Plant-Water Relations

• Importance of Water to Plants

  • Photosynthesis: source of electrons and carbon uptake
  • Structural: mechanical support; tension from water allows water to grow vertically
  • Transport: transfer of materials
  • Temperature regulation: prevent desiccation of plants at a certain amount; hence, plants do not dry up
• Properties of Water

  • Polar molecule - due to differences in electronegativity
  • Forms hydrogen bonds with itself - strongest among the Intramolecular Forces
• Cohesion - water molecules stick with themselves by pulling on one another
• Adhesion - water molecules stick to surfaces / other surfaces
• Surface Tension - occurs between the polar and non-polar structures
• High-Specific Heat Capacity - water absorbs the heat from sunlight received by plants, hence it can regulate the temperature of plants
• Capillary Action - movement of water across pores
• What governs water transport?

  Diffusion, Osmosis, Bulk-Flow (Mass Flow)
• Diffusion

  • random of movement of molecules in which net flow of matter moves along the concentration gradient (area of high concentration to lower concentration)
• Factors that affect diffusion:

  • concentration gradient - the steeper the gradient, the faster diffusion
  • Size and density of molecules - the larger molecules and denser, the slower the diffusion
  • Distance - the higher the distance, the slower the diffusion
  • Temperature - the higher the temperature, the faster the diffusion
  • Resistance - depends on the medium where molecules travel
• Osmosis - a solvent flow across a semipermeable membrane from lower to higher solute concentration regions
• Bulk-Flow (Mass Flow) - overall movement of water in response to differences in the potential of water

  • Greatly affected by gravity from higher to lower potential energy
  • Pressure from positive to negative pressure
• Water Potential

  • potential energy of water per unit volume relative to pure water
  • determines the tendency of water to move from one area to another
• Pure Water - 1 atm at 25 degrees - no solute no pressure
• Higher potential energy = higher likelihood for water to move
• Water always moves from a higher to a lower water potential
• Solute Potential (s)

  • also called as osmotic potential
  • Tendency of water molecules to move from a hypotonic solution to a hypertonic solution across a semi-permeable membrane
  • Plant cells will always have a negative solute potentialsince there are ions inside the plant cells (more solutes in the cytoplasm). 
• Hypertonic environment - cell shrink (plasmolyzed)

  Isotonic - flaccid

  Hypotonic - turgid
• Increase in solutes will result to negative Solute potential and hence lowering total water potential
• Pressure Potential (P)

  • turgor pressure
  • Hydrostatic pressure to which water in a liquid phase is subjected
  • May be positive or negative
  • Positive pressure: molecules push each other
  • Negative pressure: molecules pull on each other
  • At the cellular level, pressure and solute potential is always connected
  • Hypertonic = water diffuse out
  • Hypotonic = enters the cell
  • Isotonic = 0 pressure
• Gravitation Potential (G)

  • Gravitational potential energy of water /unit vol
  • Height difference of the water column; 
  • Becomes more positive by 0.1 MPa for each 10m of elevation (higher height; positive G)
• Matric Potential (M)

  • caused by the attraction of water molecules to itself (cohesion) or to other molecules (adhesion)
  • adhesion of water molecules to undissolved structures of the system (Soil particles); most often has a negative value
  • More soil = slow movement of water
  • negative water potential = higher soil
• Solution A has more solutes than Solution B, which solution has a more negative and less negative water potential?

  Solution A has more solutes, more negative solute potential and more negative water potential.

  Solution B has less solutes, less negative solute potential, and less negative water potential
• Sol A has more solutes than Sol b, what will be the direction of the movement of water?

  Water will move from Solution B to Solution A. Water will always move from less negative water potential to more negative water potential.
• Sol A has more solutes than Sol B. What will happen to the solute  potential of Solution B if the solute concentration is increased?

  Since solute is increased, the solute potential will become more negative, and overall lowering water potential
• Sol A has more solutes than Sol B. What will happen to the water potential of Solution A if the pressure is applied to it?

  Since positive pressure is applied, the solute potential will become more negative, and hence the water potential is also more negative
• Sol A has more solute than Sol B. What will happen to the water potential of Solution B if its elevation is increased?

  The increase in elevation causes an increase in gravitational potential, and hence increasing water potential
• Sol A has more solutes than Sol B. What will happen to the direciton of the movement of water with the use of sand in Solution B?

  None. Sand is insoluble in water so it does not affect the osmotic potential and water potential of cells.