Water Movement

• Diffusion - movement of solutes (molecules) from an area of higher concentration to lower concentration - want to spread out
• • Osmosis - movement of water from low solute concentration to high solute concentration
• • Isotonic - concentration same inside as outside of cell
  • Hypertonic - lots of solute in solution, water moves out of cell to dilute
  • Hypotonic - solution has less concentration of solute, water moves into cell
• Water Potential

  • Net movement of water results from differences in water potential
  • Potential energy of the water compared to that of pure water at atmospheric pressure and room temperature (reference conditions)
  • Involves: osmosis (solute concentration), gravity, mechanical pressure, matrix effects including capillary action
  • Measured in megapascals (MPa), SI unit of pressure = 1 N/m2
  • Represented by ψ
• Water Potential (Part 2)

  • Pure water at atmospheric pressure & room temperature has a water potential of 0 Mpa
  • Water potential has two components, water solute potential and pressure potential
  • Solutions that have high solute concentration have a low solute potential
  • Pressure potential is essentially water movement according to pressure applied to it
  • Wall pressure - cellulose resists movement
  • Turgor pressure - expanding volume of cell pushes membrane out
• Water Potential in Trees: Roots

  • Moist soil has a water potential that’s higher in the soil than in the roots
  • The roots have more solute in them - water wants to go from water in soil to plant cell via osmosis
  • Xylem cells also actively uptake minerals - use ATP energy to pump into the cells to get greater solute concentrations
• Water Potential in Trees: Tree

  • Compare tree potential to atmosphere potential
  • Atmosphere has water pressure of 0 Pa
  • Temperature + humidity affects it
  • Transpiration - water leaves plant via stomata
• Water Transportation Hypotheses

  • Root pressure - pressure potential in roots drive water up against gravity
  • Capillary action draws water up the xylem
  • Cohesion-tension - transpiration pulls water up - one molecule goes, drags others with it
• Endodermis - inside, surrounds vascular bundles
• Water gets into plant via 1 of 3 routes.

  • Symplastic route - through plasmodesmata, very small, don’t allow transport of things like chloroplasts
  • Transmembrane route - goes across the membrane through water channels called aquaporins
  • Apoplastic route - within porous cell wall - gets blocked by endodermis so has to go through one of the other two routes to get through
  • Endodermis has Casparian strip - waxy suberin, waterproof
  • Has to cross through 2 membranes, into endodermis and out of it
• Water Movement via Root Pressure

  • Root pressure is from water and ions entering root
  • Pushes water up xylem
  • Influx of ions into the roots lowers water potential, drawing in water from nearby cells and creating a positive pressure that forces water up the xylem
  • Guttation due to root pressure can force water droplets out of leaf margins
• Water Movement via Capillary Action

  • Involves three forces - surface tension, adhesion, cohesion
  • Surface tension - water doesn’t want to be disturbed, has surface tension - creates upward pull
  • Adhesion - ability for water molecules to adhere to something - creates meniscus - smaller vessel, lower meniscus
  • Cohesion - water is polar molecule, molecules stick together (hydrogen bonds)
  • Root pressure + capillary action can only move water a few meters against gravity
• The Cohesion-Tension Theory

  1. Water Vapour. Atmosphere is drier, warmer, evaporation happens. Stomata are open, water wants to leave and enters atmosphere.
  2. Water comes out of cells that surround guard cells on the stomata because of evaporation. Fills up the space left behind.
  3. Water is pulled out of the xylem. Because it’s attached to other water molecules farther down, it brings it with it.
  4. Root cells actively pumping in ions to make themselves have a higher concentration of solutes so that water will want to leave the soil to go into the roots.
• Xylem Cells and Water Movement

  • Xylem cells are dead at maturity
  • Two types of cells: tracheids (have pits), vessel elements (have pits and perforations that allow for a continuous flow of water)
  • Cells reinforced by lignin (very strong and rigid) - allowed for evolution of really tall trees
• Transpiration

  • Occurs whenever two conditions are met - stomata are open, and the atmosphere is drier than the air inside leaves
• Factors Reducing Soil Water Potential

  • When soil water potential drops, water is less likely to move into the roots
  • Salty soils have low Ψ due to high solute concentrations 
  • Dry soils water adheres tightly to soil particles, lowering Ψ 
  • Irrigation increases solute concentration over time, lowering Ψ
• Photosynthesis-Transpiration Compromise

  • Salt-adapted species accumulate and tolerate solutes in roots, lowering their Ψs
  • Dry-adapted species tolerate low Ψs (Ψs of leaves drops during summer)
• Features That Reduce Water Loss by Transpiration

  • Thick, waxy cuticle to reduce water loss
  • Position of the stomata - full-bearing sun is going to accentuate transpiration loss - plants with exposed leaves have stomata on the underside to reduce transpiration loss
  • Trichomes - fine hair that coat the leaves, disrupt wind flow so less evaporation
  • Needle-like leaves - less surface area
  • Crassulacean acid metabolism (CAM) plants (succulents) open stoma at night
• Stomata Opening (Part 1)

  • Open and close by turgor pressure
  • Conditions with higher carbon dioxide demands increase turgor pressure, open up stomata
  • Light is dominant stimulus - as light levels increase, stomata opens
  • Guard cells are the only epidermal cells with chloroplasts
• Stomata Opening (Part 2)

  • Blue light specifically activates phototropin (photoreceptor kinase protein) - kinase is a type of enzyme that adds a phosphate to another group
  • Turned on by blue light, activate a proton pump in the cell membrane of the guard cells + protons are pumped out of the cell - setting up an electrochemical gradient
  • Potassium comes in, triggers breakdown of stored starch that breaks down into sugars which create an osmotic potential, water comes into guard cells via osmosis
• Stomata Opening (Part 3)

  • Cellulose microfibrils - bands that prevent even distribution and opening of guard cell - restrict movement so pore in the middle doesn’t always open
  • When there’s no light, photosynthesis stops
  • No blue light, cell isn’t pumping out ions, starch isn’t breaking down
  • Guard cells have a lower solute concentration than neighbouring cells, and water moves out of guard cells by osmosis
  • Guard cells become flaccid