Comparison of the effects of freezing and thawing on the cysts of the two potato cyst nematode species, Globodera rostochiensis and G. pallida, using differential scanning calorimetry

Differential scanning calorimetry was used to examine and to compare the effect of freezing and thawing on the eggs of the two potato cyst nematode (PCN) species, Globodera rostochiensis and G. pallida. No thermal events were recorded for either PCN species when dry cysts were cooled to -45°C and thawed to +5°C. The cyst walls of hydrated cysts of both species were found to contain ice-nucleating agents that significantly increased the temperature at which external and internal water in contact with the cyst wall froze compared with control water droplets. The freezing of hydrated eggs of both species of PCN eggs produced a very broad exotherm ranging from -30°C to -42°C. A significant positive correlation between cyst weight and supercooling point for G. pallida eggs was identified, indicating that cyst size and egg location within the cyst may influence the freezing of PCN eggs on an individual level. Integration of these egg exotherms identified a supercooling point of -37.45°C and -39.05°C for G. pallida and G. rostochiensis, respectively. The supercooling points of eggs of G. pallida but not G. rostochiensis were found to increase significantly on the second freeze cycle of a freeze-thaw-freeze programme. Rehydration of these G. pallida cysts restored the supercooling point to its original level when the cysts were re-analysed, indicating that water was lost from the egg complex of G. pallida during the initial freeze-thaw-freeze programme. The results of this study indicate that the ability of eggs of both species of PCN to supercool appears to be a trait that was developed prior to divergence of the two species.