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How Hardy Trees Survive Winter
| Article
# : |
14178 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
1 / 1988 |
756 Words |
| Author
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Richard W. Tinus
Richard W. Tinus is a plant physiologist with the USDA Forest
Service, Rocky Mountain Forest and Range Experiment Station,
in Flagstaff, Arizona. He leads a project on stress physiology
of Southwest tree species. |
In the Temperate Zones, a tree in summer pulses with life, yet in winter the same tree may have to endure for months in a frozen world.
To survive through a cold winter, the tree's cells must be modified so they are not damaged with the onset of freezing temperatures.
Why would cold weather damage a tree? An important cause of damage is ice formation within the living cells. When water freezes, it increases in volume by about 9 percent. The sudden formation of ice may rupture the cell membranes. Damage can also occur during thawing. Low temperatures cause components of the cell membrane to rearrange, which renders the membrane less flexible. During thawing, water enters the cell faster than the membrane can stretch, and it breaks.
Another cause of damage is the dehydration and denaturation of proteins. Proteins perform most of the cell's life-sustaining functions. Proteins are composed of long chains of amino acids strung together like beads on a necklace. To perform its job, a protein must be curled and twisted into a very special configuration and held that way with the aid of surrounding helper molecules, of which water is one of the most important. When water freezes in a cell, the growing ice crystal draws water away from the protein. Stripped of these important helper molecules, the protein molecule loses its ability to function as its special three-dimensional structure comes undone, often irreversibly.
If a tree is to avoid damage it must avoid intracellular ice formation. This process of becoming resistant to cold involves a variety of physical and chemical mechanisms that collectively are called hardening. The degree of resistance to cold is called hardiness.
Except at extremely low temperatures, ice crystals do not form spontaneously; they must have a crystalline nucleus on which to start. The nucleus may be a bit of ice or a tiny crystal of almost anything with a structure similar to that of ice. However, if the growing ice crystal cannot penetrate the cell membrane, then each and every cell must be separately nucleated. Lacking nucleation, the water in such a cell remains liquid even at temperatures well below the usual freezing point of water, a phenomenon referred to as deep supercooling.
With the arrival of cool temperatures in fall, other changes occur. New proteins that work better at low temperatures than their
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