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 and several days or weeks of mild subfreezing temperatures at night, followed by above-freezing temperatures in the day. Hence, the wheat plants usually are first exposed to potentially damaging subfreezing temperatures when they have high moisture content, are growing in very wet soil, and have been exposed to freezethaw cycles for a period of time. These conditions are conducive to freezing stresses and plant responses that are different from those that occur under lower moisture conditions without freeze-thaw cycles. This study was conducted to investigate the impact of mild subfreezing temperature and a freeze-thaw cycle on the ability of 22 winter wheat cultivars to tolerate freezing in saturated soil. Seedlings that had been acclimated at +4°C for 5 weeks in saturated soil were frozen to potentially damaging temperatures under three treatment conditions: (1) without any subzero pre-freezing treatment; (2) with a 16-h period at −3°C prior to freezing to potentially damaging temperatures; and (3) with a freeze-thaw cycle of −3°C for 24 h followed by +4°C for 24 h, followed by a 16-h period at −3°C prior to freezing to potentially damaging temperatures. In general, plants that had been exposed to the freeze-thaw cycle survived significantly more frequently than plants frozen under the other two treatments. Plants that had been exposed to 16 h at −3°(without the freeze-thaw cycle) before freezing to potentially damaging temperatures survived significantly more frequently than plants that were frozen to potentially damaging temperatures without a subzero pre-freezing treatment. These results indicated that cold-acclimated wheat plants actively acclimate to freezing stress while exposed to mild subfreezing temperatures, and further acclimate when allowed to thaw at +4°C for 24 h. The cultivar Norstar had the lowest LT 50 (temperature predicted to be lethal to 50% of the plants) of the 22 cultivars when frozen with either of the subzero pre-freezing treatments, but several cultivars had lower LT 50 scores than Norstar when frozen without a subzero pre-freezing treatment. We conclude it may be possible to improve winterhardiness of wheat grown in saturated soil by combining the ability to effectively respond to mild subzero pre-freezing temperatures with a greater ability to withstand freezing to damaging temperatures without a subzero prefreezing exposure.

Winter wheat (Triticum aestivum L. em. Thell.) is sown in the autumn and harvested the following summer, and therefore must survive subfreezing temperatures for several months. Because of autumn rains and winter snows, the plants usually are subjected to these subfreezing temperatures while growing in saturated soil. As the plants freeze, they are subjected to freezing episodes that may vary in the cooling rate, the minimum temperature, the time at the minimum temperature, and the warming rate as the freezing episode ends. We investigated the impact of each of these freezing process components on the ability of 22 winter wheat cultivars to survive freezing in saturated soil using logistic regression and cluster analyses. The 22 cultivars formed three distinct groups when clustered on the odds ratios associated with the freezing process components. The distinctiveness of the three clusters indicated the cultivars within each cluster responded differently to the freezing process than cultivars in the other clusters. Some cultivars occurred in different clusters but had equal levels of freezing tolerance. We conclude the logistic regression/clustering analysis identified cultivars that differed in the mechanisms used to respond to freezing stress, and that equal levels of freezing tolerance can be attained through these different mechanisms. It may be possible to improve winterhardiness by genetically combining these disparate responses to the freezing process components.

on the phospholipid profiles. It has been suggested that alterations in concentration of specific phospholipids Phospholipid (PL) composition is known to change in plants explay a role in cold acclimation. Horvath et al. (1979) posed to cold temperature. The dynamics of PL acyl chain pairs and genes encoding phospholipase enzymes were studied in winter wheat observed an inverse relationship between survival and (Triticum aestivum L.) during cold acclimation. Mass spectrometry the loss of phosphatidylcholine by conversion to the corwas used to characterize PL dynamics, and quantitative real-time responding phosphatidic acid in field-grown wheat PCR was used to characterize phospholipase gene mRNA transcript plants. A similar conversion of phosphatidylcholine to dynamics during cold acclimation. The proportion of PLs with misphosphatidic acid was reported by De La Roche and matched acyl chains decreased concomitantly with an increase in total Andrews (1973) in plants grown at 2ЊC and morphologi-PLs during the first week of cold exposure. Proportions of mismatched cally equivalent plants grown at 24ЊC, again suggesting acyl chains then increased, while total PLs varied little. Numbers of these phospholipid dynamics are a function of plant phe-mRNA transcripts of phospholipase (PL)D, PLC, and PLA 2 increased nology rather than cold acclimation. in response to cold and remained at elevated levels throughout a 4-wk The proportion of unsaturated fatty acids, particularly period. Lysophosphatidylcholine (LPC) increased as much as 14-fold over the 5-wk period and increased significantly less in a less cold linolenic acid (18 carbons, three double bonds [18:3]), tolerant cultivar than more tolerant cultivars. It appeared that newly 209 Johnson Hall, Washington State Univ., Pullman, WA 99164; S. Halls and W.F. Siems,

As the seasons progress, autumn-planted winter wheat plants (Triticum aestivum L.) first gain then progressively lose freezing tolerance. Exposing the plants to freeze-thaw cycles of −3/3°C results in increased ability to tolerate subsequent freezing to potentially damaging temperatures. This study was conducted to determine to what extent the length of time that a plant is grown at low temperatures influenced the effectiveness of this freeze-thaw enhancement of freezing tolerance. Plants from six winter wheat lines were grown at 4°C for 1-18 wk, exposed to 0-2 cycles of freezing to −3°C for 24 h, then thawed for 24 h at 3°C, then tested for their ability to tolerate freezing to −10°C to −17°C. The freeze-thaw treatments resulted in increased freezing tolerance after 6-12 wk of growth at low temperatures, but had no significant effect before or after that time period. Two cycles of −3/3°C freeze-thaw was consistently more effective than one cycle. Variation in the extent and timing of the effectiveness of the freeze-thaw treatments was found among the wheat lines, suggesting genetic variation that may be useful for prolonging freezing tolerance further into the winter months could be found in winter wheat.

Freezing tolerance resulting from cold hardening is critical to survival of fall‐seeded winter wheat. Exposure of winter wheat plants to cycles of freeze–thaw at temperatures just below, and just above freezing results in incremental improvements of freezing tolerance. Changes in the concentrations of carbohydrates in the cellular fluids of wheat crowns, and of lipids extracted from wheat crown tissue, were quantified following exposure to 24 h freeze(s) at –3°C, with or without a following thaw of 24 h at 3°C. Concentrations of simple sugars, and sucrose and related carbohydrates, increased in the first 24 h of exposure to –3°C, and continued to increase whether the plants were then exposed to a further 24 h at –3°C or were exposed to 3°C for 24 h. The concentration of fructans decreased during the first 24 h of exposure to –3°C, and continued to decrease if the plants were exposed to 3°C for 24 h, but increased if the plants were exposed to a continuous –3°C for 48 h. The frequencies of occurrences of numerous lipid species changed with each 24 h temperature treatment, some consistent with the behavior of lipid signaling molecules. Carbohydrates and lipids in wheat crowns are actively restructured at subzero temperatures as the plants acquire greater freezing tolerance. Identifying plant lines especially able to effect this restructuring may enable the development of plant lines with improved freezing tolerance.