Cold hardened seedlings of winter wheat (Triticum aestivum L. em Thell) show an hypoxic hardening response: an exposure to low temperature flooding increases the tolerance of plants to a subsequent ice encasement exposure. Seedlings of winter barley (Hordeum vulgare L.) do not show such a response in similar experimental conditions. During ice encasement, there are general declines in adenylate energy charge (AEC), total adenylates and ATP:ADP ratios in the crown tissues of two winter wheat cultivars, and a winter barley, but rates of decline are faster in the barley. When the ice period is preceded by low temperature flooding of the whole plant, levels of the adenylate components are raised significantly in the wheats, and to a lesser extent in the barley. The survival of plants in ice preceded by flooding is related to the increased initial level of adenylates at the onset of the ice encasement stress, and the maintenance of higher levels of adenylates and ATP in the early stages of ice encasement as a result of accelerated rates of glycolysis. Higher survival of both winter wheat and barley plants during ice encasement in the light is also associated with significantly higher levels of AEC and adenylates in the early stages of ice encasement.of flooding increases tolerance to the subsequent more severely hypoxic or anoxic stress of ice encasement. Ice encased plants which were previously flooded accumulate higher levels of ethanol and have greater alcohol dehydrogenase (Adh)2 activity, and it was proposed that they enter the ice encasement stress period with an accelerated rate of glycolysis (6). A similar metabolic acclimation at warm temperatures has recently been described in root tips of Zea mays (23) In field conditions, low temperature flooding frequently precedes ice encasement. Rakitina (22) reported that with flooding at 2°C and ice encasement at -5°C, flooding reduced the tolerance ofwinter wheats to subsequent ice. In conditions in eastern Canada, ice normally forms just below the freezing point, and we found that flooding at 2°C promoted the subsequent survival of winter wheat, but not winter barley plants in ice at -1°C (5). The flooding effect could be simulated by exposing the plants to a nitrogen atmosphere (6). This is a metabolic acclimation, in which the hypoxic stress '

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