Water and Plant Cells• Properties of Water• Water Potential• Components of Water

Potential• Water relations

Water in plant life

• Corn yield as a function of water availability.• Productivity of ecosystems as a function of

annual precipitation.• Most water lost through transpiration.

Properties of Water

• Polar molecule • Thermal Properties• Specific Heat • Melting Point • Heat of fusion • Boiling point • Heat of vaporization

Solvent properties

• Dielectric constant • Cohesion and

Adhesion

Water Transport Processes

• Diffusion • Pressure-driven bulk

flown (Aquaporins)• Osmosis

Water movement

• Fick’s law of diffusion• Js = - Ds (∆Cs / ∆x)• Js is rate of transport (mol/m2s1)• Ds = diffusion coefficient through a medium• (∆Cs / ∆x) = concentration gradient of a substance

separated by distance x.

Water Potential

• Chemical activity of water • Components of water potential

• Ψw = Ψs + Ψp + Ψg

Components of water potential: Solutes (osmotic potential)

• Ψs = 0 for pure water Or always negative

• Ψs = - R T Cs

• R = 8.32 J/mol K• T = absolute

temperature• Cs = solute

concentration (mol/L)

Pressure potential

• Hydrostatic pressure (Ψp )• Positive (turgor pressure)• Negative (tension)• Ψp = 0 for water at ambient pressure (0 MPa)

Gravity

• Ψg = pwgh

• Pw = density of water• g = acceleration due to gravity• pwg = 0.01 MPa m-1

• At cell level we can ignore gravity or for small plants.

Concepts of water potential

Plant cell volume• Small changes in

volume cause large changes in turgor

• 10-15% change in volume, turgor goes to zero

• Modulus of elasticity (ε) or rigidity of cell wall.

• High ε wall is stiff, low εwall is soft

Physiological changes during drought