Abstract
The processes of freezing injury in Jerusalem artichoke (Helianthus tuberosus L.) tubers were studied using protoplasts isolated from cold-acclimated and deacclimated tubers. Prior to freezing, protoplasts were pre-loaded with 10^ M PDA (fluorescein diacetate) in an isotonic sorbitol solution. After freeze-thawing cycle at various temperatures, cell viability was evaluated under a fluorescence microscope according to the fluorescence images. In cold-acclimated tubers, more than 70% of protoplasts survived freezing to -25℃. By contrast, in deacclimated tubers, only 39% of protoplasts survived freezing to -10℃. Thus, freezing tolerance of proto-plasts differed significantly between cold-acclimated and deacclimated tubers. Two distinct types of injury were observed depending on freezing temperatures, which were caused by damage to plasma membrane (cell burst type) and damage to vacuolar membrane (abnormally stained type), respectively. The decrease in freezing tolerance of protoplasts after deacclimation was suggested to be mainly due to destabilization of vacuolar membrane against freeze-induced dehydration stress.
