Low temperatures cause cellular damage in flower buds of the sweet cherry (Prunus avium L. cv. Satohnishiki). In this study, the redox states within the cherry buds suffering freezing damage were non-destructively observed by a unique in vivo electron spin resonance (ESR) technique with a spin probe such as carbamoyl-PROXYL. The ESR signals of carbamoyl-PROXYL-treated bud were continuously recorded under freezing and thawing condition, which was decreased to approximately -4degreesC and maintained for 1.5 h, and then returned to room temperature. Most of the buds began to freeze at -2.5 to -3.9degreesC. The peak areas of the ESR signals significantly increased during the period of temperature rise. These results show that the reduced carbamoyl-PROXYL within the frozen bud was re-oxidized and became ESR-detectable while the bud was thawing. Our in vivo ESR technique has confirmed the oxidative transition of the redox states within the buds during thawing. (C) 2003 Elsevier Inc. All rights reserved.