An experiment was conducted to examine the interaction between chilling exposure (0, 50, 100, 200, 400, and 800 hours at 3C) and hydrogen cyanamide (H2CN2) concentration [0%, 1.25%, and 2.50% (v/v)] on the budbreak of dormant grape buds (Vitis vinifera L. `Perlette') collected in late fall before the onset of temperatures ≤13C. Budbreak at 22C was most rapid for cuttings exposed to 800 chill hours and least rapid for cuttings that received no chilling. Budbreak of cuttings receiving 50 to 200 hours of chilling was similar and lagged behind that of cuttings exposed to 400 or 800 hours. Maximum observed budbreak improved with increased chilling exposure. Hydrogen cyanamide hastened the growth of all chilling treatments and increased the percent budbreak of cuttings receiving ≤400 chill hours. When cuttings were not treated with H2CN2, the number of days required for 50% budbreak declined sharply as chilling exposure increased from 0 to 400 hours. In contrast, this interval was reduced only slightly as chilling increased from 400 to 800 hours. Hydrogen cyanamide-treated buds exhibited a more gradual decline in the number of days required for 50% budbreak with increased chilling exposure. In this study, the physiological efficacy and economic benefits of H2CN2 applications diminished with increased chilling exposure.
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