Low-temperature impacts on plants can be classified as chilling or freezing. Chilling injury or chilling stress, as it is often termed, is associated with temperatures above freezing but low enough to cause injury. Chilling injury is due to a sudden drop in temperature during an active period of plant growth or development. The greater the drop in temperature from normal, the more likely injury will occur. The extent of the injury may cause a reduction in photosynthesis or other metabolic processes, an alteration in growth or death of the exposed plant tissue. For example, a substantial drop in temperature during new shoot growth and leaf expansion may result in damage to the terminal meristem and a loss in shoot growth.
Other signs of chilling injury may include wilting, desiccation and/or a physical distortion of plant parts. Chilling injury is less serious than freezing injury on most of our herbaceous and woody plants. Plants can usually grow out of the damage inflicted by chilling injury. Understanding low-temperature injury begins with an examination of cold hardiness.
Cold hardiness refers to a plant’s ability to withstand cold temperatures without sustaining injury. The level of hardiness that a plant can attain is based on genetics, preseason conditioning and its current condition. The genetic origin of a plant contributes to its ability to "harden". Plants of a given species originating from warmer climates may not be as cold hardy when moved into northern regions as the same species from northern regions. The same is true for heat tolerance when northern plants are moved to the South. Origin, not production location, is the governing factor in determining hardiness capability.
Preseason conditioning refers to the physiologic and metabolic changes that occur within a plant as it begins hardening. The process leading to cold hardening is generally considered a three-stage process. The first stage occurs at the end of the growing season. Growth has ceased, plants have formed terminal buds, and carbohydrates are accumulating in stem and root tissues. Short days and cool night temperatures of early autumn begin the acclimation. During acclimation, metabolic changes occur within the plant that allow it to withstand lower temperatures. During the initial stages of acclimation, however, hardiness levels may be lost as easily as they were obtained if temperatures rise or other environmental factors promote the resumption of growth. Acclimation continues and hardiness tolerance increases with consistent exposure to temperatures at or slightly below freezing.
The final stage, the deepest level of cold hardiness, is achieved after prolonged exposure to subfreezing temperatures. Plants are said to be at midwinter levels when they reach their maximum cold tolerance for a given season. In the Northeast, maximum levels of hardiness are usually achieved by early January. As spring approaches, plants begin to deharden or lose cold tolerance with increasing temperatures. Late frosts can cause injury during this dehardening phase when plants begin to break bud and initiate shoot growth.
Remember that plant parts differ in their cold tolerance. Flower buds usually do not achieve the hardiness levels that vegetative buds and stem tissue do. The last factor contributing to a plant’s ability to achieve maximum cold hardiness is plant health. Healthy plants achieve their cold hardiness potential; stressed plants may not. It is important to note that plants stressed by drought, flooding, nutrient deficiencies, transplant shock or pest problems may not become fully hardy. Any situation that reduces a plant’s ability to produce or store carbohydrates may influence its ability to attain levels necessary to avoid low-temperature injury. The best insurance for hardiness is plant health. In summary, genetics set the foundation levels for low-temperature tolerance of a plant. Preseason conditioning and plant health determine whether a plant can approach these levels.
Information and photo from a section of "Abiotic Plant Disorders - Symptoms, Signs and Solutions A Diagnostic Guide to Problem Solving" by Robert E. Schutzki and Bert Cregg, Departments of Horticulture and Forestry, Michigan State University Michigan State University. Go to http://www.ipm.msu.edu/cat08land/pdf/9-19abiotic.pdf for the full factsheet with photos.
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