Changes in the histology of cold-hardened oat crowns during recovery from freezing

The survival of cereal crops during winter depends primarily on the ability of tissue in the crown to withstand various stresses encountered during freezing. Freeze-induced damage to specific regions of oat (Avena sativa L.) crowns was evaluated by sectioning plants at various stages of recovery after they had been grown and frozen under controlled conditions. Our results confirmed those reported for barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.) that the apical meristem was apparently the tissue in the crown most tolerant of freezing. Photographs of sections during recovery provided evidence that the apical meristem within the crown survived freezing in plants that were rated as nonsurvivors. Closer examination revealed abnormal nuclei in many cells of plants that had been frozen. These cells with condensed and dark red chromatin resembled the description of nuclear pycnosis found in mammalian cells damaged by radiation, extreme abiotic stress, and various carcinogens. The crown meristem complex was separated from the crown and fractionated into two regions: the upper portion of the crown meristem complex, called the apical region, and the lower portion called the crown core. The dry weight of both the apical region and crown core increased during cold-hardening but the increase in dry weight was higher in the crown core than in the apical region. During cold-hardening the percentage of total water freezing at -2 degrees C became lower and after 3 wk was 50 and 47% in the apical region and the crown core, respectively. The initial freezing rate of the apical region was higher than that of the crown core and reached equilibrium about 2 h earlier than the crown core. Differences are discussed in relation to the freezing survival of specific tissue.