Frosts, supercooling, and other fun stuff

Tue, 13 Nov 2007 20:42:42 PST
 Hi Gang,
1. I'm still confused about the sunshine vs. warm water notion of reducing 
damage to plants that have gotten too cold. Wives' tale or note, the notion 
persists and some folks seem to have good luck by pouring water over their 
plants before the sun shines on them. I can't figure it out.

2. I like the term I first encountered in email from a South African email 
friend. It was immediately obvious what she meant when she referred to a 
"black frost."

3. In the absence of ice forming nuclei (e.g., tiny particles), water can 
supercool to about -38 C, the homogeneous nucleation point. Dissolved salts 
or sugars can depress this temperature a few degrees more. Thus, with or 
without "antifreeze" type solutions, trees (not sure about bulbs) can 
prevent ice crystals from forming in their cells if they can achieve 
supercooling of their cellular liquids (apoplast and symplast).

There is some correlation between the northern limit of certain trees, and 
the homogeneous nucleation point. It is proposed (but I don't know if it's 
true) that some species are limited in their northward ranges because -40 C 
is about the limit for this effect.

Oddly, some evidence indicates that water can supercool to -70 C under 
laboratory conditions. Pehraps some plant out there can perform such a 
trick, but I've not hear of it.

4. To survive temperatures lower than -40 C, plants may resort to 
dehydration, pulling water out of their living cells. This my be the 
antifreeze-like effect that has been mentioned because water outside the 
cells freezes and serves to pull even more water out of living cells, 
leaving their cytoplasm ever more saturated with proteins, lipids, sugars, 

The reason ice can "pull" water out of cells is that frozen water has a 
concentration of zero, in terms of liquid water, and water moves from higher 
concentrations (i.e. inside cells) to areas of lower concentration. Just 
think of the water pulled out from foods in some freezers-eventually foods 
can become nearly freeze dried.

I think dehydration mechanisms are responsible for the survival of some 
plants (not all plants) in areas where they are routinely exposed to 
temperatures below -40 C (or so).

5. The heat of fusion refers to the extra energy that must be released by 
water if it is to freeze. As was mentioned, sprinklers over citrus can often 
be useful because the ground water keeps hitting the leaves and even 
freezing. However, the whole process keeps all the ice at about 32 F (0 C) 
even when air temperatures might be in the mid- to low-20s.

As long as you can keep new water freezing on plants you can generally keep 
the plants at about 32 F. If no new water arrives the ice on the plants, and 
the plants too, will continue to cool to ambient temperatures. Ground water 
offers two benefits: 1) it brings some warmth with it (perhaps it is 40-50 
F) and it has energy to release (heat of fusion) before it will freeze.

LINK: Enthalpy of fusion, Wikipedia

LINK: Supercooling Water to -70 C…



Conroe TX, where a bit of cool weather would be appreciated.

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