Low Temperature Stress
 
Optimum temperature for growth
 
 

 
Maintaining the integrity of the turfgrass crown is essential for freezing stress survival. Leaf, Root, and lateral shoot regeneration occurs from the turfgrass crown the following spring.
 
The freezing tolerance of a grass depends on the degree of injury and the region of the crown affected.
 
The lower region of annual bluegrass crowns is more susceptible to freezing stress injury than the upper region.
 

Turf injury or loss results from

  1. Freezing stress
  2. Chilling stress
  3. Desiccation
  4. Frost heaving
  5. Ice encasement
  6. Winter diseases
 
Freezing stress
 

Mechanisms of freezing protection
1) Freezing point depression by solutes
2) Deep supercooling - used to protect critical meristematic tissues of woody species.
3) Most important and common mechanism is intercellular ice formation. Water leaves the cells and crystallizes is intercellular spaces.
 
 
Freezing first occurs around the vessels and in the interstellar spaces of the shoot (Levitt, 1980).
 
During freezing, the protoplast and cell wall contract as water moves from within the cell to the ice loci in the intercellular spaces.
 
This extracellular ice formation can split cells apart as they shrink.
 
Once thawing begins, a surviving cell can reabsorb the intercellular water and recover its turgor, while dead cells expand to near their original shape, but the protoplast remains constricted.
 
The result is that the stress the plant feels is actually very similar to drought stress since the cell water content is lowered by the freezing process. In fact, the point at which a cell dies from intercellular ice formation is roughly the same point at which a plant dies from drought stress!
 
Intracellular ice formation is always lethal to the cell.
 
Extracellular freezing results in a reduction in the water vapor pressure in the intercellular spaces, thus drawing water from the cell.
 
The concentration of solutes within the cell increases as the water moves to the extracellular ice loci.
 
This increases the concentration of cell contents further lowering the freezing temperature of the cell.
 
The role of water movement in enhancing freezing stress tolerance may partially explain how moderate water stress exposure often improves freezing stress tolerance.
 
The permeability of the plasma membrane is important to the cell's ability to transfer enough water to establish an equilibrium concentration and to prevent intercellular freezing.
 
Intercellular freezing occurs with rapid temperature reductions and is almost certainly lethal.
 
Low temperature hardening - brought on by temperatures around 5° C (41° F) for several days.
 
Cell membranes are known to change and become more fluid. Also, soluble sugars are produced and thought to be important for freezing point depression and may also function as protectants by preventing damage to macromolecules, enzymes, etc.

 
3 to 4 weeks of hardening leads to maximum hardiness.
 
Freezing stress tolerance range
 
Considerable variation in freezing stress tolerance exists among the turfgrasses.
 
Among the cool-season species, creeping bentgrass and rough bluegrass will tolerate exposure to the lowest temperatures, while perennial ryegrass is generally found to be the least tolerant.
 
Gusta et al. (1980) determined crown LT50 values (the temperature at which 50% of the crowns survive) for several cool-season turfgrasses.
 
 
 
Gusta et al. (1980) measured electrolyte leakage following low temperature exposure of various portions of 'Fylking' Kentucky bluegrass plants.
 
With this technique, leaves were determined to have the greatest freezing tolerance during the mid-Winter ( LT50 = -40° C.), followed by crowns (LT50 = -28° C.), and roots and rhizomes (LT50 = -20° C.)
 
Soil and moisture conditions prior to sampling could impact low temperature exposure and freezing stress tolerance.
 
Considerably less is known about the freezing stress tolerance range of warm-season turfgrasses.
 
 
Seasonal variation
 
Beard (1966) observed fluctuations in Turfgrass freezing stress tolerance throughout the fall and winter.
 
Turfgrass freezing stress tolerance was obtained and lost somewhat gradually with peak hardiness occurring in early winter. A similar seasonal distribution of warm-season turfs has been observed for bermudagrass.
 
Exposure to 5° C. day and 2° C. night conditions for 14 days dehardened Kentucky bluegrass, such that it failed to regain its mid-winter freezing stress tolerance levels.
 
Many studies point out that typically turfgrasses lose their freezing stress tolerance in late winter or early spring.
 
