Plant Physiology

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 m³m^-2s^-1 or simplified m s^-1. Lp is the hydraulic conductivity of the membrane (in units of m³m^-2s^-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 Ψ_s (MPa) from the van't Hoff equation (see Chardakov lab).
Obtain the water potential of the potato tubers (Ψ_tuber) from the Gravimetric experiment. Then, calculate ΔΨ (Ψ_soln � 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)	Ψ_solution (MPa)	ΔΨ (Ψ_s_oln � Ψ_tuber; 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.