Introduction. The fruit of `Encore' mandarin (Citrus deliciosa
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Citrus nobilis) usually shows chlorotic spots in the epicarp from the first stages of development. This
peel disorder triggers off, thereafter, the occurrence of a large quantity of dark stains (pre-harvest
`peel pitting'). Materials and methods. A biochemical and ultra-structural study of the flavedo,
using epicarp with and without stains, was carried out. Quality assays were assessed on the edible portion
of the fruit or on juice. Results. The cell structure of pitted and unpitted epicarp analysed by
electron microscopy revealed that dark spots were associated with the degradation of cellular membranes.
The peroxidase system that shields the epicarp against photooxidative stress showed an inhibition of
catalase activity and an increase of superoxide dismutase functioning. Moreover, the activities of
ascorbate peroxidase and glutathione reductase remained unaffected in stained tissues. In sunshade trees,
it was also found that the number of dark stains per fruit decreased but the internal quality of the
endocarp was not significantly affected. The weight and the caliber of the fruit also slightly decreased.
Discussion. High light intensities induce an oxidative stress in the epicarp, which is characterized by an
increased peroxidation and degradation of biological membranes. On the other hand, protection against
solar radiation alleviates the stress, but, although not affecting the internal quality of the fruit,
slightly decreases the fruit weight and size.