Anatomical Investigation of Abscissed Avocado Flowers and Fruitlets

The anatomy of abscissed avocado fruitlets of the Hass varietywas compared with that of actively growing control fruitletsto investigate the reason for abscission. Control fruitletshad a small embryo (4–14 cells) and numerous endospermcells by seven days after pollination. Cotyledon developmenthad commenced by 21 days and a root/shoot axis was present at28 days. Over 90 per cent of the fruitlets which abscissed during thefirst week after the end of flowering were unfertilized and18 per cent were abnormal. By the fourth week after the endof flowering all abscissed fruitlets were fertilized and noneabnormal. Abnormalities included ovaries with more than oneembryosac or ovule, with an immature embryo-sac, with an ovulein an abnormal position or with a deficiency in ovule structure.In a few fertilized abscissed fruitlets either the embryo orendosperm had ceased development first. In the majority of casesembryo and endosperm were anatomically normal. The majorityof fertilized abscissed fruitlets had reached a stage correspondingto 14 days after pollination of the control tree. Degenerationwas often observed in both unfertilized and fertilized abscissedfruitlets. It is suggested that this degeneration occurred inthe period between the cessation of development and abscissionof the fruit. This period was approximately one week. The proportion of abnormal ovaries was too low to significantlyaffect yields, and sufficient flowers had been fertilized togive an adequate crop. No anatomical reason for the high rateof fruitlet abscission was observed. Persea americana Mill, avocado, abscission, embryo-sac, embryo, endosperm