An Overview of Cold Hardiness in Woody Plants: Seeing the Forest Through the Trees

Abstract

D. Siminovitch and his graduate student, Keith Pomeroy also were among the fi rst to document that the protoplasm underwent distinct biochemical changes during cold acclimation that presumably played a direct role in conferring stress tolerance (Pomeroy and Siminovitch, 1971). Research by the Russian scientist I. Tumanov (as reviewed in Sakai and Larcher, 1987) also recognized the importance of biochemical changes during cold acclimation. Truly, the research conducted by these pioneers in the fi rst half of the century formed the conceptual basis of much of what was to follow. Regarding cold hardiness research, it is also important to recognize the important contribution that was made by the Plant Cold Hardiness Laboratory at the University of Minnesota, St. Paul, beginning in the 1960s, under the leadership of Conrad J. Weiser. Research (as reviewed in this article) conducted by scientists and graduate students at this laboratory dominated the literature for over 25 years (1960–85). A greater understanding of deep supercooling, the biophysics of water at low temperatures, dormancy, the role of sugars in cold hardiness, and more, all grew from the activities of this laboratory. People such as Paul Li, Leslie Fuchigami, Harvey Quamme, Lawrence Gusta, Milon George, Michael Burke, Jiwan Palta, John Carter, Cecil Stushnoff, Margaret Smithburg, Tim Hall, Stan Howell, Phil Graham, Robert McLeester, Bob van Huystee, and others were associated with the Plant Cold Hardiness Laboratory and would go on to staff many of the horticulture departments in the U.S. and Canada. The seminal paper by Weiser (1970) often is cited for its recognition that changes in proteins during cold acclimation also implied changes in gene expression. “Bud” Weiser later went on to chair the Department of Horticulture at Oregon State University. In the latter part of the 20th century, characterizing the genetic regulation of cold hardiness has dominated the literature (Thomashow, 1999; Xin and Browse, 2000). Strategies allowing plants to survive freezing temperatures have been organized into two categories (Levitt 1980): 1) freezing tolerance and 2) freezing avoidance. Mechanisms representing both categories are common in woody plants (Burke and Stushnoff 1979). In comparison to herbaceous plants, woody species are extremely freezing tolerant. Many species native to boreal forests tolerate –40 °C and some can even tolerate –196 °C in midwinter. In contrast, very few herbaceous plants tolerate temperatures less than –25 °C or prolonged exposure to –15 °C for a period of several weeks, whereas certain woody species can tolerate –40 °C for months. In addition to a historical review, the present contribution will cover the main factors involved in woody plant cold hardiness, although reference will also be made to herbaceous plants where applicable. This is especially true for recent advances in the molecular biology of cold acclimation where research with Arabidopsis and other herbaceous crop species has dominated the literature.