ABSTRACTa period of low, above-freezing temperatures, termed cold-hardening. Cold-hardening is a critical event in theThe survival of cereal crops during winter depends primarily on the over-wintering physiology of plants (Levitt, 1980 with an increase in freezing tolerance (Steponkus, 1978).stages of recovery after they had been grown and frozen under controlled conditions. Our results confirmed those reported for barleyHowever, it has been difficult to establish cause and

The complex nature of freezing in plants may be easier to understand if freezing is studied in nonacclimated plants at temperatures just below freezing. Thermal patterns of model systems frozen at –2.6 °C were compared with those of crown tissue from oat ( Avena sativa L.). Thermal patterns of live crowns more closely resembled those of fructan and sugar solutions with filter paper than of plain water or a BSA solution. When the percentage of water freezing in nonacclimated plants at –2.6 °C was manually limited to 10%, the survival was reduced from 100% in supercooled plants to 25%. During cold acclimation, the percentage of water freezing at –2.6 °C went from 79% to 54% after 3 weeks of cold acclimation and resulted in 100% survival. The nucleus of cells in the primary apical meristem of nonacclimated plants appeared to have disintegrated, an effect that was not observed in any cold-acclimated (unfrozen controls) plants. Nuclear pycnosis was observed in leaf sheaths surrounding the meristem and in cells directly below the meristem. Cells of secondary meristems and in the crown core appeared undamaged, but vessels in plants frozen for as little as 30 min were ruptured and appeared plugged. The distinctive nature of injury in the apical meristem and the rapid ability of the plant to acclimate during cold to the stress causing this injury indicate that specific tissue, namely the apical region of the crown, should be the focus of attention when attempting to determine cause and effect between genetics or metabolism and cold acclimation in winter cereals.