Water Movement through Plant Cells HORT 301 – Plant Physiology September 24, 2007 Taiz and Zeiger, Chapter 3 (p. 41-49), Web Topics 3.3 through 3.7 paul.m.hasegawa.1@purdue.edu

Diffusion, pressure-driven bulk flow and osmosis Water potential drives water movement (transport) into and out of cells Cell volume homeostasis Water movement (transport) in plants Plant water status affects critical physiological processes

Diffusion, pressure-driven bulk flow and osmosis – water movement (transport) processes Diffusion – net movement of molecules from high concentration to low concentration, down a concentration gradient

Diffusion rate - movement (transport) of a substance, concentration dependent, Fick’s law , Js = -Ds Δcs/Δx Δcs – concentration gradient of substance s Δx – distance moved Ds – diffusion coefficient - capacity of a substance to move through a medium Minus (-) sign - indicates movement is down a concentration gradient (high to low concentration) Diffusion facilitates substance/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

Pressure-driven bulk flow (mass flow) – movement of a large number of molecules en masse that is driven by pressure Example - pressure driven movement of water through a garden hose Bulk flow is the primary process for long distance water transport in plants (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 Plasma membrane (other cell membranes) - semi-permeable to water and solutes Osmosis is the process by which water moves into and out of a plant cell Water diffuses across a plant membrane from high to low water concentration Solutes decrease the water concentration of a solution i.e. water moves to the side of a membrane with higher solute conc

Water potential, driving force of water movement (transport) into and out of plant cells – water potential (Ψw), nomenclature used by plant physiologists, principal force responsible for osmosis into and out of plant cells Thermodynamically, water potential (Ψw) gradient defines the free energy/chemical potential gradient for passive movement of water, higher to lower free energy

Water potential (Ψw) is the free energy of water per volume, expressed as pressure units, bar or pascal (Pa), one bar = 0.1 megapascal (MPa)

Reference state for water potential (Ψw) - pure water at ambient temperature and pressure, Ψw = 0, i.e. water containing solutes has a negative water potential (Ψw) Pure water diffuses into aqueous solutions containing solutes (negative water potential (lower free energy), Ψw < 0 (-MPa)

Water potential (Ψw) in plants is affected primarily by solute concentration, pressure and gravity Ψw = Ψs + Ψp +Ψg Ψw (water potential) - free energy of water, Ψw of pure water = 0, more negative water potential (Ψw) is lower free energy Ψs (solute (osmotic) potential) – solute concentration effect on Ψw, dissolved solutes lower free energy of water by reducing the concentration of water van’t Hoff equation - Ψs = RTcs R – gas constant, T – Kelvin, cs – osmolality

Ψp (hydrostatic pressure/pressure potential/turgor pressure) Ambient pressure = 0 Positive hydrostatic pressure/pressure potential (turgor in cells) - push Negative hydrostatic pressure/pressure potential (tension) - pull

Ψg (gravity, gravitational pull) - causes water to move downwards but has negligible impact on water transport in plants i.e., the 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 - Ψw = Ψs + Ψp See Web Topic 3.5 for discussion of matric potential, also disregarded for our consideration of water potential in plants

Water transport into and out of plant cells – passive, along (down) the water potential (Ψw) gradient, higher (less negative) to lower (more negative) Ψw and free energy Water potential equilibrium (outside and inside of the cell) - Ψw(outside/apoplast) = Ψw(inside/symplast), no water movement

Water potential (Ψw) of a solution becomes more negative by the addition of sucrose Pure water - water potential (Ψw) = 0, Ψw = Ψs + Ψp = 0 Ψs = 0 and Ψp = 0 Solution of 0.1 M sucrose - Ψw = -0.244 MPa Ψs = RTcs = -0.244 MPa, Ψp = 0 MPa

Water movement into a cell along the water potential (Ψw) gradient - flaccid cell immersed into a 0.1 M sucrose solution Water moves into the cell because the Ψw inside the cell (symplast) is more negative than outside of the cell (apoplast) Flaccid cell - Ψw = -0.732 MPa, Ψs = -0.732 MPa and Ψp = 0 (no turgor) Cell immersed into 0.1 M sucrose solution (Ψw = -0.244 MPa) Water moves into the cell until the symplastic and apoplastic Ψw reach equilibrium, -0.244 MPa

Initially, the protoplasm volume increases until constrained by the cell wall Water potential (Ψw) equilibrium is established due to hydrostatic pressure/pressure potential/turgor (Ψp) build-up, Ψp = 0.488 MPa

Water movement from the cell - turgid cell in a 0 Water movement from the cell - turgid cell in a 0.1 M sucrose solution, then sucrose concentration (apoplast) is increased to 0.3 M Turgid cell in 0.1 M sucrose solution, Ψw(apolast) = Ψw(symplast) = -0.244 MPa Cell after increasing solution to 0.3 M sucrose, Ψw(apoplast) = -0.732 MPa,Ψs = -0.732 MPa and Ψp = 0 MPa) Water potential equilibrium, - Ψw(apoplast) = Ψw(symplast) = -0.732 MPa, turgor decreases to zero, water is lost until Ψw equilibrium (-0.732 MPa) is reached

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

Cell volume homeostasis - rigid cell wall regulates cell volume Positive hydrostatic pressure/pressure potential/turgor (Ψp) – facilitates cell expansion Increases the capacity of a cell to retain water (minimize volume reduction) as the apoplastic water potential Ψw fluctuates, e.g. light period of the day/night cycle, drought

Water loss (cell volume decrease) increases as Ψp approaches 0 Plant wilts (becomes flaccid) when cell turgor approaches 0

Water Movement through Plant Cells HORT 301 – Plant Physiology September 24, 2007 Taiz and Zeiger, Chapter 3 (p. 41-49), Web Topics 3.3 through 3.7 paul.m.hasegawa.1@purdue.edu

Diffusion, pressure-driven bulk flow and osmosis Water potential drives water movement (transport) into and out of cells Cell volume homeostasis Water movement (transport) in plants Plant water status affects critical physiological processes

Water movement (transport) in plants – transport rate is dependent on the driving force (water potential) and hydraulic conductance Jv = Lp(ΔΨw), Lp – hydraulic conductivity (resistances to water movement), ΔΨw – water potential gradient Membranes are principal resistances to hydraulic conductance

Water channels facilitate water movement (transport) across the plasma membrane – into and out of the cell Water channels (aquaporins) – membrane protein pores that facilitate water movement across lipid bilayer of membranes Aquaporin pore opening and closing (gating) - regulated by different stimuli, e.g. pH, Ca2+, etc, movement (transport)

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