Plant Freezing and Damage

Imaging methods are giving new insights into plant freezingand the consequent damage that affects survival and distributionof both wild and crop plants. Ice can enter plants through stomataand hydathodes. Intrinsic nucleation of freezing can also occur.The initial growth of ice through the plant can be as rapidas 40 mm s^-1, although barriers can limit this growth. Onlya small fraction of plant water is changed to ice in this firstfreezing event. Nevertheless, this first rapid growth of iceis of key importance because it can initiate further, potentiallylethal, freezing at any site that it reaches. Some organs andtissues avoid freezing by supercooling. However, supercooledparts of buds can dehydrate progressively, indicating that avoidanceof freezing-induced dehydration by deep supercooling is onlypartial. Extracellular ice forms in freezing-intolerant as wellas freezing-tolerant species and causes cellular dehydration.The single most important cause of freezing-damage is when thisdehydration exceeds what cells can tolerate. In freezing-adaptedspecies, lethal freezing-induced dehydration causes damage tocell membranes. In specific cases, other factors may also causedamage, examples being cell death when limits to deep supercoolingare exceeded, and death of shoots when freezing-induced embolismsin xylem vessels persist. Extracellular masses of ice can damagethe structure of organs but this may be tolerated, as in extra-organfreezing of buds. Experiments to genetically engineer expressionof fish antifreeze proteins have not improved freezing toleranceof sensitive species. A better strategy may be to confer toleranceof cellular dehydration.Copyright 2001 Annals of Botany Company Freezing, dehydration, infrared video thermography, low temperature scanning electron microscopy, NMR micro-imaging