Anatomical observations of the citrus fruit abscission zone and morphological changes of the cells during secondary physiological fruit drop

Histological investigations of the fruit abscission zone and morphological changes in abscission zone cells in ponkan (C. reticulata Blanco), hyuganatsu (C. tamurana hort. ex Tanaka), 'Kiyomi' (C. unshiu × C. sinensis (L.) Osbeck), and satsuma mandarin (Citrus unshiu Marcow.) were conducted using detached fruits incubated in agar medium under 25℃, 4 weeks after anthesis during secondary physiological fruit drop. In the 96 h after the fruit abscission induction by detaching the fruits, the cumulative abscission ratio was 100% in ponkan, 22% in 'Kiyomi', and below 10% in hyuganatsu and satsuma mandarin. Fruit abscission began at 36 h in ponkan and 'Kiyomi', at 54 h in satsuma mandarin, and at 60 h in hyuganatsu after the fruit detachment. The fruit abscission zone was located on the connected part between the fruit and the disc in ponkan and hyuganatsu on the disc tissue in satsuma mandarin and 'Kiyomi'. During the fruit abscission process, no abscission layer was observed at the abscission zone in these species and cultivars. Morphological changes in the abscission zone cells were determined by scoring cell changes (a score from 0 to 4) at five positions of the abscission zone. In ponkan, the morphological changes in the abscission zone cells, which began 30 h after fruit abscission induction, were synchronized in a symmetrical position in the abscission zone. The changes in 'Kiyomi' began at 30 h, and they consisted of a one sided collapse of the symmetrical position of the abscission zone. The changes in satsuma mandarin were similar to those in 'Kiyomi'. This implies that the different patterns of morphological changes in the abscission zone cells in ponkan, 'Kiyomi', and satsuma mandarin depend on the different locations of their abscission zones. Overall, the results suggest that the cue for fruit abscission in early abscised fruit occurs until 30 h after blocking the carbohydrate translocation to the fruit under 25℃.