Abscisic acid levels and the influence of ethylene, humidity and storage temperature on the incidence of postharvest rindstaning of 'Navelina' orange (Citrus sinensis L. Osbeck) fruit

'Navelina' (Citrus sinensis L. Osbeck) oranges are prone to develop postharvest rindstaining during storage at a non-chilling temperature (22 degrees C). This physiological disorder is manifest as extensive collapsed and dry areas of the flavedo (outer coloured part of the peel), and part of the albedo (inner part of the peel), that becomes dark with time. In the present study we have examined the effect of humidity, temperature and applying ethylene to harvested fruit on the abscisic acid (ABA) content in the flavedo and on their susceptibility to develop rindstaining. The incidence of this physiological disorder was considerably reduced by treating the fruit with ethylene or by keeping them at low temperature (2 degrees C) but was enhanced by increasing the relative humidity (RH) during storage at 22 degrees C. Fruit treated with air and kept at 22 degrees C under low RH (55-60%) had higher ABA contents than fruit kept under high RH (85-90%) at the beginning of the storage period. However, after 14 days storage little difference in ABA content between fruit held under both treatments was found and the rindstaining index of fruit was different. The beneficial effect of keeping the fruit at 2 degrees C appeared to be more related to the influence of low temperature on delaying the mechanism of peel pitting than to the changes occurring in ABA content in the flavedo of fruit. Ethylene induced a sharp increase in ABA content in the flavedo and considerably reduced rindstaining. However, no differences in ethylene-induced ABA levels were found despite the efficacy of ethylene in controlling rindstaining increasing with concentration of gas applied and duration of the treatment. The overall results indicate that ethylene, but not ABA, may be an important factor protecting 'Navelina' fruit from the development of rindstaining.