35
then used with intact hydroponically grown soybean plants and the resultant estimated values of nodule gas permeability compared with estimates from other sources.
Review of Current Techniques
Nodule Respiration and Fick's First Law Approximation
Sheehy et al. (1983) modeled whole nodule respiration using a
steady-state one-dimensional diffusion equation similar to Equation 3.1 but rearranged to solve for nodule gas permeability such that
P = J / (O O. ) (4.1) ex in
-I
where P is the gas permeability (mm s ) of the diffusion barrier, J is
3 -2 -1
the oxygen flux density crossing the nodule cortex (mm mm s ), O ex
3 -3
is the external oxygen concentration (mm mm ) and 0. is the oxygen in
concentration in the intercellular air spaces internal to the cortical
3 -3
barrier (mm mm- ). Since O is experimentally defined, P can be ex
estimated from Equation 4.1 if the oxygen flux (J) into the nodule interior is known and given certain assumptions about 0. Since 0. is many orders of magnitude lower than O its absolute value is not very ex
3 -3
significant and these authors assumed it to be equal to 0.001 mm nm oxygen.
To estimate J in Equation 4.1, Sheehy et al. (1983) measured nodule carbon dioxide evolution and made certain assumptions to convert this into an oxygen flux. The first assumption concerned the fraction of total respiration representing a respiratory flux across the diffusion