Frost Damage And Control

Frost is a climatic hazard that causes serious damage to standing crops in temperate and subtropical climates. In Australia, crop losses due to frost are huge. In New South Wales (NSW) alone, a big portion of the state's fruit, vegetable, and grain crops is lost to low-temperature damage (Degan, 1989). The losses to the wheat crop vary between 5 and 35 percent due to heavy frost in late September and early October (Boer, Campbell, and Fletcher, 1993). Much distress can be avoided by properly understanding the characteristics of the frost, by using early warning information on frost, and by adopting frost protection measures. For this, the planning should begin before the crop is planted.

Frost is a weather hazard that occurs when the environmental temperature drops below the freezing point of water. It can be a white frost (also known as hoarfrost) or a black frost. White frost occurs when atmospheric moisture freezes in small crystals on solid surfaces. Black frost occurs when few or no ice crystals are formed because air in the lower atmosphere is too dry, but the damaging effect of the low temperatures on vegetation is the same as that of white frost.

Frost is formed through the physical processes of radiation and advective cooling. These are referred to as radiation frost and advection frost, respectively (Wickson, 1990).

• Radiation frost occurs when a clear sky and calm atmosphere (winds less than 8 kph) allow an inversion to develop, and temperatures near the surface drop below freezing. The thickness of the inversion layer varies from 10 to 50 m. The term inversion comes from atmospheric conditions being inverse to the normal daytime condition in which air temperature decreases with height. Plants can be successfully protected from radiation frost.

• Advection frost occurs when a cold air mass invades a relatively warm area suddenly. Under advection frost, winds may be above 7 kph and clouds may exist. The advected cold air mass may be 150 to 1,500 m deep. Plants can be protected from advection frost to a limited extent.

Frost Damage to Plants

Damage to plants from frost occurs because it results in freezing of the plant tissues. Freezing of plant tissues is a physical process triggered by ice-nucleating bacteria, the intensity and duration of the night temperature to which the plants are exposed, and the plant growth stage (Jamieson, 1986; Woodruff, Douglas, and French, 1997). Green plants contain mostly water, and on freezing, the water expands and ruptures the cell walls of the plant tissues. Because of the presence of chemicals in the sap, plant tissues freeze at temperatures lower than 0°C, the freezing temperature of water. When frozen water melts, it leaks away from the cells. The rupturing of the cells and leakage of water results in the death of tissues, giving a typical "burn" appearance to the plants.

Plants show different symptoms of frost injury, depending on the stage at which freezing occurs (Table 3.3). In the case of wheat, freezing stress can cause foliar injury and tiller death. Injury to developing foliage will not affect the crop yield because the plants can compensate. However, freezing injury during stem elongation can substantially reduce the final yield. Leaf injury can occur at any stage of development, and frozen leaves will appear dark in color. Slightly injured leaves will have yellow tips that should not be confused with the symptoms of nutrient stress (Youiang and Ellison, 1996). Injured stems appear discolored and often distorted near the nodes. Injury to young ears can cause the whole ear to die. At the booting stage, frost injury can damage the reproductive parts of some ears. The injury is easily detected after ear emergence because growth of floret and spikelet look stunted. During flowering, the reproductive parts of the plant may be damaged in some ears, and although they appear unaffected, they produce no grain.

Methods of Protection against Frost

Frost protection methods may be divided into passive and active forms (Powell and Himelrick., 1998; Mavi, 2000). Passive protection involves methods such as site selection and variety selection and several cultural practices such as brushing and soil surface preparation. These methods do not require expenditure of outside energy sources. Active protection systems replace radiant energy loss by using methods such as irrigation, heaters, and wind machines. Active methods require outside energy to operate. The proper choice of a protection method depends on many factors, such as site, crop, advantages and disadvantages of the protection methods, relative costs, and operating principles of the method.

TABLE 3.3. Frost damage to crops, vegetables, and fruits

Temperatures (°C) harmful to plants in the developmental phases

Temperatures (°C) harmful to plants in the developmental phases

TABLE 3.3. Frost damage to crops, vegetables, and fruits

Plants

Germination

Flowering

Fruiting

Crops

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