Water Transport into Plant Cells & Cell Expansion HORT 301 – Plant Physiology September 3, 2008 Taiz and Zeiger - Chapter 3 (p ) & Chapter 15 (p ), Web Topics 3.3 through 3.7, Passioura (2001) Encyclopedia of Life Sciences Diffusion, Bulk Flow and Osmosis – water transport processes in plants Water Potential Drives Water Transport Into and Out of Cells – chemical and pressure potentials that drive water movement Water Potential and Turgor Pressure: Cell Volume Regulation/Cell Expansion – plant fresh weight growth

Diffusion, Bulk Flow and Osmosis – major water transport processes in plants Diffusion – random motion of molecules that results in net movement from high to low concentration Diagram depicts red and blue molecules each concentrated to one side (initial), at an intermediate period and after equilibrium Note: random motion continues even after equilibrium

Diffusion rate – concentration dependent Fick’s law, J s = -D s Δc s /Δx J s – transport rate/flux density, amount of s crossing a unit area x per time Δc s – concentration gradient of substance s Δx – distance between the separated concentrations D s – diffusion coefficient - capacity of a substance to move through a specific medium - (minus sign) - indicates movement is down a concentration gradient (high to low concentration), mol m -2 s -1 Diffusion facilitates solute movement over short distances, i.e. into and out of cells and within cells, but is too slow for long distance transport (root to the shoot) Glucose diffusion across the cell (~50 µm) takes 2.5 seconds but over one meter takes 32 years

Bulk flow (mass flow) – movement of a large number of molecules en masse usually driven by pressure, but other forces (e.g. gravity) may drive bulk flow Examples of bulk/mass flow of water – river flow, rainfall, and pressure- driven movement of water through a garden hose Pressure-driven bulk flow is the primary process for long distance water transport, movement from roots to shoots through the xylem and water transport in soil

Osmosis – movement of water (any liquid) through a selectively permeable membrane, diffusion or pressure-driven bulk flow Osmosis is the process that moves water into and out of living plant cells and, in most instances, is mediated by diffusion Water diffuses across a plant membrane from high to low water concentration Solutes decrease the water concentration of a solution Water move across the plasma membrane (into or out of the cell) from higher → lower water concentration (lower → higher solute concentration)

Water channels (aquaporins) facilitate water (transport) across the plasma membrane – diffusion of water into or out of the cell from high to low water concentration Aquaporin pore opening and closing (gating) - regulated by different stimuli, e.g. pH, Ca 2+, etc, movement (transport) Water channels (aquaporins) – transmembrane protein pores across membrane lipid bilayers Water does not readily diffuse across the hydrophobic lipid bilayer

Water Potential (Ψ w ) Is the Principal Force for Water Transport in Plants – nomenclature used by plant physiologists for the force that drives water transport Water potential (Ψ w ) gradient defines the free energy gradient for passive movement of water, higher to lower free energy Water potential (Ψ w ) is the free energy of water per volume, expressed as pressure units, bar or pascal (Pa), one bar = 0.1 megapascals (MPa)

Reference state for water potential (Ψ w ) - pure water at ambient temperature and pressure, Ψ w = 0, Water containing solutes has a negative water potential (-Ψ w ), lower free energy than pure water Pure water diffuses into aqueous solutions containing solutes (negative water potential, Ψ w < 0 (-MPa) Water transport Summary: higher → lower water concentration (lower → higher solute conc) higher → lower Ψ w (more negative)

Plant cell water potential (Ψ w ) is affected primarily by solute concentration, pressure and gravity Ψ w = Ψ s + Ψ p +Ψ g Ψ w (water potential) - free energy of water, Ψ w of pure water = 0, more negative water potential (Ψ w ) is lower free energy Ψ s (solute (osmotic) potential) – solute concentration effect on Ψ w, dissolved solutes lower free energy of water by reducing the concentration of water van’t Hoff equation - Ψ s = RTc s R – gas constant, 8.32 J mol -1 K -1 T – Kelvin (K), 0°C = 273 K c s – osmolality (mol of solute per liter, ideal solute = 1, includes dissociation constant correction)

Ψ p (hydrostatic pressure/pressure potential/turgor pressure) Pure water at ambient pressure - Ψ p = 0 Positive hydrostatic pressure/pressure potential (push/turgor in cells) Negative hydrostatic pressure/pressure potential (tension) Ψ g (gravity, gravitational pull) - causes water to move downwards but has negligible impact on water transport in plants Effect of gravity on water at the top of 10 m column is 0.1 MPa, seawater has a solute potential of ~-2.8 MPa So for functional simplification, the following equation defines water potential: Ψ w = Ψ s + Ψ p See Web Topic 3.5 for discussion of matric potential, also disregarded for our consideration of water potential in plants

