Imaging methods are giving new insights into plant freezing and the consequent damage that affects survival and distribution of both wild and crop plants. Ice can enter plants through stomata and hydathodes. Intrinsic nucleation of freezing can also occur. The initial growth of ice through the plant can be as rapid as 40 mm s −1 , although barriers can limit this growth. Only a small fraction of plant water is changed to ice in this first freezing event. Nevertheless, this first rapid growth of ice is of key importance because it can initiate further, potentially lethal, freezing at any site that it reaches. Some organs and tissues avoid freezing by supercooling. However, supercooled parts of buds can dehydrate progressively, indicating that avoidance of freezing-induced dehydration by deep supercooling is only partial. Extracellular ice forms in freezing-intolerant as well as freezing-tolerant species and causes cellular dehydration. The single most important cause of freezing-damage is when this dehydration exceeds what cells can tolerate. In freezing-adapted species, lethal freezing-induced dehydration causes damage to cell membranes. In specific cases, other factors may also cause damage, examples being cell death when limits to deep supercooling are exceeded, and death of shoots when freezing-induced embolisms in xylem vessels persist. Extracellular masses of ice can damage the structure of organs but this may be tolerated, as in extra-organ freezing of buds. Experiments to genetically engineer expression of fish antifreeze proteins have not improved freezing tolerance of sensitive species. A better strategy may be to confer tolerance of cellular dehydration. Copyright 2001 Annals of Botany Company

Received: 24 February 2000 ; Returned for revision: 9 May 2000 . Accepted: 22 November 2000


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