water you aren’t shooting yourself in the foot by limiting heat transfer or mineral transport.