Microscale mechanisms of gas exchange in fruit tissue.

* Gas-filled intercellular spaces are considered the predominant pathways for gas transport through bulky plant organs such as fruit. Here, we introduce a methodology that combines a geometrical model of the tissue microstructure with mathematical equations to describe gas exchange mechanisms involved in fruit respiration. * Pear (Pyrus communis) was chosen as a model system. The two-dimensional microstructure of cortex tissue was modelled based on light microscopy images. The transport of O(2) and CO(2) in the intercellular space, cell wall network and cytoplasm was modelled using diffusion laws, irreversible thermodynamics and enzyme kinetics. * In silico analysis showed that O(2) transport mainly occurred through intercellular spaces and less through the intracellular liquid, while CO(2) was transported at equal rates in both phases. Simulations indicated that biological variation of the apparent diffusivity appears to be caused by the random distribution of cells and intercellular spaces in tissue. Temperature does not affect modelled gas exchange properties; it rather acts on the respiration metabolism. * This modelling approach provides, for the first time, detailed information about gas exchange mechanisms at the microscopic scale in bulky plant organs, such as fruit, and can be used to study conditions of anoxia.

Validation of predictive growth models describing superatmospheric oxygen effects on Pseudomonas fluorescens and Listeria innocua on fresh-cut lettuce.

Two microbial growth models predicting the growth of Pseudomonas fluorescens and Listeria innocua at superatmospheric oxygen and carbon dioxide concentrations at 7 degrees C were validated on fresh-cut butterhead lettuce. Cut lettuce was inoculated with the same strain of L. innocua as the in vitro experiments. The P. fluorescens strain was tagged with a gene encoding green fluorescent protein (GFP) in order to distinguish the inoculated strain from contaminating Pseudomonaceae. Also growth of aerobic mesophilic and lactic acid bacteria was monitored during the experiments. The suggested P. fluorescens model was appropriate to predict growth on cut lettuce. L. innocua on the other hand, grew considerably slower under in vivo circumstances than predicted. CO(2) had a growth promoting effect on L. innocua growing on cut lettuce, whereas in vitro an inhibiting effect was observed. Validation parameters are calculated and hypotheses to explain the discrepancy between predicted and observed growth of L. innocua are provided.

Predictive modelling and validation of Listeria innocua growth at superatmospheric oxygen and carbon dioxide concentrations.

The effect of superatmospheric oxygen and carbon dioxide concentrations on the growth of Listeria innocua, which was used as a model organism for the pathogen Listeria monocytogenes, was evaluated. The bacteria were grown on a nutrient agar surface at 7 degrees C. Three carbon dioxide levels (0%, 12.5% and 25%) were combined with different levels of high oxygen concentrations (above 20%) based on a mixture design. The applied oxygen concentrations did not significantly influence the growth. High CO2 concentrations, on the contrary, reduced the maximum specific growth rate and prolonged the lag time. An overall model to describe the growth of L. innocua under high carbon dioxide conditions was constructed based on nine growth experiments, using a weighted one-step regression procedure. The influence of carbon dioxide on lag time and maximum specific growth rate was described using Ratkowsky-type models and inserted in the Baranyi equation. The model described the growth very well. To assess the validity of the model, 14 additional experiments were carried out. There was a good correlation of the model predictions and observed validation data.

Comparative study of the O(2), CO(2) and temperature effect on respiration between "Conference" pear cell protoplasts in suspension and intact pears.

The influence of the O(2) and CO(2) concentration and the temperature on the O(2) uptake rate of cool-stored intact pears and pear cell protoplasts in suspension was compared. Protocols to isolate pear cell protoplasts from pear tissue and two methods to measure protoplast respiration have been developed. Modified Michaelis-Menten kinetics were applied to describe the effect of the O(2) and the CO(2) concentration on the O(2) uptake rate and temperature dependence was analysed with an Arrhenius equation. Both systems were described with a non-competitive type of CO(2) inhibition. Due to the inclusion of gas diffusion properties, the Michaelis-Menten constant for intact pears (2.5 mM) was significantly larger than the one for protoplasts in suspension (3 microM), which was in turn larger than the Michaelis-Menten constant obtained in mitochondrial respiration measurements described in the literature. It was calculated that only 3.6% of the total diffusion effect absorbed in the Michaelis-Menten constant for intact pears, could be attributed to intracellular gas diffusion. The number of cells per volume of tissue was counted microscopically to establish a relationship between the pear cell protoplast and intact pear O(2) uptake rate. A remarkable similarity was observed: values of 61.8 nmol kg(-1) s(-1) for protoplasts and 87.1 nmol kg(-1) s(-1) for intact pears were obtained. Also, the inhibitory effect of CO(2) on the respiration rate was almost identical for protoplasts and intact pears, suggesting that protoplast suspensions are useful for the study of other aspects of the respiration metabolism.