Diffusion and Reaction in Root Nodules

Diffusion of PAH in potato and carrot slices and application for a potato model

A method for quantifying the effect of medium composition on the diffusive mass transfer of hydrophobic organic chemicals through thin layers was applied to plant tissue. The method employs two silicone disks, one serving as source and one as sink for a series of PAHs diffusing through thin layers of water, potato tissue, and carrot tissue. Naphthalene, phenanthrene, anthracene, and fluoranthene served as model substances. Their transfer from source to sink disk was measured by HPLC to determine a velocity rate constant proportional to the diffusive conductivity. The diffusive flux through the plant tissue was modeled using Fick's first law of diffusion. Both the experimental results and the model suggest that mass transfer through plant tissue occurs predominantly through pore water and that, therefore, the mass transfer ratio between plant tissue and water is independent of the hydrophobicity of the chemical. The findings of this study provide a convenient method to estimate the diffusion of nonvolatile organic chemicals through various plant materials. The application to a radial diffusion model suggests that "growth dilution" rendersthe concentration of highly hydrophobic chemicals in potatoes below their equilibrium partitioning level. This is in agreement with field results for the bioconcentration of PAHs in potatoes.

Fruit Tree model for uptake of organic compounds from soil and air

The current European risk assessment for chemicals considers only tap water, while in reality other beverages play an important role. A good part of beverages are made from fruits, for example apple juice and vine. A mathematical model was developed to predict uptake of neutral organic chemicals from soil and air into fruits. The new fruit tree model considers eight compartments, i.e. two soil compartments, fine roots, thick roots, stem, leaves, fruits, and air. Chemical equilibrium, advective transport in xylem and phloem, diffusive exchange to soil and air and growth dilution are the main processes. The parameterization is for a square-meter of an apple orchard. The model predicts that polar, non-volatile compounds will effectively be transported from soil to fruits, while lipophilic compounds will preferably accumulate from air into fruits. Results from various experiments show no disagreement with the model predictions.

Bacteroids from soybean root nodules: respiration and N 2 -fixation in flow-chamber reactions with oxyleghaemoglobin

Suspensions of bacteroids prepared from root nodules of glasshouse-grown soybeans were studied in a stirred chamber supplied with variable flows of solutions containing dissolved air, oxyleghaemoglobin and various energy sources. In experiments of up to 6 h duration, several steady states were established in which frequent measurements were made of the concentration of free, dissolved O 2 in the range 5–200 nM and of rates of O 2 consumption, CO 2 efflux and N 2 fixation. The principal findings were: (i) bacteroids were capable of efficient N 2 fixation without the supply of exogenous energy sources when respiring at 10–50 nM free O 2 . This endogenous respiration was enhanced after periods of supply of succinate or malate. (ii) Exogenous energy sources were of three types, those that were poorly utilized (glucose), those that enhanced endogenous respiration and supported efficient N 2 fixation (glutamate, 2-oxoglutarate) and succinate and malate, which promoted respiration but supported N 2 fixation inefficiently or in some circumstances inhibited it. It is proposed that succinate and malate act primarily to increase endogenous reserves that become the principal source of reducing power for N 2 fixation, (iii) Bacteroids have a terminal oxidase system with very high apparent affinity for O 2 ( s 0.5 ≈ 5–8 nM) and complex kinetics (plots of v against s are sigmoidal; n app > 1.8). This system was active with endogenous substrates and when glutamate or 2-oxoglutarate were supplied. With succinate or malate, respiration appeared to be the sum of endogenous activity plus O 2 consumption by a system with lower affinity for O 2 ( K s = 38 nM) and simple kinetics ( n app ≈ 1). (iv) During the first hour of reactions there were changes in O 2 demand and oscillations in O 2 demand and CO 2 efflux followed supply or withdrawal of exogenous substrates. It is proposed that these changes are examples of phenotypic plasticity in these symbiotic bacteria.