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 RCO2RCO2, metabolic efficiency (Rq/RCO2)(Rq/RCO2) 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, RCO2RCO2, Rq/RCO2Rq/RCO2 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), RCO2RCO2 (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 Rq/RCO2Rq/RCO2 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.