| Sumario: | The ultrastructural changes in the ovary of Citrus sinensis (L.) Osbeck, cv. 'Navelate', during the transformation of the ovary into young developing fruit were studied. Epicarp and inner mesocarp cells have a large central vacuole surrounded by a peripheral cytoplasm. Outer mesocarp cells vary in their degree of vaculation containing abundant cytoplasm. Prominent organelles within cytoplasm of all these cells are the nucleus, mitochondria and plastids. Small amounts of rough endoplasmic reticulum, dyctyosomes and free ribosomes are also present. At petal fall, polysomal groups of ribosomes in outer mesocarp cells are observed. The main ultrastructural changes were alterations of plastid structure. Few days before anthesis, plastids of epicarp and outer and inner mesocarp, contain single or multiple starch granules. Their number decrease gradually until petal fall. Concomitantly an increase in the internal chloroplasts are visible, containing a number of small plastoglobuli. At the end of the fruit set period thylakoid breakdown in inner mesocarp chloroplasts is evident.
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