Pollen development is an important process in male flower development, the timing of which may be correlated with time of pollen shed in hazelnut (Corylus avellana L.). Early to very late blooming cultivars were identified and the relationship of microsporogenesis and microgametogenesis, and time of pollen shed were studied in nine hazelnut cultivars. Most advanced catkins from a single tree of each cultivar were collected each week from 4 Aug. to 6 Dec. 2002, and on 17 Jan. 2003, stained and analyzed by light microscopy. The phenology part of this dissertation studied the role of the chilling requirement as chill units (CU) and heat requirement as growing degree hours (GDH) in pollen shed. Hazelnut twigs of three cultivars; 'TGDL', 'Barcelona', and 'Hall's Giant' were collected at weekly intervals starting from early Fall 2006 through the time of anthesis in the field in winter 2007. Twigs were then held at a different constant temperature 0, 5, 10, 15, or 20 °C. Observing these twigs weekly, the time of anthesis (50% pollen shed) was recorded. A parallel study was conducted in more controlled conditions by collecting hazelnut twigs of the same three cultivars on 1 Nov. 2006 and holding them at 5 °C in a cold room. Five twigs of each genotype were brought out to room temperature at 5-day intervals in order to force them to bloom. Numbers of catkins that shed pollen were recorded every 5 days. From the results, we propose a model of hazelnut staminate flower development that begins with catkin differentiation concurrent with early stages of pollen development. Catkin length increased steadily and reached a lag phase at the end of microsporogenesis. While there is no external change, microspores continue microgametogenesis, the catkins are endodormant, unable to be induced to shed pollen, and accumulate chilling during this period of time. We propose that the chilling requirement is met when pollen reaches the mature stage and when catkins are in the latter stages of the lag phase of growth. At this point, GDH accumulate and catkins become ecodormant. As chilling continues to accumulate, the amount of GDH required for pollen to shed is reduced. A similar quantity of GDH was required for all genotypes from pollen grain maturity to catkin elongation.
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