Effects of water potential and thermal history on survival of second-stage juveniles of Meloidogyne hapla in frozen soil.

摘要

Second-stage juveniles of Meloidogyne hapla Chitwood undergo cold hardening. Low temperatures induce physiological changes that increase the ability to survive freezing. Temperatures tested ranged from {dollar}-{dollar}4 to 24 C, and the greatest freezing tolerance developed in juveniles exposed to 4 C. Cold hardening is rapid; the percentage of juveniles surviving freezing (i.e. freezing tolerance) increased to about 80% and plateaued within 12 hours of exposure to 4 C. Cold hardening was demonstrated for juveniles in polyethylene glycol solutions and soil. Long-term exposure (1 to 15 days) to low temperatures did not increase freezing tolerance as compared to the 12-hour exposure, but did increase the stability of freezing tolerance. Freezing tolerance lost when nematodes were returned to 24 C for 48 hours was related to the duration of prior exposure to 4 C.; Survivorship of juveniles in frozen soil was increased by exposure to soil-water potentials of {dollar}-{dollar}1910 to {dollar}-{dollar}167 kPa prior to freezing. Saturating the soil immediately before freezing reduced survivorship, but it was still greater than survivorship of juveniles exposed only to high water potentials ({dollar}-{dollar}7 to {dollar}-{dollar}2.5 kPa). Thus, it appears that low soil-water potentials increased survival 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.; The increase in freezing tolerance in response to low water potentials was studied further using polyethylene glycol to control water potential ({dollar}-{dollar}2050 to 0 kPa). The effects of water potential and temperature were interactive. Exposure to low water potentials increased freezing tolerance of juveniles at 24 C, but not at 0 C.; Freezing generates extremely low water potentials, and juveniles extracted from frozen soil or solutions appeared desiccated. Survivorship in frozen solutions was compared to survivorship of water potential and temperature combinations equivalent to those of frozen solutions. Survivorship was the same in both situations, indicating that desiccation may be an important stress responsible for mortality in frozen conditions.