SECONDARY‐FRUIT ONTOGENY IN NAVEL ORANGE

Ontogeny of the secondary fruit of navel orange (Citrus sinensis [L.] Osbeck) was studied using flower buds and fruit from groves in north central Florida. The secondary fruit, or navel, develops as a whorl of secondary‐carpel primordia within the primary‐carpel whorl when the flower bud is 1.5 to 2.0 mm in length. A complete secondary gynoecium with fused ovary but separate styles and stigmas develops within the primary gynoecium before anthesis. The stigma and style of secondary carpels are not as distinct as those of primary carpels. Three types of tissue protrusions from the secondary fruit into primary‐fruit locules were abnormal placentae, free secondary carpels, and secondary‐carpel outgrowths, the last being the most common. An abscission layer is present in the central axis of the primary fruit near the base of the secondary ovary. This layer is indistinguishable from adjacent tissues prior to commencement of secondary fruit abscission. Parenchyma cells in the abscission layer, however, contain large numbers of starch grains during abscission. Secondary and primary fruit have similar sigmoid growth patterns; however, onset of the cell‐enlargement stage is approximately 2 wk later in secondary fruit.