Surviving the winter

by Helen M. Scott
Tuesday, February 12, 2008 1:27 PM MST

Hardy plants have devised a wide variety of mechanisms to survive the extreme conditions of winter.

In northern climates, perennial plants go dormant in winter. Entry into dormancy requires a reduction of physiological activities, including discarding vulnerable parts, such as leaves and even stems.

Energy needed for regeneration in springtime is stored in the roots, which are protected underground at temperatures many degrees warmer than the air.

The ability of a tree or shrub to survive winter depends on the seasonal change in its metabolism to a dormant state, known as acclimation. The first stages of acclimation are induced in early autumn by exposure to short days and nonfreezing chilling temperatures, both of which combine to stop growth.

But to survive the kind of low midwinter temperatures Wyoming recently has experienced, woody plants must be exposed to temperatures at or below freezing for some time before they become fully acclimated. Once acclimated, most landscape plants are capable of tolerating temperatures well below zero.

Many trees have the ability to suppress ice crystal formation in their cells, even at temperatures far below the freezing point. This deep “supercooling” is seen in species such as oak, elm, maple, beech, ash, walnut, apple, pear, peach and plum.

However, cellular water can “supercool” only to about minus-40 degrees F, at which temperature ice formation occurs spontaneously, resulting in the death of a cell. This minus-40 degrees F limit explains the existence of timber lines at high elevations, and also why low elevation timberlines exist in Alaska and increasingly higher timberlines occur in Wyoming.

It is important to remember here that plants of all kinds are composed primarily of water, and that freezing of water inside living cells is fatal to individual cells and potentially deadly for the entire plant.

To avoid this, numerous biochemical changes occur, which allow a plant to tolerate low temps and the presence of ice in their tissues. One such change is the production of antifreeze proteins (AFPs).

These unique proteins have the ability to lower the temperature at which ice forms within a plant, and allow the water, moving out of the cells in response to freezing temps, to freeze in areas between the cell walls where ice formation is not destructive.

What accounts for the differences in freezing tolerance among plants and the increase in freezing tolerance that occurs with cold acclimation? Determining the answers to these questions has potential practical applications.

Freezing temps limit the geographical locations where crop and horticultural plants can be grown. Knowledge of the molecular basis of freezing tolerance and the cold acclimation process could potentially lead to the development of new ways to improve plant freezing tolerance and result in expanded areas of agricultural production.

Promising research is being done on AFPs, so we can attempt to gain sufficient understanding of the freezing-tolerance mechanism to design more freezing-tolerant plants. Also, further research indicates that investigators are closing in on genes that are likely to have major roles in cold acclimation.

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