Freezing tolerance of winter wheat plants frozen in saturated soil

Authors: Skinner, Daniel; Mackey, Bruce

Submitted to: Field Crops Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/22/2009
Publication Date: 7/28/2009
Citation: Skinner, D.Z., Mackey, B.E. 2009. Freezing tolerance of winter wheat plants frozen in saturated soil. Field Crops Research 113:335-341.

Interpretive Summary: Five winter wheat lines and all possible progeny were evaluated for their ability to survive two levels of freezing stress. The freezing episodes were described as cooling rate, warming rate, minimum temperature, time at minimum temperature, and "degree minutes" (temperature multiplied by the time at that temperature, summed over the freezing episode). Generally, the genes with the strongest effects increased sensitivity to freezing. Measurements of the significance of the components of the freezing process indicated that cooling rate and warming rate were important to survival under mild freezing stress, but not under severe stress. The minimum temperatures reached, and the lengths of time at those temperatures were important to survival under severe freezing stress. The levels of significance of these components were not consistent among the populations, indicating that genetic variation for the ability to tolerate changes in warming and cooling rate may exist in the wheat lines examined. This approach provides a new way to assess the ability of wheat to withstand the freezing process, providing new tools to use in the plant improvement process.

Technical Abstract: 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 two minutes 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.