Thus, a turf is increasingly susceptible to injury from low temperature conditions and improperly timed cultural practices during this period.
 
This is also a time when desiccation potential is high.
 
Cool soils and warm winds combined to decrease the water absorption capability of the root system while the draw of water from the plant is increased.
 
fertility
 
Nutritional studies are not conclusive or in complete agreement, but they have demonstrated the importance of avoiding excessive nitrogen (49 kg ha-1 or more) fertilization during the fall.
 
Late-season fertilization (just prior to warm-seasoned turfgrass dormancy or around the time of last fall mowing for cool-season turfs) generally results in improved fall color retention and earlier Spring greenup of turfs.
 
Unfortunately, increased applications of N also results in marked reductions in winter survival and freezing stress tolerance of warm-season turfs.
 
This is also true for cool-season turfgrass species, but with their much higher freezing stress tolerance there is minimum impact on winterkill.
 
The damaging effects of increased nutrition on freezing stress tolerance, winter survival, or both of warm-season turf grasses has been offset by increased K nutrition (Gilbert and Davis, 1971, Palmertree et al., 1974)
 
Potassium's role in carbohydrate synthesis and translocation, protein synthesis, regulation of transpiration, and enzyme activity is likely important during the cold hardening period.
 
Moisture content
 
Dehydrated tissues are known to withstand exposure to very low temperatures. Wheat seed exposed to -196° C for 120 seconds terminated if the moisture content was 10.6%, while seed at 25.1% moisture failed to germinate (Lockett and Luyet, 1951).
 
All seed at moisture contents above 30% froze when exposed to -25° C.
 
The tolerance of several cool-season turfgrass species was found to be inversely proportional to the crown moisture content (Beard, 1966). December crown moisture content range from 54% from congressional creeping bentgrass to 85% for annual ryegrass.
 
 
Gusta et al. (1980) found that ryegrass crowns tended to have the highest water content and the poorest freezing stress tolerance among the six genera they evaluated.
 
Crown hardening
 
The main feature that distinguishes a low-temperature hearty plant or tissue from tender tissue is the ease which ice penetrates into the protoplasts.
 
Many types of physiological or morphological responses are correlated with the degree of low-temperature hardiness or tolerance including cell size, carbohydrate level, types of proteins, and hydration level.
 
Actively growing plants generally have a minimum level of low temperature tolerance. Shoot growth of the turfgrass slows and eventually ceases as the autumn soil temperatures decrease below 45°F.
 
Carbohydrate accumulation occurs during this period of minimal shoot growth.
 
Enzymes convert the insoluble carbohydrates to soluble sugars that accumulate in the vacuoles and cause an increase in the osmotic potential.
 
Changes in the protoplasmic proteins result in an increase capability to bind water.
 
The net result is a very significant reduction in the water content of the protoplasm that enables the tissues to achieve a maximum level of low temperature hardiness.
 
A 3 to 4 week period of soil and air temperatures slightly above freezing is required for cool-season turfgrass is to achieve maximum low temperature hardiness.
 
The degree of low-temperature hardiness varies through the winter.
 
Maximum hardiness generally occurs during early winter followed by a slight decrease in hardiness during February.
 
A dramatic reduction in low-temperature hardiness is evident in late winter.
 
The thawing of snow during this period frequently results in standing water and increased crown hydration levels.
 
Physiological alterations
 
Much research has been devoted to identifying the primary site(s) of freezing injury within the cell.
 
The cell membrane has been found to be greatly affected by low temperature stresses.
 
Alteration of membrane permeability, composition, ultrastructure, and electrical characteristics have all been observed.
 
Some of the many physiological changes during freezing stress include increased soluble protein concentration, increased starch, total nonstructural carbohydrates are both, increased sulfhydryl content, increased N content, increased amino acid content, decreased hydration levels, increased photosynthetic activity, increased amylolytic enzymes activity, increased fatty acid content, and an increased ratio unsaturated to saturated fatty acids.
 
Carbohydrate reserves
 
Starch and total nonstructural carbohydrate content increases during the fall hardening period.
 
The increasing carbohydrates is likely a result of decreased shoot growth during the cool fall conditions while the photosynthetic rate declines more slowly.
 
