Effect of chilling on calorimetric responses of dormant vegetative apple buds

Chilling exposure is important in apple buds to overcome dormancy. The obje ctive of this work was to determine the effect of different chilling treatm ents on apple bud calorimetric responses. One-year-old Golden Delicious tre es were kept in a controlled temperature chamber at 4 degrees C for up to 1 600 h. At 200 h intervals, five trees were transferred to forcing condition s at 25 degrees C. Isothermal calorimetry was used to determine metabolic a ctivity (q), respiration rate (R-CO2), respiration efficiency (q/ R-CO2) an d specific growth rate (R-SG Delta(HB)) in vegetative buds. DSC was used to estimate activation energy (E-a). Calorimetric assays were performed at 0, 15 and 30 days at forcing conditions and budbreak was recorded. No signifi cant differences (p less than or equal to 0.05) in the above respiration pa rameters were found between chilling treatments during cold storage and imm ediately when the trees were transferred to growing conditions (0 days at 2 5 degrees C). Also, the lowest values registered for q (2.5 mu W mg/ dw) an d R-CO2 (8 mmol CO2 mg/dw) were found during the above two periods. However , after 15 and 30 days at forcing conditions, buds increased their metaboli sm at a slow, but significant pace, as a direct function of chilling exposu re time. After 15 days under forcing treatments q increased from 2.5 to 4 m u W mg(-1) dw, R-CO2 from 9 to 13.5 mmol CO2 mg(-1) dw and R-SG Delta(HB) f rom 1 to 2.5. After 30 days, these responses kept increasing, q from 2.5 to 5 mu W mg(-1) dw, R-CO2 rose from 10 to 17 mmol CO2 mg(-1) dw and R-SG Del ta(HB) from 2 to 3. On both periods (15 and 30 day), highest values were fo und after chilling exposures more than 1000 h at 4 degrees C, which coincid es with an increased budbreak of 12%. No consistent pattern was observed in q/R-CO2, regardless of exposure to forcing conditions. Cumulative chilling resulted in decreasing E-a of 29.8 and 33.3 J mol(-1) K after forcing for 15 and 30 days, respectively. In general, dormant vegetative apple buds inc reased their calorimetric responses after forcing for 15 and 30 days, but t his was significant only when chilling equaled or exceeded 1000 h. Calorime tric parameters agree with information available elsewhere on chilling requ irements for this variety (C) 2000 Elsevier Science B.V. All rights reserve d.