Spring.wmf (18300 bytes) 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. 


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

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)  


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