There may also be a decrease in TNC after December due to the use of carbohydrates as an energy source until shoot growth resumes.
 
Nucleic acid and protein synthesis
 
Davis and Gilbert (1970) found an increase in total protein concentration in bermudagrass during cold heartening.
 
While it is thought that freezing stress tolerance is accomplished by accumulation of proteins and sugar, protein accumulation alone is not always enough to improve freezing stress tolerance.
 
Membrane alterations
 
Membranes are important structural components of the cell that are capable of significant environmentally induced modifications.
 
Phospholipids and their fatty acid portions are major components of biological membranes.
 
The fatty acyl chains of phospholipids can exist in a gel or liquid crystalline structure state depending on their phase transition temperature. This transition temperature depends, in part, on the length and degree of unsaturation of the fatty acyl chains.
 
 
Membrane rigidity is favored by the presence of long chain, saturated fatty acyl chains as components of the membrane lipids.
 
Membrane fluidity is enhanced by the incorporation of short chain polyunsaturated fatty acyl components of the membrane lipids.
 
A more flexible membrane system could limit or decrease membrane disruption during freezing.
 
There is a significant correlation between membrane liquid constituents in the activity of the membrane-bound enzymes.
 
Organelle effects
 
The protoplast will typically freeze prior to the vacuoles.
 
Rogers et al. (1977) found bermudagrass chloroplasts changed from an elongated shape in late summer to a globular appearance during the fall. This was compared to zoysiagrass chloroplasts which maintain a consistent shape from summer through fall.
 
Chilling Stresses
 
 
Chilling stress- low temperature injury without freezing only occurs on warm-season turfgrasses.

Chilling injury in warm season turfgrasses - symptoms necrotic lesions, loss of chlorophyll, reduction in photosynthesis, stoppage of growth.

Chilling injury appears to be related to a loss of membrane function. Lipids in membranes need to become more saturated at chilling temperatures. Loss of membrane permeability results in leakage of cell constituents I.e. ion leakage similar to heat stress.

Bermudagrass shows chilling injury when exposed to chilling temperatures and high light intensity. Appearance of anthocyanin pigments In leaf tips is a sign of chilling injury.

Iron applications can help reduce chilling injury in bermudagrass.
 
Turfgrass chilling injury has been characterized by the presence of necrotic lesions, loss of chlorophyll, reduction in photosynthetic rate, and the secession of growth.
 
Direct showing injuries characterized by the rapid development of symptoms within 24 hours. Soybean injury has been observed at 2° C. after only a 5-minute exposure.
 
Such injury is too rapid to be accounted for by metabolic disturbances.
 
In contrast, indirect chilling injury involves a slow development of symptoms after several days exposure to stress conditions.
 
Physiological alterations
 
Chilling injury appears to involve several physiological dysfunctions including a loss of amylolytic activity, decreased carbon dioxide exchange rate, reduced net photosynthesis/dark respiration ratio, and increased photo-oxidation of chlorophyll.
 
Leakage of K+ ions is a commonly observed manifestation of direct chilling injury.
 
Ion leakage is slow at first, followed by rapid leakage phase that occurs currently with lesion appearance.
 
Increases in membrane permeability due to indirect chilling injury also result in increased leakage of ions and amino acids.
 
Starvation
 
Chilling indirect injury due to starvation has been postulated, but unconfirmed.
 
Thylakoid membranes of chill-sensitive species are less able to maintain a light-induced high energy state at chilling temperatures.
 
Such damage and chlorophyll photo-oxidation at chilling stress temperatures could result in a much greater respiration rate than photosynthetic activity.
 
This imbalance can be reason to lead to starvation, but observed cases of chilling injury occur long before plant food reserves are depleted.
 
Desiccation
 
Desiccation- drying out of the plant most severe on exposed sites

a) Elevated sites
b) areas with excessive wind
c) where surface runoff is high
d) air temps above 32° F

Snow cover is very important in reducing winter desiccation

Atmospheric winter desiccation
a) Shallow root system
b) Viscosity of water increases at lower temperatures
c) Root permeability to H2O decreases at lower temperatures
d) Soil water is frozen

Non-creeping turfs have more tendency to be injured by desiccation- no stolons or rhizomes that are more tolerant to stress conditions

Prevention

1) Apply water! - Water wagons

2) Reduce ET
a) windbreaks - sticks and branches can be used as windbreaks and to increase snow cover
b) Mulching or topdressing - used to insulate crown
c) synthetic covers - used to prevent desiccation

 
 
 
Covers have become more and more popular particularly in more northern climates than ours. Although some people do use them in Illinois.
 
