Effects of water potential and temperature on survival of the nematode Meloidogyne hapla in frozen soil

The effects of water potential on the survival of Meloidogyne hapla second-stage juveniles in frozen soil were studied. Soil-water potentials and freezing temperatures tested ranged from 0 to −1910 kPa and from −1 to −4 °C, respectively. Survivorship was greater in soil at water potentials of −1910 to −520 kPa than in soil at higher water potentials. Saturating the soil immediately before freezing reduced survivorship, but it was still greater for juveniles exposed to low water potentials before saturation. Thus, low water potentials increased survivorship directly by reducing the pore space filled with ice and indirectly by causing physiological changes that increased the ability of juveniles to survive frozen conditions. Exposure to low water potentials in polyethylene glycol solutions at 24 °C also caused an increase in the percentage of juveniles that survived frozen conditions. Regardless of water potential experienced (−2050 to 0 kPa), subsequent exposure to 4 °C caused an additional increase in the percentage surviving frozen conditions. However, exposure to a low water potential (−1110 kPa) at 0 °C did not cause an increase in tolerance of frozen conditions. Juveniles extracted from frozen soil or solutions appeared desiccated. Survivorship in frozen solutions was compared with survivorship at water potential and temperature combinations equivalent to those of the frozen solutions. Survivorship was the same in both situations, indicating that desiccation may be an important stress responsible for mortality in frozen conditions.