Plant Physiology (Biology 327)  - Dr. Stephen G. Saupe;  College of St. Benedict/ St. John's University;  Biology Department; Collegeville, MN  56321; (320) 363 - 2782; (320) 363 - 3202, fax;    ssaupe@csbsju.edu

Water Conductivity of Potato Tubers

Flux density, Js,  is a measure of the volume of water crossing the membrane per unit area per unit time.  It is expressed in units of  m3 m-2 s-1 or simplified m s-1.  Lp is the hydraulic conductivity of the membrane (in units of m3 m-2 s-1 MPa-1 or simplified to m s-1 MPa-1) which is a factor that relates permeability of the system to water flow.

Ultimately, the flow into (or out of) a system is a function of hydraulic conductivity.  The driving force for transport is the water potential gradient.  These factors can be mathematically expressed as:   Js = Lp (ΔΨ).

Flux density and hydraulic conductivity can be calculated from our Gravimetric data.  However, we will also need to know the surface area of a core.

Procedure:

1.  Set up the Gravimetric experiment.  Measure & record  in Table 1 the diameter and length of the cores.  Also record the duration of the experiment (Table 1) and then complete the calculations.

2.  Complete Table 2

• Calculate the solution Ψs (MPa) from the van't Hoff equation (see Chardakov lab).

• Obtain the water potential of the potato tubers (Ψtuber) from the Gravimetric experiment.  Then, calculate ΔΨ (Ψsoln Ψ tuber).

• Transfer the change in weight data from the Gravimetric experiment (column 4) into column 4 (water uptake).

• Calculate flux density

3.  Plot Js (m s-1) vs. ΔΨ (MPa).

4.  Calculate the slope of this line which will yield Lp = cm sec-1 MPa-1. For potato, the expected values for L are 1.5 10-7 m MPa-1 s-1.

 Table 1:  Core/Experimental Data average core length (cm) average core width (cm) average core surface area (cm2; 2 π r2 + 2 π r h ) total core surface area per treatment (surface area/core times the number of cores per treatment; cm2 ) time experiment start time experiment end incubation time for cores (s)

 Table 2.  Water Uptake by Potato Tubers [Sucrose] (molal) Ψsolution (MPa) ΔΨ (Ψsoln � Ψtuber ; MPa) Water Uptake (Final wt � Initial wt; cm3) Js = flux density = (cm3 / (core SA in cm2 x time in s) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

References:

• Barcelo, AR, AA Calderon and R Munoz.  1994.  Measuring water conductivity coefficients in plant tissues.  Journal of Biological Education 28: 83 � 85.

 | Top | SGS Home | CSB/SJU Home | Biology Dept | Biol 327 Home | Disclaimer |

Last updated:  01/07/2009     � Copyright  by SG Saupe