FREEZE TOLERANCE, PART II. Editor’s Note: This is part two of a two-part series. Read part one Here.
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 single 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 cultivars 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).
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’ crowns or stems are not “feeling” the sub-zero temperatures. The temperature under the snow cover is considerably warmer.
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 “rises” 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 4) shaded areas tend to be wetter, which may contribute to the severity of freeze/thaw cycles in late winter.
COLORS OF THE SEASON: PURPLE & YELLOW
Discoloring of Creeping Bentgrass. The up and down weather conditions that have occurred over the last few weeks has resulted in creeping bentgrass going off color. These patches represent segregated-out clones of the creeping bentgrass. The purpling is especially evident on Penncross creeping bentgrass, and the older types of bentgrass cultivars and mixes, but will likely occur on all seeded bentgrass cultivars. The purplish color occurs when temperatures get cold in late fall. During relatively warm sunny days (60 to 65 F), the plants are actively photosynthesizing creating photosynthates (sugars). At night, the sugars are translocated down into the storage areas of the plant. However, if nighttime temperatures are cold, some of the sugars fail to translocate. The molecules left in the leaf attach to a pigment called anthrocyanin (purple or blue pigment). The buildup of anthrocyanin from this process results in the expression of the purplish color. The purplish color will persist through the winter and into early spring. Once temperatures rise and turf growth begins again, the purplish color will disappear. The most common misdiagnosis that I have seen is that superintendents may think they have Drechslera melting-out. This misdiagnosis could result in an unnecessary fungicide application.
Yellow Patch. Yellow patch, also known as cool-weather brown patch, cool temperature brown patch and winter brown patch is caused by the fungus Rhizoctonia cerealis. Symptoms appear as light brown, yellowish rings or patches with lesions often absent. The disease is most prevalent on annual bluegrass/creeping bentgrass greens, but may also occur on bermudagrass. Yellow patch is usually most obvious in spring, but this disease becomes active in the late fall when annual bluegrass/creeping bentgrass turf ceases top growth.
SOIL TEMPERATURES.
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