BIO203: Lecture 6 - Water Relations 1

BIO203: Lecture 6 - Water Relations 1

• Overview: Plant cell water relations, water loss (transpiration), C4 and CAM water use efficiency.

• Water & Plants: Why Care? Lab 4 discussion. Human civilization relies on 6 inches of soil and rain.

• Water Relations: Key Concepts

  • Water relations (at the cellular level)
  • Water potential (Ψ)
  • Movement: Water uptake (roots), transport (xylem), and loss (stomata).
  • Plants grow where water is available.

• Why is water important to plants?

  • Cell contents are mostly water (80-95%).
  • Water is a solvent, crucial for biochemical reactions (photolysis, hydrolysis).
  • Necessary for cell expansion and growth.
  • Over 99% of water entering a well-watered plant is transpired directly.

• Water: Plant shape & growth

  • Water loss via transpiration regulates temperature (e.g., cool grass).
  • Adaptations for water conservation include narrow leaves, thick cuticles, and sunken stomata in grasslands.

• Water: Cell elongation & growth

  • Water movement into cells and vacuoles drives cell expansion, loosening cell walls.
  • Vacuoles are mostly water. Cell expansion requires minimal extra cytoplasm.

• Water: Calculating water content

  • Most cells contain 90%+ water; Fresh cut wood is over 65% water.
  • FW (fresh weight) = water + cell wall + cell contents
  • DW (dry weight) = cell wall + cell contents
  • Water content = FW - DW.
  • Oven temperature is approximately 110°C.

• Water relations of a single cell

  • Selective membrane permeability (plasmalemma and tonoplast).
  • Osmosis = water movement across membranes based on relative solute concentrations.
  • Plant cells have cell walls and vacuoles.
  • Cell walls maintain plant cell integrity.

• Selective Permeability

  • Water passes freely through membranes (aided by aquaporins).
  • Salts and other solutes pass through selective channels and transporters.
  • Differing concentrations inside/outside cells/organelles create gradients for water movement, membrane potentials and signals.

• Key Points: Water relations of a cell

  • Membranes are selectively permeable (not semi-permeable).
  • Water moves down a water potential gradient (high to low).
  • Relationship between the vacuole and the surrounding medium.
  • Cell walls provide rigidity.

• Water Potential (Ψ)

  • "Drives" water movement in/out of cells/organelles.
  • Ψ is influenced by osmotic potential (solute concentration) and pressure potential (physical pressure on the water, e.g., turgor pressure).

• Osmotic potential (Ψπ or Ψs)

  • Pure water has a water potential of zero.
  • Solutes decrease water potential.
  • Difference between pure water potential and water with solute equals Ψπ

• Pressure potential (Ψp)

  • Pure water has a water potential of zero.
  • Water gains energy under pressure, becomes more positive.
  • Difference between pure water potential and water under pressure = Ψp

• Osmometer

  • The principle of osmosis. Water moves from lower to higher solute concentrations across a semipermeable membrane.

• The cell as an osmometer

  • Ψπ of liquid (water + sugars/salts) in vacuoles, more negative than surrounding solutions.
  • Water flows into cell.
  • If water is drawn from the surrounding cells, the cell wall will eventually become rigid and support the cell shape (as turgor builds up).

• Water Loss

  • Cuticular transpiration.
  • Lenticular transpiration.
  • Stomatal transpiration (Most of water loss).

• Transpiration (evapotranspiration)

  • Vapor pressure difference between inside leaf and atmosphere.
  • Simulation: water loss from a dish.

• Stomata

  • Stomatal opening/closing controlled by guard cells.
  • Size of stomatal opening is controlled by guard cells.

• Water loss from stomata

  • Water is lost by evaporation.
  • Vapor pressure difference determines evaporation rate.
  • Water movement from surrounding leaf cells replaces lost water.

• Stomata Opening and Closing

  • Proton pumps move H+ out of guard cells.
  • K+ and Cl− ions move into guard cells, resulting in water entry and stomata opening.
  • Conversely, K+ and Cl− ions moving out causing stomata to close

• Factors affecting stomatal activity

  • Opening: Light, Low CO2, Overheating.
  • Closing: Water loss, Abscisic acid (ABA), Root to shoot signal

• Factors affecting transpiration

  • Air Movement: Increased air movement increases transpiration rate.
  • Temperature: Higher temperature increases transpiration rate.
  • Pollutants: SO2 can cause stomata to remain open, dust can block stomata

• Photosynthesis II

  • Calvin Cycle overview: Ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco), steps in C3 fixation (CO2 addition to 5-C RuBP), outputs (G3P)

• RuBisCO

  • RuBisCO is also an oxygenase, leading to photorespiration when O2 is high; this is a wasteful process.

• Photorespiration costs

  • RuBisCO is most efficient at low O2 : high CO2.
  • Increased oxygenase activity reduces the efficiency of photosynthesis in C3 plants.

• RuBisCO & Water-Use Efficiency

  • Plants open stomata to take in CO2, leading to water loss.

• C3 Plants

  • CO2 is fixed in mesophyll chloroplasts by Rubisco.

• C4 Plants

  • Light reactions and the Calvin cycle are physically separated.
  • CO2 is fixed in mesophyll cells using PEP carboxylase.
  • C4 pathway generates malate and releases CO2 in bundle sheath cells for the Calvin cycle.

• Comparison of C3, C4, and CAM plants (Summary table in page 55).

• C4: Adaptation to environment

  • Higher percentage of C4 in tropics.
  • C3 plants are more common in temperate environments

• C4 Water Use Efficiency

  • C4 typically uses less water to produce the same amount of dry matter compared to C3.

• Photosynthetic adaptations to drought & arid environments

  • Open stomata at night to collect CO2 and fixing it in the form of malate, then releasing CO2 during the day for use in the Calvin cycle.

• Crassulacean Acid Metabolism (CAM)

  • CAM plants import CO2 at night, with stomata open.
  • Malate is stored in the vacuole.
  • Released CO2 during day for Calvin cycle.

• CAM: Costs & Benefits

  • Dramatically reduced water loss.
  • Energetically expensive.

• Coping with dry conditions

• Metabolic approaches: C4 and CAM.

• Morphological and developmental approaches: Trichomes, stomata position, thickened epidermal cells, thickened cuticle, bulliform cells.

• Reducing surface area for water loss

  • Reduction in the number of leaves, succulents, and cacti; less permeable cuticles; thicker epidermis; bulliform cells.

• Key points summary water loss

  • Types of water loss (cuticular, lenticular, stomatal)
  • Stomatal regulation of water loss
  • Water loss effects at a cellular level.
  • Adaptations to reduce water loss.