Water Transport in Plants: Xylem

xylem is the tissue primarily response for

movement of water

plants are able to transport water through a combination of

• water potential

• evapotranspiration

• stomatal regulation

all without using any cellular energy

water potential

(Ψ) measure of the potential energy in water based on potential water movement between 2 systems

• water potential = difference in potential energy between any given water sample and pure water (at atmospheric pressure + ambient temp)

• expressed in megapascals (units of pressure)

• can be positive or negative

• Ψsystem = Ψs + Ψp, where Ψs = solute potential, and Ψp = pressure potential

add more dissolved solute with ___ the water potential

decrease

removing dissolved solutes will ___ the water potential

increase

adding more pressure (positive pressure) will ___ the water potential

increase

removing pressure (including creating a vacuum, or negative pressure) will ___ the water potential

decrease

water always moves

from a region of high water potential to an area of low water potential

• until it equilibriates the water potential of the system

  • at equilibrium, there is no difference in water potential on either side of the system

  • Ψ^soil must be > Ψ^root > Ψ^stem > Ψ^leaf > Ψ^atmosphere

potential energy of pure water =

zero

in order for transpiration to occur:

Ψ^soil must be > Ψ^root > Ψ^stem > Ψ^leaf > Ψ^atmosphere

solute potential (osmotic potential) Ψ_s

• proportion to the number of dissolved molecules

• is 0 for pure water

• dissolving more solutes = decreased water potential → solute potential is negative bc of high solute concentration of cytoplasm

• as long as the water pot in the plant root cells is lower than the water pot in the soil, then water will move from the soil into a plant’s root cells via osmosis

• plant cells can metabolically manipulate Ψ_s by adding/removing solute molecules to increase water uptake from the soil during drought conditions

pressure potential (turgor potential) Ψ_p

• the physical pressure on a solution

• may be positive or negative

• positive pressure (compression) increases Ψ_p and negative pressure (vacuum) decreases Ψ_p

• pos pressure inside cells is contained by a rigid cell wall → produces turgor pressure

• plant cells can indirectly manipulate Ψ_p via their ability to directly manipulate Ψ_s and by the process of osmosis: if a plant cell increases the cytoplasmic solute concentration, then Ψ_s will decline and water will move into the cell by osmosis, causing Ψ_p to increase. 

• stomatal openings allow water to evaporate from the leaf, reducing Ψ_p and Ψ_total of the leaf and increasing the water potential difference between the water in the leaf and the petiole, thereby allowing water to flow from the petiole into the leaf

• example of the effect of turgor pressure is the wilting of leaves and their restoration after the plant has been watered. Water is lost from the leaves via transpiration (approaching Ψ_p = 0 MPa at the wilting point) and restored by uptake via the roots

hypotonic

• the solution on one side of the membrane where the solute concentration is less than the other side

hypertonic

• the solution on one side of the membrane where the solute concentration is greater than the other side

transpiration

evaporation of water from the plant stomata resulting in the continuous movement of water through a plant via the xylem, from soil to air, without equilibrating

• Ψ^soil must be > Ψ^root > Ψ^stem > Ψ^leaf > Ψ^atmosphere

• ATP is not required- energy source that drives the process of transpiration is the extreme difference in water potential between the water in the soil and the water in the atmosphere

  • relies on a water potential gradient- water potential decreases at each point from soil to atmosphere as it passed through plant tissues

impact of soil and environmental conditions on the plant water potential gradient

• if soil becomes too dry → decreased solute potential (dt same amount of solutes dissolved in smaller quantity of water) as well as decreased pressure potential in severe droughts (dt negative pressure/vacuum in soil due to loss of water volume)

• if water potential becomes sufficiently lower in the soil that in the plant’s roots → water will move out of the root and into the soil

pathways of water and mineral movement in the roots

• symplast

• transmembrane pathway

• apoplast

the symplast

“shared cytoplasm”

• pathway where water and minerals move from cytoplasm of one cell into the next via plasmodesmata that physically join diff plant cells until eventually reaching the xylem

the transmembrane pathway

water moves through water channels present in plant cell plasma membranes, from one cell to the next until reaching the xylem

the apoplast

“outside of the cell”

• pathway where water and dissolved minerals never move through a cell’s plasma membrane but instead travel through the porous cell walls that surround plant cells

• water and minerals in this pathway do not encounter a filtering step (within plasma membrane for other pathways) until they reach a layer of cells known as the endodermis (separate vascular tissue from the ground tissue in the outer portion of the root)

  • endodermis is only present in roots and serves as a checkpoint for materials entering root’s vascular system

    • waxy region of suberin (waxy substance) on the walls of endodermal cells = casparian strip- forces water and solutes to cross the plasma membranes of endodermal cells instead of slipping between the cells → ensures only materials required by the root pass through the endo dermis

explanation for movement of water up against gravity

• root pressure pushes water up

• capillary action draws water up within the xylem

• cohesion-tension pulls water up the xylem

root pressure

• relies on positive pressure that forms in the roots as water moves into the roots from the soil

• water moves into the roots from the soil by osmosis → intake of water increases Ψ_p in root xylem → pushes water up

• when stomata are closed at night preventing water from evaporating from the leaves, root pressure results in guttation, or secretion of water droplets from stomata in the leaves

• root pressure can only move water against gravity by a few meters, so it is not sufficient to move water up the height of a tall tree

capillary action

tendency of a liquid to move up against gravity when confined in a narrow tube (capillary)

• ex: a meniscus is an effect of capillary action

• occurs due to three properties of water:

  • surface tension: hydrogen bonding between water molecules is stronger at the air-water interface than among molecules within the water

  • adhesion: molecular attraction between unlike molecules- molecules of xylem cell walls and water molecules

  • cohesion: molecular attraction between like molecules- in water, it is due to hydrogen bonding between water molecules

• on its own, capillarity can work well within a vertical stem for up to approximately 1 meter, so it is not strong enough to move water up a tall tree

cohesion-tension

• most widely accepted model for movement of water in vascular plants

• combines the process of capillary action with transpiration (evaporation of water from the plant stomata) → transpiration is the main driver of xylem water movement combined with the effects of capillary action

• cohesion-tension model:

1. transpiration (evaporation) occurs bc stomata in leaves are open to allow gas exchange for photosynthesis → as transpiration occurs, evap of water deepens meniscus of water in the leaf → creates negative pressure (tension/suction)

2. tension created by transpiration pulls water in the plant xylem → drawing water upward like sucking water through a straw

3. cohesion causes more water molecules to fill the gap in the xylem as the top-most water is pulled toward end of the meniscus within the stomata

• transpiration results in a phenomenal amount of negative pressure within the xylem vessels and tracheids, which are structurally reinforced with lignin to cope with large changes in pressure. The taller the tree, the greater the tension forces (and thus negative pressure) needed to pull water up from roots to shoot