2005 Annual Report 4d.Progress report. This report serves to document research conducted under a Specific Cooperative Agreement (Project #5306-21000-015-07S) between ARS and University of California, Davis. The objective is to determine chilling requirements in Juglans regia germplasm. We used the methodology recommended by a recent workshop on standardizing methods for evaluating chilling requirements. In early December, after the leaves had fallen and about 185 chill units had accumulated, shoots, approximately one meter in length, were harvested from 11 genotypes of a diverse set of germplasm. Items were chosen to represent terminal and lateral bearing habit, early, mid and late phenologies and reportedly low and high chill requirements. Shoots were placed at 5C in plastic bags with damp wood shavings in a dark incubator. At time 0, and approximately every 200 hours later, up to 1728 hours, two shoots of each genotype were placed in pails of water in an incubator set at 15C with a 16 hour photoperiod. Shoots were examined every several days and bud break was recorded. The standard walnut descriptors were used to assess when bud break had occurred. The shoot bases were trimmed and water was changed every week. The terminal bud usually broke first. Bud break, however, was sporadic and for several genotypes only one bud broke on the total of 20 shoots included in this experiment. Early leafing types had the highest proportion of buds expanding and late leafing genotypes had the fewest, suggesting that the shoots requiring more heat units could not survive the time required at 15C. There was a trend of fewer heat units needed after more chilling and chill units required were within the expected range for walnuts. Modifications for 2005-2006 will be to increase the growing temperature to 22C, incorporate a bactericide into the water, and to harvest shoots from the field at specified chill unit accumulations. Additional details of this research can be found in the report for the parent CRIS #5306-21000-015-00D: Management of Fruit and Nut Crop Genetic Resources.