| Summary: | The nature of ice-sheet damage to overwintering forage plants was studied in a controlled environment at non-injurious freezing temperatures. The soil atmosphere was analyzed in a gas-chromatograph and the plants were assessed for injury by histological examination and recovery rates in the greenhouse. Under experimental ice-covers carbon dioxide accumulated in the soil in some instances to as high as 10% while oxygen was depleted to less than 4% of the atmosphere. Plants rooted in such soils were killed after 7 weeks of ice-cover. When the soil under the ice-sheet was flushed with carbon dioxide the plants were killed after periods as short as 21 days. In both cases injury appeared to be physiological rather than mechanical. Furthermore, carbon dioxide accumulation rather than oxygen depletion was responsible for the injury since the plants were able to withstand periods of 3 weeks in a nitrogen-saturated soil. A freeze-thaw-freeze cycle, with moderate freezing temperatures and associated with an ice-sheet, did not appear to be damaging to alfalfa. Continuous ice-covers resulted in a greater accumulation of carbon dioxide and consequently more injury suffered by the plants than where the cover was temporarily broken by a thaw. High soil-moisture conditions which are usually associated with ice-sheets did not result in an increased hydration level in the tissue and consequently did not make the plants more susceptible to cold injury. A technique was developed to determine the ability of plants to withstand ice-encasement. Several varieties and species that were tested exhibited no clear-cut correlation between resistance to ice encasement and frost hardiness. Land and Food Systems, Faculty of Graduate
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