TechNotes Nov. 21 - It's Getting Cold Outside

FREEZE TOLERANCE, PART I. With fall temperatures arriving attention will turn shortly to the potential for winter damage, specifically freeze injury on both warm and cool season turfgrasses. This is part I of a two part series on freeze injury and what can be done to increase the potential for turfgrass survival
 
Freeze tolerance of plants is not constitutive but induced in response to low, nonfreezing temperatures (< 50 F) during the fall or early winter. This process is known as cold acclimation.  It explains why a plant species growing at a warm temperature is exposed to freezing is killed, while that same plant exposed to a cold acclimation prior to sub zero temperatures survives.
 
Management practices in the autumn can influence the cold acclimation process.  For example, if you are trying to promote turf growth through the fall by covering greens, it might be advisable during late fall to expose these greens to cool temperatures below 50 F but above 32 F. This will allow your turf to cold acclimate, or sometimes referred to as hardening off. 
 
Freeze injury is a potential problem on warm season turfgrasses like bermudagrass and seashore paspalum and cool season turfgrasses primarily annual bluegrass and ryegrass along their northern range of adaptation. Freeze tolerance or conversely injury is due in a 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. Unfrozen water within the cell moves down this gradient out of the cell toward the ice crystals in the intercellular spaces. Thus, the descriptive term dehydration for the loss of water from the cell. The colder the temperatures the more water travels down the gradient toward the frozen water. At 14 F, 90 percent of the osmotically active water will move out of the cell into intercellular spaces (Thomashow, 1998). 
 
The freezing point is believed to be 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 (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.
 
The most prevalent type of freeze injury that occurs on golf courses in the United States occurs at relatively high freezing temperatures 25 to 28 F during late winter/early spring. This type of freeze injury is sometimes described as “expansion-induced lysis” because it occurs during freeze/thaw cycles. The expansion/contraction of the plasma membrane in plants that have broken cold acclimation can lead to death. Ice rapidly forming or 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.
 
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 and more likely around 14 F 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 however with decreasing temperature. 

CHILLING. Chilling is a low temperature stress that occurs in the absence of freezing. Chilling injury is most common on warm season turfgrasses like bermudagrass. The scenario for chilling injury is that it often occurs under sunny skies and warm temperatures followed by a rapid drop in temperature below 54 F at sunset. Injury symptoms appear as necrotic lesions, or "bleached out" leaf blades from loss of chlorophyll. Chilling injury also results in a reduction in the photosynthetic rate and a stoppage in growth.

Bermudagrass will often undergo a change in color with that arrival of cool temperatures that do not reach that of chilling.  In these cases the bermudagrass will often go off color and appear purplish.

SOIL TEMPERATURES.