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Home TurfNotes Freeze tolerance of plants is not constitutive but induced in response to low, nonfreezing temperatures (< 50 F (10C). This process is known as cold acclimation, which occurs during the fall or early winter, explains why a plant species growing at a warm temperature then exposed to freezing is killed, while that same plant exposed to a cold acclimation period prior to sub-freezing temperatures survives.

Scientists have identified and studied the roll of specific plant genes in freeze tolerance. A group of genes called cold-response (COR) genes apparently plays an important role. The activation of these genes requires a period of low but nonfreezing temperatures (32 to 50 degrees F (0 to 10 degrees C). The activation of these genes is then associated with the hardening or freeze tolerance of the plants. A possible reason why plants in effect die when exposed to freezing temperatures without a hardening period is due to the lack of COR gene activation. Interestingly, light in addition to low but nonfreezing temperatures is needed for gene activation, which may explain partially why we see turf in shade more susceptible to freeze injury (Danneberger, 2006).

Photograph:Freeze injury on a bermudagrass fairway in northern Kentucky (picture was taken in June).

Although turfgrasses undergo cold acclimation, freeze injury is still a potential problem especially on warm season turfgrasses like bermudagrass and seashore paspalum. Regarding cool season turfgrasses, annual bluegrass (Poa annua and ryegrass (Lolium perenne, Lolium multiflorum are the most likely to experience freeze injury along their northern range of adaptation. (Top photograph is freeze injury on a Poa annua green in northwestern Ohio)

Freeze injury and conversely tolerance is due in large part to how the turfgrass plant reacts to cell dehydration. During freezing temperatures water freezes intercellularly, causing a decrease in water potential outside the cell. As water freezes intercellulary (between cells) unfrozen water within the cell begins to move out of the cell toward the ice crystals in the intercellular spaces and subsequently freezes. Thus, dehydration occurs within the cell. The colder the temperatures the more water travels down the gradient toward the frozen water. At 14 F (-10C), 90% of the osmotically active cellular water will move out of the cell into intercellular spaces (Thomashow, 1998). The freezing point is higher intercellularly than intracellulary, which is a good thing because intracellularly freezing is fatal.

As water leaves the cell, the plasma membrane (syn. plasmalemma) contract and pull away from the cell wall. With the arrival of warm temperatures the ice present intercellularly melts and the water flows back into the cell where hydration takes place. If no damage has occurred to the plasma membrane (ex. punctured, ruptured) then the cell is alive and well. However, if the cell rehydrates and damage has occurred to the plasma membrane cell death is eminent.

Photograph: A common scenario for freeze injury is a warming (thawing) in late winter where the turf, in this case Poa annua, loses its cold hardiness, followed by water from precipitation or a poorly drained site where water stands followed by a rapid freezing.

The most common type of freeze injury in the United States occurs at relatively high freezing temperatures 24 to 28 F (-2 to -4 C) during late winter/early spring. This type of freeze injury is sometimes described as "expansion-induced lysis" because it occurs during freeze/thaw cycles. In this freeze/thaw scenario, the plant loses its cold hardiness through warming temperatures which leads to the expansion of the plasma membrane.

Photograph: Upon the arrival of warmer temperatures, the ice (in this case) or snow melts and within a few days injury symptoms occur.

Following the warming/thawing period, a rapid drop in temperature can case the plasma membrane to contract. Should water rapidly freeze or a rapid collapse of the plasma membrane can result in ruptures in the membrane. Excessive water around the crown of the plant during these freeze/thaw cycles in late winter increases the severity of the damage.

Photograph:In extreme winter conditions freezing can occur on freeze hardy plants. In this case freeze injury has happend on a creeping bentgrass (Agrostis stolonifera) putting green in a high altitude situation in the Rocky Mountains. I would believe this type of freeze injury would be due to extreme cell dehydration described below.

