Crown dehydration

The crown is a continuation of the vegetative stem leaves, and the spirally arranged leaflets have a similar morphology. The stomata are located in furrows on the underside of the leaf, which is also covered by tri-chomes. The stomata in mature leaves open at night and are closed during the day, associated with crassulacean acid metabolism (Sideris et al., 1948). However, the stomata of young leaves such as on the crown, may not show this same diurnal function (R.E. Paull, 1985, unpublished results), as water-loss rate appears to be constant during storage. At 10°C, stomatal opening and closing are slowed and CO2 uptake is reduced, compared with 17°C (Kent, 1967). Ethylene increases leaf respiration, causes stomatal opening and enhances dehydration (Dull and Staruszkiewicz, 1966). Opening of stom-ata requires illumination on the preceding day and ethylene negates this requirement (Kent, 1967). Hence, what has been reported for plant leaves may also occur with the crown leaves, such as sensitivity to low temperatures and ethylene response.

The crowns of fresh fruit soon lose their green healthy look and the leaves become dehydrated and necrotic. The oldest leaves are generally most affected, and this is possibly due to ethylene. Postharvest ethylene treatment leads to greater water loss than if crowns are untreated. Rate of dehydration is also related to the relative humidity of storage (Dull and Staruszkiewicz, 1966). A well-ventilated crown can lose up to 10% of its weight in 14 days, while a crown treated with 200 |il l-1 ethylene can lose up to 64% of its fresh weight. Waxing of the crown frequently leads to phytotoxicity and excessive water loss, and some paraffin-wax formulations can slightly reduce weight loss (Schappelle, 1941).

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