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Water Relations. Steady- State. Surface. Area. “Tho’ vegetables have not an engine, which, by its alternate dilatations and contractions, does in animals forcibly drive the blood through the arteries and veins; yet has nature wonderfully contrived other means, most powerfully,

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Water Relations

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Water Relations

Steady- State



“Tho’ vegetables have not an engine, which,

by its alternate dilatations and contractions,

does in animals forcibly drive the blood through

the arteries and veins; yet has nature wonderfully

contrived other means, most powerfully,

to raise and keep in motion the sap.

I shall begin with an experiment upon roots,

which nature has providently taken care to

cover with a fine thick strainer; that nothing

shall be admitted into them, but what can

readily be carried off by perspiration,

vegetables having no other provision for

discharging their recrement.”

Stephan Hales, Vegetable Staticks, 1727

Plant Growth: Getting to the Root of the Problem



Absorb water

Absorb nutrients

Filter out the unwanted



Communicate with shoot


Surface Area



Growing root tips

Zone of Maturation

Continued Root

growth is critical

to root function.




Root Cross Section






Endodermis is location of filter


Longest roots recorded are for mesquite, Prosopis glandulosa (80’)

Primary roots of an adult rye plant Secale cereale were measured and found to be 380 miles in length.

Cohesion-Tension TheoryMechanism of water movement in xylem is driven by changes in  from soil through plant to air

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

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

tubes = cohesion

The very negative 

of the air tugs on

the water column,

causing the H2O

molecules to

move up through

the plant.

(Water molecules, not Disney symbols)


Rhizoshere (rootzone)

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

Transpiration at leaves

Water 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 Movement

Many 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 Control

  • 3. 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 Stomates

    • Area of higher RH develops

    • in 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 Water

  • 3. 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

Plant Adaptations to Save Water

  • 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

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