CHILLING REQUIREMENTS OF HAZELNUT CULTIVARS

Chilling requirements of catkins, female inflorescences, and vegetative buds of 45 genotypes of Corylus avellana L. and 13 genotypes of other Corylus species and interspecific hybrids were estimated by cutting shoots in the field at weekly intervals and forcing them in a warm greenhouse for 4 weeks. Chilling requirements were lowest for catkins and highest for leaf buds, and ranged from less than 100 to 860 h for catkins, 290-1550 h for female flowers, and 365-1690 h for leaf buds. The lowest chilling requirements were observed for the leading C. avellana cultivars of Turkey and southern Italy. The yellow-leafed ornamental C. avellana cultivar 'Aurea' had very high chilling requirements for all plant parts. Chilling requirements of vegetative buds of clones heterozygous for the non-dormant trait were less than those of clones homozygous for the dormant allele. Chilling requirements were comparable or slightly higher for three groups of other Corylus species, but were considerably higher for all plant parts of Corylus cornuta var. californica. Time of catkin elongation, stigma exsertion, and leaf bud swell in the field were highly and positively correlated with estimated chilling requirements of these plant parts. The chilling requirements of female inflorescences and vegetative buds were highly correlated. Genotypes with the same estimated chilling requirement leafed out as much as 3 weeks apart in the field, indicating differences in post-rest heat requirements.