Changes in respiration pathways during hydrogen cyanamide-induced terminal budbreak of Cripps’ Pink

Apple trees that do not receive adequate chill during winter undergo improper release of endodormancy resulting in decreased bud break, and uneven and delayed blooming that impacts negatively on yield. To overcome this, chemical rest breaking agents are used to artificially release the buds from endodormancy and synchronise bud break. This study was conducted to investigate the effects of hydrogen cyanamide (HC) (Dormex™) and oil on the total respiration rate and the rate of four respiratory pathways (tricarboxylic acid cycle (TCA), pentose phosphate pathway (PPP), alternative pathway (ALT) and cytochrome C (CYT) pathways). The HC-treated endodormant terminal buds of mature Cripp’s Pink trees were randomly collected from commercial orchard in Elgin (warm winter area; ±700 CU). Control shoots were covered with plastic bags during the treatment. The respiration rate was determined by using a Clark-type oxygen electrode fitted to a liquid filled chamber containing the buds. The respiration rate of the different pathways was measured by using inhibitors targeting the specific pathways. Means were compared using a two-way ANOVA. The results showed that Dormex™ and oil treatment cause an increase in the respiration rate in the TCA and CYT pathways reaching a maximum at the budbreak. The untreated buds continued to show a crippled respiration rate with an increasing use of the ALT pathway. The study concludes that the treatment with Dormex™ and oil prevents delayed foliation and unsynchronised, poor bud break by increasing the respiration rate of bud tissue via the TCA and CYT pathways to provide energy for growth.