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By the end of this section, you will be able to:
  • Define water potential and explain how it is influenced by solutes, pressure, gravity, and the matric potential
  • Describe how water potential, evapotranspiration, and stomatal regulation influence how water is transported in plants
  • Explain how photosynthates are transported in plants

The structure of plant roots, stems, and leaves facilitates the transport of water, nutrients, and photosynthates throughout the plant. The phloem and xylem are the main tissues responsible for this movement. Water potential, evapotranspiration, and stomatal regulation influence how water and nutrients are transported in plants. To understand how these processes work, we must first understand the energetics of water potential.

Water potential

Plants are phenomenal hydraulic engineers. Using only the basic laws of physics and the simple manipulation of potential energy, plants can move water to the top of a 116-meter-tall tree ( [link] a ). Plants can also use hydraulics to generate enough force to split rocks and buckle sidewalks ( [link] b ). Plants achieve this because of water potential.

 Photo (a) shows the brown trunk of a tall sequoia tree in a forest. Photo (b) shows a grey tree trunk growing between a road and a sidewalk. The roots have started to lift up and crack the concrete slabs of the sidewalk.
With heights nearing 116 meters, (a) coastal redwoods ( Sequoia sempervirens ) are the tallest trees in the world. Plant roots can easily generate enough force to (b) buckle and break concrete sidewalks, much to the dismay of homeowners and city maintenance departments. (credit a: modification of work by Bernt Rostad; credit b: modification of work by Pedestrians Educating Drivers on Safety, Inc.)

Water potential is a measure of the potential energy in water. Plant physiologists are not interested in the energy in any one particular aqueous system, but are very interested in water movement between two systems. In practical terms, therefore, water potential is the difference in potential energy between a given water sample and pure water (at atmospheric pressure and ambient temperature). Water potential is denoted by the Greek letter ψ ( psi ) and is expressed in units of pressure (pressure is a form of energy) called megapascals (MPa). The potential of pure water (Ψ w pure H2O ) is, by convenience of definition, designated a value of zero (even though pure water contains plenty of potential energy, that energy is ignored). Water potential values for the water in a plant root, stem, or leaf are therefore expressed relative to Ψ w pure H2O .

The water potential in plant solutions is influenced by solute concentration, pressure, gravity, and factors called matrix effects. Water potential can be broken down into its individual components using the following equation:

Ψ system = Ψ total = Ψ s + Ψ p + Ψ g + Ψ m

where Ψ s , Ψ p , Ψ g , and Ψ m refer to the solute, pressure, gravity, and matric potentials, respectively. “System” can refer to the water potential of the soil water (Ψ soil ), root water (Ψ root ), stem water (Ψ stem ), leaf water (Ψ leaf ) or the water in the atmosphere (Ψ atmosphere ): whichever aqueous system is under consideration. As the individual components change, they raise or lower the total water potential of a system. When this happens, water moves to equilibrate, moving from the system or compartment with a higher water potential to the system or compartment with a lower water potential. This brings the difference in water potential between the two systems (ΔΨ) back to zero (ΔΨ = 0). Therefore, for water to move through the plant from the soil to the air (a process called transpiration), Ψ soil must be>Ψ root stem leaf atmosphere .

Questions & Answers

What is cytoplasm
Nitta Reply
Parasitic adaptation of tapeworm
Angela Reply
Describe osmoregulator or osmoconformers and how these tools allow animals to adapt to different environment.
Alick Reply
describe the secondary function of a leaf to a plant.
Twizera Reply
hi I'm asking a question about HIV infection, can HIV infection transmitted from Mother to unbron child? please help me I So confused.
Khushboy Reply
no it can't
Ahmad
yes it can but their is an injection that can be used to prevent it.
Gift
Ahmad how no?
Khushboy
Hi im new
innocent
no, unless when giving birth
Bright
YES
Kaole
it is can be transmitted but there is an injection that the mother is injected to prevent the disease
Nia
OK
Kaole
I agree with nia
peter
what is the difference between primary and secondary active transport in detail? I didn't understand the steps in the textbook specifically
Fathima Reply
you are a doctor?
Mohammed
what is the meaning of connective tissue?
Mohammed Reply
what are the characteristics of living things
Owolo Reply
hi
Mohammed
what's the meaning of connective tissue?
Mohammed
Reproduction, adaptation, interaction, movement, growth, respiration, made of cells, responsive to environment (homeostasis), metabolic action (consumption of food converted into energy)
garret
Movement, reproduction, nutrition, irritability, growth, excretion, respiration, death, adaptation, competition
peter
state two most important factors that favour exponential growth of population of a gazelle in a pack
Eliza Reply
growth
Ben
what are the two types of electron microscope
Sharlom Reply
light microscope and early microscope
Sama
Enzymes are biological catalyst which alter any reaction and protein in nature
Nkoue Reply
Thanks
Gaudi
Your welcome sir
Nkoue
guyz you enjoying
Royd
What is translation and transcription
Nkoue
Transcription is making RNA from DNA. Translation is going from RNA to proteins.
Eric
full meaning of RNA and DNA
Charity
Nkoue what homeostits means?
Mohammed
Mohammed khalfan, In biology, homeostasis is the state of steady internal, physical, and chemical conditions maintained by living systems.[1] This dynamic state of equilibrium is the condition of optimal functioning for the organism and includes many variables, such as body temperature and fluid bal
Khushboy
what is the definition of enzymes
Royd Reply
enzymes are biological catalyst that speed up chemical reaction.
Alale
What are enzymes?
Gaudi
Enzymes are made of proteins and lower the energy of activation. In other words, they bring things together which helps to lower the amount of energy for a reaction to go forward.
Eric
they are catalyses that speeds up chemical reaction.... e.g they break down the food we consume.
Azeez
These are catalyst that speeds up the chemical reaction.
Micheal
What is connective tissue?
Mohammed
what homeostits means ?
Mohammed
what is a spirogyra
Talabi Reply
Spirogyra is a filamentous chlorophyte green algae of the order Zygnematales. It is named for the helical or spiral arrangement of the chloroplasts. That is characteristic of the genus. It is commonly found in freshwater habitats. And there are more than 400 species of Spirogyra in the world.
Eric
what is the mean of biology
Bello Reply
what is cell
Bello
A cell is the smallest living unit.
Eric
Hi I'm new in this group can someone please help with the list features shared by plants and charopytes that are not shared with most other eukaryotes
Wendy
iz a basic units of a living thing?
simongc
what is eutrophication
Chinaza Reply
hi.. I'm asking a question about HIV infection.... Can HIV infection transmitted from Mother to unbron child?
Khushboy
Eutrophication is an enrichment of water by nutrient salts that causes structural changes to the ecosystem such as: increased production of algae and aquatic plants, depletion of fish species, general deterioration of water quality and other effects that reduce and preclude use.
Khushboy

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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