Effect of Chilling on Budbreak in `Thompson Seedless' and `Carignane' Grapevines

Shortly before leaf fall (November 10), potted `Thompson Seedless9 and `Carignane9 vines were put in a coldroom (35 ± 2°F) for various periods ranging up to 77 days. They were then removed to a heated greenhouse for determination of rate and amount of budbreak. Respective times to initial budbreak of `Thompson Seedless9 and `Carignane9 were reduced from 5 and 6 weeks in unchilled vines to 2 and 3 weeks in vines chilled 9 weeks. The buds that broke per vine on `Thompson Seedless9 were 2-3 times as numerous on vines that received one or more weeks of chilling as on unchilled vines. For `Thompson Seedless9, 7 or more weeks of chilling resulted in maximum percentage of budbreak per vine and minimum time between first and last bud to break. Gibberellic acid prolonged the time required for the first bud to break dormancy on vines that received 0 or 7 days of chilling, but had little effect on vines that were chilled longer. Neither benzyladenine nora 1:1 mixture of gibberellin and benzyladenine significantly affected the time or amount of budbreak.