Effectiveness and mechanisms of dormancy breaking treatments for walnuts

As with other high chill orchard crops, walnuts (Juglans regia) need tools to support current varieties to sustain production through warmer winters created by climate change, while lower chill varieties are developed, tested and adopted. While many chemical products have been shown to influence budbreak, the challenge is not just to find a chemistry that can break dormancy, but to understand how to do so consistently and predictably. Recent research is showing that dormancy emergence involves long-distance transport, indicating that studying cut shoots only tells part of the physiological story, while also limiting phytotoxicity and fruit set measurements. However, using intact trees in the field introduces the possible complication that trees may respond differently to dormancy breaking treatments if that have received adequate winter chill compared with when they have not. To understand the response of mature trees to dormancy breaking treatments both under adequate and inadequate winter chill conditions, a novel whole-tree, open top field heating system was designed. Bearing-age walnut trees (‘Chandler’) were utilized to test dormancy breaking treatments, with trees heated to experience projected mid-century winter conditions paired with trees experiencing ambient winter conditions. Scaffolds within each tree were treated with one of three dormancy breaking treatments, hydrogen cyanamide (e.g., Dormex^®), Erger^® (a blend of nitrogen compounds) and Forchlorfenuron (CPPU), an analog of the plant hormone cytokinin, along with a water control. Timing of budbreak was found to vary significantly based on chemical treatment and whether trees received adequate chill, with a significant interaction between chill experienced and phenological response to chemical treatment. These results indicate that conclusions regarding efficacy of dormancy breaking treatments based on their use in adequate chill winters may not reflect their utility under the warmer winters of climate change.