Visual Assessment of Gas-exchange Sites in Harvested Tomato Fruit

Gas exchange in fruits generally occurs at sites located over the entire epidermal surface. By contrast, atmospheric  exchange in tomato fruit is largely restricted to the stem-scar region. Application of gas-impermeable substances to the stem-scar area of mature-green fruit prevents gas transfer and strongly suppresses ripening. In spite of the importance of the stem-scar region to postharvest attributes including ripening and pathogen proliferation, no information is available regarding the nature or location of the structures that facilitate this exchange. The purpose of the present study was to determine the nature of the gas egress sites. Specifically, the interest was to determine whether gas exchange occurs uniformly over the scar region or is localized to specific areas. Gas egress was visualized by immersing fruit in water at temperatures generating pressure differences of approximately 5 kPa between the fruit atmosphere and external atmosphere (fruit > external). Within seconds of immersion, gas egress was observed at a small number of sites situated in the corky ring and at the outer rim of the stem scar. These egress sites were consistently located over the radial pericarp walls and have been termed 'radial apertures." Egress was rarely noted in tissue of the central stem-scar. Egress patterns were unaffected when fruit were subjected to greater pressure differentials (16.9 kPa). When predicted egress sites were blocked with epoxy resin, gas egress was observed at alternative sites within the interaperture corky ring and to a lesser extent the stem scar (secondary egress sites).  Fruit with primary egress sites blocked with epoxy resin showed a reduced rate of ripening, indicating that these sites represent the preferential gas ingress/egress in tomato fruit. In addition to serving as gas-exchange portals, these egress points can be breached by aqueous suspensions of soft rot bacteria, leading to development of internal lesions. Speculation can be drawn that these sites, being peripheral to the stem attachment region, may serve as pathogen-ingress sites under preharvest as well as postharvest conditions.