The dog didn’t seem to be cold. I wasn’t either, but it was no warmer than —15°F. It was a clear, still night; we had gone out for a walk before bed and nothing much seemed to be moving. The only sound I was aware of was the brittle, dry snow crunching beneath our feet. As we came to the corner, under the street light, I saw a puddle of water, unfrozen. I wondered at it and reflexively kicked a dusting of snow into it. It froze almost instantaneously from the points of contact with the snow.
Why the puddle was unfrozen in the first place I don’t know. The temperature had been well below freezing for a week or more, so the puddle must have existed for that long anyway. On first glance, freezing water seems like the simplest thing in the world. But a closer look exposes more complexity. The same is true with the interaction of plants and freezing temperatures.
During the course of the year, plants from temperate regions change in their capacity to tolerate and survive freezing temperatures. Very few herbaceous plants can tolerate low temperatures for long; some have little or no tolerance at all. But after a period of cold acclimation, some perennial species are extraordinarily tolerant. Trees native to the boreal zone withstand temperatures of –40°F for months and in midwinter survive temperatures lower than that. But even species that are quite cold hardy in midwinter can be damaged by frost during their growing season.
Everybody knows that ice melts above 32°F. What’s less well known is that pure water (not rain water or tap water) is unlikely to freeze at temperatures much warmer than —40°F; the attribute of water to remain liquid at temperatures well below the freezing point is called “supercooling”. When water freezes, it becomes crystalline, but this transition does not usually occur spontaneously at temperatures warmer than —40°F. A germ or seed that initiates or “nucleates” ice crystal formation and from which crystals grow is required. This is how the snow triggered the puddle to freeze on my walk.
“Ice nucleators” are important, if not essential, in the formation of snow and rain, and they are ubiquitous in the atmosphere. Most are bacteria; a diverse range of bacterial species presumably deposited by snow can be isolated from high mountain peaks that would otherwise seem sterile. Air-borne dust also plays a role in ice nucleation, and some plant constituents are ice nucleators. Water in nature has an abundance of ice nucleators, and so it generally freezes at or slightly below 32°F.
Most plants, even very tender ones, have some tolerance for cold temperatures, and for most, frost during their growing season is a real possibility. Think of northern orchard crops, apples or cherries, whose flowers are frozen (and possibly killed) in a late frost. Global warming notwithstanding, it’s not at all uncommon in the Upper Midwest where I’m from to see frost every month of the year. Across the Sun Belt, citrus growers routinely face frost as their crops ripen.
Most plants normally supercool to a few degrees below 32°F, but generally ice nucleators cover plant surfaces—leaves, stems, flowers—and initiate freezing. On clear, windless nights, heat loss into the open skies causes plants and other objects to become colder than the surrounding air. The air temperature may never dip below 32°F, but temperatures of leaves and the soil surface may fall below freezing and ice nucleators initiate freezing (radiation frost).
If moisture is present on a plant surface and there is an entry point (a wound, a broken epidermal hair, or a stomate), ice can form and propagate within the plant’s intercellular spaces. Ice crystals within cells are always lethal. But that’s not how damage is generally caused by frost, and the ability of a plant to adapt to seasonal cold plays no part. Instead, ice in the intercellular spaces causes water to flow out of the neighboring living cells into the intercellular spaces where it too freezes. As the amount of intercellular ice increases, more and more water flows out of cells. Ultimately, dehydration rather than freezing per se injures or kills the plant.
Many woody plants are not much susceptible to this sort of frost damage. Yews and oaks are examples. Research indicates that morphological features such as thick, waxy cuticles act as barriers to ice nucleation and propagation in these plants. In some plants, the propagation of intercellular ice is blocked from entering tender lateral shoots or blossoms. There has been some success on an experimental basis spraying hydrophobic particle films on the surface of tomato plants, which are tender and can be killed by frost, to block ice nucleation. But the current procedure is probably not worthwhile on a commercial basis and impractical for a home garden.
An obvious practical approach a gardener can take is to be sure that plants are well watered before a period of expected frost. Having fully turgid, nonstressed plants may prevent killing cellular dehydration that can accompany a growing-season frost. In the cranberry bogs of New England and Wisconsin, when there is threat of frost, commercial growers continuously apply water to cranberry vines with sprinklers. The rationale behind this is that as water freezes, heat is released. This is what is known as the “latent heat of fusion”, and it is enough to keep the vines from freezing. Citrus growers avoid allowing cool air to pool by keeping air moving with giant fans.
As days grow shorter and nights colder, annual herbaceous plants senesce and die, but perennial plants that are adapted to the temperate and boreal zones enter a period of dormancy and begin to acclimate to the cooler and ultimately freezing temperatures that a month or so earlier might have killed them. Despite 100 years of study, our understanding of how plants perceive low temperatures and respond by regulating gene expression and metabolism is incomplete. This is not too surprising really since cold adaptation is an exceeding complex trait that is controlled by a myriad of genes that in turn are influenced by a myriad of factors, and it is nearly impossible to model adequately.
The concept of “cold adaptation” is implicitly presented as if it were a unique, one-time event. This of course is a grand oversimplification. The climate in winter is no more stable than it is in summer. Winter begins on a particular calendar date, but cold temperatures do not usually conform. There are periods of intense cold, followed by warming trends, followed by intense cold. Plants experience these events and respond to them. In midwinter, a warming trend may induce a plant to partially deacclimate, but the next week the same plant may be subjected to intense cold. To survive, it must reacclimate.
In a purely descriptive simplistic sense, as a woody plant adapts to cold, water is flushed out of the cells to the intercellular spaces, where it freezes, and water further flows out in response to the intercellular ice. The composition of fats and proteins in the increasingly permeable cell membrane changes, and salts, sugars, and proteins are synthesized that are concentrated in the living cells and increase the solute concentration, acting as “antifreeze”. Freezing and killing dehydration do not occur.
The ability to maintain this state, where intercellular water is frozen but the adjacent cells remain viable and intracellular ice nucleation is suppressed at very low temperatures is called “deep supercooling”. What allows small quantities of water within cells to avoid freezing, despite the proximity of extracellular ice and low temperature, is poorly understood. The ability to supercool seems to be related to the cell wall structure and composition, but there are also adaptive features that must be under genetic control.
Over the last two decades, increasingly sophisticated molecular biology techniques have been developed for plants. More and more these tools have been applied to teasing apart the genes and their roles in cold acclimation of the weedy species Arabidopsis, the fruit fly of plants. Lots of progress has been made. But from an anthropomorphized view of evolution, the goals of Arabidopsis are quite different from those of woody temperate plants, and Arabidopsis has the capacity to survive only a few degrees of cooling below freezing. The knowledge gained from Arabidopsis will certainly aid in breeding more frost tolerant plants and crops, but understanding cold adaptation and deep supercooling may remain elusive.