Resistance of Citrus Fruit to Mass Transport of Water Vapor and Other Gases

Abstract

The resistance of oranges (Citrus sinensis L. Osbeck) and grapefruit (Citrus paradisi Madf.) to ethylene, 02, CO2, and H20 mass transport was investigated anatomically with scanning electron microscope and physiologically by gas exchange measurements at steady state. The resistance of untreated fruit to water vapor is far less than to ethylene, CO2 and 02. Waxing partially or completely plugs stomatal pores and forms an intermittent cracked layer over the surface of fruit, restricting transport of ethylene, 02, and C02, but not of water, whereas individual sealing of fruit with high density polyethylene films reduces water transport by 90% without substantially inhibiting gas exchange.Stomata of harvested citrus fruits are essentially closed. However, ethylene, 02 and CO2 still diffuse mainly through the residual stomatal opening where the relative transport resistance (approximately 6,000 seconds per centimeter) depends on the relative diffusivity of each gas in air. Water moves preferentially by a different pathway, probably through a liquid aqueous phase in the cuticle where water conductance is 60-fold greater. Other gases are constrained from using this pathway because their diffusivity in liquid water is 10'-fold less than in air.The commercial practice ofwaxing fruits inadequately reduces transpiration, and yet it is so effective in restricting 02 and CO2 transport that off-flavors sometimes result (4-7). Conversely, sealing fruits individually in HDPE4 film reduces water loss 10-fold without changing the fruit's endogenous 02, C02, or ethylene content (6). Consequently, seal-packaging is more effective than waxing in preventing shrinkage and in extending the storage life of citrus and certain other fruits (6,7,10,20).The different effects which seal-packaging with a plastic film 10 ,um in thickness, and waxing, which forms a noncontinuous membrane i ,um in thickness, have on gas exchange are not easily explained by previous morphological studies concerning the distribution of applied waxes (4,7,8,31), nor by most current theories of gas mass transport in citrus and other fruits. Opinions differ widely concerning the relative contributions of the various mechanisms proposed to account for gas exchange in harvested fruits, although usually it is tacitly assumed that water and gases move by the same pathway. Air-filled stomata

Epicuticular wax content and morphology as related to ethylene and storage performance of ‘Navelate’ orange fruit

“…Likewise, water potential and turgor pressure of freshly harvested fruit reflected turgid nonstressed cells, indicating that fruit were not stressed at harvest and, therefore, that they did not develop non-chilling peel pitting as a consequence of re-hydration, which has been shown to induce this disorder (Alférez et al, 2003). Waxes may have a more marked effect on inhibiting transport of gases than transport of water through the cuticle in citrus fruit (Ben-Yehoshua et al, 1985). The mode of action by which external or internal gases may alter the susceptibility of citrus fruit to develop peel damage remains unknown, though postharvest treatments modifying internal gas composition or the O 2 environmental levels may also modify non-chilling peel pitting susceptibility (Petracek et al, 1998;Ben-Yehoshua et al, 2001;Porat et al, 2004).…”

“…In citrus fruit, it has been shown that the amount, composition and structure of the epicuticular wax varies among cultivars, changes with fruit development, maturation and age (Freeman et al, 1979;El-Otmani and Coggins, 1985a;Sala et al, 1992), and has an effect on both water loss (Albrigo, 1972a) and transport of gases through the cuticle (Ben-Yehoshua et al, 1985;El-Otmani et al, 1986). Furthermore, redistribution of epicuticular wax layer may improve physical barriers to pathogen penetration and reduce disease incidence caused by Penicillium digitatum (Pers.:Fr.)…”

The fading distinctions between classical patterns of ripening in climacteric and non-climacteric fruit and the ubiquity of ethylene—An overview

“…Burg and Burg (1965b) and then Solomos (1987) developed relationships between rates of ethylene production by fruit and the internal concentrations and these relationships were surprisingly found similar across many fruit species. However, more has to be learned about the retention and release of ethylene in plant tissues in relation to its resistance towards the diffusion across the boundaries of fruit and physiological activity of ethylene bound to its receptor (Ben-Yehoshua et al 1985;Goldschmidt et al 1993). So, to get better understanding of ethylene emission by tomato fruit Genard and Gouble (2005) developed a simulation model called 'ETHY'.…”

Role of internal atmosphere on fruit ripening and storability—a review

“…But, usually, this aspect has not been taken into consideration in the studies dealing with resistance of the fruits to diffusion and exchange of gases (Ben-Yehoshua et al 1985). So, more has to be learned about the retention and release of bound ethylene in plant tissues in relation to physiological activity of ethylene (Goldschmidt et al 1993).…”