The first stages of pre-harvest 'peel pittings' development in 'Encore' mandarin. An histological and ultrastructural study

'Peel pitting' in the flavedo (the coloured epicarp) of 'Encore' mandarin arises as chlorotic spots. These spots correspond to parenchymal flattening and collapse of a variable number of sub-epidermal cell layers, which increase in parallel with the epidermis, extending between apparently healthy zones. The first signs of cellular damage are associated with internal membrane disorganization of the plastids. Great vesiculation of the cytoplasm occurs, followed by degradation of cytoplasmic membranes and accumulation of abundant osmiophilic material, yielding amorphous and dense masses. The disorder started by September. Spotting was only observed on the outer side of the fruit and on portions of the rind most exposed to external factors. Radiation and high temperature of the rind over long periods may induce localized dehydration, plasmolysis and cell collapse. (C) 1999 Annals of Botany Company.