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witchazel.jpgThe End of Winter?

We experienced a relatively warm January here in western Washington (record-setting, in fact). Some daffodils are already coming up and the witch hazels are blooming.

But most of the trees haven’t started to leaf out because they haven’t yet emerged from winter dormancy.

Most woody perennial plants (mainly trees and shrubs) that must survive winter in the temperate latitudes do so by becoming dormant in the fall.

The deepest state of this dormancy called “endodormancy” – often referred to in horticulture as “rest” – allows them to survive cold temperatures, sometimes well below -30o F. (This adds new meaning to the phrase “chilling out”.)

Cold Acclimation (a.k.a., Cold Hardening)

Some plants, but not all, can become more cold tolerant or cold hardy simply by being exposed to near freezing temperatures. This is sometimes called cold hardening or cold acclimation. Interestingly, plants that are more cold hardy are generally also more tolerant to cellular desiccation. Indeed, improved cold hardiness may also be induced by mild drought. So, it appears that some of the cellular things the plant does to become cold hardy are the same as it does to become more drought tolerant. Icy_Tree.jpg

What happens at the cellular level during cold acclimation to make the plants more cold tolerant was described in a previous post. Briefly, the plants may synthesize new proteins that inhibit ice crystal formation or stabilize cellular structures against the cold.

For some plants, it may take only a few days to become cold-hardened. For other plants, especially those entering endodormancy, it may take weeks to become fully cold-hardened.

Of course, the degree of cold hardiness varies greatly among plant species. (USDA Plant Hardiness Zone Map) “So-called chilling-sensitive plants, such as the tropical banana and the semitropical avocado, can be severely injured or even killed by long-term exposure to temperatures (50 degrees Fahrenheit, for example) that are well above freezing. By contrast, chilling-resistant plants, such as garden peas and potatoes, survive brief periods of frost but are killed when freezing conditions continue for more than about four hours. Cold-hardy plants, on the other hand, tolerate extended periods of freezing, and laboratory tests indicate that cold hardness in some of these plants permits them to survive at temperatures as low as minus 75 degrees Fahrenheit.” (from Ref. 1 below)

pussy_willow.jpgThe Big Chill

Plants that exhibit the greatest cold hardiness are those that become dormant. When these plants make the transition to dormancy, one of the first stages in this process is similar to cold acclimation. This actually may be triggered in many plants by longer nights and reinforced by cooler temperatures.

But these plants go beyond chilling tolerance into a so-called “resting state” (endo-dormancy or mid-winter hardiness) that renders them extremely cold hardy. This resting state is characterized by a temporary suspension of growth and a lower rate of metabolism.

Dormancy is broken after exposure to long periods (weeks to months) of cool temperatures (typically below 40o F) and shorter nights (among other factors), though cold deacclimation and reacclimation may take place. (see Ref. 3 below)

How global warming may affect the complex process of tree winter dormancy has certainly been on the minds of some plant scientists (e.g., see Ref. 4 below).

The plant hormone ABA is generally thought to be a chemical signal that induces cold acclimation and dormancy in many plants. (Lots of new information about how ABA works has been published in the past few months, so maybe that would be a good subject for next time.)

References

1. John W. Einset “What Determines a Plant’s Cold Hardiness?” Arnold Arboretum, Harvard University (PDF)

2. Mark Longstroth Dormancy and Cold Hardiness in Fruit Crops

3. Kalberer, S.R., M. Wisniewski and R. Arora. (2006) “Deacclimation and reacclimation of cold-hardy plants: Current understanding and emerging concepts.” Plant Science vol. 171, pp.3-16. (PDF)

4. Legave, J.M., I. Farrera, T. Almeras and M. Calleja. (2008) “Selecting models of apple flowering time and understanding how global warming has had an impact on this trait.” Journal of Horticultural Science & Biotechnology vol. 83 pp. 76–84. (PDF)

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