Study On Avocado (Persea Americana) Pollination Mechanism

The avocado (Persea americana Mill) is an economic fruit tree native to Mesoamerica and has an unique flowering behavior known as synchronous dichogamy where the pistils in a flower expose at times different than the stamens. Therefore interplanting cultivars of various flower types has long been used in practice to enhance the pollination rate through matching the timing of blooming. 'Hass' is the most important avocado cultivar in the United States, but the best pollinizing cultivars for 'Hass' are still unknown due to the difficulty in identifying the pollen doner for 'Hass' fruits.The simple sequence repeats of SSR (i.e. microsatellite) technology is an efficient tool of determing the pollen donor of progeny of known maternal genetic background. This study consists of two parts. In the first part, 9 interplanted avocado cultivars were evaluated in terms of their ability of pollination in a 'Hass'-dominated orchard from 2003 to 2005. Microsatellite markers were used to identify potential pollen donors for 'Hass' fruits. DNA typing with different primer combinations was applied to avocado embryos collected from 'Hass' trees. Pollen donors were determined by paternity analysis based on the parental contribution of alleles in the genotypes of the embryos. Of 3541 avocado embryos collected during the three years of study period, 57.8% were pollinated by 'Hass'. It indicates selfing-pollination remains a dominant way for avacao in a single-cultivar orchard despite the fact of its unique flowering behavior known as synchronous dichogamy. On the other hand, cross-pollination by interplanting cultivars accounted for inappropriatedly high proportion during the period of study. 'Zutano' and 'Ettinger' contributed to the pollination of about 12% of fruits, significantly higher than other 6 cultivars. Pollen outreach index is the highest for 'Zutano'.In the second part of this project, the role of honeybee in the pollination process for 'Hass' avocado was evaluated in two experiments. The heavy avalcado pollen is sticky to the body of most pollinating insects and its scented nectars are also attractive to pollinating insects. So bee has long been considered as a major media in avocado pollination and the introduction of bee hive is a common practice in many avocado orchards. One is supplemental 'Zutano' pollen experiment conducted in 10 different orchards during two years of study. The other is caged and uncaged experiment. The results show that honeybees play a negative role in avocado pollination.