NEWS for North Dakotans
Agriculture Communication, North Dakota State
University
7 Morrill Hall, Fargo, ND 58105-5665
June 4, 1998
Frost damage to small grains is highly dependent on the stage of the crop, the actual sub-freezing temperature reached and duration of the low temperature, and environmental conditions that existed before the frost occurred, says a plant scientist with the North Dakota State University Extension Service.
"Small grains are among the most frost tolerant of crops," says Michael Peel, NDSU extension agronomist. "From early developmental stages to jointing, small grains can easily withstand temperatures of 25 F."
After jointing, the growing point moves above the soil surface and is more susceptible to frost damage. From boot through flowering, small grains are most sensitive to frost and will not tolerate temperatures below 28 F, Peel notes. But when temperatures are below 50 F for several days before the frost, small grain crops go through a hardening process and develop more frost tolerance. Drought stress prior to frost can also cause hardening of small grains and increase frost tolerance. Prior to jointing, hardened small grains have been shown to withstand temperatures as low as 14 F.
Once a plant's growing point is above ground, it is more likely to be injured by frost, but Peel says small grains in the jointing stage have been shown to withstand temperatures of 25 F without damage to the growing point, although injury is more likely than at earlier stages.
Small grains are most susceptible to frost from late boot through flowering, the stage when reproductive tissue is developing. Anthers, the plant's pollen-producing structures, are easily damaged by frost, which prevents or severely reduces pollen production. Frost injury occurs during boot through flowering when temperatures fall below 28 F. Barley is more susceptible to frost injury than wheat during the boot stage because it flowers while still in the boot. The longer low temperatures persist, the greater the potential for injury, Peel says.
Small grain plants can avoid injury through a process called super cooling, particularly when temperatures cool slowly, explains Peel. Super cooling occurs when water from within plant cells moves out—the effect is to lower the temperature at which water within the cells will freeze. This reduces the potential for ice crystal formation and physical damage to cell membranes.
Evaluating Damage
Before evaluating frost damage, producers should wait two or three days so injury is more easily observable. Most winter wheat fields presently are in the boot to flowering stage and are at the greatest risk for frost injury. Evaluating these crops for damage involves dissecting flowers and looking at the anthers, says Peel. Darkening with a watery appearance indicates frost injury. Damaged anthers will not shed pollen. If pollen shed has already occurred, florets need to be evaluated for kernel development, which may require several days.
During early development, the growing point of a small grain plant is below the soil surface, thereby making it less susceptible to injury. With this protection, plants can suffer loss of above-ground foliage without dying. Again, Peel advises producers to wait several days to make an accurate determination of injury. At this point, new growth on plants should have begun.
"If no regrowth is observable, the stem of the plant may be split to inspect the growing point," explains Peel. "The growing point should be white or cream colored. Darkening or softening, with a watery appearance of the growing point, indicates injury and usually precedes plant death."
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Source: Michael Peel (701) 231-8037
Editor: Dean Hulse (701) 231-6136