Factors controlling fruit set of rabbiteye blueberry (Vaccinium ashei Reade)
Brevis, Patricio Alejandro
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Rabbiteye blueberry (REB) is the primary blueberry species cultivated in Georgia and other southern states. Poor fruit set is one of the most important problems of the REB industry, and the failure to set adequate fruit loads has been attributed, among other factors, to insufficient pollination. The research conducted as part of this dissertation addressed selected aspects of the pollination biology of REB in an effort to identify limiting factors for fruit set and yield. Pollen grain production per ovule averaged 402 ± 35, which is very low for a xenogamous species. REB is a buzz-pollinated plant, and its low pollen-ovule ratio may be an indicator of the high efficiency of its pollen dispensing mechanism. Total pollen production and release varied among commercial cultivars. High percentages of in vitro tetrad germination (>80%) suggest that pollen viability does not contribute to REB reproductive failure. REB requires cross-pollination for optimum fruit yield. Transport of cross-pollen by bumblebees was assessed in the field using a novel technique based on frequency distributions of pollen diameter, measured with a particle counter. The study was conducted in 2003 and 2004 in a blueberry planting with alternating rows of ‘Brightwell’ and ‘Climax’ plants. Bumblebees carried low proportions of cross-pollen, which indicated that these pollinators visited ‘Brightwell’ and ‘Climax’ flowers in a non-random fashion. The likelihood for cross-pollination was low and limited to the period of maximum bloom overlap. Specimens collected from ‘Climax’ in 2004 carried more cross-pollen than those from ‘Brightwell’, which may be related to the difference in pollen release between these cultivars. The length of flower receptivity or effective pollination period (EPP) was established for the cultivars Brightwell and Tifblue. The EPP was 7 days at day/night temperatures of 23/10°C. Although low stigmatic receptivity limited tetrad germination in flowers pollinated on the day of anthesis, this variable and fruit set were not positively associated. Therefore, age-related factors other than stigmatic receptivity (ovule longevity and/or pollen tube growth rate) likely limit the EPP of REB. This study provided the first quantitative evidence of late stigma maturation in blueberry.