Chilling sensitivity of cucumber cotyledon protoplasts and seedlings

Cucumber (Cucumis sativus L.) seedlings are more sensitive to chilling stress when transferred to low temperature from the night cycle than from the day cycle. However, greater damage occurs when chilling is carried out in light than in dark. Freshly isolated protoplasts are extremely sensitive to damage when chilled at 4 degrees C in light, but suffer significantly less injury when chilled in dark. If freshly isolated protoplasts are pre-chill conditioned at 27 degrees C in either light or dark for a few hours prior to exposure to various chilling stresses, subsequent chilling damage is markedly reduced. Damage to chilled protoplasts also is reduced if cultures are placed in dark instead of light immediately following removal from low temperature. Experiments utilizing the cell wall synthesis inhibitor, dichlorobenzonitrile, showed that cell wall regeneration during the pre-chill conditioning period at 27 degrees C does not appear to be associated with the enhanced chilling tolerance observed in these cultures. The results obtained in this investigation suggest that the physiological properties of cucumber cotyledon protoplasts accurately reflect those of intact seedlings, and hence provide a good system for studies into the mechanism of chilling damage in plants.