Hibru Kelemu Mebatsion

Three-Dimensional Gas Exchange Pathways in Pome Fruit Characterized by Synchrotron X-Ray Computed Tomography

Our understanding of the gas exchange mechanisms in plant organs critically depends on insights in the three-dimensional (3-D) structural arrangement of cells and voids. Using synchrotron radiation x-ray tomography, we obtained for the first time high-contrast 3-D absorption images of in vivo fruit tissues of high moisture content at 1.4-μm resolution and 3-D phase contrast images of cell assemblies at a resolution as low as 0.7 μm, enabling visualization of individual cell morphology, cell walls, and entire void networks that were previously unknown. Intercellular spaces were always clear of water. The apple (Malus domestica) cortex contains considerably larger parenchyma cells and voids than pear (Pyrus communis) parenchyma. Voids in apple often are larger than the surrounding cells and some cells are not connected to void spaces. The main voids in apple stretch hundreds of micrometers but are disconnected. Voids in pear cortex tissue are always smaller than parenchyma cells, but each cell is surrounded by a tight and continuous network of voids, except near brachyssclereid groups. Vascular and dermal tissues were also measured. The visualized network architecture was consistent over different picking dates and shelf life. The differences in void fraction (5.1% for pear cortex and 23.0% for apple cortex) and in gas network architecture helps explain the ability of tissues to facilitate or impede gas exchange. Structural changes and anisotropy of tissues may eventually lead to physiological disorders. A combined tomography and internal gas analysis during growth are needed to make progress on the understanding of void formation in fruit.

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.

Fruit Microstructure Evaluation Using Synchrotron X-Ray Computed Tomography

Apple and pear are stored under controlled atmosphere conditions. Too low internal oxygen (O2) or too high carbon dioxide (CO2) concentrations may lead to storage disorders such as browning. The internal gas concentration is mainly determined by the fruit’s gas exchange properties, which depend on the structural arrangement of fruit cells and tissues. We used (for the first time) submicron synchrotron X-ray computed tomography (CT) to investigate how pome fruit tissues are spatially organized to facilitate or impede gas exchange. The experiments were conducted on beamline ID19 at the European Synchrotron Radiation Facility (ESRF, Grenoble, France), i.e., on a long (150 m) imaging beamline where the spatial coherence of the beam is particularly large (transverse coherence length in the order of 100 μm). The method allows for imaging in phase contrast, which, as opposed to absorption contrast, is a powerful method to distinguish, in absorbing materials, phases with very similar X-ray attenuation but different electron densities. In this study, it is efficiently used for edge detection at cell-cell interfaces where absorption images have insufficient contrast. We visualized 3-D networks of gas-filled intercellular spaces in two fruits, apple (cv. Jonagold) and pear (cv. Conference), that provide the main routes for exchange of O2 and CO2 with the environment, using absorption as well as phase contrast synchrotron X-ray CT at a pixel resolution ranging from 0.7 to 5.0 μm. The differences in void dimensions and connectivity between tissues and fruits helped explain imbalances in gas exchange that may result in internal disorders and structural degradation. We also showed that tomography with synchrotron radiation operated in phase-contrast mode and is able to visualize the 3-D geometry of voids, cells, and cell walls of biological tissues with high water content at submicron voxel resolution. In terms of facilitating gas exchange, the network pattern of the voids indicated a large size and volume fraction difference with the unconnected void structure found in apple. The partial breakdown of such networks would quickly lead to an internal gas imbalance leading to internal disorders. The achievement of high-resolution 3-D microstructural properties of cells and tissues is an important breakthrough for the study of gas exchange mechanisms in fruits stored under controlled atmosphere conditions. In addition to gas exchange, the results will benefit the study of water relations and mechanics of foods.

Multiscale Modelling of Gas Transport in Pome Fruit A paper from the State-of-the-Art in Application of Finite Element Numerical Solutions to Engineering Problems: A Session Honoring Pioneering Contributions of Professor Kamyar Haghighi of Purdue Universi

Pome fruit such as pear and apple are often stored under controlled atmosphere conditions with reduced oxygen and increased carbon dioxide levels to extend their commercial storage life. When the gas conditions are too severe, fermentation may occur in some fruit, eventually leading to physiological disorders such as brown discoloration in pear circumference. Knowledge of gas exchange mechanisms would be very valuable to guide commercial storage practices for stored fruits such as apple and pear, since disorders related to fermentation are a prime cause of concern. In the past, mathematical models have been by us to predict the internal gas concentrations including permeation, diffusion and respiration and fermentation kinetics. However, these models are essentially based on the continuum hypothesis which is not likely to hold for cellular tissue such as that of fruit.