Covers provide good protection against desiccation damage; however, they provide minimal protection against freeze damage. Why?
 
Answer: Freeze damage occurs most likely at night when temperatures are the coldest. Under these conditions, blankets provide only minimal protection.
 
What to look for in a synthetic cover. A high degree of light, air, and water permeability.
 
Covers that are too thick or solid prevent good exchange of air and most importantly light. Turf can become stemmy and etiolated under these conditions.
 
Frost Heaving
 
The upward lifting of the plants from their normal position that exposes the root and crowned tissues to atmospheric desiccation is called heaving.
 
Young turfgrass seedlings can be lifted completely above the soil.
 
Specific prerequisites are necessary for frost heaving to occur.
  1. Surface soil temperatures must be below 32°F.
  2. The capillary pores size in the soil must be sufficiently large to permit a reasonable amount of water movement to the basal site of freezing.
  3. Free water must be available in the soil for movement by molecular cohesion to the site of freezing.
 
Frost heaving is most common:
  1. in fine textured soils
  2. in soils having a high water content
  3. during periods when the temperature fluctuates above and below freezing
  4. when no snow cover exists
 
A thin layer of ice develops at the soil surface during gradual temperature drops below freezing.
 
Vertically oriented ice lenses or columns are formed as water freezes at the bottom of this layer.
 
They continue to grow in length as water moves through the soil to the site of freezing.
 
A frozen mass of segregated water lifts the first layer of ice higher and higher.
 
Heaving continues as long as water is available and the heat released during freezing of the water escapes upward.
 
Ice Encasement
 
Direct ice cover injuries due to the ice functioning as a barrier to the exchange of gases between the turfgrass tissue and the atmosphere.
 
Two mechanisms are hypothesized for direct ice cover kill.
  1. The kill results from oxygen suffocation caused by the exhaustion of the oxygen supply required for respiration processes
  2. The accumulation of toxic gases such as carbon dioxide and cyanide evolved from the respiration or oxidation of living tissues, thatch, soil organic matter, soil organisms, or certain fungi.
 
Is ice cover a big problem on turf?  Minor in comparison to winter desiccation injury.
 
Ice removal techniques
  1. milorganite
  2. solid core aerification etc
 
Most cool-season turfs can survive 60 days or more without injury

However repeated freeze thaw cycles can lead to freezing injury more than ice damage.

Ice rinks on Kentucky bluegrass up to 150 days under frozen conditions Creeping bentgrass 150 days Annual bluegrass 75 days.
 
Winter Diseases
 
Pink snow mold - microdochium nivale - often called copper spot because of the copper color of the infection centers. Can go much farther into the growing season than most superintendents expect. See in early June at times. Can occur without or without snow cover. Patches tend to be larger with snow cover.
 
Grey snow mold - Typula incarnta and T. Ishikariensis - Usually requires 3-4 months of continuous snow cover. Occurs mostly in the northern states of Minnesota, Wisconsin, Michigan, New York, and New England.
 
Cultural practices
 
Effects of cultural practices on hardiness
  1. Improve drainage - poor drainage can lead to wet conditions that prevent the crown from removing excess water.
  2. Alleviate soil compaction and poor percolation.
  3. Proper timing of N fertility. Avoid applications in Oct or after hardening has begun.
  4. Adequate or above K. Helps reduce tissue water contents and ensures proper hardening.
  5. Raise the cutting height
  6. Remove excessive thatch. Reduces crown insulation
  7. Shade can reduce carbohydrate levels that impact winter hardiness. Poor drainage is probably the biggest cause of freezing injury.
  8. Manage traffic, both foot and vehicle, when turf tissue is frozen.
  9. Use protective covers where practical.
 
Tissues damaged are the meristem or crown tissues at or below soil surface.
 
Crown hydration in March caused by environment.