Dormancy in Grapes

By Dr. Mike Campbell, Director, Lake Erie Regional Grape Research and Extension Center

As winter sets in plant growth in the vineyard stops. The onset of short days and cooler temperatures results in a state of no growth and reduced metabolic activity termed dormancy. The apparent dormant state of perennial plants is composed of multiple stages that have major significance to the plant’s ability to withstand environmental stress, particularly the cold of winter in Pennsylvania, or the drought found in deserts or Mediterranean climates. Grapes, as a perennial plant, enter a developmental state in the fall called ecodormancy. Ecodormancy is brought on by shorter days and cooler temperatures and is characterized by reduced growth that can be reversed by altering growing conditions. Ecodormant vines slowly transition to a state of endodormancy through a process called acclimation (Figure 1).  

Figure 1: The stages of dormancy in a perennial plant. The process of acclimation involves shorter days and gradually cooling temperatures. Deacclimation results from reaching the time and cool temperatures to meet the chilling requirement. 

Endodormancy, sometimes referred to as deep dormancy, is a state where there is an internal suppression of growth. This internal suppression includes an increase in the plant hormone abscisic acid, an inducer of dormancy, as well as cryoprotectants that provide cold hardiness. Once placed into endodormancy exposing the vines to optimal conditions of temperature and light will not result in growth. If autumn weather involves gradual cooling conditions, and the vines are in excellent health, the plants will reach a maximum depth of endodormancy and a state of maximum cold hardiness. The depth of endodormancy is genetically determined and different varieties of grapes reach a specific maximum cold hardiness.  This is one reason why certain varieties of grape, particularly varieties of Vitis vinifera L., have limited use in Pennsylvania and the Lake Erie region; the winters reach a cold temperature that is below the threshold of endodormancy where freeze damage occurs (Figure 2). 

Figure 2: Comparison of the maximum cold hardiness between chardonnay and Concord grapes illustrating the impact of deep dormancy on freeze tolerance.    

Once a vine enters endodormancy a combination of time and cold temperatures (chilling hours) is required to remove the internal mechanism that prevents growth. The chilling requirement is not necessarily temperatures below freezing, and winters in Pennsylvania usually provide more than enough chilling hours to terminate endodormancy in grapes. In very warm climates, where chilling hours are not met for termination of endodormancy, application of chemicals such as hydrogen cyanamide are used to initiate bud growth. In the northern United States, including Pennsylvania, chilling requirements to break endodormancy, and initiate growth if warm weather occurs, are often met by mid-winter.  This creates a challenge for growers. Once endodormancy terminates vines are in a condition of ecodormancy, which is characterized by a condition where growth is largely suppressed by environmental conditions such as air and soil temperature. Ecodormancy vines also begin to lose cryoprotectants found in the plant tissues resulting in a reduced level of cold hardiness. (Figure 3). Thus, if endodormancy is terminated early in winter, and that termination is followed by a spell of warm weather, vines will begin to grow. Bud break in vines leaves them susceptible to freeze damage. This means that cold damage to grape vines can occur if the temperature falls below the maximum for cold hardiness in endodormancy but also at higher temperatures when vines have left endodormancy and lost cryoprotectants.

Figure 3: Lack of chilling requirements can result in prolonged endodormancy and delayed bud break (A). If chilling requirements are met warm winter weather can result in premature termination of endodormancy and early bud break increasing the risk of freeze damange in early spring (B).

Changes in climate have implications to the process of dormancy in grapes. The concept of a warmer climate suggests that maximum cold temperature will be on average higher. While it would take a significant amount of global warming to result in a climate in Pennsylvania where chilling requirements are not met, as in the warmer Mediterranean regions where grapes are grown now, there other challenges that climate change presents to growers. Higher temperatures bode well for grape varieties that have a higher sensitivity to cold damage in endodormancy. Increasing average winter temperatures through climate change may mean the ability to successfully grow more varieties sensitive to cold in Pennsylvania. However, there is another more insidious downside to climate change and that is the impact of changes on chilling requirement. A warming climate will also result in an increase in warm spells mid-winter, which will result in earlier termination of endodormancy, increasing risks of vines damage from late winter and spring frosts. Growers can expect new challenges as climate change impacts the dormancy cycle in grape varieties growing in our region. 

Citations

Davenport J, Keller M, Mils L. 2008. How cold can you go? Frost and winter protection for Grape. HortScience 43:1966-1969. 

Ferguson J, Moyer M, Mills L, Hoogenbom G, Keller M. 2014. Modeling dormant bud cold hardiness and budbreak in 23 Vitis genotypes reveals variation by region of origin. Am. J. Enol. Vit. 65:59-71. 

Horvath D, Anderson J, Chao W, Foley M. 2003. Knowing when to grow: signals regulating bud dormancy. Trend in Plant Science 8:534-540. 

Kalberer S, Wisniewski M, Arora R. 2006 Deacclimation and reacclimation of cold-hardy plants: Current understanding and emerging concepts. Plant Science 171(1)3-16. 

Or E, Vilozny I, Eyal Y, Ogrodovith A. 2000. The transduction of the signal for grape bud dormancy breaking induced by hydrogen cyanamide may involve the SNF-like protein kinase GDBRPK. Plant Molec Biol 43(4):483-494. 

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