Reproductive biology of avocado (Persea americana)

Abstract

Avocado is a subtropical fruit tree that shows protogynous dichogamy. As a consequence, each bisexual flower opens twice: first functionally as female (stigmatic surface receptive), then the flower closes and reopens the next day functionally as male (dehiscence of anthers). In avocado, more that 99% of the flowers produced at anthesis are not able to set fruits. In order to study why most flowers prematurely abscise while some of them remain in the tree, we analyzed the progamic phase (from pollination to fertilization) under the environmental conditions of southern Spain. Our results showed that pollination is a critical step in avocado fruit set since most of the flowers had received no pollen at the time of closing in the female stage. Although the probability of a flower developing into a fruit was significantly affected by the number of pollen grains adhered to the stigma, fertilization and subsequent fruit set can take place with a low number of pollen grains on the stigma. Moreover, a high pollen adhesion also occurs during the male stage, and, although the stigma maintains the capacity to support pollen germination, no fertilization was observed in flowers pollinated during the male stage. Even after hand-pollination and, in spite of the higher fruit retention observed, fruit set rate was still very low. This suggests that additional factors might be involved in fruit set. The results are discussed in terms of the implications of the environmental conditions on reproductive success and fruit set in avocado.