The floral biology of the European Hazelnut (Corylus avellana L.) exhibits several unusual characteristics.
It is a monoecious species.
During flowering it is possible to observe male and female inflorescences on one-year-old shoots.
They are either present individually, associated together, or associated with vegetative buds.
Male flower induction occurs in mid-May, more than one month before female flower induction.
Female flower induction depends on three main factors: the level of light received by the one-year-old shoots during the previous growing season, and the vigor and origin of these shoots.
The floral organs grow slowly until flowering which takes place thoughout the winter.
Many clones are dichogamous.
Pollen shedding and anthesis of pistillate flowers occurs over a period of a few to several weeks and, if pollination is prevented, stigmatic surfaces may remain receptive for up to three months.
Pollination is anemophilous.
Except for some rare cultivars, the Hazelnut is self-incompatible and inter-incompatibility between varieties frequently occurs.
This incompatibility is determined sporophytically and depends on an allele series controlled by one incompatibility locus S. In the pistil, alleles exhibit independent action whereas in the pollen alleles exhibit either dominance or co-dominance.
The dominance relationship is linear.
At the time of pollination, the ovary is not formed and grows only if the flower is pollinated.
The formation of ovules begins in March and fertilization occurs by the end of May or during the first three weeks of June, four to five months after pollination, when the diameter of the nuts is 7–10 mm.
Ripeness begins three months later, in September or October.
A high level of flower cluster drop may be observed before fertilization, in late April or May.
This drop is dependent on the apical dominance which exists along one-year-old shoots and along the peduncle of catkins.
The formation of seedless nuts is a complex phenomenon.
Beyond genetic and cultural factors, some climatic parameters could play a major role in these phenomena, particularly in the case of brownstain disorder which is due mainly to a cold weather during the two weeks following the fertilization time.
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