In article ,
Alan Meyer wrote:
I'm trying to understand better how plants survive the winter in
cold climates.

I know about leaf abscision, annuals and perennials, the
withdrawal of sugars into storage roots, reduction in metabolic
and growth rates, etc., but I don't understand how plants avoid
or survive the destructive effects of freezing water in the
cells.

There are several methods.

If I've got the numbers right, ice is about 9% less dense than
liquid water. I would think that ice crystals, as they form,
would rupture membranes and cell walls, causing irreverible
damage to the cells. Isn't this what happens to animal tissue
when it is frozen?

Yes, that happens to plant cells too. It's a sad sight when an
unexpected frost turns your frost-sensitive garden plants into
soggy dripping rags.

As we have seen in the news, this is a problem for fruit trees in
California and Florida - with freezing destroying the fruits and,
if it gets cold enough, the trees.

But how do cold weather plants handle it?

Do cold weather plants continue metabolism and generate enough
internal heat to keep from freezing? Do they manage to expel
enough water that freezing is not a problem? Do they have cell
walls that are tough enough to contain the freezing water by
compression and prevent crystalization (i.e., leading to
"supercooled" water)?

The usual method is to dry out the exposed tissues so that the
solute concentration in the cells increases enough to lower the
freezing point. This method is used by evergreen trees and many
other plants. It can be quite visible. For example, I've got
some Opuntia cacti that are native to Ontario. When the days get
short, despite the rains, they shrivel up their pads and let them
droop until they are flat on the ground. They benefit from the
insulating property of snow cover this way, too.

Some plants enhance this anti-freeze effect by specifically increasing
some solute, usually sugar, as well. Some vegetables, such as
brussels sprouts, are well-known to taste sweeter after a hard frost.

Another method is to force water out of the cells and into the spaces
between them where it can freeze without rupturing the cells. The
leaves may appear frozen, with the characteristic dark, glassy look,
and may even droop pitifully. However, they go back to normal when
the temperature rises. I've observed this in brassicas, leeks, and
a number of evergreen perennials and weeds.

Apart from the spring shoots of skunk cabbage (an aroid) there aren't
many plants that can generate much heat from metabolism, and certainly
not all winter. I don't know about cell walls that can compress water
so well as to prevent freezing. It's risky to say anything is impossible
in biology!

Note also that a lot of plants that don't freeze in warmer climates
can take a lot of damage, even freezing to the ground, and still
come back year after year. I'm always surprised when I read British
garden books that plants I think of as herbaceous perennials don't
die to the ground in winter there.