Diffusion barriers have been shown to restrict fluxes of O2 and CO2 to cells in bulky plant organs such as roots, tubers, stems, inflorescences, seeds and fruit (Sinclair & Goudriaan, 1981; Drew, 1997; Geigenberger et al., 2000; Seymour, 2001; Van Dongen et al., 2003; Armstrong et al., 2006). Restricted fluxes of respiratory gases may lead to anoxia, eventually resulting in cell death (Malik et al., 2003; Franck et al., 2007). Differences in gas exchange properties of plant organs have been observed in response to a wide range of factors covering nutrient availability, water status, environmental conditions, development stage, cultivar and crop management (Malik et al., 2003; Armstrong et al., 2006; Ho et al., 2006a). Knowledge of gas exchange mechanisms would be very valuable to guide commercial storage practices for stored fruits, such as pears, since disorders under controlled atmosphere related to fermentation are a prime cause of concern (Franck et al., 2007). The intercellular free space is believed to affect gas exchange in the fruit tissue greatly (Raven, 1996, Armstrong et al., 2006), as it provides a low resistance pathway for gas supply. At the fruit tissue level, the hypothesis is that O2 is transported through the intercellular space and subsequently permeates through the cellular membrane to the cytoplasm. Finally, the O2 diffuses within the cytoplasm into the mitochondria. Through respiration, O2 is reduced to water and CO2 is produced. CO2 essentially follows the reversed path. However, the air volume fraction of fruit such as pear is smaller than 10% of the fruit volume (Schotsmans et al., 2002). Further, the solubility of CO2 in water is higher than that of O2 and the magnitude of the CO2 concentration in the liquid phase is comparable to its concentration in air; therefore, the exchange mechanisms of the two gases may be significantly different. The relative importance of intra- and intercellular gas exchange rates and metabolic reaction rates has not yet been quantified.
Gas exchange in fruit and other bulky storage organs has been described macroscopically with Fick's laws (Burg & Burg, 1965, Cameron & Yang, 1982; Lammertyn et al., 2001a; Schotsmans et al., 2003; Ho et al., 2006a,b). Typically, the gas exchange at the microscale is not modelled explicitly; instead a macroscopic diffusion equation is used containing empirical apparent diffusivities that implicitly incorporate the microscale topology (Wood et al., 2002, Ho et al., 2006b). This averaging procedure essentially hides the microscale phenomena that are essential to understanding gas exchange. A microscale model of gas exchange in fruit tissue is therefore essential. Microscale exchange of CO2 in leaves has been investigated using theoretical models (Vesala et al., 1996, Aalto & Juurola, 2002). However, O2 transport, which is essential to respiration processes in fruit was not addressed, and the geometrical model was relatively crude compared with the actual irregular microstructure of the tissue.
Mass transport of a component gas occurs in both the gas phase of the intercellular space and the liquid phase of the cytoplasm. Gas exchange between the intercellular space and the cell can be described by gas permeation through the plasma membrane (Nobel, 1991). Gas equilibrium between the gas and liquid phase follows Henry's law (Lide, 1999). CO2 transport is complicated by the fact that there is hydration of CO2 into . Moreover, the equilibrium between soluble forms of CO2 in the liquid phase is affected by the cytoplasmatic and vacuolar pH (Bown, 1985; Boron, 2004). Transport of CO2 in biological liquids in the form of dissolved CO2 and was discussed for blood and mammal tissue by Geers & Gros (2000). A recent review by Teskey et al. (2008) indicates that the high CO2 storage capacity through hydration and dissociation of CO2 to other forms in the liquid phase can affect CO2 exchange considerably. Hydration and dissociation of CO2 to other forms in the liquid phase have not yet been investigated quantitatively in relation to CO2 transport in plant tissue.
In this manuscript, a new approach for the detailed study of O2 and CO2 exchange through the intercellular spaces, cell wall and cytoplasm of cells in fruit tissue is introduced, using a microscale model and pear as a model system. The objectives were: (i) to verify the applicability of the microscale model of gas transport at the tissue level based on measured gas exchange rates and gas concentration profiles in intact fruit; and (ii) to quantify the pathways of gas exchange in relation to the microstructure of fruit tissue.