DIFFERENTIAL SCANNING CALORIMETRY STUDIES ON THE CYSTS OF THE POTATO-CYST NEMATODE GLOBODERA-ROSTOCHIENSIS DURING FREEZING AND MELTING

Differential scanning calorimetry (DSC) was used to determine the ther mal events associated with freezing and melting of the cysts of the po tato-cyst nematode Globodera rostochiensis, There were no thermal even ts during the cooling of dry cysts from 5 to -60 degrees C and warming back to 5 degrees C, indicating the absence of water freezing in the dry cysts, During heating of dry cysts from 5 to 80 degrees C, two ove rlapping endothermic events were observed at 55 degrees C, indicating the irreversible destruction of the permeability barrier of the eggshe ll by the melting of the lipids which constitute the lipid layer. The first exothermic event (T=-9 degrees C) during the cooling of hydrated cysts indicates the presence of an ice-nucleating agent, A broad exot herm at -38 degrees C is due to the freezing of eggs, DSC thus confirm s that the eggshell prevents exogenous ice nucleation and allows the e ggs to supercool in the presence of external ice. The enthalpy of the egg exotherm increased during hydration of the cysts. The temperature of the egg exotherm was elevated after heating of the sample to 70 deg rees C. This is thought to be due to the loss of trehalose from the eg gs following the destruction of the permeability barrier of the eggshe ll. During melting, three endothermic events were observed, These beca me merged after the destruction of the permeability barrier of the egg shell by heating, and only two peaks were observed in isolated eggs, T he sample is thus considered to consist of three freezable compartment s: (1) the water surrounding the cyst, (2) the solution between the cy st wall and the eggs and (3) the egg contents.