The Importance of Water

BISC 367 - Plant Physiology Lab Spring 2009 Plant Biology Fall 2006 Notices: • O2 electrode data • IRGA data • Reading material (Taiz & Zeiger): • Chapter 3, Water and Plant Cells • Chapter 4, Water Balance of Plants BISC 367 BISC 367 The Importance of Water • Physiological aspects Movement of water in plants • Molecular diffusion BISC 367 – Water moves from an area of high free energy to area of low free energy • i.e. down a conc. gradient – Described by FICKS LAW Js = -Ds dcs/dx Js = flux density for s (mol m-2 s-1) Ds = diffusion coefficient dcs/dx = difference in water conc. over distance x Movement of water in plants • Bulk flow – Movement of water in response to a pressure gradient • Analogous to water flowing in a pipe BISC 367 – Affected by: • Radius of pipe (r) • Viscosity of liquid (h) • Pressure gradient dyp/dx – Described by POISEUILLE’S equation: vol. flow rate (m3 s-1) = (pr4/8h)(dyp/dx) Movement of water into a plant cell occurs by osmosis • 2 mechanisms: BISC 367 – Diffusion across the membrane – Bulk flow across aquaporins (water filled pores) Movement of water into a plant cell occurs by osmosis • Water uptake is driven by a free energy gradient composed of: – Concentration gradient – Pressure gradient BISC 367 Free energy gradient for water movement is referred to as a Water Potential Gradient BISC 367 Water Potential • Water potential (Yw) is equivalent to the free energy of water & influenced by: – Concentration (or activity) – Pressure – Gravity • Yw is the free NRG of water per unit volume (J m-3) – Divide chem. pot. of water (J mol-1) by the partial molal vol. (m3 mol-1) – Units equivalent to pressure (Pa) BISC 367 Water Potential • Yw (Mpa) is a relative quantity and defined as: Chemical potential of water (in pressure units) compared to the chemical potential of pure water (at atm. pressure and temp.), which is set to zero BISC 367 Water Potential Yw = Ys + Yp + Yg Ys = Solute component or osmotic potential Result of dissolved solutes that dilute water (entropy effect) Estimated using van’t Hoff’s eqtn (see p.44) Yp = Pressure component or pressure potential Yp inside a cell is positive = turgor pressure Yp in the apoplast is negative Note: Yp of pure water is zero, therefore not a measure of absolute pressure BISC 367 Water Potential Yg = Gravity component Ignored unless considering vertical water movement >5m Dependent on: Height of water above ref. state (h) density (rw) acceleration due to gravity (g) Yg = rwgh rwg = 0.01MPa m-1 Plant Water Relations Cell (protoplast) water relations Yw = 0 Ys(p) Yp(p) Yw = 0 BISC 367 Cell wall (apoplast) water relations Ys(a) Yp(a) Yw(p) Yw(a) Whole plant water relations p = protoplast a = apoplast Ys(a) Yp(a) Ys(p) Yp(p) Yw = 0 Yw(p) Yw(a) Yp is sensitive to small changes in cell volume • BISC 367 Relates to rigid cell wall, illustrated by Hofler diagram – Plot of Yw & its components against relative cell vol. Initial drop in cell vol (5%) is accompanied by a sharp drop in Yp and Yw As cell vol falls <90%, decreased Yw is accounted for by a lowered Ys as [solute] increases • • Yp is sensitive to small changes in cell volume • BISC 367 Slope of Yp curve yields the volumetric elastic modulus (e) – e is a function of the rigidity of the cell wall – High value indicates a rigid wall for which a small vol. change translates into a large drop in Yp – e decreases as Yp falls b/c walls are rigid only when Yp is high BISC 367 Typical values for Yw • • Yw = -0.2 to -0.6 MPa – Plants are never fully hydrated due to transpiration Ys = -0.5 to -1.5 MPa – Plants living in saline or arid environments can have lower values • Yp = 0.1 to 1.0 MPa – Positive values needed to drive growth and provide mechanical rigidity Measuring Yw Scholander’s pressure bomb BISC 367 A leaf or shoot is excised and placed in the chamber • Cutting the leaf breaks the tension in the xylem causing water to retreat into the surrounding cells Pressurizing the leaf chamber returns water to the cut surface of the petiole • The amount of pressure to return water to the cut surface equals the tension (Yp) present in the xylem (but is opposite in sign) before excision From Plant Physiology on-line (http://4e.plantphys.net/) Values obtained approximate the tension in the xylem and are used as a measure of Yw • Strictly speaking to know the actual Yw some xylem sap should be collected to Measuring Yw Relative water content Assesses the water content of plant tissues as a fraction of the fully turgid water content • relevant when considering metabolic / physiological aspects of water deficit stress Considered to be a better indicator of water status and physiological activity Captures effects of osmotic adjustment • Osmotic adjustment lowers the Yw at which a given RWC is reached BISC 367 Simple technique: • Leaf disks are excised, weighed (W) then allowed to reach full turgidity and re-weighed (TW). disks are dried to obtain their dry weight (DW) . RWC (%) = [(W – DW) / (TW – DW)] X 100 Leaf