Respiratory response of apple buds treated with budbreaking agents

Isothermal calorimetry was used to evaluate the effect of budbreaking agents (BBA) on metabolic rates Rq and image, metabolic efficiency image and specific growth rate (RSG·ΔHB) in apple flower buds during late winter. Three BBA were tested (hydrogen cyanamide, HC at 1.0% (v/v); thidiazuron (1-phenyl-3-(1,2,3-thiadiazol-5-yl)urea), TDZ at 0.3% (v/v) and mineral oil, MO at 1% (v/v)). Water was used as a control. BBA application was performed at two Rq rates (4.8 and 5.3 μW mg−1 dw, corresponding to 30 and 15 days before budbreak, approximately). Rq, image, image and RSG·ΔHB were measured during 18 days at 3 day intervals under forcing conditions. Budbreak percentage was scored at the end of the experiment. Significant differences in the calorimetric parameters (p ≤ 0.05) were found between BBA treatment and BBA application date. The highest Rq (11.4 μW mg−1 dw), image (32.2 mmol CO2 mg−1 dw), RSG·ΔHB (2.8 μW mg−1 dw) and budbreak percentage (95%) values were found for HC treated buds when application was performed 15 days before budbreak. These samples showed the lowest image value (368 kJ mol−1) confirming an increase in bud metabolic efficiency. These results indicate that HC is an effective BBA to overcome blooming problems associated with insufficient winter chilling on apples.