Midori Yoshida

The LT₅₀ values and soluble carbohydrate levels in wheat (Triticum aestivum L.) crowns and leaves were monitored throughout autumn and winter in cultivars varying in freezing tolerance and snow mold resistance during 1993/1994 and 1994/1995 in the field at Sapporo, Japan. During the first stage of hardening, from sowing to mid‐November, the pattern of accumulation of mono‐ and disaccharides was similar for all cultivars. During the second stage of cold hardening, from mid‐November to mid‐December, the greatest accumulation of mono‐ and disaccharides, without a corresponding increase in fructan, was observed among the freezing‐tolerant cultivars; and levels of simple saccharides rapidly decreased under snow cover. Conversely, levels of mono‐ and disaccharides in snow mold‐resistant cultivars were less than 70% of those in freezing‐tolerant cultivars before snow cover and maintained low levels throughout winter, while polysaccharide levels in snow mold‐resistant cultivars were about 120% of those in freezing‐tolerant cultivars in December. Sugar metabolism during the winter was examined using 18 cultivars in 1994/1995. LT₅₀ values were correlated to the greatest extent with total mono‐ and disaccharide and fructan content among wheat cultivars excluding snow mold‐resistant cultivars in December. Snow mold‐resistant cultivars tended to metabolize carbohydrates more slowly until the end of the snow cover period. This result suggested that the enzymatic metabolism of the synthesis of sugars and the conversion of fructan to cryoprotective sugars in response to low temperatures, especially subzero ones, might be different between the two contrasting types in resistance to winter stress.

The relationship between freezing tolerance (expressed as LT₅₀, the lethal freezing temperature for 50% of plants) and the amount and physical state (as determined by spin‐lattice [T₁] and spin‐spin [T₂] relaxation times of protons) of water in crown tissue was examined in contrasting winter wheat (Triticum aestivum L.) varieties grown under field conditions from 1992 to 1994. During acclimation, the LT₅₀ values decreased from around ‐7 to ‐17, ‐20 and ‐27°C in PI 173438, Chihokukomugi and Valuevskaya, respectively. Tissue water content decreased continuously through autumn to reach a plateau around 3 g H₂O (g dry weight)^‐1 in early winter when LT₅₀ was still decteasing, and then gradually increased under snow cover. A significant negative correlation was found between mean minimum air temperatures and freezing tolerance prior to the establishment of continuous snow cover. In contrast, a positive association between mean minimum temperatures and crown tissue water content was significant only when air temperatures were above 0°C, as water content did not decrease further at sub‐zero temperatures. Seasonal changes in T₁ were closely related to changes in freezing tolerance. T₁ decreased until January even though water content stopped decreasing. Further tests on 15 field‐grown varieties confirmed a strong negative association between freezing tolerance and T₁. The results suggest that cold hardening is comprised of two stages, with the transition occurring at ca 0°C. Development of hardiness was related to (1) a reduction in water content in the first stage (at minimum temperatures > 0°C), and (2) a change in physical state of water without much reduction in water content in the second stage. Varietal differences in hardiness thus arise due to changes in both water content and physical state of water.