ASHS 2025 Conference

The mechanism controlling dormancy in buds of woody perennial plants remains largely unknown. However, it is known that exposure to cold temperatures (chilling) promotes the transition from a non-responsive to a responsive status to growth-conducive temperatures (i.e., endo- to ecodormancy transition). In horticulture, hydrogen cyanamide (HC) has been used for decades to overcome chilling accumulation deficiencies for temperate fruit crops grown in subtropical climates. Given the connection between cold hardiness loss and budbreak, we hypothesized that HC would increase the rate of cold hardiness loss (deacclimation rate). To test this, we collected grapevine (Vitis hybrid ‘Petite Pearl’) cuttings from field conditions in Madison, WI in approximately bi-weekly intervals from December to April. Single node cuttings were prepared and randomly separated into two treatments: a control group [0.5% surfactant (Regulaid®, KALO, Inc.)], and an HC group [5% hydrogen cyanamide (Dormex®, Alzchem Group AG) and 0.5% surfactant]. Both groups were treated by submersion for 10s. Cuttings were then placed in cups of water, and under forcing conditions. The forcing conditions used for all collections were 22ºC and 16h light. In later collections, two additional forcing temperatures were used: 8ºC and 16ºC. Cold hardiness was measured using differential thermal analysis on the day of treatment application, from field collected buds (n>10), and in semi-regular intervals from cuttings under forcing conditions, with interval length depending on temperature [e.g., quasi-daily at 22ºC (T0 1d, T0 2d, …); about every two days for 16ºC (T0 2d, T0 4d, …), about every five days for 8ºC]. At each collection time and for each temperature and treatment, 10 cuttings were set apart to observe time to budbreak. As expected, budbreak occurred earlier in HC treated buds compared to control. Using measurements of cold hardiness over time under forcing, we determined deacclimation rates as the slope of linear regressions. The rate of deacclimation in the control group increased progressively with each collection, as chilling accumulated in the field (1.0ºC/d in December to 1.4ºC/d in March). However, the deacclimation rate of the HC-treated group was always greater than the control (1.6ºC/d in December and 1.7ºC/d in March). In March, at 16ºC, there was also a difference between control and HC group in deacclimation rate (1.2ºC/d and 1.4ºC/d), while there were no differences at 8ºC. HC increases the rate of deacclimation in grapevines. We anticipate that understanding the interplay between cold hardiness, deacclimation, and budbreak will be helpful in uncovering the dormancy mechanism.