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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. The Importance of Water. Physiological aspects.

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slide1

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
the importance of water
The Importance of Water
  • Physiological aspects
movement of water in plants
Movement of water in plants
  • 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 LAWJs = -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

slide4

Movement of water in plants

  • 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 (m3 s-1) = (pr4/8h)(dyp/dx)

movement of water into a plant cell occurs by osmosis
Movement of water into a plant cell occurs by osmosis
  • 2 mechanisms:
    • Diffusion across the membrane
    • Bulk flow across aquaporins (water filled pores)
movement of water into a plant cell occurs by osmosis1
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 PotentialGradient

water potential
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)
slide8

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

slide9

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

water potential1
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 = 0.01MPa m-1

plant water relations
Plant Water Relations

Cell wall (apoplast) water relations

Yw = 0

Yw = 0

Ys(a)

Cell (protoplast) water relations

Ys(p)

Yp(a)

Yw(p)

Yw(a)

Yp(p)

Whole plant water relations

p = protoplast

a = apoplast

Yw = 0

Ys(a)

Yp(a)

Ys(p)

Yw(p)

Yw(a)

Yp(p)

p is sensitive to small changes in cell volume
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
p is sensitive to small changes in cell volume1
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
typical values for y w
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
slide15

Measuring Yw

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
  • 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

From Plant Physiology on-line (http://4e.plantphys.net/)

slide16

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
  • Simple technique:
    • Leaf disks are excised, weighed (W) then allowed to reach full turgidity and re-weighed (TW). Leaf disks are dried to obtain their dry weight (DW).
  • RWC (%) = [(W – DW) / (TW – DW)] X 100