Freezing tolerance of winter wheat plants frozen in saturated soil

Winter wheat is sown in the autumn and harvested the following summer, necessitating the ability to survive subfreezing temperatures for several months. Autumn months in wheat-growing regions typically experience significant rainfall. Hence, the wheat plants usually are exposed to freezing temperatures when they have high moisture content and are growing in very wet soil. Both of these conditions are conducive to freezing stresses different from those that occur under lower moisture conditions. This study was conducted to seek genetic variability among winter wheat lines and their progeny in the ability to tolerate freezing in saturated soil. Fully acclimated seedlings in saturated soil were frozen to a narrow range of temperature conditions that resulted in about 50% mortality of the most freezing tolerant lines studied. The temperature of the soil near the crowns of the plants was recorded every 2 min throughout each freezing episode. The following components were then determined for each freezing episode: the amount of time the plants remained in subfreezing temperature before all freezable water had been converted to ice; the rate of cooling from the freezing temperature to the minimum temperature; the minimum temperature; the length of time the plants remained at the minimum temperature; the rate of temperature increase from the minimum to 0 °C after freezing; and the total amount of time the plants were actually frozen. Partial regression analysis revealed the minimum temperature significantly influenced survival in all of the progeny populations, while the other five components significantly influenced survival in some, but not all of the populations, suggesting genotypic differences in the ability to tolerate variation in specific aspects of the freezing process. Evidence from progeny populations suggested that improved freezing tolerance was associated with decreased sensitivity to the length of time held at the minimum temperature and increased responsiveness to the post-freezing warming rate. Further analysis of this kind of variation may enable the genetic combining of sources of tolerance of the stresses imposed by specific components of the freezing process, leading to cultivars with improved tolerance of freezing in saturated soil.