Effect of chilling on calorimetric responses of dormant vegetative apple buds A.A. Gardea * , E. Carvajal-Milla Ân, J.A. Orozco, V.M. Guerrero, J. Llamas Centro de Investigacio Ân en Alimentacio Ân y Desarrollo, Unidad Cuauhte Âmoc. Apdo. Postal 781, 31570 Cuauhte Âmoc, Chih. Mexico Received 30 June 1999; received in revised form 28 September 1999; accepted 28 September 1999 Abstract Chilling exposure is important in apple buds to overcome dormancy. The objective of this work was to determine the effect of different chilling treatments on apple bud calorimetric responses. One-year-old Golden Delicious trees were kept in a controlled temperature chamber at 48C for up to 1600 h. At 200 h intervals, ®ve trees were transferred to forcing conditions at 258C. Isothermal calorimetry was used to determine metabolic activity (q), respiration rate (R CO2 ), respiration ef®ciency (q/ R CO2 ) and speci®c growth rate (R SG D 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 signi®cant differences (p0.05) in the above respiration parameters were found between chilling treatments during cold storage and immediately when the trees were transferred to growing conditions (0 days at 258C). Also, the lowest values registered for q (2.5 mW mg/ dw) and R CO2 (8 mmol CO 2 mg/dw) were found during the above two periods. However, after 15 and 30 days at forcing conditions, buds increased their metabolism at a slow, but signi®cant pace, as a direct function of chilling exposure time. After 15 days under forcing treatments q increased from 2.5 to 4 mW mg 1 dw, R CO2 from 9 to 13.5 mmol CO 2 mg 1 dw and R SG D HB from 1 to 2.5. After 30 days, these responses kept increasing, q from 2.5 to 5 mW mg 1 dw, R CO2 rose from 10 to 17 mmol CO 2 mg 1 dw and R SG D HB from 2 to 3. On both periods (15 and 30 day), highest values were found after chilling exposures more than 1000 h at 48C, which coincides 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 increased their calorimetric responses after forcing for 15 and 30 days, but this was signi®cant only when chilling equaled or exceeded 1000 h. Calorimetric parameters agree with information available elsewhere on chilling requirements for this variety # 2000 Elsevier Science B.V. All rights reserved. Keywords: Chilling; Calorimetry; Dormancy; Apple; Budbreaking 1. Introduction Perennial plants from the temperate zone developed dormancy as a mechanism to overcome winter adverse conditions [1]. Dormancy can be described as a pro- cess that inhibits growth of several meristematic tis- sues. Dormancy allows plants to synchronize growth and development so that making it possible to survive under harsh environments [2]. Many apple (Malus domestica Borkh) varieties need exposures from 1200 to 1500 h under temperatures below 78C to ful®ll chilling requirement to overcome dormancy. Once Thermochimica Acta 349 (2000) 89±94 * Corresponding author. Tel.: 52-158-12921; fax: 52-158- 12921. E-mail address: gardea65@hotmail.com (A.A. Gardea) 0040-6031/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII:S0040-6031(99)00500-6