Water potential (Ψ w ) of a solution becomes more negative by the addition of sucrose Solution of 0.1 M sucrose: - Ψ w = MPa Ψ s = RTc s = MPa, Ψ p = 0 MPa MPa (Ψ w ) = (Ψ s ) + 0 (Ψ p ) Pure water: water potential (Ψ w ) = 0 Ψ w = Ψ s + Ψ p = 0 Ψ s = 0 and Ψ p = 0 Ψ w = Ψ s when Ψ p = 0

Water transport into and out of plants cells is driven by the water potential (Ψ w ) gradient – passive, water moves from higher to lower (more negative) Ψ w Water movement into a cell - flaccid cell (Ψ w = MPa) immersed into a 0.1 M sucrose solution (Ψ w = MPa) Water moves into the cell because the Ψ w inside the cell (symplast) is more negative than outside of the cell (apoplast) Flaccid cell: - Ψ w = MPa, Ψ s = MPa and Ψ p = 0 (no turgor pressure) Cell immersed into 0.1 M sucrose solution (Ψ w = MPa): water moves into the cell until the symplastic and apoplastic Ψ w reach equilibrium, MPa

Initially, the protoplasm volume increases until constrained by the cell wall Water potential (Ψ w ) equilibrium is established due to hydrostatic pressure/turgor pressure ( Ψ p ) build-up, Ψ p = MPa Cell walls facilitate Ψ p increase Ψ p – driving force for cell expansion (volume increase), which is due to uptake of water, Ψ w gradient between the apoplast and symplast (0.1 M sucrose solution)

Water movement from the cell - turgid cell in a 0.1 M sucrose solution, then immersed into 0.3 M sucrose solution causing water loss and volume reduction Turgid cell in 0.1 M sucrose solution: Ψ w(apolast) = Ψ w(symplast) = MPa, Ψ s = MPa and Ψ p = MPa Cell after immersion into 0.3 M sucrose solution:, Ψ w(apoplast) = MPa,Ψ s = MPa and Ψ p = 0 MPa Water potential equilibrium, - Ψ w(apoplast) = Ψ w(symplast) = MPa, turgor pressure decreases to zero, water is lost until Ψ w equilibrium ( MPa) is reached

Water movement into and out of cells summary: Water moves from higher (less negative) to lower (more negative) Ψ w Ψ w(apolast) = Ψ w(symplast) No water movement Ψ w(apoplast) higher (less negative) than Ψ w(symplast) Water moves into the cell and turgor pressure increases Ψ w(apoplast) lower (more negative) than Ψ w (sympast) Turgor pressure decreases and water moves out of the cell

Water Transport into Plant Cells & Cell Expansion HORT 301 – Plant Physiology September 3, 2008 Taiz and Zeiger - Chapter 3 (p ) & Chapter 15 (p ), Web Topics 3.3 through 3.7, Passioura (2001) Encyclopedia of Life Sciences Diffusion, Bulk Flow and Osmosis – water transport processes in plants Water Potential Drives Water Transport Into and Out of Cells – chemical and pressure potentials that drive water movement Water Potential and Turgor Pressure: Cell Volume Regulation/Cell Expansion – plant fresh weight growth

Water Potential and Turgor – Cell Volume Regulation and Cell Expansion – semi-rigid cell wall mechanically restricts water loss (cell volume reduction) as the plant is subjected to more negative Ψ w, e.g. during daylight, drought Turgor pressure (Ψ p ) – “buffers” the cell from water loss as apoplastic Ψ w decreases (more negative) Ψ p decreases and Ψ w equilibrium is established with minimal symplastic water loss

Water loss from the cell and reduction in cell volume increases as Ψ p (turgor pressure) approaches 0 Plant wilts (becomes flaccid) when cell turgor pressure approaches 0 ~15% volume reduction in this example Ψ w decreasing Ψ p decreasing

Turgor pressure (  p ) pushes on walls forcing cell wall expansion  w gradient facilitates water uptake for cell volume increase/expansion (fresh weight gain) Turgor and growth rate T Turgor Pressure (MPa) Growth rate is defined by the formula: GR = m(  p –Y) GR – growth rate m – wall extensibility  p – turgor pressure Y – yield threshold m is due to turgor pressure and biological processes (hydrolysis and biosynthesis

Plant water status affects critical physiological functions – water potential (Ψ w ) is a “signal” for numerous physiological processes