Plant freezing and damage

Imaging methods are giving new insights into plant Freezing and the consequ ent damage that affects survival and distribution of both wild and crop pla nts. Ice can enter plants through stomata and hydathodes. Intrinsic nucleat ion of freezing can also occur. Thr 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 freezi ng 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. Ho wever, supercooled parts of buds can dehydrate progressively, indicating th at avoidance of freezing-induced dehydration by deep supercooling is only p artial. 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 t olerate. In freezing-adapted species, lethal freezing-induced dehydration c auses damage to cell membranes. In specific cases, other factors map also c ause damage, examples being cell death when limits to deep supercooling are exceeded, and death of shoots when freezing-induced embolisms in xylem ves sels persist. Extracellular masses of ice can damage the structure of organ s but this may be tolerated, as in extra-organ freezing of buds. Experiment s to genetically engineer expression of fish antifreeze proteins have not i mproved Freezing tolerance of sensitive species. A better strategy may be t o confer tolerance of cellular dehydration. (C) 2001 Annals of Botany Compa ny.