plant cell vs. ice crystal

Plants are AMAZING. Can YOU produce your own antifreeze?

(the picture above is a row of carrots, frozen)

Winter’s cold has arrived, bringing light frosts to the farm several nights within the past week. Temperatures have been dipping into the low 30s, and one night even below 30. This long of a frost streak can be tough on plants … especially because it was 70 degrees just a few weeks ago.

So of course the tomato, cucumber, and squash plants, frozen through, drooped to the ground as they thawed and are now quite dead. Tomatoes are notorious for having no sense of timing. They were still flowering and producing fruit that would never ripen right up until the first frost last week.

Then there are other plants, like peas and the least cold tolerant lettuces that show some browning from frost damage, and will not die in this weather, but also will not grow much.

And then there are the carrots, the collards, and all other manner of roots and leafy greens, including most lettuces, that will keep growing, albeit slowly, and even taste better after being frozen a few times.

But how does this work??? How is it that a tomato plant will be downed by a frost while the carrot tops, after laying flat on the ground in the morning, thaw and rise back up by noon???

Here are a couple of ways that vegetable plants have adapted to live through winter’s freezing temperatures.

First, how can a plant actually avoid freezing if the temperature is below 32F? How on earth does a broccoli plant manage to freeze solid each night, and then defrost and go on growing by the next afternoon? (In the picture below, broccoli which froze over night begins thawing out. The leaves that flopped down with frost are back to their normal position by afternoon)

Some plants have proteins that inhibit growth of ice crystals in the plant, called antifreeze proteins. The production of those proteins actually lowers the freezing point of the plant’s extracellular matrix (the liquid surrounding the plant cells), so that the plant does not freeze.

But what if the plant is going to freeze despite producing its own antifreeze proteins?
When frost causes damage, it often looks like the plant is wilted:

And they are wilted in a sense, because the formation of ice crystals in the plant creates a deficit of liquid water, by drawing water out of the plant cells to join the ice crystals. (Some of the adaptations that help a plant survive frost also help the plant in times of drought.) The other danger in ice crystals is that they have such a firm crystalline structure that as they grow they actually severely damage the plant’s cells, eventually causing death. Luckily there is a group of proteins that helps to prevent the cells from being damaged or killed by stabilizing the cell membranes. Yet other proteins prevent ice from forming large crystals which would damage the cells.

The below picture of is a row that had basil on the right (now killed by frost, dill on the left (slightly damaged) leaks further down the row (totally impervious to frost), and parsnips mixed in (taste better after frost).

And as if that isn’t awesome enough, lets take a look at the cell membrane of plant cells, and how the membrane differs in cold tolerant and intolerant plants:

The membrane that surrounds each cell in any organism is made up mostly of lipids, the building block of fats. In some plants, these lipids could be loosely compared to bacon fat, which is firm at room temperature and freezes into a crystaline structure at a fairly high temperature, while in others they are more like olive oil, liquid at room temperature, and freezing at a lower temperature. The lipids in a fat like in bacon are mostly saturated, they are straight and skinny like a popsicle stick. In olive oil there are more unsaturated fats, maybe like the branches of a tree. Now, under freezing temperatures, the saturated lipids, nice straight popsicle sticks stacked right next to each other freeze to one another making the cell membrane rigid. On the other hand, the unsaturated lipids, which have branches sticking out at odd angles, freeze together only where they touch, allowing the membrane to remain somewhat flexible.

When the cell membrane becomes rigid, the proteins in the membranes cannot function, and the contents of the cell leak out, the cell cannot adjust to light conditions, it can’t maintain itself, and the cells and possibly whole plant die. The plants whose cells have more unsaturated lipids though, like many leafy greens, maintain the function of their cell membranes.

In many plants, tolerance to freezing is built up over time. They adapt to colder weather over the season, so when frost comes they are prepared. Thy can withstand some dehydration and ice formation if it takes place slowly, while if they were frozen in july, they might very well die.

Now, is that not absolutely AMAZING??!!!

Here is the winter rye cover crop in the South Garden. As one of the most hardy winter grains, a light frost has no affect on it.
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