A second type of freeze injury occurs at lower temperatures involves changes in the plasma membrane. Where expansion-induced lysis is a result of mechanical damage, at temperatures below 25 F (-4 C) and more likely around 14 F (-10 C) loss of cell responsiveness occurs because of membrane changes. The plasma membrane becomes more rigid, and loses its ability to be pliable through structural or phase changes (Gordon-Kamm and Steponkus, 1984). Technically, the plasma membrane undergoes a phase transition from lamellar-to- hexagonalII. Actually it is this work (Gordon-Kamm and Steponkus, 1984) that demonstrated that freeze-induced phase transitions are a consequence of dehydration rather than subzero temperatures per se. The severity of dehydration increases with decreasing temperature.

Freeze resistance is comprised of two components - freeze tolerance and freeze avoidance. Freeze tolerance is the plant's response to the freeze temperature. Without a doubt the singular most important tolerance mechanism of plants is plasma membrane stabilization through cold acclimation. Where plasma membranes from nonacclimated plants suffer expansion-induced lysis and phase transition, membranes from cold acclimated plants do not (Thomashow, 1998).

Turfgrass investigations that looked at plasma mebrane bilayer constituents found that cold tolerance of cultivars of both bermudagrass and seashore paspalum involved fatty acids. The presence of unsaturated fatty acids like linolenic acid tend to be associated with lower freeze tolerance than those cultivars with proportionally higher saturated fatty acids (Cyril et al., 2002). This and other similar type of studes provide insight into the mechanism of freeze tolerance and could be helpful in identifying or breeding more freeze tolerant varieties.

Freeze avoidance is where the plant is present, but not exposed to the freeze. For example, if the air temperature is sub zero but the turf is covered with snow, the plants crown or stems are not "feeling" the freezing temperatures. The temperature under the snow cover is considerably warmer. From a management perspective focusing on freeze avoidance strategies can help increase the survival of turfgrasses at risk.

Turf managers have some control of increasing the likelihood of winter survival by:

* Raising the mowing height on warm season turfgrasses during the fall. This will provide more some protection to the growing point during freezing temperatures.

* Provide drainage for removal of water from excessively wet areas. During freeze/thaw cycles the presence of excessive moisture can enhance freeze injury.

* Reduce thatch. A significant thatch layer results in the plant's growing point to lose contact with the soil as it elevates into the thatch layer. This will expose the plant more readily to freezing temperatures.

* Potassium fertilization. On warm season turfgrasses potassium is often applied for increasing the chances of winter survival. Potassium is an ion that helps lower the osmotic potential of the cell decreasing water the potential for water flow from the cell.

* Reduce the likelihood of excessive growth going into the winter. Overstimulation of growth promotes succulent high water content cells that are more likely to encounter freeze injury.

* Minimize shading. Although not fully researched, a degree of correlation has occurred with freeze injury and degree of shading. Shading may contribute to increased freeze injury due to plant cells tend to be 1) more succulent in shade and have larger intercellular spaces, 2) lower carbohydrate levels, which may influence water potential, as well as the energy requirements of the turf and 3) shaded areas tend to be wetter, which may contribute to the severity of freeze/thaw cycles in late winter.

Reference

Cyril, J., G.L. Powell, R.R. Duncan, and W.V. Baird. 2002. Changes in membrane polar lipid fatty acids of seashore paspalum in response to low temperature exposure. Crop Science 42:2031-2037.

Danneberger, T.K. 2006. Another brick in the winter fortress. Golfdom 62(11):38.

Gordon-Kamm, W.J. and P.L. Steponkus. 1984. Lamellar-to hexagonalII phase transitions in the plasma membrane of isolated protoplasts after freeze-induced dehydration. Proceeding of the National Academy of Science 81:6373-6377.

Thomashow, M.F. 1998. Role of cold-responsive genes in plant freezing tolerance. Plant Physiology 118:1-8.

Posted by Karl Danneberger