BISC 367 Plant Physiology Lab Spring 2009 Plant

BISC 367 - Plant Physiology Lab Spring 2009 Plant Biology Fall 2006 Notices: • O 2 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 BISC 367 • Molecular diffusion – 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 BISC 367 • Bulk flow – Movement of water in response to a pressure gradient • Analogous to water flowing in a pipe – Affected by: • Radius of pipe (r) • Viscosity of liquid (h) • Pressure gradient dyp/dx – Described by POISEUILLE’S equation: vol. flow rate (m 3 s-1) = (pr 4/8 h)(dyp/dx)

BISC 367 Movement of water into a plant cell occurs by osmosis • 2 mechanisms: – Diffusion across the membrane – Bulk flow across aquaporins (water filled pores)

BISC 367 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 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. (m 3 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 >5 m Dependent on: Height of water above ref. state (h) density (rw) acceleration due to gravity (g) Yg = rwgh rwg = 0. 01 MPa m-1

BISC 367 Plant Water Relations Cell (protoplast) water relations Cell wall (apoplast) water relations Yw = 0 Ys(p) Yp(p) Yw = 0 Ys(a) Yp(a) Yw(p) Yw(a) Whole plant water relations p = protoplast a = apoplast Ys(a) Yw = 0 Yp(a) Ys(p) Yp(p) Yw(a)

BISC 367 p is sensitive to small changes in cell volume • 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

BISC 367 p is sensitive to small changes in cell volume • 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 w • 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 w BISC 367 Scholander’s pressure bomb 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: //4 e. 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 measure Ys

Measuring w BISC 367 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 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