Injury within the crown of winter wheat seedlings after freezing and icing stress

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Abstract
The cells in the crown of winter wheat cv. Fredrick critical for the survival of freezing and icing stress were identified using tetrazolium staining as a viability test. In acclimated seedlings, a freezing stress which lowered regrowth (-12.degree. C) also lowered tetrazolium staining in the vascular transition zone in the basal portion of the crown but generally did not affect the staining of the apical meristem. The majority of cells in the crown, including the apical meristem, were able to reduce tetrazolium after a lethal freezing stress. Thus, survival was limited by the freezing tolerance of a relatively small number of cells in the basal region of the crown. These observations were confirmed using plasmolysis and mitotic figures as alternative indices of viability. No significant variability was observed among winter wheat cultivars. However, in seedlings not acclimated to freezing stress, there was quite a different pattern of injury. In these seedlings, the sensitivity of the apical and basal regions to freezing was similar. Thus, these 2 regions appeared to differentially acclimate and the cells of the apical meristem developed greater cold hardiness than that of the basal area. After a lethal icing stress, all regions within the crown were able to reduce tetrazolium, but the crown was unable to regrow. The ability to reduce tetrazolium was gradually lost during the regrowth period. Unlike freezing stress, no differential sensitivity was observed within the crown, and there was no variability among the cultivars of winter wheat examined.