Dormancy and time of bud burst in the sultana vine

Abstract

Time of bud burst for sultana vines at Merbein has been closely related to sums of daily temperatures at four periods during bud dormancy by a multiple regression covering 17 years, the result being supported by field trials of vine heating.

Bud burst is delayed by higher daily maximum temperatures in May. Higher daily minimum temperatures in late June and in July, or higher daily maximum temperatures in August and until burst, are associated with earlier bud burst.

Studies of bud dry weight and moisture have suggested that the change-over between the association of higher daily minimum temperatures and higher daily maximum temperatures with earlier bud burst occurs at a specified sum of degrees by which daily minimum temperatures fall below 50°F, giving dates ranging from July 5 to August 7 for the years of the regression. This appears to be the beginning of imposed dormancy. Use of the regression for predicting time of bud burst is discussed.